Failures thermal

The occurrence of thermal failures in a plastic depends not only on the cyclic frequency and applied stress level but also on the thermal and damping characteristics of the material. For example, polycarbonate has very little... 

The level of mean stress also has an effect on the occurrence of thermal failures. Typically, for any particular sUess amplitude the stable temperature rise will increase as the mean stress increases. This may be to the extent that a stress amplitude which causes a stable temperature rise when the mean stress is zero, can result in a thermal runaway failure if a mean stress is superimposed. 

ASTM D-3151. Standard Test Method for Thermal Failure of Solid Electrical Insulating Materials Under Electric Stress. 

Plastics, including polymer blends, are relatively high damping and low thermal conductivity materials. Thus, repeated straining of an article leads to a temperature rise within and throughout its body. Rapid stress-strain cycling can significantly heat up the article and thereby induce thermal failure — the phenomenon is frequency-dependent. Where the thermal effect is to be minimized, much lower frequencies, of the order of a few Hz, should be employed. 

FIGURE 3.41 Typical fatigue behavior of a thermoplastic at several frequencies, f, fatigue failure T, thermal failure, O, 5.0 Hz A, 1.67 Hz , 0.5 Hz. (Adapted from Crawford, R. J. 1981. Plastics Engineering, Pergamon, London.)... 

Finally, the researchers considered the PO s contact with copper wire over a cable s 50-year lifetime, and how this contact can contribute to "metal catalyzed thermal failures " Although the real-world tendency of a cable to oxidize over the long term is difflcult to measure in testing, these research ers evaluations showed that phenolic AOs, combined with a metal deactivator or chelator (see Table 3.1, for example), are effective in providing long oxidative Induction times at high temperatures. 

The PPQ failures were characterized as gradually changing from cohesive to adhesive. This type of thermal failure also appeared in Phase I exposure and test. Surface failure analysis had shown that an incompatibility between PPQ resin and the titanium oxide layer had occurred. The PPQ resin was pulling clear from the oxide. Conclusions at this point were that the polymer was thermally stable but an unknown factor was creating an incompatibility with the anodized, prepared oxide. 

Ca-doped LaCrOs, for example. The thermal expansion coefficients of the components must be as close to one another as possible to minimize thermal stresses which could lead to cracking and thermal failure. Optical fleximetry is a valuable tool for studying these systems. Details can be found at www.expertsystemsolutions.com. 

Fig. 6. Changeover stress from mechanical to thermal failure as a fimction of specimen surface area-to-volume ratio /3 and test frequency (24). To convert MPa to psi, multiply by 145.

The cr-N behavior in tension-compression at 7 Hz of plasticized PVC containing 10 phr of dioctyl adipate exhibits several features characteristic of other polymers (141). Under conditions of high strain amplitude, high ambient temperature, and specimen cooling by natural convection, thermal failures are observed ... 

Fracture is defined as stress-biased material disintegration through the formation of new surfaces within a body. Fracture starts out as a localized event that eventually encompasses the whole object (Fig. 1). Fracture is synonymous with rupture and breakage but not with failure. The latter term is more general and also encompasses nonmechanical breakdown through heat (thermal failure) or environmental degradation (chemical attack, irradiation). 

In the case of walls formed by parts with high thermal properties (such as those containing slag or basalt aggregates) thermal failure occurs because the temperatures reached in the mortar exceed those permitted by the standard [14]. 

A mechanical failure, for example, could occur as the result of an impact between a ship and another moving vessel or a stationery object. An electrical failure could occur as the result of internal partial discharges that degraded the insulation of a ship s propulsion motor. A chemical failure could occur as the result of corrosion of poorly protected parts/components on an offshore wind turbine. Finally, a thermal failure could be the result of heat generated by current flowing in an electrical conductor, causing insulation degradation. 

It should also be noted that even if any temperature rise upon dynamic fatigue is insufficient to cause thermal failure the properties of the adhesive, and possibly the substrates, will obviously be affected. Indeed, increases in the local temperature may cause blunting of the crack tip, due to the adhesive s yield stress being lowered, and so actually improve the mechanical fatigue resistance of the joint. Thus, thermal effects during dynamic fatigue are often important but frequently complex to interpret. 

Y. C. Zhou, S. G. Long, and Y. W. Liu, Thermal failure mechanism and failure threshold of SiC particle reinforced metal matrix composites induced by laser beam, Mechanics of Materials, vol. 35, no. 10, pp. 1003—1020, 2003. 

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