Sudden Stress

In an experiment which is the opposite of stress relaxation after sudden strain, a shear stress a may be applied within a very brief period before time 0 and then maintained constant (Fig. 1-6). The dependence of the strain y on time can be derived from equation 9 exactly as equation 12 is obtained from equation 7, with the result 

This shows the physical meaning of the creep compliance J(t), which has the dimensions of reciprocal modulus and is a monotonically nondecreasing function of time. For a perfectly elastic solid J = l/G. However, for a viscoelastic material, J t) 7 1/G(0. because of the difference between the two experimental time patterns interrelations between the two functions will be given Chapters 3 and 4. 

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The first rule for long belt life is to maintain correct belt tension. Loose belts slip, causing belt and pulley wear. The snapping action of loose belts adds sudden stress and often breaks the belt. To test for tension, press down firmly... 

Impact tests Such tests reveal the resistance of coatings to deformation and destruction by concentrated sudden stresses. They thus throw considerable light on the integrity of the metal-coating bond. Changes in adhesion through chemical reaction at the paint/metal interface will be reflected in the impact-test values. 

Although the reinforcements are intended to bear the loads, the matrices play a significant role in composite failure by damping sudden stress variations, by distributing the loads and by protecting the reinforcements. 

Other less obvious factors are also considered while choosing a suture. For instance, the age and health of the patient. An older person in poor health generally requires a longer period to heal, thus for absorbate sutures, the required degradation time needs to be increased. Conversely, for an active child, while the degradation time may be less, the suture may be exposed to more frequent sudden stresses and the body tissues are generally thinner. The presence of immunodeficiency is also an important factor since they are more susceptible... 

The main feature about molten high polymers (molecular weights higher than about 104) concerns the broadness of the relaxation spectrum that characterises the viscoelastic response of these systems. This broad two-dispersion spectrum may spread over a range of relaxation times going from about 10 9 up to several seconds [4]. It is well illustrated from the modulus of relaxation observed after applying a sudden stress to the polymer the resulting sudden deformation of the sample is then kept constant and the applied stress is released in order to avoid the flow of the polymer. For example, the release of the constraint oxy(t) is expressed as a function of the shear modulus of relaxation Gxy(t) ... 

Many materials, particularly polymers, exhibit both the capacity to store energy (typical of an elastic material) and the capacity to dissipate energy (typical of a viscous material). When a sudden stress is applied, the response of these materials is an instantaneous elastic deformation followed by a delayed deformation. The delayed deformation is due to various molecular relaxation processes (particularly structural relaxation), which take a finite time to come to equilibrium. Very general stress-strain relations for viscoelastic response were proposed by Boltzmann, who assumed that at low strain amplitudes the effects of prior strains can be superposed linearly. Therefore, the stress at time t at a given point in the material depends both on the strain at time t, and on the previous strain history at that point. The stress-strain relations proposed by Boltzmann are [4,39] ... 

Mental stress also causes the inner layer of the blood vessels to constrict, which may increase the risk of sudden cardiac death. Sudden stress leads to endothelial dysfunction, the medical term for a malfunction of the arterial linings, and the artery s ability to dilate is impaired. Swiss researchers found a significant decrease in blood vessel dilation after mental stress. Diastolic blood pressure shot up from 83 to 96 and heart rate rose from 63 to 81 beats per minute. 

Shear stress relaxation after sudden stress application in a capillary rheometer [131]. 

When a subject is suddenly strained and then the strain is maintained constant afterward, the corresponding stresses induced in the subject decrease with time, this phenomenon is called stress relaxation. If the subject is suddenly stressed and then the stress is maintained afterwards, the subject continues to deform, this phenomenon is called creep. Creep and stress relaxation are another two important phenomena in the arterial viscoelasiticity. Langewouters et al [46] studied the creep responses of human thoracic and abdominal aortic segments. The pressure in the... 

Cavitation can also lead to local destruction of protective layers. In rapidly flowing liquid and on solid surfaces that oscillate in the liquids, gas or steam bubbles are produced at sites at which the pressure in the liquid is briefly lowered to vapor pressure due to flow in excess of the critical flow rate threshold. When the pressure is raised again, these bubbles collapse suddenly (implosion) and a jet of liquid hits the material surface at a high rate of speed. This sudden stress load pattern is continuously exposing or creating active surfaces on which increased corrosion (cavitation corrosion) takes place in an aggressive medium. 

FIGURE 5.5 A guide to the application of consolidant to an object If the application is carried out under vacuum, the container should be evacuated before the addition of the liquid consolidating material. A cova- should be placed over the container to prevent premature evaporation or reaction with atmospheric moisture. The object is lifted off the bottom of the container to reduce the pos-sibiUty of air pockets (a). A small amount of polymer in Uquid form is added to start the penetration process. This Uquid is drawn up into the porous material by capillary absorption (b). The object is soaked in consoUdant aUowing time for trapped air to dissolve and the diffusion of the various components in the Uquid to approach equdibrium. Any vacuum that has been applied is broken slowly and carefully to prevent sudden stresses being appUed to the object (c). Excess consolidant is allowed to drain out before the drying ot setting takes place. This reduces excess consoUdant and reduces the fomation of a surface skin with solvent-appUed polymers. 

Between the extremes of viscous fluids and elastic solids are materials that seem to exhibit both traits. These are called viscoelastic materials or memory fluids, and their dual nature becomes most evident when we subject them to time-dependent (unsteady) tests. The three major types of unsteady tests are the so-called relaxation, creep and dynamic tests. In the previous sections, we gave definitions and descriptions for stress, strain and deformation rates. These quantities are now used in defining the various unsteady tests. Thus, in a relaxation test the sample is subjected to a sudden, constant, strain. The stress shoots up in response and then gradually decays ( relaxes ). In the creep test, a sudden stress is applied and held constant. Now the strain picks up quickly and then, while continuing to increase, slows down on its rate of increase. We say the material creeps under the constant stress. In dynamic tests, one confining wall is made to move periodically with respect to another. One monitors both the strain and the stress as a function of time. 

Original iPP is a highly cohesive polymer in the crystalline state (see Section 15.1), a property resulting from low intermolecular distances in the crystalline phase. It is much more impact-sensitive than PE, particularly at temperatures below ambient. This can be due to a relatively difficult flow of the chains under sudden stress near the glass transition temperature, in relation with their helical structure. 

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