Creep resistance tests

Polymers of this type have exceptional good values of strength, stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a tensile load of 70 MPa the creep modulus drops only from 4200 to 3(K)0 MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500 and 2500 MPa respectively. If the test temperature is raised to 150°C the creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa in 100 h. 

The acetal resins show superior creep resistance to the nylons but are inferior in this respect, to the polycarbonates. It is to be noted, however, that limitations in the load-bearing properties of the polycarbonates restrict their use in engineering applications (see Chapter 20). Another property of importance in engineering is abrasion resistance—a property that is extremely difficult to assess. Results obtained from various tests indicate that the acetal polymers are superior to most plastics and die cast aluminium, but inferior to nylon 66 (see also Section 19.3.6 and Chapter 18). 

One of the other benefits of incorporating polar monomers in the PSA is the enhancement in cohesive strength. This can be observed in the form of higher shear holding in a static shear test and/or better creep resistance of the adhesive when subject to a constant load. 

SI temperatures are given for a number of tests, including some carbon and low-alloy types for comparison, in Table 7.8. As well as the types listed in Table 7.7, a selection of creep-resisting grades is included. In addition some of the special stainless steels (see Section 3-3) are also included to demonstrate the effects of some other alloying elements. 

This test on rigid plastics indicates their ability to withstand continuous short-term compression without yielding and loosening when fastened as in insulators or other assemblies by bolts, rivets, etc. It does not indicate the creep resistance of a particular plastic for long periods of time. It is also a measure of rigidity at service temperatures and can be used as identification for procurement. Data should indicate stress level and the temperature of the test. 

Butyl rubber - This material generally had the least endurance in fatigue tests, but it may be adequate for some cardiovascular applications. Advantages include less sensitivity to stress concentrators than Pellethane, a very low permeability to fluids, a moderate creep resistance and widespread availability at low cost. Disadvantages include a relatively low fatigue resistance compared to the elastomers specifically designed for these applications. The rubber tested was not designed for medical applications and had standard rubber additives and modifiers that were cytotoxic unless the material was extracted after manufacture. 

In Figure 4.3(a) we can see a broad difference between two grades tested under a light load (2-3 MPa). The third grade tested under 8.75 MPa is probably more creep resistant. 

Even though the above work is providing a stable, non-sintering, creep-resistant anode, electrodes made with Ni are relatively high in cost. Work is in progress to determine whether a cheaper material, particularly Cu, can be substituted for Ni to lower the cost while retaining stability. A complete substitution of Cu for Ni is not feasible because Cu would exhibit more creep than Ni. It has been found that anodes made of a Cu - 50% Ni - 5% A1 alloy will provide long-term creep resistance (36). Another approach tested at IGT showed that an "IGT" stabilized Cu anode had a lower percent creep than a 10% Cr - Ni anode. Its performance was about 40 to 50 mV lower than the standard cell at 160 mA/cm. An analysis hypothesized that the polarization difference could be reduced to 32 mV at most by pore structure optimization (37). 

FTIR has shown the close similarity of most resins based on Bis-Phenol A and has helped narrow the focus of development on the curative as the principal contributor to successful formulation. For present applications oligomeric polyamide amines appear successful in meeting present criteria. However, the only objective analysis of cured resin to date exhibiting a correlation of measured value with success in creep resistance as well as adhesion is heat distortion temperature. The following presents a correlation of heat distortion temperatures and adhesion for several formulations tested. In most cases, pass/fail criteria was based on the majority of six samples tested. 

LX-08 is an extrudable, curable expl developed for use in Dautriche timing tests LX-09 is similar to the LASL expl PBX-9404, but with significantly improved thermal stability and slightly poorer physical props LX-10 is in the same energy class as LX-09 and PBX-9404, but utilizing HMX and Viton A like LX-04, and having excellent thermal characteristics. It also exhibits high creep resistance, but may be somewhat more sensitive... 

Recent tests have revealed surprisingly good fatigue and creep resistance for carbon/carbon composites. Figure 29 presents some results of torsion and flexure tests in which the fatigue properties of carbon-fiber-reinforced carbon (CFRC) 3D composites are compared with those of carbon-fiber-reinforced polymer (CFRP) 3D composites (53). 

Fig. 4.22 A comparison of the measured lifetime of NC132 (solid line) with projected lifetimes of NT154 (dashed line). Test temperature, 1300°C. The improvement of lifetime of =5 orders of magnitude is mainly a consequence of the improved creep resistance of the NT154.

Mechanical tests using bending, tensile and compressive load conditions including the determination of elastic constants of the orthotropic material were carried out under room and high temperature conditions. For creep tests in tension four testing devices were established and creep tests longer than 6,000 h were carried out with different CMC qualities. The WHIPOX CMCs show much better creep resistance compared to state-of-the-art metallic combustor materials. 

For all the temperatures tested, it is again seen that no approach to steady state is reached during the entire creep test time and increase in creep resistance is mote than two orders of magnitude with three orders of magnitude increase in time with strain rates of the order of - 4 x lO s . The experimental data corresponding to creep test temperatures 1400 °C, 1450 °C and 1500 °C (as... 

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