Failure Measurements

Experience has also shown that in cases such as stress rupture and thermal ageing the test parameters may have to be designed progressively. Shorter tests at higher loads (or temperatures) are set up first and the times to failure measured. The test conditions for longer lifetimes are then set on the basis of these results. This is particularly important where the validity of the final result depends on obtaining a failure within a particular time interval, e.g., over 5,000 h as in IEC 60216 [2], or where the measurements must be completed within a set time. 

Uniaxial tension testing with superposed hydrostatic pressure has been described by Vernon (111) and Surland et al. (103). Such tests provide response and failure measurements in the triaxial compression or tension-compression-compression octants. 

Some materials might produce a unique failure surface providing measurements could be conducted under first stretch conditions in a state of equilibrium. Tschoegl (110), at this writing, is attempting to produce experimental surfaces by subjecting swollen rubbers to various multiaxial stress states. The swollen condition permits failure measurements at much reduced stress levels, and the time dependence of the material is essentially eliminated. Studies of this type will be extremely useful in establishing the foundations for extended efforts into failure of composite materials. 

Based on our experience, the strength of membrane/MEA, particularly strain-to-failure, measured ex situ has the potential to serve as a metric/indicator for PEMFC life prediction and accelerated testing when the failure mode is membrane crossover. It is an... 

Elongation to failure measured in tensile stress-strain measurements at room temperature (i.e., approximately 25 C) vary significantly as the composition of the polymer is varied from the glassy PS to the elastomeric PB (Figure 6). Also it is clear that the tensile properties of the random copolymer is significantly different from the same composition block copolymer. This difference is due, in part, to the differences in molecular weight (Table 1). 

Williams, J. C., and A. H. Birks. 1967. The comparison of the failure measurements of powders with theory. Powder Technol. 1 199-206. 

Precision of time-to-failure measurements in controlled conditions, when the said test procedure is closely followed, is rather fair. ASTM D 3012-00 lists an example with three polypropylene samples, apparently, of different origin, tested in seven different laboratories (a ronnd robin test). They showed oxidative stability of the samples (time to failnre) at 150°C as 14.0 0.8,35 3, and 63 5 days, respectively, for within-laboratory standard deviations of the average, that is, within 6-9% of the average, and 14 + 3, 35 + 7, and 63 + 19 days, respectively, for between-laboratory standard deviations of the average, that is, within 20-30% of the average. 

Frequency is not the same as probability. An item has a frequency of failure measured in inverse time units such as once in a 100 years. The consequences of that event may be mitigated by a safeguard, which has a (dimensionless) probability of occurrence. For example, high level in a tank may occur once every 2 years. However, the tank has level control instruments that detect high level and stop the flow of liquid into the tank. These instruments may have a probability of failure of 0.01 or 1%. Therefore, the likelihood of a system failure is 0.005 year , i.e., once in 200 years. 

In reliability engineering, the primary statistical variable of interest is Time To Failure (T). The time to failure measurement can be analyzed to generate another important measurement, failure rate. Instantaneous failure rate is a commonly used measure of reliability that gives the number of failures per unit time from a quantity of components exposed to failure. 

Driver s front airbag ignition loop Enable the failure Measure the failure of loop... 

Table 1. Safety integrity levels target failure measures for a safety function operating in low demand mode.

In the present paper, only the low demand mode of operation is assumed (i.e. the safety function is only performed on demand, and not more than one per year). The target failure measure is then defined by the average probability of dangerous failure on demand of the safety function (PFDavg), and the associated SIL is determined by Table 1. Note that these definitions stem from lEC 61508, 2009 revision, which are more precise than in the first edition. 

One broad class of outcome measures commonly used to measure safety performance is called failure measures. These measures are generated from the injury recordkeeping system. Included in this group can be performance measured by dollar criteria. Losses and cost savings attributed to safety performance will often mean more to management than other safety measures such as frequency and severity rates. 

K. N. Mathes, Surface failure measurements, in Engineering Dielectrics, Vol. IIB, Electrical Properties of Solid Insulating Materials Measurement Techniques, ASTM Special Techiiical Publication 926, Ed. R. Bartnikas, 1987, p. 221. 

A common cause of death in Western Europe is chronic heart failure. Measures for its severity are hemodynamic parameters, including stroke volume (SV). Until now, the gold standard for measuring these parameters is the thermodilution technique using pulmonary artery catheters. However, the risks of estimating cardiac output via catheters include infections, sepsis and arrhythmias, as well as increased morbidity and mortality. An alternative technique to assess SV easily and cost-effectively is the use of impedance cardiography (ICG). 

The time to failure measured on non-precracked specimens represents the sum of the initiation and propagation times of a crack. Unfortunately, crack initiation, which often is a slow step, is a random phenomenon. As a result, the reproducibility of corrosion fatigue tests earried out with non-precracked specimens is usually mediocre. Furthermore, to determine a Wohler eurve, a large number of specimens is needed. Statistical methods are usually applied to determine the endurance limit in corrosion fatigue tests. 

NOTE 1 The target failure measures for the safety integrity levels are specified in Tables 3 and 4. 

The probability of failure on demand of each safety instrumented function shall be equal to, or less than, the target failure measure as specified in the safety requirement specifications. This shall be verified by calculation. 

NOTE 1 In the case of safety instrumented functions operating in the demand mode of operation, the target failure measure should be expressed in terms of the average probability of failure to perform its design function on demand, as determined by the safety integrity level of the safety instrumented function (see Table 3). 

NOTE 4 The target failure measure may be a specified value of average probability of failure on demand or dangerous failure rate derived from a quantitative analysis or the specified range associated with the SIL if it has been determined by qualitative methods... 

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