Underlying Failure Mechanisms Research Articles

Recent progress in thermal/environmental barrier coatings and their corrosion resistance

Thermal/environmental barrier coatings (T/EBCs) play important roles in jet and/or gas turbine engines to protect the Ni-based superalloys and/or ceramic matrix composite substrates from the high-temperature airflow damage. Great efforts have been contributed to searching for enhanced T/EBC materials to improve the efficiency of the engines, which is the key of improving thrust-to-weight ratio and energy saving. The practical candidates, rare earth-contained materials, are widely used for T/EBCs in gas turbines due to their excellent properties such as low thermal conductivity, high melting point, high-temperature strength and durability as exhibited in yttria-stabilized zirconia, pyrochlore oxides and rare earth silicates. In addition to the intrinsic properties, the microstructures obtained by different synthesis processes and the service performances, as well as the underlying failure mechanism, are also significant to this specific application. However, the main challenges for T/EBCs developments are T/EBC materials selection with balanced properties and their anti-corrosion performances at higher operating temperature. In this review, we summarized the progress in their fabrication techniques and mechanical/thermal properties of typically rare earth-contained T/EBCs, together with their anti-corrosion performance under the condition of molten salts or oxides (such as, Na2SO4, V2O5 and NaVO3), calcium–magnesium–alumina–silicate (CMAS) and high-temperature water vapor.

Application of Remote Sensing Data for Evaluation of Rockfall Potential within a Quarry Slope

In recent years data acquisition from remote sensing has become readily available to the quarry sector. This study demonstrates how such data may be used to evaluate and back analyse rockfall potential of a legacy slope in a blocky rock mass. Use of data obtained from several aerial LiDAR (Light Detection and Ranging) and photogrammetric campaigns taken over a number of years (2011 to date) provides evidence for potential rockfall evolution from a slope within an active quarry operation in Cornwall, UK. Further investigation, through analysis of point cloud data obtained from terrestrial laser scanning, was undertaken to characterise the orientation of discontinuities present within the rock slope. Aerial and terrestrial LiDAR data were subsequently used for kinematic analysis, production of surface topography models and rockfall trajectory analyses using both 2D and 3D numerical simulations. The results of an Unmanned Aerial Vehicle (UAV)-based 3D photogrammetric analysis enabled the reconstruction of high resolution topography, allowing one to not only determine geometrical properties of the slope surface and geo-mechanical characterisation but provide data for validation of numerical simulations. The analysis undertaken shows the effectiveness of the existing rockfall barrier, while demonstrating how photogrammetric data can be used to inform back analyses of the underlying failure mechanism and investigate potential runout.

Mechanical behaviors of SLM additive manufactured octet-truss and truncated-octahedron lattice structures with uniform and taper beams

In this paper, three-dimensional lattice structures composed of octet-truss and truncated-octahedron unit cells with regular and tapered beams are proposed, and their mechanical properties are investigated through experiments and simulation comparisons. Firstly, analytical closed form expressions of the relative density functions are derived for different lattice structures. Secondly, in-situ compression samples are fabricated via Selective Laser Melting (SLM) 3D printing technique, monotonic compression experiments are performed for studying the effects of tapered beams on the mechanical properties of lattice structures with ordinary uniform beams, and SEM characterizations of fracture surface morphology are performed for understanding the underlying failure mechanisms. Afterwards, asymptotic homogenization (AH) method is used for evaluating the anisotropy properties of these lattice structures, closed form expression of the mechanical behaviors of unit cells is derived and solved with ABAQUS® using surface traction method. Asymptotic homogenization analysis shows that node enforcement can significantly enhance the modulus and reduce the anisotropy of the lattice.

