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## Über dieses Buch

Fracture, Fatigue, Failure and Damage Evolution, Volume 8 represents the eighth of nine volumes of technical papers presented at the Society for Experimental Mechanics (SEM) 15th International Congress & Exposition on Experimental and Applied Mechanics, held at Costa Mesa, California, June 8-11, 2015. The full set of proceedings also includes volumes on: Dynamic Behavior of Materials, Challenges in Mechanics of Time Dependent Materials, Advancement of Optical Methods in Experimental Mechanics, Experimental and Applied Mechanics, 16th International Symposium on MEMS and Nanotechnology, International Symposium on the Mechanics of Composite and Multi-functional Materials, 5th International Symposium on the Mechanics of Biological Systems and Materials, International Symposium on the Mechanics of Composite and Multi-functional Materials; and Residual Stress, Thermomechanics & Infrared Imaging, Hybrid Techniques and Inverse Problems.

## Inhaltsverzeichnis

### Chapter 1. Reflection-Mode Digital Gradient Sensing Technique for Experimental Fracture Mechanics

In this work, the reflection-mode Digital Gradient Sensing (r-DGS) method is extended for visualizing and quantifying crack-tip deformations in solids under quasi-static and dynamic loading conditions. The r-DGS technique employs digital image correlation principles to quantify two orthogonal surface slopes simultaneously in specularly reflective solids by quantifying small deflections of light rays. For the first time, r-DGS has been implemented here to study both mode-I and mixed-mode (I/II) problems and quantify fracture parameters. Under dynamic loading conditions, r-DGS is implemented in conjunction with high-speed digital photography to map surface slopes in edge cracked plates subjected to one-point impact. The measured surface slopes have been used to successfully evaluate stress intensity factor histories by pairing measurements with the corresponding asymptotic crack tip field descriptions using overdeterministic least-squares analyses.

Amith S. Jain, Hareesh V. Tippur

### Chapter 2. Experimental and Computational Investigation of Out-of-Plane Low Velocity Impact Behavior of CFRP Composite Plates

Strength of composite materials under transverse loading has remained a major weakness despite numerous advancements in composite technologies. Most frequent and critical result of this characteristic is internal delamination damage, which is undetectable and lead to major strength reduction in the structure. This condition is usually encountered in low-velocity impact situations which frequently occur during the maintenance of aircraft. Past studies have successfully developed experimental and analysis methods for accurately predicting impact force history and damage footprint based on the comparison with post-impact results. However, there is almost no experimental work on the progression sequence of damage during impact in the literature. This paper focuses on experimental and computational investigation of the damage initiation and growth process during low-velocity impact of [07/904] s and [907/04] s cross-ply CFRP laminates. In the experiments, through-the-thickness direction is tracked using ultra-high speed camera and DIC technique to record damage progression and dynamic strain fields. In the numerical part of the study 3-D explicit, finite element analysis is conducted to model matrix crack initiation and propagation. The finite element results are then compared with experiments in terms of failure modes and sequence.

O. T. Topac, B. Tasdemir, B. Gozluklu, E. Gurses, D. Coker

### Chapter 3. Prediction of Incipient Nano-Scale Rupture for Thermosets in Plane Stress

There is limited experimental evidence that fracture nucleation in polymers includes a small number of covalent bond scissions followed by rapid void growth by chemo-mechanical processes. Generalized criteria for predicting such bond scission, then, would help anticipate fracture in polymer matrix composites. Strain states at incipient bond scission for thermoset resins in plane stress are here predicted by atomistic simulation. Several cured epoxy systems were examined, each having a different chain length. For biaxial extension and a portion of the shearing regime, scission occurs at a critical value of the larger principal strain. This value increases with increasing chain length. The corresponding dilatation is largest for biaxial extension and decreases to nearly zero for pure shear. Results are compared with strain invariants at fracture measured from experiments in which polymer matrix composites having various ply stacking sequences were loaded to rupture.

