Single Edge Notched Tensile Specimens Research Articles

Acoustic emission–stress intensity factor relations for tensile deformation of notched specimens of AISI type 304 stainless steel

Interrelation between acoustic emission (AE) and stress intensity factor ( K) has been examined using data generated during deformation of a series of single edge-notched tensile specimens. The experiments were carried out on nuclear and commercial grade AISI type 304 stainless steel sheet specimens of different thickness. Analyses of AE total counts ( N) versus K in log–log scale indicate N to bear the popular relationship with K as N= AK m where A and m are constants; but the magnitudes of A and m are different at low and high K regimes. Both the steels indicate higher values of m up to macroyielding than those obtained from analysis of AE data between macroyielding and the stress corresponding to K max values in the experiments. The magnitudes of m were found to be higher for the commercial grade steel than that for the nuclear grades one. In addition to the new observation of two regimes for the N= AK m relation, it was also noted that the value of m is dependent on the thickness. The variation of N with specimen thickness in the two steels appears to provide further evidence of acoustic emission from inclusion decohesion.

An acoustic emission study of the temperature-dependent fracture behavior of polypropylene composites reinforced by continuous and discontinuous fiber mats

The temperature-dependent fracture toughness, K Q, and acoustic emission, AE, behavior of glass-mat-reinforced thermoplastic polypropylene (GMT-PP) composites with continuous (GM-C) and discontinuous (GM-D) fiber mats were studied at three different temperatures: −40°C, room temperature (RT) and +90°C. Apart from the mat type, the matrix viscosity and the fiber/matrix adhesion were also varied in the GMT-PP samples. K Q was determined on statically loaded single-edge notched tensile (SEN-T) specimens. During loading of the SEN-T specimens, the burst-type AE signals were monitored. It was established that the mat type and the testing temperature strongly affect the fracture-toughness response. The K Q values of the GMT-PPs with GM-C reinforcements were superior to those with GM-D. Matrix and interfacial variables marginally affected the K Q values, however. Making use of mathematical statistics, correlations were found between K Q data and selected AE signal parameters.

Wood fracture, acoustic emission, and the drying process Part 1. Acoustic emission associated with fracture

Acoustic emission (AE) signals were collected during fracture tests in order to analyze them for characteristics that could be used as parameters in a reactive control system for the wood drying process. Ponderosa pine and California black oak single edge notch tensile specimens were tested in the TR, TL, and mixed mode configurations, at 12 and 18 percent moisture content and at temperatures of 20, 40 and 60 °C. AE was observed in both opening mode and mixed mode tests, but cumulative events to maximum load in mixed mode were 7.4 times greater than in opening mode for oak and 3.4 times greater in pine. It was concluded that mixed mode AE signals were most promising for pattern recognition analysis, which will be the subject of Part II of this study.

Application of the finite volume method to the analysis of dynamic fracture problems

A detailed description of a new numerical method for the solution of dynamic fracture problems is presented. The method employs finite volume discretization of the equilibrium equations. The present work considers the analysis of rapid crack propagation (RCP) in two-dimensional geometries only. The simulation of steady-state RCP in a peeling-strip geometry, and an economical approach which allows the calculation of the crack driving force from a ‘snapshot’ computation of the displacement field are described. Also presented is the modelling of transient RCP in single edge notch tensile specimens, based on a fixed-mesh ‘node release’ technique and a ‘holding back’ force concept. It is shown that finite volume results are in very good agreement with both analytical and finite element predictions. The accuracy, simplicity and efficiency of this novel method are also demonstrated.