Effects of water content on the shear behavior and critical state of glacial till in Tianmo Gully of Tibet, China

Glacial tills are widely distributed in Tibet, China, and are highly susceptible to landslides under intense rainfalls. Failures of the slope during rainfall are closely related to the shear behavior of glacial tills at different moisture conditions. This study investigates the shear behavior and critical state of saturated and unsaturated glacial tills through a series of drained direct shear tests. The tests were conducted on two types of compacted glacial tills with different water contents and total normal stresses. A strain softening mode of failure is observed for all water content conditions accompanied by noticeable dilation. Dilatancy is found to decrease with increasing water content. Unsaturated samples showed increased rates of dilation as water content is decreased for all applied normal stresses a behavior which cannot be predicted well by classical stress-dilatancy models. Furthermore, it was found that the Critical State Line (CSL), plotted on the (e-lnσn) plane, can be used to define the shear behavior of unsaturated glacial tills at different water contents. The CSL of saturated glacial tills run parallel to this line. The experimental results in this study are aimed to provide a basic understanding to the underlying failure mechanisms of glacial tills.

A model for high temperature hydrogen attack in carbon steels under constrained void growth

Petrochemical vessels exposed to high temperature and high pressure hydrogen gas may suffer from high temperature hydrogen attack (HTHA). HTHA is a hydrogen-induced degradation of carbon steels whereby internal hydrogen reacting with carbides forms methane gas bubbles, mainly on grain boundaries (GBs), with an associated loss in strength that can result in premature fracture of structural components. The design of equipment against HTHA is primarily based on the use of the empirical Nelson curves which are phenomenological and do not account for the underlying failure mechanisms and the material microstructure. Starting from the underlying deformation and fracture mechanisms, we present a simple constraint-based model for failure of steels by HTHA which involves growth of GB voids due to coupled diffusion of atoms along the GBs and creep of the matrix surrounding the voids. Since voids form only on some of the GBs, the uncavitated GBs geometrically constrain the growth of voids on the cavitated ones. The model is used to study void growth in HTHA of 21/4Cr–1Mo steel both in the presence and absence of externally applied stress. In the latter case, the model predictions are in good agreement with experimental results. Lastly, the model is used to develop a Nelson-curve type diagram in the presence of external stress in which the curves demarcating the safe/no-safe regimes are functions of the time to failure. This diagram though should be viewed as the result of the application of a new methodology toward devising mechanism-based Nelson curves and not as proposed new Nelson curves for the steel under investigation.

Entanglement Density-Dependent Energy Absorption of Polycarbonate Films via Supersonic Fracture.

The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as entanglement density as well as extrinsic parameters such as strain rate and test temperature. Here we use laser-induced projectile impact testing (LIPIT) to study the extreme strain rate (≈107 s-1) puncture behavior of free-standing polycarbonate (PC) thin films. We demonstrate that changes to the PC molecular mass and the degree of plasticization can lead to substantial changes in the specific puncture energy. We relate these changes to the alteration of the entanglement density of the polymer that determines the underlying failure mechanism as well as the size of the deformation zone.

Damage characterizations and simulation of selective laser melting fabricated 3D re-entrant lattices based on in-situ CT testing and geometric reconstruction

In recent years, metal additive manufacturing (AM) are widely employed for industrial applications, such as: biomedical, aerospace, automotive, marine and offshore sections. AM demonstrated superior manufacturing efficiencies and economic advantages for advanced lightweight industrial components with unlimited arbitrary topological layouts and complex internal microstructures, and are also employed for fabrication of auxetic materials and structures. In this paper, damage characterizations and mechanical behaviors of selective laser melting (SLM) fabricated 3D re-entrant lattices are investigated based on in-situ interrupted micro-CT test, and simulation based on geometric reconstructed models are performed for exploring the underlying failure mechanisms. Firstly, theoretical models for predicting the mechanical properties of 3D re-entrant lattice are developed, such as stiffness, Poisson's ratio and strength, etc. Secondly, the geometrical errors and fabrication defects of 3D re-entrant lattices are analyzed based on 3D micro-CT scanning, in-situ micro-CT interrupted compression tests are performed for studying the deformation process and failure mechanisms. Finally, image finite element models with the detailed information of the shape, position and distribution of defects of the 3D reentrant lattices are constructed from 3D tomographic images, and numerical simulations are performed for studying the effects of the defects on the mechanical performances of the SLM additive manufactured 3D re-entrant lattice structures. It is shown that the failure behavior of the reentrant lattice is governed not only by its topology, but also by the geometric defects and surface defects. Moreover, the proposed interrupted in-situ micro-CT mechanical loading experiments and image finite element approaches can also shed lights on the relations between fracture failure around the edge and the powder adhesion. The damage evolution process is compared with the numerical simulation results to verify the materials failure modes.

Dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation

With a validated fluid-structure-fracture coupling approach, this paper studied the dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation. Fracture of tube was judged by a bivariate failure criterion derived from the underlying failure mechanism at high strain rate conditions. A programed burn approach based on the CJ theory was applied to simulate gaseous detonation. The coupling between detonation wave and tube was realized by penalty contact algorithm with an improved contact stiffness calculation formula. It was demonstrated that the peak pressure at tube edge is 29% higher than that at the middle of tube face. The dominant crack driving force comes from the specific vibration and deformation modes of square tube, where the deformed round section of tube corresponds to the maximum stress wave that travels behind the flexural waves on the tube. Above mechanism makes the backward cracks branch or turn before the forward cracks and the speeds of front and back branch cracks comparable to each other, which is opposite or different from the cases of round tubes. The crack behaviors with different initial flaw locations and detonation pressures were summarized and identified in detail. The forward crack speed can be up to 900 m/s, while the backward crack speeds are generally 65%–85% of above and the branch cracks run at about 100 m/s. In addition, the crack speed has a certain increase immediately after crack branching or turning. Among the three initial flaw location cases, the tube with initial flaw at the middle of face is most resistant to crack propagation under internal detonations.

A micro-mechanical model of reinforced polymer failure with length scale effects and predictive capabilities. Validation on carbon fiber reinforced high-crosslinked RTM6 epoxy resin

We propose a micro-mechanical numerical model able to predict the nonlinear behavior and failure of unidirectional fiber reinforced high-crosslinked epoxy subjected to transverse loading conditions. Statistical microstructural volume elements (SMVE) of a realistic composite material are generated from the statistical characterization of the fibers distribution and fiber radius estimated from SEM images of a similar material system. The fibers are assumed to be transversely hyperelastic isotropic and the matrix obeys a hyperelastic viscoelastic–viscoplastic constitutive model enhanced by a multi-mechanism nonlocal damage model. This polymer model captures the pressure dependency and strain rate effects. Besides, it also accounts for size effects through its internal length scales, allowing capturing, with the same unique set of parameters, the behaviors of the epoxy as pure material as well as matrix phase in composites, which are experimentally observed to be different. Additionally, since fiber/matrix interfaces of the considered composite material are categorized as strong ones, the true underlying failure mechanism is located in the matrix close to the fibers, and the interface does not need to be explicitly introduced in the model. The model prediction is found to be in good agreement with experimental results in terms of the global nonlinear stress–strain curves over various strain rates and pressure conditions, on the one hand for pure matrix samples, and on the other hand for the composite coupons, making the proposed framework a predictive virtual testing facility for material design. Finally, using this model, we study the localization behavior in order to characterize the post-failure behavior of the composite material: the cohesive strength is given by the stress–strain curve peak stress while the critical energy release rate is estimated by evaluating the dissipated energy accumulated during the post-peak localization stage. Finally, different SMVE realizations are considered allowing assessing the discrepancy in the failure characteristics of composites.

