Implementation Challenges for High-Temperature Composites

ABSTRACT

Some issues that influence the implementation of ceramic (CMC) and titanium (TMC) matrix composites in high-temperature systems are addressed. Commonalities and differences are brought out by comparing and contrasting their thermomechanical characteristics, with examples. Emphasis is given to the role of design and the use of mechanism-based models of material behavior. Factors addressed include the relative significance of toughness and ductility for thermostructural applications, including their manifestation in the notch sensitivity. The detriments caused by anisotropy are examined, through discussions of mechanisms such as delamination in CMCs and transverse debonding in TMCs.

INTRODUCTION

The implementation of new high-temperature materials involves an integration of manufacturing, design, and life prediction that attains improved system performance at costs lower than those for existing systems. In turbines for propulsion and power generation, various composite materials, intermetallics, ceramics, and coatings are all vying for implementation. Each promotes specific performance benefits, mostly based on reduced weight or higher gas temperatures. The rationale for evaluating this spectrum of materials is described. Among these, only polymer matrix composites (PMCs) and ceramic thermal barrier coatings (TBCs) are in routine implementation. Technological issues restricting wide implementation are discussed.

An underlying technical theme is the relative sensitivity of many candidate materials to strain concentrators, particularly manufacturing flaws, holes, impact sites, and fretting contacts. This sensitivity is reflected in the toughness and ductility and manifest in the engineering notch performance [1-9]. The mechanisms involved are those governing inelastic strains. Such strains enable redistribution of the



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IMPLEMENTATION CHALLENGES FOR HIGH-TEMPERATURE COMPOSITES Implementation Challenges for High-Temperature Composites ABSTRACT Some issues that influence the implementation of ceramic (CMC) and titanium (TMC) matrix composites in high-temperature systems are addressed. Commonalities and differences are brought out by comparing and contrasting their thermomechanical characteristics, with examples. Emphasis is given to the role of design and the use of mechanism-based models of material behavior. Factors addressed include the relative significance of toughness and ductility for thermostructural applications, including their manifestation in the notch sensitivity. The detriments caused by anisotropy are examined, through discussions of mechanisms such as delamination in CMCs and transverse debonding in TMCs. INTRODUCTION The implementation of new high-temperature materials involves an integration of manufacturing, design, and life prediction that attains improved system performance at costs lower than those for existing systems. In turbines for propulsion and power generation, various composite materials, intermetallics, ceramics, and coatings are all vying for implementation. Each promotes specific performance benefits, mostly based on reduced weight or higher gas temperatures. The rationale for evaluating this spectrum of materials is described. Among these, only polymer matrix composites (PMCs) and ceramic thermal barrier coatings (TBCs) are in routine implementation. Technological issues restricting wide implementation are discussed. An underlying technical theme is the relative sensitivity of many candidate materials to strain concentrators, particularly manufacturing flaws, holes, impact sites, and fretting contacts. This sensitivity is reflected in the toughness and ductility and manifest in the engineering notch performance [1-9]. The mechanisms involved are those governing inelastic strains. Such strains enable redistribution of the