Experimental and Applied Mechanics, Volume 6

The dynamic crack velocity is a key-parameter involved in the micro-mechanics based modelling of the tensile damage of geomaterials under impact loading. However, because of technical difficulties, very few experimental methods have been proposed in the literature. In this work, a new set-up is presented based on spalling tests. A compressive pulse is transmitted to a parallelepipedic specimen by means of a Hopkinson bar. It is reflected as a tensile wave on the opposite free surface of the sample. A short notch is used to trigger a single crack whereas a larger notch provides a rocking effect of the rear part of the specimen. This experimental configuration has been optimized using numerical simulation analysis. Finally, a series of tests have been conducted on dry and wet concrete. Crack gauges and ultra-high speed camera coupled to Digital Image Correlation have been used to determine the crack speed in this material.

Numerical and experimental results are presented from a project aimed at predicting the fatigue life of a rotorcraft airframe component subjected to flight load spectrum. The airframe component is a riveted joint used on cabin frame cap splices of several civilian and military helicopters which hereafter is modeled as lap-joined nested-angle plates. This component is fatigue sensitive due to the highly cyclic and vibratory rotorcraft mission spectrum and as such prediction of its fatigue life is an important part of the design cycle. This paper presents a systematic approach that combines 3D finite element simulation in ABAQUS and 2D damage analysis in NASGRO to estimate the life of the component. In the numerical analysis, fatigue crack growth rates for throughthe- thickness crack initiating from fastener holes is computed using 2D standard and weight function models with the crack plane stress field obtained from 3D FEA analysis. Effect of load interaction due to tensile overload is included using strip-yield retardation model. Finally, results of the numerical simulations are compared with representative experimental data obtained under similar spectrum loading condition.

Hydroxyapatite (HAP) displays very excellent biocompatibility in the body and is for the most part used in biomedical applications thanks to well biocompatibility for replacement of bone. In fact, Boron and Ti containing HAP are the bioactive materials and it can incorporate into bone structures, supporting bone in-growth without breaking down or dissolving, and it interacts with the living tissue due to the presence of free calcium and phosphate compounds. Boron containing HAP are also extensively useful for the manufacturing of bio-ceramics in order to improve the physical and chemical properties of biomaterials. Boron nowadays used in HAP applications is a very successful candidate material for bioceramic engineering. Generally, Al2O3 powder is added to HAP powder in order to obtain high fracture toughness. Al2O3 has good mechanical properties as compared with HAP, and exhibits extremely high stability with human tissues. In this paper, the effect of microwave sintering temperature on the relative density, hardness, and phase purity of compacted bovine Hydroxyapatite (BHA) powder was reported. This research is a comprehensive attempt to develop Hydroxyapatite bio composite ceramics reinforced with alumina - Al2O3, pure metallic titanium and pure pulverised boron powders. A Finite Element (FEM) analysis is also used to simulate the macroscopic behaviour of this material, taking into account the relevant microscopic scales. Generally, microwave-sintered samples showed much small grain size and a uniform microstructure. For this reason, the behaviour of bio ceramics in case of rapid heating in microwave was also discussed. Recent results revealed that microwave processing was a promising method for sintering porous bio ceramics thanks to clean and shorter sintering time regarding to conventional sintering methods.

This paper reports damage analysis of TiB2 (ceramic particles) reinforced steel matrix composite sheets. This new steel composite receives much attention as potential structural materials due to their high specific strength and stiffness. The goal of the research described in this paper is to study the usage of this new steel family in the manufacture of light structures. Therefore, titanium diboride TiB2 reinforced steel matrix composite sheets were characterized by optical and scanning electron microscopes after the mechanical tests carried out on the base metal and welded specimens under dynamic and static test conditions. However, the non homogeneity of the structure in this type of composites makes deeply complexity of their numerical and analytical modelling to predict their damage during the loading. For example, the interfaces essentially play a key role in determining mechanical and physical properties. A Finite Element (FEM) analysis is also used for modelling to simulate the macroscopic behaviour of this material, taking into account the relevant microscopic scales.

Gaskets/seals in PEM fuel cells are exposed to acidic, humid air, mechanical compressive pressure and cyclic temperature environment. Chemical degradation of three elastomeric gasket materials in a simulated and an aggressive accelerated fuel cell solution at 80oC up to 63 weeks was investigated in this work using dynamic mechanical analysis (DMA) which assesses the change of dynamic mechanical properties of the three material samples as they aged. The three materials tested are copolymeric resin (CR), liquid silicone rubber (LSR), and fluorosilicone rubber (FSR).

Planar Solid Oxide Fuel Cells (SOFCs) are made up of repeating sequences of thin layers of cermet electrodes, ceramic electrolytes, seals, and current-collectors. For electro-chemical reasons it is best to keep the electrolyte layers as thin as possible. However, for electrolyte-supported cells, the thin electrolytes are more susceptible to damage during production, assembly, and operation. The latest-generation electrolyte-supported SOFCs employ a honeycomb-type support structure that includes both thick and thin regions within the electrolyte. The thin regions are more electro-chemically efficient, while the thick regions provide mechanical support. The performance of the electrolyte within the context of a mechanically and thermally loaded stack is being investigated. Temperature profiles are obtained from CFD models. Mechanical and thermal material properties are obtained from experiments with individual components. Single-component models are used to further characterize components with complex geometries, particularly the electrolyte and the compliant, electrically conducting metal foams. A 2-D stack model uses the data from experiments and single-component models to evaluate the mechanical response of the cells to loads comparable to those experienced in assembly and operation. The model is run both at room temperature and at operating temperature to determine which parameters can best reduce the demands on the brittle electrolyte without sacrificing electro-chemical efficiency.

The impulse imparted onto a structure is a major component in the study of fluid-structure interactions during a blast loading event. A better understanding of the impulse transferred to a structure will lead to an improved evaluation of an object’s blast performance. However, limited experimental studies have been performed to determine the impulse imparted to a structure during a blast event. In this study, a comprehensive experimental study on the impulse imparted to free standing monolithic plates under blast loading is conducted. A series of aluminum and steel cylindrical plates were subjected to various experimental loading conditions using a shock tube apparatus. The motion of the specimens was captured using a high speed camera, Photron SA1, to determine their velocity and momentum. The relationship between the impulse transferred to the specimens and the shock wave pressures was analyzed. These results were compared with the theories developed by Xue and Hutchinson, 2004 and Kambouchev, et al, 2006, 2007. The comparisons show that the current fluid-structure model needs to be modified in air blast when the compressibility of the fluid cannot be ignored.

Nano-engineered intermetallic composites have been recently developed and identified as an emerging technology for a number of breakthrough downhole applications at Baker Hughes. This type of intermetallic composite is composed of particulate metallic phases with specifically designed intermetallic intergranular interphases, which may behave very differently from conventional metal-based composites and alloys. The customized design of these materials yields unique mechanical and chemical properties preferred for various downhole applications. To characterize the shear properties of the material that often needs special attention, a proven shear test method, the V-notched beam or Iosipescu shear test method (ASTM D5379), was used for the targeted materials. However, when this test method was applied to the selected intermetallic composite, complicated off-axial twopath fractures, rather than a desired pure shear failure mode, were observed. To investigate the failure mechanism and the effectiveness of the ASTM testing method, a full 3-D finite element model was created and investigated using an extended finite element method (XFEM). Based on the finite element analysis results and the additional shear tests of a reference alloy with similar base metal composition, the true material systemdependent failure mechanisms were identified and the test method was further validated. A special mechanical design guideline for this innovative nano-engineered intermetallic composite material was also recommended.

The double cleavage drilled compression (DCDC) fracture test uses axial compression to drive stable cracks in glasses and brittle polymers. The cracks are generated by regions of tension in a rectangular column of material containing a central hole. The observed relationship between crack length and the applied axial stress is fitted with a two-dimensional finite element model to estimate fracture toughness. The model is applied to previous DCDC experimental results for poly(methyl methacrylate) (PMMA) samples of varying thicknesses. Both plane stress and plane strain cases are considered. Three dimensional finite element models of the DCDC test indicate plane stress analysis is the most applicable condition and suggest explanations for the effect of sample thickness.

