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Stress Concentration Factors for Finite Orthotropic Laminates With a Pin Loaded Hole

INTRODUCTION

The stress concentrations at fastener holes are widely recognized as having
a degrading effect on structural fatigue life. In conventional metallic struc—
tures, fatigue cracks commonly initiate at these stress-concentration sites.
In recognition of this, the effects of stress concentrations on crack initiation
and growth have been incorporated into current methods for predicting fatigue
life. However, in composite structures, the stress concentration effects are
much more complicated and are not well understood. For example, holes can pro-
duce either composite delamination, matrix crazing, fiber failures, or various
combinations of these. Furthermore, for a pin—loaded hole, these different
types of damage can produce three different modes of failure: net—tension,
shear—out, or bearing. Before analytical models can be developed for these
various failures, the stress concentrations at loaded holes must be better
understood. To date, the few stress analyses that have dealt with this problem
have been limited to rather specific cases. The purpose of the present paper
is to present some of the needed background on stresses at loaded holes in
composite laminates.
This study was based on a two—dimensional finite—element analysis of
stresses in an orthotropic laminate that is loaded by a frictionless steel pin
in a circular hole of the same diametero six graphite/epoxy laminates were
analyzed: quasi—isotropic [OD/:450/90015, 00, 900, [GO/90°15, [i450]5, and
[OD/:4SOis. Finite—element models were developed for wide ranges of width~to—
diameter ratio w/d and edge—distance—to—diameter ratio e/d. First, each
laminate was analyzed for different values of w/d using one large value of
e/d. Next, a range of e/d was used with one large w/d valueo

The results are presented in two ways: first as stress distributions near
the hole and then as stress—concentration factors for ranges of w/d and e/do
Results for the orthotropic laminates were compared with the quasi—isotropic
laminate which served as a reference case. Similarly, the w/d and e/d
effects were evaluated relative to an infinite—laminate reference case.