Post-failure Analysis Research Articles

Protecting the extremities of military personnel: fragment protective performance of one- and two-layer ensembles

In order to provide protection from fragmenting ballistic threats, combat body armour contains multiple layers of fabric. The garment covers the torso, but may provide (removable) protection to the upper arms, neck and groin. Such garments are thick, stiff, impede movement and increase the thermophysiological loading of the dismounted soldier. Examination of wound locations from recent conflicts has suggested it would be advantageous to provide protection to the extremities. Current modular systems can be expanded with strap-on coverings to the arms and legs, but this further exacerbates the mass, mobility and thermal problems already observed. Soldiers already wear coverings on their arms and legs in the form of a combat uniform, and the provision of a hierarchical protection system incorporated in the existing uniform has been discussed. Not all areas of the body would be protected to the same level. In the current work, the fragment protective capabilities of one or two layers of commercially available para-aramid woven fabric. Specifically, 1.1 g chisel-nosed fragment simulating projectile V50 data were obtained. The aim was to establish whether the incorporation of such one or two layers of para-aramid woven fabric into current combat clothing could provide a level of fragment protection with only a minimal associated increase in stiffness, mass and thermal resistance. Post-failure analysis was conducted to investigate inter-layer interaction and failure mechanisms. This work suggests that the use of one- and two-layer para-aramid woven fabric layers incorporated into clothing could offer some protection against wounding to the extremities from fragments.

Failure Analysis of Hydraulic Fitting Brazed Connections

Progressive fracture occurred in a number of brazed joints on hydraulic fittings during service, resulting in a series of fires. Post-failure analysis revealed that the brazed connections that failed were of poor quality and inconsistent with properly manufactured connections of this type. The brazed connections on used, exemplar fittings removed from identical service were also examined and were found to possess similar manufacturing deficiencies. In the absence of these manufacturing deficiencies, the observed fracture modes of the fittings would not have occurred.

Evolution of crack path and fracture surface with degradation in rubber-toughened epoxy adhesive joints: Application to open-faced specimens

The crack path and fracture surface in the mixed-mode fracture of two different rubber-toughened epoxy adhesives were evaluated using double-layered open-faced double cantilever beam (ODCB) specimens in which the primary adhesive layer had been environmentally aged. The crack path in the mixed-mode fracture of unaged ODCB specimens was unexpectedly in the secondary adhesive layer, and several hypotheses were examined to explain this. It was concluded that a reduced residual stress in the secondary adhesive layer produced stable crack growth in the secondary layer instead of the expected path in the primary layer. The average crack path depth, fracture surface roughness and maximum elevation in the fracture surface profiles were then measured using optical profilometry as a function of the degree of aging. The results showed a strong relationship between all these parameters and the critical strain energy release rate, G cs , irrespective of the type of adhesive. In the case of adhesive A where significant irreversible degradation was observed, all these parameters varied approximately linearly with G cs . In the case of adhesive B, aging did not result in permanent degradation ( G cs was unchanged) and so all these fracture surface parameters also remained unchanged after aging. The results indicate that quantifying fracture surface parameters as a post-failure analysis can be of use in the estimation of the fracture toughness at which a practical joint fails.

Computational post failure analysis with a second gradient theory Gradient elastoplasticity

Computational post failure analysis with a second gradient theory Gradient elastoplasticity

Computational post failure analysis with Cosserat continuum

Computational post failure analysis with Cosserat continuum

Computational post failure analysis with a second gradient theory

ABSTRACT This article presents a theory of Gradient Elastoplasticity for strain-softening materials, in which internal length scales are present in both the elastic and the elastoplastic regime. A numerical discretisation based on the displacement formulation and a C1 finite element is used. It is shown that the formulation enables robust modelling of the post-peak material behaviour leading to localisation of deformation, spontaneously tracking emergent instabilities and quantitatively capturing details of the final localised failure mechanism.

Computational post failure analysis with Cosserat continuum

ABSTRACT We present computational analysis of geomechanics problems in the post failure regime based on a Cosserat material modelThe introduction of internal length (related to grain size) in the Cosserat model improves the computational stability and allows for robust progressive localization modelling. The model is capable of predicting advanced deformation modes such as surface buckling and localization of deformation in shear bands that may cause failure in a structure. The obtained results show clearly a progressive failure mechanism and the computed modes are in a good qualitative agreement with laboratory and field observations. As illustration we present results of two applications from petroleum engineering: the modelling of failure in thick walled cylinder test and the stability of elliptical shape perforations.

Post-Hydrogen Permeation Characterization of V-Based Crystalline Alloy Membranes

The surface preparation and hydrogen embrittlement in particular are research challenges facing the practical application of vanadium alloy membranes. These two issues are addressed by surface characterization and fracture analysis in order to find the reasons why delamination and crack failures occur during hydrogen permeation. Post-failure analysis of the hydrogen-induced cracking membrane specimen suggests a new failure mechanism for hydrogen embrittlement.

