Section 2.4.3. Material Properties
The material properties enter the damage integration package in
the form of constant amplitude crack growth rate data. Crack growth data are generated in the
laboratory under constant cyclic loading on simple specimens with accepted
characterizing stress intensity factors.
Crack growth rate data are developed and correlated on the basis of
growth rate (da/dN) as a function of
stress intensity factor range, DK, (DK = Kmax
- Kmin), as defined in Figure
2.4.7. The ASTM defines Kmin = 0 and thus DK = Kmax
whenever R < 0 (R = smin/smax); see Section 5.1 for additional
Figure 2.4.7. Stress-Intensity Factors – Cyclic Loading
For a given DK,
the crack growth rate increases with increasing stress ratio, R for R > 0. Hence, the
constant amplitude crack growth rate properties for a given material or alloy
consist of a family of curves as illustrated in Figure 2.4.8. The crack mechanics approach described in Section
2.2.1 considers that for a given DK,
R combination, there is a da/dN that is independent of
geometry. Thus, the damage integration
package has available a growth rate for each DK
determined for the given crack configuration and loading.
Figure 2.4.8. Constant Amplitude Crack Growth Rate Data for 7075-T6 Aluminum.
When necessary, thermal or chemical environment and time
(frequency of loading) effects are also included in the crack growth rate data
generated for use with the damage integration package.
Section 7 presents a summary of the currently available
procedures and techniques which are used to establish crack growth rate data.