Fracture, Fatigue, Failure and Damage Evolution, Volume 8

Several fatigue failure modes originate at the microstructural level by the activation, interactions and development of what are referred to as “damage precursors” long before the formation of dominant cracks that grow as a function of loading and crystallographic parameters. In this context, this work presents new developments of an in-house developed experimental mechanics approach to evaluate aspects of microstructure evolution and identify validated damage precursors that are active during fatigue loading by combining Nondestructive Evaluation (NDE) methods with ex situ and in situ Scanning Electron Microscopy (SEM). The used NDE methods include real time Acoustic Emission (AE) monitoring from inside the SEM chamber and Digital Image Correlation (DIC) for strain evolution directly at the grain scale. The coupling between quasi in situ microscopy with actual in situ nondestructive evaluation falls into the ICME framework and the idea of quantitative data-driven and multiscale characterization of material behavior. To demonstrate this approach, Aluminum 2024-T3 specimens were tested using a SEM mechanical testing stage under low cycle fatigue to identify and validate the presence of damage precursors, while correlating their presence with specific parameters extracted by the available NDE data. The reported results show how load information could be correlated with both AE activity, DIC strain maps, and microscopic observations of precipitate fracture and microcracks.

The fracture processes of three tropical species: Aucoumea klaineana, Malicia excelsa and Pterocarpus soyauxii, are investigated with the grid method. These species are widely used in many sub-tropical countries, in timber building construction, as well as in semi-finished products and paper fabrication. However their fracture behaviour must still be investigated, data being scarcely available on this subject. Modified Mixed Mode Crack Growth specimens are used in order to obtain a stable crack growth evolution in opening, shear and mixed mode ratios. The images of the grid are analysed to provide the crack opening displacement and the crack tip location. The stress intensity factors and the critical energy release rates for each species are then obtained by using the compliance method in imposed displacement. The semi-experimental energetic method is also applied in order to show the efficiency of the proposed technique to characterize the fracture properties of the tropical species under study.

Heterogeneous materials are ubiquitous in nature, and are increasingly being engineered to obtain desirable mechanical properties. Naturally, the bulk properties of a heterogeneous material can be different from those of its constituents. Thus, one needs to determine its overall or effective properties. For some of these properties, like effective elastic modulus, the characterization is well-known, while for other such as effective fracture toughness, it is a matter of ongoing research. In this paper, we present a method to measure the effective fracture toughness. For the method, we apply a time-dependent displacement condition called the surfing boundary condition. This boundary condition leads the crack to propagate steadily macroscopically but in an unconstrained manner microscopically. We then use the grid method, a non-contact full-field displacement measurement technique, to obtain the displacement gradient. With this field, we compute the macroscopic energy release rate via the area J-integral. Finally, we interpret the effective toughness as the peak of the energy release rate. Using this method, we investigate the influence of heterogeneity on effective fracture toughness. We find that the effective toughness can be enhanced due to the heterogeneity. Consequently, engineered heterogeneity may provide a means to improve fracture toughness in solids.

A hybrid specimen was developed to minimize material used when assessing bending fatigue behavior with a vibration-based experimental procedure. Motivation for this work comes from the eagerness to quantify critical three-dimensionally printed gas turbine engine components at low cost under representative operating and stress conditions. The original vibration-based fatigue specimen (a whole, square plate) is capable of providing airfoil representative data at a lower cost than failing a fully manufactured airfoil. The reduced cost, though significant, was further reduced with the hybrid specimen. The most recently published iteration of the hybrid specimen produced an insert-plate system that required 95 % less material than the original specimen in order to gather fatigue data, and the hybrid specimen data compared favorably within a 95 % prediction interval of the original for Aluminum 6061-T6. Despite the successful comparison, improvements to the hybrid plate specimen were still necessary for accurately assessing fatigue behavior of more commonly used aerospace alloys. Specifically, improving repeatability in the experimental response, increasing the hybrid specimen excitability (i.e. reduce specimen system damping), and minimizing damage accumulation on the carrier plate of the hybrid specimen were critical to the efficiency of the characterized bending fatigue behavior. The proposed investigation addressed these issues for Titanium 6Al-4V, resulting in a more repeatable experimental procedure and results that agree with whole plate data.