J. C. Moller, S. A. Barr, T. D. Breitzman, G. S. Kedziora, A. M. Ecker, R. J. Berry, D. Nepal

### Chapter 4. Effect of Degree of Cure on Damage Development in FRP

We have developed a fiber-optic sensing system for measurement of degree of cure (DOC) of FRP laminates. By applying this system to monitoring of cure process of FRP laminates, molding time cam be optimized. However, it is considered that some part of the composite has incomplete DOC due to non-uniformity of molding temperature when the optimized molding process is conducted. Therefore, the effect of DOC on quality of FRP must be clear to manufacture high quality FRP using the optimized molding process. In the present paper, GFRP cross-ply laminates were manufactured with arbitrary DOC. Tensile tests to investigate the effect of DOC on the damage development behavior of the cross-ply laminates were conducted. DOC of the specimen was monitored by the fiber-optic sensing system developed by our laboratory. Matrix cracks were observed during the tensile tests. From the experimental results, it was found that the lower DOC specimen showed the lower crack initiation stress and the slower development rate of transverse crack in 90° layer. In addition, it appeared that many splitting cracks were generated in 0° layers when the DOC was less than 100 % due to the low toughness of resin. From these results, it was found that DOC governs damage development behavior of FRP laminates strongly.

Takahiro Ozaki, Tatsuro Kosaka, Kazuhiro Kusukawa

### Chapter 5. Stochastic Discrete Damage Simulations of Laminate Composites

In this paper, probabilistic failure response and damage patterns in polymer matrix composite laminates was investigated by considering spatially varying strength properties. For this purpose, an efficient random field modeling framework for multiple cross-correlated random fields is proposed whereby different a set of uncorrelated random variables in the Karhunen-Loève (KL) expansion are generated by Latin hypercube sampling technique and transformed to sets of correlated random variables. Discrete Damage Modeling (DDM) is performed by means of Regularized eXtended-Finite Element Method (Rx-FEM). The strength properties represented by spatially varying cross-correlated random fields define the matrix crack insertion patterns, whereas their propagation as well as the delamination growth is governed by cohesive law models with constant fracture toughness properties. One composite laminate a quasi-isotropic carbon/epoxy (Hexply IM7/8552) [45/90/−45/90]s was modeled by using probabilistic DDM. The effect of statistical parameters such as the correlation length, variance and correlation coefficient between random fields of normal and shear strength within DDM framework was examined for the first time. Significant effects of the statistical parameters on the failure behavior and ultimate component strength was observed, manifesting importance of accurate definitions of the statistical properties for predicting probabilistic failure behavior and damage tolerance of laminate composites.

Gunjin Yun, Li Zhao

### Chapter 6. Development of a Specimen for In-Situ Diffraction Planar Biaxial Experiments

In this paper, the design of cruciform shaped, planar biaxial loading specimens using finite element analysis, mechanical testing, and digital image correlation is discussed. The specimens were designed to be capable of arbitrary combinations of tension and compression loading. Digital image correlation results from uniaxial tension tests of first-generation specimen infer key design attributes of second-generation specimen. Finite element results are compared with a plane stress analytical formulation and differences between the two are attributed to stress concentration fields originating at the intersection of specimen arms. These results motivate a parametric finite element geometry optimization of second-generation specimen.

G. M. Hommer, A. P. Stebner

### Chapter 7. V-Notched Rail Test for Shear-Dominated Deformation of Ti-6Al-4V

Evermore sophisticated ductile plasticity and failure models demand experimental material characterization of shear behavior; yet, the mechanics community lacks a widely accepted, standard test method for shear-dominated deformation and failure of ductile metals. We investigated the use of the V-notched rail test, borrowed from the ASTM D7078 standard for shear testing of composites, for shear testing of Ti-6Al-4V titanium alloy sheet material, considering sheet rolling direction and quasi-static and transient load rates. In this paper, we discuss practical aspects of testing, modifications to the specimen geometry, and the experimental shear behavior of Ti-6Al-4V. Specimen installation, machine compliance, specimen-grip slip during testing, and specimen V-notched geometry all influenced the measured specimen behavior such that repeatable shear-dominated behavior was initially difficult to obtain. We will discuss the careful experimental procedure and set of measurements necessary to extract meaningful shear information for Ti-6Al-4V. We also evaluate the merits and deficiencies, including practicality of testing for engineering applications and quality of results, of the V-notched rail test for characterization of ductile shear behavior.

Sharlotte Kramer, John Laing, Thomas Bosiljevec, Jhana Gearhart, Brad Boyce

### Chapter 8. A Statistical/Computational/Experimental Approach to Study the Microstructural Morphology of Damage

The fractural behavior of multi-phase materials is not well understood. Therefore, a statistic study of micro-failures is conducted to deepen our insights on the failure mechanisms. We systematically studied the influence of the morphology of dual phase (DP) steel on the fracture behavior at the onset in two ways: (i) in a numerical setting by statistically averaging over the micro-structural arrangements around the damage sites in no less than 400 randomly-generated idealized microstructural models loaded in pure shear; and (ii) in an experimental setting by statistically averaging, similar to the numerical simulations, over the damage sites found in a large collection of large field-of-view SEM images of DP steel deformed in uniaxial tension, where deliberately-overexposed backscattered electron images sharply mark the damage location, while simultaneously-recorded secondary electron images are used to identify the material phases. The numerical and experimental analyses were validated and tested for accuracy. Application of both techniques to DP showed a similar single topological feature to be most sensitive to damage: a small region of soft matrix material with hard inclusion particles on opposing sides. These results are representative for and yield insight in damage evolution in a wide variety of multi-phase materials.