REIF: A novel active-learning function toward adaptive Kriging surrogate models for structural reliability analysis

Structural reliability analysis is typically evaluated based on a multivariate function that describes underlying failure mechanisms of a structural system. It is necessary for a surrogate model to mimic the true performance function as the brute-force Monte-Carlo simulation is computationally intensive for rare failure probabilities. To this end, the paper presents an effective active-learning based Kriging method for structural reliability analysis. The reliability-based expected improvement function (REIF) is first derived based on the folded-normal distribution. To account for the modulating effect of the joint probability density function of input random variables on the scattering geometry of candidate samples, an improvement of the REIF active-learning function, i.e., the REIF2 is further presented. Then, the low-discrepancy samples and adaptively truncated sampling regions are combined together to initiate efficient active-learning iterations. The truncated sampling region is directly related to a structural failure probability result, rather than subjectively fixed by an analyst. Numerical validity of the proposed active-learning functions in conjunction with adaptively truncated sampling region and low-discrepancy samples is demonstrated by several structural reliability examples in the literature.

Comparison between cohesive zone and coupled criterion modeling of crack initiation in rhombus hole specimens under quasi-static compression

Numerical predictions of initiation loading and crack length in a rhombus hole specimen made of a brittle material under quasi-static compression using either cohesive zone model or coupled criterion based on finite fracture mechanics are compared. Both methods lead to the conclusion that crack initiation results in a crack jump over a finite length at a given loading level, which depends on the material parameters and on the specimen geometry. The initiation load level and crack extent depend on the cohesive zone traction-opening displacement profile which represents the underlying failure mechanism. The ranges of initiation force and crack lengths obtained with various traction-opening profiles respectively comprise the initiation force and lower bound for crack arrest derived from the coupled criterion. For the configuration under investigation, similar initiation forces and crack arrest lengths as those predicted using the coupled criterion are obtained using a bilinear cohesive zone model.

Analysis of failures after the Bristow-Latarjet procedure for recurrent shoulder instability.

Despite good clinical results and low recurrence rates, post-operative complications of coracoid process transfer procedures are not well understood. This study aims to evaluate the underlying failure mechanism in cases requiring major open revision surgery after prior Bristow or Latarjet stabilization. Between January 2006 and January 2017, 26 patients underwent major open revision after primary Bristow or Latarjet procedure. Clinical notes and radiographic images were retrospectively reviewed for all cases to determine underlying pathology. Choice of treatment and clinical and radiographic outcome were similarly reported for all cases. The underlying failure mechanism was associated with non-union in 42.3%, resorption in 23.1%, graft malpositioning in 15.4%, and trauma or graft fracture in 19.2% of cases. Although none of the patients reported any dislocations, mean subjective shoulder score was 60.2% and WOSI scores averaged 709.3 points at final follow-up. Radiographic signs of deteriorating degenerative arthritis were seen in 34.6%. Graft non-union resulting in recurrent instability was the main indication for open revision surgery after Bristow or Latarjet procedure, followed by resorption, malpositioning, and graft fracture in this retrospective case series. Revision surgery consisted of a structural iliac crest bone graft in the majority of cases. Clinical and radiographic outcomes are predictably variable in this population of multioperated patients.

The effect of off-axis angles on the mesoscale deformation response and failure behavior of an orthotropic textile carbon-epoxy composite

The deformation response and failure behavior of an orthotropic textile carbon-epoxy composite were investigated under off-axis tensile loading. Digital image correlation (DIC) was utilized to effectively capture the full-field and mesoscale strain distribution. The macroscale mechanical performance was strongly sensitive to the fiber bundles orientation relative to the loading direction. Based on the experimental data, a quantitative relation between the rotation angle and off-axis angle was established, and a negative correlation between the failure strength and the rotation angle was observed. The underlying failure mechanisms of the specimens with different off-axis orientations were analyzed using scanning electron microscopy (SEM) and DIC techniques. The load-bearing mechanisms were different between the on- and off-axis cases. High local shear strain eventually resulted in the brushy shear-type fracture in the off-axis case, and the local micro cracks developed during the loading caused the reduction and imbalance of local load-bearing capacity.

