Underlying Failure Mechanisms Research Articles

Reply to the Editor— Inside-Out Insulation Failure of a Defibrillator Lead with Abrasion-Resistant Coating

Our report1 correlated characteristic nonphysiological implantable cardioveter-defibrillator electrograms with a specific short circuit within the Durata lead to alert clinicians that simultaneous cyclical signals on both shock and sensing channels may indicate a cable-coil abrasion. The images of “inside-out” abrasion of cables through the silicone walls of their lumens show the root cause of this short circuit. They illustrate the underlying failure mechanism common to St Jude Medical’s silicone-body, multilumen defibrillation leads—whether or not they are surrounded by the external Optim™ copolymer tubing.

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.

A Lesson from the Failure of Intramedullary Fixation of Atypical Subtrochanteric Fractures: A Report of Two Cases.

Surgical fixation of subtrochanteric fractures remains technically challenging, even to experienced fracture surgeons. These fractures are often treated with intramedullary devices; fixed-angle plating is an alternative. Recently, there have been an increasing number of reports about bisphosphonate-related atypical femoral fractures occurring at the subtrochanteric or proximal diaphyseal region1-4. These fractures are characterized by cortical beaking, thickening of the femoral cortex prior to fractures, and transverse or short oblique orientation in people who have been on long-term bisphosphonate therapy2,3,5,6. As with the vast majority of trauma-related subtrochanteric fractures, these bisphosphonate-related fractures are usually stabilized with cephalomedullary devices. Surgeons often do not pay adequate attention to the length of the intramedullary nail, which in effect is the working length of the fixation. Short nails are sometimes used for ease of insertion and their availability in most operating rooms. Spanning the distal femoral region with a longer nail is erroneously viewed as unnecessary. In fact, the less biomechanically sound fixation achieved by a short nail will predispose to the occurrence of delayed union, nonunion, and implant breakage. We report two cases of atypical subtrochanteric fractures in patients who had been on long-term bisphosphonate therapy; treatment with short cephalomedullary nails failed in both. We also discuss the underlying failure mechanism. Both patients were informed that data concerning their cases would be submitted for publication, and they provided consent. Case 1. An eighty-two-year-old woman who had taken alendronate for osteoporosis for six years was admitted to the hospital after a …

Shear strength behavior of human trabecular bone

The shear strength of human trabecular bone may influence overall bone strength under fall loading conditions and failure at bone-implant interfaces. Here, we sought to compare shear and compressive yield strengths of human trabecular bone and elucidate the underlying failure mechanisms. We analyzed 54 specimens (5-mm cubes), all aligned with the main trabecular orientation and spanning four anatomic sites, 44 different cadavers, and a wide range of bone volume fraction (0.06–0.38). Micro-CT-based non-linear finite element analysis was used to assess the compressive and shear strengths and the spatial distribution of yielded tissue; the tissue-level constitutive model allowed for kinematic non-linearity and yielding with strength asymmetry. We found that the computed values of both the shear and compressive strengths depended on bone volume fraction via power law relations having an exponent of 1.7 (R2=0.95 shear; R2=0.97 compression). The ratio of shear to compressive strengths (mean±SD, 0.44±0.16) did not depend on bone volume fraction (p=0.24) but did depend on microarchitecture, most notably the intra-trabecular standard deviation in trabecular spacing (R2=0.23, p<0.005). For shear, the main tissue-level failure mode was tensile yield of the obliquely oriented trabeculae. By contrast, for compression, specimens having low bone volume fraction failed primarily by large-deformation-related tensile yield of horizontal trabeculae and those having high bone volume failed primarily by compressive yield of vertical trabeculae. We conclude that human trabecular bone is generally much weaker in shear than compression at the apparent level, reflecting different failure mechanisms at the tissue level.

Environmentally Assisted Cracking of Magnesium Alloys

The propensity of the magnesium alloys AM30 and AZ91 to environmentally assisted cracking, and in particular to hydrogen embrittlement, was assessed in constant extension rate tensile tests on smooth and pre-cracked specimens which were subjected to monotonic loading in corrosive environment. The experimental findings can be rationalized by model approaches: A meso-scale fibre bundle model was employed to simulate the results obtained in tests on smooth AZ91 tensile specimens, assuming a combination of pitting and subsequent hydrogen embrittlement as the underlying failure mechanism. The experiment data as well as the model results revealed the effect of hydrogen embrittlement on crack growth resistance. The model calculations generated fracture surfaces which were in remarkable correspondence with those observed in the experiments, and stress-strain curves similar to the experimental ones, both reflecting the influence of the applied strain rate on hydrogen induced failure. The effect of hydrogen embrittlement on cracking in AM30 was assessed using a fracture mechanics based approach. A cohesive model which accounts for hydrogen enhanced crack extension and which earlier has been successfully applied to HE of steels is currently readjusted to EAC of magnesium.

