This report was prepared as a description of work sponsored by an agency of the United States Government. Fiber volume fraction was also found to have a significant effect on the creep behavior of CMC.
Introduction and Objective
Background on Melt Infiltrated SiC-SiC Composites
Mechanics
- Ultimate Tensile Strength
- Proportional Limit
- Strain at UTS
- Modulus
As expected, the UTS scaled linearly with the fiber volume fraction in the test direction. Assuming that the effective modulus of the fiber in the transverse direction is zero, that the modulus of the transverse composite layer is 9.
Ply-Level Behavior
Laminate theory predicts that the modulus of a composite with cross-strands in the 0 or 90 directions will follow a rule of mixtures based on the moduli of the 0 and 90 plies and the ratio of 0 to 90 plies in the composite.
Creep Rupture
There are many studies in the literature on the creep behavior of siliconized silicon carbide.31-36 This material consists of interconnected networks of silicon and silicon carbide. The creep exponent n of siliconized silicon carbide has been shown to increase with applied stress.
Overview
Design of Experiments
- Room-Temperature Bulk Mechanical Properties
- Effect of Ply Thickness and Stacking Sequence
- Ply-Level Behavior
- Testing at 1315 C (2400°F)
Unidirectional composites were also tested to obtain basic values of the layer-level elastic bending properties. To allow a comparison between the flexural and tensile properties of the CMC, two samples from each panel were also tested in tension.
Sample Fabrication
For each fiber volume fraction, six samples were tested for failure at different stress levels.
Test Methods
- In-Plane Tension
- Interlaminar Shear
- Room-Temperature Interlaminar Tension
- Residual Stress
- Measurement of the Coefficient of Thermal Expansion
- Flexure Test Method
- In situ Cracking
- Creep Rupture 1315°C (2400°F)
- Post-Test Characterization
The ILS was calculated by dividing the failure load by the mid-plane area of the composite between the two points. The ends of the two pins were then captured and the CMC pulled to failure.
Microstructures for Various Fiber Volume Fractions
Room-Temperature Tension
Ultimate Tensile Strength
In an ideal CMC, above the proportional limit the matrix is cracked, and the cracks are bridged by the fibers. When a crack extends the entire cross-section of the specimen, the fibers carry the entire load, and the ultimate tensile strength is simply the. As predicted, the UTS of all the samples in this study scaled linearly with the directional fiber volume fraction, and the balanced and unidirectional samples followed a common trend line, as shown in Figure 5 .
The strength reported by the vendor of the fiber quantity used in this study is 2.6 to 2.8 GPa. The agreement between the fiber strength in CMC and the fiber strength supplied by the vendor should be treated as a coincidence, since the gauge length and the number of filaments under load are different in CMC compared to fiber tensile tests and the statistical parameters of equation 1 is not taken into account.
Modulus
The slope of this line is the fiber strength realized in the CMC, which in this case is 2.6 GPA or 373 ksi. Using these values for Em and Ecf tends to slightly overpredict the measured modulus. The effect of fiber volume fraction on modulus is much greater for the balanced architecture samples, as shown in Figure 6 .
The modulus of the balanced 0-90 composites can be calculated by a simple rule of mixtures using the modulus of the 0 and 90 plies. The moduli of the balanced composites from DOE I were calculated by combining the results of Equations 6 and 13.
Failure Strain
The fiber is elastic in nature, and an understanding of the stress in the fiber can be obtained from models that address its stress. For the purposes of this discussion, the stress in the fiber bridging a crack will be assumed to be equal to the applied stress divided by the fiber volume fraction. As the matrix crack spacing, Z, decreases, the slip fields of neighboring cracks will overlap, the fiber will not return to the far-field stress/strain state, and the bulk strain in the composite will increase.
In this example, the stress in the fiber at the crack equals the UTS of the fiber and the composite fails. As the load is further increased, the far-field stress in the matrix will continue to increase until either another matrix crack forms or the stress in the bridge fibers exceeds the fiber UTS.
Measurement of τ and Γ m
Proportional Limit
The resulting specimens had fiber volume fractions in the test direction similar to that of the balanced architecture specimens. Chemical reactions and phase changes that occur at temperature, as well as CTE mismatches between different components, can result in residual stress in the compound at room temperature. The residual stress in HiPerComp® CMC measured on three samples using the ply stripping technique ranged from 7 to 28 MPa (1-4 ksi) of compressive residual stress acting on the matrix relative to the fiber.
The modulus of the (0-M-0-M)s sample is higher than the balanced sample because the matrix modulus is approximately twice that of the 90-layer. The PL of the unidirectional sample is also higher because the matrix is stronger than the 90 layer.
