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 = K_max - K_min), as defined in Figure 2.4.7.  The ASTM defines K_min = 0 and thus DK = K_max whenever R < 0 (R = s_min/s_max); see Section 5.1 for additional discussion.

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.