Assessment of stable tearing on fatigue fracture surfaces

Fatigue fracture surfaces in a range of structural metals sometimes exhibit bands, visible as dull crescent-shaped regions which contrast to the “bright” background fatigue surface. Such bands are believed to be created within a single load cycle under either constant-amplitude or variable-amplitude load conditions. Microscopically, crack advance by tearing has similarity to the final unstable fracture of the component, with the difference that the tearing is stable: it arrests after a certain distance of crack front advance, which is then followed by further fatigue crack growth. Multiple stable tearing bands, separated by regions created by fatigue crack growth, may be visible on a single fracture surface, to the extent that they may even make up the majority of the fracture surface area. Post-failure analysis of fracture surfaces often relies on quantitative fractography to relate various crack-front progression markings to specific loads in the load history, in order to estimate the crack growth history in the component. Matching this data to predicted crack growth, however, is complicated by the fact that stable tearing is not included in fatigue predictive models, and so the presence of significant tearing can greatly complicate the derivation of a crack growth history which can be matched to a crack growth model. The post-failure analysis of fracture surfaces is particularly difficult in cases where the load history record is poor, and that process has been observed to be much more difficult when significant tearing is present. Several empirical models and concepts have been proposed to assist with post-failure analysis of tearing, but as yet we do not have a good understanding of the parameters which influence both the onset and the arrest of the tearing, and the factors which control crack shape change as tearing progresses. As a result, the interpretation of tearing on service fracture surfaces remains difficult. This paper reviews the existing empirical models for tearing analysis, and describes a series of tests which produced tearing in an aircraft aluminium alloy, with the aim of developing a better understanding of these fracture surface features. The analysis of the tearing shapes indicates that one of the empirical models may have broad value as an engineering analysis tool, since it correlates reasonably well with tearing characteristics in these tests. The study reveals that the stress intensity factor is one of the key controlling parameters in tearing onset and arrest, although simple methods for estimation of relevant stress intensity values are problematic because of the key role of crack front curvature. The main conclusions relate to the notable differences between tearing under constant-amplitude and variable-amplitude loading. The constant-amplitude conditions confer some resistance to stable tearing, and this is believed to result from the high loading prior to tear onset; several mechanisms could be involved. This research is part of a program which is developing improved analytical and prognostic models for stable tearing.

Professor Ioannis Vardoulakis (1949–2009)

Professor Ioannis Vardoulakis (1949–2009)

Damage Assessment Due to Single Slamming of Foam Core Sandwich Composites

Foam core sandwich composite panels were slammed onto the body of calm water as a function of slamming energy (161—779 J) and deadrise angle (0°—45°). Higher slamming energy and lower deadrise angle resulted in greater damage to the material. Discrete pressure data though critical in ship design, failed to yield any relevant damage information. Catastrophic failure was observed to occur beyond a threshold strain of 0.0035 mm/mm. With the introduction of a hydroelasticity function, quasi-static analysis accurately predicted strain behavior under slamming. Post-failure analysis of noncatastrophically damaged specimens indicated very little reduction in flexural capacity in spite of a measurable change in the acoustic emission activity. Core shear along the interface with the facesheets and local buckling of the facesheet and resin fragmentation were observed to be the dominant modes of failure under slamming.

Failure Analysis of a Temporary Power Line Anchor

Delayed fracture occurred to a temporary anchor supporting an overhead powerline during construction. The fracture resulted in loss of the overhead transmission line and resulted in a singe vehicle incident in the adjacent roadway. Post-failure analysis conducted as part of personal injury litigation revealed that the fracture was precipitated by a pre-existing weld related crack. Closed form and numerical stress analyses were conducted in order to assess the role of anchor installation in the failure, with the results indicating that installation was not conducted outside of the parameters intended by the anchor manufacturer.

Collapse of a Reinforced Concrete Dome in a Wastewater Treatment Plant Digester Tank

Exponent Failure Analysis Associates was retained by a municipal government to perform an independent study of a fatal collapse in a local sewer treatment plant. The reinforced concrete dome shell covering a cylindrical prestressed concrete digester tank collapsed due to sludge overfilling. The collapsed dome, similar to domes on two adjacent identical tanks, showed radial and circumferential cracking prior to the collapse. Analysis showed the dome cracks were a result of temperature and shrinkage stresses, and of internal overpressure happening at some time prior to the overpressure resulting in the collapse. Inspections of the postfailure debris and analysis of the dome cracking preceding the collapse show practically no relationship between the dome condition prior to collapse and the failure. Failure of the sludge level monitoring system and the lack of a sludge overflow system are cited as the primary reasons for the failure.

Experimental study of composite T-joints under tensile and shear loading

An experimental study of a new type of composite T-joint with transverse stitching was conducted under tensile and shear loading. Fixtures were designed to load the T-joint specimens under simple support and fixed support boundary conditions, respectively. During experimentation, load–displacement and strain–load measurements were recorded and post-failure analysis was accomplished on failed specimens using optical microscopy. T-joint behavior under simple-support tension boundary conditions was shown to be linear up to initial failure. Specimens failed initially due to matrix cracking at the fillet region of the web-to-flange interface. Onset of ultimate failure was attributed to fiber insertion bridging leading to insertion breakage and pullout. The mechanical behavior under an applied shear load was marked by nearly linear behavior up to ultimate failure. Specimens fail by interfacial matrix cracking at the web-to-flange interface leading to T-joint catastrophic failure with complete separation of the web and flange.

