ISSN: 1945-7553
CODEN: JTEVAB
Page Count: 8

Computer-Controlled Decreasing Stress Intensity Technique for Low Rate Fatigue Crack Growth Testing

Saxena, A
Senior engineersMembers of ASTM, Westinghouse Research Laboratories, Pittsburgh, Pa.

Hudak, SJ
Senior engineersMembers of ASTM, Westinghouse Research Laboratories, Pittsburgh, Pa.

Donald, JK
Vice-president and staff engineermember of ASTM, Del Research Division, PSC-Professional Services Group, Inc., Hellertown, Pa.

Schmidt, DW
Vice-president and staff engineermember of ASTM, Del Research Division, PSC-Professional Services Group, Inc., Hellertown, Pa.

Abstract

An automated test system utilizing computer control was developed to obtain crack growth rate data down to the fatigue crack growth threshold with a decreasing stress intensity technique and compact type specimens. The starting stress intensity range ΔK₀ was chosen to yield crack growth rates in the range of 2.54 × 10⁻⁸ m/cycle (10⁻⁶ in./cycle) and subsequent values of ΔK are controlled to the equation ΔK = ΔK₀ exp [C(a − a₀)] (a₀ and a are the initial and instantaneous crack lengths and C is a test variable). Crack length is continuously monitored by using the elastic compliance technique, thereby enabling ΔK to be decreased continuously. Comparison crack growth data were also obtained by the more conventional constant load amplitude or K-increasing method. Excellent agreement was observed between data obtained from the two procedures for a Society of Automotive Engineers 1045 steel at load ratios R of 0.1 and 0.5, an A356-T6 sand-cast aluminum alloy at load ratios of 0.1 and 0.8, and a 2219-T851 aluminum alloy at a load ratio of 0.1.

Keywords:
fatigue (materials), crack propagation, stresses, threshold, computers, stress intensity, fracture mechanics, 1045 steel, 356 aluminum

Paper ID: JTE10938J
DOI: 10.1520/JTE10938J
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