Analysis of Crack Closure by Different Measurement Methods

Methods for crack opening load and crack tip shielding determination: a review

“…Huang, Lang, Wu and Doker 9 used the NCTG, CMOD and potential drop measurement to obtain the compliance data. The tangent lines method was used to calculate P op .…”

“…According to Elber, U is dependent solely on the load ratio, R = P min / P max . The most common indirect techniques which utilized the compliance curve are: back face strain (BFS), 4–5 , 9 , 11 , 24 , 45 , 49,50,51 , 77–81 crack mouth opening displacement (CMOD), 1 , 4–5 , 11 , 24 , 44,45 , 50 , 52 , 77–92 near‐crack tip gage (NCTG) 1 , 11 , 44,45 , 54 , 75 , 78,79 , 81 , 90 , 92–96 , 101 and ‘push rod’ gage 19 , 77 measurements. In addition, other methods such as acoustic emission (AE), 1 , 80 , 102,103 eddy current, 97,98 potential drop (PD), 9 , 99–101 ultrasonic isoscanning 102 and dynamic compliance 88 are also used.…”

“…A common assumption is that when the crack starts to close at the surface, the crack tip is fully shielded from further damage. The direct techniques include the following: optical and scanning electron microscopy observations, [11][12][13][14][15][16][17][18][19][20][21][22][23]52 photography, 24,25 optical and laser interferometry, 1,5, [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42]54 replica, 4,11,13,[20][21][22][23]52 surface strain, 5,9,11,24,[44][45][46][47]…”

“…According to Elber, U is dependent solely on the load ratio, R = P min /P max . The most common indirect techniques which utilized the compliance curve are: back face strain (BFS), [4][5]9,11,24,45,49,50,51,[77][78][79][80][81] 102 and dynamic compliance 88 are also used. Usually, the CMOD, BFS, and 'push rod' methods give similar results, 19,45,77,80 whereas the NCTG 19,45,77 and AE measurements result in somewhat higher values of P op .…”

“…A number of concerns surfaced during those round robins, starting an additional ‘wave’ of studies associated with new methods and suggested improvements to the existing methods. The most recent reviews 9–11 , 118 concentrated on specific type of methods such as curve fitting, finite element, smoothing of raw data, etc. Until now, there is no systematic review of the existing methods (developed before and after the ASTM round robins) and in this article it is intended to fill this gap.…”