Causes of failure

All materials tend to be somewhat permeable to chemical molecules, but the permeability rate of some elastomers tends to be an order of magnitude greater than that of metals. Though permeation is a factor closely related to absorption, factors that influence the permeation rate are diffusion and temperature rather than concentration and temperature. Permeation can pose a serious problem in elastomer-lined equipment. When the corrodent permeates the elastomer, it comes into contact with the metal substrate that is then subject to chemical attack. 

Bond failure and blistering, caused by an accumulation of fluids at the bond when the substrate is less permeable than the lining or from the formation of corrosion or reaction products if the substrate is attacked by the corrodent 

Loss of contents through lining and substrate as the result of eventual failure of the substrate 

The degree of permeation is affected by lining thickness. For general corrosion resistance, thicknesses of 0.010-0.020 in. are usually satisfactory, depending on the elastomeric material and the specific corrodent. Thick linings may be required when mechanical factors such as thinning because of cold flow, mechanical abuse, and permeation rates are taken into consideration. 

Increasing lining thickness will normally decrease permeation by the square of the thickness. However, this is not necessarily the answer to 

Like other plastics, PP is not free from product failure during service. In a recent study [34], the main reasons for failure in plastics, in general, are stated in decreasing order of failures as environmental stress cracking, dynamic fatigue, static notch failure, creep related failure, chemical attack, UV attack, heat degradation and wear/abrasion. 

Environmental stress cracking, which is the largest reason of failure in other plastics, is not very significant for PP. PP is virtually free from the problems of environmental stress cracking. 

Another major reason for the failure of plastic products is dynamic fatigue. However, being a semi-crystalline material, PP offers better fatigue resistance than amorphous thermoplastics. It should be noted that dynamic fatigue strength of PP is lower than that of engineering plastics. 

Like other semi-crystalline materials, PP imdergoes significant creep. Elastomer- 

PP offers good chemical resistance in general. However, it is strongly reconunended that the effect of chemicals at service temperature should be checked. 

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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. 

Sihcone contamination has been impHcated as a cause of failure in telephone switching systems and other devices that contain relay switch contacts (507). Analysis of airborne particulates near telephone switching stations showed the presence of siUcones at these locations. Where the indoor use of sihcones is intentionally minimised, outdoor levels were found to be higher than inside concentrations (508). Samples of particulates taken at two New Jersey office buildings revealed sihcone levels that were considerably higher indoors than outdoors. In these cases, indoor sihcone aerosols are beheved to be generated primarily by photocopiers, which use sihcone fuser oils. 

Most fractography can be conducted with simple instmments such as a pocket magnifying eyepiece or an optical microscope. As with any detective work, it is important to maintain careful records and to pay close attention to details in the reconstmctions of the conditions under which fabrication and failure occurred. Seemingly unimportant details of fabrication, service, and/or the conditions under which failure occurred can frequently be the key to determining the cause of failure. 

Failure Mode and Ejfect Analysis (FMEA) This is a systematic study of the causes of failures and their effects. All causes or modes of failure are considered for each element of a system, and then all possible outcomes or effects are recorded. This method is usually used in combination with fault tree analysis, a quantitative technique. FMEA is a comphcated procedure, usually carried out by experienced risk analysts. 

Wastage was caused by crevice corrosion, accelerated by the difference in tube and tube sheet metallurgies. The brass tube, being more noble, was cathodically protected by corrosion of the surrounding mild steel tube sheet. However, the galvanic effect was secondary to the primary cause of failure, namely, crevice corrosion. 

SCC has been defined as failure by cracking under the combined action of corrosion and stress (Fig. 9.1). The stress and corrosion components interact S3mergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state. They may run in any direction but are always perpendicular to the principal stress. Longitudinal or transverse crack orientations in tubes are common (Figs. 9.2 and 9.3). Occasionally, both longitudinal and transverse cracks are present on the same tube (Fig. 9.4). Less frequently, SCC is a secondary result of another primary corrosion mode. In such cases, the cracking, rather than the primary corrosion, may be the actual cause of failure (Fig. 9.5). 

The closer one is to the failure, the more its direct effects are apparent. The cumulative effects of failure are often overlooked in the rush to fix the immediate problem. Too often, the cause of failure is ignored or forgotten because of time constraints or indifference. The failure or corrosion is considered just a cost of doing business. Inevitably, such problems become chronic associated costs, tribulations, and delays become ingrained. Problems persist until cost or concern overwhelm corporate inertia. A temporary solution is no longer acceptable the correct solution is to identify and eliminate the failure. Preventative costs are almost always a small fraction of those associated with neglect. 

