Sliding mode crack closure and mode III fatigue crack growth in mild steel

Abstract Mode III fatigue crack growth behaviour in AISI C1018 steel ( R = −1, 1 Hz) is investigated in circumferentially notched cylindrical specimens. The crack surfaces in contact glide against each other, the friction, abrasion and mutual support between them reducing the effective stress at the crack tip (“sliding mode crack closure”). Crack growth rates under the effective stressing (so-called “true” crack growth rates) can be obtained by extrapolation to zero crack length ( c → 0), when no friction can be present. This way the effect of “sliding mode crack closure” can be quantified. It increases with increased plastic strain intensity, reaches a maximum for ΔΓ 111 ~1.5 × 10 −2 mm for the tested specimens and decreases drastically for stressing beyond that value. For these large loadings the large plastic zones formed in mode III open the crack in mode I and eliminate friction. Fractographic evidence for the processes involved is obtained.