Estimation Of Fracture Strength Research Articles

Optical and mechanical properties characterizations of transparent polycrystalline MgAl2O4 spinel

ABSTRACT The microstructure and mechanical properties of polycrystalline spinel ceramics (MgAl2O4) with high translucency were investigated. Several kinds of polycrystalline spinel ceramics were prepared under various sintering conditions, and single-crystal spinel and sapphire were used for comparison. The bending strength of polycrystalline spinel with smaller grain size fabricated without sintering aids exceeded that of single-crystal spinel. The bending strength at 1000°C tended to be lower than that at room temperature. The fracture toughness of polycrystalline spinel prepared without sintering aids measured by SEVNB was higher than that of spinel with sintering aids. By estimation of fracture strength considering different fracture origins, the polycrystalline spinel prepared without sintering aids might be fractured mainly from defects around residual spherical voids. If the size of voids can be minimized in polycrystalline spinel fabricated without sintering aids, both high transmission and good mechanical properties could be achieved.

Sizing limitations of ultrasonic array images for non-sharp defects and their impact on structural integrity assessments

Existing structural integrity assessment procedures typically assume flaws to be infinitely sharp when they cannot be considered as local thinned areas. This assumption is often over-conservative, resulting in a pessimistic assessment of structural components and a significant underestimation of their margins of safety against fracture. One of the main challenges while adopting non-destructive evaluation techniques is distinguishing between sharp cracks (e.g., fatigue) and non-sharp defects and identifying the more severe ones. Towards this broader challenge, the present work aims to examine the sizing limitation and accuracy of ultrasonic array image-based techniques for non-sharp defects (surface breaking u-notches) and investigate how these measurements would affect the structural integrity assessment of components. Parametric numerical simulations and experimental measurements are performed to generate full-matrix capture datasets, which are then processed using the total focusing method to form an image. The image-based sizing approach is shown to perform efficiently for notch depths higher than the inspection wavelength (λL), i.e. as small as 0.2 mm, and semi-notch widths as small as 0.1 mm. The influence of ultrasonic measurements on structural integrity assessments is highlighted using different case studies in the context of non-sharp defects of fatigue and fracture strength estimates. For the cases under analysis, resolving non-sharp defects led up to 5× and 3× values of effective fracture toughness and fatigue strength, respectively. We have also seen that a 30 % uncertainty in semi-notch width sizing would result in a 30 % and 20 % error in fatigue strength and fracture toughness estimations, respectively.

Estimation of the fracture strength of a spatial beam–rod structure

The estimation of fracture strength of a spatial beam–rod structure is demonstrated in the sample of a locomotive cab. The calculated case of collision of the locomotive with an obstacle is considered. The estimation consists of three interrelated tasks. At the first stage, the finite-element model of a cab frame, taking into account a buffer device, is built and the structure rigidity determined. At the second stage a two-mass design diagram (locomotive–obstacle) is considered and the interaction force is determined. At the third stage, the finite-element model describing the stressed–deformed state of the cab design at the collision with the obstacle is built. The resulting load, determining the bearing capacity of the cab, is compared with different variants of calculated forces defined for interaction of the locomotive with various obstacles.

Fracture behavior evaluations for ferritic steel piping with circumferential double flaws on the inner surface

Methods for assessing the structural integrity of nuclear components having some flaws are provided in the Rules on Fitness-for-Service for Nuclear Power Plants of the JSME code (JSME FFS code). Although the JSME FFS code provides such methods for piping with a single flaw, it does not describe any method for fracture assessment of piping with multiple flaws including flaw coalescence criteria. Some investigations on the fracture behavior of mainly austenitic stainless steel piping with multiple flaws, whose fracture mode is plastic collapse, have recently been reported and fracture assessment methods have been proposed. In the present study, fracture tests and analyses of carbon steel piping with a single and two circumferential flaw(s) on the inner surface were conducted to investigate a method for fracture assessment of ferritic steel piping with multiple flaws. It was found that fracture assessment based on the twice elastic slope method and the plastic collapse mechanism gave inadequate results for a large single flaw. Including this case, two kinds of elastic–plastic fracture assessment method, one using the Z-factor in the JSME FFS code and the other by ductile instability analysis, gave conservative estimates of fracture strength even when the structural factor SF was not considered (i.e. SF = 1).

Calibration of a probabilistic cohesive zone model for generating a fracture nomograph

ABSTRACTIn the recent years, Cohesive Zone Models (CZMs) have gained increasing popularity for modelling the fracture process [1] and also in other applications like composite de‐lamination [2] solder failures [3] in circuits, etc. This can be attributed to the ability of the CZM to adapt to the nonlinearities in the process it represents by adjusting the model parameters. These parameters that are selected to represent the material behaviour in the vicinity of the crack or a damage zone are non‐deterministic in nature resulting in random fracture strength estimates. Currently, there are no standardized tests for measuring the CZM parameters and their random scatter. Numerous researchers in the literature suggest values for the CZM parameters based on their experience from limited test data. Traditionally, fracture toughness is determined through coupon tests for any material system that is being analysed using Linear Elastic Fracture Mechanics to determine the fracture strength of a specimen. Since data for fracture toughness are available, this research is aimed at determining the probability density functions (PDFs) for the cohesive zone parameters that would give the same scatter in fracture strength as that obtained from the test statistics. Correlations between the model parameters were introduced to improve the accuracy of the fracture strength PDF. A finite width cracked plate was selected as a test case to demonstrate the process. This paper also presents evidence that material scatter can be isolated from the geometric effects to determine a normalized PDF of fracture strength for a given material. This normalized PDF can then be scaled, using mean fracture strength, to any crack configuration to develop a nomograph that can be used to rapidly assess risk without the need for a probabilistic fracture analysis.