In this work, the correlation between local damage and residual stiffness in carbon fiber composites is investigated experimentally. The study presents the interaction between local damage mechanisms and the global response of the material, with the aim to capture the gradual local failure phenomenon. High resolution digital image correlation technique at micro length scale is used to measure the local deformation in the specimen made of carbon fiber composite, subjected to tension-tension fatigue loading. During testing, image of a speckled surface inside the gage length of the specimen is captured at specified number of loading cycles. Using digital image correlation, the acquired images are processed and the local deformations and strains are extracted. The growth of local plastic deformation as a function of loading cycle is acquired and different damage modes as a function of loading cycle is explored. The study has able to elucidate the event that shows the gradual growth of matrix cracking into inter-ply debonding. Furthermore the degradation of modulus of elasticity as a function of loading cycle is determined and the corresponding type of damage incorporated within the plastic strain distribution around it is presented.

Resistance against delamination failure and through the thickness tensile properties of curved carbon fiber reinforced plastics composites are investigated experimentally by conducting the curved beam strength tests. Effect of novel material thin ply non crimp fabric (NCF) architecture on delamination resistance of carbon fiber reinforced composites are investigated and compare with that of standard UD layups. In order to determine through the thickness tensile properties of curved carbon fiber composites, standard test method is carried out, namely four-point bending tests. The dynamic delamination propagation and failure sequences under curved beam bending is captured using Photron© Fastcam SA5 ultra high speed system. For the non-crimp fabric configuration an increase in the curved beam strength is observed in comparison with [0] and [0/45/-45/0] laminates by unidirectional (UD) tape material. For the UD tape, the initial defects caused by the out-of-autoclave manufacturing process is found to be the potential failure sites. The test results and observations suggest that thin-ply NCF is much less vulnerable to the existence of manufacturing voids in contrast to standard thickness UD tape. Finally, TPNCF is shown to have superior properties in regard to delamination resistance and curved-beam strength.

In aerospace industry, high demand for the lightweight structures are fostering the use of carbon fiber reinforced polymer composites in a wide variety of shapes, as primary load carrying elements. However, once a composite laminate takes a highly curved shape, such as an L-shape, interlaminar stresses augmented in the curved region cause highly dynamic delamination nucleation and propagation. This paper provides experimental observations of dynamic delamination failure in cross-plied L-shaped composite laminates under quasi-static shear loading for varying laminate thickness. In the experiments, load-displacement curves are recorded and dynamic delamination events areas captured using a million fps high speed camera. In our previous work, two distinct types of failure modes have been identified depending on the laminate layup: (i) formation of multiple delaminations leading two single load drop in its load-displacement curve during the failure of unidirectional laminates, [0]₁₇, and (ii) formation of sequential delaminations associated with each discrete load drop in its load-displacement curve were during the failure of cross-ply laminates, [90/0]₁₇. Accordingly this current study shows that formation of sequential delaminations is independent from the laminate thickness.

The fracture properties of TRIP780 and magnesium AZ31B-H24 alloy sheets were investigated in this paper. Mechanical experiments were performed for TRIP780 under uniaxial tension, notch tension, punch test, and plane strain tension. Mechanical experiments were performed for magnesium AZ31B-H24 sheets under different loading conditions, including monotonic uniaxial tension, notch tension, in-plane uniaxial compression, wide compression (or biaxial compression), plane strain compression, through-thickness compression, in-plane shear, punch test, and uniaxial compression-tension reverse loading. The stress invariants based Modified-Mohr-Coulumb (MMC) fracture model was transferred into an all-strain based MMC (eMMC) model under the plane stress condition, predicting the fracture strain in the space of strain ratio or Φ angle, instead of stress triaxiality and Lode angle parameter. The strain ratio or Φ angle could be directly measured by digital image correlation (DIC), while the latter required finite element analysis to be determined. This method made it possible to study fracture of materials while bypassing plasticity. Using the fracture strain measured by DIC, fracture locus was calibrated by the all-strain based MMC model. The fracture strain was extended by using a linear transformation operating to the plastic strain tensor to incorporate the anisotropic fracture behavior. Good prediction capability has been demonstrated for these two materials.

Polyimides and fiber reinforced polyimide matrix composites are used in demanding applications requiring mechanical performance at high temperatures (300 + °C). When exposed to moisture and elevated temperature for extended periods of time polyimides may undergo hydrolytic degradation in which bonds are broken at a rate dependent on the temperature and moisture absorbed into the material. These broken bonds will be reflected in reductions in mechanical properties such as moduli, glass transition temperature and flow strength. In this project the polyimide HFPE-II-52 is aged under fully moisture saturated conditions at temperatures up to 250 °C. Following the temperature and moisture exposure, cube shaped samples are tested in compression at room and elevated temperatures to measure the reductions in stiffness and yield stress. This research is aimed at providing a means for monitoring and predicting hydrolytic degradation and its effect on mechanical performance.