In order to validate an existing energy-based fatigue life prediction understanding, the strain energy accumulation for interrupted loading cycles was analyzed. The life prediction method being validated was developed based on the understanding that strain energy density accumulated during monotonic fracture is a physical damage quantity that is equal to total cumulative hysteresis strain energies in a fatigue process. If this understanding is true, it is possible to suspend cyclic loading for long periods during a fatigue procedure, and then resume the procedure to failure, resulting in the same fatigue life as if the fatigue test was conducted continuously to failure. This assumption, along with critical analyses such as surface roughness and loading frequency, is tested empirically on Titanium 6Al-4V (Ti64) axial tension-compression specimens in the lifetime regime of 5x103 and 6x104. The failure results are compared, with encouraging results, to the aforementioned energy-based prediction method, thus validating the theory and the prediction capability.

The ultrasonic impact treatment (UIT) is one of the new and promising processes for fatigue life improvement of welded elements and structures. In most industrial applications this process is known as ultrasonic peening (UP). The beneficial effect of UIT/UP is achieved mainly by relieving of harmful tensile residual stresses and introducing of compressive residual stresses into surface layers of a material, decreasing of stress concentration in weld toe zones and enhancement of mechanical properties of the surface layers of the material. The UP technique is based on the combined effect of high frequency impacts of special strikers and ultrasonic oscillations in treated material. Fatigue testing of welded specimens showed that UP is the most efficient improvement treatment as compared with traditional techniques such as grinding, TIG-dressing, heat treatment, hammer peening and application of LTT electrodes. The developed computerized complex for UP was successfully applied for increasing the fatigue life and corrosion resistance of welded elements, elimination of distortions caused by welding and other technological processes, residual stress relieving, increasing of the hardness of the surface of materials. The UP could be effectively applied for fatigue life improvement during manufacturing, rehabilitation and repair of welded elements and structures. The areas/industries where the UP process was applied successfully include: Shipbuilding, Railway and Highway Bridges, Construction Equipment, Mining, Automotive, Aerospace. The results of fatigue testing of welded elements in as-welded condition and after application of UP are considered in this paper. It is shown that UP is the most effective and economic technique for increasing of fatigue strength of welded elements in materials of different strength. These results also show a strong tendency of increasing of fatigue strength of welded elements after application of UP with the increase in mechanical properties of the material used.

The paper describes the development of an experiment with associated analysis to determine the toughness of a molecular adhesive formed by bringing together two functionalized silicon surfaces. Si (111) surfaces were coated with amine and carboxyterminated self-assembled monolayers (SAMs). The areal density of these termini was varied in order to modulate the bonding interactions between the two surfaces. The silicon beams were pressed together to form miniature laminated beam beam specimens and then fractured in high vacuum using a specially developed fracture tester. Normal crack opening displacements were measured using infra red crack opening interferometry.

Experimental study was performed to investigate the fracture behavior of relatively brittle polymer foam. A single notch bending specimen made of a PVC core cell foam A-series, A 800 and A 1200, are used for the investigation. To measure the strain around the defect section, a 2D digital image correction (DIC) technique was used. The fracture initiation toughness was calculated from the load displacement curve and a strain fields obtained from DIC technique. Furthermore a study was performed to investigate the failure behavior of foam core with sharp cracks, notch and circular hole. To reduce the size effect, the net cross-sectional areas of the specimen for all the geometries considered are kept constant. An Instron tensile loading machine was used and the tensile load was measured directly through the load cell. The full strain field around the section was measured using DIC and the data points at the interest location were extracted. The result was compared with the dog-bone tensile experiment of intact specimen. It was observed that, the net section strength for specimen with cracks, notch and circular hole is higher than that of the intact foam core.

The problem of crack growth in a three-point bend specimen made of polymeric foams is investigated. Polymeric foams are anisotropic materials and cracks, generally, propagate under mixed-mode loading. The axes of material anisotropy are inclined with respect to the crack plane. Due to the anisotropy of the material crack kinking occurs even though the applied load is symmetrical with respect to the crack plane. The study will take place within the frame of linear elastic fracture mechanics of anisotropic media. The strain energy density criterion will be used for the determination of the critical load of crack initiation and crack growth path under mixed-mode loading. The opening-mode and sliding-mode stress intensity factors are determined by a finite element program. A special circular core element surrounding the crack tip with 19 nodes is used. The core element is joined to the conventional 12-node quadrilateral element by requiring that the displacement at the nodes on the circumference of the core element match the corresponding singular solution. Results are obtained for the critical load of crack initiation and crack growth trajectories as a function of the orientation of the axes of material symmetry and the anisotropy of the material.

Tin whisker growth has become a reliability concern during the course of Pb-free solder transition. A common mitigation strategy is to use conformal coatings to prevent electrical shorts by tin whiskers. However, recent research indicates that under elevated temperature and humidity, or in areas of thin covering, whiskers can grow and penetrate conformal coatings. Additionally, for long environmental exposure, the effectiveness of conformal coatings may be compromised.

Planar Solid Oxide Fuel Cells (SOFCs) are composed of repeating cathode-electrolyte-anode units separated by electrically conductive interconnects. With the reduction in fuel cell operating temperature to approximately 800°C, it has become possible to use chromium-based, ferritic stainless steel or Crofer for interconnects. These interconnects must survive the high temperature oxidizing and reducing environments while maintaining electrical conductivity. Unfortunately the formation of chromium oxide scale poisons the cell by significantly reducing cathodic activity. Chromium scale formation can be inhibited by applying an electrically conductive manganese cobalt oxide (MCO) spinel coating to the interconnect prior to its installation in the fuel cell. The most costeffective way to apply protective coatings to interconnects involves spray coating. To investigate the quality of the coatings and the coating adhesion, four-point bend experiments were undertaken at room temperature. Tensile cracking patterns on the convex surfaces of the bend specimens were used to determine the interfacial shear strengths of the coatings. SEM images of the cracked coating surfaces were processed to analyze the interface failure mechanisms, the crack spacing, and areas that spalled at higher strains. These investigations were able to show distinct differences between coatings formed with different processes parameters.

This paper presents parameter-estimation methods developed to determine crack-tip field parameters in anisotropic elastic solids from full-field experimental data. The crack-tip field parameters of interest include not only stress intensity factors but also effective crack-tip positions. Two approaches that are based on least-squares method and conservation integrals were presented. In the approach based on the least-squares method, the Stroh representation of anisotropic elasticity was used to determine the coefficients in the asymptotic expansion of crack-tip fields in anisotropic solids. On the other hand, conservation integrals in fracture mechanics, such as J-integral, M-integral, Interaction integrals, were also used to extract the parameters including the effective crack-tip position. Both approaches were employed to analyze crack-tip displacement fields obtained by using Digital Image Correlation technique during translaminar fracture test of a fiber- reinforced polymer matrix composite. The applicability of homogeneous isotropic plane-elasticity models to translaminar fracture processes in laminated composite materials is also discussed.

In this study, 152.4 mm by 101.6 mm (6”×4”) GLARE 5 plates with various thicknesses were impacted by a 0.22 caliber bullet-shaped projectile using a high-speed gas gun. Velocities of the projectile along the ballistic trajectory were measured at different locations throughout the test using (1) a pair of laser-beam optoelectronic paths near the gun muzzle, (2) two sets of chronographs before and after the specimen, and (3) a high speed camera for recording the trajectory history of the projectile. The high speed camera yielded the most reliable way of measuring the projectile speed and could record the projectile orientation before and after the impact. It was experimentally detected that the measured speed of the projectile at the muzzle of the barrel was less than the actual impact speed of the projectile. The phenomenon can be explained by the well-known intermediate ballistics. The incident projectile impact velocity versus the residual velocity was plotted and numerically fitted according to the classical Lambert–Jonas equation for the determination of ballistic limit velocity, V50. The results showed that V50 varied in a parabolic trend with respect to the metal volume fraction (MVF) and the specimen’s thickness.

