Fracture and Fatigue Failure

Case studies in fast fracture and fatigue failure... 

Case studies in fast fracture and fatigue failure 165 Integration gives the number of cycles to grow the crack from to 02 ... 

Many steel structures - like bridges, storage tanks, and ships - are held together by welds. And when incidents arise from fast fractures or fatigue failures they can often be traced to weaknesses in the welds. The sinking of the Alexander Keilland oil platform in 1980 is an example. 

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. 

J. E. Ritter, Jr, Engineering design and fatigue failure of brittle materials, pp. 667-86 in Fracture Mechanics of Ceramics, Vol. 4, edited by R. C. Bradt et al.. Plenum Press, New York, 1978. 

Fracture of massive brittle and ductile pieces are rather well understood. By taking proper account of the microstructure as well as the micro- and macro-defects, most catastrophic and fatigue failures find a satisfactory explanation within the scope of the linear elastic fracture mechanics or the elasto-plastic fracture mechanics. Metallic filaments are particular and in many respects deserve a treatment of their own. Particular fabrication methods, such as drawing, melt spinning or crystallization from the vapor phase for whiskers are needed to obtain their small lateral dimensions. These processes may give rise to particular textures, intrinsic and extrinsic defects. Thermal treatments may modify or eliminate such defects but in many cases fracture is initiated by defects that stem from the fabrication process. Moreover, the small lateral dimensions, especially in micro-wires, make metallic filaments prone to external influences. Corrosive attacks may rapidly affect an important fraction of their cross-section. Hydrogen, for instance, which usually results in a severe embrittlement, may diffuse up to the core in a rather short time. 

The stress intensity factor forms the central part of a great deal of the fracture and fatigue literature. Once the stress intensity factor is known, it is possible to determine the minimum stress value that would cause failure. This... 

It is explained that most rubber products due to the development of cracks, as a result of being subjected to environmental factors. This paper reviews in depth the underlying failure mechanisms, and discusses ways and means of minimising them. It presents the effects of polymer molecular weight, the styrene content of SBR, and the diene microstructure on fracture and fatigue life of the vulcanisates. It also discusses the role played by carbon black and vulcanising systems on fracture and fatigue. 56 refs. 

The time-temperature dependence of the flexural constant strain-rate (CSR), creep, and fatigue strengths of various carbon fiber-reinforced plastics (CFRP) has been studied by McMurray et aV and Miyano et al. It was observed by Enyama et al that the fracture modes are almost identical for the above three types of loading over wide ranges of time and temperature. Similar results were also reported by Karayaka et al at room temperature. The literature survey indicates the validity of the two hypotheses for CFRP the same failure process and the same time-temperature superposition principle for CSR, creep, and fatigue failure. 

The third statistical argument (fracture as the result of a large number of molecular processes) is very old. Its application to polymers definitely was stimulated from many sides including fracture and fatigue of metals [4] or glasses [17] and the thermodynamics of reactions [19]. The interpretation of fracture as a sequence of individual steps will be mainly used in the following. It necessitates a consideration of the correlation of such steps and of the final failure criterion. 

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... 

We shall now examine material selection for a pressure vessel able to contain a gas at pressure p, first minimising the weight, and then the cost. We shall seek a design that will not fail by plastic collapse (i.e. general yield). But we must be cautious structures can also fail by fast fracture, by fatigue, and by corrosion superimposed on these other modes of failure. We shall discuss these in Chapters 13, 15 and 23. Here we shall assume that plastic collapse is our only problem. 

Here is the number of cycles to fracture under the stress cycle in region i, and Nj/Nf is the fraction of the lifetime used up after N, cycles in that region. Failure occurs when the sum of the fractions is unity (eqn. (15.4)). This rule, too, is an empirical one. It is widely used in design against fatigue failure but if the component is a critical one. Miner s Rule should be checked by tests simulating service conditions. 

Broeklehurst [37] has written an exhaustive review of the early work (prior to 1977) on fracture in polyerystalline graphite. Mueh of this work foeused on the fraeture behavior of nuclear graphites. In most investigations eonsidered, conventional fracture meehanies tests and analysis were performed for maeroseopie craeks. LEFM provided an adequate eriterion for failure. Additionally, results on work of fraeture, strain energy release rate, and fatigue eraek propagation were reported. 

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... 

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