Marginal failure

Margaric acid, physical properties, 5 29t Marginal failure, 26 982 Marginal regeneration process, 12 13 Margin of exposure, 25 244 Margin of safety determination, 25 234-235... 

A marginal failure is a failure that can degrade performance or result in degraded operation. Special operating techniques or alternative modes of operation involved by the loss can be tolerated throughout a mission but should be corrected upon its completion. 

Figure 4.3 illustrates the thin-shell packed-particulate design chosen as a reference container for this study. The mathematical procedure to combine various probability functions and arrive at a probability of failure of a hot container as a result of crevice corrosion at a certain temperature is illustrated in Fig. 4.4. The failure rate due to HIC was arbitrarily assumed to have a triangular distribution in order to simplify the calculations, given that HIC is predicted to be only a marginal failure mode under the burial conditions considered.

There is still a substantial safety margin up to the ultimate tensile strength, which amounts to 60 to 90 percent, depending on the steel (Kirby, Siwek, Treventing Failures of Equipment Subject to Explosions, Chemical Engineering, Jtine 23, 1986). 

In summary, for a eomponent/eharaeteristie it is possible to define an area of aeeeptable design on a graph of oeeurrenee versus severity. The aeeeptability of the design ean be enhaneed somewhat by the addition of inspeetion and test operations. The requirements of proeess eapability may be relaxed to a degree, as the eonditional probability of failure reduees, but this should be subjeet to a generous safety margin. 

Figure 4.31 Failure probability (per application of load) versus safety margin for various loading roughness values (adapted from Carter, 1997)...

The failure determining stresses are also often loeated in loeal regions of the eomponent and are not easily represented by standard stress analysis methods (Sehatz et al., 1974). Loads in two or more axes generally provide the greatest stresses, and should be resolved into prineipal stresses (Ireson et al., 1996). In statie failure theory, the error ean be represented by a eoeffieient of variation, and has been proposed as C =0.02. This margin of error inereases with dynamie models and for statie finite element analysis, the eoeffieient of variation is eited as Q = 0.05 (Smith, 1995 Ullman, 1992). 

In designing the eon-rod, we wish to ensure that the pin will fail, in the ease of an overload, in preferenee to the eon-rod. To realize this, the mean values of their individual strength distributions are to be set apart by a margin to ensure this requirement. In this way, the probability of eon-rod failure will beeome insignifieant to that of the pin. The foree to shear the pin in an overload situation is a funetion of the ultimate shear strength, t, of the material. The relationship between the ultimate tensile and shear properties for steel is (Green, 1992) ... 

Again, a preferred value for the aetual seetion width would be Z) = 110 mm. In a more simplified way than that presented in Seetion 4.8.4, we have separated the failure of the pin from the eon-rod by approximately 3 + 3 strength standard deviations. The aetual separation ean be modelled for the distribution of the shear foree in the pin and tensile foree in the eon-rod, as illustrated in Figure 4.69. The safety margin, SM, is ealeulated to be 4.61, or defined another way, the reliability R = 0.999998 whieh is adequate for the applieation to avoid overdesign of the eon-rod. 

Bonded-bolted joints have good load distribution and are generally designed so that the bolts take all the load. Then, the bolts would take all the load after the bond breaks (because the bolts do not receive load until the bond slips). The bond provides a change in failure mode and a sizable margin against fatigue failure. 

A possible adjunct to the laminate design procedure is a specific laminate failure criterion that is based on the maximum strain criterion. In such a criterion, all lamina failure modes are ignored except for fiber failure. That is, matrix cracking is regarded as unimportant. The criterion is exercised by finding the strains in the fiber directions of each layer. When these strains exceed the fiber failure strain in a particular type of layer, then that layer is deemed to have failed. Obviously, more laminae of that fiber orientation are needed to successfully resist the applied load. That is, this criterion allows us to preserve the identity of the failing lamina or laminae so that more laminae of that type (fiber orientation) can be added to the laminate to achieve a positive margin of safety. 

Table 18.4.1 smiinuuizes another inetliod of risk assessment tliat can be applied to an accident system failure. Both probability and consequence have been ranked on a scale of 0 to 1 witli table entries being the sum of probability and consequence. The acceptability of risk is a major decision and can be described by dividing tlie situations presented in Table 18.4.1 into unacceptable, marginally acceptable, and acceptable regions. Figiue 18.4.2 graphically represents tliis risk data. ... 

The set burst pressure should be selected to permit a sufficiently wide margin between it and the vessel s used or design operating pressure and temperature to avoid premature failure due to fatigue or creep of metal or plastic coatings. 

Note When rupture disk devices are used, it is recommended that the design pressure of the vessel be sufficiently above the intended operating pressure to provide sufficient margin between operating pressure and rupture disk bursting pressure to prevent premature failure of the rupture disk due to fatigue or creep. 

Develop final Perform structural design of analysis of component acceptable accuracy Determine structural response—stresses, support reactions, deflections, and stability—based on a structural analysis of acceptable accuracy. Determine acceptable accuracy based on economic value of component, consequences of failure, state-of-the-art capability in stress and stability analysis, margin of safety, knowledge about loads and materials properties, conservatism of loads, provisions for further evaluation by prototype testing... 

Develop practical test program to demonstrate components ability to meet structural and performance criteria. Extent of such test program, if any, depends on economic value of component, number of units to be produced, consequences of failure, accuracy of structural analysis and design, margins of safety used in design, knowledge about service loads and environments, and difficulty of duplicating service loads and conditions in test... 

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