A Comparison of the Initiation With the Rapid Propagation of a Crack in a Mild Steel Plate

Abstract

Recent indications of a direct proportionality between plane strain fracture toughness KIc and the strain hardening exponent n provide a new basis for comparing the initiation with the propagation of a crack. A mild steel from the University of Illinois wide plate studies was employed because of its extensive crack propagation data. With this, the proportionality KIc/n was established at low temperatures. The n values were then measured at service temperatures and also at extremely high strain rates, to 6000/sec, with bar impact loaders. The additional speed range delineates a series of minimum fracture toughness levels even at temperatures above the NDT or CV15. A plot of adiabatic minima KIc(Q) as pertinent to the crack arrest temperature and isothermal KIc(T) to crack initiation permits a more quantitative interpretation of the Pellini fracture diagram.

A model for unstable shear crack propagation in pipes containing gas pressure

“…s is the static flow stress and is regarded as the average between the yield stress and ultimate tensile stress for the purposes of this paper, and F~2 psi sec. Comparisons with recent measurements [13][14][15][16][17][18][19] show that Equation (12) description of actual test data. The interpretation is complicated by twinning, which is not very prevalent in fine-grained pipe steels but may have reduced the strength levels observed by Rhode [19] and by Oxley and Stevenson [17] at the highest strain rates.…”

“…This means that ~ 1.2 x 104-1.5 x 105 sec-1 for a crack propagating at 500 fps. The flow stress, t?, of steel under these conditions is likely to be 2 x to 3 x the value measured in an ordinary tensile test [13][14][15][16][17][18][19][20], and this will influence the magnitude of the COD, as described in the next section. Following the procedure in Reference 21, the rate dependence is approximated by a simple linear relation :…”

Temperature rise in a viscoplastic material during dynamic crack growth

“…x:l (g/~°)~ } u: I(K=IE<7o) and 6~ = a j a o (2) Here, o0 is the initial yield stress, r, o = ao/E is the initial yield strain, E is the Young's modulus and K is the remote dynamic stress intensity factor. Also, rn, = v/c, will denote a normalized crack speed, where c, = (11/to) 1/2 is the elastic shear wave speed and 11 is the shear modulus of the material.…”

“…Some early experiments by Krafft and Irwin [1] and Eftis and Krafft [2] on fracture toughness display the influence of temperature. It is important to investigate this temperature rise because it may affect the material behaviour.…”

“…It is important to investigate this temperature rise because it may affect the material behaviour. Eftis and Krafft [2] observed (by using a combined rate scale) that a minimum in fracture toughness for mild steel is reached for a crack velocity that is only a small fraction of the elastic wave speed. Some early experiments by Krafft and Irwin [1] and Eftis and Krafft [2] on fracture toughness display the influence of temperature.…”