The effect of underwater shock loading on an E-Glass / Vinyl-Ester composite material has been studied. The work consists of experimental testing, utilizing a water filled conical shock tube and computational simulations, utilizing the commercially available LS-DYNA finite element code. The plates consist of elliptically curved geometry with 0/90 biaxial laminates. The plates are held with fully clamped boundary conditions and are subjected to underwater explosive (UNDEX) loading. The transient response of the plates is captured in real time through the use of a Digital Image Correlation (DIC) system. The DIC data and computational results show a high level of correlation for both the plate deformation and velocity histories using the Russell error measure. The finite element models are also shown to be able to simulate the onset of delamination mechanisms.

A load applied to a composite is transferred from the matrix to the dispersed phase through their shared interface. The performance of the composite depends strongly on the interfacial adhesion between the constituents. Shear strength, resistance to delamination, and effective load transfer between the matrix and the inclusion are improved when the interfacial bond is optimized. The interfacial adhesion strength between the metallic inclusions, titanium (Ti) and a nickel titanium shape memory alloy (NiTi), and different polymeric matrices, namely polycarbonate (PC), polypropylene (PP), and high-density polyethylene (HDPE), was measured using pullout testing. The adhesion strengths were determined by dividing the force needed to produce debonding of the interface of the polymer/metal strip over the embedded strip area. Results show that PC exhibits the highest adhesion with no difference for either NiTi or Ti. Comparison of the adhesion strength values for PP/Ti and PP/NiTi show a higher and very distinct value for the PP and Ti. In the case of HDPE, the interfacial bonding between the polymer and the NiTi is stronger than with the Ti but the difference is not as big as the one found for the PP composites.

Identification of modal parameters from response data only is studied for structural systems under nonstationary ambient vibration. By assuming the ambient excitation to be nonstationary white noise in the form of a product model, the modal parameters of a system could be identified through the correlation method in conjunction with a technique of curve-fitting. However, the error involved in the approximate free-decay response would generally lead to a distortion in the modal parameters of identification. It is shown that, under appropriate conditions, the ambient response corresponding to nonstationary input of various types can be approximately expressed as a sum of exponential functions, so that we can use the Ibrahim time-domain method in conjunction with a channel-expansion technique to directly identify the major modes of a structural system without any additional treatment of converting the original data into the form of free vibration. To further distinguish the structural modes from non-structural modes, the concept of mode-shape coherence and confidence factor is employed. Numerical simulations, including one example of using practical excitation data, confirm the validity and robustness of the proposed method for identification of modal parameters from general nonstationary ambient response.

Future United States Air Force (USAF) high-speed vehicles will require innovative, non-contacting full-field measurement techniques to validate analysis and design practices. In this experimental investigation, the authors explore the feasibility of using high-speed 3D digital image correlation (DIC) to measure the geometrically nonlinear and stochastic response of a compliant panel representing thin-gauge aircraft-like structure. Existing measurement techniques typically employed for this application include laser vibrometry, accelerometers, and discrete strain gages. However, these approaches are limited to a few points or direct contact resulting in altered structural response. The possibility of full-field noncontact displacement and strain measurement is an attractive alternative for this type of dynamic response testing, particularly as one is not limited to predetermined sensor location. The technical challenges of using DIC for this application include extending the technique from quasi-static or extremely short duration transient dynamic measurement technique to steady-state, long-duration (seconds of data) random response. Multiple, long-time sample records are desired for ensemble averaging, and correspondingly high sample rates generate appreciable volumes of digital images never before attempted with this type of analysis. DIC displacement and strain results are compared to the more traditional measurement methods to establish accuracy. Results demonstrate the feasibility of using DIC for nonlinear dynamic displacement and strain response measurements. The ability to obtain full-field displacement data was beneficial towards identification and differentiation of the dynamic panel response from the inherent dynamic response of the experimental facility.

Modal testing of an extremely flexible self-rigidizing inflatable torus with regular pattern of hexagonal domes was carried out. For the first time the feasibility of using a non-contact in-house fabricated electromagnetic excitation in modal testing of such an ultra-flexible inflatable structure was investigated. Non-contact transducers, laser displacement sensors, were used in this study. Non-contact excitation and measurement technique were used to avoid frequency shift problems due to inertial loading on the structure. Within the frequency bandwidth of 1-25 Hz, four in-plane and four out of plane modes’ damped natural frequencies were extracted and compared with prior studies.

In recent years, it has been demonstrated that the ultrasound radiation force can be used as a noncontact technique for modal excitation. A novel capability of this method is the ability to perform selective excitation. A phase shift is imparted between the modulation signals emitted from a pair of ultrasound transducers focused at different positions on an object. When there is no phase difference between the radiation force produced by each transducer, they push in unison on the object, which induces symmetric eigenstates. When there is a 180° phase shift between the radiation force from the transducers, they will excite antisymmetric or torsional eigenstates. We describe some of the first demonstrations of this selective excitation using high-frequency focused ultrasound transducers.

There is an economic imperative to increase the level of innovative activities in developed and developing countries. Universities have been encouraged to increase the number of science and technology graduates. One of the approaches used by universities is the encouragement of minority students, in particular, Black African- American and African-Caribbean students into undergraduate engineering. However, to realize the potential of these students, programs need to build understanding of engineering principles in a manner that appeal to multiple learning styles. The steelpan, a percussion instrument invented in Trinidad and Tobago, can provide a possible solution. The steelpan also known as the pan or steel drum is produced by creatively deforming metal sheet. The pan is a unique musical device and offers an opportunity to teach engineering concepts using an instrument that is a part of the cultural heritage of some of these students. The technology of the steelpan is multidisciplinary and requires knowledge in the areas of materials science, production processes, acoustics, vibrations and music. By decomposing the production of the instrument into these underlying bodies of knowledge, it provides an ideal opportunity to explain and demonstrate engineering principles at low cost. This presentation demonstrates options for courses that use the steelpan to encourage elementary and high school students into the engineering profession.

This paper presents the development of a test procedure and application of non-contacting strain measurement in dynamic tensile testing. The strain time histories of test specimens measured by a laser extensometer were derived by a phase-shift technique based on zero-crossing method. The accuracy of proposed procedure was verified by comparing the strain values measured by the laser extensometer with the actuator measurement of a servo-hydraulic high speed machine using aluminum alloy (AA) 6061-T6, and then applied to Alkaline Resistant (AR) glass fabric reinforced cement composite. Comparison between these two measurements shows a good agreement. The strain rate effect on the mechanical properties of AR-glass fabric-cement composite was found. The failure behavior of the composite was also discussed using the images captured by high speed camera.

This paper describes a method for quantifying defects in fiber based composite materials created during the fabrication process. The defects include voids that arise during manufacturing, micro-cracks produced during cure, and delaminations which occur between lamina. In this method, photomicrography technology is used in conjunction with computer aided design (CAD) software. The photomicrographs, stored as bitmap images, provide a visualization of the bond behavior between the fiber and the resin materials. The CAD software transforms each bitmap image into a two-dimensional work space where the areas of the defects are computed by tracing them using a closed polygon feature. Micro-crack density measurements are made by dividing the sum of the individual micro-crack areas by the total specimen area. The approach is applied to measure the micro-crack densities in materials used for ablative nozzle applications.

An innovative Hopkinson pressure bar system for testing the shear response of materials at high strain rates has been developed. A novel single-lap specimen of a polymer bonded explosive (PBX) is used. Instead of strain gauges mounted on the bars, one quartz force transducer is sandwiched between the clamp and the transmission bar to directly measure the weakly loading forces. A laser gap gauge is employed to monitor the shear strain of the specimen, which is based on the luminous flux method. Finite element code ANSYS is used to analyze the stress state in the specimen. Experimental results show that this new method is effective and reliable for determining the shear stress-strain responses of the soft materials at high strain rates.

The electronic industry continues to dramatically reduce the size of electrical components. Many of these components are now small enough to allow shock testing with Hopkinson pressure bar techniques. However, conventional Hopkinson bar techniques must be modified to provide a broad array of shock pulse amplitudes and durations. For this study, we evaluate the shock response of accelerometers that measure large amplitude pulses, such as those experienced in projectile perforation and penetration tests. In particular, we modified the conventional Hopkinson bar apparatus to produce relatively long duration pulses. The modified apparatus consists of a steel striker bar, annealed copper pulse shapers, an aluminum incident bar, and a tungsten disk with mounted accelerometers. With these modifications, we obtained accelerations pulses that reached amplitudes of 10 kG and durations of 0.5ms. To evaluate the performance of the accelerometers, acceleration-time responses are compared with models that use independent stress and strain measurements. Comparisons of data from all three measurements are in good agreement.

