Cyclic stress testing

Bills (7) has applied an adaptation of this law to solid propellants and propellant-liner bonds for discrete, constantly imposed stress levels considering U to be the time at the ith stress level and tfi the mean time to failure at the ith stress level. A probability distribution function P was included to account for the statistical distribution of failures. For cyclic stress tests the time is the number of cycles divided by the frequency, and the ith loading is the amplitude. The empirical relationship... 

A configuration frequently used in cyclic stress testing is the reduced area adhesive bond (RAAB) specimen. This is similar to the blister detection specimen, but is 1/2 inch wide and a 1/4-inch hole is drilled through the center of the overlap region this provides more environmental exposure of the bondline. Potter has reported using RAAB specimens to compare the durabilities of several modified epoxy adhesives.Specimens subjected to cyclic stressing failed in fewer hours than specimens held at a constant stress, even when cycled at a lower stress level. He also found that a slow cyclic rate was more damaging than a fast cyclic rate. 

Figure 25. Cyclic stress testing cycles and time to failure. After ref. 103.

Here Acto is the cyclic stress range for failure in Nf cycles under zero mean stress, and Acr m is the same thing for a mean stress of a .) Goodman s Rule is empirical, and does not always work - then tests simulating service conditions must be carried out, and the results used for the final design. But preliminary designs are usually based on this rule. 

It is often difficult to conduct laboratory tests in which both the environmental and stressing conditions approximate to those encountered in service. This applies particularly to the corrosive conditions, since it is necessary to find a means of applying cyclic stresses that will also permit maintenance around the stressed areas of a corrosive environment in which the factors that influence the initiation and growth of corrosion fatigue cracks may be controlled. Among these factors are electrolyte species and concentration, temperature, pressure, pH, flow rate, dissolved oxygen content and potential (free corrosion potential or applied). 

Dynamic properties are taken to mean the results from mechanical tests in which the plastic is subjected to a deformation pattern from which the cyclic stress-strain behaviour is calculated. These do not include cyclic tests in which the main objective is to fatigue the material. 

Cyclic fatigue measurements require the specimen to be subjected to cyclic stress or strain of a higher amplitude than that employed for the simple dynamic test just described. The deformation must be of sufficient intensity to bring about specimen failure after a certain number of cycles, N. The value of stress leading to failure for a given N is typically 20% to 40% of the static tensile strength. 

Another important factor for tensile testing is cyclic testing or applying force and releasing (or relaxing) force on a test specimen to evaluate its ability to endure cyclic fatigue. A typical stress vs. strain relationship is shown in Fig. 3.16 and a cyclic stress relationship in Fig. 3.17. 

Carstensen JT, Rhodes CT. Cyclic temperature stress testing of pharmaceuticals. Drug Dev Ind Pharm 1993 19 401 -03. 

Environmental aging is usually less severe in service (laboratory tests tend to accelerate aging so that the testing can be completed in a reasonable time). However, the effects of the actual service environment are generally more complex. For example, there may be simultaneous exposure to cyclic stress, cyclic temperature, and humid environments. 

The bond length of the specimens under cyclic tensile test was 20mm. Cyclic tensile tests were performed under the condition of stress ratio 7 = 0.1 and a loading frequency / = 5 Hz. Cyclic tensile loads applied to the co-cured single lap joint specimens were 30%, 40%, 50%, 60%, and 70% of the tensile load bearing capacity obtained from the static tensile load test. 

Figure 6. Stress-strain curves obtained from cyclic tensile tests. Samples of soybean flour protein and pectin containing without PEO (A) and with PEO (B). For the PEO free films, the loop created in the first cycle is larger than following cycles. For the PEO included films, the size of loops gradually decreased as cycled, then became constant in the last two cycles.

In a typical fatigue test, a sample is subjected to an alternating stress of a given amplitude and frequency. The cyclic stress amplitude is defined as... 

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