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In precrack Charpy slow-bend and impact tests of D6aC high-strength steel at 300 to 500 F, there was a reduction in slow-bend fracture energy as the test temperature was increased; whereas, there was an increase in impact fracture energy over the same temperature range. Thus, in high-strength steels, it is sometimes found that the precrack Charpy slow-bend and impact test results are approximately the same at room temperature, but attain a significant spread at moderately elevated temperature. In 6Al-4V titanium, the difference between slow-bend and impact was most pronounced at room temperature, again with the lowest fracture energy in slow bend; whereas, at 300 F, the slow-bend test indicated the material to be as tough or tougher than impact. In all-beta 3Al-13V-11Cr titanium in the solution-treated condition tested at room temperature, the slow-bend test result showed markedly higher toughness than impact, but, after aging, the slow-bend test result was appreciably lower than the impact value.
Differences in fracture appearance attending slow versus impact rates of loading confirm the above observations. The lower toughness in slow bending is believed to be the result of a combination of factors, including an adiabatic temperature rise in the plastic zone on impact, and a time-dependent-embrittlement mechanism occurring in slow bending. In a material with a sharply defined transition curve at or around room temperature, it is easy to see how an adiabatic temperature rise in the plastic zone ahead of the crack could markedly increase the resistance to crack propagation. At the same time, it is well known that embrittlement involving a diffusion mechanism is not detected by impact testing; for example, hydrogen embrittlement and strain aging. In 6Al-4V titanium, the difference between slow-bend and impact at room temperature is consistent with a hydrogen-embrittlement mechanism; namely, maximum embrittlement around room temperature and a decreasing embrittlement at higher (and lower) temperatures. In D6aC steel, on the other hand, the loss of toughness in slow-bend occurs at moderately elevated temperature, which is consistent with spontaneous strain aging as a time-dependent embrittling mechanism.
From these findings, it appears that the concept of strain-rate sensitivity should not be limited to the traditional interpretation that impact is the most severe test of a material. In a number of common structural metals, the situation is reversed; that is, the materials are in fact more “sensitive” to slow rates of straining.
impact tests, dynamic tests, steels, low-strain-rate embrittlement, hydrogen embrittlement, spontaneous strain aging, evaluation, tests
Associate scientistPersonal member, Aerojet-General Corp.ASTM, Sacramento, Calif.