The dynamic equilibrium equation was derived for the film under the test of impact by coated bullet (ICB) to include the effect of the residual stress. Then, the finite element modeling was carried through to investigate the impact responses of the film of different initial stress states. The preliminary results revealed that the residual stresses will influence both the film stress and interface stress of the sample under the ICB test.

Continuous fiber reinforcement in a polymer matrix can create a composite material with excellent strength-to-weight and stiffness-to-weight ratios. In a manufacturing environment, however, it is sometimes necessary to remove portions of the composite to repair defects like wrinkles and voids, particularly for complex part geometries. This results in a discontinuity of the reinforcement and the potential for the repaired portion to prematurely fail in service. The current work details an effort to optimize a resin for scarf repair of glass reinforced polyester composite structures. Results from Double Cantilever Beam (DCB) tests are presented along with details of micro-structural examination of the fracture surfaces. Additional optimization work to be carried out is discussed.

The objective of this investigation was to develop, process, and test hybrid nano/microcomposites with nano-reinforced matrix and demonstrate an enhancement in thermomechanical properties, with emphasis on damage tolerance measured in terms of fracture toughness, impact damage, residual strength, and fatigue life. The material investigated was carbon fabric/epoxy with the matrix reinforced with multi-walled carbon nanotubes (CNTs). A solvent-based method with a dispersion enhancing block copolymer was used to prepare composites with and without CNTs. It was first shown that CNT reinforced composites have higher matrix dominated properties, such as compressive modulus and strength, in-plane shear modulus and strength, interlaminar shear strength, and interlaminar fracture toughness. The composite with 0.5 wt% of CNTs showed noticeably improved resistance to indentation damage by about 16 % and increased damage tolerance in terms of residual compressive strength by about 35 % over the composite without nanotubes. A significant enhancement was also shown under interlaminar fatigue testing with fatigue lives an order of magnitude longer than those of the reference material. The high increase in fatigue life was related to an increase in static interlaminar shear strength, the logarithmic dependence of the fatigue-life (S-N) curves, and an increase in interlaminar fracture toughness.

Impact responses and damage induced by a drop-weight instrument on GLARE 5 fiber-metal laminates with different layup configurations and geometries were studied. The damage characteristics were evaluated using both the nondestructive ultrasonic and mechanical sectioning techniques. Only the contour of entire damage area could be obtained using ultrasonic C-scan whereas more details of damage were provided through the mechanical cross-sectioning technique. It was found that failure mode changed with varying stacking sequence. GLARE 5 made of unidirectional fibers had the worst impact resistance; followed by cross-ply and angle-ply configurations, while the quasi-isotropic lay-up showed the best resistance to impact. Finally, influence of different geometries was considered. The results show that by introducing circular geometry, damage patterns and impact behaviors were changed. This was especially apparent for panels with the quasi-isotropic layup configuration.

Finite element analysis of an extremely flexible inflatable self-rigidizing torus (SRT) with regular pattern of hexagonal domes was carried out. Owing to the large number of hexagonal domes in the SRT, a simplified sub-structuring technique was proposed. In this method, each hexagonal dome was replaced with a statically equivalent flat hexagon with the same mass and stiffness as the hexagonal dome. Then the finite element modal analysis of the SRT was carried out for an equivalent torus made of flat film. The geometric nonlinearity and the effect of the follower load on the stiffness were included in the analysis. Natural frequencies and mode shapes determined by the finite element modal analysis were compared with those obtained from the earlier modal testing.

Biologically inspired wings of micro munition vehicles are constructed with the pre-strained hyperelastic membrane, attached with composite reinforcing structures. Finite element models are developed for the modal characteristics of the flexible wings of micro munition vehicles and validated by experimental results. The effect of added mass, damping, and aerodynamic loads on the modal characteristics (natural frequencies and mode shapes) of the wings is investigated. The wings are vibrated in the vacuum and air environments for investigating the effect of added mass and damping on their modal characteristics. Aerodynamic loads are calculated from the wind tunnel test data where the angle of attack of the wings and free stream velocity of air are varied. Natural frequencies increase with mode, pre-strain level of the membrane, and aerodynamic loads but decrease in air from those in the vacuum environment due to the added mass of air. Damping of air is low and has minimal effect on the natural frequencies of the wings but helps to reduce the out-ofplane modal amplitude of vibration. The effect of added mass, damping, and aerodynamic loads on the mode shapes of the wings is also presented in the paper.

The rapid advancement of computing power and recent advances in numerical techniques and material models have resulted in accurate simulation of ballistic impacts into multi-layer transparent armor configurations. Transparent and opaque materials are used in protective systems for enhancing survivability of ground vehicles, air vehicles, and personnel. Transparent materials are utilized for face shields, riot gear, and vehicle windows, in addition to other applications for sensor protection, including radomes and electromagnetic (EM) windows. For both transparent and opaque protective systems, low velocity impact damage can compromise structural integrity and increase the likelihood of further damage or penetration from a high velocity impact strike. Modeling and simulation of material impact by various threat types has proven to be a significant analysis tool in the identification of damage mechanisms and the failure process. The impact of laminate targets consisting of a series of glass layers adhered to each other by polyurethane and backed by a polymeric backing layer, was modeled and simulated. The failure mechanism of laminate targets was studied by ANSYS/AUTODYN [1] commercial software and the results were compared to available experimental data from various nondestructive techniques. Successful output of this modeling effort will provide useful information for the mitigation of damage propagation through targets used in protective systems and it will help to establish an economical damage acceptance criterion for any future material prior to its fielding.

Many steel truss railroad bridges use tension members consisting of eyebars. After years of service, these eyebars may exhibit significant wear at the pin holes, leading to the development of unequal tension stresses among eyebars within the same set. To restore bridge performance, eyebar tensions are often equalized through flame-shortening, where the tension in any eyebar is determined by measuring its fundamental natural frequency and then converting this frequency to the corresponding tension through a simple analytical model. In practice this model assumes that the ends of the eyebar are pinned; however, for transverse vibration about the minor axis, a significant degree of rotational restraint will often exist, potentially resulting in very inaccurate tension estimates. This paper presents the results of analytical studies, model testing, and field observations that focus on improving the prediction of eyebar tensions from observed natural frequencies. Besides supporting the flame-shortening process, the ability to achieve greater accuracy in tension measurements also provides for valuable applications to bridge rating.

Cellular solids are materials comprised of an interconnected network of solid ligaments or plates which form the edges and walls of cells [1]. These types of materials are often considered in engineering applications because of their unique properties relative to their fully-dense counterparts, including elastic moduli, specific strength, and thermal conductivity [1]. Classical cellular mechanics models, such as the Gibson-Ashby model, have been extensively used to describe the mechanical response and idealize the behavior of foam-like materials by modeling unit-cell level deformation. Scaling equations provided by such mechanics formulations allow for comparison between theory and experiment through readily measurable quantities such as relative density. These mechanics models are applied to cell sizes on the order of several micrometers or larger, and often to materials with random cell structures in the absence of a complete understanding of the mechanics governing local deformation. Direct experimental characterization of unit-cell level deformation has been scarce and generally limited to either two-dimensional or post-mortem analyses. Extensions to micro- and nanostructured systems are further complicated by size-dependent mechanical behavior where the constitutive response of the ligament material is not always known a priori.

Recrystallization of the grain structure of metals into nano-sized grains by using mechanical means, has received wide attention in the last two decades. It is well known that materials with a fine-grain crystal structure have favorable properties compared to the same materials with course-grained crystal structure. Surface Mechanical Attrition Treatment (SMAT), a technique developed in the early part of this decade, has been successfully used to recrystallize the surface grains of metals into nanocrystals of the order of 10 to 100 nanometers from their original grain sizes on the order of 10 to 30 microns. Resulting enhancement in surface properties has quite interesting applications, varying from materials with improved fatigue resistance to medical devices. In this study, our focus is on experimental characterization of the enhancement in mechanical properties of surface nanocrystallized metals. Copper, Aluminum and Titanium samples are subjected to SMAT under different conditions followed by appropriate heat treatment. Microindentation and nanoindentation techniques are conducted to characterize various mechanical properties. Microindentation test shows significant improvement in surface hardness due to SMAT process on these samples. Our initial results from nanoindentation also show significant enhancement in materials surface properties. However, several other interesting characteristics obtained in the nanoindentation tests require further studies for verification.

