Sustained stress

Rupture. Rupture strain decreases steadily with increases in the duration of stress. Alternately, the magnitude of stress needed to cause rupture decreases as the duration of stress increases. Figure 2-31 shows the development first of damage and then of yielding in a PVC compound as a function of its being under sustained stress. The decay at the onset of the first damage and of yield... 

Change in strength under sustained stress from beginning to end of decade, or unit change in log time. 

Recovery is the strain response that occurs upon the removal of a stress or strain. The mechanics of the recovery process are illustrated in Fig. 2-34, using an idealized viscoelastic model. The extent of recovery is a function of the load s duration and time after load or strain release. In the example of recovery behavior shown in Fig. 2-34 for a polycarbonate at 23°C (73°F), samples were held under sustained stress for 1,000 hours, and then the stress was removed for the same amount of time. The creep and recovery strain measured for the duration of the test provided several significant points. 

For the designer it is not important whether cracking develops upon exposure to a benign or an aggressive medium. The important considerations are the embrittlement itself and the fact that apparently benign environments can cause serious brittle fractures when imposed on a product that is under sustained stress and strain, which is true of certain plastics. 

Stress factors Sustained stress, cyclic stress, compression set (in rubbers) under continuous loading... 

The rapid cooling of certain plastic products can result in frozen in stresses and strains (particularly with injection molding). The stresses may decay with time in a viscoelastic manner. However, they will act like any other sustained stress to aggravate cracking or crazing in the presence of aggressive media and hostile environments like UV radiation. 

The central nervous system, and the brain in particular, is the prime candidate for both being the sensor and normally the major controller of the response to physiological and psychological stimuli. However, under pathological circumstances, such as the influence of sustained stressful, unhealthy psychosocial environments and different damaging lifestyles such as those observed in unhealthy societies, the brain is altered... 

Special precautions during high-risk times [e.g., severe traumatic life events, sustained stress, transmeridian travel, seasonal changes, premenstrual period)... 

For example, creep has an adverse effect on the behaviour of a prestressed concrete member but is irrelevant when considering curbs and gutters since these are subjected to very small sustained stresses. 

The X-ray diffraction pattern for the electrolyte of the anode-supported cell is measured and the residual stresses were estimated by the sin2 diffracting planes of a crystallite in a microscopic grain can be expressed as... 

In past studies (3), Electron Spin Resonance, ESR, has been used to investigate bond rupture associated with stress-ozone degradation in unsaturated rubbers. It was believed that it might be enlightening to perform similar experiments on plastics ( 5, 8, 10, 11). In the study reported here the combined effects of environment (O3, N02 and S02) and sustained stress on the ultimate properties of Nylon 6 fibers are presented. These agents were found to have a profound effect on strength, toughness and the deformation-bond rupture kinetics. 

For AK >10, the crack growth rate increases rapidly with increase in AK, the increase starting at higher AK the higher the frequency. The slope of the data, which are initially approximately linear, decreases toward a plateau similar to that observed for sustained-stress SCC. 

Creep and the Effects of Temperature. When sulfur or sulfur composites are subjected to sustained stress, creep is observed. Since the magnitude of this phenomenon is significant from the engineering point of view, the causes of creep are of interest. 

On the other hand, when a part is subjected to a permanent deformation (Figure 2.12 b), a gradual decrease of stress is observed due to stress relaxation. Creep and stress relaxation often act together and strongly influence the dimensional stability of plastics-based products they are of interest to engineers in any application where the material must sustain stress or strain over a long period of time. 

Creep rate varies with stress level - generally the higher the stress, the greater the creep rate. It has also been suggested (29) that when the sustained stress is lower than some equilibrium value then indefinite creep will not occur. When stressed above this value the material will creep to failure. What happens to adhesive joints at the lower stress levels is perhaps more important in civil engineering than rapid creep to failure at high stress levels. This is so because structural adhesive joints tend to be designed to withstand low mean stresses, for example 10% of ultimate, but which have to be sustained for many years. Creep rate also varies with temperature. An increase above room temperature results in an increase in creep rate until the Tg is reached, when there is a marked further increase in creep rate. 

A further major consideration in the repair of structural concrete elements is the decision as to whether to relieve the element of load whilst the repair process is being performed. If the application of live or imposed load is prohibited during the process of concrete removal but dead load is not temporarily resisted by propping then there will be a redistribution of dead load stresses within the residual section. Further, the patch repair material will not subsequently contribute to the support of dead load. Alternatively, if both dead and live loads are supported by temporary props whilst the repair is carried out then the reapplication of dead load will result in subsequent creep of the repair material under the sustained stress, whilst any cyclic live loads will induce fatigue loading on the repaired section. A third possibility, for example in the case of columns... 

Durability creep rupture D A DILLARD Creep under sustained stress Durability fatigue D A DILLARD Effect of cyclic loads... 

Stress distribution shear lag solution D A DILLARD Stresses in shear joints Creep A D CROCOMBE Occurrence protection against creep failure Durability creep rupture D A DILLARD Creep under sustained stress Durability fatigue D A DILLARD Effect of cyclic loads... 

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