A. R. Rosenfield

Mixed-mode fracture in biaxial stress state: Application of the diametral-compression (Brazilian disk) test

Abstract Mixed-mode fracture of soda-lime glass was studied using a diametral-compression test that features disk specimens with symmetric through-cracks. The test enables one to study fracture under pure mode I loading, pure mode II loading, or any combination of mode I and mode II loading by a simple alignment of the crack relative to the diameter of compression loading. The disk specimens were precracked with the aid of both chevron notches and water-assisted subcritical crack growth. The directions of noncoplanar crack extensions and the relative magnitudes of mode I and mode II stress-intensity factors for mixed-mode fracture under inert conditions were compared to the predictions of three different mixed-mode fracture theories. None of the theories was completely adequate to explain the experimental observations, but a maximum hoop stress criterion modified to include second order, nonsingular term in the series solution for the crack-tip region stress gave reasonable agreement with the experimental results.

Structure of the 〈100〉 Edge Dislocation in Iron

The positions of atoms around the core of a 〈100〉 edge dislocation in iron have been calculated using atomic potentials developed by Johnson, boundary conditions based on anisotropic or isotropic elasticity and the GRAPE computer program. The dislocation is quite narrow and the atoms below the extra half‐plane relax into a microcrack. It is shown that particular care must be taken in introducing the dislocation and choosing the potential. The configuration of this particular dislocation is only slightly sensitive to the choice of boundary conditions and the size of the model.

Combined mode I-mode III fracture toughness of a spherodized 1090 steel

Abstract The aim of this investigation was to determine the fracture behavior of a spherodized 1090 steel under combined mode I-mode III loading conditions. Suitably defined formulations of the J integral denoted Jic and Jiiic were used to characterize the elastic-plastic fracture of this steel. As the mode III component in the system is increased, the resolved mode I J integral at initiation decreases, its mode III counterpart increases and the total J value remains nearly a constant. This implies a constant energy requirement for fracture initiation under mixed mode loading. As the crack plane becomes less inclined to the load line, the slopes of the mode I and total J resistance curves increase from their pure mode I values until a crack inclination angle of about 65° is reached. Somewhere in the region of 65-55°, a maximum in these values is reached and they fall off rapidly for larger mode III components. This drop is accompanied by the breakup of the crack front into mode I and mode III steps, which is shown to be an energetically more favorable process for this steel.

Microstructure of a quenched and tempered Cu-bearing high-strength low-alloy steel

The grain structure, inclusion content, and precipitate types were characterized for six heat treatments of a copper-bearing HSLA steel. A higher austenitizing temperature combined with water-quenching resulted in an acicular ferrite microstructure, while lower temperatures produced equiaxed ferrite or ferrite-pearlite structures. Refinement of equiaxed ferrite was observed in material austenitized at a high enough temperature to dissolve a portion of the carbonitrides, allowing reprecipitation during the austenite-ferrite transformation. Age-hardening precipitated body-centered cubic copper clusters; face-centered cubic copper precipitates were observed in overaged material.

Combined mode I - mode III fracture toughness of a high carbon steel

Abstract 1. 1. Brittle materials exhibit minimum energy dissipation under pure mode I loading 2. 2. In these materials, the mode IIIcomponent essentially provides redundant plastic work and increases J totalc . 3. 3. This behavior contrasts with more ductile materials where combined mode I - mode III loading produces The minimum energy dissipation.

Cleavage fracture of steel in the upper ductile-brittle transition region

Abstract Stable-crack growth precedes cleavage fracture in the upper ductile-brittle transition region. It is suggested that the conversion from one fracture mode to the other occurs at random locations in the microstructure, denoted trigger points. It also is suggested that K Ic for cleavage should be calculated from the elastic energy in the specimen at the point of conversion. Experiments are described that showed that the amount of stable-crack growth increases with increasing temperature and with decreasing load-train compliance.

Analysis of R-curve behavior of non-phase-transforming ceramics

Abstract An analysis is presented of the phenomenon of increased crack resistance with crack growth ( R- curve behavior) exhibited by ceramics such as alumina, which do not undergo phase-transformation in the process and by ceramic-matrix composites. These materials contain a wake of unbroken ligaments and crack bridges, and these are modeled in this paper to account for R- curve behavior. The analysis is based on a modified Dugdale strip yield model. Various sizes and geometries of specimens are analysed, and a small-scale yielding estimation is also presented. Good correlation is obtained between predicted and experimental R- curve behavior. The model is also used to predict the dependence of saturation fracture toughness on grain size.

Effects of microstructure on fracture toughness of a high-strength low-alloy steel

Fracture toughness and ductile-brittle transition behavior were measured for a copper-bearing HSLA steel. The value ofKlc for cleavage failure was independent of heat treatment, whileJlc for ductile failure decreased monotonically with increasing strength level. With both failure modes, fracture appears to be controlled by cracking of sulfide inclusions. The decrease in ductile-failureJlc is caused by decreased work-hardening rates that suppress cleavage and facilitate void coalescence. Both higher austenitizing temperature and quenching rate after austenitization influence the ductile/brittle transition temperature, either through grain-size and precipitate refinement or through an increase in the resistance of the steel to shear failure.

A Crack Arrest Measuring Procedure for K Im , K ID , and K Ia Properties

New developments are described which offer a flexible procedure for measuring the fracture resistance values, K I m , K I D , and K I a which characterize the crack arrest behavior of materials. The procedure relies on a dynamically stiff, wedge-loading which limits dynamic energy exchanges between the test specimen and the testing machine. This makes it possible to interpret both small and large crack jumps without recourse to crack velocity measurements. The procedure is general and can be applied to ordinary and duplex rectangular and contoured double-cantilever-beam (DCB) specimens, and to compact and single-edge-notched (SEN) specimens when dynamic analyses for these shapes become available. The paper examines the virtues of the different specimen configurations, size and thickness requirements, possible upper and lower bounds on the size of the crack jump, and the problem of branching and side grooves. Measurements of specimen machine interactions are described. Results obtained for A533B steel with a series of rectangular DCB specimens illustrate the dependence of K I a on the size of the crack jump as well as the previously proposed relations between K I a and K I m or K I D . The results confirm that the K I m (or K I D ) values obtained with the new procedure from static measurements of load point displacement and the crack length at arrest agree with values derived from crack velocity measurements. The experiments also illustrate that duplex DCB specimens with modest dimensions (400 by 140 by 50 mm) have the capacity for measuring K I m and K I D values in excess of 150 MPa.m 1 /2.

Crack arrest in steels

Abstract This paper examines 3 theories that have been used to characterize the arrest capabilities of steels and structures: (1) The static analysis, arrest toughness ( K Ia ) theory; (2) The dynamically loaded/stationary crack toughness ( K Id ) theory, and (3) The dynamic analysis, propagating crack energy or toughness ( R ID or K ID ) theory. These three concepts are examined in the light of measurements of unstable fracture and crack arrest in wedge-loaded DCB test pieces together with a fully dynamic analysis of the experiments.