Time-to-failure

Early failures may occur almost immediately, and the failure rate is determined by manufacturing faults or poor repairs. Random failures are due to mechanical or human failure, while wear failure occurs mainly due to mechanical faults as the equipment becomes old. One of the techniques used by maintenance engineers is to record the mean time to failure (MTF) of equipment items to find out in which period a piece of equipment is likely to fail. This provides some of the information required to determine an appropriate maintenance strategy tor each equipment item. 

In usiag these failure rates an exponential distribution for time to failure was assumed. Such an assumption should be made with caution. Parallel Systems. A parallel (or redundant) system is not considered to be ia a failed state unless all subsystems have failed. The system rehabihty is calculated as... 

For example, if the time to failure is given as an exponential distribution, then... 

Example 3. A centrifugal pump moving a corrosive Hquid is known to have a time-to-failure that is well approximated by a normal distribution with a mean of 1400 h and a standard deviation of 120 h. A particular pump has been in operation for 1080 h. In order to plan maintenance activities the chances of the pump surviving the next 48 h must be deterrnined. 

Example 3 illustrated the use of the normal distribution as a model for time-to-failure. The normal distribution has an increasing ha2ard function which means that the product is experiencing wearout. In applying the normal to a specific situation, the fact must be considered that this model allows values of the random variable that are less than 2ero whereas obviously a life less than 2ero is not possible. This problem does not arise from a practical standpoint as long a.s fija > 4.0. 

Creep tests are ideally suited for the measurement of long-term polymer properties in aggressive environments. Both the time to failure and the ultimate elongation in such creep tests tend to be reduced. Another test to determine plastic behavior in a corrosive atmosphere is a prestressed creep test in which the specimens are prestressed at different loads, which are lower than the creep load, before the final creep test (11). 

Rotating Beam Fatigue Test for Steel Cords. The purpose of this test method is to evaluate steel cord for pure bending fatigue (121). The test sample consists of a 3-mm diameter mbber embedded with steel cord. Different bending stress levels are appHed and the time to failure is recorded. The test stops at 1.44 million cycles. The fatigue limit is calculated from S—N (stress—number of cycles) curve. 

Analysis methods for hydrogen absorbed in the deposit have been described (65), and instmments are commercially available to detect hydrogen in metals. Several working tests have been devised that put plated specimens under strain and measure the time to failure. A method for cadmium-plated work has been described (66) as has a mechanical test method for evaluating treatments on AlSl 4340 Steel (67). Additional information on testing for hydrogen embrittlement is also available (68). 

As was cited in the case of immersion testing, most SCC test work is accomplished using mechanical, nonelecdrochemical methods. It has been estimated that 90 percent of all SCC testing is handled by one of the following methods (1) constant strain, (2) constant load, or (3) precracked specimens. Prestressed samples, such as are shown in Fig. 28-18, have been used for laboratory and field SCC testing. The variable observed is time to failure or visible cracldng. Unfortunately, such tests do not provide acceleration of failure. 

To tackle any of these we need constitutive equations which relate the strain-rate e or time-to-failure tf for the material to the stress ct and temperature T to which it is exposed. These come next. 

It is not surprising - since creep causes creep fracture - that the time-to-failure, tf, is described by a constitutive equation which looks very like that for creep itself ... 

Times-to-failure are normally presented as creep-rupture diagrams (Fig. 17.9). Their application is obvious if you know the stress and temperature you can read off the life if you wish to design for a certain life at a certain temperature, you can read off the design stress. 

The life-time of a component - its time-to failure, tf - is related to the rate at which it creeps. As a general rule ... 

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

Fig. 2-17 Relation between the time to failure by intergranular stress corrosion cracking and potential for tensile specimens of soft iron (a) boiling 55% Ca(N03)2 solution, 5 = 0.65 R a = 0.90 R (b) 33% NaOH, a = 300 N mm, at various temperatures.

Fig. 2-19 Time to failure vs. potential for X70 pipeline steel in pH 5.5 buffer solution containing 150 mg L sulfide ions at various loads, 15°C = -0.53 V...

