INVESTIGATION OF THE ROLE OF RESIDUAL-STRESSES IN THE WARM PRESTRESS (WPS) EFFECT .1. EXPERIMENTAL

The role of residual stresses in the warm prestress (WPS) effect has b een investigated. Three types of specimen have been tested in this inv estigation: smooth uniaxial tensile specimens, blunt notched single ed ge notched bend (SENB) specimens and sharply precracked SENB specimens . Room temperature prestraining of uniaxial tensile specimens leads to a dramatic decrease in the measured nominal fracture stress at -196 d egrees C. Such an embrittling effect may be expected to reduce the ben eficial increase in subsequent fracture toughness commonly observed in WPS sequences. The blunt-notched specimens were prestressed in tensio n and compression. Compressive prestressing was found to lead to a dec rease in subsequent fracture load whereas tensile prestressing leads t o an increase. The load decrease following a compressive WPS was great er than the load increase following a tensile WPS. Various sequences o f loading, unloading and cooling have been investigated and the differ ences in the subsequent fracture behaviour of specimens have been expl ained qualitatively by superposition arguments. The theories of Chell and Curry have been supported by the general trend of results.