Numerical analysis of the mobility of the Palos Verdes debris avalanche, California, and its implication for the generation of tsunamis

Analysis of morphology, failure and post-failure stages of the Palos Verdes debris avalanche reveals that it may have triggered a significant tsunami wave. Our analysis of the failure itself indicates that the slope is stable under aseismic conditions but that a major earthquake (with a magnitude around 7) could have triggered the slide. A post-failure analysis, considering the debris avalanche as a bi-linear flow, shows that peak velocities of up to 45 m/s could have been reached and that the initial movement involved a mass of rock less than 10 km wide, 1 km long and about 50–80 m thick. Initial wave height estimates vary from 10 to 50 m. Tsunami waves propagating to the local shoreline would be significantly smaller. Such a range demonstrates our lack of proper knowledge of the transition from failure to post-failure behavior related to mass movements. Further investigations and analyses of terrestrial and submarine evidence are required for a proper hazard assessment related to tsunami generation in the Los Angeles area.

Elevated temperature short crack fatigue behaviour in near eutectic Al–Si alloys

This paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent short crack growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue crack initiation and early growth. Large primary Si particles were found to act as preferential initiation sites by cracking or decohesion (dependent on test temperature) and are also sought out preferentially during short crack growth.

Post failure analysis of 0 degrees /90 degrees ultra high molecular weight polyethylene composite after ballistic testing

The ballistic performance of a 0 degrees /90 degrees ultra high molecular weight polyethylene (UHMWPE) composite was investigated. Two areal densities were considered: one for fragmentation protection and one for high velocity bullet protection. The role of microscopy in the post-failure analysis of the UHMWPE was explored. Stereo, optical and environmental scanning electron microscopy were utilized and attempts were made to correlate the results obtained with other characterization techniques available. These included high speed photography, visual inspection and ultrasonic inspection. Visual inspection revealed the gross effects of ballistic testing. These primarily included deformation and delamination of the composite plate. The result obtained corresponded with the predicted impact event and, for high velocity bullet impact, corresponded well to the images obtained using high speed photography. Optical, stereo and electron microscopy allowed for fibre and matrix characterization. Fibre displacement and pull-out were also observed on impact and exit faces.

Relationship between fatigue striation height and stress ratio

In order to obtain stress ratio as well as stress intensity factor range which was applied on the material before failure from the fracture surface observation, the effect of stress ratio on fatigue striation height has been investigated on two different alloys (an Fe-3% Si single crystalline alloy and a 7075 polycrystalline aluminium alloy). Striation height has been measured both microscopically using a scanning tunnelling microscope and macroscopically using a scanning electron microscope. Striation height increased with decreasing stress ratio on both the materials at a given crack growth rate. This tendency was found by all the measuring methods used. This suggests that not only stress amplitude but also stress ratio which were applied on the material before the failure could be obtained by post-failure analysis.

Post-Failure Analysis of Composite Cylindrical Panels under Compression

In this study, the most conservative stiffness degradation method called the complete unloading model is used for the progressive failure analysis after the buckling of composite shell structures. To incorporate the complete unloading-failure model into the post-failure analysis of shell structures under compression, a modified arc-length control method is presented. In the method, unlike the previous arc-length methods, discontinuity of equilibrium path due to the abrupt change in stress and stiffness by failure of laminates is included in the determination of arc-length. Numerical examples considered are composite cylindrical panels under compression with various stacking sequences. The results of the numerical examples show the proposed numerical method can be extensively used for the progressive-failure analysis of composite shell structures after snap-through compression buckling.

Testing of Reinforced Slopes in a Geotechnical Centrifuge

Abstract An evaluation of the use of centrifuge modeling as a tool for analyzing the behavior of reinforced soil slopes is presented in this paper. A review of the state-of-the-art indicates that previous centrifuge studies have focused mainly on the performance of reinforced soil vertical walls and that limit equilibrium approaches (used in the design of reinforced soil slopes) have not been fully validated against the failure of models in a centrifuge. As part of an evaluation of the conditions of similarity governing the behavior of reinforced soil structures at failure, scaling laws are specifically derived by assuming the validity of limit equilibrium. It is demonstrated that an Nthscale reinforced slope model should be built using planar reinforcements having 1/N the strength of the prototype reinforcements in order to satisfy similarity requirements. A description of the experimental testing procedures implemented as part of a recent centrifuge testing program is presented, and an example dataset from this investigation is used to illustrate typical results. These include the g-level at failure, visual observation of failure development, and post-failure analysis of reinforcement breakage. The pattern observed in the geotextile reinforcements retrieved after testing indicates that the boundary effects were negligible.