Ceramic Matrix Composites

J. Paulo Davim, J. Paulo Davim

• Editorial: De Gruyter
• Año: 2016
• ISBN:
  9783110383461
• Edición: 1
• Categoría: Technik & Maschinenbau
• Categoría: Werkstoffwissenschaft

Composite materials are engineered from two or more constituents with significantly altered physical or chemical properties within the finished structure. Due to their special mechanical and physical properties they have the potential to replace conventional materials. This book, written by experts from all over the world, presents fundamentals and recent advances on ceramic matrix composites.

Nathalie Godin, Pascal Reynaud, Mohamed R’Mili, and Gilbert Fantozzi

1Mechanical behavior of ceramic matrix composite (CMCs) and lifetime prediction by acoustic emission

1.1Introduction

Space and aeronautic applications need light and nonbrittle materials even at high temperature. For example, in airplane engines, materials have to maintain their good long-term mechanical properties under high temperature and oxidizing atmospheres even when subjected to complex mechanical loads. Nonoxide ceramicmatrix composites (CMCs) and more particularly SiC/SiC composites are very attractive candidates for many high-temperature structural applications because of their excellent creep resistance, high-temperature strength, and light weight [1–15]. Damage tolerance is achieved through the use of low shear strength fiber coatings that deflect cracks along the interfaces. Future engine applications in civil aircrafts are foreseen for such composites. These applications require very long lifetimes under in-service conditions.

CMCs are composite materials made with long ceramic fibers cloths embedded in a ceramic matrix deposited by chemical vapor infiltration [6] (CVI) (Fig. 1.1). In these composites, the fracture strain of fibers is higher than the fracture strain of the matrix. Then, when a load is applied on a composite, matrix cracks first, and fibers bridging this crack sustain the load. Due to the difference of stresses between bridging fibers and matrix at the level of the matrix crack, interface between fiber and matrix is subjected to a shear stress leading to a debonding of fibers and matrix [7]. The concept of functional multilayered matrix was therefore recently introduced in the new generations of SiCf/[Si-B-C] composites in order to improve the lifetime under intermediate and high temperatures thanks to the formation of sealant glasses [8, 9].

Various authors have studied its mechanical behavior and degradation mechanisms at high temperatures [10–13]. A lot of studies are done to understand the links between microstructure, damage, and durability of these materials [14]. Today, the challenge is to predict components service lifetime. To achieve this goal, quantification of damage as well as identification of the various damage modes are required. The acoustic emission (AE) technique [15] may be a useful method for the investigation of local damage in materials. In the case of composite materials, many mechanisms have been confirmed as AE sources including matrix cracking, fiber-matrix interface debonding, fiber fracture and delamination. The acoustic emission (AE) technique is widely used for studying the damage mechanisms in composite materials [16–20]. Modal AE seems to be a very interesting approach if wide-band sensors are used, because AE signals are less modified by wide-band sensors than by traditional resonant sensors. The frequency content of each signal waveform can then be analyzed in order to distinguish different types of events and attribute themto a damage mechanism [21].

Fig. 1.1: (a) SEM micrograph of the cross-section of SiC/SiC composites (b) Typical fracture surface of SiC/SiC composite.

Another approach consists in describing the AE signals by using some parameters such as the amplitude, duration, energy, and rise time (for example) of each signal. Statistical multiparameter analysis can then be performed to classify the data by using classification algorithms or neural networks [22–26].

The main purpose of this chapter is to consider the possibility of predicting the fracture time of CMC from damage evolution recorded by AE technique. Two kinds of analysis based on acoustic emission recorded during mechanical tests are investigated.

In the first analysis, based on individual AE signals analysis, acoustic signature of each damage mechanism is characterized. So with a clustering method, AE signals having similar shapes or similar features can be grouped together into a cluster. Afterwards, each cluster can be linked with a main damage [27–29].

In this way, a careful analysis of acoustic emission signals can lead to the discrimination of the different damage mechanisms occurring in a composite material. It is a possible solution for identification of damage during service with a view to component lifetime control.

The second analysis is based on a global AE analysis, on the investigation of liberated energy, with a view to identify a critical point. In 1962, pioneering experiments on rocks were carried out by Mogi [30]. Acoustic emissions associated with microcracks were monitored, and power law frequency-magnitude statistics were observed. Many researchers investigated the elastic energy release during the failure process of materials [31–39]. Smith and Phoenix [31], Curtin [32], and Newman and Phoenix [33] studied the critical point hypothesis (CPH) using the fiber model. Turcotte et al. [34] and Ben-Zion and Lyaldaovsky [35] also performed analogous investigations on CPH. Johansen and Sornette [36] and Guarino [37] also found similar results on the acoustic emission release prior to failure of composites. They all observed that the energy release accelerated in the form of a power law. The objective of the second approach is to propose amethod based on acoustic energy in order to evaluate the remaining lifetime during long-term-mechanical tests. This approach is based on the determination of energy released and identification of a critical point in energy release during mechanical test. Thus, beyond this characteristic point the criticality can be modeled with a power-law in order to evaluate time to failure. These models are significant to avalanche behavior very similar to that observed in seismicity.

1.2Acoustic emission: Analysis and methodology

1.2.1Location of the AE signal

To perform a uniaxial tensile test, AE is usually monitored by using a two-channel acquisition system. The resonant sensors were attached on the specimen inside the grips for the tests at intermediate temperature. Medium viscosity vacuum grease was used as a coupling agent. Two heat-resistant steel wave guides with a length of 140 mm and a diameter of 8 mm were put onto the specimen for the tests at high temperature above 700°C.

The location of sources has been calculated using the difference in times of arrival on each sensor. Only the signals coming from the working length of the specimens are analysed.

The AE wave velocity was calibrated before the test Ce0, according to a pencil lead break procedure: several breaks were performed on the specimen at several locations x between the two sensors. The difference in time of arrival Δt(x) between the two sensors was calculated by using the first peak of each signal. The velocity Ce(ε) of an extensional wave in a thin plate is proportional to the square root of the elastic modulus E of the material...