Location Of Failure Initiation Research Articles

Fatigue life prediction of friction-stir-welded aluminum bridge decks – Experimental tests and numerical framework

This paper presents the results of experimental tests and proposes a numerical framework for the prediction of the fatigue behaviour of friction stir welded (FSW) butt-lap joints of aluminium bridge deck extrusions. Fatigue test specimens made of welded real roadway bridge deck extrusions were subjected to cyclic bending under different loading conditions. The fatigue failure modes were thoroughly analyzed. A numerical framework incorporating a 2D finite element (FE) model based on the theory of critical distances (TCD) and a simplified linear elastic fracture mechanics (LEFM) model was developed to predict the fatigue failure initiation location and the fatigue life of the specimens under the test conditions. The location of the failure initiation played a key role in the choice of the numerical approach adopted in the prediction of fatigue life. For fatigue failures initiating from the base metal, the framework utilizes the TCD formulations while for failures in the weld joint, the LEFM model was applied. The proposed numerical framework showed a good agreement with the experimental data.

An Oxide Growth-Coupled Viscoplasticity Model and Its Application to Interfacial Stress Analysis near an Air Hole within a Thermal Barrier Coating

Strength assessment for thermal barrier coatings (TBCs) is vital in the safety design of hot-section components in engines. However, several crucial factors, including thermally grown oxide (TGO) growth and creep–plasticity interaction, have been less considered in thermo-mechanical analyses for TBCs near air holes. In this study, a unified viscoplastic constitutive model incorporating TGO growth is developed and integrated into a finite element framework. The model considers multiple factors, including TGO growth, creep–plasticity interaction, interface undulation, and temperature gradient. Additionally, an analytical solution for the non-uniform temperature field of a TBC is derived. The model is then applied to calculate interfacial stresses and accumulated strain energies in the TBC near an air hole, which promote interface debonding. The obtained results can be utilized to investigate the mechanisms of hole edge delamination in TBCs, considering the combined effects of multiple complex factors. A competition for the potential failure initiation location is revealed between the first oxide layer and the evolving TGO/bond coat interface. The developed viscoplasticity model demonstrates effective capability in modelling a range of dynamic behaviors that collectively contribute to hole edge delamination failure.

FE Micromodel of a Cord-rubber Composite Test Specimen Subjected to Uniaxial Tension

The aim of this study is to examine displacements, strains and stresses as well as to predict possible failure mechanisms arising at the yarn-matrix level of a test specimen of a railway composite cord-rubber air brake tube undergoing uniaxial tension by microscale modelling.Furthermore, this paper also aims to verify the material properties of the micromodel of the test specimen.The micromodel is based on macromodels (by matching the boundary conditions of the micromodels with displacements of the macromodels) created previously by authors of this article. The reinforcing yarns are described by an orthotropic, elastic material model, whereas the matrix has been described by a 2 parameter Mooney-Rivlin model, which all have been validated before by a uniaxial tensile test and a three-point bending test.Force-displacement curves of the micromodel and experimental results show a considerably good agreement.Yarns have a less dominant role in the load transfer mechanism of the reinforcement layers, because of the short-yarn reinforced nature of the specimen. Shear strains are high at free yarn ends marking the possible locations of failure initiation in debonding in the shear mechanism of the reinforcement layers. High shear strain values imply that the dominant mode of load transfer is shear in the matrix in the reinforcement layers.

Strength of asymmetric tapered composite foam core sandwich structures subjected to thermomechanical loading

An experimental test program and companion analytical study were designed and conducted to gain understanding of the tensile failure mechanisms in asymmetrical tapered sandwich core structures at cold temperatures. Representative test coupons were subjected to a tensile load in an environmental chamber to induce foam tensile failure in the core. Finite-element modeling with a proposed maximum principal stress criterion was used to predict the failure loads of the tapered test coupon design at the test temperatures. The predicted failure loads were in good agreement with test results. An important finding is that the cure stresses in the foam are significant and should not be ignored. Additionally, the analysis correctly predicted the failure initiation location, which was verified using high speed photography during the tensile test. The study identified the critical failure regions of asymmetrical tapered sandwich core designs and the failure load dependence on the temperature gradient in the structure.

