Fracture, Fatigue, Failure and Damage Evolution , Volume 3

This work investigates the fracture behavior of preflex beams submitted to time-dependent effects and long-term loadings. However, it is necessary to consider delayed effects such as shrinkage and creep on these structures because their influence is not negligible. The different materials used in mixed construction are presented. The flowchart applied to compute creep and relaxation effects versus the time is also described. It is interesting to note that the analytical results obtained for a preflex beam are very similar to those obtained using finite element modeling on the LUSAS software. It is also observable that the cracking of the concrete that encases the lower fiber, due to creep, causes an increase of stresses in the steel profile, which can break the beam. The use of reinforcement on the preflex beam decreases the crack opening after 100 years loadings.

Aluminum materials of various grades are utilized across many industries, spanning from the cycling, automotive, aerospace, and the marine industry. In the marine grade, aluminum materials are utilized to construct entire vessels of various lengths or portions of them by taking advantage of the lower weight characteristics of the materials and impact on stability of the structures. In particular, 5xxx series aluminum materials are relied on by the marine industry for these purposes, taking advantage of the ability of these series to resist marine corrosive environments. However, during its lifetime, a marine vessel will experience a multitude of variable amplitude loading conditions, with occasional overloads and underloads depending on the operation and environmental conditions. In some cases, these overloads/underloads can result in the failure of the structures by reaching its ultimate capacity, but in other instances, they can systematically affect the growth rate of localized cracks. Existing models, like the Wheeler, Willenborg, plus variations of these, have been utilized to predict the crack growth behavior with varying degrees of success. We created an experimental matrix to explore the effects of overload/underload combinations on fatigue crack growth in 5xxx aluminum. Both visual inspection of crack tip location and digital image correlation (DIC) characterization of the crack tip deformation fields were used to characterize the crack growth in center crack tension (CCT) panel specimens. DIC also enabled additional analysis of strain fields to elucidate on the conditions responsible for change in the crack growth behavior. This chapter outlines some of the ongoing results of this work, which built on past experimental work conducted. Future phases of this work will utilize this data to develop new models for fatigue crack growth, and application of multiple pulses sequences.

Dynamic fracture analysis of aluminum alloys is a significant research area due to their uses in the aerospace and defense industry, where catastrophic failure can occur suddenly. The crack mouth opening displacement (CMOD) is an important tool in determining the various fracture parameters like stress intensity factor. Quasi-static and dynamic three-point bend (TPB) experiments were performed to investigate the effect of high loading rates on CMOD and fracture behavior of the Aluminum 6063-T6 alloy. A 3D Digital Image Correlation (DIC) was utilized in measuring the CMOD experiments. Quasi static three-point bend experiments were performed using UTM and high-resolution stereo camera system for low loading rates. For dynamic three-point bend experiment, a modified Hopkinson pressure bar setup, two high speed digital cameras, and a dedicated data acquisition system were utilized in measuring load point data and the full field displacement profile. Authors have also evaluated the time response of static and dynamic stress intensity factor from the CMOD with the help of the standard formulations as used by several other researchers. Authors found the values of dynamic fracture toughness are significantly high as compared to static fracture toughness.

Additive manufacturing (AM) has proven itself to be an effective and versatile solution in replacing aircraft structures and components. However, the AM process still requires the necessary structural reliability as well as the technology to assess operational longevity. In this work, a fatigue performance and damage progression assessment framework is proposed to achieve a fundamental understanding of the fatigue damage mechanisms and its progression in as-built treated electron beam melted (EBM) Ti-6Al-4V at the macroscopic structural scale as well as at the microscopic constituent scale. The work presented utilizes digital image correlation (DIC), an optical strain measurement technique, as a method to detect crack initiation sites occurring on the material’s surface and propagating throughout the specimen. A comprehensive testing framework and experimental procedure is developed to generate fatigue data for AM material Ti-6Al-4V as-built specimens. Characterization and simulation of the fatigue progress due to AM process defects (voids, surface roughness, etc.) are also performed using damaging energy progress and damage evaluation.

Several reinforced concrete structures fail by fatigue loads. The effects of this type of loading are complex. Many mechanical models based on the damage theory could be used to represent the behavior of reinforced concrete under cyclic loading. Their use requires, among others, knowledge of the material characteristic parameters and its related uncertainties that could be determined from experimental tests. However, the models are time-consuming and the experimental data scarce. In this chapter, we propose a Bayesian network–based methodology to propagate uncertainties in the damage theory model. The proposed methodology is useful to identify the uncertainties of the damage model used when some parameters are measured. The methodology is illustrated with a reinforced concrete beam subjected to cyclic loading. The results obtained were compared with those of the experimental tests to validate the proposed methodology. The good agreement indicates that our approach is capable of propagating uncertainties and integrating data from experimental tests. The proposed approach could be also used to identify the uncertainties of the model used by introducing experimental measurements.

