Cumulative material damage

1 Cumulative Material Damage Cumulative material damage 

The concept of cumulative damage states that a failure must be considered to be the direct result of accumulation of damage with time (1). 

When a sample is subjected to a dynamic stress level r imder constant frequency, temperature, moisture content, etc., the damage D can be formulated as a function of the number of load cycles n and the stress level r, 

The damage function is normalized to start at 0 and reaches 1 at failure. A particularly simple case of stress independent damage is 

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A less common use of accelerated tests is to estimate remaining lifetime. Heat ageing of two compounds and applying the time temperature superposition principle and models based on Miner s rule of cumulative material damage is described by Gillen and Cehna (41). However, Sun and co-workers (50) found that Miner s rule did not apply for fatigue of black filled NR and SBR. 

Damage (aging) is a cumulative process in plastics. Ultimate failure therefore results from an accumulation of material damage over the course of time. The defined shift factor (Eq. 1.10) generally depends not only on temperature T, but also on damage Q ... 

Cumulative Damage. Pressure vessels may be subjected to a variety of stress cycles during service some of these cycles have ampHtudes below the fatigue (endurance) limit of the material and some have ampHtudes various amounts above it. The simplest and most commonly used method for evaluating the cumulative effect of these various cycles is a linear damage relationship in which it is assumed that, if cycles would produce failure at a... 

Immediately dangerous to life or health (IDLH) conditions pose a threat of severe exposure to contaminants, such as radioactive materials, that are likely to have adverse cumulative or delayed effects on health. Two factors are considered when establishing IDLH concentrations. The worker must be able to escape (1) without loss of life or without suffering permanent health damage within 30 minutes and (2) without severe eye or respiratory irritation or other reactions that could inhibit escape. If the concentration is above the IDLH, only highly reliable breathing apparatus is allowed. 

Two variables are fundamental to assessing the flow across complex fault zones. The first variable is the cumulative fault-rock thickness across the fault zone, i.e., the total thickness of fault-rock from all faults along the flow path. This depends upon the fault frequency along the flow path and is not equivalent to the fault damage zone thickness (cf. Knott, 1993) unless the fault zone is invaded by cements. The second variable is the connectivity of the faults or deformation features with low permeabilities in the fault zone. In the case of a completely connected array with no windows of undeformed material along possible flow paths, the flow is controlled by the permeability of the fault rocks. Where a more open network of faults is present then the flow will depend upon the tortuosity associated with flow around the low permeability zones and the ratio of matrix to fault-rock permeability. The interaction of these two factors will control the effective transmissivity of the zone. We have constructed a database on... 

Chronic or cumulative toxicity is manifested as a result of continuous exposure to a chemical. A common example is the genotoxicity of benzene, a chemical present in car exhausts and cigarette smoke. The metabolism of benzene in the fiver results in the formation of highly reactive free-radicals. These in turn may cause damage to the genetic material of a cell, in some cases leading to cancer. 

Chronic As with subacute, chronic exposures are multiple exposures however, with chronic exposme, there can be cumulative effects. Cumulative effects are simply a buildup of poison in the body. After the first exposure, some or all of the toxic material stays in the body. The first exposure may not cause any illness or damage. As additional exposmes occur and the poison builds up in the body, it can reach toxic levels where illness, damage, or death can occur. 

Because radiation exposure can be cumulative, there are no truly safe levels of exposure to radioactive materials. Radiation does not cause any specific diseases. Symptoms of radiation exposure may be the same as those from exposure to cancer-causing materials. The tolerable limits for exposure to radiation that have been proposed by some scientists are arbitrary. Scientists concur that some radiation damage can be repaired by the human body. Therefore, tolerable limits are considered acceptable risks when the activity benefits outweigh the potential risks. The maximum annual radiation exposure for an individual person in the United States is 0.1 REM. Workers in the nuclear industry have a maximum exposure of 5 REMs per year. An emergency exposure of 25 REMs has been established by The National Institute of Standards and Technology for response personnel. This type of exposure should be attempted under only the most dire circumstances and should occur only once in a lifetime. 

Based on the theory of damage mechanics, when the material is in one-dimensional state, the relationship between the damage development rate and the cumulative plastic strain rate satisfies the following equation (Li, S.C et al. 2009). 

Although the stiffness reduction curves offer a reasonable basis on which to establish damage curves for constant amplitude loading at different stress levels, these damage curves may not reflect all physical effects which are important for the development of fatigue damage. That, however, would mean that the development of realistic cumulative damage laws for composite materials would require considerable further research and development work. 

Z Hashin, Cumulative damage theory for composite materials residual life and residual strength methods . Compos Sd Technol 1985 23 1-19. 

Hwang, W. and K. Han (1986). Cumulative damage models and multi-stress fatigue life prediction. Composite Materials 20, 125-153. 

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