Material defects

USCT in problems of non-destructive testing (NDT) of material defective state ... 

A considerable assumption in the exponential distribution is the assumption of a constant failure rate. Real devices demonstrate a failure rate curve more like that shown in Eigure 9. Eor a new device, the failure rate is initially high owing to manufacturing defects, material defects, etc. This period is called infant mortaUty. EoUowing this is a period of relatively constant failure rate. This is the period during which the exponential distribution is most apphcable. EinaHy, as the device ages, the failure rate eventually increases. 

The failure rate changes over the lifetime of a population of devices. An example of a failure-rate vs product-life curve is shown in Figure 9 where only three basic causes of failure are present. The quaUty-, stress-, and wearout-related failure rates sum to produce the overall failure rate over product life. The initial decreasing failure rate is termed infant mortaUty and is due to the early failure of substandard products. Latent material defects, poor assembly methods, and poor quaUty control can contribute to an initial high failure rate. A short period of in-plant product testing, termed bum-in, is used by manufacturers to eliminate these early failures from the consumer market. 

Enamel Defects. Characterization of defects in porcelain enamel surfaces frequently requites detailed examination via microscopy to determine the sources of the defects. Defects ate divided into processing and material defects. The greatest number of defects result from processing bhsters, pinholes, black specks, dimples, tool marks, and chipping. Defects often occur from unobserved sources at almost every stage of the enameling process, but they ate not recognizable until the ware is fired. Conscientious process control helps to minimize the incidents of unacceptable finishes. 

The percentage of failures that occur solely from material defects is quite low—less than 1% of all failures. What is more common is that defects may act in conjunction with specific environmental factors to produce failures, such as cracking or localized corrosion. 

No other evidence of deterioration, such as corrosion, was apparent on either surface. The cracks are probably material defects. They may be laps or seams that were present on the external surface prior to the fin-rolling operation and were exaggerated during the rolling process. 

Figure 14.6 Material defect, perhaps a lap or seam, on the external surface of a finned copper tube.

The subject of weld defects is quite extensive, and complete coverage is well beyond the scope of this chapter. Therefore, this chapter will focus on specific types of weld defects of general concern in cooling water systems. Defects of seam-welded tubes are considered under material defects in Chap. 14. 

Liquefied ammonia is delivered in rail tankcars to Fisons Limited for storage in a 1,900 tonnes spherical tank at -6° C. Several hundred tonnes of liquefied ammonia could be released on land if either of the two storage tanks, at Shell UK Oil and at Fisons Limited failed. The consequences of f lilure of the Shell tank would be minimal, because a high concrete wall to contain the contents and limit the heat transfer and consequently the rate of evaporation of the liquid. Such protection has not been provided. Because of the storage under pressure there are numerous ways the tank could fail from material defect to missile. The spillage of 50 to 100 tonnes, could kill people if noi [imrnp( , evacuation. 

Avoiding structural failure can depend in part on the ability to predict performance of materials. When required designers have developed sophisticated computer methods for calculating stresses in complex structures using different materials. These computational methods have replaced the oversimplified models of materials behavior relied upon previously. The result is early comprehensive analysis of the effects of temperature, loading rate, environment, and material defects on structural reliability. This information is supported by stress-strain behavior data collected in actual materials evaluations. 

The best would be to use all of the preceding actions to find the optimum conditions. Most often, however, the economical constraints and possibly the time put limits to the efforts that can be invested. Moreover the need to find optimum processing conditions may not be critical for many applications. Very often, however, the situation can be the opposite (i.e., the curing process generates material defects or takes too much time, hence actions must be taken to find solutions to these problems). 

Cure problems can be divided into problems that give material defects and problems that make the process inefficient. Some examples of common problems of the first type will be discussed in this section. 

Particular attention should be paid to fatigue failures as they may reveal design deficiencies rather than material defects. 

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