Failure Regime Research Articles

Hindu Kush intermediate-depth seismicity: A possible explanation for the preferential asymmetry displayed by river catchments within the corresponding epicentral area

Some conceptual models proposed for explaining the Hindu Kush intermediate-depth seismicity assumed that a subhorizontal fracture was migrating laterally, from east toward west, along the strike of a subducted tectonic plate. The present study has identified a distinct cluster of earthquake hypocenters, likely corresponding to the advancing tip of the subhorizontal fracture. Those hypocenters belonged to a set of fairly moderate earthquakes (5.6 ≤ M ≤ 6.5), which had not been, so far, explicitly addressed in the Hindu Kush seismo-tectonic interpretations, and which were proven to be virtually the only events displaying a failure regime characteristic to a laterally propagating breakoff – namely, reverse-fault focal mechanisms with subhorizontal compression axes parallel to the slab strike. A separate, upper-positioned hypocenter cluster was noticed to include all the Mw ≥ 7 intermediate-depth earthquakes: it was conjectured, relying on a previously published numerical model, that those strong events could be triggered by upward transfer of the viscoelastic stress induced by the slab pull increase caused by an episode of lateral breakoff advance. At the same time, as the inferred breakoff fracture enables the deeper segment of the plate to detach from the shallower segment, the plate region situated above the severed region is relieved of the gravitational pull that was imposed by the deeper, and currently detached, plate segment. In response to the resulting crustal rebound, the surface topography is expected to exhibit down-tilting to the west. Such an anticipation seems to be confirmed by the catchments of several streams which flow roughly perpendicular to the intermediate-depth earthquakes epicentral domain: relevant geomorphic indices (the swath profile average and maximum elevation curves; the transverse hypsometric integral – THi; the transverse topographic symmetry factor – T-vector) computed for the concerned valleys consistently suggested larger uplift rates in the east, as compared to those manifest further to the west.

Topology optimization using the discrete element method. Part 2: Material nonlinearity

Structural Topology Optimization typically features continuum-based descriptions of the investigated systems. In Part 1 we have proposed a Topology Optimization method for discrete systems and tested it on quasi-static 2D problems of stiffness maximization, assuming linear elastic material. However, discrete descriptions become particularly convenient in the failure and post-failure regimes, where discontinuous processes take place, such as fracture, fragmentation, and collapse. Here we take a first step towards failure problems, testing Discrete Element Topology Optimization for systems with nonlinear material responses. The incorporation of material nonlinearity does not require any change to the optimization method, only using appropriately rich interaction potentials between the discrete elements. Three simple problems are analysed, to show how various combinations of material nonlinearity in tension and compression can impact the optimum geometries. We also quantify the strength loss when a structure is optimized assuming a certain material behavior, but then the material behaves differently in the actual structure. For the systems considered here, assuming weakest material during optimization produces the most robust structures against incorrect assumptions on material behavior. Such incorrect assumptions, instead, are shown to have minor impact on the serviceability of the optimized structures.

Controlling failure regimes in Brick-and-Mortar structures

Brick-and-Mortar structures have a highly tunable mechanical response, offering the possibility to achieve exceptional combinations of properties such as strength and toughness. These properties markedly depend on the failure mechanism. However, the effect of geometric and material parameters on failure is not fully understood. In this work we report the existence of a ‘two-peak’ and a ‘peak-plateau-peak’ failure regime, differing in the ability of the structure to distribute damage in the layers prior to failure. A transition from the ‘two-peak’ to ‘peak-plateau-peak’ regime is observed in 3D-printed Brick-and-Mortar structures by increasing the aspect ratio (brick width over height) in the lower “layer failure” aspect ratio range. Further control of the two regimes is investigated with the help of a semi-analytical model of finite-sized structures. Theoretical predictions suggest that the failure regime can be controlled by tuning the relative shear and normal layer materials. This is confirmed experimentally by testing Brick-and-Mortar structures made with different materials for the shear and normal layers. Our work demonstrates that the transition from the ‘two-peak’ to the ‘peak-plateau-peak’ failure regime significantly increases the toughness, without compromising strength or stiffness of the structure, highlighting the importance of controlling these regimes.

