Mechanical Behavior and Damage Mechanisms of an Oxidized Carbon/Carbon Composite with Acoustic Emission and Digital Image Correlation
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Théo ZUBIAURRE (FRANCE), Zoheir ABOURA (FRANCE), Jérôme FAVERGEON , Florent BOUILLON (FRANCE), Guillaume ROUSSEAU (FRANCE)Abstract :
Context
Carbon/carbon (C/C) composites have become widely used in the aerospace industry thanks to their excellent thermo-mechanical properties and low density. One of the main threats to C/C composites is their oxidation by atmospheric oxygen, resulting in the transformation of their solid carbon into gas. No protective effect arises, and weight loss is used as the oxidation rate. Understanding the mechanical behavior of these materials under the effect of oxidation is essential to improve their design. This study focuses more particularly on the damage mechanism, following static tensile loading tests, in C/C materials having undergone oxidation.
Materials and Methods
The studied material is a 2.5D needled C/C composite. The matrix is made of pyrolytic carbon and densified by a CVI (Chemical Vapor Infiltration) process and is naturally porous. The graphite-liked fibers were obtained from a PAN (polyacrylonitrile) precursor. Three oxidation temperatures (480°C, 650°C and 850°C) are studied for an iso-weight loss of 5%. A multi-physic instrumentation, including Digital Image Correlation (DIC) on both faces and sides of the samples and Acoustic Emission (AE) sensors are used to record and follow damage mechanisms.
Results
Fig.1 compares the tensile behavior of unoxidized and oxidized samples. Both of them exhibit an elastic behavior with an incremental damage progression and residual strains. A significant difference of the material’s residual properties does exist between low oxidation temperatures (480°C/650°C) and high oxidation temperature (850°C). 480°C/650°C samples show lower stiffness and strength. The larger size of their hysteresis loops and their higher residual strains reflects a more severe damage. On the contrary, the 850°C sample remains very close to the unoxidized sample, except for the Ultimate Tensile Strength (UTS) which is notably lower. DIC post-treatment based on the research of non-linearities (Fig.2a) reveals some areas of interest which are likely correlated with incipient damage sites. We post-traited AE with the Kmeans algorithm in an unsupervised clusterisation in order to identify signals clusters related to damage mechanisms. One class of signals shows a good correlation with the damage parameter d (Fig.2b) described by equation (1) :
d =1 - E_i/E_0 (1)
with E_i the secant modulus of each loop and E_0 the Young's modulus.
This class shares similar characteristics in both oxidized and unoxidized cases, which supports the existence of a similar damage mechanism. The proposed approach enabled an identification of different damage thresolds depending on the oxidation temperature. Two groups of behaviours, the 480°C/650°C and the Unoxidized/850°C, were identified with differences in damage kinetics. Thus, it is necessary to specify the oxidation temperature with the weight loss criterion to properly to properly assess the way that oxidation affects the mechanical properties of the C/C composite.
Carbon/carbon (C/C) composites have become widely used in the aerospace industry thanks to their excellent thermo-mechanical properties and low density. One of the main threats to C/C composites is their oxidation by atmospheric oxygen, resulting in the transformation of their solid carbon into gas. No protective effect arises, and weight loss is used as the oxidation rate. Understanding the mechanical behavior of these materials under the effect of oxidation is essential to improve their design. This study focuses more particularly on the damage mechanism, following static tensile loading tests, in C/C materials having undergone oxidation.
Materials and Methods
The studied material is a 2.5D needled C/C composite. The matrix is made of pyrolytic carbon and densified by a CVI (Chemical Vapor Infiltration) process and is naturally porous. The graphite-liked fibers were obtained from a PAN (polyacrylonitrile) precursor. Three oxidation temperatures (480°C, 650°C and 850°C) are studied for an iso-weight loss of 5%. A multi-physic instrumentation, including Digital Image Correlation (DIC) on both faces and sides of the samples and Acoustic Emission (AE) sensors are used to record and follow damage mechanisms.
Results
Fig.1 compares the tensile behavior of unoxidized and oxidized samples. Both of them exhibit an elastic behavior with an incremental damage progression and residual strains. A significant difference of the material’s residual properties does exist between low oxidation temperatures (480°C/650°C) and high oxidation temperature (850°C). 480°C/650°C samples show lower stiffness and strength. The larger size of their hysteresis loops and their higher residual strains reflects a more severe damage. On the contrary, the 850°C sample remains very close to the unoxidized sample, except for the Ultimate Tensile Strength (UTS) which is notably lower. DIC post-treatment based on the research of non-linearities (Fig.2a) reveals some areas of interest which are likely correlated with incipient damage sites. We post-traited AE with the Kmeans algorithm in an unsupervised clusterisation in order to identify signals clusters related to damage mechanisms. One class of signals shows a good correlation with the damage parameter d (Fig.2b) described by equation (1) :
d =1 - E_i/E_0 (1)
with E_i the secant modulus of each loop and E_0 the Young's modulus.
This class shares similar characteristics in both oxidized and unoxidized cases, which supports the existence of a similar damage mechanism. The proposed approach enabled an identification of different damage thresolds depending on the oxidation temperature. Two groups of behaviours, the 480°C/650°C and the Unoxidized/850°C, were identified with differences in damage kinetics. Thus, it is necessary to specify the oxidation temperature with the weight loss criterion to properly to properly assess the way that oxidation affects the mechanical properties of the C/C composite.