J. P. M. Hoefnagels, C. Du, T. W. J. de Geus, R. H. J. Peerlings, M. G. D. Geers

### Chapter 9. Prediction of Ductile Fracture Through Small-Size Notched Tensile Specimens

In this study, a combined experimental and numerical procedure that based on extended analysis of stresses, strains, and damage of the specimen notch region, was proposed to investigate high-strength steel and pure nickel metals. Miniaturized notched tensile specimens with different notch radii were used to generate various levels of triaxial stress, and to evaluate stress-dependent failure. It was shown that the triaxiality plays a major role in the damage evolution demonstrated by decreasing ductility. The experimental investigation was supplemented by scanning electron microscopy observations of fractured surfaces. The deformation mechanisms leading to the failure ware linked with the extensions of the Gurson model for porous ductile metals. The evolution of damage in both materials was compared and discussed.

L. Farbaniec, H. Couque, G. Dirras

### Chapter 10. Development of a Generalized Entropic Framework for Damage Assessment

Data-driven stochastic and probabilistic methods that underlie reliability prediction and structural integrity assessment remain unchanged for decades. This paper develops an alternative approach to reliability assessment in terms of fundamental concepts of science within the irreversible thermodynamic framework. The common definition of damage, which is widely used to measure the reduction of reliability over time, is based on observable markers of damage at different geometric scales. Observable markers are typically based on evidences of any change in the physical or spatial properties or the materials, and exclude unobservable and highly localized damages. Thermodynamically, all forms of damage share a common characteristic: “energy dissipation”. Energy dissipation is a fundamental measure of irreversibility that within the context of non-equilibrium thermodynamics is quantified by “entropy generation”. The definition of damage in the context of thermodynamics allows for incorporation of all underlying dissipative processes including unobservable markers of damage. Using a theorem relating entropy generation to energy dissipation associated with damage producing failure mechanisms, this paper presents an approach that formally describes and measures the resulting damage.Having developed the proposed damage model over time, one could determine the time that damage accumulates to a level where the component or structure can no longer endure the damage and fails. Existence of any uncertainties about the parameters and independent variables in this thermodynamic-based damage model leads to a time-to-failure distribution. Accordingly, such a distribution can be derived from the thermodynamic laws rather than estimated from the observed failure histories.

Anahita Imanian, Mohammad Modarres

### Chapter 11. Modelling of Experimental Observations of Electrical Response of CNT Composites

A physics based analytical models on prediction of electro-mechanical behavior of carbon nanotubes (CNTs) embedded epoxy composite are developed to investigate the change in resistance under quasi-static tensile and compression loading conditions. Two different types of contacts namely in-line and lateral contacts between CNTs were considered in predicting electrical tunneling of current. It was identified from experiments that the extent of these contacts vary during deformation and lateral contacts predominates after composite reaches maximum stress during deformation, resulting decrease in resistance. The non-linear constitutive response of the composite obtained from experimental results is incorporated into model to accommodate decrease/increase in distance between above contacts. Under tensile loading conditions, the model made decent predictions against experimental results for composites of three different weight fractions (0.1, 0.3 and 0.5 %) of CNTs. Later, this model is extended to predict electro-mechanical response for intermediate weight fractions. Our efforts are currently focused on developing model to predict electrical response under compression loading and the results of this model along with under tensile loading will be presented at the conference.

K. Shkolnik, V. B. Chalivendra

### Chapter 12. Effect of Micro-Cracks on the Thermal Conductivity of Particulate Nanocomposite

The effect of micro-cracks on the thermal conductivity of particle-reinforced nanocomposites is investigated. Two different particles (Carbon nanotube and Silicon dioxide) with different geometries are considered to account for the effect of particle aspect ratio. Three batches of specimens, two with and one without nano-fillers are fabricated. First, the thermal conductivity of the as-fabricated samples were measured using steady state linear heat transfer unit. Afterwards, the samples were subjected to cyclic loading and at the end of every 5000 cycles the samples were taken out and the thermal conductivity was measured. At the same time, the Modulus of Elasticity of the specimens were determined using uniaxial compression test. Based on these results, the effect of micro-cracks on the thermal conductivity of the nanocomposites is presented. In addition, the relation between micro-cracks, stiffness, and thermal conductivity are presented.

