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Long-term loading

Posted by admin on July 25th, 2012
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Long-term loading
Long-term loading occurs when parts are placed under high external loads, within the proportionallimit, for extended periods of time. This term also refers to parts that must withstand high internal or residual stresses that result from either the molding process or from the following assembly processes: Press-fit and snap-fit assemblies Tapered fit between......Read more

Short-term loading

Posted by admin on July 25th, 2012
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Short-term loading
Short-term loads are those imposed during handling and assembly, and during usage where the load is applied occasionally with short durations. The following suggestions apply to parts that will be subject to short-term loading conditions. Use proportional limit in stress-strain curve Designers should consider the stress-strain behavior of the plastic materi......Read more

Design for strength

Posted by admin on July 25th, 2012
9660 Views
Design for strength
Predicting part strength The success of failure of the plastics part design is often determined by how accurately the part strength (stiffness) can be predicted. The types of strength correspond to the load and restraint conditions to which the part is subjected, such as tensile, compressive, torsional, flexural, and shear. The strength of a plastics part w......Read more

Impact strength

Posted by admin on July 24th, 2012
2575 Views
Impact strength
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 materia......Read more

Fatigue

Posted by admin on July 24th, 2012
3798 Views
Fatigue
Fatigue has to be considered when designing plastic parts that are subject to Repeated loading. The cyclical loading application is relatively infrequent and there is a long time between applications. If the loading is cyclical, use the proportional limit for design calculations. If the loading is repeated at short intervals and for long periods, use the S-......Read more

Creep and stress relaxation

Posted by admin on July 24th, 2012
3833 Views
Creep and stress relaxation
Creep and stress relaxation are critical concerns when designing structural parts that are subject to Long-term loading. Creep Regardless of the rate at which the initial load is applied, if a constant load is continued, the structure will continue to deform. This long-term, permanent deformation is called creep, as plotted in Figure 9. FIGURE 9. A typical......Read more

Stress-strain curves for fiber-filled polymers

Posted by admin on July 24th, 2012
86775 Views
Stress-strain curves for fiber-filled polymers
The stress-strain curves for a pair of thermoplastic compounds are shown in Figure 7. The base resin is the same for both compounds, except one compound is unfilled while the other contains 30 percent glass fiber as a reinforcement. You can see that the glass fibers significantly increase the ultimate strength, yield strength, proportional limit, and the Yo......Read more

Stress-strain curves for Unfilled polymers

Posted by admin on July 23rd, 2012
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Stress-strain curves for Unfilled polymers
below shows a typical stress-strain curve for short-term loading of a typical unfilled thermoplastic material. Figure 6 depicts the same curve as shown in Figure 5, except it is stretched horizontally to show the details within the elastic region. Several important material properties, such as Young’s modulus, proportional limit, elastic limit, yield ......Read more

Stress-strain behavior

Posted by admin on July 23rd, 2012
2842 Views
Stress-strain behavior
Part strength The stress-strain behavior of a material determines the material contribution to part strength (or stiffness), the relationship between load and deflection in a plastic part. Other factors that affect part strength include part geometry, loading, constraint conditions on the part, and the residual stresses and orientations that result from the......Read more

Material properties for part design

Posted by admin on July 23rd, 2012
2435 Views
Material properties for part design
Plastics are sensitive to operating conditions The plastics molding processes allow parts designers more freedom than working with metals because plastics materials are so versatile. Unlike metals, however, the mechanical properties of plastics are very sensitive to the type, rate, duration, and frequency of loading, the change in operating temperature, and......Read more