Evaluation of Tensile Properties of SiC/SiC Composites with Miniaturized Specimens

Abstract

Mechanical testing after neutron irradiation is a critical research tool for evaluating materials for fusion systems, such as silicon carbide fiber silicon carbide matrix (SiC/SiC) composites. However, single-axis tensile testing, which is required to build a fundamental database, requires large specimens. Therefore miniaturization of tensile test specimens has long been pursued as a method to reduce the irradiation volume to fit the capsule size limitation. The objective of this study is to identify specimen size effects on tensile properties of SiC/SiC composites from the viewpoints of the influences of fabric architecture and tensile loading axis, with a final goal to establish a small specimen test technique for tensile testing of the composites. The axial fiber volume fraction plays an important role in achieving good tensile properties. However the size dependent change of the axial fiber volume fraction gives specimen size effect. The composites with much fiber volume content tended to have superior tensile strength, elastic modulus and proportional limit stress. Contrarily, the tensile properties of the composites with the same axial fiber volume fraction were almost independent of the specimen size. This type of size effect is generally common in any types of architecture. The size-relevant fracture mode in off-axis tension: detachment in shorter widths vs. in-plane shear at larger widths, also gives specimen size effect on tensile properties, resulting in strict limitation of miniaturization of the tensile specimen. Finally we proposed a miniature tensile specimen for the composites.

The effect of neutron irradiation on the fiber/matrix interphase of silicon carbide composites

“…Specimen size effect is a potential concern when testing these specimens. Considering the structural minimum unit width (or thickness) of the unidirectional composites is larger than the width (or thickness) of the mono fiber bundle (<1 mm), a very minor effect of specimen size on tensile properties, which depend on the axial fiber volume fraction, is expected in the size range of concern [17]. Indeed, no significant size effect was found in this study.…”

Analysis of SiC/SiC composites for energy applications at ambient conditions

“…Furthermore, the majority of PW specimens studied in literature exceed 2 mm in gauge thickness with an average ply thickness of ~0.25 mm, 12,27 which is over twice the gauge thickness of the planar CMCs studied here (~0.89 mm). It is suspected that gauge thickness below 1 mm would increase sensitivity to tow architecture or internal porosity/gap defects, which may reduce the ultimate strain of the CMCs 27,28 . Additionally, thin CMC samples and those with surface texture are prone to an increased percentage of dimensional measurement error, which would potentially reduce ultimate failure stress.…”

“…It is suspected that gauge thickness below 1 mm would increase sensitivity to tow architecture or internal porosity/gap defects, which may reduce the ultimate strain of the CMCs. 27,28 Additionally, thin CMC samples and those with surface texture are prone to an increased percentage of dimensional measurement error, which would potentially reduce ultimate failure stress.…”

Current status and critical issues for development of SiC composites for fusion applications

“…Primarily for studying the neutron irradiation effects, small specimen test techniques (SSTT's) for room and elevated temperature tensile properties [71], in-plane shear strength [72], and transthickness tensile strength [73] have been developed in fusion SiC/SiC programs. Reliable tensile properties were obtained using specimens with typical dimensions of 40 · 4 · 2 mm 3 ; whereas the conventional tensile tests typically require >10 times larger specimen volume.…”