INDICATORS FOR THE DAMAGE EVOLUTION AT INTERMEDIATE TEMPERATURE UNDER AIR OF A SIC/[SI-B-C] COMPOSITE SUBJECTED TO CYCLIC AND STATIC LOADING

Elie Racle1,2, Nathalie Godin1, Pascal Reynaud1, Mohamed R'Mili1, Gilbert Fantozzi1, Lionel Marcin2, Florent Bouillon3, Myriam Kaminski4

1 MATEIS, INSA-Lyon, F-69621 Villeurbanne, France

2 SNECMA - Groupe SAFRAN, rond-point René Raveau, F- 77550 Moissy Cramayel, France

3 HERAKLES - Groupe SAFRAN, Les Cinq Chemins, F- 33185 Le Haillan, France

4 ONERA, 29 avenue de la Division Leclerc, F- 92322 Chatillon, France

ABSTRACT

The low density and the high tensile strength of Ceramic Matrix Composites (CMC) make them a good technical solution to design aeronautical structural components. To fully understand damage mechanisms and be able to design components, its behavior has to be analyzed during fatigue tests. The aim of the present study is to compare behavior of this composite under static and cyclic loading. Tests are realized under the same conditions of temperature and maximal load levels in order to determine the effects of cycles on the sequence of damage mechanisms. Hence the evolution of mechanical parameters is analyzed. Nevertheless the complexity of mechanisms and duration of tests make the use of complementary damage indicators necessary. Different approaches based on acoustic emission can be taken into consideration in order to quantify damage along the tests. In this case the analysis of acoustical energy is studied by comparing to the evolution of strain energy. This method enables to point out different damage levels during tests.

INTRODUCTION

To optimize design of parts, the mechanical behavior has to be fully understood. The aim of the study consists in analyzing and comparing material behavior under cyclic and static fatigue loadings, at the same temperature and under air, to determine effects of a cyclic loading on damage and lifetime. CMCs seems to be promising material for aeronautical applications, even if its constituent materials are brittle, the strain at failure is rather high due to considerable matrix cracking and cracks deflection at interfaces1. However, these materials behavior is affected by oxidation of interphases and fibers and the ultimate failure is governed by slow crack growth in fibers2. Self-healing material has been developed to protect fibers against oxidation, which increased largely the lifetime of material. Nevertheless, under air and for temperature above 550°C, self-healing is not significant enough to fully protect the material. This is why it is important to understand the material behavior for those temperatures.

As specified above, the lifetimes under these types of loadings are rather long which makes it hard to realize tests on laboratory equipment. This kind of studies needs to be done with a limited number of tests, thus the use of different techniques to monitor the damage in real time is mandatory. Acoustic emission (AE) appears to be a good candidate in this case. It consists of recording transient elastic waves on the material created by damage mechanisms. There are several studies referring to this kind of method for different types of CMCs under tensile tests3,4,5. For fatigue tests, damage can be analyzed from different points of view, first by linking each acoustical event to the damage mechanism which generated it6. This process needs clustering algorithms7. Another approach consists in considering the evolution of released energy8-10. It is generally accepted that the energy of an AE signal is related to the energy released by the source. Consequently, AE energy gives information about material damage; it is then possible to point out precursory elements to ultimate failure or to simulate AE energy evolution with a power law to determine lifetime.

In the current study, the global AE is analyzed and compared to the mechanical energy during cyclic and static fatigue tests. This process is based on studies realized by Minak11,12 on organic matrix composites. The goal of this process is to determine new damage markers for CMCs.

EXPERIMENTAL

The material is composed of Nicalon SiC fibers coated with PyC and a self-healing [Si-B-C]. The fibers reinforcement is composed of several layers of 2D satin fabrics linked together by strands of fibers in the third direction. In this study all the specimens have a dog-bone shape with a thickness of 4 mm and a gauge section of 60 mm × 16 mm.

