Elastic compliance expressions for compact type (CT) and WOL specimens have been formulated for a wide range of crack lengths (0.2 ⩽a/W ⩽ 0.975) using results from Newman's modified boundary collection techniques and Wilson's deep crack analysis. The location of the axis of rotation of the specimen arms at various crack lengths has been calculated and subsequently used in a proposed extrapolation technique to predict compliance at any location of the specimen convenient for measuring deflection during a crack growth test. The predicted compliances were found to be in excellent agreement with expreimental values for the two specimen types considered. Compliance expressions are also included for the center crack tension specimen.

Review and extension of compliance information for common crack growth specimens

Tensile, fracture toughness and fatigue crack growth rate properties of silicon carbide whisker and particulate reinforced aluminum metal matrix composites

Quantitative analysis of microstructural effects on fatigue crack growth in widmanstätten Ti-6A1-4V and Ti-8Al-1Mo-1V

Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851 aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs through a “hydrogen embrittle ment” mechanism.

Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy

Fatigue crack growth rates in commercial purity Ti-6A1-4V can be substantially reduced with a beta anneal from levels associated with the mill anneal, owing primarily to crystallographic crack bifurcation in the Widmanstatten packets. This microstructurally sensitive fatigue crack growth occurs when the reversed plastic zone is less than the packet size and results in a fracture surface with a faceted morphology. It appears from replica and scanning electron microscopic examination that the facets are comprised of three superposed features,viz cleavage-like river-line patterns, very fine striations and traces of slip lines (and slip-band cracks). Limited X-ray evidence suggests a facet orientation some 8 to 10 degrees off the basal plane.

Observations on microstructurally sensitive fatigue crack growth in a widmanstätten Ti-6Al-4V alloy

To provide the data necessary to develop a recommended practice for fatigue crack growth rate testing, an extensive interlaboratory (round robin) program was conducted and the variability and bias associated with the current state of the art of fatigue crack growth rate testing was determined. Fatigue crack growth rate data (expressed in terms of linear elastic fracture mechanics parameters) were generated for a 190-ksi (1310-MPa) yield strength 10 Ni-8Co-1Mo steel at 15 different laboratories with several test specimen geometries. The results were evaluated statistically and on the basis of a graphical comparison, and the variability and bias associated with both the experimental and analytical aspects of crack growth rate testing were determined. In general, the overall interlaboratory variability was found to be approximately 3 to 1 (on crack growth rate at a given stress intensity range). The intralaboratory variability was typically 2 to 1. No significant geometry or data processing bias was encountered. The results of this study show that the primary source of variability associated with fatigue crack growth rate testing is the experimental procedure used to obtain the raw test data (crack length versus elapsed cycles).

W G Clark Jr

Papers

Variability in fatigue crack growth rate testing