On the analysis of delamination fractures in high-strength steels
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I.
INTRODUCTION
D E L A M I N A T I O N has been observed in a variety of structural steels. It is due, in part, to the transverse stresses developed in either necked tensile specimens or sharply notched geometries, such as Charpy impact specimens. The occurrence of delamination has been correlated with reductions in both impact energy t~'2] and the through thickness strength of rolled steel plates; t3} therefore, it is important to understand the conditions under which it occurs.
The microstructural mechanisms of delamination may involve a variety of processes, including cleavage, t4] ductile tearing along sheets of either inclusions or precipitates, t3,51or grain boundary decohesion, t61 Due to the range of possible mechanisms, no single criterion has been established to describe the conditions under which delamination occurs. In addition, the range of stress and strain states attainable in either tensile or impact specimens is too narrow to develop or critically evaluate any quantitative models. Thus, in the present study, we examined the delamination behavior of two high-strength steels (AISI 4340 and AISI O1 tool steel) over a wide range of stress states. This was accomplished by conducting tensile tests on both uniform cylindrical samples under superimposed pressures up to 690 MPa and on prenotched samples at atmospheric pressure. The influence of the yield stress of the material on the occurrence of delamination was also studied by varying the heat treatments of the two alloys. II. STRESS STATES IN C Y L I N D R I C A L TENSILE SAMPLES
Here, we briefly review the stress states in axisymmetric tensile samples. The results are subsequently F. ZOK, formerly with the Department of Materials Science and Engineering, McMaster University, is Assistant Professor, Department of Materials, University of California, Santa Barbara, CA 93106. J.D. EMBURY, Professor, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. Manuscript submitted August 1, 1989. METALLURGICAL TRANSACTIONS A
used to evaluate the role of the transverse stresses in the delamination process. The stress state in a cylindrical tensile sample prior to necking is trz=t~-p
ando'0=crr=-p
[1]
where # is the effective (or deviatoric) stress, p is the superimposed hydrostatic pressure, and trz, o'0, and trr are the principal stresses in the axial, tangential, and radial directions, respectively. Once a neck forms, there is an additional hydrostatic tension, o'r, which varies across the sample section. A similar effect occurs in prenotched tensile samples. In this case, the stress state is ~rz = # -
p + cr r
[21 cr0 = crr = - p + o"r where t7l Crr=~ln
[ 1 + 2 Ro
[3]
and where a is the minimum sample radius, R is the radius of curvature at the neck or notch, and r is the distance from the center along the plane of the neck. To evaluate ~, the average hydrostatic tension (o'r - p) is subtracted from the average axial stress (O'z - p). The result is t71 Ogz
e = (1 + ~ )
[4]
In (1 + 2-~)
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