Crack initiation and near-threshold surface fatigue crack propagation behavior of the iron-base superalloy A-286

  • PDF / 3,212,197 Bytes
  • 8 Pages / 613 x 788.28 pts Page_size
  • 33 Downloads / 251 Views

DOWNLOAD

REPORT


I.

INTRODUCTION

I T has become increasingly clear in recent years that design methods are not yet adequate for fatigue regimes where the initiation and propagation of small cracks occur. These are either physically short cracks with regard to microstructural features or cracks which are smaller than the plastic zone size and are thus not of a valid size to apply linear elastic fracture mechanics.1 The observation that surface cracks behave differently, and in particular that they can grow at a rate faster than equivalent long cracks at the same AK, has led to a great interest in the nearthreshold fatigue crack propagation behavior of such cracks.l More specifically, such anomalous surface crack behavior brings into doubt conventional life predictions such as those which begin with the.Paris law, 1 da/dN = C(AK) r"

Ill

and then integrate to obtain a C(AK) m - N s - N c

with

AK=I(a)

[2]

where a = crack length, AK -- stress intensity factor range, N = number of cycles, the subscripts i and f indicate initiation and failure, and c and m are constants. These predictions become nonconservative for most microcracks because at submillimeter crack size, growth is often an order of magnitude or more faster than the extrapolated value inferred from the Paris law, which implies for a log-log plot a continuing linear relationship between crack growth rate and stress intensity factor range down to threshold values of AK, called AKth. Recent studies have shown that small surface cracks can not only propagate at an accelerated rate in the nearM.A. DAEUBLER, formerly with Carnegie Mellon University, is at MTU-Munich, Postfach 500640, 8000 Miinchen 50, West Germany. A.W. THOMPSON is Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. I.M. BERNSTEIN is Provost, Illinois Institute of Technology, Chicago, IL 60616. Manuscript submitted September 19, 1986. METALLURGICALTRANSACTIONS A

threshold regime compared to through cracks but that such surface cracks can also show an intermediate minimum or dip in crack growth rate.1 Lankford 2 has suggested that such crack retardation can be correlated with crack lengths equaling the grain size, while other authors 3 have suggested that the minimum propagation rates do not occur for crack depths on the order of the grain size. Surface crack studies by Brown and Taylor4 on fine grained, T-L orientated, Ti-6A1-4V showed a drop in crack growth rate at a AK of 4 ksi~ln. While a possible grain size dependence was not discussed, their crack depth at the minimum rate was about 30/zm, much greater than the grain size. Their result remains ambiguous, however, because they observed similar crystallographic orientations for neighboring or-grains, which could act to increase the effective slip length. Other factors such as surface cyclic hardening 3 or residual stresses s have also been put forward as possible explanations for this drop in crack growth rate. It appears that the role of grain size and grain boundaries in the exten

Data Loading...