Failure Mechanisms in Vertically Aligned Dense Nanowire Arrays.

Nanowires are an increasingly prevalent class of nanomaterials in composites and devices, with arrays and other complex geometries used in various applications. Little investigation has been done regarding the mechanical behavior of micron-sized nanowire structures. We conduct in situ microcompression experiments on vertically aligned dense microbundles of 300 nm diameter single-crystalline zinc oxide nanowires to gain insights into their structural failure. Experiments demonstrate that bundles containing approximately 10-130 nanowires experience two failure regimes: (1) localized noncatastrophic interfacial splitting and (2) global structural failure. Utilizing Weibull statistics and experimental results, we develop a technique for analyzing flaw distribution and use it to predict the expected range of bundle failure stress. This analysis provides guidelines for nanowire arrays' susceptibility to failure, sensitivity to flaw size, interfacial interactions of constituents, and degree of alignment. This work develops insights to understand and predict fundamental failure mechanisms in highly aligned, dense structures.

TEM investigation of SCC crack tips in high Si stainless steel tapered specimens

ABSTRACT The stress corrosion cracking (SCC) mechanism is investigated in high Si duplex stainless steel in a simulated PWR environment based on TEM analysis of FIB-extracted SCC crack tips. The microstructural investigation in the near vicinity of SCC crack tips illustrates a strain-rate dependence in SCC mechanisms. Detailed analysis of the crack tip morphology, that includes crack tip oxidation and surrounding deformation field, indicates the existence of an interplay between corrosion- and deformation-driven failure as a function of the strain rate. Slow strain-rate crack tips exhibit a narrow cleavage failure which can be linked to the film-induced failure mechanism, while rounded shaped crack tips for faster strain rates could be related to the strain-induced failure. As a result, two nominal strain-rate-dependent failure regimes dominated either by corrosion or deformation-driven cracking mechanisms can be distinguished.

Capillary and viscous fracturing during drainage in porous media.

Detailed understanding of the couplings between fluid flow and solid deformation in porous media is crucial for the development of novel technologies relating to a wide range of geological and biological processes. A particularly challenging phenomenon that emerges from these couplings is the transition from fluid invasion to fracturing during multiphase flow. Previous studies have shown that this transition is highly sensitive to fluid flow rate, capillarity, and the structural properties of the porous medium. However, a comprehensive characterization of the relevant fluid flow and material failure regimes does not exist. Here, we used our newly developed multiphase Darcy-Brinkman-Biot framework to examine the transition from drainage to material failure during viscously stable multiphase flow in soft porous media in a broad range of flow, wettability, and solid rheology conditions. We demonstrate the existence of three distinct material failure regimes controlled by nondimensional numbers that quantify the balance of viscous, capillary, and structural forces in the porous medium, in agreement with previous experiments and granular simulations. To the best of our knowledge, this study is the first to effectively decouple the effects of viscous and capillary forces on fracturing mechanics. Last, we examine the effects of consolidation or compaction on said dimensional numbers and the system's propensity to fracture.

Model for ancient Greek and Roman coinage production

Coinage production of ancient powers such as Athens and Rome is usually inferred from die statistics of monetary issues. The present work applies a Kaplan-Meier analysis of resistance to failure to 29 sets of well-documented monetary issues. The failure rate function assumes a U-shaped form known in reliability engineering literature as the ‘bathtub curve’. With the geometric distribution of die failure being demonstrably violated for a large fraction of the data sets, the die distribution of each data set was instead fitted by a mixture of two Weibull distributions corresponding to two failure regimes. Dies can be divided into bad dies, failing early for various reasons, and good dies, failing late by fatigue. The dual populations reflect the efforts of the smiths at the time to produce bronze dies that would meet two conflicting needs: the reduction of premature die failure (= infant mortality) and the limitation of ductile deformation during minting. The variable proportions of the two populations suggest that not all workshops had fully mastered die technology. Because of the dichotomy induced by contrasting mechanical properties, corrections for missing dies based on singletons and causes of die failure must be carefully assessed for each data set.

