Force For Fatigue Crack Growth Research Articles

New developments in fibre metal laminates (ARALL): Vermeeren, C.A.J.R., Vogelesang, L.B. and Gunnink, J.W. Technische Hogeschool Delft Report No N91-11860/4/XAB June 1990 5 pp

Experiments were carried out to demonstrate the validity of a new concept for the evaluation of the effective fatigue threshold values for loading conditions with Kmin below the crack opening value. For this, load-CTOD curves were determined for different R-ratios keeping Kth, max constant and reducing Kmin below Kth, min, which resulted in a resumption of the fatigue crack growth. This observation is explained on the basis of a new closure evaluation, which takes into account the total range CTOD to affect the driving force for fatigue crack growth. Three methods to measure the effective threshold values are proposed and critically compared. Experimental data are given for Cu, high-strength Al alloys and a dual-phase steel.

Crack closure in 2219-T851 aluminum alloy

The effects of specimen thickness, stress ratio ( R) and maximum stress intensity factor ( K max ) on crack closure (or opening) were studied using a 2219-T851 aluminum alloy. The crack length and the occurrence of crack closure were measured by an electrical potential method. The experimental work was carried out within the framework of linear-elastic fracture mechanics. The experimental results show that the onset of crack closure (or opening) dependes on R, K max ), and specimen thickness. In terms of the “effective stress intensity range ratio” ( U), as defined by Elber, the results show that U tends to increase for increasing R, decrease for increasing K max , and decrease with increasing specimen thickness. From these trends, it is shown that the “effective stress intensity range” ( ΔK eff ) does not always increase with increasing stress intensity range ( ΔK). The experimental results show that crack closure cannot fully account for the effects of stress ratio, specimen thickness and K max on fatigue crack growth. The use of ΔK eff as a parameter for characterizing the mechanical driving force for fatigue crack growth is questioned.