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Über dieses Buch

Fracture, Fatigue, Failure and Damage Evolution, Volume 7 of the Proceedings of the 2017 SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the seventh volume of nine from the Conference, brings together contributions to this important area of research and engineering.

Session organizers include: Jay Carroll, Shuman Xia, Allison Beese, Ryan Berke, Garrett Pataky, Samantha Daly, Kavan Hazeli, Antonios Kontsos, Omer Ozgur Capraz, Scott Grutzik, Onome Scott-Emaukpor

The collection presents early findings and case studies on a wide range of areas, including:

Mechanics of Energy & Energetic Materials

Vibration Effects in Fracture & Fatigue

Fracture & Fatigue of Additively Manufactured Materials

In Situ Techniques for Fatigue & Fracture

Microscale & Microstructural Effects on Mechanical Behavior

Fracture & Fatigue of Composites

Integration & Validation of Models with Experiments

Fracture & Fatigue in Extreme Environments

Novel Experimental Methods for Fatigue and Fracture

Fracture of Brittle & Ductile Materials

Interfacial Fracture



Chapter 1. Interface Mechanical Strength and Elastic Constants Calculations via Nano Impact and Nanomechanical Raman Spectroscopy

Interfaces are ubiquitous in important natural and manmade materials. Research evidence has shown that interface chemistry, structure, and thickness together strongly influence material microstructure and mechanical properties. The focus of the present work is on presenting an experiment based theoretic advancement to predict thickness dependent elastic properties of materials interfaces by treating the interfaces and the area around them in a material as an elastic continuum. The experiments are based on the nanomechanical Raman spectroscopy (NMRS) developed by authors earlier with a capability to simultaneously measure stress components in orthogonal directions during an in-situ nanomechanical loading. An analytical model is developed based on boundary conditions of interface to predict thickness dependent interface elastic constants. The interface elastic constants are compared with the relations provided in literature.

Devendra Verma, Vikas Tomar

Chapter 2. Effect of Strain Rate and Interface Chemistry on Failure in Energetic Materials

We study the failure at interfaces between Hydroxyl-terminated polybutadiene (HTPB)-Ammonium Perchlorate (AP) based energetic material. In this work, interface mechanical strength of a set of HTPB-AP interfaces is characterized using nano-scale impact experiments at strain rates up to 100 s−1. A power law viscoplastic constitutive model was fitted to experimental stress-strain-strain rate data in order to obtain constitutive behavior of interfaces, particle, and matrix. A mechanical Raman spectroscopy is used to analyze the effect of binding agent at different temperature. A tensile fracture experiment combined with In-situ Mechanical Raman Spectroscopy was used to obtain fracture properties. Stress maps are obtained near the interface using In-situ Mechanical Raman Spectroscopy to analyze the changes in the stress distribution around interfaces for different loads till failure. Cohesive zone model parameters were obtained from the consideration of local stress during failure and the cohesive energy required for delamination of AP from HTPB matrix. Effect of binding agent on the interface strength is found to be quite significant. The cohesive zone parameters and the viscoplastic model obtained from the experiment were then used in the cohesive finite element method to simulate the dynamic crack propagation as well as the delamination. Results show that interfacial properties are affected by the rate of loading and are also dependent upon the binding agent.

Chandra Prakash, I. Emre Gunduz, Vikas Tomar

Chapter 3. Characterization of Crack Tip Plasticity in IN-617 Using Indentation and Nano-Mechanical Raman Spectroscopy

This research focuses on work with emphasis on direct measurements of stresses during mesoscale microstructural deformation of nickel based super alloys during 3-point bending tests at elevated temperatures. A novel nano-mechanical Raman spectroscopy measurement platform was designed for temperature, stress, and chemistry mapping at micro to nano-scale for different temperature and loading conditions. During the 3-point bending test, notch tip plastic stresses as a function of microstructure, load, and temperature, with micron scale resolution were measured. The temperature field distribution was correlated to stress distribution and residual microstructure stresses around the area of the notch tip. Grain boundaries are the stress concentrated area but with lower temperature due to the contact thermal resistant. Grain boundary slide or grain rotation can result in stress concentration but enhance the ability of heat conduction and result in lower temperature. The mechanical properties which include the elastic modulus, hardness and stress-strain relation at the plastic zone around the notch tip were also measured. Instead of considering actual grain structures with different material properties, a new FE method was adopted to predict stress distribution applying the material properties which were obtained from indentation experiments around the same notch area as scanning. Predictions from theory and simulations matched closely in stress concentration area with experimental measurements. However, away from notch area a slight deviation due to microstructural effects was observed.

