Fatigue crack growth testing at negative stress ratios: discussion on the comparability of testing results

Abstract

A B S T R A C T It is an accepted fact in fatigue community that compressive loads contribute to fatigue crack growth. Evidences range from fatigue crack growth under fully compressive loads to effects of compressive underloads to negative stress ratio loading. Because the crack closes under compression and the crack flanks transmit compressive stresses, the loading situation is completely different to those of tensile loading. The present paper addresses the comparability of crack growth testing procedures at negative stress ratios. It reveals that compressive loading at the crack tip differs in different specimens for an equal maximum stress intensity factor K max and negative stress ratio R. Furthermore, the crack length can significantly influence the loading conditions at the crack tip for tensioncompression loading. Depending on the specimen type and crack length, a negative force ratio may lead to a change of algebraic sign of the stresses at the crack tip or not. As a consequence, the comparability of available literature results for R ≤ 0 tests is not ensured. Proposals to improve the comparability of tension-compression crack growth testing will be given.

Reconsiderations of fatigue crack growth at negative stress ratios: Finite element analyses

“…The σ tip concept was proposed and intensively discussed in [11]. Therefore, only a brief description of the concept is given within the present work.…”

“…Since R = -1, the compressive stresses in the crack plane are lower for the crack length a 2 , although K max and R are identical. The compressive stresses in the crack plane can be determined by linear finite element analyses, according to [11]. Figure 2c shows the compressive stresses in the crack plane corresponding to the minimum load at a 1 and Figure 2d shows the distribution for a 2 .…”

“…As a consequence it was suggested to run tests at negative stress ratios, not with a constant value of R, but with a constant value of σ tip [11]. Yu et.…”

“…Yu et al [9] studied a center cracked specimen at negative stress ratios and claimed that the controlling variables are K max and S min (minimum external stress) rather than K max and R. Zhang et al [10] investigated the crack length dependency for an unnotched center cracked specimen at negative stress ratios by finite element analysis. They showed that the local elasto-plastic response is controlled by K max and S min rather than by K max and R. Benz and Sander [11] suggested to use a more general parameter which quantifies the compressive loading at the crack tip for arbitrary geometries and loading conditions. This parameter was verified experimentally [5] and it is intended to give further evidence within the present investigation.…”

Short‐crack thresholds and propagation in an AISI 4340 steel under the effect of SP residual stresses

“…This approach found a discrete success, even though some specific questions remain to be addressed. In fact, negative load ratios have been shown to profoundly change the crack closure levels, and the correlation with the classical parameters of the fracture mechanics, the stress intensity factor (SIF) range Δ K , and the negative load ratio R , is difficult to verify . However, the effect of the compressive stress and negative load ratios is controversial.…”