What is stress

In simple terms, stress is a psychological and physiological reaction to events or situations in our environment. Whatever triggers the reaction is called a stressor. So stress is the reac- 

We usually talk about stress in negative terms. It is unwanted and uncomfortable, but the first definition of stress in my copy of The American Heritage Dictionary (1992) is importance, significance, or emphasis placed on something (page 1205). Similarly, The New Merriam-Webster Dictionary (1989) defines stress as a factor that induces bodily or mental tension. .. a state induced by such a stress. . . urgency, emphasis (page 701). 

The bad state is distress. Distress is defined as anxiety or suffering. . . severe strain resulting from exhaustion or an accident The American Heritage Dictionary 1991, page 410) or suffering of body or mind pain, anguish trouble, misfortune. .. a condition of desperate need The New Merriam-Webster Dictionary, 1989, page 224). 

Psychological research supports these distinctions between stress and distress. Stress can be positive, giving us heightened awareness, sharpened mental alertness, and an increased readiness to perform. Certain psychological theories presume that some stress is 

The Yerkes-Dodson law states that, up to a point, performance will increase as arousal, or pressure to perform well, increases, but the best performance comes when arousal is optimum rather than maximum. Push a person too far and his performance starts to deteriorate. In fact, at exceptionally high levels of pressure or tension a person might perform as poorly as when he is hardly stimulated at all. Ask someone who is hysterical and someone who is about to fall asleep to do the same job, and you will not be pleased by either one s results. 

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Fig. 3 shows results of Wirotest 202 A indications as a function of compressive stress for cylindrical samples d> 18 mm made of steel LH 15, what is turn indieates that the dependence of indications on compressive stress is linear in the range up to 180 MPa with regard of quenched (1) and up 120 MPa with regard to annealed (2) samples. 

A problem obviously exists in trying to characterise anomalies in concrete due to the limitations of the individual techniques. Even a simple problem such as measurement of concrete thickness can result in misleading data if complementary measurements are not made In Fig. 7 and 8 the results of Impact Echo and SASW on concrete slabs are shown. The lE-result indicates a reflecting boundary at a depth corresponding to a frequency of transient stress wave reflection of 5.2 KHz. This is equivalent to a depth of 530 mm for a compression wave speed (Cp) of 3000 m/s, or 706 mm if Cp = 4000 m/s. Does the reflection come from a crack, void or back-side of a wall, and what is the true Cp ... 

Shock-compression processes are encountered when material bodies are subjected to rapid impulsive loading, whose time of load application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. In our everyday experience, such loadings are the result of impact or explosion. To the untrained observer, such events evoke an image of utter chaos and confusion. Nevertheless, what is experienced by the human senses are the rigid-body effects the time and pressure resolution are not sufficient to sense the wave phenomena. 

What is the form of G in the presenee of a nonisotropie stress tensor (i.e., nonzero shear stress) and... 

To see what is going on physically, it is easier to return to our first condition. At low stress, if we make a little neck, the material in the neck will work-harden and will be able to carry the extra stress it has to stand because of its smaller area load will therefore be continuous, and the material will be stable. At high stress, the rate of workhardening is less as the true stress-true strain curve shows i.e. the slope of the o/e curve is less. Eventually, we reach a point at which, when we make a neck, the workhardening is only just enough to stand the extra stress. This is the point of necking, with... 

Explain what is meant by the ideal strength of a material. Show how dislocations can allow metals and alloys to deform plastically at stresses that are much less than the ideal strength. Indicate, giving specific examples, the ways in which metals and alloys may be made harder. 

Explain briefly what is meant by a dislocation. Show with diagrams how the motion of (a) an edge dislocation and (b) a screw dislocation can lead to the plastic deformation of a crystal under an applied shear stress. Show how dislocations can account for the following observations ... 

It is required to find the torque without slippage that can be transmitted by a hub that is assembled by an interference fit to a powered shaft. The hub outside diameter D = 070 mm, and the shaft diameter d = 050 mm, as shown in Figure 4.55. The length of the hub is 100 mm. Both hub and shaft are machined from hot rolled steel SAE 1035 with a yield strength Sy A(342,26) MPa (see Table 4.6). Given that the hub is stopped suddenly in service due to a malfunction, and considering only the torsional stresses, what is the probability that the shaft will yield ... 

What is important to realise is that a polymer may be tough when exposed to tensile load but brittle when assessed by an Izod-type test where a notched sample is subjected to a bending load. Table 9.3 attempts to summarise the behaviour of typical polymers to different stresses. 

Most materials scientists at an early stage in their university courses learn some elementary aspects of what is still miscalled strength of materials . This field incorporates elementary treatments of problems such as the elastic response of beams to continuous or localised loading, the distribution of torque across a shaft under torsion, or the elastic stresses in the components of a simple girder. Materials come into it only insofar as the specific elastic properties of a particular metal or timber determine the numerical values for some of the symbols in the algebraic treatment. This kind of simple theory is an example of continuum mechanics, and its derivation does not require any knowledge of the crystal structure or crystal properties of simple materials or of the microstructure of more complex materials. The specific aim is to design simple structures that will not exceed their elastic limit under load. 