The mechanism of slow crack growth in polyethylene by an environmental stress cracking agent

Using single edge notch tensile specimens under plane strain conditions, the time to fail by slow crack growth was accurately measured in an ethylene-octene copolymer in Igepal and air. The primary result was the observation of a critical time for failure called the ‘Igepal transition’. Only for failure times greater than the ‘Igepal transition’ did Igepal accelerate fracture. For failure times shorter than the ‘Igepal transition’ the temperature dependence and stress dependence of the failure times in air and Igepal were the same and secondary crazes were observed in Igepal but not in air. By relaxing the local stress the secondary crazes made the failure time in Igepal longer than in air. For times longer than the ‘Igepal transition time’ the Igepal accelerated fracture. The activation energy in Igepal was reduced to 69 kJ mol −1 as compared to 118 kJ mol −1 in air and the dependence of failure time on stress was changed by the Igepal. The Igepal has two distinct effects: it enhances crazing by plasticizing the amorphous region of the bulk polymer; and it enhances fracture of the craze by plasticizing the crystalline region of the fibrils. The ‘Igepal transition time’ was related to the difference between the diffusion coefficients in the crystalline and amorphous regions.

The stress and strain fields in the neighbourhood of a notch in polyethylene

The stress and strain fields were determined in the neighbourhood of a sharp notch in a single edge-notched tensile specimen. The effects of notch opening, distance from the notch and unloading the specimen and the difference between plane stress and plane strain were investigated. From the measurements of the strain field, the stress was inferred directly from the stress-strain curve of the material. In general, the stress field is fairly uniform between the notch and the craze tip, in good conformity with the assumption of the Dugdale theory. Complexities in the strain field arise from the stress concentration at the corners of the bottom of the notch and from the characteristics of the craze.

Discussion of a scalelike fracture surface mark in fine ceramics.

Scalelike marks are usually observed in the fracture surface of a Si3N4 specimen tensioned to fracture at 20°C and 500°C. They are the regularly shaped marks, ellipselike and parabolalike ones along with other irregularly shaped marks. The regularly shaped marks are carefully observed and analyzed in the same manner of defining an ellipselike mark (both ends closed mark, BECM) and a parabolalike mark (one end opened mark, OEOM) observed in unsaturated polyester resins (UP) and PMMA, as reported previously. the marks in a smooth specimen are mostly the OEOMs where the crack velocity often reached about 4500m/s at maximum and the BECMs are dominated in the fracture surface of a single edge-notched tensile specimen in which the crack velocity reached the maximum value that is less than the above. The variation of the marks and the mechanism of forming these marks seem to be the same with the cases in UP and PMMA, and they are discussed in detail from the points of fracture strength and the crack velocity change. The shapes of the marks are approximated by the equation of fracture surface marks.

A note on the ductile fracture of single edge-notch tensile specimens loaded through frictionless pins

A note on the ductile fracture of single edge-notch tensile specimens loaded through frictionless pins

K-calibration and advantages for fatigue testing of single edge-notch tensile specimens with restrained ends

In studies of fatigue crack propagation, it is desirable that the portion of the width over which crack growth measurements are feasible be as large as possible and that K not vary strongly with crack length for a constant load. A long specimen can also be desirable since the space available on both sides of the crack plane facilitates the utilization of a travelling microscope to follow the crack tip, the positioning of displacement gauges on the sides of the specimen in order to measure crack opening, or the fixing of an environmental chamber to the specimen. From these points of view, the single edge-notch (SEN) tensile specimen and the central notch (CN) specimen are more interesting than the more often employed compact tension (CT) specimen. Of course, both the SEN and CN specimens require larger loads for given values of a/W and K than the CT specimen. In comparison to the SEN specimen, the CN specimen presents the practical difficulty of necessitating the initiation and propagation of two symmetrical cracks, which can lead to an important percentage of rejects. The major difficulty encountered with SEN tensile specimens is that the non-symmetrical line of action of the applied load with respect to the non-cracked ligament can, depending on the loading or gripping conditions employed, lead to modifications in the stress distribution. This results in uncertainty in the true value of K obtained. The loading or gripping conditions employed with SEN specimens can be assumed to lie between two extremes. At one limit, the specimen is pin-loaded, employing pins which are considered to be frictionless, so that complete freedom of rotation of the ends of the specimen can be assumed. At the other limit, rigidly fixed grips are employed and are assumed to completely restrain the ends of the specimen and prevent any rotation.