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Modern Packaging Magazine - September 1958 - Return to Main Search
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MERIT NUMBER VS. TENSILE IMPACT TOUGHNESS

TENSILE IMPACT TOUGHNESS

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1. Special jigs and fixtures were made to fit into the standard impact machine and support the specimen as shown. The ordinary hammer is removed from the swinging arm of the impact machine, so that the arm passes over the specimen and hits the striking clamp, thus elongating the specimen. The energy required to break the specimen is read from the semi-circular scale, as is usual in Izod testing.

Tensile impact testing

Before the tensile impact testing of films can be discussed, the general procedures of impact testing should be explained (3, 4, 5). The "effective gauge length55 of the dumbeJl-shaped specimen is computed from the theory of elasticity (6). In order to compute this effective gauge length, the strain energy of the dumbell-shaped specimen is equated to the strain energy of a ⅜-in.-wide rectangular specimen, which, under a given load, extends by exactly the same amount as the dumbell specimen does between grips. By means of this conversion it is possible to reduce the energy readings to a unit number in a relatively simple and standard manner. The effective volume is computed to Ev = 0.067t.

Actual loss correction and energy to break in the tensile impact test. In performing an impact test by using a pendulum-type machine, there are cer

tain corrections which are inherent to the apparatus itself. These corrections are intended to account for such energy losses as pendulum friction, pointer friction and windage. In addition, in the tensile impact test an adjustment must be made to account for the energy consumed in tossing the broken half of the specimen and its end clamp. This energy which is imparted to the free fractured half of the specimen and its accompanying end clamp is referred to as toss energy or actual toss correction.

The energy to break a specimen can be calculated by using the formula:

f - (S - T) U " (2 - T)

Where S = scale reading for breaking and tossing the specimen (corrected for friction and windage)

T = scale reading for free toss (corrected for friction and windage)

U = pendulum capacity

Tensile impact toughness

Theoretically, impact toughness is equal to the product of tensile strength and elongation, with various correction factors introduced to account for different shapes of the stress-strain curve, different rates of testing and different shapes of specimens. For simplicity, the product of tensile strength and elongation has been called the "Merit Number.55 The "Merit Number55 of several resins, plotted as a function of the tensile impact toughness, is shown in Figure 2. The correlation is seen to be quite good despite the fact that the "Merit Number55 was obtained at very slow speeds and the tensile toughness at a speed of ll½ ft. per second. This graph is principally of value in assessing the relative importance of the stress-strain curve. Variations in either strength or elongation will produce variations in impact toughness. It should be noted that the Izod test is deliberately designed to introduce the notch effect; the tensile impact test, to eliminate it.

Figure 3 shows tensile impact toughness plotted in terms of two fundamental resin parameters: density and melt index. We found that with these resins it was possible to divide the total data into approximately three categories, based on density: low, medium and high density. At any given melt index, for example, the lowest-density resins prove toughest. In any given density category, a decrease in melt index produces a far tougher resin. This is consistent with the general behavior of the resins, since the low-density materials are softer and usually elongate more than the more-crystalline samples.

As with sheet material, the results obtained from the tensile impact test are resolved in units of energy per unit volume; e.g., foot pounds per cubic