Impact strength calculation

It has been shown that for acrylic, glass-filled nylon and methyl pentene there is reasonable correlation between the reciprocal of the stress concentration factor, K, and impact strength. However, for PVC good correlation could only be achieved if the finite dimensions of the sample were taken into account in the calculation of stress concentration factor. 

Figure 6.8 plots the reinforcement ratios for short glass fibre reinforced polyamide (PA-GF) versus neat polyamide for six important characteristics calculated versus density and material cost. These characteristics are tensile strength, tensile and flexural modulus, impact strength, HDT A and B. 

The Izod impact strength values are useful in giving a rough ranking of materials for quality control purposes, since the tests are simple and quick to carry out. The test is hardly of any use for design calculations for plastic parts. Neither has the test any physical base. 

So is the case in polymers containing additive systems. As will be shown, some additive systems can impart so much light scattering to the base resin that certain colors can no longer be achieved. Or if they can be achieved, other properties may be adversely affected, such as impact strength and cost. In either case, the practical color gamut or palette that is obtainable with this particular resin system is reduced. The discussion below presents the effects that the polymer and its additives can have on colorability. Color data presented in the following tables have been calculated under illuminant D-65,10° observer, specular included, expressed in CIELAB units, unless otherwise noted. 

It is noted that attempts to apply composites theory to the materials investigated have not been entirely successful. While upper and lower bounds on, e g., moduli can be established there is little quantitative ediction of the impact strei th or fracture toughness parameters of the composites. Hence, the systems cannot be considered as optimized, for example, with regard to impact strength versus particle size, shape, or distribution or matrix-particle adhesion. The complexity is, of course, due to the statistical structure of the dispersed phase and the resultant uncertainties in the calculations of local stress fields, which in turn imply uncertainty in the local mode of yielding or rate of yielding. 

Notched Charpy impact strength acN is calculated from the absorbed energy W, related to the smallest initial cross-section of the specimen at notch base ... 

Both logarithmic plots of Izod impact strength versus excess volume fraction over the critical stress volume fraction determined by the critical ligament distance gives the slope of 0.45, which is comparable with the critical exponent of 0.44 calculated for monodisperse particles by Bug and coworkers [12]. 

Figure 12.8. Theoretical curves for the impact strength (energy under the stress-strain curve) of filled polymers (Nielsen, 1966). Curve for case of no adhesion was calculated using stress concentration factor S = 1.

In Nielsen s treatment, the constant K is equal to unity, and the value of S, the stress concentration factor, is assumed to be equal to about 0.5 for typical cases. The curves for predicted tensile and impact strengths are plotted in Figures 12.7 and 12.8, assuming that S = 1. [For the curve in Figure 12.8 the value used for was calculated using the Sato and Furukawa (1963) relationship.] Clearly, for more realistic values of S ( 0.5) the predicted strength and toughness curves would tend to lie below the curves for the case of S = 1. 

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