Sample 13464-17-06-1 Mechanical Testing

Initial sample mechanical testing is needed to establish an understanding of the

mechanical properties of the SiCf-SiCm composite architecture. Due to the great variety in

this material solely based upon the material. Any slight change in tubing design or material

can result in a massive change in material strengths. After initial table mounted impulse

excitation, sample 13464-17-06-01 (or simply sample 1) was prepared for mechanical

testing beginning with tension and torsion testing under low loads followed by internal

pressure testing until tubing burst.

Adapter nuts were fabricated and adhered to each end of sample 1. Two linear strain

gauges and one shear strain gauge were attached prior to tension and torsion testing. Using

the ADMET tension system mentioned previously, Sample 1 was loaded to 400 N with

axial strains recorded. Figure 3.5, below, indicates the stress-strain relationship for the

tension test. The Young's Modulus calculated based upon the recorded data was 109.7 GPa

along the axial direction of the SiCf-SiCm tube. This value comes within 2% of the average

Young's Modulus found between the Free-Free and Longitudinal vibration modes using IE

for this sample.

Figure 3.5: Stress-Strain plot of tension test performed on Initial SiCf-SiCm composite sample

13464-17-06-01 0.0 3.0 6.0 9.0 12.0 0 25 50 75 100 125 Ax ia l Str es s( MP a) Axial Strain(με)

3.8b Torsion

After sample had been mechanically tested under tension, the sample was placed in

the torsion rig to determine its Shear Modulus. To ensure no damage to the composite

specimen would the maximum shear stress that the composite tubes will be allowed to

experience during torsion tests will be 32.1 MPa. This is well within expected elastic range

for this material without causing significant material cracking. To aid in monitoring of

structural damage during mechanical tension and torsion testing, acoustic emission was

recorded. During all tests for sample 13464-17-06-1 under the previously mentioned

parameters, a negligible number of AE hits were recorded per test (less than 100 hits). As

mentioned in previous sections, the adapter nuts adhered to the end of the sample are

compatible with the torsion rig and therefore make testing more time efficient. Sample 1,

was manually loaded on the torsion rig with the load recorded by the load cell, and the

strains recorded via the Micro-II system as parametric voltages. A priority during loading

was to avoid any drastic changes to load that could potentially damage the sample or the

exposed strain gauges. The stress-strain plot for this torsion test, seen in Figure 3.6, shows

a linear correlation between the shear stress and shear strain of the composite specimen.

Figure 3.6: Shear Stress-Strain plot of sample 13464-17-06-1 torsion test

3.8c Internal Pressure

Internal pressure testing provides excellent characterization of a tubing sample by

determining the material's Modulus of Elasticity along the hoop direction of the tube.

Internal pressure testing will be performed on all three composite samples and have

recorded stress, strains, and moduli compared to other publications dealing with similar

testing. Due to the few number of available samples, it is advantageous to conduct as many

tests of a specimen as possible before completely damaging it beyond adequate testing

condition. As such, internal burst testing must be conducted last in the line of mechanical

testing. In order to perform internal pressure testing on sample 13464-17-06-1, it is required

that the adapter nuts and the sample be separated. Since heating the adapter nuts and the

sample to sufficient adhesive melting temperatures would potentially damage the integrity

of the composite tube, it was elected that the inner most section of the tube be cut away.

Sample 1 was cuts using a South Bay Technology Model 650 Low Speed Diamond Wheel 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 0 100 200 300 400 500 600 700 Sh ea r Str es s( MP a) Shear Strain(με)

Saw and 4" diamond wheel.Figure 3.7, shows the cuts made to the sample in preparation of burst testing. The cuts were made at 90o of the composite tube's axis. The length of the

center piece to be used in burst testing was 31.75 mm, which is comparable to lengths of

SiCf-SiCm composite tubes tested by Shapovalov in similar burst testing. Similarly sized

sections will be cut from the remaining two samples for burst testing after all other testing

on those samples is completed.

Figure 3.7: Cut section of sample 13464-17-06-1 used for internal burst testing

Sample 13464-17-06-1 was loaded via internal pressure and ruptured at

approximately 8.52 ksi with a Proportional Limit Stress (PLS) occurring roughly between

3.5 and 3.7 ksi. Nozawa and Shapovalov indicated that PLS for ceramic composite material

could consistently be determined when the highest Absolute Energy in AE events occurred.

[17,19] In addition, it was seen that a rapid densification of high amplitude and energy AE

events occurred shortly before or at PLS. Based upon these characteristics, PLS for SiCf-

SiCm composite sample 13464-17-06-1 was predicted. This PLS was confirmed by the

stress-strain curve for this sample. .Figure 3.8, below, outlines the predicted PLS for the

sample. After 3.5ksi, the number of AE events with Absolute Energy of 1.0E6 aJ

Figure 3.8: AE Absolute Energy and pressure found in burst test of sample 13464-17-06-1

The recorded pressure and hoop strain indicate that the tubular specimen has a

Young's Modulus of 120.4 GPa along the hoop direction of the sample. This value, at first,

was considered to be particularly low compared to other CVI produced SiCf-SiCm

composite tubing studies. Maximum UTS calculated for the external face of sample 1 was

found to be 58.7 MPa while the UTS for SiCf-SiCm composite tubes found by Jacobsen

and Shapovalov were approximately 400 and 370 MPa, respectively. [17,18] In addition,

the tubing tested using the same internal pressure method by Shapovalov burst at pressures

close to 14 ksi. [17] Both points suggest that the 6" composite tubing used in this study are

composed of a much weaker architecture. Let it be noted that mechanical properties, such

as moduli and strength, are architecture dependent. To clarify, the architectures tested in

3.9 Samples 13464-17-06-2 and 3 Pressure Loading and IE testing