Back analysis of the Joss Bay Chalk Cliff Failure using numerical modelling

Use of numerical modelling for back analysis of slope failure can provide a valuable insight into the underlying failure mechanisms and improve our understanding of factors controlling instability and geomorphology of slopes. This paper provides a brief overview of the range of methods that are available for slope instability analyses, and their respective benefits, using the back analysis of the Joss Bay Chalk cliff failure as a case example. A variety of analytical and numerical approaches are used to back analyse the failure to assess the potential impact of both the rock mass strength and discontinuity-controlled behaviour on the potential failure mechanism. Limit equilibrium analysis approaches are used to assess the sensitivity of the simulated model to variation in input parameters. Numerical simulation, constrained by the limit equilibrium models, is then used to evaluate the progressive nature of failure mechanism and directly simulate the trigger mechanism. Importantly, the modelling demonstrates that shear strain initiates at the base of the cliff and subsequently extends upwards resulting in tensile failure in the upper part of the slope. The case example demonstrates the capability of numerical models to simulate the progressive nature of slope failure, and their ability to evaluate the detrimental effects of a wave-cut notch at the base of a cliff.

Thermal Fatigue Damage Evaluation of EB-PVD TBC Specimens - Its Potential and Problems

Thermal fatigue damage evolution behavior in thermal barrier coatings (TBCs) was studied, by employing the originally designed two dimensional ring-shape TBC specimen. The TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by electron-beam physical vapor deposition (EB-PVD) method with 250 micron-meters in thickness. Three kinds of MCrAlY bond coat alloys were specified as an experimental variable. Through the work, special attention was paid not only to the failure life of TBC specimen, but also to the underlying failure mechanisms. Some problems have been also pointed out, on feeding back these experimental findings to engineering applications.

Effect of Bond Coat Materials on Thermal Fatigue Failure of EB-PVD Thermal Barrier Coatings

Effect of MCrAlY bond coat alloy systems on thermal fatigue failure of thermal barrier coatings (TBCs) was investigated, where the TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by EB-PVD method with 250 μm in thickness. Three kinds of MCrAlY alloys were studied as the bond coat material. Employing the originally developed test equipment, thermal fatigue tests were carried out, by applying thermal cycles between 400 and 950°C in air. Special attention was paid not only to the failure life of the TBC specimen, but also the underlying failure mechanisms. The experimental results clearly demonstrated that the effect of MCrAlY bond coat alloys on the thermal fatigue life was very significant. Some discussions were made on the experimental results based on the measurements of mechanical and metallurgical properties of the bond coat alloys: i.e., elastic stiffness, thermal expansion coefficient and high temperature oxidation resistance.

Longitudinal Compressive Strength of Continuous Fiber Composites

The in-plane longitudinal compressive strength of continuous fiber-reinforced composites is significantly less than their tensile strength, a phenomenon that is still not well understood. Dow and Rosen attempted in 1965 to explain this phenomenon through a fiber microbuckling model, in which each fiber was modeled as a beam-on-an-elastic-foundation. The model, however, grossly overestimates the compressive strength; researchers have since focused on introducing factors that reduce the predicted compressive strength to fit experimentally observed values. These include initial fiber waviness, fiber and matrix properties, the interfacial strength, and interphase properties. While empirical results have demonstrated the influences of some of these factors, an analytical model that can accurately predict the strength while taking into account the underlying failure mechanism is still lacking. The present study treats the compressive strength prediction of continuous fiber-reinforced composites using a different approach. Here, the matrix surrounding the fiber is not assumed to be homogeneous, but is divided into a discrete interphase region adjacent to the fiber and bulk matrix phase beyond the interphase. The three-phase model is shown to provide a more accurate prediction of the compressive strength of typical high-performance composites, such as glass and carbon fiber-reinforced epoxy. The present model's prediction agrees well with the trends shown in the experimental database available in the literature.