Addis Tessema, Dan Zhao, Addis Kidane, Sanat K. Kumar

### Chapter 13. Fatigue Tests on Fiber Coated Titanium Implant–Bone Cement Interfaces

The goal is developing an efficient bond interface between the implant and the cement by applying micron to nano size fibers to the surface of the implant through an electrospinning process, utilizing biocompatible fibers. Experimental models have been developed to evaluate the forces experienced on a cemented cylinder shape titanium implant through a static and cyclic tests. Finite element analysis (FEA) model for an uncoated cylindrical cemented titanium model was developed and tested under static and fatigue conditions. Our experimental study on cylindrical model found increase of pull out static strength for fiber coated implant (Mean strength = 1.308 MPa) compare to uncoated implant (Mean strength = 1.098 MPa) for 2 samples. Our experimental study also found no noticeable increase of pull out fatigue life for fiber coated implant (Mean fatigue life = 2019 cycles) compare to uncoated implant (Mean fatigue life = 2015 cycles) for 2 samples. Our FEA study on cylindrical model found the design life to be 1690 cycles with element size of 3.0E-3 m under the minimum stress of 112 kPa and maximum stress of 9.71 MPa according to Modified Goodman theorem.

M. Khandaker, Y. Li, P. Snow, S. Riahinezhad, K. Foran

### Chapter 14. Fatigue Behavior of Carburized Steel at Long Lives

The fatigue behavior of two carburized steels is investigated with rotating bending tests for lives between approximately 105 and 108 cycles. Cracks are observed to start at sub-surface inclusions within carburized cases and form “fish eyes” often seen in high cycle fatigue of hardened steels. Optically dark areas (ODAs) are found surrounding the inclusions that acted as crack origins. Fatigue strengths are evaluated by Murakami’s area $$\sqrt{area}$$ model, but accounting for the influence of residual stress and hardness at the different depths at which cracks started. Values of stress intensity factor range at the periphery of inclusions and the outer border of ODAs are computed accounting for the influence of residual stress. Possible factors that may influence crack growth within ODAs are discussed.

D. V. Nelson, Z. Long

### Chapter 15. Fatigue Behavior of Fluid End Crossbore Using a Coupon-Based Approach

Fracture or mud pumps are known as the heart of the drilling and hydraulic fracturing. Crossbore geometries are central to the design of fluid end module in these positive displacement reciprocating pumps. Intersection between bores emerges as a stress concentrator and because the fluctuating pressure history is extreme, fatigue limits the useful life of the pump. Approaches such as autofrettage are typically used to extend fatigue lives through the imposition of compressive residual stresses at crossbore intersections. Direct investigation of the impact of residual stresses in working pumps is not typically possible. In order to improve understanding of the impact of residual stresses on fatigue life and to optimize the fatigue-strength improvement provided to fluid ends, unique sample geometry was designed to simulate the stresses in the crossbore. These samples are tested on laboratory-based servohydraulic fatigue frames and eliminate the need for complicated in-situ stress analysis on the fluid ends. Using notch strain analysis and modified Smith–Watson–Topper approach a life prediction algorithm was also developed to calculate the fatigue life of the coupon. To optimize the autofrettage load and cyclic loading simulation, elastoplastic FEA was accomplished utilizing a combined nonlinear isotropic/kinematic hardening material model for 4300-series alloy steel.

Mahdi Kiani, Rayford Forest, Steven Tipton, Michael W. Keller

### Chapter 16. Notch Strain Analysis of Crossbore Geometry

Many high-pressure components have intersecting bore geometries, such as fluid end module of fracture pumps. Imposition of compressive residual stresses at crossbore intersections can extend the fatigue life, thus approaches such as autofrettage are typically used. Understanding the stress–strain response during autofrettage in the crossbore is critical for fatigue life estimation and design. Crossbore geometry is frequently complex and no closed-form analytical solution is available for prediction of residual stresses. As such, numerical methods like FEA are frequently used. Applying FEA to complicated geometries requires extensive parametric studies which are computationally expensive and time consuming, thus notch strain analysis methods are promising. Elastoplastic stress–strain responses due to varying internal pressures in a crossbore geometry were evaluated using FEA and notch strain analysis formulas including both Neuber and Glinka approaches. To define the theoretical elastic stress concentration factor based on ratios between maximum Mises or hoop stress and pressure or nominal stress four different values were calculated and imported into the notch strain analysis formulas. It was observed that the results of Glinka approach match better to the FEA results particularly by applying the ratio between maximum Mises stress and nominal stress as elastic stress concentration factor.

