B Power

Evaluation of the J Integral for the Compact Specimen

Methods for determining the J integral from an experimental load versus load point displacement curve for the compact specimen are discussed. The original analysis by Merkle and Corten, which accounted for the tension component in the compact specimen, is presented along with a simplified version (of the analysis) that is shown to be essentially equivalent to the original formulation. Based on experimental results from Landes, Walker, and Clarke, a further simplified expression is recommended as the best expression to use for determining J for the compact specimen.

Generalized Application of Neuber's Rule

An analysis is presented concerning problems of correct application of approximation formulas such as Neubers rule for the case of inelastic net section behavior of notched members. The rules developed allow a generalized consideration of elastic-plastic net section behavior for any type of loading and notch geometry. The analysis is illustrated by discussion of experimental and calculated load-notch strain curves of different specimens.

Optimization of the Electrical Potential Technique for Crack Growth Monitoring in Compact Test Pieces Using Finite Element Analysis

Finite element procedures are used to optimize the efficiency of the electrical potential technique for monitoring the initiation and slow growth of cracks, as applied to the compact tension fracture test piece. An analysis of various configurations of current input and potential measurement lead placement is performed to optimize the accuracy, sensitivity, and reproducibility of measurement and to maximize output voltages. Numerical calibration curves are computed for selected configurations and are confirmed by experimental measurements.

Relationships Between Low-Cycle Fatigue and Fatigue Crack Growth Rate Properties

A fracture mechanics model is developed to describe crack growth in a low cycle fatigue test specimen. The model involves a J integral analysis and a growth rate hypothesis in terms of ΔJ. A relationship for low cycle fatigue is derived that has strain energy density as the controlling variable. This relationship reduces to well-known low cycle fatigue equations in terms of elastic and plastic strains for the limiting conditions of fully elastic and fully plastic strain fields. These equations in turn define relationships between the material properties commonly employed to describe low cycle fatigue and fatigue crack growth rate data. The latter are used to demonstrate the facility of predicting fatigue crack growth rate curves from standard low cycle fatigue properties.

Comparisons of Fracture Toughness Measurements by the Short Rod and ASTM Standard Method of Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399-78)

A multilaboratory blind comparison testing program evaluated the accuracy of the short rod method of measuring the fracture toughness of metallic materials. Valid comparisons between values measured according to ASTM Standard Method of Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399-78) and values of the plane-strain critical stress intensity factor as measured by the short rod method of fracture toughness measurement (KIcSR) were obtained for several steels, aluminum alloys, and titanium. The KIcSR values measured by the short rod method were consistently low, averaging 6% below the measurements according to ASTM Standard Method E 399. A 4% adjustment in the short rod calibration constant that had been previously evaluated only to ±7% brings the two sets of measurements into very good agreement. The short rod method thus appears to be a viable alternative for measuring the fracture toughness of metallic materials.

Effects of Stress Ratio on Fatigue Crack Growth Rates in X70 Pipeline Steel in Air and Saltwater

Fatigue crack growth rates in an X70 pipeline steel were measured over a wide range of stress intensities in laboratory air and in 3.5% sodium chloride solution under free corrosion and cathodic (coupled to zinc) potentials. The effects of stress ratio in all these environments and of cyclic frequency in the sodium chloride solution were studied. The results are compared with fatigue crack growth rates measured previously in lower strength X65 steel. The stress ratio strongly influences the fatigue crack growth thresholds and close-to-threshold growth rates. With increasing stress intensity range, the stress ratio effect decreases similarly in both air and aqueous environments. The lower cyclic frequency enhances corrosion fatigue crack growth rates at intermediate stress intensity ranges, as was observed earlier in X65 steel.

Tensile Properties of A357-T6 Aluminum Castings

Consideration of high-strength aluminum alloy castings as primary aircraft structure requires assurance that these products will possess structural integrity. Current industrial design practices place factors on properties or loads to offset the unknown characteristics of castings in recognition that the only absolute guarantee of properties is achievable through destructive tests. This paper presents an evaluation of tensile properties and their relationships to physical parameters for A357-T6 castings as an attempt to improve product assurance through nondestructive inspections. By improving both production controls and the measurements of significant physical conditions, it is believed that adequate product assurance can be developed to make these materials competitive with various wrought aluminum alloy fabricated structures.

A Single Specimen Determination of JIc for Different Alloy Steels

A simplified JIc test procedure has been proposed for determining JIc experimentally from a single specimen. The experimental results have shown that the crack initiation property directly influences the crack propagation behavior of the materials. Measurement of a critical J value corresponding to a small finite amount of crack growth instead of to an assumed blunting line procedure is proposed.

A Hydrostatic Stress-Sensitive Relationship for Fatigue Under Biaxial Stress Conditions

Failure criteria for elastic- and plastic-strain controlled fatigue under biaxial loading are proposed. The criterion for elastic strain is defined in terms of the equivalent strain from the distortion energy theory and that for the plastic part as the corresponding equivalent plastic strain, but modified to include a function of the hydrostatic stress component. These criteria are incorporated into the derivation of a relationship for fatigue life in terms of the principal strain amplitude, analogous to a conventional form for uniaxial loading. An analysis of some limited available data from the literature shows the potential predictive capability of the relationship.

Computer-Controlled Stress Intensity Gradient Technique for High Rate Fatigue Crack Growth Testing

An automated test system utilizing a computer for data acquisition and machine control was used to obtain crack growth rate data from 2.5 × 10−8 to 2.5 × 10−6 m/cycle (1 × 10−6 to 1 × 10−4 in./cycle) on a compact type specimen. The crack length was monitored continuously by using the elastic compliance technique, enabling the stress intensity K to be increased according to the equation ΔK = ΔK exp [C(a − a0)] (where a0 and a are the initial and instantaneous crack lengths and C is a test variable). Crack growth data were obtained on a Ni-Cr-Mo-V rotor steel by using both different, programmed K gradients and the more conventional constant-load-amplitude method where the stress intensity increases as a function of increasing crack length. Excellent agreement was observed between the two test procedures.