Interlaminar fracture is a failure mode that fiber-reinforced polymer composites (FRPC) are commonly susceptible to during loading. The strain energy release rate associated with delaminations can be the limiting factor in a laminate’s design. Standard test methods have been developed to measure the critical strain energy release rates using precracked coupons, such as the double cantilever beam (DCB). However, since the adherends in these coupons are laminates themselves, often the crack can initiate a secondary crack within one of the adherends and propagate along a secondary interface as well as the primary, precracked, interface. Deconvoluting the effects of the two cracks, a bridged ply, and multiple crack tips can turn a relatively simple test in determining a material property into a very complicated structural problem. In most cases, it is best to scrap the data collected after the crack has jumped interfaces and start with a fresh specimen. For fabric composites, this phenomenon can be quite common due to the variation in bond line thickness between plies resulting from the architecture of the fabric itself (tow size, weave architecture) as well as manufacturing flaws (voids, foreign object debris). This study aims to use the crack jumping phenomenon to learn more about the characteristics of the process zone through the insertion of designed flaws as well as design a method for evaluating the fracture properties of a toughened film adhesive in a co-cured context.

In all widely used fracture test specimens, the compressive or tensile stress parallel to the plane of growing crack is negligible, and thus its effect cannot be revealed. The classical fracture models, including the cohesive crack model, cannot capture any effect of such crack-parallel normal stress and strain, except parametrically, because they do not figure such stress and strain as the basic thermodynamic variable. To capture this, the fracture process zone whose 3D stress and strain state is fully described must be implemented. Here it is shown experimentally, and documented by crack band finite element simulations with microplane model M7, that the crack-parallel normal stresses have a major effect on quasibrittle materials such as concrete. They are shown to cause a major decrease or increase of the Mode I (opening) fracture energy G_f (or fracture toughness K_Ic). The experiments introduce a modification of the standard three-point bend test, the idea of which is to use plastic pads with a near-perfect yield plateau to first generate compression and a gap at end supports to close later and generate bending. The experiments show and the microplane model confirms that a moderate crack-parallel compression greatly increases G_f (even doubling it), but a higher compression reduces G_f greatly, which represents the case of compression splitting. Through numerical extrapolation, it shows that crack-parallel tension reduces G_f and further that a high compressive or tensile stress normal to the specimen plate has a similar major effect on G_f. While mild parallel stresses arise in shear failure of reinforced concrete beams or slabs and prestressed concrete, high crack-parallel stresses will be impactful in hydraulic fracturing of shale when the effective stress state in the solid phase changes at the presence of a nearby borehole or fluid diffusion.

This work is supplementary to extending the use of digital image correlation (DIC) to the experimental method of traveling wave excitation (TWE). The current state of the art for measuring the modal response of a component using TWE is with scanning laser vibrometry (SLV). Although SLV is highly accurate, only one point may be measured during a frequency sweep, therefore potentially causing excessively long experimental testing time when measuring the full-field modal response of a component. The replacement of SLV with DIC has the ability to significantly reduce the time to conduct TWE tests as DIC is capable of measuring full-field data during one sweep when compared to SLV which requires one sweep for often many key points on the component identified by the user. Within this work, DIC measurements are compared and validated against SLV for the measurement of the model response of an academic finger disk undergoing frequency sweeps via single point excitation.

Explosive bonding is a dynamic joining method used to rapidly create metallurgical bonds between two metals. These interfaces can exhibit strengths greater than the parent materials and contain little porosity. Bond quality, however, is highly dependent on processing parameters. Explosive bonds fueled by ammonium nitrate have been extensively characterized, but processing windows for plastic explosives have not. Here, bond strength in 304L stainless steel plate explosively bonded using rubberized/plastic explosives are assessed using shear by tension loading of single-lap-joint specimens. The effects of collision velocity and collision angle on bond quality and strength are investigated and used to define a processing window. Failure modes varied across both the process space studied and within individual bonds. Microstructural analysis across fractured interfaces is combined with fractography to describe the different failure modes and variable strength across bond interfaces.

Insight into damage progression within glass fiber reinforced composites (GFRCs) contributes to understanding failure of composites by interaction of various damage modes, developing physics-based canonical theoretical models, and finally manufacturing desired compositions. In this work, dynamic singe-edge notched bending (DSENB) experiments were performed on a modified Kolsky compression bar, impacting the notched composite beam onto an indenter mounted in front of a load cell. The high-speed X-ray phase-contrast imaging (PCI) technique was used to penetrate the opaque composite and capture in real time the detailed damage initiation and evolution inside the GFRC. Experimental results were compared with those obtained by optical imaging technique, revealing high-speed X-ray PCI technique was able to characterize the inner layers of composite and capture the damage progression among multiple composite layers.

