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Handbook for Damage Tolerant Design

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Section 5.1.3. Factors Affecting Crack Growth

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]