Mahdi Kiani, Steven Tipton, Michael W. Keller

### Chapter 17. Opto-acoustic and Neutron Emissions from Fracture and Earthquakes

TeraHertz phonons are produced in condensed matter by mechanical instabilities at the nano-scale (fracture, turbulence, buckling). They present a frequency that is close to the resonance frequency of the atomic lattices and an energy that is close to that of thermal neutrons. A series of fracture experiments on natural rocks has recently demonstrated that the TeraHertz phonons are able to induce fission reactions on medium weight elements with neutron and/or alpha particle emissions. The same phenomenon appears to have occurred in several different situations and to explain puzzles related to the history of our planet, like the ocean formation or the primordial carbon pollution, as well as scientific mysteries, like the so-called “cold nuclear fusion” or the correct radio-carbon dating of organic materials.Very important applications to earthquake precursors, climate change, energy production, and cell biology can not be excluded.

Alberto Carpinteri

### Chapter 18. Field Theoretical Description of Shear Bands

Spatiotemporal behaviors of the shear band have been analyzed. Based on a recent field theory of deformation and fracture, it has been hypothesized that (a) a shear band is formed along the boundary of opposite rotational displacements of a specimen. (b) When the propagation velocity of movable dislocations along the front of a shear band is higher than the phase velocity of the rotation wave the shear band appears continuously whereas when the velocity of the dislocations is lower than the rotation wave the shear band travels at the same velocity as the rotational wave appearing intermittently. Electronic-Speckle-Pattern Interferometric setup has been used to monitor the formation and movement of shear bands under various tensile strain rates. The observed spatiotemporal characteristics of the shear bands have been found to support the above hypotheses.

Sanichiro Yoshida, Tomohiro Sasaki

### Chapter 19. Measuring the Effective Fracture Toughness of Heterogeneous Materials

Heterogeneous materials where the scale of the heterogeneities is small compared to the scale of applications are common in nature. These materials are also engineered synthetically with the aim of improving performance. The overall properties of heterogeneous materials can be different from those of its constituents; however, it is challenging to characterize effective fracture toughness of these materials. We present a new method of experimentally determining the effective fracture toughness. The key idea is to impose a steady process at the macroscale while allowing the fracture process to freely explore at the level of microstructure. We apply a time-dependent displacement boundary condition called the surfing boundary condition that corresponds to a steadily propagating macroscopic crack opening displacement. We then measure the full-field displacement using digital image correlation (DIC) method, and use it to obtain the macroscopic energy release rate. In particular, we develop a global approach to extract information from DIC. The effective toughness is obtained at the peak of the energy release rate. Finally, the full field images also provide us insight into the role of the microstructure in determining effective toughness.

Chun-Jen Hsueh, Guruswami Ravichandran, Kaushik Bhattacharya

### Chapter 20. Local Strain Analysis of Nitinol During Cyclic Loading

Various approaches for obtaining a cyclic stress-strain curve for superelastic nitinol are evaluated. Smooth test specimens made from seamless drawn tubing and equivalently processed as medical device materials are tested according to one of three methods. The technique of phase shifted moiré interferometry is used to provide in situ strain data with automated data analysis using spatially correlated fringe patterns during the cyclic stress-strain testing. Selected results demonstrate the exceptional spatial and temporal resolution of the measurement system over a wide range of deformations and time scales. General trends in the testing and observations on the feasibility and application of the various methods to medical device design are discussed.

Kenneth Perry, Alex Teiche, Izak McGieson

### Chapter 21. Environmental Protection by the Opto-acoustic and Neutron Emission Seismic Precursors

The acoustic (AE) and electromagnetic (EM) emission signals detected during the failure of brittle materials are analogous to the anomalous mechanical and geoelectromagnetic waves observed before major earthquakes. These phenomena reinforce the idea that opto-acoustic emissions can be applied as a forerunning tool for seismic events.The elastic energy released by micro-cracking eventually yields to form macroscopic fractures, whose mechanical vibrations are converted into electromagnetic oscillations over a wide range of frequencies, from Hz to THz. This excited state of the matter could be the cause of resonance phenomena at the nuclear level producing neutron bursts, in particular during stress-drops or sudden catastrophic failures.The authors present the results they are obtaining at a gypsum mine located in Northern Italy. The observations revealed a strong correlation between AE/NE events and the closest and most intense earthquakes. Thanks to the position of the monitoring station (100 m under the ground level), the acoustic and electromagnetic noise from human activities is greatly reduced, as well as the neutron background. An integration of AE/EME/NE data with CO2 and Radon variations, that are considered as additional seismic precursors, is planned.