Seals or gaskets under compressive stress are used in PEM fuel cells (PEMFC) or stacks to prevent leaking of the liquid and gas inside the cell. The fuel cells are normally assembled with bolts or a combination of bolts and springs. As the seal is typically made of polymers, the level of the compressive stress on the seal during long term operation of the fuel cell depends on the stress relaxation property and any potential chemical degradation of the seal materials. In addition, the amount of compression applied to the seal may vary due to temperature changes during the fuel cell operation which causes thermal expansion and contraction of all components in the cell. To understand the sealing force existed in a fuel cell during operation, all these factors must be fully understood. In this study, the compression of the seal in a PEMFC was investigated experimentally. Specifically the compressive amount was measured in-situ, i.e. immediately after the assembly and during the normal operation of the PEMFC. The objective of this study is to gain an understanding of the variation of compressive strain applied to the seal as the temperature of the PEMFC changes and cycles. This information can then be used to estimate the sealing force in the cell and consequently the life prediction of the seal. Both the temperature and pressure are monitored during the tests. An interesting observation is that both the gap spacing and the outside dimensions jumped initially and did not follow the cell temperature’s later rise to a maximum of 80oC. It’s effect by the first gas inlet pressure.

Neutron emission measurements, by means of 3He devices and bubble detectors, were performed during three different kinds of compression tests: (i) under monotonic displacement control, (ii) under cyclic loading, and (iii) by ultrasonic vibration. The material used for the tests was Green Luserna Granite. Since the analyzed material contains iron, our conjecture is that piezonuclear reactions involving fission of iron into aluminum, or into magnesium and silicon, should have occurred during compression damage and failure. This hypothesis is confirmed by the direct evidence of Energy Dispersive X-ray Spectroscopy (EDS) tests. It is also interesting to emphasize that the anomalous chemical balances of the major events that have affected the geomechanical and geochemical evolution of the Earth’s Crust should be considered as an indirect evidence of the piezonuclear fission reactions.

Acoustic emission (AE), electromagnetic emission (EME) and electrical properties measurements have been performed during laboratory compression tests on Green Luserna Granite dry rock loaded with increasing pressure up to their failure. Using a servo-controlled testing machine (MTS) each compression test was carried out at constant displacement rate and under ordinary room conditions. AE signals were detected by applying to the sample surface a piezoelectric (PZT) transducer, sensitive in the frequency range from 50 to 500 kHz. EME signals were detected using a loop antenna (80.0 dB gain at 500 kHz). The time-dependent outputs of the PZT transducer and the electromagnetic tester were connected to a DL708 Yokogawa oscilloscope (10 MSa s–1) in order to acquire simultaneously AE and EME signals associated with the same fracture event. Electrical properties, in terms of relative electrical resistance variation, were monitored by using copper electrodes coupled to specimens by conductive silver paint, and connected to a multimeter (Agilent model 34411A). The recorded AE, EME and electrical properties variation were related to the load vs. time diagram of the tested specimens. The experiments conducted in this research confirm that AE and EME are failure precursors in quasi-brittle materials, moreover EME signals and electrical property variations are related to sharp drops in stress vs. time diagram.

Monotonic tensile tests were conducted following ASTM Standards D3039 (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials) and D3518 (Standard Test Method for In-Plane Shear Response of Polymer Matrix Composite Materials by Tensile Test of a ??45° Laminate), on hybrid and non-hybrid plain weave composite materials. Strips of non-hybrid IM-7 Graphite/SC-79 epoxy called GR for short, non-hybrid S-2 Glass/SC-79 epoxy called GL for short, hybrid GR/GL/GR and hybrid GL/GR/GL specimens were tensile tested. The tests were conducted at –60°C, –20°C, room temperature, 75°C and 125°C. The rule of mixtures was used to predict the Young’s moduli of GL/GR/GL and GR/GL/GR using the experimental values obtained from the stress-strain curves of the GL and GR specimens. The predicted Young’s moduli of GL/GR/GL and GR/GL/GR were then compared to those obtained experimentally. It was found that the calculated Young’s and shear moduli match closely (within 6 %) to those obtained experimentally.

In the present investigation, fibers from banana stem and Bagasse are used in addition to low wt% of silica to cast bio composites. Extraction of the banana fibers, Bagasse fibers and preparation of both banana and Bagasse fibers were carried out as explained elsewhere in literature. The lengths of the fibers are kept between 180 to 425 micrometer. Composites, containing lower fiber content of Bagasse and banana (10 wt %) and higher fiber content (20 wt %) were used with silica (2 wt%) and mixed with epoxy CY 230 epoxy resin. Different mechanical properties such as tensile, compressive, impact, flexural, fracture strength etc are discussed and characterized.

Nature is a popular source of inspiration for the development of new materials and structures. Palmetto wood has been found to be a potential bio-inspiration due to its historically successful mechanical performance to develop materials with enhanced mechanical properties. To understand the basis of mechanical performance of Palmetto wood, its failure mechanism and energy absorbing capacity is elucidated at multiple length scales. The quasi-static and dynamic three-point bend tests has been used to reveal the leading failure mechanisms like shear dominated debonding and pore collapse. The damage evolution under quasi-static and dynamic impact is determined. The sandwich material systems are yet to be investigated in detail to utilize its potential in advanced engineering applications. We present some work on development of sandwich composites with improved mechanical behavior using Palmetto wood as a biological template. The quasi-static and dynamic characterization of nano-enhanced sandwich materials is presented. The leading failure mechanism in Palmetto wood are found to be shear dominated delamination at the macrofiber-matrix interface caused by shear strain concentration and shear cracking in the porous cellulose matrix caused by pore collapse. Reinforcement in the bio-inspired core and the nano-enhancement in the adhesive increase the mechanical properties of the sandwich structure.

Woven fabrics are used in many applications, including ballistic armors, propulsion engine containment systems and fabric reinforced composites. In order to facilitate the design and improvement of such applications, this paper presents the experimental programs of quasi-static uniaxial tension, biaxial tension and picture frame test to obtain the material properties of Kevlar® 49 fabric. This study discusses the stress-strain response in both the warp and the fill directions of the fabric under uniaxial tension, the effective Poisson’s ratio and the in-plane shear response, and also investigates the possible effect of specimen size on the responses of the fabric. The results show that the fabric exhibits non-linear and orthogonal behavior in tension, and can deform up to 20% before the complete failure. The effective Poisson’s ratio is a nonlinear function of strain. The shear response is nonlinear to the shear angle, but not dependent upon the specimen size after normalization. Images were used to demonstrate the deformation and failure behavior of the fabric. All these results are very important for the numerical modeling of the Kevlar® 49 fabric used in engine fan containment systems and other ballistic protection.

In recent years, there has been a large increase in the use of polymers in engineering applications. Silica-styrenebutadiene rubber filled hybrid composites is a material designed with different weight % of silica and styrene-butadiene rubber. Five crosshead speeds (0.01 mm/min, 0.1 mm/min, 1mm/min, 10mm/min and 100mm/min) are considered in this study. The results showed that yield stress and elastic modulus decreased with decreasing crosshead speed. An artificial neural network model is presented to predict the mechanical properties. The result of the analysis of variance (ANOVA) shows a good interaction between filler materials and testing condition by 99% confidence level. In the present investigation Scanning Electron Microscope studies has been done to see the dispersion of silica and styrene-butadiene rubber particles in resin.

Composite sandwich structures find applications in many aerospace systems due to their lightweight and high strength. However, these structures are susceptible to low energy impact damage. Our studies of the damage resistance and tolerance of honeycomb core sandwich structures shows that the performance of the core plays a key role. Thus we have undertaken a study of the compressive and shear behavior of the aluminum honeycomb cores used in several space systems. The study consists of uniaxial and compression/shear tests of the core. Using a novel ring specimen loaded in compression and torsion, we can load along any line in compression/shear stress space in order to map out the core’s yield surface and its evolution. Results from the experiments are used to validate computational models that are part of a larger simulation of the compression after impact strength of honeycomb core composites.

