Numerical Analysis of Mode-I SIF of an Arc Shaped Specimen Under Fatigue Loadingand Comparison of Maximum SIF of Present Method with BCM.

Abstract

Practically, high risk products like nuclear plant steam generators, thick cylinders etc. do  fail either by Mode-I or Mode-II or Mode-III. Mode- I failures are more and hence studied about the same very often. Root cause of most of these products are the presence of inbuilt or in-service flaws in the structures. Although designers give more importance to design featuring, notches and grooves to minimize the stress, fatigue failures of the components repeatedly take place during the service in the industries. Hence it is necessary to understand the mechanism of Fatigue crack growth (FCG) in metals through the tests using the suitable experimental setup. After vigorous literature survey, it is noticed that, an Arc shaped Tension specimen (AT) can be used as it is very useful for testing the properties of thick wall pipes.

To meet the set objectives, present work is begin with FCG testing of AT specimen of Al alloy using the Experimental setup. Stress Intensity Factor(SIF) range, number of fatigue cycles are recorded and Paris’ constants are determined. To study the effect of Mode-I loading on the SIF at the crack tip , an AT specimen with 2 crack geometries , i.e crack on inner surface and outer surface are considered independently. In both the cases , same radius ratio and crack size of the specimen is maintained. To execute fatigue load, maximum and minimum loads so obtained from the experimental study is used in Finite Element analysis (FEA). AT specimen with 1mm inner surface crack is modeled and SIF is evaluated. The process is continued in a step of 1mm crack length increment, up to the stage when maximum SIF reaches the critical value of the material. Using the Paris’ constants, FCG rate is computed. Number of stress cycle is calculated using the integral formula. The outcome in terms of relative crack length  vs maximum SIF as obtained by the present method is compared with Boundary Collocation Method (BCM) results of literature. Deviation in the maximum SIF of 2% and 9.49% between the present method and BCM results is observed for inner and outer crack respectively.

Also, it is been observed that , stress required to widen the crack at the inner surface of the specimen is more than the stress required to widen the crack at the outer surface. Al alloy  can sustain more number of fatigue cycles,  if the crack is at the outer surface than the crack at inner surface  under the similar mode of loading. This is due to the fact that, as  outer crack induces comparatively less stress gradient and thus crack can extend to larger size before reaching to the unstable growth.