Quantification of the Warm Prestressing Effect in a Shape Welded 10 MnMoNi 5-5 Material

Abstract This work describes investigations of the warm prestressing (WPS) effect in a 10 MnMoNi 5-5 shape welded material. Three point bend tests of precracked specimens were carried out for the following load cycles: cooling and loading to fracture without prestressing (CF), warm prestressing with loading, cooling, fracturing (LCF), and loading, unloading, cooling, fracturing (LUCF). The specimens were prestressed at room temperature (RT) and at a load level corresponding to K WPS of 70–80 MPa m 1/2 , and fractured in the temperature range of −130°C to −158°C. Fracture surfaces of a number of specimens were observed in detail to measure the stretch zone width, the size and distance of cleavage origins from the blunted crack tip and to determine the character of the origins. For various loading cycles the stress distribution ahead of the crack tip and the crack tip opening displacement were numerically calculated. An increase in the apparent fracture toughness after WPS can be correlated with the geometry of the specimens, the loading cycle and, in particular, with the blunting of the original crack during prestressing. Based on the numerical results, the local fracture stress, σ f , was estimated. The local fracture stress turns out to be almost constant (2400–2500 MPa) for different specimens and load cycles. This suggests the use of σ f as a criterion to predict cleavage fracture after WPS. A statistical model is developed to predict failure loads from the calculated stress and strain fields. It is based on the Beremin concept and evaluates the incremental fracture probability, which depends not only on the maximum principal stress, but also on the equivalent plastic strain and its increment. This incremental probabilistic failure model predicts realistic fracture loads after WPS compared to the experiments, whereas the original Beremin model yields values which are too low.