Field experience has revealed that one of the major causes of failure of an HT motor is weak insulation, caused by environmental pollution and ageing. 

SYMPTOMS AND CAUSES OF FAILURE FOR POSITIVE DISPLACEMENT PUMPS... 

Potential Causes of Failure. What would make the eomponent, produet, proeess or system fail in the way suggested by the potential failure mode ... 

Figure 4 Pareto chart of RPN values against potential cause of failure for the rear brake lever design...

The main causes of failure in gear couplings are wear or surface fatigue caused by lack of lubricant, incorrect lubrication, or excessive surface stresses. Component fracture caused by overload or fatigue is generally of secondary importance. 

This represents the locus of all the combinations of Ca and Om which cause fatigue failure in a particular number of cycles, N. For plastics the picture is slightly different from that observed in metals. Over the region WX the behaviour is similar in that as the mean stress increases, the stress amplitude must be decreased to cause failure in the same number of cycles. Over the region YZ, however, the mean stress is so large that creep rupture failures are dominant. Point Z may be obtained from creep rupture data at a time equal to that necessary to give (V cycles at the test frequency. It should be realised that, depending on the level of mean stress, different phenomena may be the cause of failure. 

It is unclear whether previously published fire risk analyses have adequately ircaicd dependent failures and systems interaetions. Examples of either experienced or postulated system interactions that have been missed include unrelated systems that share common locations and the attendant spatially related physical interactions arising from fire. Incomplete enumeration of causes of failure and cavalier assumptions of independence can lead to underestimation of accident l rci uencies by many orders of magnitude,... 

Detection in the range 1 to 10, with 10 meaning that the control will not detect the potential failure and 1 meaning that the control will almost certainly detect the potential cause of failure. 

The pressure and temperature of a container s contents at the time of failure will depend on the cause of failure. In fire simations, direct flame impingement will weaken container walls. The pressure at which the container fails will usually be about the pressure at which the safety valve operates. This pressure may be as much as 20 percent above the valve s setting. The temperature of the container s contents will usually be considerably higher than the ambient temperature. 

The project began with an extensive evaluation of 900 reported incidents involving failures of fixed pipework on chemical and major hazard plant. As part of the analysis a failure classification scheme was developed which considered the chief causes of failures, the possible prevention or recovery mechanism that could have prevented the failure and the underlying cause. The classification scheme is summarized in Figure 2.13. A typical event classification would be... 

Data from an existing collection system were analyzed for failure modes and distribution. The results of Pareto analyses indicate the principal causes of failure. A few values of mean times to maintenance action (MTBM) are given for ethylene plant pumps (85 electric driven centrifugal pumps over a 19-month period), and ethylbenzene-styrene monomer plant equipment from 10 months data 4 gas compressors, 3 screw conveyors, 121 pumps, and 235 other items... 

Predictive maintenance utilizing vibration signature analysis is based on the following facts, which form the basis for the methods used to identify and quantify the root causes of failure ... 

Many of the common causes of failure in machinery components can be identified by understanding their relationship to the tme mnning speed of the shaft within the machine-train. 

Cause of failure Ply separation caused by substandard pulley diameter (Figure 58.11). 

Cause of failure Rough pulley sidewalls cause the cover to wear off in an uneven pattern (Figure 58.12). 

Cause of failure Belt has evenly spaced deep bottom cracks from use of substandard backside idler (Figure 58.13). 

Cause of failure Back of the belt rubbing on a belt guard or other component (Figure 58.14). 

Cause of failure Tensile breaks caused by high shock loads, foreign objects lodged under the belt, or damage sustained during installation (Figure 58.15). 

Cause of failure Excessive exposure to oil or grease causing the belt to become soft and swell, resulting in the bottom envelope seam splitting open (Figure 58.16). 

Cause of failure Weathering or crazing caused by the elements and aggravated by small pulleys (Figure 58.17). 

Cause of failure Cut bottom and sidewall indicates that the belt was pried over the pulley and damaged during installation (Figure 58.18). 

Cause of failure Spin burn caused by a frozen or locked driven pulley (Figure 58.19). 

Cause of failure Worn pulley grooves allow the joined belt to ride too low thus cutting through to the top band (Figure 58.20). 

Cause of failure Split on side at the belt pitch-line indicates use of a pulley with a substandard diameter (Figure 58.21). 

Cause of failure The load-carrying member has been broken by a shock load or damaged during operation (Figure 58.22). 

Cause of failure Web fabrics wear caused by improper belt and pulley fit (Figure 58.23). 

Cause of failure Flange wear on belt (Figure 58.24). 

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