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Metal Matrix Composites
Damage and Failure Behavior under Biaxial Loading
Demands on structural materials for better performance under more severe loads and environments are increasing to the point that monolithic materials cannot fully satisfy the requirements. One approach that has emerged is to develop composite materials whose properties can be tailored for the specific needs. Metal matrix composites (MMCs) are being proposed for increased use in structures that require strength at elevated temperatures, damage tolerant behavior, and high performance-to-weight ratios. The particular material system studied in this program is a SiC SCS-6 fiber/Timetal 21S MMC. This material consists of rows of silicon carbide fibers imbedded in a matrix of titanium. The microstructure of the material tested is shown in Figure 1.
Because SiC fiber/Titanium matrix composite materials are relatively new, little is known about their mechanical performance and failure behavior. The overall goal of this program is to measure and model the complex material constitutive and failure behaviors of the MMC under multiaxial loading conditions. To measure the MMC behavior, a combination of off-axis tensile tests and biaxial cruciform material characterization experiments are being performed. The off-axis tests were performed in a standard MTS testing machine. The biaxial cruciform test were peformed by adapting two independent testing machines to apply the loads as shown in Figure 2. This set of tests produces a wide range of multiaxial load ratios that produce a variety of material constitutive behaviors.
A summary of the biaxial loading applied to the cruciform specimens is shown in Figure 3. Using the applied loads as well as measured specimen strains allows the biaxial stress-strain behavior to be determined. Similarly the failure stresses and strains under biaxial loading were helpfull in the development of failure criteria for MMCs.
The approach used for the material modeling is to develop a microstructurally based constitutive and damage model for the MMC material. This approach analyzes separately the deformation processes of the fiber and matrix materials on a microscopic level. These microstructural material deformation and failure processes can then be combined to obtain a model capable of predicting the measured macroscopic material response. A comparison of the measured data and the model predictions for the off-axis tensile tests is shown in Figure 4.
The final phase of the program was to develop failure criteria for the MMC. Both simple design type failure criteria and more complex micromechanical damage models were considered. The design criteria were found to give a good comparison for a limited set of data as shown in Figure 5. However, the complexity of the failure processes over the full range of test conditions required micromechanics based failure criteria.
References
- S.W. Kirkpatrick, "Damage and Failure Behavior of Metal Matrix Composites Under Biaxial Loads," Ph.D. Thesis, Department of Mechanical Engineering, Stanford University, 1999.
For inquiries or comments, please contact:
Dr. Steven Kirkpatrick
Principal Engineer
e-mail: skirkpatrick@ara.com
This research program was sponsored by the Air Force Office of Scientific Research and performed at SRI International and Stanford University by Dr. Steven Kirkpatrick as his PhD Thesis. The Stanford University advisor for this research was Prof. Fu-Kuo Chang of the Stanford Structures and Composites Laboratory.
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Figure 1 - SEM Micrographs of the SiC SCS-6/Timetal 21S MMC Microstructure
Figure 2 - Schematic of the Biaxial Cruciform Testing Machine
Figure 3 - Summary of Biaxial Cruciform Tests
Figure 4 - Model Comparison for the Off-Axis Tensile Tests
Figure 5 - Comparison of a Quadratic Failure Criterion with Biaxial Data
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