Doctoral Dissertations

Date of Award

8-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Peter K. Liaw

Committee Members

Carl McHargue, Lance Snead, John Landes, Thomas Meek

Abstract

Fabric orientation effects on the monotonic and fatigue behavior of two commercially available continuous fiber-reinforced ceramic-matrix composites (CFCCs) were investigated by performing flexure tests at room temperature in air, and at 1,000°C in an argon environment. The two CFCCs used in the study were: (i) a Nicalon woven-fabric reinforced alumina (Al2O3) matrix composite fabricated by the directed metal oxidation (DIMOX) process, and (ii) a Nicalon woven-fabric reinforced silicon carbide (SiC) matrix composite fabricated by an isothermal chemical vapor infiltration (ICVI) process.

Specimens of square cross-section (3 mm x 3 mm of Nicalon/Al2O3 and 2 mm x 2 mm of Nicalon/SiC) were subjected to four-point bending loads to perform the monotonic and fatigue tests at room and elevated temperatures. The specimen configurations were designated as edge-on and transverse, depending on whether the load was applied parallel or perpendicular to the fabric plies, respectively.

The monotonic and fatigue behavior of the Nicalon/Al2O3 composite was remarkably affected by the fabric orientation at room and elevated temperatures. The ultimate flexural strength (UFS) was significantly higher in the edge-on orientation, as compared to that in the transverse orientation, at RT and 1,000°C. Also, the stress at which the samples survived one million load cycles was higher in the edge-on orientation, relative to that in the transverse orientation, particularly at RT. Under monotonic and fatigue loadings, the samples tested in the edge-on orientation failed by specimen severance into two pieces, while the transversely oriented samples failed by specimen collapse. Due to the interlaminar weakness of the material, delamination cracks propagated in the transversely oriented samples, and the specimens failed by a complex combination of tensile, compressive and shear stresses. In contrast, the specimens tested in the edge-on orientation failed in a predominantly tensile mode.

The fabric orientation did not noticeably affect the monotonic and fatigue behavior of the Nicalon/SiC composite, either at RT in air, or at 1,000°C in the argon environment. The UFS was comparable in the edge-on and transverse orientations, at both temperatures. Under fatigue loading, the specimens survived one million load cycles at stress levels as high as 80% of the UFS, in both orientations, at room and elevated temperatures. The interlaminar pores in the material appeared to have acted as stress concentration sites, and the cracks initiated from these sites linked up the interlaminar pores, resulting in specimen severance under monotonic and fatigue loadings, in both edge-on and transverse orientations.

While the test temperature did not influence the flexural stress-strain behavior of the Nicalon/Al2O3 composite in either edge-on or transverse orientations, there was a significant degradation in the fatigue (S-N) life of the material at 1,000°C, particularly in the edge-on orientation. The fiber-strength degradation contributed, at least in part, to the decrease in the fatigue performance of the material at the high temperature.

The flexural stress-strain or fatigue (S-N) behavior of the Nicalon/SiC composite was not noticeably affected by the test temperature. The UFS values in the two orientations, and at the two testing temperatures, were comparable. Although there was some degradation in the strength of the fibers due to the extended exposure to 1,000°C in the argon environment during the fatigue tests, the fatigue life of the material was not noticeably affected by the fiber-strength degradation.

The flexural behavior of a multi-layered composite was modeled using analytical and numerical (finite element) techniques. The results of the two models showed that, when a composite specimen with interlaminar weakness (i.e., low interlaminar shear strength) is subjected to flexure loads, there can be a significant increase in the normal-stress and midspan-deflection values, resulting in specimen failure occurring at lower loads in the transverse orientation than in the edge-on orientation.

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