Fatigue is a multistep process where cyclic loading causes damage within materials that eventually leads to crack formation and propagation. In nanocrystalline metals, a dominant damage mechanism is the abnormal growth of grains up to 100 times their original size. Previous in situ synchrotron experiments have revealed that this grain growth process precedes crack formation and takes up a majority of the fatigue lifetime. The growth of nanocrystalline grains leads to the formation of protrusions on the surface of a material, which can be resolved in scanning electron microscopy. Based on this concept, an automated in situ scanning electron microscope tension–tension fatigue test method has been developed to observe the evolution of crack formation and propagation in materials. In this study, this method was applied to understand the high-cycle fatigue behavior in nanocrystalline Ni- and Pt-based alloys. Fatigue tests between 10⁵ and 10⁷ cycles were performed, and in combination with postmortem characterization through grain orientation mapping and transmission electron microscopy, we identified differences in resistance to damage and crack propagation in the various alloys, and observed varying damage levels prior to crack formation, strongly dependent on the number of cycles to failure.

In many engineering applications, Mode-III type loading in the crack tip region is more common. Since loading in structures oftentimes is quite complex, the crack tip region generally experiences Mixed-Mode conditions. In this work, torsional loading experiments are performed by employing a modified spirally cracked cylindrical specimen. The cylindrical specimen used in all experiments is machined to incorporate a full revolution, spiral v-notch crack. The v-notch crack is inclined at an angle of 67.5° with respect to the specimen centerline to obtain Mixed-Mode (I/III) crack tip conditions under torsional loading. By combining the experimental measurements with detailed numerical simulations, the Mixed-Mode (I/III) fracture parameters for polycarbonate (PC) are quantified using an interaction integral method. The elastic-viscoplastic material response of the PC material, required for numerical simulations, is determined by performing standard tensile loading experiments. The Mixed-Mode (I/III) fracture toughness, as well as the stress intensity factors for Mode-I and Mode-III crack tip conditions are presented and discussed.

The primary objective of this research is to investigate, through fundamental experiments, the dynamic multiaxial deformation, failure and strength degradation mechanisms that govern individual ballistic fiber failure. Predicting ballistic impact performance of armor systems requires an understanding of fiber failure under complex multiaxial loading conditions. This study examines the failure behavior of ultrahigh molecular weight polyethylene (UHMWPE) Dyneema® SK76 single fibers under dynamic transverse impact as a function of varying loading rates and projectile geometry. A novel single fiber transverse impact experiment is developed by modifying the Kolsky bar to characterize failure of fibers to create the foundation for a failure model. Experiments are performed with load cells at the fiber ends and with high speed imaging for determining average stresses and strains. Post-test microscopy imaging of failure surfaces are compared to determine the dominant fiber failure modes for each experimental group.

Linear elastic fracture mechanics theory predicts a parabolic crack opening profile. However, direct observation of crack tip shape in situ for brittle materials is challenging due to the small size of the active crack tip region. By leveraging advances in optical microscopy techniques and using a soft brittle hydrogel material, we can measure crack geometry on the micron scale. For glasses and ceramics, expected crack opening displacements are on the order of nanometers. However, for hydrogels, we can achieve crack opening displacements on the order of hundreds of microns or larger while maintaining brittle fracture behavior. Knowing the elastic properties, we can use crack geometry to calculate the stress intensity factor, K, and energy release rate, G, during propagation. Assuming the gel is hyperelastic, we can also approximate the size of the nonlinear region ahead of the crack tip. Geometric measurement of fracture properties eliminates the need to measure complex boundary and loading conditions, allowing us to explore new methods of inducing crack propagation. Further, this allows us to define measures of fracture resistance in materials that do not fit the traditionally defined theories of fracture mechanics.

The polyurea coating is found very useful in strengthening structures ranging from helmets to concrete structures under impact or blast loading. We believe that the hierarchical architecture of nano and microstructures is the bases of the strengthening mechanism, which provides scale-dependent stress laxation and energy dissipation. Here, a challenge is to characterize the strengthening mechanisms not only in the bulk of the copolymer but also at the coating/substrate interface. To this end, we have found that the tapping-mode images of an atomic-force-microscope (AFM) are ideal markers for digital image correlation (DIC) analysis of nano/micro-scale deformation. The tapping-mode images typically exhibit clustered hierarchical structures of hard and soft domains that can trace multiscale deformation mechanisms. To study the role of the hierarchical deformation mechanisms in dynamic toughening, we have developed a line-image shearing interferometer (L-ISI) for plate impact experiments of dynamic fracture testing. The L-ISI measures the variation of the normal-displacement-gradient over time along a line on the back surface of a pre-cracked specimen loaded by plate impact. The time history of the displacement gradient forms fringes on the streak-camera image, and the fringes are inverted to determine the time history of the crack speed and the dynamic toughness.