An assessment of various crack tip displacement based elastic-plastic fracture parameters to characterise ductile crack growth resistance

Abstract

There is continuing interest in defining a fracture parameter which is capable of characterising the ductile crack growth resistance o~" a material under conditions of large scale plasticity. The aim of this study is to assess the capability of a numberof, elastic-plastic fracture parameters for the case of a C-Mn steel. The fracture parameters considered are: the crack opening displacement at the original crack tip (CTOD); the crack opening angle at the growing crack tip (CTOA); and the crack opening displacement at the growing crack tip (6°). The parameters were assessed by determining experimentally whether they were affected by changes in test specimen size and geometry in cases where there was no change in the crack tip constraint. Changes in the latter were assessed from changes in the CTOD at crack initiation (CTOD i). Only 6~ was capable of characterising the crack growth resistance, as unlike both CTOD~ and 6a, dCTOD/da and CTOA were dependent on specimen geometry.

A numerical study on the crack growth behavior of a low and a high strength steel

“…For ST37, direct CTODi measurements in [9] show that within certain ranges of geometry variations the CTODi value is quite constant. In [10] it was shown that for deeply notched bend type specimens the crack opening displacement at the growing crack tip (which is equivalent to CTOAc) measured in the central region of the specimen is unaffected by the specimen geometry. The material tested in [10] was a normalized C-Mn steel (BS 4360 43A) which has similar mechanical properties as ST37.…”

“…In [10] it was shown that for deeply notched bend type specimens the crack opening displacement at the growing crack tip (which is equivalent to CTOAc) measured in the central region of the specimen is unaffected by the specimen geometry. The material tested in [10] was a normalized C-Mn steel (BS 4360 43A) which has similar mechanical properties as ST37. In fact, the influence of the geometry variations on the local quantities such as CTODi and CTOAc, if any, may be quite different from that on the global load-displacement response of the specimens.…”

A numerical investigation on the geometry dependence of the crack growth resistance in CT specimens

“…A similar formula to calculate the J-integral of a plain sided specimen was proposed in [ 14] and [ 15] where the J-integral of the total specimen was divided into a contribution of flat fracture in the center region of the specimen and a contribution of shear fracture near the side surface. In place of Bs, two times of the shear lip width were used.…”

“…According to our model, the increment of the J-integral dJ due to an increment of the load-line displacement dVLL Can be calculated from the increments dJs and dJe of J in the plane stress part and plane strain part of the specimen with (5) dJ = ~dJ~ + (1 -~s)dJ~. (13) Similarly, due to (4) the increment of the crack extension da due to dVLL is da =/3~das + (1 -~)dae, (14) where da~ and da~ are crack extension increments of the plane stress part and the plane strain part, respectively. All d J, dJs, dJ~, da, das and dae are measured at the same load-line displacement.…”

“…All d J, dJs, dJ~, da, das and dae are measured at the same load-line displacement. Combining (13) and (14) leads to an expression of the slope of the JR-curve dg /3sdJs + (1 -fls)dJ~ da -/3sdas + (1 -/3s)dae" (15) In the beginning stage of the stable crack growth, usually, the crack grows only in the center part of the specimen. If we omit the crack extension due to the blunting of the crack tip, then das = 0 and we get dJ dJ¢ fl~ dJ~ 1 dJ da -da--~ + 1-~----~ da~ -1-13~ dae" (16) With a further increase of the load, the crack begins to grow in the surface region, too, and then da~ > O.…”