On local intrinsic dimensionality of deformation in complex materials

We propose a new metric called s-LID based on the concept of Local Intrinsic Dimensionality to identify and quantify hierarchies of kinematic patterns in heterogeneous media. s-LID measures how outlying a grain’s motion is relative to its s nearest neighbors in displacement state space. To demonstrate the merits of s-LID over the conventional measure of strain, we apply it to data on individual grain motions in a set of deforming granular materials. Several new insights into the evolution of failure are uncovered. First,s-LID reveals a hierarchy of concurrent deformation bands that prevails throughout loading history. These structures vary not only in relative dominance but also spatial and kinematic scales. Second, in the nascent stages of the pre-failure regime, s-LID uncovers a set of system-spanning, criss-crossing bands: microbands for small s and embryonic-shearbands at large s, with the former being dominant. At the opposite extreme, in the failure regime, fully formed shearbands at large s dominate over the microbands. The novel patterns uncovered from s-LID contradict the common belief of a causal sequence where a subset of microbands coalesce and/or grow to form shearbands. Instead, s-LID suggests that the deformation of the sample in the lead-up to failure is governed by a complex symbiosis among these different coexisting structures, which amplifies and promotes the progressive dominance of the embryonic-shearbands over microbands. Third, we probed this transition from the microband-dominated regime to the shearband-dominated regime by systematically suppressing grain rotations. We found particle rotation to be an essential enabler of the transition to the shearband-dominated regime. When grain rotations are completely suppressed, this transition is prevented: microbands and shearbands coexist in relative parity.

Spatiotemporal slope stability analytics for failure estimation (SSSAFE): linking radar data to the fundamental dynamics of granular failure

Impending catastrophic failure of granular earth slopes manifests distinct kinematic patterns in space and time. While risk assessments of slope failure hazards have routinely relied on the monitoring of ground motion, such precursory failure patterns remain poorly understood. A key challenge is the multiplicity of spatiotemporal scales and dynamical regimes. In particular, there exist a precursory failure regime where two mesoscale mechanisms coevolve, namely, the preferred transmission paths for force and damage. Despite extensive studies, a formulation which can address their coevolution not just in laboratory tests but also in large, uncontrolled field environments has proved elusive. Here we address this problem by developing a slope stability analytics framework which uses network flow theory and mesoscience to model this coevolution and predict emergent kinematic clusters solely from surface ground motion data. We test this framework on four data sets: one at the laboratory scale using individual grain displacement data; three at the field scale using line-of-sight displacement of a slope surface, from ground-based radar in two mines and from space-borne radar for the 2017 Xinmo landslide. The dynamics of the kinematic clusters deliver an early prediction of the geometry, location and time of failure.

Critical dynamics of damage-failure transition in wide range of load intensity

The self-similarity of characteristic stages of damage-failure transition has been studied both theoretically and experimentally, with the damage localization kinetics analyzed according to the nonlinearity of free energy release of solids with defects: Free energy in the generalized Ginzburg–Landau form reflecting the specific criticality of solids with defects, and the structural-scaling transition in the presence of two critical points separating qualitatively different material responses corresponds to the types of collective modes of defects. These collective modes have the nature of the self-similar solutions of the damage evolution equation. The final damage stage is associated with the singular self-similar blow-up solutions localized on a set of spatial scales. The presence of two singularities (the stress field at the crack tip and the blow-up damage modes) allows the interpretation of self-similarity of damage localization kinetics to be construed as a criticality sign of damage-failure transition and has been illustrated by the original experimental statements: transition from the steady state to branching crack dynamics, dynamic fragmentation statistics, resonance regimes of failure in shocked materials.