Tests are realized at a temperature of 450°C. This temperature is critical for the material since SiC can be degraded by oxidation without any self-healing effect. Three different types of tests are performed: tensile tests, static fatigue tests and cyclic fatigue tests. Cyclic fatigue tests are made on hydraulic tensile test machine whereas tensile and static tensile tests are made on a pneumatic tensile machine which has been designed to allow a long constant load. Strain is measured using extensometer. In the case of static fatigue tests, in order to determine what stress level creates damage on the material because of oxidation, the imposed stress increases every Ti (time for 18% of Nf, number of cycles to failure) of 6% of tensile strength. At the same time cyclic fatigue tests are realized with imposed stress oscillating at a frequency of 2 Hz between 0 and an incremental maximum value which is increasing of 6% every 18% of Nf (Figure 1)

Figure 1. Applied stress for a. Static fatigue test b. Cyclic fatigue test

ACOUSTIC EMISSION

Acquisition System

Two piezoelectric sensors (Micro80, Mistras Group) are maintained on the specimen surface. Medium viscosity vacuum grease is used to ensure a good coupling between the specimen and sensors. Each sensor is connected to the data acquisition system (PCI2, Mistras Group) via a preamplifier with a 40 dB gain and 20-1200 kHz bandwidth (Mistras Group). For each detected signal, with an acquisition threshold of 45 dB on the pneumatic tensile machine and 55 dB on the hydraulic tensile machine, the data acquisition system records waveform descriptors such as amplitude or duration and mechanical information (stress, strain).

Location of AE sources

During tests, AE is recorded with 2 piezoelectric sensors, one on each side of the reduced section (Figure 2). The position of a detected source can be determined linearly knowing the wave velocity in the material using the formula described in eq. (1)

Figure 2. AE setup on a composite specimen

(1)

where v is the wave velocity in the material, tsens1 and tsens2 respectively the arrival times of signals generated by the same source on sensor 1 and sensor 2. The velocity is determined simulating an AE source with the Hsu-Nielsen pencil lead breakage procedure; it depends on sensor setup and the detection threshold. It is evaluated to 8500 m/s on the pneumatic setup and 7500 m/s on the hydraulic setup. The velocity also evaluates when the material is being damaged. It has been seen that v could be corrected using the evolution of the secant elastic modulus4 as shown in eq. (2)

(2)

where E(t) is the secant elastic modulus at time t and E0 the Young's Modulus of the material. The velocity is thus evaluated for each cycle during cyclic fatigue tests. Unloading/loading cycles are realized regularly for tensile and static fatigue tests to estimate evolution of secant elastic modulus. It allows to consider only events which are emitted from the reduced section, and to eliminate noise from the equipment. In the following, AE events will be considered if the distance between its source and the middle of the specimen is less than 30 mm.

Acoustic energy

Since the acoustic energy is related to material damage, cumulated acoustic energy progression during a test can be considered as an indicator of damage evolution. It is obtained by summing the energy of each event localized in the gauge section of the specimen. Nevertheless for an event, different energy values can be chosen depending on the sensor. These values also depend on the propagation on the material as seen in eq. 3.

(3.a)

(3.b)

with × defined in Figure 2, L the distance between the 2 sensors, Usource the acoustic energy released by the source, Asensi the coupling effects between sensor i and the material and B the attenuation factor. This is why for each localized source, the equivalent energy is Ue considered (eq.4)

(4)

This makes that Use is not function of the location of the source position.

To evaluate the evolution of acoustic energy, the Rae rate can be calculated using eq. 5

(5)

Where Tue. is the cumulated energy of N signals, and ?t the time between the first and the last signals of the N signals. This rate is calculated every n signals. N is chosen depending on the acoustic activity (N~1% of number of signals). It has to be high enough to get a smooth curve. n is usually chosen weaker than N but close enough to have enough accuracy (n~0.90-0.95.N).

Sentry function

Sentry function has been defined by Minak in order to compare mechanical evolution during a test to acoustical activity. Variations of the function can show different levels of damage in the composite. Sentry function is defined in eq. (6)

(6)

where Us is strain energy and Uae the...