Mode of failure

Even in the best of designed machinery and plant failures can occur. When they do, the cause is normally investigated to determine the mode of failure so that repetition can be prevented. Modes of failure relate to the type of stress to which the component has been subjected and the characteristic features of failures due to tension, compression, shear and torsion are well known. Sometimes the failure is related more to the operating process than to the stress, particularly when there is repeated stress cycling of the part when it can suffer fatigue failure. 

In processes using certain chemicals, stresses have been shown to make the parts prone to corrosive attack which can reduce their strength. Similarly failures have occurred where a chemical in contact with the part has affected its ability to carry stresses through causing embrittlement such as zinc embrittlement of stainless steel and hydrogen embrittlement of grade T chain.  

While there are common legislative requirements for the testing of finished products where, in normal work, tiie equipment is stressed, such as 

In the manufacture of plant and machinery, and following subsequent periodic inspections, should they become necessary testing is normally non-destructive and a number of specialised techniques have been developed. The aim is to check the condition of the material of which the plant is constructed and to identify faults that cannot be seen by eye. Special detection techniques used to highlight the weaknesses or faults in the material include the use of magnetic particles, penetrant dyes. X-ray and gamma-ray sources, ultrasonic vibrations, microwave and infrared rays. Equipment is now available to enable visual inspections to be carried out of inaccessible places using fibre optics and remote-controlled television. 

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A crack in a body may grow as a result of loads appHed in any of the three coordinate directions, lea ding to different possible modes of failure. The most common is an in-plane opening mode (Mode I). The other two are shear loading in the crack plane (Mode II) and antiplane shear (Mode III), as defined in Figure I. Only Mode I loading is considered herein. 

The fracture surfaces, revealed when the tube is broken open, are found to be smooth with a rippled appearance characteristic of fatigue. This type of behavior is sometimes known as leak before break. On the other hand, if the material lacks toughness, the propagation of the fatigue crack may be intermpted part way through the wall by the intervention of fast fracture, resulting in what is sometimes known as the break before leak mode of failure. 

The purpose of the criticaUty rating is to provide guidance as to which failure modes require resolution. However, critical modes of failure resulting in unsafe operation should be given special attention, and design/verification actions should be taken to ensure that they never occur. 

The toughness induced in ceramic matrices reinforced with the various types of reinforcements, that is, particles, platelets, whiskers, or fibers, derives from two phenomena crack deflection and crack-tip shielding. These phenomena usually operate in synergism in composite systems to give the resultant toughness and noncatastrophic mode 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. 

Review of planned operation of process, especially the possibility of upsets, modes of failure, unexpec ted delays, redundancy of equipment and instrumentation, critical instruments and controls, and worst-credible-case scenarios... 

We shall now examine material selection for a pressure vessel able to contain a gas at pressure p, first minimising the weight, and then the cost. We shall seek a design that will not fail by plastic collapse (i.e. general yield). But we must be cautious structures can also fail by fast fracture, by fatigue, and by corrosion superimposed on these other modes of failure. We shall discuss these in Chapters 13, 15 and 23. Here we shall assume that plastic collapse is our only problem. 

Rupture discs should be removed from service at predetermined intervals for visual inspection. Depending on the condition of the disc and recommendations by the manufacturers, they are either replaced or returned to service. The most common mode of failure is case (c), premature rupture below the minimum bursting pressure. An analysis of this mode of failure indicates that this can be the result of ... 

Potential mode of failure (i.e., open, elosed, on, off, leaks)... 

The mode of failure in a test is examined carefully before the failure is included in the database. In the diesel-generator example, unsatisfactory performance may have been reported because of a trip on a low oil pressure signal, high oil temperature, or both. 

Analysis may show one mode of failure to be most likely in which case further analysis conteiunilcs on ihat mode thereby considerably reducing the scope of the analysis. If there are several etiiiallv likely modes, they must all be analyzed. 

Analyses are types of calculations but may be comparative studies, predictions, and estimations. Examples are stress analysis, reliability analysis, hazard analysis. Analyses are often performed to detect whether the design has any inherent modes of failure and to predict the probability of occurrence. The analyses assist in design improvement and the prevention of failure, hazard, deterioration, and other adverse conditions. Analyses may need to be conducted as the end-use conditions may not be reproducible in the factory. Assumptions may need to be made about the interfaces, the environment, the actions of users, etc. and analysis of such conditions assists in determining characteristics as well as verifying the inherent characteristics. (See also in Part 2 Chapter 14 under Detecting design weaknesses.)... 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

One of the procedures used to determine which sensors are needed to sense process conditions and protect the process is called a Failure Mode Effect Analysis—FMEA. Every device in the process is checked for its various modes of failure. A search is then made to assure that there is a redundancy that keeps an identified source or condition from developing for each potential failure mode. The degree of required redundancy depends on the severity of the source as previously described. Table 14-2 lists failure modes for various devices commonly used in production facilities. 

The failure rates and times-to-restore developed used a variety of data sources and data construction methodologies and are presented in Section 2. The principal methodology used is a kind of failure mode analysis for each component several principle modes of failure are analyed by characteristics including frequency of occurence, repair time, start-up time, and shut-down time. From these an average failure rate is developed and expressed as failures per million hours and mean time between failure(MTBF). 

The process of analyzing designs includes the modes of failure analysis. At an early stage the designer should try to anticipate how and where a design is most likely to fail. A few examples of potential problems due to loading conditions on products are reviewed. 

Construction of test samples that can be subjected to actual exposure to the end-use environments which are tested to destruction to determine the possible modes of failure and the conditions that cause the failure. 

In many cases, a product fails when the material begins to yield plastically. In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute failure. The criterion for failure may be based on normal or shear stress in either case. Impact, creep and fatigue failures are the most common mode of failures. Other modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed to eliminate failure. 

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