Section 3.2.1. Description of Equivalent Initial Quality Method
The Equivalent Initial Quality Method is presented for fastener
holes since this is the most prevalent source of cracking in aircraft
structures [Rudd & Gray, 1978].
Quality may be defined as a measure
of the condition of the structure relative to imperfections, flaws, defects, or
discrepancies that are either inherent in the material or introduced
during manufacturing of the structure.
The approach is to quantify these
imperfections by representing them with fatigue cracks of a particular size and shape, such as the corner cracks
illustrated in Figure 3.2.1. Also illustrated in Figure
3.2.1 are some of the parameters that can contribute to the initial
quality of fastener holes. If an
initial quality representation is performed for each of a number of fastener
holes, an equivalent initial quality statistical distribution can be used to
quantify the quality of the fastener holes produced by certain manufacturing
and processing procedures [Potter, 1978].
Figure 3.2.1. Parameters that Affect Fastener-Hole Initial Quality [Rudd &
The initial quality representation, defined as the equivalent
initial quality, can be obtained in the following manner. Consider a piece of structure with a
fastener hole containing the defect of characteristic dimension l (Figure 3.2.2).
This defect results in fatigue crack initiation and propagation when
subjected to some known load history.
Upon failure of the structure, a fractographic examination of the
fracture surface is performed to obtain as much of the crack growth curve as
possible. Analytical crack propagation
analyses are performed until there is good agreement between the analytical
prediction and the fractographic test data.
The initial crack length (crack length when the load history is first
applied), ai, of the
analytical crack growth curve that correlates best with the fractographic test
data is defined as the equivalent initial quality. Hence, ai is
said to be the analytical equivalent of the actual defect of characteristic
dimension, l, if each results in a crack size ae after Ne
cycles of the same load history have been applied. Hence, fastener holes that contain actual crack lengths less than
ae after Ne cycles have been applied
are of better quality than those that contain actual crack lengths equal to or
greater than ae after Ne cycles.
Figure 3.2.2. Definition of Equivalent Initial Quality [Rudd & Gray, 1978]