Composites Creep test

Flat tensile creep specimens were machined from the blocks so that the longitudinal specimen axes were either parallel to the plane containing the majority of the long axes of the fibres for the squeeze-cast composites or parallel to the extrusion direction for powder metallurgy materials. Constant stress tensile creep tests were carried out at temperatures from 423 to 523 K. The applied stresses ranged from 10 to 200 MPa. Creep tests were performed in purified argon in tensile creep testing machines with the nominal stress maintained constant to within 0.1% up to a true strain of about 0.35. Almost all of the specimens were run to final fracture. 

Figure 14.10. Master curve for short-term creep tests on polyamideimide-carbon fiber composites, made at various temperatures. This shows the over-prediction for creep at higher temperatures, caused by physical aging having stiffened the polymer.

Specific creep tests have not been developed for composites at the coupon level. Data have been obtained using several of the standard test geometries described above, but creep is normally considered less of a problem than fatigue. The creep tests for plastics. ISO 899 [31], can be used for guidance with the standard composite specimens. 

Maharaj, G.R. and Jamison, R.D. 1993. Creep testing of a composite material human hip prosthesis. In Composite Materials for Implant Applications in the Human Body Characterization and Testing, ASTM STP 1178. R.D. Jamison and L.N. Gilbertson, Eds. Am. Soc. Testing, Materials, Philadelphia, PA, 86-97. 

Several tensile creep tests were carried out on SCS-6[o]/MoSiz-50Si3N4 composite specimens between 1273 and 1473 K in vacuum. Test durations of-1000 hr were achieved and some idea of long term durability was obtained. Specimens tested at these temperatures exhibited a short primary creep stage and an extended secondary stage. The minimum creep rates ranged from 1.0 x 10 to 2.0 x 10 at 70 MPa between 1373 and 1473 K. 

Creep rupture tests are used to measure the long-term response of a material to a continuously applied stress at a given temperature. The ideal material should be able to support significant stresses for extended periods of time without accumulated permanent strain or breakage. The SiOC-Nextel 312 BN 2-D composites were tested in limited stress... 

All the creep tests were performed under constant load to determine the creep rates, maximum strains and times to failure. Tests were performed at 566°C (1050"F) and at stresses of 55, 69, 83, and 96 MPa (8, 10, 12, and 14 ksi). Four types of specimens were tested. Composites with five and eight infiltrations were examined. Sample specimens infiltrated 5 times were given specimen designations beginning with A while those infiltrated eight times were designated with B . Specimens were tested after a 600°C oxidation exposure in flowing air for 100 hours. The results of this work are summarized in Table 6. 

SiOC-Nextel 312 BN 2D Composites (5X) Specimen A-7 Tensile Creep Test at 566C... 

Creep testing is a test to determine the strain-time dependence of a material under a constant load. This test is also used to determine the long-term behavior of the material luider combination of load and temperature. For ceramic matrix composites, when this testing is done in air, there is an added complexity of environmental exposure. A total of 12 creep tests were done on straight-sided tensile bars with two different central hole sizes (2.286 and 4.572 mm) at 1204 C. 

Extrapolation of creep test data to characterise the long term performance of the composite is best achieved by fitting a mathematical equation to the available short term test data. The creep of polymers can be modelled by a power law function identified by the linear nature of the data when plotted on a logarithmic axes. 

As high performance ceramic matrix composite systems, such as Melt Infiltrated (MI) SiC/SiC, are being considered for advanced gas turbine engine applications, the characterization of the material becomes more important. A series of tests were conducted where Pt and Ni sheathed Pt thermocouples were used to monitor temperature for short and long duration fast fracture, fatigue and creep tests. While it is known that Si forms eutectics with Pt and Ni, this was initially not considered an issue. But since Ml SiC/SiC composite achieves much of its performance from the infiltrated phase of Silicon (for high conductivity and low porosity), it was felt that further study of possible interactions of the Si phase has to be considered. 

Fig. 8.81 Relationship between time-to -failure and steady-state creep rate for the monolithic (filled square box) and composite (filled circle) materials. The downward arrows indicate tests with large accelerated creep (tests at 1350 °C). The upward arrows indicate interrupted tests [40]. With kind permission of John Wiley and Sons...

Fig. 9.36 Creep strain (b) recorded as a function of time (a) and creep rate as a function of creep strain measured during compression creep test for composites with different amounts of CNTs [19]. With kind permission of Elsevier...

A large number of tests have shown that the creep strength of CFRP is far superior to that of other fibre composites. In a creep test performed on CFRP rods under alternating load, no failure was observed in the rods up to a stress level of 70% of their short-term tensile strength after 10,000 hours [34]. In another experimental study, a creep strength of 79% of their short-term tensile strength is extrapolated for CFRP rods for a period of 50 years [35]. 

Figure 1. Scanning electron micrograph showing (a) the porous matrix within fibre bundles and (b) the fibre bundle configuration of the standard SiCf/SiC composite before creep testing.

Fig. 10.7 Variation of power law exponent for an epoxy resin used in composites with length of creep test and length of recovery test. (Data from Kiel et al (1984))...

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