Local Plastic Deformation and Failure Behavior of Nd:YAG Laser Welds in AA5182-O and AA6111-T4

An experimental methodology that quantifies tensile properties and damage accumulation in Nd:YAG laser welds in aluminum sheet metals is presented. Coupons identified as AA5182-O and AA6111-T4, with bead-on-plate and lap welds in various configurations, were quasi-statically strained to fracture in a miniature tensile stage. Two-dimensional strain fields, measured with a state-of-the-art digital image correlation (DIC) algorithm, served as the basis for the calculation of true stress–true strain behavior and the salient tensile properties of the weld materials. Scanning electron microscopy (SEM) analysis provided estimates of fracture strength, while ultrasonic microscopy was used to quantify weld porosity and establish a qualitative linkage with observed deformation behavior. Although containing substantial porosity, the AA5182-O all-weld material was found to have greater ductility than the AA6111-T4 material. The nominal strength of the AA5182-O/AA6111-T4 lap welds exceeded that of welds in AA6111-T4/AA5182-O, up to 50 pct strain. This suggests that bimaterial lap-weld strength at large plastic strains is dependent upon the placement of each Al alloy relative to the laser beam. Atomic force microscopy confirmed that fracture consistently occurred along weld/base metal interfaces, irrespective of the alloy type.

Failure behaviour of an ultra high strength low alloy steel

An ultra high strength low alloy steel having the composition Fe - 0.3C - 1.35Cr - 0.9Mo - 0.25V - 0.1Nb (wt-%), has been developed using the electroslag refining process. In order to understand the failure behaviour of the material, tensile coupons, surface crack tension (SCT) specimens, compact tension (CT) specimens, and pressure chambers were fabricated and tested. The fracture parameters of the material were evaluated from the test data. Using these parameters a failure assessment diagram was generated to quantify the effects of the presence of cracks on material strength. Fracture strength estimates on SCT and CT specimens compare well with test results. The effects on fracture properties owing to weld/heat treatment processes have also been examined. To select a suitable failure theory for this material, failure assessment has been carried out on pressure chambers and verified with test results. Fracture analysis has also been carried out on the cylindrical vessels containing axial surface cracks and it was found that failure pressure decreases with increasing in crack size.

Fracture strength of composite laminates containing surface notches

An empirical relationship developed recently between the failure stress and stress intensity factor at failure is examined for the estimation of fracture strength of thick graphite/epoxy laminates containing surface notches. The results are found to be reasonably in good agreement with the existing test results.

Interaction of process variables with the fracture behavior of amorphous polystyrene during attrition

AbstractThis study investigates the fracture of oriented general‐purpose polystyrene. Specimen microstructure was controlled using an extrusion‐sheeting technique where draw ratio, polymer melt temperature, cooling roll temperature, and nip roll pressure changed according to a designed experiment. Sharp‐notched tension results display a 4X difference in longitudinal vs. transverse specimen values. Shrinkage measurements show contraction in the machine direction amplifies with increasing draw ratio. Craze formation is visible only in those specimens tested in the longitudinal direction. Fracture mechanisms have strong orientation dependence and the extrusion‐sheeting process produces an ideal system to study the effect on attrition. Comminution studies illustrate draw ratio has an effect on the final median particle size and width distribution of the powder. Increasing the draw ratio reduces the volume median particle size and minimizes the formation of fine particles. A monotonic fracture mechanism was initially considered but rejected in favor of a low cycle fatigue model based on estimates of fracture strength.

Effect of surface treatments on the repair strength of a light-activated denture repair resin using censored data

Objectives. The aim of this study was to evaluate the effect of three different chemical surface modifications on the bond strength of light-activated denture repair resin (Triad, Dentsply International) to a heat-cured resin (Lucitone, Dentsply International). Methods. Transverse flexure test specimens were made, with surface treatment of 1) Triad bonding agent, 2) unreacted Lucitone monomer, and 3) a 1:1 mixture of methylene chloride and monomer, using two different application times (2 and 4 min). For all except one group, 18 specimens were made. The controls were solid single composition bars, 18 for each material. The elastic modulus in transverse flexure was measured for each material. The statistical analysis employed the Weibull distribution. Fracture strength values were obtained in a three-point flexure test. Results.It was noted that many fractures occured away from the interface and the center member of the test fixture. For those fractures, the observed fracture strength is a censored value of the fracture strength. Algorithms for the statistical treatment of censored data were used to get the maximum likelihood estimates of the fracture strength. A mechanical model showed that under three-point bending of a beam of two joined materials, fractures will occur in the part with the higher elastic modulus. This findingsis indeed observed in the data. The 4 min monomer treatment showed the highest estimate of the bond strength (187.8 MPa). Significance. Censored data can be used to obtain fracture strength estimates, and the censoring indicates that the bond strength may exceed the strength of the bulk material.

Prediction of Bending Strength of Long Bones From Measurements of Bending Stiffness and Bone Mineral Content

The relationship between fracture strength in bending, bending stiffness and bone mineral content for 56 fresh canine radii, ulnae and tibiae was determined from three- and four-point bending tests on a MTS testing machine and from measurements of bone mineral content at the fracture site using the Norland-Cameron Bone Mineral Analyzer. The high correlation of bending stiffness (0.962) and mineral content (0.901) with fracture bending moment permits estimates of fracture strength using current noninvasive methods of measuring stiffness and mineral content.