Failures fatigue

Strength degradation - ereep failure, fatigue, wear, eorrosion. 

Fine cracks spread under stress and result in fatigue failure. Fatigue-related failure may be slow or immediate, depending upon stress conditions. Fatigue cracks generally run transverse to the grain flow... 

Note that the service stress in any loaded machine or structure is the algebraic sum of the applied stress, due to the service load, and any residual stress that may have existed before the service load was applied. If the residual stress is not known, neither is the service stress. When the service stress reaches dangerous levels, failure occurs. Interest in residual stress stems mainly from the role it plays in three kinds of metal failure fatigue failure, brittle fractures in general, and stress-corrosion cracking. 

Pressure vessels are also subject to cyclic stress. Cyclic stress arises from pressure and/or temperature cycles on the metal. Cyclic stress can lead to fatigue failure. Fatigue failure, discussed in more detail in Module 5, can be initiated by microscopic cracks and notches and even by grinding and machining marks on the surface. The same (or similar) defects also favor brittle fracture. 

A thorough understanding of the mechanics of UHMWPE is important for efforts to improve the performance of orthopedic components. Elastic properties, resistance to plastic deformation, stress and strain at failure, fatigue behavior, and wear resistance of UHMWPE are believed to play roles in the life expectancy of an UHMWPE bearing. There exists a fundamental relationship between a material s intrinsic mechanical properties, akin to state variables, and how a structure made of the material will respond under mechanical stimuli. This material-specific fundamental relationship is referred to as a constitutive model. A validated constitutive model is a required input to a finite element (FE)-based simulahon of a structure made of the material in question. 

Corrosion-fatigue cracks are typically transgranular. They are often branched (Fig. 8.16), and several cracks are usually observed at the metal surface in the vicinity of the major crack accounting for failure. Fatigue cracks are similarly transgranular (exception lead and tin), but rarely is there evidence of more than... 

Fatigue ductility Fatigue failure Fatigue limit Fatigue strength... 

Erythropoietin derivatives are associated with an increased frequency of thrombo-vascular events in a variety of tumor types [97 ]. However, whether patients with susceptibility factors should receive prophylactic antithrombotic treatment has not been confirmed. The adverse effects of combination therapy with epoetin beta and all-trans retinoic acid (ATRA) in 59 patients with myelodysplastic syndromes were muscle pain, raised liver enzymes without hepatic failure, fatigue, headache, dry skin, and dry mucosa [98 ]. 

Component failure analysis (CFA) It looks at the piece of the machine that failed, such as a bearing or a gear to determine the specific cause of the failure (fatigue, overload, or corrosion) and that there were these x, y, and z influences. 

Synergistic degradation effects can also lead to early joint failure. Fatigue causes faster crack growth than do steady-state stresses. Mechanical stress in combination with environmental stresses like standing water, heat, ultraviolet light, acid rain, ozone or solvents can reduce service life many fold. 

Structural plastic part failures occur for a variety of reasons including ductile failure, brittle failure, fatigue, and others. Ductile failure sometimes results in catastrophic failure of the plastic part in end use, but more often the ductile failure causes excessive deformation that may be rectified by increasing the stiffness as discussed in the preceding section. Perhaps the most undesired failure mode is unexpected brittle failure, which can occur in plastic products due to unexpected load cases, stress concentrations. 

McDiarmid, D.L. A general criterion for high cycle multiaxial fatigue failure. Fatigue Fract. Eng. Mater. Struct. 14, 429-454 (1991)... 

For many metals and alloys, stress decreases continuously with increasing number of cycles at failure fatigue strength and fatigue life are parameters used to characterize the fatigue behavior of these materials (Figure 8.19b). 

---