Yang Zhang, Vikas Tomar

Chapter 4. The Two-Way Relationship Between Residual Stress and Fatigue/Fracture

Most mechanicians are aware that residual stresses affect fatigue and fracture. Few are aware of the full extent of the effects residual stress have on myriad aspects of those and other structural processes AND of the reverse relationship: fracture concepts are hugely intertwined with residual stress measurements. This talk first reviews the straightforward effect that residual stress has: it acts just like an external load and, depending on the sign, accelerates or retards failure processes. The talk then describes one exemplar less well-known effect: residual stress can make it very difficult to measure true material properties like fracture toughness. The talk then describes the influence of fracture concepts, most notably the mode I stress intensity factor K I , on residual stress measurement. A short and elegant derivation reveals that incremental slitting measurement strain data can be converted to residual stress profiles using a K I analysis and a conceptually simple calibration factor that is independent of the stress distribution. The talk next shows that several error sources for slitting and contour method stress measurements can be directly correlated with the K I caused by residual stress. The talk then lays out Bueckner’s superposition principle, the most powerful but misunderstood and mistrusted tool for analyzing stress fields around cracks. Originally developed for fracture mechanics, Bueckner’s is used to calculate the K I from residual stresses and to calculate calibration coefficients for hole drilling and slitting. More notably, the contour method for measuring residual stress is a direct experimental embodiment of Bueckner’s principle. In fact, we review an experimental demonstration that misfit measurements between mating fracture surfaces can be used to calculate the residual stresses that existed prior to the fracture.

Michael B. Prime

Chapter 5. Designing Brittle Fracture Specimens to Investigate Environmentally Assisted Crack Growth

Subcritical crack growth can occur in a glass when the stress intensity factor is less than the fracture toughness if water molecules are present. A novel bi-material beam specimen is proposed to investigate environmentally assisted crack growth (EACG). Two materials with different coefficients of thermal expansion are diffusion bonded at high temperature and cooled to the room temperature which introduces residual stress in the beam. A Finite element (FE) model is developed and initially validated with an analytical model. Steady-state crack (SSC) depth at which mode II stress intensity factor (KII) is zero and the corresponding mode I stress intensity factor (KI) value are obtained for different material pairs and thickness ratios of the top and bottom materials using the FE model. Crack propagation path is also predicted. We finally modify the geometry of the specimen to generate non-constant KI values as the crack propagates.

Sunday Aduloju, Wenjia Gu, Timothy Truster, John Emery, Dave Reedy, Scott J. Grutzik

Chapter 6. Flexible Energy Harvesting/Storage Structures for Flapping Wing Air Vehicles

Morphing structures are needed for many challenging missions for flapping wing air vehicles (FWAVs), but they will increase electrical power consumption in order to drive actuation systems and will require additional support systems to deliver and control the power. These power demands and associated support systems will comprise the ability of FWAVs to operate for longer periods of time without opportunities for refueling. Harvesting energy, such as solar, and increasing energy storage capacity appears to be an attractive solution to address this challenge. Although research has been conducted on integrating powering existing into aerospace components such as the body, tail and wings, they are typical rigid and can reduce performance because of parasitic mass. We envision compliant multifunctional skins with built-in capabilities for harvesting and storing energy can be used in highly compliant aerospace components, such as morphing wings. These skins will be able to deform and conform to the underlying structure, much like the way skins have evolved to work with components of biological actuation systems such as muscle and bone. They will increase the range of operation by utilizing readily available solar energy to increase electrical power and also increase storage capacity for excess electrical power conversion. Compliant multifunctional structure-energy harvesting-power concepts, once completely developed, may have an enormous impact not only on FWAVs, but on the field of UAVs in general, because of the current energy limitations that exist for these flight platforms. Once energy sources are no longer limited, the field of UAVs, especially Micro Air Vehicles (MAVs), should be able to expand exponentially.

Alex Holness, Hugh A. Bruck, Satyandra K. Gupta

Chapter 7. The Influence of Formulation Variation and Thermal Boundary Conditions on the Near-Resonant Thermomechanics of Mock Explosives

The thermomechanics of energetic and inert particulate composite materials are of pronounced interest in the defense community. This work seeks to further characterize the macroscale, thermal and mechanical response of these materials under various near-resonant mechanical excitations. The fabrication of mock energetic samples based on the PBXN-109 formulation, comprised of hydroxyl-terminated polybutadiene (HTPB) binder with 85% solids loading and varying additive content (0%, 15%, and 30%) of sucrose and/or spherical aluminum crystals, enabled a systematic investigation into the effect of formulation variation on the thermal and mechanical response. Experiments were also performed on insulated plate samples of identical composition to examine the effect of varying thermal boundary conditions. In each of these experiments, the samples were mechanically excited using an electrodynamic shaker, while their thermal and mechanical responses were recorded using an infrared camera and scanning laser Doppler vibrometer, respectively. The investigation of these responses aids in the effort to characterize and understand the behavior of polymer-bonded explosives under mechanical excitation.