Tensile Ply Level Properties
For the balanced samples at PL, the voltage in the 0 layers is much smaller than the PL voltage of the unidirectional 0 material. By extension, the stress in the 90 layers at PL can therefore be interpreted as the cracking strength of the 90 layers. This result validates the expectation that the 90 layers crack first in the cross-layer laminates, and therefore the 90 layers control the PL of the composite material.
As shown in Table VII, the strength of the 90 inserts decreases as Vf increases. It follows that the modulus and strength of the 90 insert will decrease when these matrix bands are removed.
Effect of 90 Ply Thickness and Stacking Sequence on Proportional Limit
The fiber bands in the intermediate pattern tend to be tight, but there are still areas where 90 fibers span the entire width of the 90 ply. These microstructures show that the 90-layer thickness of the thick sample is about 2 times that of the thin sample. However, as shown in Figure 17, the results did not show a large effect of layer thickness on PL.
For example, microstructurally, two adjacent 90 layers will look very similar to a single 90 layer with twice the nominal layer thickness. Therefore, the assumption of a pre-existing fault spanning the width of the 90 layer is probably not true.
Interlaminar Properties
This result, together with the results from DOE III, which also showed no predicted effect of inlay thickness, demonstrates that the assumption of pre-existing defects spanning the inlay width of 90 is not valid in the HiPerComp® prepreg MI SIC-SiC system. In this system, the fiber and the matrix are based on silicon carbide, so they have well-matched thermal properties. As discussed above, the residual stress present compresses the die and the 90 ply somewhat, making it unlikely that the die will have microcracks after processing.
In the case of glass matrix composites for which Equation 3 is validated, due to the CTE mismatch, the matrix is in residual tension relative to the fiber, making it more likely that the matrix will microcrack after processing. Matrix cracking of cross-layered HiPerComp® CMC should still be based on the presence of flaws and is proposed to be treated as a monolithic ceramic using Weibull statistics.
Ply-Level Behavior
The stacking sequence of the cross-layer composites had a dramatic effect on the bending stress-strain response. The flexural modulus was calculated for each sample based on the slope of the linear portion of the curve. As predicted, the measured flexural modulus of the cross-layer samples varied by a factor of ∼2 based on the stacking sequence.
Using m=15 and equation 17, we expect that the flexural strength should be 90 of the insert. As indicated in Table IX, the strength of the 90 inserts in the cross-insert tensile tests averaged 69 MPa ksi (10 ksi).
In situ Cracking
An important caveat to this technique is that only one face of the sample is visible for observation. Given that the samples are 10 mm (0.4”) wide, it is possible that other cracks may be present in the sample before the cracks are observed at the polished edge. However, it is likely that any cracks will propagate more quickly across the thickness of the sample (in the plane of highest stress) than through the thickness of the sample.
The box labeled area A is the area of the composite under the loading pin shown earlier in Figure 31. Optical micrographs (0-0-90) of the specimen before (top) and after (bottom) failure in three-point bending.
Coefficient of Thermal Expansion
Elevated Temperature
- Tensile Tests
- Creep-Rupture Results
- Post-Rupture Microstructures
- Creep Rupture Normalized for Fiber Loading
This tertiary creep is more evident in high Vf samples, but is also present in medium Vf samples. The first, group A, is for cracks in the matrix-rich layer on the CMC surface. The result of this is that for a given stress, 90-layer cracks in a high Vf material will not extend as far into the adjacent 0 inlay as in a medium Vf material.
In addition, as shown in the previous section, all the specimens in this study cracked during creep exposure. This result illustrates how the creep rate of the composite can be adjusted by changing the fiber volume fraction in the test direction.
Conclusions
Assuming that the entire load is carried by the fibers, the composite creep rate as a function of the fiber tension agrees with published data30 for the creep rate of the raw fiber. Post-fracture analysis revealed cracks in the 90 layers for all the creep-fracture specimens in this study. Some of these cracks are suspected to be creep-induced cavitation, where the fibers running in the transverse direction have separated from the matrix in the radial direction on the outside of the fiber coating.
Suggestions for Future Work
Morscher, “Stress-dependent matrix cracking in 2D woven SiC fiber-reinforced melt-infiltrated SiC matrix composites,” Compos. Takeda, “Modeling of brittle and ductile stress strain behavior in unidirectional ceramic matrix composites,” Acta Mater. Smith, “Matrix Cracking in Unidirectional Ceramic Matrix Composites under Quasi-Static and Cyclic Loading,” Acta Metall.
Pujar, “Design guidelines for in-plane mechanical properties of SiC fiber reinforced melt-infiltrated SiC composites,” Int. Percevault, “Probabilistic-statistical approach to matrix damage and stress strain behavior of 2-D woven SiC/SiC ceramic matrix.