A woven carbon/epoxy composite was subjected to fatigue crack growth under mixed Mode-I/Mode-II loading to obtain crack growth behavior at different cyclic strain energies. Owing to the woven structure of the material, pure Mode-II fracture is usually a difficult proposition because of friction, interference, and interlock of woven tows in adjacent plies at an interlaminar crack. These limitations were overcome by the use of a novel form of mixed Mode-I/Mode-II specimen, which imposes sufficient crack surface opening (Mode-I) to alleviate ply–ply interactions, but not so much as to obscure the sliding (Mode-II) response. Comparison with pure Mode-I fatigue crack growth data, in conjunction with a fracture interaction criterion, provided a means to extract the Mode-II behavior.

Damage evolution in fatigue tests (R = 0.5, −1, 2) conducted on carbon fiber reinforced plastic (CFRP) composites has been characterized using digital image correlation (DIC). Since damage initiation/delamination is a local phenomenon affecting transverse strain more than the longitudinal, local transverse strain is a better indicator of onset of delamination and its propagation. Variation of transverse strain near the initiated delamination with cycles indicates that the damage evolution occurs over 2–3 stages. Each stage has a stable damage growth with sudden increase between the stages. Waviness and the associated error due to the lag between image and load data acquisition was overcome by plotting the maximum transverse strain obtained from a curve fit to each set of continuous cycles. Error due to large relative deformations was avoided by choosing different reference images for different stages. Extent of damage zone and its evolution was characterized by the length over which the transverse strain exceeds a limiting value, which was taken to be that at the end of first stage in the plot. Rate at which the damage propagates shows similar variation as that of the local transverse strain, which shows that the latter can be used as an indicator of fatigue damage evolution. This also provides a method to quantify the damage in terms of local transverse strain, which can in turn be used to validate any developed damage models.

A new experimental method has been develop to characterize critical interfacial damage parameters of solder interconnects subjected to high strain-rate mechanical loading, simulating shock or impact loading. A test apparatus and a test specimen were devised to experimentally characterize such critical damage parameters, particularly interfacial shear strength and fracture toughness. The test fixture was designed to easily mount and unmount test specimens, and accommodate various sizes of electronic components and solder layer thicknesses. Test specimens were fabricated with both metallic and polymeric solder materials, and tests were conducted under various shear load rates. It was found that both strength and fracture toughness exhibit significant rate-dependency.

Through thickness reinforcement (TTR) technologies have been shown to provide effective delamination resistance for laminated composite materials. The addition of this reinforcement allows for the design of highly damage tolerant composite structures, specifically when subjected to impact events. The aim of this investigation was to understand the delamination resistance of Z-pinned composites when subjected to increasing strain rates.

Z-pinned laminated composites were manufactured and tested using three point end notched flexure (3ENF) specimens subjected to increasing loading rates from quasi-static (~0 m/s) to high velocity impact (5 m/s), using a range of test equipment including drop weight impact tower and a split Hopkinson bar (SHPB).

Using a high speed impact camera and frame by frame pixel tracking of the strain rates, delamination velocities as well as the apparent fracture toughness of the Z-pinned laminates were measured and analysed. Experimental results indicate that there is a transition in the failure morphology of the Z-pinned laminates from quasi-static to high strain rates. The fundamental physical mechanisms that generate this transition are discussed.

This study explores a probable correlation between the degradation of bond line toughness in adhesively bonded joints and the degradation/changes in measurable mechanical properties of the adhesive layer by indentation hardness due to contamination. The proposed framework utilizes the large scale bridging of interfacial fracture proposed by Tvergaard and Hutchinson (Philos Mag A 70:641–656, 1994). A typical adhesive/adherend material system (EA9394/Hexcel IM7-G/8552) exposed to different contaminants at the same concentration was examined. Nano indentation technique, considered as a non-destructive testing methodology compared to the bond line thickness, was utilized to measure the adhesive mechanical properties. In addition, macroscopic mode-I fracture toughness was independently measured by double cantilever beam test. Finite element method employing cohesive zone method was used to rationalize the experimental results and the prospective scaling-laws. The combined experimental results of macroscopic properties and the numerical results of the interfacial properties suggest a scaling between the interfacial cohesive fracture toughness and the measurable flow stress. While the proposed scaling is verified to a common adhesive-adherend system in aerospace industry, with additional examination of other systems, the proposed scaling law facilitates the utilization of the non-destructively evaluated indentation hardness to serve as an indicator for the bond line macroscopic fracture toughness.

