ON THE USE OF COUPLED PRESSURE BARS TO MEASURE THE DYNAMIC FRACTURE INITIATION AND CRACK PROPAGATION TOUGHNESS OF PRESSURE VESSEL STEELS

Une méthode expérimentale est introduite pour la détermination de la ténacité dynamique reliée aux procédés d'initiation et de propagation d'une fissure utilisant des petites éprouvettes compactes de rupture. Un couplage de barres d'Hopkinson préchargées en tension est employé dans l'expérience. Le relachement rapide de l'énergie emmagasinée dans les barres vers les éprouvettes est obtenu par l'intermédiaire de la rupture d'un échantillon fragilisé à entaille. L'initiation dynamique de la fissure est caractérisée par une vitesse d'augmentation du facteur d'intensité de contrainte KI de 3 x 106 Mpa√m s-1. La propagation dynamique de la fissure s'ensuit avec une vitesse constante de l'ouverture en fond de fissure. La conception et le développement de l'appareillage expérimental sont décrits avec l'évaluation de leur efficacité basée sur des expériences conduites sur un acier 4340 et sur un acier utilisé pour les cuves nucléaires A533B. Les essais sur l'acier A533B ont été conduits à des températures d'essai de 50 et de 64 °C au-dessus de la température de transition.An experiment method is presented for the evaluation of dynamic initiation and propagation toughness using small compact type specimens. The experiment involves coupled Hopkinson pressure bars preloaded in tension. A fast release of the energy stored in the pressure bars to the two compact specimens is obtained through the failure of a brittle notched specimen. The dynamic crack initiation event is characterized by a stress intensity rate KI of 3 x 106 Mpa√m s-1. The crack then rapidly propagates under a constant crack opening displacement rate. The design and development of the experimental apparatus is described, along with an evaluation of its effectiveness based on dynamic fracture experiments conducted on 4340 steel and A533B nuclear pressure vessel steel. The experiments with the A533B steel were performed at testing temperature of 50 and 64 °C above the nil-ductility transition temperature

Evaluation of a Threshold-Based Model of the Elevated-Temperature Fatique of Impact-Damaged γ-TiAl

Step-loading fatigue tests have been conducted on two γ-TiAl alloys with differing microstructures following quasi-static indentations intended to simulate assembly-related impact damage to low-pressure turbine blades. Fatigue tests were conducted at 600 °C using computer-controlled servohydraulic loading at a frequency of 20 Hz. Reasonably good agreement was achieved between the fatigue data and calculated fatigue strength based on the fatigue threshold and measured impact severity. In certain cases, the fatigue threshold model fails to completely describe the data. These discrepancies may be related to residual stresses, variations in crack-shape morphology, and small-crack effects. Residual stresses could not be directly measured, given the small size of the damage zones. However, a comparison of fatigue threshold approximations based on a through-thickness crack geometry and a corner-crack geometry suggests that these two models may represent the upper and lower bounds of the actual fatigue behavior. In addition, the behavior of small cracks was examined by modeling the stress-lifetime response of lightly damaged specimens of the duplex alloy. This effort indicates the need for small-crack fatigue threshold values when designing fatigue-critical γ-TiAl components