A coupled plasticity-damage model of K418 nickel-based superalloy with pressure and temperature dependence

The issue of containment stems from the aero-engine field, and there is little research in the turbocharger field. In order to reduce the number of tests, development cycle and cost, it is necessary to predict the burst speed of the turbine accurately. In response to this problem, a coupled plasticity-damage model is developed for turbine materials according to the stress state and operating temperature of the turbine. The effect of stress triaxiality, temperature are considered in the plastic constitutive and the failure criterion. The stress update algorithm of plasticity-damage model is given. According to the operating temperature and stress state of the turbine, the experimental scheme of K418 nikel-base superalloy is designed to calibrate the plasticity-damage model. Finally, the coupled plasticity-damage model is verified by the specimens with the notch radius R6 at 550 °C and 600 °C. The results show that the maximum error of proposed model is about 7.3%, which is better than the 14.2% prediction error of the constitutive model without any corrections. Moreover, the failure displacement can be predicted by the proposed constitutive model (the maximum error is about 8.2%), which is beyond the ability of traditional model. The location of failure initiation, fracture pattern in numerical studies have also showed close correspondence to the experimental results.

Pure shear plastic flow and failure of titanium alloys under quasi-static and dynamic torsional loading

The pure shear responses of Ti6Al4V alloy and Ti3Al2.5V alloy, conceived for unique applications in jet engine components, are compared using Digital Image Correlation technique from quasi-static (10−3/s), medium strain rate (101/s) and high strain rates (103/s). A series of bespoke torsion tests have been performed on a screw driven mechanical system, a fast hydraulic Instron machine and a Campbell split Hopkinson torsion bar, equipped with high speed photographic equipment. Observations have provided the strain rate dependence and the relation of pure shear failure at quasi-static and high strain rates. The quasi-static shear constitutive relationships including shear modulus, yield stress and failure strain of both alloys are compared at the location of failure initiation, using a bespoke four camera system. The Ti3Al2.5V alloy presents lower shear flow stress and strain rate sensitivity with higher ductility, as opposed to the Ti6Al4V alloy with limited plastic deformation capacity. Associated with modest estimated overall temperature rise until final collapse of the specimen, both ductile Ti3Al2.5V alloy and brittle Ti6Al4V alloy fail by adiabatic shear banding at high strain rates. This is different from the void growth induced failure at medium strain rate with mild temperature rise and at quasi-isothermal condition. The present results show a general description of brittle and ductile alloys in engineering design. Likewise, this study will guide the characterization of pure shear flow and failure for current and future developed impact resistant alloys.

Failure behaviors of rock masses around highly pressurized cavern: Initiation and modes of failure

Reasonable failure path including the failure initiation location and propagation direction is essential in uplift evaluation of highly pressurized caverns. This study investigates the failure initiation behavior of a rock mass that encloses a highly pressurized cavern, taking into account the diverse strength properties of the rock mass. A series of stress analyses in conjunction with failure criteria were performed in the framework of a 2D axisymmetric model. The analysis results indicated that the initial mode of failure strongly depends on the rock strength properties, regardless of the initial stress condition. In contrast, the failure initiation location was influenced by both the rock strength properties and the initial stress condition. Diagnostic charts were proposed for determining the failure mode using the relationship between the tensile and shear strengths of the rock mass and for identifying the failure location using the appropriate parameters. Tests of a physical model with a 200-fold scaling-down, in which a digital image correlation technique is utilized to monitor the full-field displacement during the experiments and identify the failure mode, were also performed to verify the effectiveness of the proposed charts.

Numerical modeling and prediction of adhesion failure of adhesively bonded composite T-Joint structure

This study is reported the adhesion failure in adhesive bonded composite and specifically for the T-joint structure. Three-dimensional finite element analysis has been performed using a commercial tool and the necessary outcomes are obtained via an eight noded solid element (Solid 185-element) from the library of ANSYS. The structural analysis input has been incurred through ANSYS parametric design language (APDL) code. The normal and shear stress distributions along different layers of the joint structure have been evaluated as the final outcomes. Based on the stress distributions, failure location in the composite joint structure has been identified by using the Tsai-Wu stress failure criterion. It has been found that the failure index is maximum at the interface between flange and web part of the joint (top layer) which indicates the probable location of failure initiation. This kind of failures are considered as adhesion failure and the failure propagation is governed by strain energy release rate (SERR) of fracture mechanics. The different adhesion failure lengths are also considered at the failure location to calculate the SERR values i.e. mode I fracture (opening), mode II fracture (sliding) and mode III fracture (tearing) along the failure front. Also, virtual crack closure technique (VCCT) principle of fracture mechanics steps is used to calculate the above said SERRs. It is found that the mode I SERR is more dominating compared to other two modes of failure for the joint considered. Finally, the influences of various parametric (geometrical and material) effect on SERR of the joint structure are evaluated and discussed in details.