Reinforcement effect of polypropylene fiber on dynamic properties of cemented tailings backfill under SHPB impact loading

Dynamic loads such as blasting may lead to diverse levels of failure in cemented tailings backfill (CTB) under diverse strain rates and frequency spectral amplitudes. Hence, changes in dynamic properties of filling due to these loads should be studied as a function of strain rates for CTB mixtures. In this study, the dynamic characteristics of CTB samples containing polypropylene fiber contents were examined with the use of split Hopkinson pressure bar (SHPB) system and high speed photography technology. The experimental results illustrate that the backfill has a damping influence on the elastic wave. The original waveform noise of CTB with fiber is higher than the one without fiber, and the average strain rate changes the reflected waveform. The effect is greater than the impact velocity. Secondly, the dynamic strength of CTB with fiber is higher than the static strength, which obeys a great effect of the strain rate. Meanwhile, the backfill (with fiber) stress-strain characteristics show a ‘double peak’ phenomenon, and the difference between the first and the second peak stresses decreases with increasing fiber content. The failure mode analysis demonstrated that the fiber reinforcement stopped the development of big cracks during the tensile fiber pull-out failure regime. Lastly, the findings of the present study can offer an important reference to the overall design of backfilling regarding the understanding of the good dynamic characteristics and the reduction of operational costs.

Dynamics of undeforming regions in the lead up to failure: jumping scales from lab to field

Knowledge transfer from micromechanics of granular media failure to geohazard forecasting and mitigation has been slow. But in the face of a rapidly expanding data infrastructure on the motion of individual grains for laboratory samples – and ground motion data at the field scale – opportunities to accelerate this knowledge transfer are emerging. In particular, such data assets coupled with data-driven approaches enable ‘new eyes’ to re-examine granular failure. To this end, effective strategies that can jump scales from bench to field are urgently needed. Here we demonstrate one strategy that focusses on the study of deformation patterns in the precursory failure regime using kinematic data. Unlike previous studies which focus on regions of high strains, here we probe the development and evolution of near-undeforming regions through the lens of explosive percolation. We find a common dynamical signature in which undeforming regions, which are initially transient in the precursory failure regime, become persistent from the time of imminent failure. We demonstrate the robustness of these findings for data on individual grain motions in a classical laboratory test and ground motion in two real landslides at vastly different scales.

Effects of plasticity on the anisotropy of the effective fracture toughness

This paper investigates the effects of plasticity on the effective fracture toughness. A layered material is considered as a modelling system. An elastic-plastic phase-field model and a surfing boundary condition are used to study how the crack propagates throughout the material and the evolution of the effective toughness as a function of the layer angle. We study three idealized situations where only one of the properties among fracture toughness, Young’s modulus and yield strength is taken to be heterogeneous and the others are assumed to be uniform. We observe that in the case of toughness and strength heterogeneity, the material exhibits anomalous isotropy: the effective toughness is equal to the largest of the point-wise values for any layer angle except when the layers are parallel to the macroscopic direction of propagation. As the layer angle decreases, the crack propagates along the brittle-to-tough interfaces, whereas it goes straight when the layers have different yield strength but uniform toughness. We find that smooth deflections in the crack path do not induce any overall toughening and that the effective toughness is not proportional to either the cumulated fracture energy or the cumulated plastic work. In the case of elastic heterogeneity, the material is anisotropic in the sense of the effective toughness, as the latter varies as a function of the layer angle. Four toughening mechanisms are active: stress fluctuations, crack renucleation, plastic dissipation and plastic blunting. Finally, we consider a layered medium comprised of compliant-tough-weak and stiff-brittle-strong phases, as it is the case for many structural composites. We observe a transition from an interface-dominated to a plasticity-dominated failure regime, as the phase constituents become more ductile. The material is anisotropic in the sense of the effective toughness.