O. Borla, G. Lacidogna, E. Di Battista, M. Costantino, A. Carpinteri

### Chapter 22. Neutron Emissions from Hydrodynamic Cavitation

During the last few years, some investigators reported interesting results regarding neutron emissions from ultra-sonic cavitation in liquids and solids. In the present paper, the described experiments were conducted in order to evaluate neutron emissions from liquids subjected to hydrodynamic cavitation by an hydraulic circuit prototype. In particular, different aqueous iron salt solutions were tested in order to correlate neutron emissions and evolution of chemical element concentrations after different operating hours. The experiments were conducted using an hydraulic circuit fine-tuned by the authors. The pilot plant, which also includes the power supply and the electronic control of the recirculation pump, was realized entirely by plastic material (with the exception of the centrifugal pump and the hydraulic cavitator). The pump will be equipped with a system of six stages and with a maximum flow rate of 6 m3/h. The maximum working pressure is equal to 10 bar. The evidence obtained during the tests returned an appreciable neutron emission, about 30 % greater than the background level. A significant decrement in Fe concentration was detected at the end of the test, whereas a considerable amount of aluminum—previously absent—was found on the internal walls of the pipe.

A. Manuello, R. Malvano, O. Borla, A. Palumbo, A. Carpinteri

### Chapter 23. From Dark Matter to Brittle Fracture

Prompted by the intriguing results obtained by some of the rare-event searches looking for the dark matter that may make up the bulk of the matter in the Universe, we have studied brittle fracture as a background in scintillation detectors. Under conditions of ambient temperature and pressure, we have demonstrated a correlation between fracture, acoustic emission, and emission of light in several common scintillators. We present early results from an improved setup. When commissioned, it will provide additional channels to study these phenomena, in controllable atmospheres.

P. C. F. Di Stefano, C. Bouard, S. Ciliberto, S. Deschanel, O. Ramos, S. Santucci, A. Tantot, L. Vanel, N. Zaïm

### Chapter 24. Compositional Variations in Palladium Electrodes Exposed to Electrolysis

Literature presents several cases of nuclear anomalies occurring in condensed matter, during fracture of solids, cavitation of liquids, and electrolysis experiments.Previous papers by the authors have recently shown that, on the surface of the electrodes exposed to electrolysis visible cracks and compositional changes are strictly related to nuclear particle emissions. In particular, a mechanical interpretation of the phenomenon was provided accounting to the hydrogen embrittlement effects. Piezonuclear reactions were considered responsible for the neutron and alpha particle emissions detected during the electrolysis. Such effects are thoroughly studied in a new experimental campaign, where three pure palladium (100 % Pd) cathodes coupled with Ni anodes are used for electrolysis, separately exposed to processes of different duration: 2.5 h, 5 h and 10 h, respectively. In this paper, the authors intend to show the new results concerning the changes on the surface of the electrodes in terms of composition and presence of cracks after the electrolytic process. Measures of heat generation as well as of neutron emission will be reported.

A. Carpinteri, O. Borla, A. Goi, S. Guastella, A. Manuello, R. Sesana, D. Veneziano

### Chapter 25. Strain-Rate-Dependent Yield Criteria for Composite Laminates

The strain-rate-dependent failure of a fiber-reinforced toughened-matrix composite (IM7/8552) was experimentally characterized over the range of quasi-static (10−4) to dynamic (103 s−1) strain rates using off-axis lamina and angle-ply laminate specimens. A progressive failure paradigm was proposed to describe the matrix-dominated transition from linear elastic to non-linear material behavior, and the Northwestern Failure Theory was adapted to develop a set of yield criteria for predicting the matrix-dominated yielding of composites using the lamina-based transverse tension (F2ty), transverse compression (F2cy), and shear (F6y) yield strengths. A verification and validation protocol was employed to evaluate the applicability of the new failure-mode-based yield criteria. Starting with the lamina, the proposed criteria were validated to accurately predict matrix-dominated yielding. Angle-ply laminates were investigated to isolate the matrix-dominated laminate behavior, and the predictions were found to be in superior agreement with the experimental results compared to the classical failure theories in all cases using simply determined average lamina yield properties.

