ISSN: 1546-962X
CODEN: JAIOAD
Published Online: 27 April 2010
Page Count: 16

Crack-Closure Behavior of 7050 Aluminum Alloy near Threshold Conditions for Wide Range in Load Ratios and Constant K_max Tests

Newman, J. C.
Dept. of Aerospace Engineering, Mississippi State Univ., Mississippi StateMS

Yamada, Y.
Dept. of Aerospace Engineering, Mississippi State Univ., Mississippi StateMS

Newman, J. A.
NASA Langley Research Center, Hampton, VA

(Received 8 May 2009; accepted 22 March 2010)

Abstract

Fatigue-crack-growth rate tests were conducted on compact specimens made of a 7050-T7451 aluminum alloy to study the behavior over a range in load ratios (0.1≤R≤0.9) and constant K_max test conditions. Previous research had suggested that differences in the threshold regime at high load ratios were attributed to K_max effects. But recent measurements of crack-closure behavior under high R and constant K_max test conditions near threshold conditions on a variety of materials have indicated that these tests may not be crack-closure free as suspected. Strain gages were placed near and ahead of the crack tip to measure crack-opening loads from local strain records. In addition, a back-face strain (BFS) gage was used to monitor crack sizes and to measure crack-opening loads from remote strain records during the same tests. The 7050 alloy produced very rough crack-surface profiles. For R=0.1, the BFS and local gages indicated very similar high crack-opening loads. For R≥0.7 and K_max test results in the threshold regime, the BFS gages indicated lower crack-opening loads than the local gages. Based on local measurements, crack-closure-free fatigue-crack-growth data (ΔK_eff against rate) were calculated. These results indicated that the ΔK_eff against rate relation is nearly a unique function over a wide range of R values even in the threshold regime, if crack-opening loads were measured from local strain gages. At low R, all three major shielding mechanisms (plasticity, roughness, and fretting debris) are suspected to cause crack closure. But for high R and K_max tests, roughness and fretting debris are suspected to cause crack closure above the minimum load. A strip-yield model was also used to correlate the data over a wide range in load ratios and rates, but required a very low constraint factor (α=1.3), due to the high crack-opening loads.

Keywords:
fatigue-crack growth, crack closure, Kmaxeffect, threshold, compression precracking, load ratio

Paper ID: JAI102490
DOI: 10.1520/JAI102490
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