Quasistatic response of a shear-thickening foam: Microstructure evolution and infrared thermography

In this work, the authors study the thermo-mechanical response of a dilatant polymeric foam in quasistatic tension and compression, focusing on the links between microstructure, mechanical response, and associated temperature rises in these materials. The authors study these links for a commercially-available shear-thickening foam, named D3O LITE D. Samples were tested under quasi-static conditions for a strain rate of 0.1 s−1 in tension and compression. Micro X-ray computed tomography (XCT) was used to study the evolution of microstructure (pore size and wall thickness) as a function of strain and this was achieved by developing MATLAB-based programs to analyze these microstructural features. The foam specimens were loaded until failure which allowed for the investigation of the elastic, inelastic, and failure regimes. From the XCT images, pore stretching and cell wall tearing are observed in tension, and buckling and pore collapse are observed in compression. These mechanisms are studied in-situ using an infrared thermal camera which record temperature profiles, and temperature measurements are linked back to stress-strain, and temperature-strain responses. For this material, the tensile yield stress was 0.57 ± 0.10 MPa and the elastic modulus was 5.47 ± 0.10 MPa respectively, at a yield strain of 0.10 ± 0.04. At the time of failure, the average temperature of the specimen was found to increase by ∼3.00°C and a local temperature increase of ∼8.00°C was observed in the failure region. In compression, the elastic collapse stress and elastic modulus were found to be 0.130 ± 0.016 MPa and 2.5 ± 0.2 MPa, respectively. The temperature increase in compression at ∼0.83 strain was ∼0.65°C. These results represent some of the first mechanical properties on shear-thickening foams in the literature, and the discoveries on the linkages between the microstructure and the mechanical properties in this study are important for researchers in materials design and modelling.

Wear characteristics of nanoporous alumina and copper filled nanocomposite coatings

Anodisation of pure aluminium under certain conditions, form nanoporous alumina (NPA) with highly ordered pores. High interface strength and hardness make NPA an ideal tribological coating. The nanopores can be filled with metals using electrodeposition to give rise to a nanocomposite coating with better tribological properties. In this study, the friction and wear behaviour of NPA and copper filled nanocomposite coatings are studied under dry reciprocation conditions with a hard zirconia ball. The coefficient of friction of NPA and the nanocomposite is found to be about 0.95. Wear particles are generated by the propagation of surface microcracks and possibly by compaction. From the SEM images of the wear tracks, it is found that these wear particles form a protective tribolayer. The wear process of the coatings is characterised by three distinct regimes of constant wear rate, low wear, and catastrophic failure. The coating fails, possibly by radial cracking, when the thickness of the unworn coating reaches a critical value. Uniform filling of copper in the nanopores is found to enhance the wear life.

Void coalescence and ductile failure in IN718 investigated via high-energy synchrotron X-ray tomography and diffraction

Ductile failure through the growth and coalescence of voids is of particular relevance for many engineering materials. Yet, the lack of experimental measurements of the mechanical state of the material at an appropriate length scale has limited further understanding of the regime of ductile failure. In this study, local grain-scale experimental measurements are used to determine their relevance for describing and predicting ductile failure. Additive manufactured materials, due to the presence of inherent porosity, as well as the ability to tailor internal porosity, provides a promising avenue to study ductile failure. Selective laser melting is used to manufacture a specially designed specimen with two large, internal voids, in addition to the natural porosity, which is characteristic to the production process. The initial porosity and its evolution upon tensile loading are characterized via micro-tomography. Several locations of void coalescence are captured in the sample, revealing activity of multiple modes of failure. Finite element simulations, with a simplified J2 plasticity model, instantiated with the initial void structure, is deployed, in which geometric localizations of stress concentration corresponded with experimentally observed sites of coalescence but was inadequate in capturing the appropriate failure mechanism. From the experimental results of the far-field high-energy diffraction microscopy, the heterogeneous micromechanical state is identified and tracked around voids due to the local grain interactions. These experiments determine narrow bands of low stress triaxiality, at the onset of failure, which highlight the path of coalescence through intervoid shearing. Diffraction spot spreading analysis aided characterization of intragranular plasticity and strain heterogeneity, which can be coupled with high triaxiality to form the conditions inducive for coalescence through intervoid necking. In this study, the experimentally determined grain-scale description of the micromechanical state provides a physical basis that can accurately capture both the sites and the mechanism of void coalescence at the onset of ductile failure.