In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate ... 

In a small polypropylene component a tensile stress of 5.6 MN/m is applied for 1000 seconds and removed for 500 seconds. Estimate how many of these stress cycles could be permitted before the component reached a limiting strain of 1%. What is the equivalent modulus of the material at his number of cycles The creep curves in Fig. 2.5 may be used. 

A slit die is designed on the assumption that the material is Newtonian, using apparent viscous properties derived from capillary rheometer measurements, at a particular wall shear stress, to calculate the volumetric flow rate through the slit for the same wall shear stress. Using the correction factors already derived, obtain an expression for the error involved in this procedure due to the melt being non-Newtonian. Also obtain an expression for the error in pressure drop calculated on the same basis. What is the magnitude of the error in each case for a typical power law index n = 0.377... 

Equations (2.9) and (2.10) are representative of all isotropic, homogeneous solids, regardless of the stress-strain relations of a solid. What is strongly materials specific and uncertain is the appropriate value for shear stress, particularly if materials are in an inelastic condition or anisotropic, inhomogeneous properties are involved. The limiting shear stress controlled by strength is termed r. ... 

In their pioneering paper on laminated plates, Reissner and Stavsky investigated an approximate approach (in addition to their exact approach) to calculate deflections and stresses for antisymmetric angie-ply laminated plates [5-27]. Much later, Ashton extended their approach to structural response of more general unsymmetrically laminated plates and called it the reduced stiffness matrix method [5-28]. The attraction of what is now called the Reduced Bending Stiffness (RBS) method is that an unsymmetrically laminated plate can be treated as an orthotropic plate using only a modified D matrix in the solution, i.e.,... 

The objective of this chapter is to address introductory sketches of some fundamental behavior issues that affect the performance of composite materials and structures. The basic questions are, given the mechanics of the problem (primarily the state of stress) and the materials basis of the problem (essentially the state of the material) (1) what are the stiffnesses, (2) what are the strengths, and (3) what is the life of the composite material or structure as influenced by the behavioral or environmental issues in Figure 6-1 ... 

The term nonlinear in nonlinear programming does not refer to a material or geometric nonlinearity but instead refers to the nonlinearity in the mathematical optimization problem itself. The first step in the optimization process involves answering questions such as what is the buckling response, what is the vibration response, what is the deflection response, and what is the stress response Requirements usually exist for every one of those response variables. Putting those response characteristics and constraints together leads to an equation set that is inherently nonlinear, irrespective of whether the material properties themselves are linear or nonlinear, and that nonlinear equation set is where the term nonlinear programming comes from. 

Eventually one should emphasize that in GM stress relaxation proceeds through a unique mechanism [11], very different from what is well established... 

A cylindrical steel pressure vessel (AlSl SAE 10.85, cold rolled) with a wall thickness of 0.1 in. and an inside diameter of 1 ft is subject to an internal pressure of 1,000 psia and a torque of 10,000 ft-lb (see Figure 2-30). What is the effective stress at point A in the wall What is the factor of safety in this design ... 

Though the literature is replete with methods of measuring the moisture content, truly accurate as well as practical methods are virtually nonexistent in the food field. The situation is well illustrated in what is probably the best compendium on this subject, the Official and Tentative Methods of Analysis of the Association of Official Agricultural Chemists (2). It becomes apparent from an examination of this volume that the stress is laid not so much on accuracy as on reproducibility and practicability of a method. Though these last two factors are for the most part the only ones of importance in the control of processing procedures and in standardization of products of commerce, the factor of accuracy is, nevertheless, of extreme importance to the research worker who endeavors to establish broad quantitative generalizations for the conditions that govern the stability of foods. 

Though it is easy to determine formally what is a yield stress, in practice its measurement faces essential methodical and principal difficulties. Here two approaches are basically used, stationary and dynamic . During stationary measurements a flow curve is measured and interpretation of the results obtained leads to the definition of a yield stress. An example of such an approach is given in Fig. 1, where experimental points are shown conventionally. They can be obtained under the con-... 

Unfortunately, a few papers are known where normal stresses during shear flow of filled polymers were measured directly. Here an additional problem is connected with the solution of the problem what is considered a one-valued measure of elasticity of a material and under what conditions to compare the measured values of normal stresses. Moreover, the data at hand often represent rather a contradictory picture. 

It seems of importance to elucidate what the stresses are that should be applied to the melt in order to (1) ensure formation of a nematic phase in the melt, and (2) attain values of > fiCI so that the crystallization caused by melt extension should proceed by the chain-extension mechanism. It is also desirable to answer the question whether the formation of the nematic phase is an indispensable intermediate stage preceding formation of ECC51). 

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