Failure characterisation of CF/epoxy V-shape components using digital image correlation and acoustic emission analyses

Testing of V-Shape demonstrator components formed from carbon fibre epoxy composites was undertaken to assess the failure mechanisms at the corner section (V-Shape) of a square beam structure under static and fatigue loading configurations. Digital image correlation and acoustic emission analyses were used to determine the time to failure, as well as the location of failure, providing additional insights for failure characterisation. Digital image correlation was used on the surface of the specimens’ side as it analysed the displacements and strains on the externally visible through-thickness region. Acoustic emission analyses identified the origin and location of stress waves generated within the sample which were related to damage events. These techniques have shown a higher sensitivity to determining the onset of damage and the time to failure, in comparison to load drops, crosshead displacement changes, and strain gauge measurements, providing a more conservative and accurate measurement of component failure. They have also proven successful at determining the location of failure initiation, highlighting the added benefits of incorporating these techniques into component analyses.

Analytical and experimental study of failure onset during a Brazilian test

The location of the failure initiation in the Brazilian test is strongly influenced by the contact angle created in the loaded area. As this angle increases, the position of the initial failure point is closer to the center of the disc, which is a result of both: uniform radial pressure and sinusoidal radial pressure; as well as adopting different failure criteria and materials. Moreover, the frictional stresses throughout the contact between the jaw (or platens) and the disc cannot be considered as a critical failure parameter for failures initiated from the origin to a radial distance of 0.8R.In this paper, firstly, the influence of the above mentioned parameters in the location of the initial failure point was analytically studied. Secondly, the analytical results have been proven through experimental tests. Finally, once the importance of the contact angle in the location of the initiation point was checked, a procedure is suggested in order to increase the contact angle with the same load applied to one specific sample, by using various jaw curvatures.The location of failure initiation does not cause great disturbances in the determination of the indirect tensile strength. However, the position is important in order to adjust the failure criteria in the tension region. According to the location of the initial failure, the stress field in terms of principal stresses (σ1 and σ3) will be different. So by knowing this position, the precise failure criteria could be established in σ1 - σ3, specifically in the tension region. Meaning that as well as the triaxial test is mostly used to calculate the failure criteria in the compression region, the Brazilian test could be used to obtain the limiting stress states in the tension region.

Role of Interfaces in Elasticity and Failure of Clay-Organic Nanocomposites: Toughening upon Interface Weakening?

Synthetic organic-inorganic composites constitute a new class of engineering materials finding applications in an increasing range of fields. The interface between the constituting phases plays a pivotal role in the enhancement of mechanical properties. In exfoliated clay-organic nanocomposites, individual, high aspect ratio clay sheets are dispersed in the organic matrix providing large interfaces and hence efficient stress transfer. In this study, we aim at elucidating molecular-scale reinforcing mechanisms in a series of model clay-organic composite systems by means of reactive molecular simulations. In our models, two possible locations of failure initiation are present: one is the interlayer space of the clay platelet, and the other one is the clay-organic interface. We systematically modify the cohesiveness of the interface and assess how the failure mechanism changes when the different model composites are subjected to a tensile test. Besides a change in the failure mechanism, an increase in the released energy at the interface (meaning an increased overall toughness) are observed upon weakening the interface by bond removal. We propose a theoretical analysis of these results by considering a cohesive law that captures the effect of the interface on the composite mechanics. We suggest an atomistic interpretation of this cohesive law, in particular, how it relates to the degree of bonding at the interface. In a broader perspective, this work sheds light on the importance of the orthogonal behavior of interfaces to nanocomposites.

A continuum damage and discrete crack-based approach for fatigue response and residual strength prediction of notched laminated composites

This paper presents a combined continuum damage and discrete crack (CDDC) modelling approach for fatigue damage characterization and post-fatigue residual strength prediction of laminated composite components with a hole. In order to capture both the fatigue cycle-driven material degradation and discrete damage-induced stress concentration and redistribution, an overlapped element approach is developed based on a combined user-defined material (UMAT) and user-defined element (UEL). An Abaqus element coupled with UMAT for fatigue damage characterization is used to detect the location of failure initiation, while the discrete crack network-based (DCN) UEL is applied to insert a crack without remeshing. The intensified stress field induced by the newly inserted matrix crack is used for the evaluation of failure initiation and stiffness degradation. The UMAT for the fatigue analysis has incorporated the stress-cycle ( S-N) curves for the damage evolution characterization associated with matrix and fiber based on the tested S-N curves for plies at their different orientations. A continuum damage mechanics (CDM) approach is used for the fatigue-driven delamination initiation and propagation by insertion of a finite thickness interface layer at each ply interface. Both the blind and recalibrated predictions are performed for specimens of three different layups under the Air Force Tech Scout 1 program. The predicted fatigue failure progression and the stiffness against cycle curves are compared with the test data provided by the Air Force Research Lab (AFRL). In addition, post-fatigue residual strength predictions are performed for these notched specimens under tension and compression.

