Micromechanical study of the warm-prestress (WPS) effect

The WPS effect has been shown to be beneficial in increasing the fracture toughness in the cleavage region after prestressing at elevated temperatures. This paper is concerned with experimental investigations to explain this phenomenon. CT specimens made of the steel 10 MnMoNi5-5 were broken at -120 C in prestressed, prestressed with subsequent stress relief annealing and nonprestressed condition, and an enhancement of fracture toughness by a factor of 3 to 4 times after prestressing has been estimated. By X-ray measurements a characteristic compressive residual stress field could be found ahead of the crack tip after prestressing. Microfractographic observations showed clearly that the WPS-induced fracture initiation is more inhibited, and the cleavage trigger point distance can be obtained as characteristic length parameter. The study demonstrates that an apparent increase in the lower shelf toughness due to warm prestressing can be caused by the compressive residual stress field including the damage process at the crack tip. (orig.)