Stress, types tensile/compressive

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. 

The method of obtaining creep data and their presentation have been described however, their application is limited to the exact same material, temperature use, stress level, atmospheric conditions, and type of test (tensile, compression, flexure) with a tolerance of 10%. Only rarely do product requirement conditions coincide with those of the test or, for that matter, are creep data available for all grades of material that may be selected by a designer. In those cases a creep test of relatively short duration such as 1000 h can be instigated, and the information can be extrapolated to the long-term needs. It should be noted that reinforced thermoplastics and thermosets display much higher resistance to creep (Chapter 2). 

The mechanical properties of a material describe how it responds to the application of either a force or a load. When this is compared to an area, it is called stress, another term for pressure. Three types of mechanical stress can affect a material tension (pulling), compression (pushing), and shear (tearing). Figure 15.27 shows the direction of the forces for these stresses. The mechanical tests consider each of these forces individually or in some combination. For example, tensile, compression, and shear tests only measure those individual forces. Flexural, impact, and hardness tests involve two or more forces simultaneously. 

FIGURE 14.7 Major types of stress tests compressive (a) pulling stress or tensile strength (b) and shear stress (c). 

FIGURE 8.14 Fundamental types of stress (ot-tensile or oc-compression) acting on the body. 

Two types of mechanical tests have been used the low rate of deformation tensile, compressive or bending tests and the high speed impact tests. Immiscibility of polymers is reflected in both. For example, in tensile tests the maximum strain at break (or the maximum elongation), and the yield stress (or the maximum strength) can be... 

Stress intensity within the body of a component is expressed as one of three basic types of internal load. They are known as tensile, compressive, and shear. Figure 1 illustrates the different types of stress. Mathematically, there are only two types of internal load because tensile and compressive stress may be regarded as the positive and negative versions of the same type of normal loading. 

Assessment of mechanical properties is made by addressing the three basic stress types. Because tensile and compressive loads produce stresses that act across a plane, in a direction perpendicular (normal) to the plane, tensile and compressive stresses are called normal stresses. The shorthand designations are as follows. 

Two types of stress can be present simultaneously in one plane, provided that one of the stresses is shear stress. Under certain conditions, different basic stress type combinations may be simultaneously present in the material. An example would be a reactor vessel during operation. The wall has tensile stress at various locations due to the temperature and pressure of the fluid acting on the wall. Compressive stress is applied from the outside at other locations on the wall due to outside pressure, temperature, and constriction of the supports associated with the vessel. In this situation, the tensile and compressive stresses are considered principal stresses. If present, shear stress will act at a 90° angle to the principal stress. 

Corresponding to the three main types of stress—tensile, compressive, and shear— three types of strain can be distinguished. Thus, tensile strain is expressed as elongation per unit length (Figure 3.1a),... 

Polymer blends must provide a variety of performance parameters. Usually it is a set of performance criteria that determines if the material can be used or not For specific apphcatimi more weight can be given to one or another material property. The most important properties of polymer blends are mechanical. Two types of tests have been used the low rate of deformation (tensile, compressive, or bending) and the high-speed impact Inuniscibility affects primarily the maximum elongation at break and the yield stress. 

There are several modes of operation of DMAs. The most common is the rotational/torsional type of instrument, although a number of linear tensile-compressive type are now available. These may operate in either a constant-strain or a constant-stress mode. In the former, the specimen is always deflected to a defined strain while the stress is measured. Constant-stress machines are the converse. These instruments are preferred for creepmode type experiments while constant-strain instruments lend themselves better to stress-relaxation studies. The decision of what type of DMA to use for a test then depends not only on the type of behavior under study but also on the mode of deformation. Torsional DMAs provide data in a shear mode, while tensile-compressive machines yield a tensile or compressive mode. With the application of proper fixtures, the linear DMAs are also able to perform flexural and cantilever-type measurements. 

This chapter has covered some of the stress loading situations other than simple tensile, compressive and bending loads encountered by plastics parts. The unique characteristics of plastic materials of certain types make them especially suited to resist the specific stress conditions. With proper part design, material selection and modification the designer can make parts that will perform well under unique stress conditions. 

In the debonding area two types of stress are present negative tensile (compressive stress) in the center of the circular detachment area and, still more important, shear stress in the annular region. The maximum diameter of the debonding area can serve as a measure of the adhesion at the interface. The larger the diameter the lower the adhesion rate (11). 

Compression stress-strain tests may be conducted if in-service forces are of this type. A compression test is conducted in a manner similar to the tensile test, except that the force is compressive and the specimen contracts along the direction of the stress. Equations 6.1 and 6.2 are utilized to compute compressive stress and strain, respectively. By convention, a compressive force is taken to be negative, which yields a negative stress. Furthermore, because /q is greater than /, compressive strains computed from Equation 6.2 are necessarily also negative. Tensile tests are more common because they are easier to perform also, for most materials used in structural applications, very little additional information is obtained from compressive tests. Compressive tests are used when a material s behavior under large and permanent (i.e., plastic) strains is desired, as in manufacturing applications, or when the material is brittle in tension. 

The particular type of thermoplastic elastomer (TPE) shown in Figure 3 exhibits excellent tensile strength of 20 MPa (2900 psi) and elongation at break of 800—900%, but high compression set because of distortion of the polystyrene domains under stress. These TPEs are generally transparent because of the small size of the polystyrene domains, but can be colored or pigmented with various fillers. As expected, this type of thermoplastic elastomer is not suitable for use at elevated temperatures (>60° C) or in a solvent environment. Since the advent of these styrenic thermoplastic elastomers, there has been a rapid development of TPEs based on other molecular stmctures, with a view to extending their use to more severe temperature and solvent environments. 

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