Development of impact testing procedure at elevated temperature.

摘要

The first part of this study dealt with a new hybrid experimental-numerical procedure for characterizing the dynamic fracture response of structural ceramic materials under impact loading at elevated temperature in excess of 1000{dollar}spcirc{dollar}C. The hybrid procedure involves the construction of a master curve which relates the transient crack opening displacement (COD) and the crack extension history during rapid crack propagation at room temperature. A similar master curve can be constructed to locate the instantaneous crack length from the transient COD data at elevated temperature by a proportional adjustment to accommodate for the changes in effective elastic moduli at elevated temperature. The transient COD is measured by a non-contacting laser interferometric displacement gage (LIDG) technique.; Dynamic fracture initiation toughness, K{dollar}sb{lcub}rm Id{rcub}{dollar}, values of the brittle materials used in this study were found to be higher than the static fracture toughness, K{dollar}sb{lcub}rm IC{rcub}{dollar}, at both room and elevated temperatures. The dynamic stress intensity factor (SIF) versus crack velocity relation was used to characterize the dynamic fracture response of brittle materials. The relations of the ceramics and CMC materials used in this study did not follow the {dollar}Gamma{dollar}-shaped relations observed in metals and polymers. While the trend, which is similar to the vertical stem of proposed {dollar}Gamma{dollar}-shaped relation, was observed for static loading, a propagating crack continued to propagate when the dynamic SIF is substantially lower than the static fracture toughness. Unlike the static counterparts, the crack continued to propagate at nearly its terminal velocity under impact loading for CMC materials.; The second part of this study dealt with the fracture resistance associated with stable crack growth in CMC materials. A combined experimental-numerical procedure was developed for determining the crack growth resistance, K{dollar}sb{lcub}rm R{rcub}{dollar}, versus crack extension relation at room temperature. The possible existence of a crack toughening mechanism associated with stable crack growth in ceramic matrix composites was investigated. This study showed that toughening mechanism was not activated during stable crack growth of TiB{dollar}sb2{dollar}-particulate/SiC-matrix composite under the monotonic and the cyclic loading and thus justified the use of linear elastic fracture mechanics (LEFM) for the dynamic fracture analysis of this material.