THE EFFECT OF GRAIN SIZE ON FATIGUE (THE FRACTURE OF DUCTILE METALS).
MASSACHUSETTS INST OF TECH CAMBRIDGE
Pagination or Media Count:
Fatigue at long lives was studied in materials of different grain sizes. Alpha brass, copper and aluminum were tested in tension-compression at constant stress-amplitude over a life range of 1000 to 10,000,000 cycles. Grain size, d, was varied by at least a factor of ten, and beyond 100,000 cycles had no effect on life in copper and aluminum. In brass, however, an order of magnitude decrease in d increased life by about the same amount. The stress for a given life, theta, was found to be proportional to d to the minus 12 power in brass. Cracks were examined in longitudinal sections, and the average crack depth, was proportional to d in brass. That dependence was the basis for concluding that the grain-size effect came from Stage 1 cracking. The differences in behavior between brass, and copper and aluminum, were traced to the differences in slip character of these materials. Cross-slip is easy in copper and aluminum, and a dislocation cell structure is formed in fatigue. In brass, difficult cross-slip limits the structure to transgranular bands of dislocations. The cell structure acts to mask the grain boundaries, so that the Stage 1 crack is not affected by grain size. In brass, however, crack growth is impeded near grain boundaries. Increasing grain size in brass lowers the frequency with which the crack enounters boundaries more rapid Stage 1 crack growth and shortened life follow from this. Author
- Metallurgy and Metallography
- Solid State Physics