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Cracking stability in tapered DCB test pieces

International Journal of Fracture
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Cracking stability in tapered DCB test pieces 292 CRACKING STABILITY IN TAPERED DCB TEST PIECES Y. W. Mai Department of Mechanical Engineering, 2046 East Engineering Building The University of Michigan, Ann Arbor, Michigan 48104 USA tel: 313/764-3383 With the increasing acceptance of DCB test pieces for fracture toughness determinations, many investigators are now interested in modifying the basic design to suit their own requirements. The arms are no longer parallel but contoured or for simplicity of manufacture, tapered so that a constant crack extension force R or stress intensity factor K is available under constant applied loads for a certain range of crack length. This particular design is very often employed in studies of environmental cracking and of standard fracture toughness testing of materials. However, experiments have shown that if not properly designed, the test piece may fail catastrophically under either monotonic increasing load or displacement. Further, the crack length over which K or R is invariant may be too short for experimental use- fulness; and added to this, the crack may veer out of the arms during propagation. Since relatively little work has been done on cracking stabil ity in these tapered DCB specimens, we have, in the present note, attempted to illustrate some of the results we have found for this par- ticular problem. Consider a tapered DCB specimen of constant thickness t, with equal and opposite forces X applied at a distance e from the apex. (See inset in Figure i.) A crack of length 0 < a < W was considered to spread from the apex and along the center line. By taking into account the crack end effects, we obtain the compliance expression as C 1 = (u lX) l = (2r21eEt) {(0.49 + 1.4/~ + i /~2)Zn [ i + (alW)ICe/W)] + 2(e/W)/(a/W+e/W) 1.5 (1) O.S(elW)21(a/W + elW) 2 } where r is the Srawley and Gross parameter [i]; e, the measure of the slope of the taper; and E, Young's modulus of the material. Figure 1

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