Three novel crack stopper designs for foam cored composite sandwich structures have been investigated with respect to their ability to deflect and arrest propagating face debond cracks. One of the new crack stoppers was similar to a previously developed design, whereas the two others were modified with layers of glass fibre fabric extending from the peel stopper tip into the face sheet, or into the face sheet/core interface. The novel designs were investigated under mode I dominated crack propagation conditions. Both quasi-static and fatigue loading scenarios were investigated. The mechanisms controlling crack propagation were studied using Thermoelastic Stress analysis (TSA) and Finite Element (FE) analysis. The TSA revealed significant new information about the local stress fields in the vicinity of the crack stopper tip as well as the fracture process zone. The first configuration in most cases was able to deflect debond cracks, albeit not in all cases, whereas it was incapable of achieving crack arrest. The two other designs performed better in that they consistently demonstrated the ability to deflect propagating cracks. Only the second design could arrest the cracks consistently as well. Detailed numerical fracture mechanics analyses confirmed and explained the experimental observations.

In this work, acoustic emission (AE) is used as a measurement technique to detect and locate the crack tip and, to monitor its propagation in a wooden specimen subjected to mechanical stresses. Tensile tests were performed on DCB specimens to generate mode I cracking. Under these stresses, the material response results in a release of energy in the form of transient elastic waves that were recorded by AE sensors. By means of the AE technique, the monitoring of material damage lies in the ability to identify the most relevant descriptors of cracking mechanisms. The latter are identified by clustering the AE data. A K-means++ algorithm was used, and two AE features—peak frequency and number of counts—represent adequately the AE events clustering. This unsupervised classification allows the AE events that were generated by crack tip growth during the test to be identified. There are many parameters that can affect the accuracy of AE monitoring such as noise signals, geometry, wood species, etc. Consequently, in this study, probabilistic approaches (Probability of Detection) were used to both characterize uncertainties and improve AE experimental protocol.

Temporary structures refer systems and assemblies without a permanent foundation, which will be removed after a certain period of time. In the past decades there have been numerous significant collapses of temporary structures in United States. To ensure structural stability and the safety, there is increasingly demand of developing an industry wide standard code, technology and procedure of temporary structure design, erecting, maintenance, monitoring and evaluation for practicing engineers and contractors. In this study, Digital Image Correlation was utilized for structure monitoring and failure detection of temporary structure during construction or loading period. The experimental results demonstrated the displacement measurement at the structural nodes could be used to determine the safety stage of the structure, predict the structural failure time and location. The aim of this research is to advance this state of the art measurement technology and facilitate a real-time automated monitoring and detection system for of structural failures of temporary structures.

The extraction of hydrocarbons from unconventional oil and gas reservoirs relies on a detailed understanding of the cracking processes in shale. Also, underground structures designed for nuclear waste repositories are often preferred to be in shale due to its characteristic low permeability. Specifically, how cracks initiate, propagate and coalesce in shale is of interest in these contexts. A series of unconfined compression tests were conducted on Opalinus shale extracted from the Mont Terri underground rock laboratory in Switzerland. These tests were performed on prismatic Opalinus shale specimens with one vertical flaw. High resolution imagery was used to capture crack initiation, -propagation and -coalescence. Digital Image Correlation (DIC) methods were used to determine deformation and crack initiation. By using static loading and unloading cycles, localized strains associated with cracking events were observed. The DIC analysis showed much more cracking than visual inspection with the naked eye. Additionally, a significant influence of bedding planes on the cracking patterns was observed.

Conventional procedures and methods used for evaluating the fatigue performance of materials require time consuming tests with high number of specimens. In the last years different methods have been developed with the aim to reduce the testing time and then the cost of the experimental campaign. In this regard, thermographic methods represent a useful tool for rapid evaluating the fatigue damage and the fatigue limit.

This work puts forward a novel procedure for assessing the fatigue limit and monitoring damage in GFRP material by means of thermography. In view of this, the proposed method allows for inferring several information related to the damage phenomena in composite materials, and, moreover, the reported results show a good agreement with those provided by the conventional procedures.

Crack initiation in a cantilevered beam subject to harmonic excitation near the beam’s second natural frequency has been determined using Nonlinear Model Tracking (NMT), a health monitoring technique. This method assumes a second order nonlinear differential equation model with cubic stiffness; the nonlinear parameter is tracked until catastrophic failure using a Continuous Time based System Identification. Previous research has shown that significant change in the value of the nonlinear parameter indicates the system’s transition from healthy to unhealthy. This study introduced Gaussian noise into the raw stimulus and response data at various signal-to-noise ratios. The results were compared with those of the original data to highlight the technique’s effectiveness in determining a change in the system’s health. The model’s robustness was also investigated by exciting the system at a range of frequencies near resonance, and the results of this test were compared to results from excitation at a single frequency. New methods of identifying crack formation in the beam were also implemented. The raw acceleration response data was plotted next to the nonlinear parameter in real time, and the system’s natural frequency was recorded before and after crack initiation.