Thermal fatigue failure

In cyclic loading of a viscoelastic solid an amount of energy AW is dissipated per cycle. For sinusoidal loading in the anelastic range, one obtains 

This hysteresis heating leads to a noticeable temperature rise. At low test frequencies and low stress levels the temperature increase of the test specimen generally approaches a finite value.. A PA 6 sample for instance fatigued at 50 Hz, at a constant stress amplitude of 8 MN m , and at an ambient temperature of 21 °C, assumed a stable temperature of 27 °C after some 10 cycles [139—140]. At this temperature mechanical energy input and thermal energy loss were in equilibrium. 

Ta designates the ambient temperature, h the coefficient of convection at the sample surface and the constants p, Cp, and X have their usual significance as density, specific heat, and heat conductivity of a material. The term X/pCp is also known as temperature conductivity number. For conventional polymeric materials this number assumes values between 1.0 10 m /s (PTFE) and about 2.1 10 m /s (HOPE, POM). 

The conditions of thermal failure are especially discussed by Oberbach [129—130] for a variety of polymers, by Zilvar [139—140] for PA 6, and by Alf [141] forPMMA and unsaturated polyester resins. 

It is only through their effect on the above constants and on the complex modulus E that the molecular chain structure influences thermal failure. In the context of this monograph this type of failure will, therefore, not be discussed any further. 

---

FIGURE 57.14 Thermal fatigue failure in eutectic Sn-Pb solder for a TSOP component. (Photo courtesy K. Gratala)... 

Frear, D. Grivas, D. Morris, J.W. A microstructural study of the thermal fatigue failures of 60Sn-40Pb solder joints. J. Electron. Mater. 1988, 17, 171-180. 

TABLE 4 Thermal Fatigue Failure Rate of Plated-Through-Hole (PTH) Solder Joints Encapsulated with Epoxy... 

Test conditions temperature cycling between —54°C and 75°C, soak time of 2 hr in each chamber. Thermal fatigue failure rate (in percent) was determined by detecting a 360° crack around the solder fillet at a magnification of 40 x at the first cycle and every 25 cycles thereafter. 

Mei, Z. Holder, H. A thermal fatigue failure mechanism of 58Bi-42Sn solder joints. Trans. AIME June 1996, 118 (6), 62-66. 

FIG. 4 Cross section depicting the thermal fatigue failure of a Pb-free CSP solder joint that failed in the ATC testing. 

But they all oxidise very rapidly indeed (see Table 21.2), and are utterly useless without coatings. The problem with coated refractory metals is, that if a break occurs in the coating (e.g. by thermal fatigue, or erosion by dust particles, etc.), catastrophic oxidation of the underlying metal will take place, leading to rapid failure. The unsafeness of this situation is a major problem that has to be solved before we can use these on-other-counts potentially excellent materials. 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

A uPVC rod of diameter 12 mm is subjected to an eccentric axial force at a distance of 3 ttun from the centre of the cross-section. If the force varies sinusoidally from — F to f at a frequency of 10 Hz, calculate the value of F so that fatigue failure will not occur in 10 cycles. Assume a safety factor of 2.5 and use the creep rupture and fatigue characteristics described in the previous question. Thermal softening effects may be ignored at the stress levels involved. 

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. 

The service life of a product can be governed by many factors. These include fatigue failure under repeated stressing, excessive creep or stress relaxation, excessive change in stiffness due to thermal ageing, and excessive change in a physical property due to the action of chemicals. 

---