Joseph D. Schaefer, Isaac M. Daniel

### Chapter 26. Experimental Fatigue Specimen and Finite Element Analysis for Characterization of Dental Composites

Dental composites are becoming more popular due to their semi white color and appearance. Mechanical damage such as cracks are causing the majority of short-term failures of dental composites. Using self-healing materials most of these failures can be prevented. Fatigue loads are a proper method to characterize the crack initiation and propagation. As healing makes uncertainty about the location of the crack tip, samples of tapered double cantilever beam (TDCB) are frequently used for their crack length independent in the measurements of healing efficiency and fracture toughness of self-healing. Due to the high cost of dental composite materials, tiny, inexpensive TDCB samples, about 30 % of the standard size, were developed and optimized with Rapid Prototyping (Objet 3D printer). FEA is also performed in order to visualize the stress field of the crack tip.

Dhyaa Kafagy, Michael Keller

### Chapter 27. Fracture Toughness and Impact Damage Resistance of Nanoreinforced Carbon/Epoxy Composites

In this study, the objective was to develop, manufacture, and test hybrid nano/microcomposites with a nanoparticle reinforced matrix and demonstrate improvements to damage tolerance via Mode-II fracture toughness and impact damage absorption. The material employed was a woven carbon fiber/epoxy composite, with multi-wall carbon nanotubes as a nano-scale reinforcement to the matrix. A direct-mixing process, aided by a block copolymer dispersant and sonication, was employed to produce the nanoparticle-filled epoxy matrix. Fracture toughness was tested by several different Mode-II and mixed Mode-I/Mode-II specimens to determine the toughness improvement. Testing and material difficulties were overcome by this approach, showing a Mode-II toughness improvement of approx. 35 % in the hybrid material. Impact tests were performed in a falling-weight drop tower at different energies to introduce interlaminar damage in samples of both materials. Impact damaged specimens were imaged by ultrasonic c-scans to assess the area of the damage zone at each ply interface. Post-mortem optical microscopy confirmed the interlaminar nature of the impact damage. These tests showed a consistently smaller absorbed energy and smaller total damage area for hybrid composite over reference material, translating to a nominally higher ‘effective impact toughness’ in the hybrid composite (approx 42 %) regardless of specific impact energy.

Joel S. Fenner, Isaac M. Daniel

### Chapter 28. Compression Testing of Micro-Scale Unidirectional Polymer Matrix Composites

This project builds on work done by Lu et al. An experimental study is carried out to characterize the failure behavior of a fiber reinforced polymer matrix composite at the micro-scale using the same test methodology. In order to address the issue of catastrophic failure observed in the previous effort, a physical stop for the indenter that limits maximum displacement to a predetermined value is integrated into the specimen design. Micron-sized specimens of IM7/BMI unidirectional composite with an integrated indenter displacement control were fabricated using Focused Ion Beam (FIB) milling. The specimens were compression tested using a custom built, SEM-based in-situ micro-testing device. During compression, SEM images are acquired continuously between displacement intervals so the deformation phenomena can be observed. Initial results showed that the integrated indenter displacement control prevents complete destruction of the specimen after the onset of failure. Damage observed includes interface failure, broken fibers, and general crushing. Parallel efforts on larger-scale compressive testing are conducted on millimeter-sized specimens using an in situ mechanical test frame located in an X-ray micro computed tomography (μCT) system. Failure response includes longitudinal splitting or brooming and kinking. A quantitative comparison of the compressive strength and modulus obtained from the two size scales specimen shows that there is no indication of a size effect. The experimental results will be used to validate the numerical models of micro-compression behavior.

Torin Quick, Sirina Safriet, David Mollenhauer, Chad Ryther, Robert Wheeler

### Chapter 29. Crack Analysis of Wood Under Climate Variations

The knowledge of crack driving forces such as energy release rate and stress intensity factors is very important in the assessment of the reliability of timber structures. This work deals with static and creep fracture tests in opening mode crack growth. They are performed in climatic chamber, which allows reproducing the real environment effects on the cracking of Double Cantilever Beam specimens machined in Douglas and White Fir species. The evolutions of the crack length are posted versus time. The aim of this work is to compare the obtained experimental results with numerical tools given by a finite element model. The numerical model is based on a new analytical formulation of the A-integral, generalized to viscoelastic orthotropic material, which allows taking into account the effect of thermal and hydric loads in the cracking process.

