Among the many factors that affect crack propagation, the
following should be taken into consideration for crack growth properties:
Material production:
·
Type of product (plate, extrusion, forging)
·
Heat treatment
·
Orientation with respect to grain direction
·
Manufacturer and batch
·
Thickness
Environmental conditions:
·
Environment
·
Temperature
·
Frequency
No attempt will be made to illustrate the effects of all these
factors with data, particularly because some factors have largely different
(and sometimes opposite) effects on different materials. Rather, some general trends will be briefly
mentioned.
Several factors pertaining to the material production affect
crack growth. The crack propagation
characteristics for a particular alloy differ for plates, extrusions, and
forgings. The latter may exhibit large
anisotropy, which may have to be considered in the growth of surface flaws and
corner cracks, which grow simultaneously in two perpendicular directions. Closely related to this are other processing
variables, particularly the heat treatment.
An alloy of nominally the same composition but produced by
different manufacturers may have quite different crack propagation properties
[Schijv & DeRijk, 1966]. This is
illustrated in Figure 5.1.8. The differences are associated with slight
variations in composition, inclusion content, heat treatment (precipitates),
and cold work. Similar variations in
crack growth occur for different batches of the same alloy produced by the same
manufacturer. Data presented in Figure 5.1.9 show that growth rates can vary with sheet
thickness [Broek, 1963; Broek, 1966; Raithby & Bibb, et al., 1961;
Donaldson & Anderson, 1960; Smith, et al., 1968].

Figure 5.1.8. Possible Variation of Crack Growth in
Materials from Different Sources [Schijve & DeRijk, 1966]

Figure 5.1.9. Example of Effect of Thickness on Crack Growth [Broek, 1963]
In view of the factors that influence crack growth properties,
predictions of crack growth should be based on material data that pertain to
the product form. Spot checks may be
necessary to account for variability in heats and/or manufacturer.
The factors pertaining to environmental conditions are
associated with the environmental circumstances. A lightly corrosive environment (humid air) gives rise to higher
crack growth rates than a dry environment [Hartman, 1965; Piper, et al., 1968;
Bradshaw & Wheeler, 1969; Dahlberg, 1965; Meyn, 1971; Meyn, 1968; Achter,
1967; Wei, 1970; Hartman & Schijve, 1970; Shih & Wei, 1974]. The effect is illustrated in Figure 5.1.10.
Although opinions differ in explaining the environmental effect, there
is concurrence that the principal factor is corrosive action, which is time and
temperature dependent. The effect of
cyclic frequency [Piper, et al., 1968; Meyn, 1971; Hartman & Schijve, 1970;
Schijve & Brock, 1961] is related to the environmental effect, with slower
cyclic frequencies usually associated with accelerated fatigue crack growth
rates.

Figure 5.1.10. Effect of Humidity on Fatigue Crack
Propagation [Hartman, 1965]
At low temperatures, the reaction kinetics are slower and the
air contains less water vapor. This may
reduce crack propagation rates in certain alloys [Broek, 1972; Tobler, et al.,
1974]. Figure
5.1.11 shows the influence of low temperature on crack growth for 7075-T6
alloy compared with growth at normal temperatures [Broek, 1972]. Temperatures higher than ambient may
increase crack growth rates [Schijve & DeRijk, 1963; Lachnaud, 1965].
In view of the effect of environment on crack growth, the data
used for life predictions should represent the effect of the expected
environment and temperature.

Figure 5.1.11. Example of Temperature Effect on Crack
Growth [Broek, 1972]