In this study, a light guide component for enhancing light harvesting efficiency of dye-sensitized solar cell (DSSC) was designed and developed through both optical simulated and experimental approaches. The light guide component is capable of being disposed on a photo electrode of DSSC to turn the light toward a dye covered nanoporous TiO2 film (D-NTF). The optical simulation results show that the light harvesting efficiency of DSSC can be effectively improved and the experimental power conversion efficiency can be enhanced as high as 39.78% for active area of 0.25 cm2.

In the last few years the use of guided ultrasonic waves (GUWs) for the health monitoring of engineering structures increased rapidly, with the most recent studies focusing on the application of GUWs to complex structures or to simple structures under varying environmental conditions. Monitoring complex structures is challenging as reflections, scattering, and mode conversion arise. In addition, sensitivity to temperature and surface wetting can degrade the performance of a GUW-based structural health monitoring system. This paper presents the results of an experimental investigation where GUWs were used for the health monitoring of a truss, that was part of a highway variable message structure removed from service. The monitoring strategy proposed here combines the advantages of GUWs with the extraction of defect-sensitive features to perform a multivariate diagnosis of damage. The effectiveness of the proposed approach was tested by monitoring the propagation of waves along one of the main chords of the truss and detecting the presence of two artificial cracks located around the welds that join two diagonal angular members to the chord.

A reliability approach applied to wood structures under seismic excitation is proposed in this paper. This approach is based on First Order Reliability Method (FORM). The mechanical, model which represents a timber roof truss used in individual housing, is detailed and the mechanical nonlinear behavior of the timber joints is defined by hysteresis elements. The uncertainties are taken into account in the numerical model through random variables representing the stiffness parameters at timber joints. The reliability analysis allows us to compute the failure probability and their sensitivity to various random parameters. Numerical examples show the performance and the efficiency of the proposed method.

Previous work has led to the publication of a standard guide for the calibration of optical instruments for making full-field measurements of in-plane strain in pseudo-static cases. A consortium of international organizations drawn from across the innovation process is engaged in extending this earlier work to allow the calibration of instruments capable of making fullfield measurements of in-plane and out-of-plane deformations resulting from time-varying loading that may or may not be repetitive or cyclic. Preliminary designs for a reference material have been completed and are being tested in a round robin exercise. Calibration of the system employed for performing measurements in experiments allows the associated uncertainty to be defined which is an important step in the validation of computational models. However no recognized procedures exist for performing validations of simulations with full-field data from experiments. The consortium is developing appropriate validation procedures based on using image decomposition to enable comprehensive and quantitative comparisons between data sets for strain from experiments and simulations. Progress will be reported and the direction of planned work will be discussed to allow input from the user-community.

Previous uniaxial tests have shown that the viscoplastic behaviour of several polymers can be accurately described by means of the Rabotnov-Suvorova integral equation for materials with memory, for a wide range of loading rates in tension. Prediction of deformation can also be made in creep and relaxation. Present investigation concerns biaxial states of stress, achieved by simultaneously applying tensile force and torsional moment to thin-walled tubes. Results show that this constitutive equation, whose parameters were determined from uniaxial tensile tests at three different loading rates, also predicts the biaxial behaviour quite well until the torque comes close to the buckling load of the specimen. As a second verification, creep tests of a polyoximethylene membrane, subjected to hydrostatic pressure, were carried out. The validity of the equation is again confirmed.

The horizontal stabilizer is a fixed wing section; it’s a key element to provide flight stability for the aircraft, to keep it flying straight. Most of horizontal stabilizers are equipped with elevators which are a movable surfaces intended to vary the amount of force generated by the tail surface and are used to generate and control the pitching motion of the aircraft. The most important requirement expected from the horizontal stabilizer is to be reliable, easy maintainable, and operable at any condition which might occur during standard flight. The main goal in this paper is to perform a static analysis for the structure of the stabilizer assembly using ABAQUS CAE to simulate and predict the behavior of the structure under various external critical loads.

Mechanical characterization of sub-micron thin films or similar small scale structures have been a continuous challenge to the mechanics community due to the difficulty in accurately quantizing the applied load and resulted deformation. In this work, a new Force Domain Analog-to-Digital Converter (F-D ADC) is adopted to perform thin film tensile tests. The key component of the F-D ADC is a quantizer-array of microfabricated buckling beams of pre-determined length. During the testing, the applied force is quantized using the critical load of the increasing number of buckling beams and the deformation of the specimen is controlled by the loading stage. The resolution and accuracy of the current method can be significantly improved by increasing the number of buckling beams.

This paper presents the use of distributed fiber optic sensing to achieve centimeter level resolution strain data along the entire length of a large composite beam. A 6.5 meter long composite beam, designed for use in a corrosive flue gas desulfurization (FGD) unit, was instrumented. A section of optical fiber was embedded into a fiberglass rope, which in turn was embedded into the composite beam during the manufacturing process. The beam was experimentally tested in four-point bending at the North Carolina State University Constructed Facilities Laboratory, and the strain profile along the entire length was measured using the embedded optical fiber. Strains of up to 6500 microstrain were measured at over 300 unique positions along the span by monitoring changes in the spectral shift of the Rayleigh scatter in the optical fiber using optical frequency domain reflectometry (OFDR). The fiber used in this test was optically equivalent to standard telecommunication fiber, allowing for low-cost, high-density strain measurements on large structures. The experiment confirms the potential of embedded fiber optic distributed sensing to be used for real-time health monitoring, or as a process feedback in an instrumented structural system. Benefits of employing distributed fiber optic sensing in structures such as the composite FGD unit include the ability to monitor and detect deterioration and damage, minimize the chance of unplanned downtime or failure, and limit exposure to consequences such as environmental contamination.

This paper presents the results of a research program aimed at developing passive and active circuit components to be used in air vehicle applications. The team of Advanced Engineering Technology (AET, Inc.) and Vanderbilt University is developing diamond sensors that will be integrated with a silicon-based integrated circuit for incorporation into air vehicles. Specifically, the team is developing the technology necessary to integrate chemical vapor deposited (CVD) diamond films with the silicon integrated circuit.

When a panel is tested in uniaxial compression in a test machine, the boundary conditions are not quite the same as they would be if it were part of a complete structure. A restraint system may be used to simulate conditions found in a complete vehicle. Quantifying the quality of the restraint with only point-measurement devices can leave an inadequate characterization of the out-of-plane behavior. However, today’s full-field displacement monitoring techniques allow for much more accurate views of the global panel deformation and strain, and therefore allow for a better understanding of panel behavior. In the current study, the behavior of a hat-stiffened and two rod-stiffened carbon-epoxy panels is considered. Panels were approximately 2 meters tall and 0.76 to 1.06 m wide. Unloaded edges were supported by knife edges and stiffeners were attached to a support structure at selected locations to restrain out-of-plane motion. A comparison is made between test results based on full-field measurements and analyses based on assumptions of boundary conditions of a completely rigid edge restraint and the absence of any edge restraint. Results indicate that motion at the restrained edges must be considered to obtain accurate test-analysis correlation.

The elastic and plastic strain data of tubular specimens undergoing rupture under internal pressure tests are presented and analyzed. Six tubular specimens were tested. The specimens were cut from longitudinally welded tubes made of API 5L X80 steel with a nominal outside diameter of 457.2 mm (18 in) and a nominal wall thickness of 7.93 mm (0.312 in). Each of the six specimens had one external longitudinal or circumferential corrosion defect that had been machined using spark erosion. Tensile specimens and impact test specimens were tested to determine material properties. Post-yielding electrical resistance strain gages were used to measure the elastic and plastic strains. The failure pressures measured in the laboratory tests were compared with those predicted by four assessment methods: the B31G method, the RSTRENG 085dL method, the DNV RPF101 method for single defects and by the Kastner equation. The paper also discusses the strength of the pipe segments used in the tests under the assumptions of following the Tresca and von Mises rupture criteria.

