Compression precracking methods to generate near-threshold fatigue-crack-growth-rate data

J.C Newman Jr,Y Yamada

doi:10.1016/j.ijfatigue.2009.02.030

J.C Newman Jr, Y Yamada

Open Access

https://doi.org/10.1016/j.ijfatigue.2009.02.030

Copy DOI

Abstract

This paper summarizes some of the fatigue-crack-growth-rate data generated in the threshold and near-threshold regimes on two aluminum alloys (7075-T651, 7075-T7351), a titanium alloy (Ti–6Al–4V β-STOA), a high-strength 4340 steel and a nickel-based superalloy (Inconel-718) using compression precracking constant-amplitude (CPCA), compression precracking load-reduction (CPLR), and the ASTM E-647 load-reduction (LR) test methods. Tests were conducted over a range in stress ratios ( R = 0.1, 0.4 and 0.7) on compact specimens. One of the aluminum alloys (T651) and the 4340 steel showed very little difference between the methods; however, the other three materials showed significant differences with the compression precracking test methods giving lower thresholds and faster crack-growth rates than the load-reduction test method. Materials that have shown significant differences exhibited either rough crack-surface profiles (7075-T7351, Ti–6Al–4V β-STOA) or produced fretting debris along the crack surfaces in the threshold and near-threshold regimes (Inconel-718).

Highlights

Accurate representation of fatigue-crack-growth thresholds is extremely important for many structural applications
The third method was compression precracking, followed by constant amplitude (CA) loading, and load reduction (LR) following E-647 procedures, except that the initial stress-intensity factor range and crack-growth rate at the start of LR test is much less than the current standard
Some of the fatigue-crack-growth-rate data generated in the threshold and near-threshold regimes on two aluminum alloys (7075-T651, 7075-T7351), a titanium alloy (Ti-6Al-4V E-STOA), a highstrength 4340 steel and a nickel-based superalloy (Inconel-718) were determined by using the compression precracking constant-amplitude (CPCA) and compression precracking load-reduction (CPLR) test methods

Summary

Introduction

Accurate representation of fatigue-crack-growth thresholds is extremely important for many structural applications. To generate fatigue-crack-growth-rate data in the threshold and near-threshold regimes, without appreciable load-history effects, a “compressioncompression” precracking method, developed by Hubbard [12], Topper and Au [13], Pippan et al [14,15], Forth et al [16] and Newman et al [17] is used. Environmental effects, such as oxide and/or fretting-debris-induced closure, crack-surface roughness-induced closure, and plasticity-induced closure would naturally develop under “constant-amplitude” loading conditions. The results determined using the compression precracking methods are compared with data generated on the same materials using the ASTM E-647 load-reduction test procedures

Background

Test Procedures

Experimental Results

Summary