Methodological and Practical Comparison of Integrated Probabilistic Risk Assessment (I-PRA) with the Existing Fire PRA of Nuclear Power Plants

Nearly half of the U.S. nuclear power plants (NPPs) are in the process of transitioning, or have already transitioned, to a risk-informed, performance-based fire protection program. For this transition, Fire Probabilistic Risk Assessment (Fire PRA) is used as a foundation for fire risk evaluation. To increase realism in Fire PRA by reducing conservative bias, the authors have developed an Integrated Probabilistic Risk Assessment (I-PRA) methodological framework that does not require major changes to the existing plant Probabilistic Risk Assessments (PRAs). The underlying failure mechanism models associated with fire events are developed in a separate module, which can be interfaced and connected to the existing plant PRA. This paper explains the areas of methodological advancements in I-PRA, comparing them with the existing Fire PRA of NPPs. This comparison is further demonstrated in a realistic case study that applies the I-PRA framework to a critical fire-induced scenario at an NPP. The core damage frequency (CDF) for the selected scenario, computed by the I-PRA framework, is compared with the results of the Full Compartment Burn screening method and the existing Fire PRA methodology. Using the I-PRA framework, the CDF for the selected scenario has decreased by a factor of 20 compared with the Full Compartment Burn screening approach and by a factor of 2 compared to the existing Fire PRA methodology based on NUREG/CR-6850 and the subsequent NUREGs that have updated the data and methods for individual steps.

Machine Learning for Predictive Estimation of Qubit Dynamics Subject to Dephasing

Decoherence remains a major challenge in quantum computing hardware and a variety of physical-layer controls provide opportunities to mitigate the impact of this phenomenon through feedback and feedforward control. In this work, we compare a variety of machine learning algorithms derived from diverse fields for the task of state estimation (retrodiction) and forward prediction of future qubit state evolution for a single qubit subject to classical, non-Markovian dephasing. Our approaches involve the construction of a dynamical model capturing qubit dynamics via autoregressive or Fourier-type protocols using only a historical record of projective measurements. A detailed comparison of achievable prediction horizons, model robustness, and measurement-noise-filtering capabilities for Kalman Filters (KF) and Gaussian Process Regression (GPR) algorithms is provided. We demonstrate superior performance from the autoregressive KF relative to Fourier-based KF approaches and focus on the role of filter optimization in achieving suitable performance. Finally, we examine several realizations of GPR using different kernels and discover that these approaches are generally not suitable for forward prediction. We highlight the underlying failure mechanism in this application and identify ways in which the output of the algorithm may be misidentified numerical artefacts.

Experimental assessment of fire-exposed RC beam-column connections with varying reinforcement development lengths subjected to column removal

This paper describes an experimental investigation into the behaviour of RC beam-column connections under a column removal scenario induced by fire. A purpose-built hybrid heating furnace is employed to carry out the fire tests on five RC connections with varying reinforcement development lengths, with and without cooling effects. The thermal response of the RC connection specimens, including the temperature field and the axial force-furnace temperature curves, is firstly described. Subsequently, push-down tests are carried out on the beam-column connections, and the horizontal support reactions are closely monitored using a specially-designed sensor system. Based on the experimental results, the joint vertical load-displacement and bending moment-rotation relationships are presented, together with an account of the failure modes observed. The mechanical behaviour is discussed in detail, including the tying and rotation capacity provided by the connection specimens, with emphasis on the effect of the reinforcement development length as well as the heating regime. The experimental capacity interaction curves are also compared with the theoretical prediction for ambient condition, and employed to carry out a detailed examination of the underlying failure mechanisms. Finally, the findings are used to provide practical recommendations for enhancing the structural robustness of structural configurations of the form considered in this study.