Two full scale crash tests were conducted on a small MD-500 helicopter at NASA Langley Research Center’s Landing and Impact Research Facility. One of the objectives of this test series was to compare airframe impact response and occupant injury data between a test which outfitted the airframe with an external composite passive energy absorbing honeycomb and a test which had no energy absorbing features. In both tests, the nominal impact velocity conditions were 7.92 m/sec (26 ft/sec) vertical and 12.2 m/sec (40 ft/sec) horizontal, and the test article weighed approximately 1315 kg (2900 lbs). Airframe instrumentation included accelerometers and strain gages. Four Anthropomorphic Test Devices were also onboard; three of which were standard Hybrid II and III, while the fourth was a specialized torso. The test which contained the energy absorbing honeycomb showed vertical impact acceleration loads of approximately 15 g, low risk for occupant injury probability, and minimal airframe damage. These results were contrasted with the test conducted without the energy absorbing honeycomb. The test results showed airframe accelerations of approximately 40 g in the vertical direction, high risk for injury probability in the occupants, and substantial airframe damage.

Since the power generation capacity of a wind turbine has a direct correlation with increased blade length there is a general trend in the wind industry to move towards larger blades. Critical design issues associated with larger blades are: weight, strength and blade stiffness, reliability, manufacturing, installation and service costs and testing. Definitely, full scale testing becomes quite expensive. Therefore, testing components reduced in size and containing critical parts like adhesive bond lines seems to be an interesting alternative. As the adhesives are one of the main load carrying materials in many modern wind turbine blades, the bond line strength investigation is of vital importance. Recently, linking the gap between coupon size material characterization and the material performance on a blade structure, a lot of effort has been invested in the development of sub-component tests for structural evaluation prior to the construction of a prototype. In the present study, two GFRP I-beams are loaded incrementally to failure. The particular subcomponents simulate the complex stress state developed in the adhesive bonding between the spar cap and the shear web connection of a blade. The bond line has a thickness of 5 mm. Typical Acoustic Emission (AE) load-hold proof tests are performed at intermediate loading stages, in order to locate and characterize damage processes at relatively low loads

Crazing is one of the dominant failure mechanisms in amorphous polymers. The micro-mechanics of crazing is often incorporated by embedding Cohesive Zone Models (CZM) into continuum models. In CZM, a traction separation law for the cohesive zone is specified with a maximum traction and a critical opening as the parameters. The main focus of the present study is to determine the cohesive parameters of crazes in thin polystyrene films. Polystyrene granules were dissolved in Toluene on a glass slide which was then transferred to a spin coater. By changing the spinning speed, polystyrene films of the desired thickness were prepared. Single Edge Notch Tension (SENT) specimens were cutout of the prepared sheets and fracture tests were conducted inside a Scanning Electron Microscope (SEM) using a tensile tester at a loading rate of 0.1 mm/min. The critical opening of the cohesive zone was directly measured from the SEM images while the

J

-Integral was calculated from the load-displacement curve. The opening traction was calculated by differentiating the

J

-Integral with the measured critical craze opening.

Adhesive bonding of aircraft primary structures has been in use for many years. For example, joining the stringers to skins of fuselage and wing structures, metallic honeycomb to the skins of elevators, ailerons, tabs, and spoilers constitute the main uses of adhesives in aircraft structures. Due to this increasing use of bonded structures in recent years, for weight saving, considerable work has been done in the fracture testing of different types of adhesive joints. However, most previous work on adhesive bonded joints deals mostly with Mode I fracture, and very little work appears to have been done on compressive delamination of adhesive joints. The Chow and Ngan test, consisting of two slender beams with a blister at the centre of each, which were bonded together and loaded in compression, is an exception. In the present work, a cohesive finite element model was developed for this blister test-piece, and the geometric non-linearity was incorporated in the strain/displacement relations. The crack propagation in the adhesive joint under compression for the proposed test-piece was found to agree with the available experimental observation. Most importantly, the delamination in adhesive joint under compression for different constrained cases was studied by cohesive finite element with different interface mesh densities. Stronger adhesive joint was achieved for less constrained Double Cantilever Beam (DCB) specimen.

A cohesive zone model (CZM) based approach is applied with 3D finite element method to simulate stable tearing crack growth events in Arcan specimens made of 2024-T3 aluminum alloy. The CZM parameter values are calibrated for a triangular cohesive law under Mode I condition. Simulation prediction of the load-crack extension curve compares reasonably well with experimental data. With the same set of CZM parameter values, simulations are performed for mixed Mode I/II stable tearing crack growth events as well. A good agreement is also reached between the simulation predictions and the experimental results. The CTOD variation with crack extension is also checked under both Mode I and mixed-mode I/II conditions. The results suggest that CZM based simulations can predict the critical CTOD value, which conventionally is used as an input in CTOD based stable tearing simulations and is obtained from experimental measurements. The findings of the current study establish a connection between CTOD and CZM based simulation approaches.

Commercially pure Fe and Ni have been diffusion bonded. Pure Cu (99.999%) and Au-20Sn eutectic alloys have been used to bond Fe while Cu has been used to bond Ni. Fe bonded using Cu at 1071°C for 10 h showed ~20 µm thick residual Cu in the bond centerline. However, at 1100°C bonding temperature for 10 h the residual Cu disappeared and the Cu content of the joint centerline was ~ 7wt.%. Although Sn forms intermetallics with both Fe and Au, no intermetallics were found when Au- 20Sn eutectic was used to bond Fe at 600-650°C for 10h. Ni bonded at 1071°C for 10h contained 40 wt.% Cu in the joint centerline with the Cu content decreasing gradually with increasing distance from the bondline. Cu content in the bond centerline decreased to 35wt.% at 1100°C for 10 h. The concentration profiles of Cu in a Ni-Cu diffusion couple were simulated with DICTRA/Thermocalc. The simulated profiles were comparable to the experimental profiles. The ultimate tensile strengths obtained for Fe-Cu system were 245 and 276 MPa at 1085±1 and 1090±1°C for 10 h, respectively.

Stable tearing with crack tunneling in ductile materials has been commonly observed, but a quantitative understanding of the 3D complicated crack tunneling phenomena is very limited. In particular, the correlation between the driving force and fracture toughness during stable tearing with crack tunneling has not been well evaluated. In the current study, modeling efforts have been made to simulate stable tearing events with crack tunneling and slanting under remote tension in A2024-T3 plate specimens of two thickness values (2.286mm and 6.35mm). It is observed that values of CTOD, stress constraint and the Lode stress parameter vary in the specimen thickness direction. For the thinner specimen, a higher stress constraint and a lower CTOD in the midsection of the crack front are found, and the critical CTOD decreases approximately linearly with an increasing constraint and is weakly dependent on the Lode stress parameter. The variations of CTOD, the stress constraint and the Lode parameter in the two plate specimens in the thickness direction are very similar, but the magnitudes of these parameters near the midsection of the crack front increase significantly in the thicker specimen. The approximate linear correlations in the two specimens between constraint and CTOD are not identical, probably due to the coupled effects of constraint and the Lode stress parameter in this midsection region of the crack front.

In this study, a phenomenological tissue damage model has been developed to describe the fatigue-induced stress softening and permanent set of biological tissues. Since damage evolution is an irreversible dissipative process, following thermodynamic principles, an equivalent strain proportional to the strain energy of the material is employed as the damage criterion. The maximum equivalent strain represents the value necessary to cause complete sample failure during one loading cycle, while the minimum equivalent strain is the value required to elicit the onset of fatigue damage. The damage parameter evolves from zero (below minimum equivalent strain) to one at maximum equivalent strain as a function of both the equivalent strain and number of loading cycles. The permanent set evolves as a function of the peak strain in the principal directions. The damage model is implemented into ABAQUS via a user defined material (UMAT) in conjunction with the nonlinear orthotropic Fungelastic model. For the purpose of this study, glutaraldehyde-treated bovine pericardium (GLBP), a collagenous tissue traditionally used for fabricating bio-prosthetic heart valve (BHV) leaflets, is utilized as a representative collagenous tissue due to its limited durability in BHV applications.

