Maximum Allowable Flaw Size Research Articles

Development and validation of a leak before break criterion for cylindrical pressure vessels

Leak before break is a fail–safe design concept for application in pressure vessels and piping of power and process plants. A quantitative maximum allowable flaw size is required to establish to set acceptance/rejection limit to predict whether the specific cracked pipe will leak or break. A new modification and its boundary based on Modified Two Parameter Fracture Criterion is capable of separating the leak and break cases distinctly in order to predict the behavior of cracked cylinders, pipelines and pressure vessels in advance for taking necessary precautions by the plant operator and also very much handy for the designers. For the given operating pressure under the observed crack dimensions, whether the crack will leak or break can be assessed from the boundary generated for the material concerned using Modified Two Parameter fracture assessment procedure.

Safe product design – the role of the NDT reliability analysis

Abstract In line with the damage tolerance design principles, an adequate non-destructive inspection system is needed to ensure structural integrity. Yet, when pushed to the limits of their detection capability, non-destructive evaluation systems do not produce consistent hit/miss indications. Their capability of detecting flaws is therefore expressed as a probability of detection. In the conventional signal response reliability analysis, the probability of detection is expressed as a function of the flaw size. The adequacy of the inspection system is verified by comparing the size of the flaw that is reliably detected with the maximum allowable flaw size, which will not undermine the structural integrity. Analyses of modern structures show that other parameters also determine the severity of the flaw for the structure. Within the multi-parameter reliability analysis, the probability of detection can be expressed as a function of diverse influencing parameters. When determining the adequacy of the non-destructive evaluation system, the capability of detecting a flaw has to be expressed and checked against the critical value of exactly that parameter that determines flaw severity for the structure. Failing to do so can lead to a rejection of the healthy, or acceptance of the bad part. The principle is demonstrated on the transmit-receive longitudinal ultrasonic inspection data of the iron cast component with surface, semi-elliptical, crack-like flaws.

Asymptotic analysis of stresses near a crack tip in a two-dimensional colloidal packing saturated with liquid

The consolidation of colloidal particles in drying colloidal dispersions is influenced by various factors such as particle size and shape, and interparticle potential. The capillary pressure induced by the menisci, formed between the top layer of particles in the packed bed, compresses the bed of particles while the constraints imposed by the boundaries result in tensile stresses in the packing. Presence of flaws or defects in the bed determines its ultimate strength under such circumstances. In this study, we determine the asymptotic stress distribution around a flaw in a two-dimensional colloidal packing saturated with liquid and compare the results with those obtained from the full numerical solution of the problem. Using the Griffith's criterion for equilibrium cracks, we relate the critical capillary pressure at equilibrium to the crack size and the mechanical properties of the packed bed. The analysis also gives the maximum allowable flaw size for obtaining a crack-free packing.

A maximum allowable flaw size for debond-resistant bimaterial layers

Free-edge debonding and subsequent interface crack extension are common modes of failure in a variety of engineering applications in which bonded layers of dissimilar materials are subject to differential expansion stresses. In this study, the susceptibility to debonding of short interface edge cracks is investigated for the global plane elasticity problem of a bimaterial strip with a uniform edge load applied to the top layer (a general model of differential expansion). The goal of this study is to determine the maximum crack length L for which interface crack extension is inhibited, or more specifically, for which the singular interface normal stresses are compressive. This is equivalent to a maximum allowable flaw size, which is of interest to both designers and inspectors of bimaterial systems. The critical crack length L has been extracted from parametric finite element analyses over a wide range of bimaterial configurations using the commercial software package ABAQUS. Results indicate that the critical crack length L increases with both the relative stiffness and thickness of the contracting layer, and may represent a significant inspectable flaw size for a variety of practical bimaterial configurations.

FRACTURE MECHANICS APPROACH FOR PREDICTION OF FATIGUE LIFE OF HIGH STRENGTH STEEL COMPONENTS

Abstract— A fatigue life estimation procedure based on a fracture mechanics approach is analytically developed for components made of a 4140 grade high strength steel. The maximum allowable flaw sizes for both static and cyclic loads are computed. For different stress levels and shapes of the defects, initially‐existing, allowable defect size versus number of operating cycles, as well as the thickness versus tolerable defect sizes curves, are presented. The maximum allowable operating cycles of components for non‐destructively detectable minimum defects are also estimated. From these estimations, a more realistic prediction of the safe service life of components is possible.

Advances in brazing of ceramics

The main parameters in direct brazing of ceramics to ceramics and to metals are reviewed, with primary emphasis on those influencing wetting of solid ceramics by liquid filler metals. In general, wetting of ceramics by conventional brazing alloys has been regarded as difficult. As a consequence, premetallization of the faying surfaces is frequently used to facilitate the brazing of ceramics. However, it is evident from the literature that recent developments in filler metals, based on active metal (e.g. Ti) additions (the amount depending on alloy composition and type of ceramic), have provided a basis for a substantial reduction of the contact angle. This favourable effect is caused by their reactivity, resulting in the formation of oxides when joining oxide ceramics (e.g. Al2O3), and nitrides or carbides and suicides in the case of nonoxide ceramics (e.g. Si3N4 or SiC). In addition to insufficient wetting, the mismatch in thermal expansion between the joint members may give rise to a low strength level due to the formation of high residual stresses on cooling. These stresses may limit the maximum allowable flaw size in ceramics to a few micrometres, i.e. of a similar size to that of pores.