Types of Stress

Flow behaviour of polymer melts is still difficult to predict in detail. Here, we only mention two aspects. The viscosity of a polymer melt decreases with increasing shear rate. This phenomenon is called shear thinning [48]. Another particularity of the flow of non-Newtonian liquids is the appearance of stress nonnal to the shear direction [48]. This type of stress is responsible for the expansion of a polymer melt at the exit of a tube that it was forced tlirough. Shear thinning and nonnal stress are both due to the change of the chain confonnation under large shear. On the one hand, the compressed coil cross section leads to a smaller viscosity. On the other hand, when the stress is released, as for example at the exit of a tube, the coils fold back to their isotropic confonnation and, thus, give rise to the lateral expansion of the melt. 

Fig. 4. Types of stress—strain curves (a) soft and weak (b) hard and brittle (c) soft and tough (d) hard and strong and (e) hard and tough.

In postulating a statistieal model for a statie stress variable, it is important to distinguish between brittle and duetile materials (Bury, 1975). For simple stress systems, i.e. uniaxial or pure torsion, where only one type of stress aets on the eomponent, the following equations determine the failure eriterion for duetile and brittle types to prediet the reliability (Haugen, 1980) ... 

Type of stress Polymers ductile at 25°C and at 1 min strain rate... 

Type of stress. A uniaxial tensile creep test would not be expected to give the required data if the designer was concerned with torsional or compressive creep. 

Classical lamination theory consists of a coiiection of mechanics-of-materials type of stress and deformation hypotheses that are described in this section. By use of this theory, we can consistentiy proceed directiy from the basic building block, the lamina, to the end result, a structural laminate. The whole process is one of finding effective and reasonably accurate simplifying assumptions that enable us to reduce our attention from a complicated three-dimensional elasticity problem to a SQlvable two-dimensinnal merbanics of deformable bodies problem. 

Nitrogen compounds These also arise from both natural and synthetic sources. Thus ammonia is formed in the atmosphere during electrical storms, but increases in the ammonium ion concentration in rainfall over Europe in recent years are attributed to increased use of artiflcial fertilisers. Ammonium compounds in solution may increase the wettability of a metaland the action of ammonia and its compounds in causing season cracking , a type of stress-corrosion cracking of cold-worked brass, is well documented. 

Electrodeposits are usually in a state of internal stress. Two types of stress are recognised. First order, or macro-stress, is manifest when the deposit as a whole would, when released from the substrate, either contract (tensile stress) or expand (compressive stress) (Fig. 12.12). Second order or microstress, occurs when individual grains or localities in the metal are stressed, but the signs and directions of the micro-stresses cancel on the larger scale. The effects of first order stress are easily observed by a variety of techniques. 

At least, in absolute majority of cases, where the concentration dependence of viscosity is discussed, the case at hand is a shear flow. At the same time, it is by no means obvious (to be more exact the reverse is valid) that the values of the viscosity of dispersions determined during shear, will correlate with the values of the viscosity measured at other types of stressed state, for example at extension. Then a concept on the viscosity of suspensions (except ultimately diluted) loses its unambiguousness, and correspondingly the coefficients cn cease to be characteristics of the system, because they become dependent on the type of flow. 

Based on this analysis it is evident that materials which are biaxially oriented will have good puncture resistance. Highly polar polymers would be resistant to puncture failure because of their tendency to increase in strength when stretched. The addition of randomly dispersed fibrous filler will also add resistance to puncture loads. From some examples such as oriented polyethylene glycol terephthalate (Mylar), vulcanized fiber, and oriented nylon, it is evident that these materials meet one or more of the conditions reviewed. Products and plastics that meet with puncture loading conditions in applications can be reinforced against this type of stress by use of a surface layer of plastic with good puncture resistance. Resistance of the surface layer to puncture will protect the product from puncture loads. An example of this type of application is the addition of an oriented PS layer to foam cups to improve their performance. 

Aircraft radomes have also been extensively studied for the effects of wind-driven water and solids. The erosion effects are very dramatic and the surfaces are usually protected with elastomeric materials that have good resistance to this type of stress. 

The next step is to determine an allowable working stress. This is done by using a safety factor usually of IV2 to 2V2 on the yield strength or tensile strength. If the type of stress is clearly defined, the IV2 factor is adequate otherwise, it should be higher (Chapter 2, Safety Factor). 

This type of stress-related corrosion process may result in boiler failure through a sudden and violent rupturing of the boiler tube metal. Austenitic stainless steels also are corroded by SCC mechanisms in the presence of concentrated chlorides (chloride-induced SCC). 

As more is learned about the chromosomal effects on HS gene expression, it is important to point out that these genes are actually a subset of inducible responses to cellular stress. Not all of these inducible responses involve HSF, and this indicates that cells have diversified transcriptional responses to cope with different types of stress. This diversification is manifested by glucose regulated genes (grp), as well as the metallothionein and oxidant-injury genes (Watswich, 1988 Storz et al., 1990 Devary et al., 1992 Skroch et al., 1993 Xu, 1993). 

Adell, A, Casanovas, JM and Artigas, F (1997) Comparative study in the rat of the actions of different types of stress in the release of 5-HT in the raphe nuclei and forebrain areas. 

Figure 1.146, Stre.ss trajectory map.s of southern Northeast Honshu in the late Cenozoic period, after Tsunakawa and Takeuchi (1986) with a slight addition. oh, . trajectory is drawn by smoothing the inferred stress orientations from the selected dike-swarms with K-Ar dates. Selected major faults with age estimation are also shown for indicating types of stress fields. T Extensional stress field, where ay > a 2>cth , and normal or gravity faulting is preferable. P Compre.ssional, oh > ay, reverse or thrust faulting...

Standard fatigue tests normally require extended periods of time to complete regardless of the type of stress/strain used. Accelerated fatigue tests change some conditions of the test in the hope of reaching the same end point in less time. Potential means of accelerating the tests include changing factors such as the temperature, frequency, chemical environment, levels of stress or strain, or the addition of stress concentrators. 

Figure 3 General types of stress-strain curves.

As regards their physiological roles, many polyphenols (because of their antimicrobial and antioxidant properties) are directly involved in the response of plants to various types of stress (Lopcz-Nicolas and others 2006 Lopez-Nicolas and Garcia-Carmona 2008). 

A higher mutation rate in response to stress is also supported by data from a study with rats showing that several different types of stress can induce sister chromatid exchange (SCE) in bone marrow cells.78 The processes for repair or removal of damaged DNA are important parts of the initiation events for tumorigenesis, since faulty DNA repair has been associated with an increased incidence of cancer.79... 

Type of Stress Reinforcing Steel Concrete Ultimate... 

To incorporate the effect of material strength increase with strain rate, a dynamic increase factor (DIF) is applied to static strength values. DIFs are simply ratios of dynamic material strength to static strength and are a function of material type as well as strain rate as described above. DIFs are also dependent on the type of stress (i.e. flexural, direct shear) because peak values for these stresses occur at different times. Flexural stresses occur very quickly while peak shears may occur relatively late in time resulting in a lower strain rate for shear. 

Steel, aluminum, concrete, and other materials that form part of a process or building frame are subject to structural failure when exposed to fire. Bare metal elements are particularly susceptible to damage. A structural member undergoes any combination of three basic types of stress compression, tension, and shear. The time to failure of the structural member will depend on the amount and type of heat flux (i.e., radiation, convection, or conduction), and the nature of the exposure (one-sided flame impingement, flame immersion, etc.). Cooling effects from suppression systems and effects of passive fire protection will reduce the impact. 

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