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Impact strength

Posted by admin on July 24th, 2012
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Toughness

Because plastics are viscoelastic, their properties strongly depend on the time, rate, frequency, and duration of the load, as well as the operating temperature. Impact strength (or toughness) of plastics can be defined as the ability of a material to withstand impulsive loading (see High velocity and impact loading). Figure 8 shows that a material’s impact strength increases with increasing rate of loading. The limit of this behavior is that as the velocity of loading increases, there is less tendency to draw and the material acts in a brittle, rather than tough fashion. Decreasing temperature shows a similar behavior, namely, at lower temperatures the plastics are more brittle.

Stress concentration

Impact response of plastic materials is also notch sensitive. In other words, a sharp internal radius will decrease the apparent impact strength of the part due to the effect of stress concentration, as plotted in Figure 12.

FIGURE 12. Stress concentration as a function of wall thickness and corner radius.

Thermal mechanical behavior

Changes in temperature can significantly change the dimension and mechanical performance of plastic parts. Therefore, you must consider both the high and low temperature extremes associated with the application. For applications subject to large temperature variation, you’ll need to take into account the dimensional change of plastics parts when assembled/bound with other materials of different coefficient of thermal expansion (e.g., metals).

Operation at extreme temperatures

Factors that need to be considered when the operating temperatures are above normal room temperature:

Part dimensions increase proportional to length, temperature increase, and coefficient of thermal expansion and contraction.

Strength and modulus will be lower than at room temperature. Figure 8 shows that strength decreases with increasing temperature.

Material may exhibit a rubber-like behavior with low modulus and high degree of drawing.

Storage at extreme temperatures

Factors that need to be considered for long-term storage at elevated temperatures:

Increased creep and stress relaxation for any components that are loaded during the storage. This includes relaxation of any residual stresses from the molding process or from assembly.

The plastic becomes brittle due to molecular degeneration.

Some of the ingredients bleed from the compound.

Factors that need to be considered when the storage temperatures are below room temperature:

Part dimensions decrease proportional to length, temperature decrease, and coefficient of thermal expansion and contraction.

Modulus increases.

Parts are more brittle.

Coefficient of thermal expansion

The coefficient of thermal expansion measures the change in dimension from a specific temperature rise. The typical values (~ 10-4 1/deg-K) are 5 to 10 times larger than that of metals. If the plastics part is rigidly joined to a metal part, the weaker plastic part will fail due to differential expansion or contraction. Depending on the strength of the plastic and the temperature rise, the failure may be immediate or delayed (see Rate- and temperature-dependency of stress-strain curves). The design must make allowances for the change in length between the plastic and the metal to which it is attached. If one end of the plastic is rigidly attached, the other end must be allowed to float. The orientation of molecules and fibers might cause the change in dimension to be anisotropic.That is, the coefficient of thermal expansion (thus the expansion or contraction) is greater in one direction (e.g., the flow direction) than in the cross direction.

Heat deflection temperature under load

This value is derived from an ASTM test that includes soaking a standard test specimen in an oil bath of uniform temperature. A flexural load is applied after the specimen reaches the constant temperature of the oil bath. The temperature at which the specimen is deflected to a specified amount is called the heat deflection temperature. The test has little other meaning than to rank materials for heat resistance. Stress-strain curves for a range of temperatures provide a more reliable way of evaluating material’s performance at elevated temperatures.

 

Author: ZHILIAN MOULD
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