Mechanical fatigue failure

Continuous flexing of a PBA will fracture component leads and, more important, surface-mounted component solder joints, due to mechanical fatigue failure. (Mechanically induced flexing or vibration in assembled PBs is used under controlled conditions to induce failures in solder joints for quality and reliability studies.)... 

Linkage of these microcracks to give a discontinuous crack propagation over and above that due to mechanical fatigue failure. 

The reason why properly designed bonded joints do not suffer from mechanical fatigue failures is that the most critical conditions are not developed where the adhesive is protected by the adherends. This can be understood by characterizing the minimum and maximum adhesive shear strains as a function first of bonded overlap and secondly as a function of adherend thickness, accounting for the effect of the environment in each case. This is illustrated in Tig. 44.17, for room temperature. 

Fracture Mechanics. Linear elastic fracture mechanics (qv) (LEFM) can be appHed only to the propagation and fracture stages of fatigue failure. LEFM is based on a definition of the stress close to a crack tip in terms of a stress intensification factor K, for which the simplest general relationship is... 

Because pure aluminum is n picaUy too soft to be drawn into a fine wine, it is often alloyed with 1° o sihcon or 1° o magnesium to provide a sofid solution-strengthening mechanism. The resistance of Al-1° o Mg wine to fatigue failure and to degradation of ultimate strength after exposure to elevated temperatures is superior to that of Al—1° o Si wine. 

Bellows can vibrate, both from internal fluid flow and externally imposed mechanical vibrations. Internal flow liner sleeves prevent flow-induced resonance, which produces bellows fatigue failure in minutes at high flow velocities. Mechanically induced resonant vibration is avoided by a bellows with a natural frequency far away from the forcing frequency, if known. Multiple-ply bellows are less susceptible to vibration failure because of the damping effect of interply friction. 

Fatigue is another property of considerable interest to the design engineer. CycHc deflections of a predeterrnined ampHtude, short of giving immediate failure, are appHed to the specimen, and the number of cycles to failure is recorded. In addition to mechanically induced periodic stresses, fatigue failure can be studied when developing cycHc stresses by fluctuating the temperature. 

To explain the mechanism by which the formation of Ni3S2 scale can result in the fatigue failure of the blade disc, the effect of sulfur on the mechanical properties of nickel base alloys must be understood. Since the fracture modes are totally intergranular with evidence of... 

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

Most reliability problems associated with positive clutches fall into two categories mechanical damage and fatigue failures. 

While lead of purity in excess of 99.99% is commercially available, it is very rarely used owing to its susceptibility to grain growth and fatigue failure by intercrystalline cracking, and indifferent mechanical properties. Because of its generally superior corrosion resistance, pure lead to BS 334 1982 type A, shown in Table4.13, is occasionally used in chemical plant, but only if there is no suitable alternative. 

If sufficient data are available, much more information can be provided when different curves for various percentages of failure are plotted. Where such data are available, reasonable design criteria would be based on some probability for failure, depending on how critical the effects of failure occur. If a large, expensive repair of a complex mechanism would result from the fatigue failure of one product, then a 10 or even 1 % probability of failure would be a more likely design criterion than the 50% suggested above. 

Fracture mechanic The fracture mechanics theory developed for metals is also adaptable for use with plastics. The basic concepts remain the same, but since metals and plastics are different they require different techniques to describe their fatigue-failure behaviors. Some of the comments made about crack and fracture influences on fatigue performance relate to the theory of fracture mechanics. The fracture mechanics theory method, along with readily... 

In many cases, a product fails when the material begins to yield plastically. In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute failure. The criterion for failure may be based on normal or shear stress in either case. Impact, creep and fatigue failures are the most common mode of failures. Other modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed to eliminate failure. 

The main failure of equipment is a loss of process containment. The consequences depend on the properties and the amount of the leaking material and the conditions both inside and outside of process equipment. Pumps and compressors (Marshall, 1987) are perhaps the most vulnerable items of pressurised systems, because they contain moving parts and they are also subject to erosion and cavitation. Pumps and compressors produce also vibration, which may lead to fatigue failure. Both seals and bearings of pumps and compressors are liable to failure. In addition agitator systems present difficulties due to mechanical stresses, though they operate at much lower speeds than pumps. 

The possible fatigue failure mechanisms of SWCNT in the composite were also reported (Ren et al., 2004). Possible failure modes mainly include three stages, that is, splitting of SWCNT bundles, kink formation, and subsequent failure in SWCNTs, and the fracture of SWCNT bundles. As shown in Fig. 9.12, for zigzag SWCNT, failure of defect-free tube and tubes with Stone-Wales defect of either A or B mode all resulted in brittle-like, flat fracture surface. A kinetic model for time-dependent fracture of CNTs is also reported (Satapathy et al., 2005). These simulation results are almost consistent with the observed fracture surfaces, which can be reproduced reasonably well, suggesting the possible mechanism should exist in CNT-polymer composites. 

Ren Y, Fu YQ, Liao K, Li F, Cheng HM (2004). Fatigue failure mechanisms of single-walled carbon nanotube ropes embedded in epoxy. Appl. Phys. Lett. 84 2811-2813. 

Pulsations and vibrations should be kept to a minimum (e.g. pumping and valve switching) because they reduce the life of plates by mechanical damage leading to fatigue failure. To ensure product security, plates should be crack tested on a regular basis (annually). 

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