Determination of displacements around fatigue cracks using image analysis of in‐situ scanning electron microscope images

Abstract

A B S T R A C T A technique to in-situ measure the displacements in the vicinity of the crack tip during fatigue crack propagation has been developed. High-resolution images of the crack tip were taken continuously throughout the fatigue load cycles with a scanning electron microscope (SEM), and an image analysis program was used to determine the displacements at different positions with respect to the crack tip. The displacements were then used to determine crack shapes and compliance curves. The measured crack shapes show a general √ r dependence versus the distance to the crack tip. However, close to the crack tip the crack shape is clearly affected by plastic deformation, even in cases when small scale yielding prevails. The compliance curve measurements close to the crack tip can be used to determine the global stress level when the crack surfaces are separated, so that the exact opening and closure stresses can be determined. N O M E N C L A T U R E A cs = specimen cross section area (m 2 )A = specimen surface area used by the cross correlation function a = projected crack length (m) E = Young's modulus (Pa) F = applied global load (N) K = stress intensity factor (Pa √ m) L = specimen length (m) v = displacement perpendicular to the load direction W = specimen width (m) α = the identified specimen surface area, used to measure the displacement δ = crack opening displacement θ = angle from crack direction ( • ) υ = Poisson's ratio σ = stress (Pa) σ ys = yield stress (Pa)

Displacement measurement of the compliant positioning stage based on a computer micro-vision method

“…Since the integer-pixel displacement measurement capability of the NCC method has been validated by the previous literature, 28 only the proposed sub-pixel algorithm is tested. As the proposed method is capable of two-dimensional displacement measure-ments, for the sake of simplicity, just one-dimensional displace-ment is verified.…”

SEM study of overload effects during fatigue crack growth using an image analysing technique and potential drop measures

“…The crack was propagated in the load frame to a crack length a of at least a = 0.7 mm, to avoid any influence from the notch. The specimen was then etched with 1/3 HCl, 1/3 H 2 O 2 and 1/3 H 3 PO 4 to create a surface structure with features suitable for recognition by the image analysis scheme 16 …”

“…Images were taken at different load levels, and using the image analysis procedure described in Ref. [16], employing a cross‐correlation scheme, pre‐specified, small sample areas along the crack line were possible to identify between consecutive images. The sample areas were chosen hands on, with centre points close to the crack line, and were of sufficient size to hold enough micro‐structural features to be possible to recognize through the image analysis procedure with a high enough certainty.…”

“…In Ref. [16], Jacobsson et al . present a method to determine the crack opening through pattern recognition between consecutive in situ SEM photographs.…”

“…In Ref. [16], Jacobsson et al present a method to determine the crack opening through pattern recognition between consecutive in situ SEM photographs. The aim of this study is to investigate the crack tip characteristics of fatigue cracks exposed to overloads, using both the PD technique and analysis of crack opening displacements to construct compliance curves.…”