This test measures the ability of a tape to resist creep under applied load. The test is covered in ASTM D-3654 and PSTC-7. A specified area (typically 12.7 mmx 12.7 mm) of conditioned tape is rolled down with a specified pressure on the substrate of choice, such as polished 302 stainless steel. The panel is fixed in the vertical position or up to 2° tilted back so that there is no element of low angle peel in the test (Fig. lb). A weight (often 1000 g) is fixed to the end of the tape and the time to failure, i.e. complete detachment from the plate, is measured. Infrequently, the time required for the tape to creep a given distance is measured and reported. 

Many attempts have been made to obtain mathematical expressions which describe the time dependence of the strength of plastics. Since for many plastics a plot of stress, a, against the logarithm of time to failure, //, is approximately a straight line, one of the most common expressions used is of the form... 

If the values for Uq and y for the material are not known then a series of creep rupture tests at a fixed temperature would permit these values to be determined from the above expression. The times to failure at other stresses and temperatures could then be predicted. 

If the material contains defects of size (2ai) and failure occurs when these reach a size (2ac) then the time to failure, tf, may be obtained by integrating the above equation. 

Creep rupture tests on a particular grade of uPVC at 20°C gave the following results for applied stress, ct, and time to failure, /. 

For the material in the previous question, use the Zhurkov-Beuche equation to calculate the time to failure under a steady stress of 44 MN/m if the material temperature is 40°C. The activation energy, Uo, may be taken as 150 kj/mol. 

The mean time to failure of various instrumentation and equipment parts would be known from the manufacturer s data or the employer s experience with the parts, which then influence inspection and testing frequency and associated procedures. Also, applicable codes and standards—such as the National Board Inspection Code, or those from the American Society for Testing and Materials, American Petroleum Institute, National Fire Protection Association, American National Standards Institute, American Society of Mechanical Engineers, and other groups—provide information to help establish an effective testing and inspection frequency, as well as appropriate methodologies. 

Equation 2.5-43 is a definition of availability." Since 1/p = MTTR (mean time to repair) and 1/A, = MTTF (mean time to failure) A more conventional definition is given by 2.5-44. 

Equipment used to process, store, or handle highly hazardous chemicals must be designed constructed, installed and maintained to minimize the risk of release. A systematic, scheduled, test and maintenance program is preferred over "breakdown" maintenance " that could compromise safety. Elements of a mechanical integrity program include 1) identification and categorization of equipment and instrumentation, 2) documentation of manufacturer data on mean time to failure, 3 ) test and inspection frequencies, 4) maintenance procedures, 5) training of maintenance personnel, 6) test criteria, and 7) documentation of test and inspection results. 

With the Industrial Revolution, life became more complex but it was not until World War II that reliability engineering was needed to keep the complex airplanes, tanks, vehicles and ships operating. Of particular concern was the reliability of radar. Prior to this time equipment was known qualitatively to be reliable or unreliable. To quantify reliability requires collecting statistics on part failures in order to calculate the mean time to failure and the mean time to repair. Since then, NASA and the military has included reliability specifications in procurements thereby sustaining the collection and evaluation of data build statistical accuracy although it adds to the cost. 

Using fire models, locations of equipment, heat transfer calculations, and environmental qualifications of the equipment, it is possible to estimate the time to failure. Fragility cuives that relate fire durations and equipment damage while considering the probability of fire suppre.ssion are produced to relate to the overall PSA. These fragility curves and their use is simitar the methods ised for seismic analysis. 

Since dependency analysis is not needed, we can go on to the BUILD program. Go to FTAPSUIT and select 5 "Run Build." It asks you for the input file name including extender. Type "pv.pch," It asks you for name and extender of the input file for IMPORTANCE. Type, for examle, "pv.ii . It next asks for the input option. Type "5" for ba.sic event failure probabilities. This means that any failure rates must be multiplied by their mission times as shown in Table 7.4-1. (FTAPlus was written only for option 5 which uses probabilities and error factors. Other options will require hand editing of the pvn.ii file. The switch 1 is for failure rate and repair time, switch 2 is failure rate, 0 repair time, switch 3 is proportional hazard rate and 0 repair time, and switch 4 is mean time to failure and repair time.)... 

Suppose that X denotes the time to failure of a bus section in an electrostatic precipitator. Then X is a continuous random variable whose range consists of the real numbers greater tlian zero. 

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