A typical pressurized thermal shock (PTS) experi­ment can be conducted by loading a thick-wall pres­sure vessel with an embedded crack by an external load and a severe thermal transient. The aim of a PTS experiment is to load the test vessel so that unstable crack initiation occurs. In connection with these large-scale PTS tests, a material test program is typically performed to produce the necessary mate­rial property data. If the thermal transient includes large temperature changes, any crack initiation after the peak temperature may be affected by the WPS effect. WPS involves the material loaded at the peak temperature during the thermal transient to a stress-intensity level that is higher than the critical fracture toughness at lower temperatures during the transient. Because the material was preloaded to a higher toughness level, the critical fracture toughness at a lower temperature of the transient is higher than without the preloading. The WPS effect is complex and is related to the crack tip stress state and is expected to be most pronounced in short transients where strain ageing will not diminish the effect.

4.14.3.2.1 PTS test by Framatome

In the 1980s, Framatome performed a thermal shock pressure test for a thick-wall pressure vessel, includ­ing a very long but shallow crack.29 The crack length (2c) in the 230-mm-thick cylinder was 1000mm and the depth (a) was 17 mm. The vessel material (A508 Cl. 3) was characterized by KIc tests conducted with large, 75 and 100-mm thick, C(T) specimens. The aim was to characterize the material directly ahead of the crack front. The data from these characterization tests have been reanalyzed using the Master Curve
method.30 The analysis was performed by making the size adjustment of the material test data to the 1000 mm crack length, corresponding to that of the vessel. After this correction, the material characteri­zation data and the crack tip load data for the test vessel for the observed first, second, and third initia­tions should fall on the same Master Curve. The comparison in Figure 24 shows that the vessel initia­tions occurred within the 5% and 95% fracture probability bounds estimated for the material, that is, two initiations fell almost on the mean Master Curve and the third close to the 95% probability level. In this case, the result indicates no WPS effect between the initiations, although some effect can be expected due to some warm prestressing during the decreasing temperature of the transient. The C(T) fracture toughness and the PTS test data generally coincide for all three initiations.