Influence of Fatigue Crack Wake Length and State of Stress on Crack Closure

Abstract

The location of crack closure with respect to crack wake and specimen thickness under different loading conditions was determined. The rate of increase of K_. in the crack wake was found to be significantly higher for plasticity induced closure 1n comparison to roughness induced closure. enRoughness induced closure was uniform throughout the thickness of the specimen en en ™ while plasticity induced closure levels were 50 percent higher in the near uu surface region than in the mid-thickness. The influence of state of stress on low-high load interaction effects was also examined. Load interaction effects differed depending upon the state of stress and were explained in terms of AK ,_. eff

Load interaction effects during fatigue crack growth under variable amplitude loading—a literature review. Part II: qualitative interpretation

“…The concept presented in Fig. 6 seems to be supported by an experimental research on the effect of fatigue crack wake removal and specimen surface removal on crack closure behaviour in Al alloy CT specimens subjected to CA loading 77 . In the latter study, incremental wake removal did not affect K op at higher Δ K levels even if the remaining wake length was only 0.33 mm, whilst under a near‐threshold Δ K a gradual decrease in K op started already at the remaining wake length of 4 mm, as schematically shown in Fig.…”

“…In this type of experiment, fatiguing a cracked specimen is interrupted and material from the specimen outer surfaces is incrementally removed, while closure is measured at each new thickness. Provided that Δ K is not near‐threshold, surface removal tests 77 , 86 reveal the trend seen at high Δ K in Fig. 7(b), indicating that after removing the ‘plane stress’ regions, the PICC mechanism is weakened, but still active.…”

“…It has been hypothesized in Ref. [77] that the dominant mechanism at near‐threshold levels must be RICC as it is homogenous throughout the specimen thickness and, thereby, not sensitive to surface removal. Conversely, the trend shown in Fig.…”

“…It could be inferred from Ref. [77] that RICC is controlled by crack surface contacts over a much longer distance behind the crack tip than in the case of PICC. However, a numerical analysis of RICC 78 indicates that this distance is of the order of material surface roughness features.…”

“… Schematic effects of (a) wake removal and (b) surface removal, on K op at high and low Δ K levels 77 …”