A prediction method of service loading from striations on fracture surface is proposed. Although it is well known that striation spacing corresponds to the crack growth rate
da/dN, it is not possible to determine
Pmax and
Pmin under service loading only from
s. This is because the value of
s is influenced not only by Δ
K but also by the stress ratio
R (=
Pmin/
Pmax).
The new idea in this study for determination of
Pmax and
Pmin is based on the anticipation that the information on
R must be included in the relative height of striation
H to
s. The results of fatigue tests on 2017-T4 Al Alloy CT specimens showed a definite correlation between
H/s and
R. The difficulty of the measurement of
H was overcome by sectioning the specimens at an inclined angle and by measuring small distance using SEM.
Striations under
R=-1 were compressed and flat, and
H/s was small. With increasing
R,
H/s became large. Therefore, the standard laboratory test results on the relationship between
s and Δ
K, and also that between
H/s and
R enables us to predict service loading from the fatigue fracture surfaces.
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