Probabilistic stability evaluation and seismic triggering scenarios of submerged slopes in Lake Zurich (Switzerland)

Subaqueous landslides and their consequences, such as tsunamis, can cause serious damage to offshore infrastructure and coastal communities. Stability analyses of submerged slopes are therefore crucial, yet complex steps for hazard assessment, as many geotechnical and morphological factors need to be considered. Typically, deterministic models with data from a few sampling locations are used for the evaluation of slope stabilities, as high efforts are required to ensure high spatial data coverage. This study presents a simple but flexible approach for the probabilistic stability assessment of subaqueous slopes that takes into account the spatial variability of geotechnical data. The study area (~2 km2) in Lake Zurich (northern Switzerland) shows three distinct subaquatic landslides with well-defined headscarps, translation areas (i.e. the zone where translational sliding occurred) and mass transport deposits. The ages of the landslides are known (~2,210 and ~640 cal. yr BP, and 1918 AD), and their triggers have been assigned to different mechanisms by previous studies. A combination of geophysical, geotechnical, and sedimentological methods served to analyse the subaquatic slope in great spatial detail: 3.5 kHz pinger seismic reflection data and a 300 kHz multibeam bathymetric dataset (1 m grid) were used for the detection of landslide features and for the layout of a coring and an in situ cone penetration testing campaign. The assignment of geotechnical data to lithological units enabled the construction of a sediment-mechanical stratigraphy that consists of four units, each with characteristic profiles of bulk density and shear strength. The thickness of each mechanical unit can be flexibly adapted to the local lithological unit thicknesses identified from sediment cores and seismic reflection profiles correlated to sediment cores. The sediment-mechanical stratigraphy was used as input for a Monte Carlo simulated limit-equilibrium model on an infinite slope for the assessment of the present slope stability and for a back analysis of past landslides in the study area, both for static and earthquake-triggered scenarios. The results show that the location of failure initiation in the model is consistent with stratigraphic analysis and failure-plane identification from sediment cores. Furthermore, today’s sediment-charged slopes are failure-prone, even for a static case. This approach of including an adaptable sediment-mechanical stratigraphy into a limit-equilibrium slope stability analysis may be applied as well to the marine realm.

Fatigue behaviour of die‐casting aluminum alloy suspension arms for scooters

ABSTRACTThe aim of the paper is at qualifying a methodology for the fatigue life assessment of structural components obtained by die‐casting for vehicle applications. Full‐scale fatigue tests were conducted on the back suspension arms of mid‐size scooters. Two loading conditions, i.e. bending and combined bending plus torsion, were considered as representative of typical operating conditions. Fatigue tests showed that the locations of failure initiation is different for the two loading configurations. Material was characterized by means of small size standard specimens extracted from the components in order to be representative of the effective material conditions, particularly of the surface. Both static (tensile) and fatigue tests were conducted, making use of three different R‐ratios. A finite element (FEM) model of the suspension arm, representative of full‐scale test conditions was set‐up in order to interpret the tests. The sub‐modelling approach was adopted in order to get accurate evaluations of the stress–strain fields with reasonable computing resources and elaboration time. Combined elastic and elastic–plastic analyses were necessary to estimate the stress cycles in the regions critical for fatigue damage. Employing the material properties given by small‐size specimens and the proposed FE analysis technique, predictions of either the locations of failure initiation and the fatigue strength of the component were obtained, in quite good agreement with full‐scale tests.

A parametric study of fiber volume fraction distribution on the failure initiation location in open hole off-axis tensile specimen

We present a model to study the effect of the spatial distributions in fiber volume fraction on the failure initiation location in the open hole off-axis tensile. These variations are introduced with a micromechanical enhancement dehomogenization process. Good agreement is obtained between our predicted failure locations and experimental results by considering a failure criterion based on the effective shear strain in the matrix. The predicted failure angle distribution is in good agreement with the experimental results when the variability in the fiber volume fraction is included in the simulations.