Allison R. Range, Nicole R. McMindes, Jaylon B. Tucker, Jeffrey F. Rhoads

Chapter 8. Detecting Fatigue Crack Closure and Crack Growth Delays After an Overload Using DIC Measurements

The closure and crack growth behaviors of a fatigue cracked DC(T) 4340 steel specimen before and after a single 100% overload on the stress intensity factor (SIF) peak, applied over an otherwise mode I loading with quasi-constant SIF range and load ratio, was studied using Digital Image Correlation (DIC) and strain gage techniques. A significant retardation in the post-overload fatigue crack growth rate was observed, and it recovered its pre-overload value only after the crack grew a distance much larger than the Irwin plastic zone induced by the overload. This behavior is attributed to discontinuous closure, since high values of crack opening loads were observed even outside the overload plastic zone. Full-field displacements and strains determined ahead of the crack tip and crack flank opening displacements measured at points along the crack faces from DIC analysis, as well as redundant back-face strain-gage measurements, support this claim.

G. L. G. Gonzáles, J. A. O. González, J. T. P. Castro, J. L. F. Freire

Chapter 9. In-Situ Observation of Damage Evolution in Quasi-Isotropic CFRP Laminates

In this work, a Digital Image Correlation (DIC) impaired with high resolution optical system is applied to capture the damage evolution in composites laminates in-situ. The developed method enables to measure the local (micro scale) deformation across the thickness on the free-edge (8-plies laminate) subjected to a quasi-static tension. Three groups of specimens are prepared by arranging plies at different stacking sequence, and the formation of strain localization, initiation of matrix crack, delamination and other damages are acquired. It is obtained that matrix cracking is the primary and dominant form of damage and it usually occurred in the 90°-plies. However, the orientation and quantity of matrix cracks are highly affected by the stacking arrangement of the plies.

Addis Tessema, Suraj Ravindran, Abigail Wohlford, Addis Kidane

Chapter 10. Contamination-Induced Degradation/Enhancement of Interfacial Toughness and Strength in Polymer-Matrix Composite Interfaces

This study explores the effect of environmental contamination on the interfacial toughness and strength of polymer-based interfaces. Double cantilever beam, end-notched flexure, and single lap joint tests are utilized to examine the contaminated bond line behavior under different fracture modes. A typical adhesive/adherend material system exposed to a common aviation hydraulic fluid at varying contamination concentrations is examined. The mode-I fracture tests reveal a significant degradation in mode-I fracture toughness and strength with increasing contaminant concentration, whereas mode-II fracture properties are found to be insensitive to the contamination level. To the contrary, single lap joint shear strength exhibits an enhancement with increasing contaminant concentration. Such contradictory result can be attributed to nucleation vs. propagation of interfacial cracks. Contamination weakens the interfacial bonding and thereby reduces the interfacial adhesion while shielding plastic dissipation within the bond line. Whereas, contamination also reduces the plastic flow stress of the bond line and thereby increases local plastic dissipation in the SLJ geometry.

Denizhan Yavas, Xu Shang, Ashraf F. Bastawros

Chapter 11. Direct and Simultaneous Extraction of Mixed-Mode Traction-Separation Relations

Traction-separation relations can be used to represent the interactions between two surfaces during separation. In the past, characterizing these interactions, particularly under mixed-mode conditions has been tedious. In this work, a direct method is proposed to simultaneously determine the normal and shear components of traction-separation relations at any mode-mix, based solely on measurements of the initial crack length, load and load-line displacement and rotation of each adherend in a laminated beam configuration.

Chenglin Wu, Rui Huang, Kenneth M. Liechti

Chapter 12. Damage Evolution in 304L Stainless Steel Partial Penetration Laser Welds

Partial penetration laser welds join metal surfaces without additional filler material, providing hermetic seals for a variety of components. The crack-like geometry of a partial penetration weld is a local stress riser that may lead to failure of the component in the weld. Computational modeling of laser welds has shown that the model should include damage evolution to predict the large deformation and failure. We have performed interrupted tensile experiments both to characterize the damage evolution and failure in laser welds and to aid computational modeling of these welds. Several EDM-notched and laser-welded 304L stainless steel tensile coupons were pulled in tension, each one to a different load level, and then sectioned and imaged to show the evolution of damage in the laser weld and in the EDM-notched parent 304L material (having a similar geometry to the partial penetration laser-welded material). SEM imaging of these specimens revealed considerable cracking at the root of the laser welds and some visible micro-cracking in the root of the EDM notch even before peak load was achieved in these specimens. The images also showed deformation-induced damage in the root of the notch and laser weld prior to the appearance of the main crack, though the laser-welded specimens tended to have more extensive damage than the notched material. These experiments show that the local geometry alone is not the cause of the damage, but also microstructure of the laser weld, which requires additional investigation.

