Strength yield

Yield strength is a measnre of the force that a material can withstand before it suffers macroscopic plastic deformation. For most materials, e.g., metal, it is taken as the point on the stress-strain cnrve when the line becomes non-linear (the elastic limit). However, for plastics, it is taken as the peak of the stress-strain curve, as that is simpler to measure. In practice, most parts are designed so that they never experience a force approaching the yield stress because yielding represents failure of the material. 

Several factors determine the level of reinforcement attained by adding filler. These include, the volume fraction of filler added, the surface area of the filler (related to particle size), particle shape, the level of adhesion between the filler and polymer [80], as well as the thickness and natnre of the interphase between the two phases. A linear correlation between yield strength and heat of crystallisation has also been reported in the case of PP filled with calcium carbonate [81]. It is well known that spherical fillers give least 

In the final analysis though, the lab scale impact tests are of limited value. They may he used to compare materials qualitatively, hut the ultimate test is to make the part or prototype and perform impact tests on that, in a manner that closely simulates the way in which the part is to be handled during manufacture, or used. 

Tensile Modulus Tensile Strength Unnotched Impact Strength Notched Impact Strength 

In the case of PP copolymer and impact modified PP, the effect of filler is very different impact strength is lowered significantly by hard particulate fillers. This is because the filler interacts with the soft or rubbery component and nullifies its ability to adsorb energy during impact. 

FIGURE 2.2 Different mechanical behavior of viscoelastic polymers under tension, (a) Stress-strain behavior of acetal copolymer, (b) normal fracture plane after 50% strain in acetal copolymer, (c) hysteresis in strain cycling acetal copolymer, and (d) longitudinal fracture of PE tube under tension to 400%. 

From the tensile test, another important property is measured and that is the Poisson ratio value (t, which is a measure of the contraction strain 8, in the transverse direction when a strain e is applied in the longitudinal direction. Stretching takes place in the load direction but shortening takes place in the other two directions for isotropic materials. 

If the material is not isotropic, the value p measured in the thickness direction (t) will be different from the value determined in the width direction (w). Such materials are either orthotropic or anisotropic. These materials are considered in Chapter 8. 

The mechanical properties under tension are not sufficient to determine total material behavior. The properties under compression, flexure, and shear can be different. For ductile materials, it is generally assumed that the behavior in compression is the same as that in tension, that is, that the compressive strength is the same and that the modulus E is the same as obtained in a simple tension test. Also, for ductile behavior, the maximum shear yield strength under tension is taken as a measure of the maximum shear strength, such that 

Furthermore, if the material is assumed isotropic, and if the stresses are within the proportional limit, the shear modulus G can be calculated from 

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The situation is more complex for rigid media (solids and glasses) and more complex fluids that is, for most materials. These materials have finite yield strengths, support shears and may be anisotropic. As samples, they usually do not relax to hydrostatic equilibrium during an experiment, even when surrounded by a hydrostatic pressure medium. For these materials, P should be replaced by a stress tensor, <3-j, and the appropriate thermodynamic equations are more complex. 

In reality most solids in contact under macroscopic loads undergo irreversible plastic defonnation. This is caused by the fact that at high nonnal forces the stresses in the bulk of the solid below the contact points exceed the yield stress. Under these conditions the contact area expands until the integrated pressure across the contact area is equal to the nonnal force. Since the pressure is equal to the yield strength of the material cr, the plastic contact area is given by... 

Tensile yield strength, 103 lb in-2 Thermal Burning rate, mm min Coefficient of linear thermal expansion, 10 °C 50-90 0.5-2.2 50-90 50-80 50-60 10-13 Self- extinguishing 40-55 46... 

Tensile yield strength, 1Q3 lb in-3 Thermal Burning rate, mm min Not Not Not Not Not Self- Self-... 

The presence of spherulites or smaller crystallites is comparable to cross-linking and affects not only the moduli and compliances, but also the ultimate properties such as yield strength and ultimate elongation. 

Tensile properties of importance include the modulus, yields, (strength at 5% elongation), and ultimate break strength. Since in many uses the essential function of the film may be destroyed if it stretches under use, the yield and values are more critical than the ultimate strength. This is tme, for example, where film is used as the base for magnetic tape or microfilm information storage. In some cases, the tensile properties at temperatures other than standard are critical. Thus if films are to be coated and dried in hot air ovens, the yield at 150°C or higher may be critical. 

Provided the design is such that it can be represented by coordinates which fall within the unshaded area, then the residual stress will not exceed the yield strength of the material. When the cylinder is built up of n components of the same material, it can be shown (35,36) that the interference per unit radius required for all cylinder mating operations is given by... 

In practice compound shrinkage is often used to prestress a high strength or corrosion-resistant liner. The optimum radius ratios of components of different yield strengths have been shown (37,38) to be... 

Creep Rupture. Metals and their alloys lose appreciable strength at elevated temperatures. For most materials, the ultimate tensile and yield strengths fall off regularly as the temperature iacreases, as illustrated ia Figure 2 (2). The exceptions are some iatermetaUics, eg, nickel aluniinide(3 l)... 

Antimony content, wt % Yield strength after aging, MPa "- Tenshe strength, MPa Elongation, %... 

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