Stress/life curves

Yet another variation of this test, which is also used to obtain creep information, is to simply attach a load to a specimen and measure its time to failure. The results are then plotted in a format identical to the one shown in Fig. 12.1 la for cyclic fatigue, and are referred to as static fatigue or stress/life curves. 

According to the shape of stress-life curve with general metal materials the following relationship can be obtained ... 

Figure 10.29 Slope of normalized stress/life curve against fibre modulus for fibre reinforced plastics. XA, HT and HM are carbon fibres with various tensile moduli (as in Figure 3.1) (Jones etal.).

The most common published fatigue data chart is the stress-life curve which is commonly called an S-N curve or a Wohler [10] curve. This is a graph of the magnitude of a cyclic stress (S), linear or log scale, against the cycles to failure (N) on a log scale. The cyclic measurement is made under constant oscillatory... 

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. 

The article takes the life time of elastomer when TE decrees to 500 MPa-% at which the elastomer is tough enough, considering the stress-strain curve in Figure 10b. 

For example, Melin et al. [20] showed that fatigue tests of impact-damaged laminates in tension—compression fatigue at R values of — 1 and —5 have almost identical stress-life (S-N) curves when the lives are plotted in terms of maximum compression stress. Tests at an J value of -1 have a mean stress of zero and equal excursions into tension and compression, whereas tests atR= -5 will have stress cycles with tension... 

Fig. 4.68 Stress-life (S-N) curves of several thermoplastics materials in mode Cctai at 23 °C [O60ss].

Fig. 49. Fatigue life curves for some lilamenis of glassy metals. Full curves indicate ribbons and broken curves indicate wires. For further details and references see Table 7. The curves for the wires and the FeCr alloy have been measured in the bending mode with imposed surface strain. In order to represent the.se on the same stress scale this strain has been multiplied by their Young modulus. The bulk amorphous alloy has also been measured in the bending mode but with imposed bending stress.

Flex life n. Informally, the number of bending-reversal cycles causing a part to fail in a particular service. Most specifically, the number of cycles to failure of a test specimen repeatedly bent in a prescribed manner. The ASTM test for plastics is D 671. The specimen, molded or cut from sheet, is subjected to load reversal at 30 Hz at a predetermined level of outer-fiber stress until it either fails or the test is discontinued. By setting up different stresses for successive specimens, one can develop a graph of stress at failure versus number of cycles to failure (usually plotted on semi-logarithmic coordinates), i.e., the flex-life curve of fatigue curve. 

Fig. 8.44. Room temperature tension-tension fatigue-life curves for yttrium, dyprosium and erbium with maximum tensile stress given in terms of the fraction of the respective ultimate tensile strengths which were not given explicitly. From Love (1960).

Fig. 3.1-122 Stress-rupture curves for 1000-h life of cast Co-based superalloys...

A further complication of creep is that it is nonlinear in strain, just as the stress-strain relationship is nonlinear for plastics. Since plastics are typically subjected to large deformations during their life, it is unfortunately essential to characterize this phenomenon. This phenomenon is best seen in the classic isochronous stress-strain curve which plots stress-strain relationships at several times, decades apart. These curves are invaluable for design and product performance evaluation. 

Calculation of the final dimensions based on the stress-time curve for the design life of the part. 

The slope of the normalized stress/fatigue life curve versus fibre modulus is summarized in Figure 10.29. This shows the effect of fibre type, modulus and lay-up for carbon and glass composites. It is clear that the higher the modulus, the lower is the rate of loss of strength during fatigue. 

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