Stress and Strain in Plastics Machinery's Handbook, 31st Edition
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Fig. 2. Isometric Plot for High-Impact Polystyrene Showing Stress Response to Imposed Strains over Three Decades
Some hygroscopic resins can absorb water from the air, with stress-strain behavior rang ing from curve B or C of Fig. 1 when very dry to curve D or E at high moisture contents. Polyamides (nylons), classified as engineering resins with superior properties, exemplify this moisture dependence, picking up about 0.3 percent in very dry climates, and 2.5 percent or more in very humid ones. Modulus and strength values reported from ASTM D638 testing should not be used in the design of plastics products that must sustain loads over long periods; the addition of reinforcements, such as glass fiber (GF), is observed to significantly increase the modulus and strength of both thermoplastics (e.g., PP) and thermosets (e.g., epoxy). Properties for various plastics are included in the plastics properties table on page 387. The Basics of Stress and Strain in Plastics.— The mechanical behavior of plastics is similar in many ways to that of metals, but there are important differences that must be kept in mind in the design of plastics products and parts. Stress: A three-dimensional body having a balanced system of external forces F 1 through F 5 acting on it such that the body is at rest is shown in Fig. 3 . Such a body develops internal forces to transfer and distribute the external loads. If the body is cut at an arbitrary cross section and one part is removed, as shown at the right in Fig. 3, a new system of forces acting on the cut surface is developed to balance the remaining external forces. Similar forces (stresses) exist within the uncut body. Stresses must be defined by both magnitude and direction. The stress S acting in the direction shown in Fig. 3 on point P of the cut surface has two stress components. One of these components, s , acts perpendicular to the surface and is called a normal stress . The other stress, t , acts parallel to the surface and is called a shear stress . Force Vectors
F 2
P S
Area A
F 1
F 3
F 1
F
F Applied Load
Cut
Cut
L
L
Increase in Length Due to Applied Load
Original Length
F 5
F 5
F
F
4 4 Fig. 3. Internal Forces and Stresses in a Body
Fig. 4. Simple Tension Load
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