Life prediction criteria

Comments on life prediction criteria and damage mechanics... 

Life prediction methodology embraces all aspects of the numerous processes that could affect the function of the element—in this case the bulk adhesive. The first step is to define the function of the adhesive clearly enough for a failure criterion to be derived. This failure criterion may be an unacceptable reduction in tensile strength, time to creep failure under a given stress, reduction in modulus due to moisture ingression, increase in modulus due to oxidation, unacceptable crack depth, or a variety of other possible criteria. It is also important that the criteria be related to practical adhesive joint performance. This is where it is difficult, and one must presume, at least for this limited analysis, that the adhesive will fail via a bulk (cohesive) property. 

After the failure criterion has been defined, the various processes that could cause this failure must be analyzed. For example, an increase in modulus could occur by thermal oxidation, increased postcure crosslinking, or the loss of plasticizer. Whatever the mechanism, each possible process needs to be identified and its rate characterized separately. Only then can interactions between different mechanisms be considered for life prediction. 

The most common approaches and criteria available for life prediction of composite laminates under multiaxial faligue have been briefly presented in the introduction to this section dedicated to continuous fiber reinforced composites. Among them, the criteria formulated by Fawaz and EUyin [76,77] and Smith and Pascoe [75], and a polynomial formulation based on the Tsai—Hill criterion [68], have already been... 

For these reasons, we decided to consider these criteria with the aim of assessing their reliability in terms of life estimation, by comparing their predictions with some of the experimental results taken from the extensive database available. This would be of help in obtaining information useful for design purposes, like strengths and weaknesses of each criterion, and, at the same time, in further clarifying the directions and the need for the development of life prediction models of general applicability. 

To verify the accuracy in the life prediction, the experimental fatigue lives are compared with those predicted by the criterion, for a given set of data. Figures 7.17 and 7.18 show the results for global and local multiaxial data, respectively. Upper and lower bounds representative of 200% and 400% error in life assessment are plotted together with the data for an easier interpretation of the results. 

The criterion is, in general, rather easy to apply, since it requires only two fatigue curves for its calibration this is of great help in overcoming the difficulty in considering, explicitly, variations of the load ratio. A limitation, however, is that the model, in the present form, cannot be applied to life prediction of unidirectional laminates due to their anisotropic response resulting in different limits for the strain energy density in the fiber direction and normal to it. 

DRF as well as Service Life Prediction itself can be related to a specific failnre criterion only. For the DRF concept, the failnre property progress is to be quantitatively measurable, additionally. ... 

Coffin-Manson predictions and test data. All predictions fall within 25% of the actual failure data. Subsequent creep-based analyses use the same fitting procedures and will produce separate fatigue life prediction equations for a given creep criterion and package type. 

Predictions of lifetime should state the units, the conditions assumed, the end-of-life criterion and the confidence limits. The value of extrapolated lifetimes is little if the confidence limits are too wide, as often happens with power laws. Prediction will always be an inexact science requiring both skill and experience. 

Volume of distribution is a conceptual pharmacokinetic parameter that scales the extent of a drug distributed into the tissues. A well-known parameter, elimination half-life, can be derived from clearance and volume of distribution. It is a very important developability criterion that warrants the desired dose regimen. It should be noted here that half-life must be discussed in the context of a biologically relevant concentration. A purely mathematically derived half-life is sometimes biological irrelevant. Some more definitive explanations and comprehensive discussion of the major pharmacokinetic parameters and their biological relevance have been extensively reviewed.25,26 These parameters should be examined across several different preclinical species to predict the behavior in humans. The DMPK topics will be discussed in Chapters 5 and 6. 

The selection of a storage site is critical, since the criterion for container life (as currently designed) is variously estimated to be between 500 and 10,000 years. Obviously, the underground storage area must be totally free of ground movement during that same time interval. Predicting... 

The most widely used potential monitoring measurements are based on protection potential criteria. Despite improvements in these methods, they only indicate if the pipeline is protected, underprotected, or overprotected. These methods can only estimate the corrosion rates, but should not be used to predict the life expectancy of the system. A more advantageous solution would be the introduction of kinetic CP criterion that... 

To conclude, the calibration parameters for the validation of the criterion in the case of local multiaxiality conditions are listed in Table 7.6, and Figure 7.21 compares experimental and estimated data. Although the number of series and fatigue data analyzed is rather limited, accuracy in life assessment is reasonable in some cases predictions are, however, even too conservative. 

Most degradation processes are temperature-activated, and they are best represented by the classic Arrhenius reaction rate equation. The application of such a model is shown in Figure 2.13. The short-term points are obtained by selecting the life criterion (for example a 50% drop in toughness) and then ageing the material at several elevated temperatures until the desired extent of degradation is achieved. Four such points are recommended. A linear extrapolation on a log(criterion) versus 1/T plot allows prediction of the life at... 

It seems reasonable to start NIV in symptomatic patients (Table 2) who have signs of respiratory muscle weakness (FVC <80% predicted or SNIP < 40 cmH20) and evidence of nocturnal desaturation or a Paco2 > 49 mmHg (18). Bourke et al. suggested that orthopnea was the most useful criterion for benefit and compliance with NIV (94), and Sivak (96) noted lack of correlation between VC and the success of NIV. In a small prospective study, Jackson et al. (19) suggested that early intervention with NIV, based on nocturnal desaturation (Sao2 < 90% for 1 min), may improve quality of life. 

The first assessment of fatigue theories to predict the OP behavior of materials will be done checking the von Mises criterion, already analyzed in Sect. 9.3 for the IP situations. In particular, Eq. (9.20) will be used combined with the Basquin-Manson-Cofhn Eq. (6.10) to yield the equivalent strain versus life N... 

Here Mq is the atomic weight of carbon and F is the Faraday constant. This criterion suggests a useful life of the melt of at least le (days) 0.86//. Thus for the mechanically cleaned coal granules with 0.17 or 1% ash, the critical time 7), is 506 or 586 days, respectively. For the solvent-extracted sources of fuel having < 0.05% ash, the time to exhaust the melt approaches 5 years—the life expected for any high temperature cell. We have not tested Weaver s criterion. Nor does this test predict the rate of fouling of the anode current collector metal surface by ash impurities, as noted by Vutetakis (1985, 1987). The ash will likely be swept from the interface and the cell by the influx of carbon, which moves at a very high relative velocity compared with the accumulation of ash in the melt. 

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