LOW-CYCLE FATIGUE PROPERTIES OF A WHISKER-REINFORCED 2124-ALUMINUM-ALLOY

Low-cycle fatigue microcracking leading to failure of smooth specimens of a powder metallurgy (PM) 2124 aluminum alloy reinforced with 20 vol pct SiC whiskers was studied. The crack size near the onset of unstable growth was inferred to be 50 to 70 mu m in the stress amplitude range of the present study (400 to 600 MPa, R = -1) from observations of the fracture surfaces of the specimens. This corresponds to stress intensities between 1/3 to 1/2 typical values of K-1c or 1/4 to 1/9 the critical length predicted from K-1c values of 12 to 14 MPa root. The microcrack size distributions and growth data were obtained from the low-cycle fatigue specimens at various stages of fatigue, using a surface replica technique. During continued cycling, microcracks formed and were lost through linkage with other cracks. At the same time, the fraction of small cracks (<5 mu m) decreased, while that of larger cracks (>5 mu m) increased. The total number of cracks increased with increasing numbers of cycles. Typical microcrack growth rates were determined to be db/dn = (3.57 to 6.11) x 10(-10) (Delta K)(2.2 to 2.48) in the lateral direction of the crack, and da/dn = (5.83 to 13.0) X 10(-11)(Delta K)(1.54 to 1.60) in the depth direction of the crack