An Assessment of Crack Closure and the Extent of the Short Crack Regime in Q1n (Hy80) Steel

Abstract

The paper addresses some aspects of the differences in fatigue crack growth rate behaviour and threshold values obtained for long through-cracks, short through-cracks and surface cracks. Attention is focused on plasticity induced closure in the wake behind the growing crack tip. For long cracks at high La,, closure is found to depend in a linear manner on K,,,, i.e. KO, increases with the size of the monotonic plastic zone. Closure increases at low AK and this is primarily a consequence of the load shedding procedure. If short through-cracks are prepared by machining specimens containing long cracks, a substantial part of the plastic wake is removed and this can produce marked effects on the closure contribution during subsequent growth. The length of crack "closed" in a long crack threshold test was found to be of the order of 1 mm. Cracks less than this length exhibited "short crack' behaviour: greater than this length, they behaved as "long cracks", with plastic wake effects apparently fully operative. Small surface cracks exhibit "long crack" behaviour at lengths as short as 0.2mm and reasons for this are discussed.

Fatigue-life assessment and validation techniques for metallic vascular implants

“…Typical crack lengths associated with small-crack growth behavior range from the order of the material grain size to as large as a millimeter [20][21][22][23]. Such microstructurally small cracks can grow significantly faster than large cracks under the same apparent crack driving force.…”

“…The ''anomalous'' small crack behavior has been attributed to inapplicability of continuum mechanics assumptions, microstructural interactions, lack of plastic wake effects, and crack-opening mode transients [19][20][21][22][23][24][25][26]. Small cracks have lengths, depths, and plastic zones of a similar size to characteristic microstructural dimensions (typically the grain size) [21].…”

“…It has been known for some time that traditional LEFM-based fatigue crack growth analyses do not conservatively predict the growth rates of microstructurally small cracks [19][20][21][22][23][24][25][26]. Typical crack lengths associated with small-crack growth behavior range from the order of the material grain size to as large as a millimeter [20][21][22][23].…”

Correlation of Fatigue Crack Growth Rate at Different Stress Ratios for Quenched and Tempered Steels and Other Alloys

“…There are two sets of results for QIN steel (both calculated from the relative displacement of growth rate curves at 10-t m/cycle). Although James and Knott [ 10] found very little influence of R on the growth rate of long. through cracks in QIN for R between 0.2 and 0.7 (similar to results for QIN in Fig.…”

On the interaction among stage I short crack, slip band and grain boundary: a FEM analysis

“…As soon as the crack enters into the adjoining grain, CTSD increases abruptly due to the activity of a new coplanar slip in the grain. The growth behavior of a short crack has been referred to as showing a lesser amount of crack closure due to the shorter plastic wake [22]. Shang and Ritchie [23] have observed that the tortuosity of the crack path in duplex structures results in a high level of crack closure near threshold level and is attributed to the sliding displacement.…”