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Crack Strength and Crack Propagation Characteristics of High Strength Metals

Report Number: ASD TR 61-207
Author: Christensen, R. H.
Author: Denke, P. H.
Corporate Author: Douglas Aircraft Company, Inc.
Laboratory: Directorate of Materials & Processes
Date of Publication: 1962-01
Pages: 258
Contract: AF 33(616)-7444
Project: 7381
Task: 738103
AD Number: AD0275395
Original AD Number: AD 275395

The purpose of this program has been to obtain engineering data on the4 crack propagation and residual strength characteristics of structural materials suitable for use in aerospace systems. Results of a fracture testing program for high strength sheet metals are presented. More than 500 sheet stock panels ranging in width from one to 18 inches and in thickness from 0.20" to.100" were tested. These panels contained centrally located cracks which were generated principally by fatigue loading at various exposure times under a variety of environments. In general the fatigue crack is shown to be a more sever stress raiser than any manufactured notch yet investigated. Fracture strengths of the cracked panels were determined for rupturing temperatures ranging from -340°F to 2,500°F.

The experimental results were studied analytically. A semi-empirical expression for residual strength is derived. This expression is based on a modification of the formula given by Crichlow for the effective width of the plastic zone. An expression for the rate of crack propagation also is presented. The crack rate formula is based on the plastic zone concept, and accounts for the observed approach to infinite cracking rates as the crack length approaches critical. These formulas are shown to agree well with test results and are suitable for design applications.

A digital analysis of the elastic and plastic stress and strain distribution in the cracked plate was performed. The analytical method is discussed and results are presented.

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