Failure Pattern Transition of Shape Memory NiTi Foam Subjected to Compression: From Quasi-Static to Dynamic

Shape memory alloy can recover its original configuration, while lightweight foam can absorb a considerable amount of energy. It is desirable to test whether shape memory foam has merits in terms of both shape memory effect and exceptional energy absorption capacity. In the present study, shape memory NiTi foam is compressed quasi-statically and dynamically in order to investigate its response and failure pattern transition with an increasing loading rate. A universal testing machine is used in a quasi-static test, while the split Hopkinson pressure bar is employed in the dynamic test. It is observed that the NiTi foam fails differently: under a quasi-static load, small foam parts of various sizes fall off from the specimen fringe; meanwhile, under dynamic load, the foam fractures in the specimen center to split it into multiple pieces. The underlying failure mechanisms accounting for the shape memory foam behavior under different loading rates are discussed.

Combining in situ monitoring using seabed instruments and numerical modelling to assess the transient stability of underwater slopes

Abstract The stability of submarine slopes is often characterized using campaign-based geophysical and geotechnical measurements in combination with numerical modelling. However, such one-off measurements do not reflect transient changes in slope stability. In situ monitoring of physical parameters critical for slope stability over periods of months to years can provide crucial information on slope stability and can also be used in an early-warning system for submarine landslides and the possibly resulting tsunamis. We review existing techniques that are capable of monitoring seafloor deformation over long periods of time. Based on numerical models we can identify the magnitude of parameters related to landslide-induced seafloor deformation. Simulations of three different failure scenarios up to the point of failure show that the development of the stress state of a slope and hence stability over time can be captured by measurements of tilt, pressure and strain at the seafloor. We also find that different failure mechanisms induce different deformation signals at the seafloor, in particular tilt. Hence, with a site- and target-specific survey design (or a large pool of instruments), seafloor deformation measurements in combination with numerical modelling can be used to determine the temporal evolution of slope stability as well as to identify underlying failure mechanisms.

C&C²-AFM - An embodiment design- and function-based approach for Analysis of Failure Mechanisms

In this article, the authors present a model-based approach that links the functional with a physical model layer based on the Contact and Channel Approach (C&C²-A), thus enabling an effective and efficient determination and assessment of the failure root causes and consequences. The approach is designed for the scenario where the failure modes are already identified, but the associated failure root causes or consequences as well as their underlying failure mechanisms have not yet been determined or assessed. First, the previous work and the foundations are introduced and discussed. They are then linked to form a procedure model, which should enable the reader to apply the approach to mechanical failure mechanisms in mechatronic systems. The resulting approach has the potential to improve efficiency and effectiveness in the case of complex mechanical failure mechanisms compared to conventional approaches such as Failure Mode Effect Analysis (FMEA) or Fault Tree Analysis (FTA).

A study on the dynamic structural behavior of Olympic sabres

Sabres used in Olympic fencing are subject to severe elastic deformations during matches and training sessions. Even though strict rules for their manufacturing are prescribed by the international fencing federation, with requirements in terms of geometrical constraints and material (steel) properties, nonetheless frequent unexpected ruptures are observed. These may cause injuries to the fencers, and involve the replacement of the blade. In this study an experimental-numerical approach is adopted to investigate the underlying failure mechanisms. To this purpose, several attacks, “bouts” in fencing, were live filmed during actual practice with digital cameras and a trajectory tracking analysis was performed on the most critical of them, taking advantage of markers fixed on the blades at different positions. The post-processed data were subsequently used as boundary conditions of a 3D finite element model of the blade. Running a non-linear transient analysis, global and local quantities such as maximum stored elastic energy, stress and strain states, strain rates and possible permanent plastic strains were evaluated. A validation of the FE model with experiments was also carried out. From the critical analysis of experimental and numerical results it was possible to speculate about the influence of materials and dynamic related effects on the structural behavior of the blade. Eventually, hypotheses on fracture mechanisms were formulated.