STP677

    Evaluation of Temperature Effects on Crack Growth in Aluminum Sheet Material

    Published: Jan 1979


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    Abstract

    Tests were conducted initially on 0.305-m (12-in.) wide center-cracked panels at room temperature to evaluate the effect of six different buckling guide configurations and three crack opening displacement (COD) measuring systems on the COD behavior of the panels. The final test configuration was selected and a series of fracture tests conducted on 1.9-mm (0.075-in.) 2024-T3 clad sheet, 1.9-mm (0.075 in.) 7475-T761 clad sheet, 4.8-mm (0.19-in.) 7075-T76 bare sheet. Full R-curves and fatigue crack propagation curves were developed for each material over a temperature range 22 to -54°C (72 to -65°F). The results show a marked drop in the apparent fracture toughness of both the 7000 series alloys with decreasing temperature while the 2024-T3 shows no apparent degradation. The entire R-curves are found to shift to lower K levels at a given Δc as the temperature decreased. Fractographic results show a change in the fracture mode of the 7000 series alloys from ductile rupture to a “quasi-cleavage” mixed with ductile rupture at temperatures of -7°C (20°F) and lower. Metallographic results indicated the “quasi-cleavage” features were due to an increased tendency for grain boundary cracking and generally reduced plastic deformation capability of the maxtrix at lower temperatures. An increase in the fatigue crack propagation rate also was observed at low temperature at stress intensity levels above Δk ∼ 22 MPam12 (20 ksi in). The R-curve and fatigue crack propagation results are discussed in terms of the fracture mode over the temperature range.

    Keywords:

    aluminum alloys, temperature, fracture toughness, R-curve, fatigue (materials), crack propagation, fracture mode


    Author Information:

    Pettit, DE
    Research scientist senior and research scientist, Rye Canyon Research Laboratory, Lockheed-California Company, Valencia, Calif.

    Van Orden, JM
    Research scientist senior and research scientist, Rye Canyon Research Laboratory, Lockheed-California Company, Valencia, Calif.


    Paper ID: STP34909S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP34909S


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