In Situ Experimental Study of Near-Tip Strain Evolution of Fatigue Cracks

Abstract

Near crack tip strain fields in a stationary and a growing fatigue crack have been studied in situ using the Digital Image Correlation (DIC) technique in a compact tension specimen of stainless steel 316 L under tensiontension cyclic loading. The evolution of near-tip strains normal to the crack plane was monitored at selected locations ahead of the crack tip in consecutive cycles during fatigue crack growth experiments. A stationary crack was examined first to provide a baseline reference whilst the evolution of the strain field ahead of a growing crack was monitored in situ at peak loads during fatigue cycling. The results show that strain accumulation with loading cycle occurred at all tracked locations in both cases, and it is particularly evident close to the crack tip. Moreover, a higher strain accumulation rate was found near the growing crack tip than that near the stationary crack tip. The results on near-tip strain evolution were collected for the first time in situ during the fatigue crack growth experiments, which may hopefully inform a physical-based modelling strategy of fatigue crack growth.

Full‐field characterisation of crack tip deformation and fatigue crack growth using digital image correlation—a review

“…Near‐tip strain accumulation has been monitored in situ during cyclic loading, and quantitative experimental evidence of normal strain ratchetting has been found ahead of the crack tip, in support of a hypothesis of strain ratchetting as a mechanism for fatigue crack growth . In addition, strains have been monitored at selected, fixed observation points in the field of a growing fatigue crack, and a critical or onset strain was identified from instantaneous strains at the crack tip as the crack approached the observation points (Figure ).…”

“…However, in practice, some found that excluding plastic region seems to be unnecessary. Nevertheless, the extent of plasticity should be considered in deciding the size of field of view, as high‐resolution DIC may have a limited field of view where the plastic component may be substantial in comparison to the elastic component; consequently, overestimation of SIF may occur because of the high total strain as a result of plastic deformation (Figure ) . Sometimes, simple estimation of SIF from discrete displacement data on the crack flank (Equation ) is deemed suffice: …”

“…A typical map of normal strain ε yy in the field of view including a crack tip at peak load ( K max = 28 MPa√m), superimposed over a random speckle pattern. An approximation of the plane‐stress plastic zone from the von Mises yield criterion is also included . Overestimation of K from the digital image correlation displacement data may occur because of the large plastic component in the field of view [Colour figure can be viewed at wileyonlinelibrary.com]…”

Local strain accumulation in fatigue crack propagation process of Ti‐6Al‐4V alloy

“…19 Through the use of in situ DIC measurements, the evolution of strain field at crack tips in stainless steel 316 L and nickel-based superalloy were also tracked. 20,21 Fatigue life prediction and crack propagation behaviour of Ti-6Al-4V alloy under different loading conditions have been widely investigated. [22][23][24] Some studies focused on the effect of microstructures and processing method of specimen on fatigue properties of Ti-6Al-4V.…”

“…The variation of crack tip plastic zone within a single cycle was also identified by DIC . Through the use of in situ DIC measurements, the evolution of strain field at crack tips in stainless steel 316 L and nickel‐based superalloy were also tracked …”