Fatigue Crack Growth Behaviour of Sm45c Steel Under Cyclic Mode I With Superimposed Static Mode Ii Loadings

Abstract

The practical applications of studies related to constant amplitude mode I loading are somewhat limited, since mode I crack growth is often influenced by mode I1 (sliding mode) or mode I11 (tearing mode) in industrial situations. For these cases, criteria, rules, and laws have to be worked out and verified by experiments. However, it is very difficult to evaluate mixed-mode fatigue cracking due to crack surface interference, crack closure, crack branching, etc. This paper, which defines the length of a branched crack as an effective slant crack with a length equal to the distance between the two crack tips, explains the influences of crack surface interference by introducing concepts of adhesive wear and scrutinizes some related researches on mixed-mode crack growth behaviour. Additionally an effective stress intensity factor range is described which considers crack closure and crack surface interference and is verified with crack growth tests under mode I fatigue loading and cyclic mode I with a superimposed static mode I1 loading. NOMENCLATURE 2a,,, 2a, = effective crack length and precrack length AK,, K,, = mode I SIF range and static mode I1 SIF, respectively A,,, = contact area under cyclic mode I with superimposed static mode I1 loading AK,,, AKeff = equivalent and effective stress intensity factor ranges, respectively K,,,,, Kll,lock = crack opening and crack locking stress intensity factors, respectively U = mode I crack opening ratio oad = adhesion strength

Influence of the loading path on fatigue crack growth under mixed-mode loading

“…with few asperities) or after removal of asperities by a fine saw-cut, but deceleration for long cracks (many asperities, so that the K II - induced decrease in K effective I becomes predominant over the intrinsic acceleration). The observations made by You and Lee (1997) on a mild steel suggest the same effects: acceleration of mode I crack growth by a static mode II, if R > 0.2 (no crack face interaction) but deceleration for lower R ratios.…”

Fatigue Crack Growth Behaviour of Sm45c Steel Under Mixed‐mode I and Ii Loading

“…Consequently, the authors have studied mode I fatigue crack with a superimposed static mode II loading, and additionally have examined historical developments [8]. This paper extends a previous report [8] to fatigue crack under mixed mode I and II cyclic loadings.…”

“…The difficulties which exist in other methods for defining the crack length and crack orientation have been fully discussed in Ref. [8].…”

“…It follows that the crack-opening ratio U, which relates to the degree of crack closure, is able to explain the real contact area. However, since A c is the contact area under a normal load, the contact zone will be more spacious under mixed-mode loading, and so the real contact area under mixed-mode loading, A mix may be related to A c by the maximum shear strain during one cycle [8],…”

“…Adhesion and shearing-off of asperities can make wear debris (see Ref. [8]), which provides additional interference between the crack surfaces. However, the effects of wear debris are not dealt with in this study.…”