Nicolas Angellier, Rostand Moutou Pitti, Frédéric Dubois

### Chapter 30. Numerical Fracture Analysis Under Temperature Variation by Energetic Method

It is known that temperature change can induce sudden crack propagation especially when the material is composed of fibers. In this fact, the crack growth process under mixed-mode coupling mechanical and thermal loads in orthotropic materials like wood is investigated in this work. The analytical formulation of A integral’s combines the real and virtual mechanical and thermal stress/strain fields under transient diet in 2D. The Mixed Mode Crack Growth specimen providing the decrease of energy release rate during crack propagation is considered in order to compute the various mixed mode ratios. By using three specific routines, the analytical formulation is implemented in finite element software Cast3m. The efficiency of the proposed model is justified by showing the evolution of energy release rate and the stress intensity factors versus crack length and versus temperature variation in time dependent material.

Rostand Moutou Pitti, Seif Eddine Hamdi, Frédéric Dubois, Hassen Riahi, Nicolas Angelier

### Chapter 31. Use of a Multiplexed Photonic Doppler Velocimetry (MPDV) System to Study Plastic Deformation of Metallic Steel Plates in High Velocity Impact

High-velocity impact experiments with a gas gun pose unique challenges, in terms of experimental setup and computational simulation, since the projectile-target interaction creates extreme pressure and temperature within few micro seconds. The objective of this study is to accurately measure the plastic deformation of plates under projectile impact that does not lead to full penetration. In this work, free surface velocities at the back side of target plates are measured using the newly developed Multiplexed Photonic Doppler Velocimetry (MPDV) system, which is an interferometric fiber-optic technique which can measure velocity from the Doppler shift of the light reflected from the moving back surface of the target. The MPDV system allows measurements of velocity from different locations on the target plate using multiple optical fiber probes oriented in specific directions and patterns. Data are reduced using a Fast Fourier transformation (FFT) technique to obtain the free surface velocity profiles. These velocity profiles can present insights into the dynamic behavior of impacted materials under shock loading conditions.

Shawoon K. Roy, Michael Peña, Robert S. Hixson, Mohamed Trabia, Brendan O’Toole, Steven Becker, Edward Daykin, Richard Jennings, Melissa Matthes, Michael Walling

### Chapter 32. In-service Preload Monitoring of Bolted Joints Subjected to Fatigue Loading Using a Novel ‘MoniTorque’ Bolt

Bolted fastening is one of the oldest, most important, and most neglected aspects in engineering design of machines and structures. One of the biggest concerns is the bolt preload control and monitoring during and after the joint assembly. A survey of automobile service managers in the United States has shown that one-quarter of all service problems were traced to loose fasteners. Therefore, fatigue behavior of in-service bolted joints is of special interest for predicting structural integrity and residual lifetime of the structure. In this work, a novel technique ‘MoniTorque’ (Monitoring Torque) that embeds fiber Bragg-grating (FBG) sensors within the bolt shaft using temporary adhesives is used to monitor pre-load changes during fatigue tests of bolted joints. The inherent small size of the FBG allows precise monitoring without affecting the intrinsic properties of the bolt. Furthermore, temporary adhesives allow easy installation, removal and reuse of FBG sensor, thus converting a simple bolt into a reliable sensing element. Results show good correlation between the number of loading cycles and preload reduction. Overall, the ‘MoniTorque’ technique demonstrated in this work shows promise in cost-effective and reliable health monitoring of in-service bolted joints.

Anton Khomenko, Ermias G. Koricho, Mahmoodul Haq, Gary L. Cloud

### Chapter 33. Fatigue Behavior of Novel Hybrid Fastening System with Adhesive Inserts

A novel joining technique that incorporates the advantages of both bonded (lightweight) and bolted (easy disassembly) techniques was invented (Provisional Patent 61/658,163) by Dr. Gary Cloud at Michigan State University. The most basic configuration of this invention consists of a bolt that has a channel machined through the bolt-shaft that allows injection of an insert compound that fills the necessary clearance between the bolt and the work-pieces and acts a structural component. More sophisticated versions of the concept incorporate additional sleeves or inserts.In this paper the fatigue behaviour of composite hybrid bolted joints was studied. Composite plates were bolted with grade-five bolts and preloaded to a torque of 35 N-m. Two types of bolted joint configurations were evaluated. In the first case, a conventional bolted joint was studied. In the second case, pristine SC-15 epoxy resin was used as the structural insert in the hybrid fastening system. The joints were subjected to different fatigue loadings with the maximum loading level up to 80 % of the joint ultimate failure load and stress ratio of R = 0.1. Results reveal considerable improvement of fatigue life of the novel fastening system compared with a conventional bolted joint.

Ermias G. Koricho, Anton Khomenko, Mahmoodul Haq, Gary L. Cloud
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