Fatigue crack closure due to surface roughness and plastic deformation

Crack closure is an important factor affecting fatigue crack growth in high strength alloy materials. Plasticity is known to be the main driver of closure, but in some materials and at some stages other mechanisms such as fracture surface roughness and debris accumulation are also important. Analytical models based on the plasticity induced closure concept have been very successful in correlating fatigue crack growth rates and lives for a range of materials under constant amplitude and spectrum loading. However, extreme values of plastic constraint factors, significantly lower than those determined from three dimensional finite element studies on similar geometry, are needed to achieve good correlation with experimental results, particularly for materials which exhibit rough and tortuous fatigue surfaces. One such material investigated here is beta-annealed Ti-6Al-4V titanium alloy. The aim of this paper is to further develop, apply and evaluate a crack closure model which combines roughness and plasticity induced closure, incorporating experimental measurements of fracture surface roughness from tests on Eccentrically Loaded Single Edge-Notch Tension (ESE(T)) specimens. The model was first proposed by Zhang et al. 2002 for short cracks in 2000 series Aluminium Alloy. The model was evaluated here for physically longer cracks in the Titanium Alloy material. Accurate surface profile and roughness measurements were made using an optical 3-D profiler with a vertical resolution of better than 0.15 pm. Verification of the proposed model was carried out by comparing the model prediction with the closure measurements by back-face strain compliance using the pin-loaded ESE(T) specimens. Results from the roughness model were then compared with results from the FASTRAN analytical crack closure code