[en] Warm prestress (WPS) behavior is the term commonly used to describe an apparent increase in material toughness of pressure vessel steels resulting from previous loading at a higher temperature. Such load histories are of interest largely due to the fact that loss of coolant accident (LOCA) and pressurized thermal shock (PTS) related load histories are expected to result in WPS behavior. While previous experimental work has demonstrated WPS behavior, insufficient attention has been given to separating material toughness variability for the WPS effect. There also appears to be a basic lack of understanding of the mechanism by which WPS behavior occurs and as a result, there is no generally accepted model or fracture criterion for predicting WPS behavior. The objectives of this study were to develop WPS data for which the enhanced toughness due to WPS could be separated from the K/sub Ic/ variability of the virgin material and to evaluate several candidate WPS models. 33 refs., 17 figs., 5 tabs