Correlation between failure mechanism and rupture lifetime of 2D-C/SiC under stress oxidation condition based on acoustic emission pattern recognition

Abstract

Creep tests of 2D-C/SiC in a wet oxidizing atmosphere were implemented for six samples. The loading process was monitored by acoustic emission (AE). Principal component analysis and a fuzzy clustering algorithm were used to perform pattern recognition of the AE data. All of the AE events were divided into four clusters and labelled as matrix cracking, interfacial damage, fiber breakage and fiber-bundle breakage respectively, according to their physical origin. It was found C/SiC has very scattered rupture lifetimes even under the same test conditions, and the evolution of AE events corresponding to fiber failure is quite different. With increasing rupture lifetime, the AE energy of fiber-bundle breakage is higher, while the number of these events is less. Thus, it is concluded that local oxidation and damage development is the controlling failure mechanism for short-lived specimens and uniform oxidation and damage development is the controlling failure mechanism for long-lived specimens.

Introduction

Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC) exhibit excellent mechanical properties at elevated temperature and have been developed for aeronautic and aerospace applications [1,2]. In service, C/SiC is subject to complex coupling effects between thermal, stress and chemical reactions, and its damage evolution and failure mechanisms are of wide interest [[3], [4], [5], [6]]. Generally, there is oxidation along with several kinds of microscopic fracture mechanisms involving stress oxidation of C/SiC in a wet oxidizing atmosphere, including matrix cracking, interface debonding, delamination and fiber breakage. Their evolution depends on the microscopic structure, especially defects in preparation, such as coating cracks, matrix cracks and porosities. Due to the random size and distribution of these defects, C/SiC usually shows a wide scatter in its performance, even under the same stress and environmental conditions.

In order to describe the failure process, researchers often rely on some physical information released from the internal changes in materials to describe the damage process, such as electrical resistance [[7], [8], [9]], mechanical parameters [[10], [11], [12]], thermal dissipation [13], acoustic emission (AE) [[14], [15], [16], [17], [18], [19], [20]] or their combinations [[21], [22], [23], [24]]. AE, which is a phenomenon whereby transient elastic waves are generated by strain energy release, provides a passive method of monitoring the changes within a material [25,26]. Since one micro-fracture event will generate one AE wave and various fracture mechanisms will generate AE signals with different characteristics, it is believed that AE is a powerful means to characterize the damage evolution of materials. Moreover, AE can be applied under conditions of elevated temperature using waveguides.

However, there are challenges in the application of AE in the study of the damage mechanisms of ceramic matrix composites (CMCs). One of the most important is that the complexity of the AE source itself, as well as the complex propagation and attenuation processes within the material, make it difficult to correlate AE events with microscopic fracture mechanisms. Researchers have tried to solve this problem with pattern recognition technology, mainly unsupervised cluster analysis [14,27,28]. The method has been used to analyze several kinds of CMCs and has been able to identify that AE signal grouping has a strong correlation with damage mechanisms [14,27].

In the present paper, a fuzzy C-means (FCM) algorithm was used instead of the more common K-means algorithm to improve the effectiveness of cluster analysis of AE data from creep tests of C/SiC in a wet oxidizing atmosphere. The damage mechanisms and their evolution were characterized by AE cluster analysis. Combined with the AE data and fracture surface observation, the dominant controlling failure mechanism for C/SiC with different rupture lifetimes under particular stress oxidation conditions is proposed.