Thermally induced failures

Where overheating effects develop, they may be of either a long- or short-term nature and may result in thermally induced corrosion or metal failure. 

Thermal fatigue is a form of metal failure whereby fracturing occurs under repeated cycles of thermally induced stress. These cycles make take place, for example, because of stop-start, lead-lag, or peak-load boiler firing arrangements. 

Fatigue corrosion occurring as Thermal fatigue cracking (thermal effect corrosion Corrosion fatigue Cycles of thermally induced stress leads to metal failure. Results from a combination of thermal cycling stress and SCC or other corrosion process. 

A form of metal failure, whereby fracturing occurs under repeated cycles of thermally-induced stress. 

Smith and Brown have pointed out, the interpretation of many of the decompositions in acidic media are subject to question due to the failure to differentiate clearly between acid-catalysed and thermally-induced reactions. Since it is known that thermal decomposition of phenyl azides occurs at appreciable rates at temperatures in the range of 140-170°, it is probable that the reactions investigated by... 

The performance of thermally loaded components (in the absence of inertial loading) is dictated by thermally induced strain e-i-= oAT, where a is the thermal expansion coefficient, and AT is the temperature difference between adjacent regions of the component. The relationship between CMC failure strain (Sf) and thermally induced strain (e-j-) can be used as a metric to rank materials for preliminary designs within regions subject to high thermal flux ... 

This relation is well suited for device-related functional failures however, for thermally induced structural failures it has to be appUed with care since these failures depend both on localized temperature as well as the history of fabrication and assembly, thermal cychng for test purposes (thermal shock testing), and powering up during operation (operational thermal stress), thus being more complicated in nature. 

Thermally induced stress Rapid variations in temperature in pressure parts, uneven heating Tube leakage, material failure... 

In this example, while the level of stress produced is below the yield stress of the TP, the presence of stress increasers can lead to failure of the product. Stress raisers can magnify the effect of thermally induced stress at the point at which the TP s tensile strength will be exceeded, which will be followed by part failure. Stress raisers may be in the form of sharply reduced section thicknesses, notches from poor trimming operations, or fastener holes. 

The test is shown schematically in Figure 8.17. The indenter penetrates the TBC and oxide layers, and plastically deforms the metallic bond coat and substrate below. As illustrated in Figure 8.18 (viewed from above), this induces an axisymmetric debonding of the TBC and oxide layers similar to that for a thermally induced failure by buckling. 

Bond wire failures which occur during service life, as opposed to the moulding operation, are the result of (i) weakening of the bond due to the growth of intermetallic compounds at the interface between the gold wire and aluminium die metallisation and/or (ii) thermally induced stresses caused by the difference between the coefficient of expansion of the plastic and the leadframe. 

In order to determine the value of 4> it is necessary to subject at least two groups of specimens to lifetests at different temperatures and to measure a parameter, such as the mean time before failure (MTBF), related to the rate of degradation. For most thermally induced failure mechanisms, the value of will lie in the range 0 45-l 2eV. 

The three main thermally induced failure mechanisms of plastic encapsulated ICs are ... 

The thermal penetration of the cable material was described by the model for Thermally Induced Electrical Failure (THIEF) implemented in FDS. The THIEF model predicts the temperature of the inner cable jacket under the assumption that the cable is a homogeneous cylinder with one-dimensional heat transfer. The thermal properties—conductivity, specific heat, and density—of the assumed cable are independent of the temperature. In reality, both the thermal conductivity and the specific heat of polymers are temperature-dependent. In the analysis, conductivity, specific heat, density, and the depth of the cable insolation were considered as uncertain parameters with relevant influence (see Table 1). 

Wheel-related derailments are generally associated with the fracture of the wheel under a vehicle, usually under conditions of high dynamic loading. High temperature/overheating related failures are associated with the change in metallurgical properties of the wheel and the formation of thermally induced cracks. 

Kletzli, D. B., C. Cusano, and T. F. Conry. Thermally Induced Failures in Railroad Tapered Roller Bearings. Tribology Transactions 42, no. 4 (1999) 824-832. 

Thermo-mechanical models are hybrids of heat transfer and finite element models of residual strength. Through-thickness temperature profiles and resin mass-loss data from the thermal components of these models have been used respectively to prediet residual modulus and increase in pressure due to accumulated volatiles within the burning composite material. It remains for such models to a) adequately represent mixed modes of mechanical failure b) accurately model ply layer delamination, and c) correctly model the directionality of thermally-induced strains as a function of ply orientation. 

Another aspect of epoxy resin mortar floorings which needs careful attention is that their coefficients of thermal expansion are approximately three times that of concrete. This, coupled with the relative low thermal conductivity of epoxy mortar, can cause stresses to be induced at the resin mortar/concrete interface under conditions of thermal shock (e.g. thermal cleaning), resulting in break-up of the flooring due to initial failure in the concrete. Two approaches have been tried to overcome this problem ... 

Obviously, the designer must take thermal expansion and contraction into account if critical dimensions and clearances are to be maintained during use where material is in a restricted design. Less obvious is the fact that products may develop high stresses when they are constrained from freely expanding or contracting in response to temperature changes. These temperature-induced stresses can cause material failure. 

In order to develop the safest process the worst runaway scenario must be worked out. This scenario is a sequence of events that can cause the temperature runaway with the worst possible consequences. Typically, the runaway starts with failure that results in an adiabatic course of exothermic reaction, inducing secondary reactions that proceed with a high thermal effect. Such a. sequence of typical events is shown in Fig. 5.4-55 (after Gygax, 1988-1990 Stoessel, 1993). It starts with, for instance, a cooling failure at time tx when the temperature is at the set level, Tv ,- Then the temperature rises up to the Maximum Temperature for Synthetic Reaction (MTSR) within the time Atn. Assuming adiabatic conditions MTSR = + ATad,R... 

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