Evaluation of an Unsuccessful Brook Trout Electrofishing Removal Project in a Small Rocky Mountain Stream

AbstractIn the western United States, exotic brook trout Salvelinus fontinalis frequently have a deleterious effect on native salmonids, and biologists often attempt to remove brook trout from streams by means of electrofishing. Although the success of such projects typically is low, few studies have assessed the underlying mechanisms of failure, especially in terms of compensatory responses. A multiagency watershed advisory group (WAG) conducted a 3‐year removal project to reduce brook trout and enhance native salmonids in 7.8 km of a southwestern Idaho stream. We evaluated the costs and success of their project in suppressing brook trout and looked for brook trout compensatory responses, such as decreased natural mortality, increased growth, increased fecundity at length, and earlier maturation. The total number of brook trout removed was 1,401 in 1998, 1,241 in 1999, and 890 in 2000; removal constituted an estimated 88% of the total number of brook trout in the stream in 1999 and 79% in 2000. Although abundance of age‐1 and older brook trout declined slightly during and after the removals, abundance of age‐0 brook trout increased 789% in the entire stream 2 years after the removals ceased. Total annual survival rate for age‐2 and older brook trout did not decrease during the removals, and the removals failed to produce an increase in the abundance of native redband trout Oncorhynchus mykiss gairdneri. Lack of a meaningful decline and unchanged total mortality for older brook trout during the removals suggest that a compensatory response occurred in the brook trout population via reduced natural mortality, which offset the removal of large numbers of brook trout. Although we applaud WAG personnel for their goal of enhancing native salmonids by suppressing brook trout via electrofishing removal, we conclude that their efforts were unsuccessful and suggest that similar future projects elsewhere over such large stream lengths would be costly, quixotic enterprises.

Fatigue of reinforced-polyurethane-based, sheet metal forming dies

Fatigue in metal forming dies is an important issue in the manufacturing industry. Fatigue failure leads to repair or replacement of the die, which interrupts and slows down the manufacturing process. Fatigue of bulk metal forming (such as, forging and extrusion) dies has been the focus of most die failure studies because the nature of the process tends to subject the die to high stresses, thus increasing the likelihood of die failure. However, as polymer composite tooling materials are making inroads into rapid tooling technologies, die failure in sheet metal forming, in which the dies are subjected to significantly lower stresses, is drawing attention. This paper presents a fatigue failure analysis of V-bending dies machined from a polyurethane-based tooling board. The mechanical properties of the tooling materials are characterized to identify the underlying failure mechanisms. Various fatigue failure criteria, namely, maximum tensile principal stress, effective stress, Smith–Watson–Topper, and critical plane approaches are investigated as die life prediction methods, among which the latter two approaches were most accurate. Finite element analyses and experiments are performed to verify the results.

A preliminary evaluation of the surface of surgically removed Proplast/Teflon interpositional disc implants from human temporomandibular joints

A preliminary evaluation of the surface of surgically removed Proplast/Teflon interpositional disc implants from human temporomandibular joints

Electrical overstress due to ESD induced displacement currents

In this paper a new failure mode is introduced, which is related to the large dV/dt of ESD pulses. It was observed after +4kV HBM stress for a 90V-BCD technology device and resulted in a gate oxide defect of a low voltage PMOS transistor, which was hidden deeper in the IC's circuitry. The underlying failure mechanisms are discussed based on experimental and simulational findings and measures for early identification and protection of potentially sensible devices are proposed.

Flexural behavior of sandwich beams fabricated by vacuum-assisted resin transfer molding

The static and fatigue behavior of vacuum-assisted resin transfer molded sandwich panels has been experimentally investigated under flexural loading. Two core materials, D-100 balsa wood and H 250 PVC foam, were used to study the effect of core material on static failure in 3-point and 4-point bending. The face consisted of a quasi-isotropic E-glass non-woven fabric cured in Derakane 441-400 epoxy vinyl ester resin. The stresses in the face and core at failure were calculated from failure loads using the simple beam theory and compared with their respective strength values to identify underlying failure mechanisms. For short beams, core failure led to failure of the beam as a whole. In long beams, however, failure of the wood core could be contained until the face failed. Only D-100 end grain balsa wood was used as core for fatigue tests. A good similarity was seen between the static and fatigue failure modes for both short span and long span specimens.

NMOSFET ESD self-protection strategy and underlying failure mechanism in advanced 0.13-μm CMOS technology

In this paper, the high current characteristics encountered during electrostatic discharge (ESD) stress using nMOS/L/sub npn/ protection devices in a 0.13-/spl mu/m CMOS technology are investigated for different device parameters: channel length, channel width, gate-oxide thickness, and drain/source contact to gate (DCG/SCG) spacing. From leakage current measurements following ESD stress, it is concluded that the shorter (0.13 /spl mu/m) devices fail because of source/drain filamentation, whereas longer (0.3 /spl mu/m) devices with thin (22 /spl Aring/) oxide gate fail because of oxide breakdown. This conclusion is consistent with and supported by numerical simulations of the electric field. It is also supported by the observed effect of hot carrier stress on I/sub t2/. Hot carrier stress experiments additionally revealed that ESD stress can and does affect subsequent hot carrier degradation of the device.