Interfacial delamination is a key reliability challenge in composites and micro-electronic systems due to (high density) integration of dissimilar materials. Predictive finite element models are used during the design and optimization stage to minimize delamination failures, however, they requires a relevant interface model to capture the (irreversible) crack initiation and propagation behavior observed in experiments. Therefore, a set of experimental-numerical tools is presented to enable accurate characterization of delamination mechanism(s) and prediction of the interface mechanics. First, a novel Miniature Mixed Mode Bending (MMMB) delamination setup is presented that enables in-situ SEM characterization of interface delamination mechanisms while sensitively measuring global load-displacement curves for the full range of mode mixities. Accurate determination of the critical energy release rate from the global load-displacement curve requires, however, identification and separation of bulk plastic contributions from the measured total energy dissipation; to this end, an analytical procedure is presented. Finally, a cohesive zone model suitable for mixed mode loading with realistic coupling is presented that can capture the range of interface failure mechanisms from damage to plasticity, as observed in-situ with SEM, as well as a parameter identification procedure. The set of experimental-numerical tools is validated on delamination measurements of a glue interface.

Stretchable electronic devices enable numerous futuristic applications. Typically, these devices consist of a (metal) interconnect system embedded in a stretchable (rubber) matrix. This invokes an apparent stretchability conflict between the interconnect system and the matrix. This conflict is addressed by shaping the interconnects in mechanistic patterns that bend and twist to facilitate global stretchability. Metal-rubber type stretchable electronic systems exhibit catastrophic interface delamination, which is investigated in this research. The fibrillation process occurring at the delamination front of the metal-rubber interface is investigated through

in-situ

SEM imaging of the progressing delamination front of peel tests of rubber on copper samples. Results show that the interface strength is dependent on the delamination rate and the interface roughness. Additionally, the fibril geometry seems highly dependent on the interface roughness, while being remarkably independent on the delamination- rate.

Flexible electronic devices (flexible displays, solar cells) are subjected to large bending loads during manufacturing and use, making delamination in the underlying thin-film structure a major reliability concern. To investigate such failures, a new miniature contactless, frictionless pure-bending device is presented that enables highly sensitive moment-curvature measurements and simultaneous in-situ SEM failure analysis. Bending tests are of particular interest as they apply nonuniform loads without geometrical instabilities (necking, buckling, etc.). Most standardized bending tests (3-point, 4-point or cantilever bending) are contact-based, however, therefore they: cannot straightforwardly impose (large amplitude) cyclic or reversed loading; may introduce local deformations (indentations); typically obstruct the field of view at top or bottom surface; and typically introduce ill-defined parasitic frictional forces and moments. Contact and friction contributions particularly form a problem for cyclic testing or miniaturization and significantly complicate experimental-numerical material model identification. The here presented miniature pure-bending device overcomes these limitations. Contactless pure bending is realized through the relative rotations of two clamps, while active piezo control is use to eliminate axial and normal clamp forces and keep the area of interest in field of view for continuous SEM observation.

To better design structures and machines, understanding of flaws and failures is essential. Stresses in the vicinity of a crack tip can be characterized by Stress Intensity Factor. The effective methods of experimentally determining the stress intensity factor for a body containing a crack is to analyze the isochromatic pattern obtained from a photoelastic model. The effect of biaxial load factor, crack angle, Crack length/width of specimen and length of specimen/width of specimen were studied and a regression model was developed for geometry correction to predict stress intensity factor for tearing mode and intensity factor for shearing mode. This approach is being used to predict crack growth trajectory under biaxial cyclic loading by assuming that the crack may grow in a number of discrete steps using the vectorial method. MTS criterion (Maximum Tangential Stress criterion) is used for prediction of crack initiating angle. The crack growth trajectory has been determined by cycle by cycle simulation procedure.

Based on the crack opening displacement, fatigue crack growth model is expressed as a function of mechanical properties. The effect of material non-homogeneity is included in the model through a random process parameter of Gaussian type. The model is validated through the experimental and predicted results from several data sets. All experimental data are taken from literature.

This paper investigates the waveforms and frequency spectra of elastic emissions (ELE), or quasi-rigid body vibration pulses, due to the formation of macrofractures in perfectly brittle, quasi-brittle and ductile materials subjected to uniaxial compression. Elastic emissions, differently from acoustic emissions, are detected in a low frequency range (i.e. below 15-10 kHz) and are characterized by high levels of released energy. Approaching to the large fractures and the final collapse of the material bursts of ELE are observed indicating the solid elastic-mechanical properties degradation and its irreversible plastic deformation. Through waveform and time-frequency analysis of the ELE spectra, measured by calibrated transducers, it is possible to provide quantitative information on the damage evolution and the strain energy released during each ELE event.

This paper is concerned with the failure characteristics and the failure load of spot welds of AHSS under combined axial and shear loading conditions. A testing fixture and a specimen are newly designed to impose the pure-shear load acting on a spot weld. The testing fixture and the specimen proposed by Song

et al.

[1] are used to impose the combined axial and shear load at the loading angle from 0° to 75° on a spot weld. Using those testing fixtures and specimens, failure tests of the spot weld of TRIP590 1.2t, DP780 1.0t, and DP980 1.2t are conducted with seven different conditions of combined loading. Based on the experimental results, failure loads and failure behaviors of a spot weld of AHSS are investigated with respect to the different loading angles. Failure loads of the spot weld obtained from failure tests are interpolated to construct the Song and Huh’s failure model [2], which facilitates the failure description of a spot weld in the macroscopic finite element analysis of autobody crashworthiness.

This paper discusses the research, development, and design considerations used to produce a Structural Information System (SIS) capable of characterizing the behavior of an adaptive reinforced concrete structure designed to withstand reverse loadings. The SIS consists of a collection of surface mounted and embedded sensors connected to a portable computer. The composite structure is reinforced with hollow carbon fiber tendons equipped with embedded strain gages and the work includes theoretical arguments, polymer concrete mix design, concrete testing, reinforcement selection and placement, sensor selection and placement, and structural testing and analysis. The primary objective is to insure that the stress in the materials remains within the elastic range so that damage does not occur. A finite element model is developed to accurately characterize the structural response in the elastic range and a hybrid approach is suggested in which displacement, strain, and stress can be obtained with a rudimentary SIS consisting of a single embedded sensor. The ability to characterize failure, once it occurs, is also demonstrated by analyzing data obtained from displacement-controlled tests. Results indicate that splices in the tendons and slippage between the tendons and the concrete help to prevent sudden failure and allow the structure to withstand relatively high service loads despite appreciable deformation.

Laser-generated ultrasonic guided and bulk waves are increasingly considered for the nondestructive testing (NDT) and structural health monitoring of engineering systems. Methods based on the use of pulsed laser or continuum laser are ideal when a non-contact approach for the generation and detection of stress waves is desired. This paper presents the initial progresses of an ongoing study where pulsed laser is used to generate stress waves in underwater structures. In particular, in this paper we show the results of few experiments where stress ultrasonic waves are generated in an aluminum block. Owing to the geometry of the specimen, ultrasonic bulk waves are generated and detected by means of either a dry or an immersion transducer. The study presented here aims at investigating the effect of certain water parameters on the ultrasonic energy propagating through the specimen. The results of three experiments are presented. In the first experiment the effect of water level on the signal-to-noise ratio of laser generated bulk waves was evaluated. Then, the effect of laser energy was studied. Finally, the effect of water temperature on the amplitude of the bulk waves was investigated. With the exception of the latter experiment, we used laser pulses at 1064 nm and 532 nm wavelengths.

This paper describes the development of a finite element model for the 7.62x39 mm mild steel core (MSC) bullet. The derivation of the numerical model is based on results of compression testing performed on bullet components and bullets. The material constants needed are obtained using an iterative approach, where numerical models of the compression tests are carried out to match the force displacement response of the tested structures. Later, these set of constants are refined by comparing predicted deformed shapes against those observed during ballistic impact experiments. Numerical simulations of the 7.62x39 mm MSC bullet impacting a semi-infinite rigid plate are carried out over a range of impact velocities, and the predicted deformed shapes compared to experimental shapes obtained with the help of high speed photography. Soft recovery of cores and jackets are also used to perform comparisons with predicted results. It was found that our constitutive and damage models, implemented in ABAQUS Explicit, were able to accurately predict deformed shapes and failure modes without any predefined defects in the element mesh.