Microscopic insights into the failure of elastic double networks

The toughness of a polymer material can increase significantly if two networks are combined into one material. This toughening effect is a consequence of a transition from a brittle to a ductile failure response. Although this transition and the accompanying toughening effect have been demonstrated in hydrogels first, the concept has been proven effective in elastomers and in macroscopic composites as well. This suggests that the transition is not caused by a specific molecular architecture, but rather by a general physical principle related to the mechanical interplay between two interpenetrating networks. Here we employ theory and computer simulations, inspired by this general principle, to investigate how disorder controls the brittle-to-ductile transition both at the macroscopic and the microscopic level. A random spring network model featuring two different spring types, enables us to study the joined effect of initial disorder and network-induced stress heterogeneity on this transition. We reveal that a mechanical force balance gives a good description of the brittle-to-ductile transition. In addition, the inclusion of disorder in the spring model predicts four different failure regimes along the brittle-to-ductile response in agreement with experimental findings. Finally, we show that the network structure can result in stress concentration, diffuse damage and loss of percolation depending on the failure regime. This work thus provides a framework for the design and optimization of double network materials and underlines the importance of network structure in the toughness of polymer materials.

Micromechanical investigation of snow failure under mixed-mode loading

An intimate understanding of snow failure, particularly under mixed-mode shear and normal loading, is a key ingredient for numerical modelling of snow avalanche release. Due to its highly porous structure, the failure of snow can result in a volumetric collapse. Despite its importance, snow failure and collapse remain poorly understood, mainly due to the fragile nature of the material, which renders systematic experimental exploration difficult and complicates observation at the microscopic level. A microstructure-based discrete element model of snow has been developed and utilized to study snow failure under mixed-mode loading. Depending on applied normal stress, three distinct failure regimes are identified, characterized by different volumetric responses. Shear-induced macroscopic collapse is observed to develop above a critical level of normal stress. A deeper understanding of the mechanisms leading to this volumetric collapse is investigated on the microscopic scale. Force chain buckling within the snow sample leads to volumetric collapse, while stable force chains result in a localized failure and the absence of a volumetric collapse. The stability of the force chains is shown to be insured by the grain contacts between the force chains and the surrounding material. The ratio of contacts close to failure is identified to control the absence or presence of collapse after failure.

Evaluation of the relationship between stress and lifetime of Pb-free solder joints subjected to vibration load using a generalized local stress approach

The article proposes a generalized high cycle fatigue evaluation methodology for solder joints based on a local stress finite element method (FEM) and compares with experimental results. A method to predict the damage initiation of surface mount technology (SMT) solder joints is accomplished in this article by making use of generic coupon level SAC387 solder specimens. High cycle fatigue (HCF) experiments conducted on generic solder specimens are followed by means of FKM guidelines, which are translated to microelectronic elements for the first time. The FEM predicted lifetime values of solder joints agree with the experimental results performed on SMT components. In addition, the overall results show that the predicted values act as a threshold damage line from which the solder joint failure regimes can be differentiated. The results also show the differences in the predicted damage line and failure line for SMT solder joints.

Adult-onset Stills disease presenting with acute liver failure caused by hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis is a well-known complication of adult-onset Still’s disease but an unusual cause for acute liver failure. Here we present a young female with the background of fever, arthritis, rash presenting with acute onset liver failure with hepatic encephalopathy and bleeding. The diagnostic evaluation was negative for infections, autoimmune disorders or malignancies and fulfilled the criteria for adult-onset Still’s disease and hemophagocytic lymphohistiocytosis resulting in acute liver failure. Treatment with liver failure regime and immunomodulation ensued a quick and complete recovery.