Piecewise linear reliability data analysis with fuzzy sets

The Weibull three-parameter model is widely used in the analysis of reliability data, since it provides a good fit to many, but not all, data cases. In this work an analysis is presented in which locally constant hazard rate functions are applied in the analysis. By defining concomitant fuzzy reliability functions as overlapping fuzzy sets, the limited ranges may be conjoined to provide an extensive continuous reliability function. The efficacy of this treatment is tested with three different typical cases of published physical data that were previously the subjects of Weibull graphical analysis. It is found that in each case the treatment is effective. Different phases of the failure pattern of each case are identified and characterized by ‘time’ constants. The treatment is not limited to three parameters, as in the Weibull case, and it is therefore more flexible. It provides a deeper insight into the underlying failure mechanisms and may be expected to provide a wider range of applications, both within reliability data analysis and in other fields, and can form part of a knowledge-based system.

A Review of Some of the Factors Affecting Fracture and Fatigue in SBR and BR Vulcanizates

Abstract This paper reviews the effects of polymer macrostructure (molecular weight and the styrene content of SBR) and diene microstructure on the fracture and fatigue life of styrene-butadiene rubber (SBR) and butadiene rubber (BR) vulcanizates. Effects of experimental parameters, such as carbon black and vulcanizing systems, on fracture and fatigue are also discussed. Investigations were performed under static (tensile and tear) and dynamic (fatigue) testing conditions. Finally, the underlying failure mechanisms are discussed.

Relationship between strength and failure mode of ceramic multilayers

The mechanical strength of ceramic multilayers for use in solid oxide fuel cell stacks was examined. The multilayers consist of a substrate, made of yttria stabilised zirconia (YSZ), with a NiO/YSZ composite coating on both faces. Under some processing conditions the mechanical strength of the multilayers was similar to that of pure YSZ, while a much lower strength was found for other processing conditions. Inspections of the fracture surfaces revealed markedly different appearances, suggesting that a transition in failure mode had occurred. Fracture mechanics models for thin films on substrates were used to interpret the observed fracture modes, providing insight to the underlying failure mechanisms.

Proportional hazards modelling in reliability analysis—an application to brake discs on high speed trains

AbstractThere has been much recent interest in the reliability field in the application of proportional hazards modelling. A number of practitioners and academics have recently been advocating its use and reviewing the basic theory. The technique is, however, in the early stages of application in reliability studies, and apparently no satisfactory examples of its application to reliability data are to be found in the open literature. An implication is that the enormous potential of this approach for reliability analysis, as well as the particular shortcomings and difficulties of applying it in this field are not well understood in the reliability community as a whole.In this article we discuss the application of the method to the reliability analysis of the brake discs on British Railways high speed trains. The need for such an approach arose owing to the paucity of current reliability analysis methods for the problem in hand. The proportional hazards methodology provided an effective means of exploratory analysis and the application of the method provided an explanation for a previously unresolved discrepancy between the statistical and metallurgical analyses. As a result of its use, the effects of the underlying failure mechanism and the various design and operating variables and their interactions are better understood, and British Rail's maintenance policy has been changed accordingly. On the basis of experience of this and similar applications, the potential for application of proportional hazards modelling in reliability, and the likely benefits to be obtained, are discussed.

Model for failure rate curves

The failure rate of electronic components is determined by the initial distribution of the critical parameters and by the nature of the degradation mechanism. Assuming the mechanism to be determined by a first order differential equation with respect to time it turns out that a simple relation exists between the life histories of components with different initial conditions. With only one failure mechanism and a (truncated) normal distribution function, with average μ and standard deviation σ, it is shown that the shape of the failure rate curve is determined by a dimensionless characteristic parameter b = n( σ/ μ) 2, in which n is typical for the underlying failure mechanism. For b < 1 4 a “bath tub” like failure rate curve will occur in general; for b > 1 4 the failure rate decreases continuously as a function of time. By considering the complete failure rate curve, including the rejects during testing, a relation between yield and failure rate can be indicated. A prediction about the temperature-dependence of the failure rate curve is given. The effect of stress testing at elevated temperature can be made quantitative by a simple scaling rule.