Sharlotte Kramer, Amanda Jones, John Emery, Kyle Karlson

Chapter 13. Cross-Axis Coupling and Phase Angle Effects Due to Multiaxial Vibration

The response of structures under combined biaxial harmonic base excitation is investigated experimentally. The effect of cross-axis coupling, especially as a function of the relative phase angle between two vibration actuators is demonstrated. The experiments are performed using a unique multiaxial electrodynamic shaker with high degree of controllability. Increasing the phase angle between the rotational and translational excitation from 0° to 135° increases the dynamic response of the structure and its nonlinear stiffness. The results are compared to both uniaxial rotational and translational vibration. The chapter provides qualitative and quantitative differences between uniaxial and multiaxial nonlinear harmonic excitation with varying phase and fatigue build up.

Ed Habtour, Abhijit Dasgupta, Sabrina Vantadori

Chapter 14. Behavior of Steel-Concrete Composite Beams Under Fatigue Loads

Besides the static load, the cyclic load caused by the vehicles always exists in the bridge structures. This kind of loading may cause failure even when the nominal maximum loads have not exceeded the ultimate resistance of the structure. So, the main objective of this paper is to evaluate the fatigue behavior of composite beams at the sagging moment regions. A numerical model is described to predict the fatigue response of each part of the composite section. The accuracy of the developed numerical model is validated using existing test data. Also, the transformed section method is used to evaluate the fatigue response of the composite beams in terms of deflection, strains in the concrete flange and the steel beam, plastic deformation, reduction in the static strength of the shear connectors, and residual capacity of the composite beam. The cumulative damage rule is analyzed through the S-N curve to assess the procedures presented in the design code. A rapid growth in the residual deformations of the composite beams is obtained at the starting and the end of the fatigue life with the number of cycles while a linear increase in the remaining part of the fatigue life occurs. In addition, a reduction in the static strength of the shear connectors is developed with the number of cycles and subsequently causes a drop in the monotonic beam capacity which can be calculated based on the new shear connection strength.

Ayman El-Zohairy, Hani Salim

Chapter 15. Studying the Fracture of Tropical Wood Species with the Grid Method

The present study consists in studying the initiation and propagation of cracks at room temperature of three tropical species: Okume (Aucoumea Klaineana), Iroko (Pterocarpus Soyauxii) and Padouk (Malicia Excelsa). A short review of the literature shows that only few studies dealing with the fracture mechanics properties of this type of wood species are available. Similar studies are however routinely performed on temperate wood species such as Beech and Douglas, using mixed-mode crack growth (MMCG) specimens for instance [1, 2]. In this paper, tropical wood specimens are studied using the grid method [3] and such MMCG specimens, but made of the three aforementioned tropical species.

B. Odounga, R. Moutou Pitti, E. Toussaint, M. Grédiac

Chapter 16. Generalization of Integral Parameters to Fatigue Loading in Room Temperature

In this paper a numerical approach coupling independent path integrals, such as M-integral, to compute the crack driving forces namely the stress intensity factors, and empirical models, for instance Paris-Erdogan’s law, to assess the cumulative fatigue damage (i.e. crack size) during the crack growth process, is proposed. The M-integral derived from Nother’s theorem combines the real and virtual mechanical deformation and stress fields. A finite element routine is developed in order to compute the energy release rate according to the stress intensity factors. Results are given for a simple standard Al7075-T6 tensile test specimen. Finally, numerical estimates are compared to experimental data for various crack length in order to prove the efficiency and the accuracy of the proposed model.

Rostand Moutou Pitti, Hassen Riahi, Mulugeta A. Haile

Chapter 17. Fracture Behavior of Unidirectional Composites Analyzed by Acoustic Emissions Technique

The aim for delamination tests is to obtain reliable results, simulating in-situ cracking and giving a fracture toughness value that can consistently characterize materials for structural applications. Standardized organizations developed and adopted similar standards to carry out tests by using proper load fixture possibly combined with optical instruments, to allow the real monitoring of delamination growth for the whole duration tests. In this work, the fracture behavior of unidirectional composites is experimentally analyzed by using acoustic emissions technique too. Sensor was placed on the sample to monitor the evolution of the acoustical events during the tests. Data obtained, in terms of energy, number of hits and hits derivative were critically analyzed to assess the capability of the technique in following the evolution of the damage process and giving information about the toughness values compared with the standardized calculation methods.

C. Barile, C. Casavola
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