Numerical Analysis on the Dynamic Response of Fuel Rods under Impact Loads

Nuclear fuel assembly is installed in the cask with the necessary protective function during fuel transportation. The structural integrity of the fuel rods which constitute fuel assembly has to be evaluated to ensure the safety even for the cask drop accident. However, the experiments of irradiated fuel rods accompany many of difficulties and the available experimental data is limited. Hence, the numerical simulation is useful method to predict and evaluate the dynamic response of the fuel rods. Generally, the mechanical properties of the fuel pellets have been assumed by a certain value in the numerical simulations, since no available data has published. However, the mechanical properties of the fuel pellets, such as yield strength and ultimate strength, will vary during service in nuclear reactor. Therefore, this paper investigates the influence of strength of fuel pellets on the dynamic behavior of fuel rods by using finite element simulation. Further, the influence of diametrical gap between cladding tube and fuel pellets is considered. It is found that the influence of the pellet strength showed a little influence on the stress propagation. However, the deformation progress of the fuel pellets were significantly different and it is expected that the location of failure initiation of the cladding tube will change depending on the pellet strength. It is shown further that the diametrical gap between cladding tube and fuel pellets may cause large difference in the buckling shape of the fuel rod.

Investigation of combined stress state failure criterion for glass fiber/epoxy interface by the cruciform specimen method

The interfacial failure criterion under combined stress state in a glass fiber/epoxy composite is investigated by the cruciform specimen method. Experiments were conducted by using specimens with a fiber whose angle from the loading direction is varied in order to make various stress state of normal and shear at the interface. Finite element analysis is performed to calculate the interfacial stress distribution. By combining the experimental measurement of the specimen stress at the interfacial debonding initiation and the finite element stress analysis, it is possible to obtain the interfacial stress state at interfacial failure. A method to determine the interfacial failure criterion and the interfacial failure initiation location simultaneously is proposed in the present study. We conclude the value of the interfacial shear strength is higher than that of the interfacial normal strength for the material system used in the present study.

Effect of the amount of adhesive on the bending fatigue strength of adhesively bonded aluminum honeycomb sandwich beams

The effect of the amount of adhesive for bonding face sheets and cores on the bending fatigue strength of aluminum honeycomb sandwich beams was analyzed. It was experimentally proved that the fatigue strength increases as increasing the amount of adhesive. Furthermore, the applied loading parameter is not correlated with the fatigue life data of all studied specimens with various amounts of adhesive because the global parameter has no clear physical meanings with respect to the failure mechanism. From the observations made during fatigue testing, debonding at the interface between the honeycomb core and face sheet is the main cause of fatigue failure. Finite element analyses were conducted to obtain the local stress states at the interface, and these simulated stresses were employed in fatigue life prediction parameters. Three local interfacial parameters were adopted and correlated with the experimental data for the studied specimens. The predicted failure locations using the three interfacial parameters were also examined by comparing the observation results in fatigue tests. Among the three studied interfacial parameters, the combined interfacial peeling and shear stress parameter is recommended for use in fatigue design as it provides good fatigue life correlations and predicts the correct locations of failure initiation simultaneously.

Finite Element Analysis of a New Custom Made Orthotropic Post Material for Dental Applications

Effects of cement properties on stress distributions at the dentin-cement-post interfaces have been studied using 3D explicit finite element simulations. The mastication and horizontal loadings are simulated with titanium and glass fiber posts. Different type of cements (moduli 2,8,18 and 22 GPa) is considered to study the interface stresses. The interface stresses are evaluated at dentin-cement-post interfaces and along the surface of the post and cement. The element deletion technique from finite element analysis has been used to predict the cement failure. The simulation results suggest that the cement with similar modulus as dentin is preferable to avoid the stress concentration at the dentin-cement interface. The types of cement do not affect the failure initiation location on the cement. Under the simulated conditions, the failure initiation is located at 1.7 to 2.2 mm from the post apex.

Structural testing and numerical simulation of a 34 m composite wind turbine blade

A full-scale 34 m composite wind turbine blade was tested to failure under flap-wise loading. Local displacement measurement equipment was developed and displacements were recorded throughout the loading history. Ovalization of the load carrying box girder was measured in the full-scale test and simulated in non-linear FE-calculations. The non-linear Brazier effect is characterized by a crushing pressure which causes the ovalization. To capture this effect, non-linear FE-analyses at different scales were employed. A global non-linear FE-model of the entire blade was prepared and the boundaries to a more detailed sub-model were extracted. The FE-model was calibrated based on full-scale test measurements. Local displacement measurements helped identify the location of failure initiation which lead to catastrophic failure. Comparisons between measurements and FE-simulations showed that delamination of the outer skin was the initial failure mechanism followed by delamination buckling which then led to collapse.