Apparent modulus of elasticity

Plastic whose apparent modulus of elasticity is not greater than 10,000 psi at room temperature in accordance with the Standard Method of Test for Stiffness in Flexure of Plastics (ASTM Designation D747). 

Nonrigid plastic Plastic whose apparent modulus of elasticity is not... 

A different design approach is used in this case. Instead of assuming an apparent modulus of elasticity using a constant creep situation covering the life of the chair, it is better to determine the actual creep deflection over a typical stress cycle, the creep recovery over a non-use cycle, and so on until the creep is determined after a series of what might be considered typical hard usage cycles for the chair. The accumulated creep after a period of two weeks can be assumed to represent the base line for an apparent modulus of elasticity to determine the design life of the chair. 

Figure 7.11 Overlay of contour plots for apparent modulus of elasticity in compression ( c)> maximum tension force (Ft) and first-cycle relaxed force (Fri). The shaded area represents optimum conditions.

One of the requirements for solution of this problem is the availability of data on the apparent modulus of elasticity as a function of time, stress level and temperature. Since this is a derived quantity based on the creep curves good creep data at several temperatures and stress levels are required. This can be used in conjunction with the WLF relation to generate a complete set of creep curves that can then be converted to apparent modulus curves, or tables, which can be used... 

Fig. 5-8. Apparent modulus of elasticity level at 4 years polypropylene homo polymer.

Fig. 5-9. Apparent modulus of elasticity polypropylene 30% glass at 4 years.

At present only special programs are available which take into account the creep performance of statically loaded plastics. The usual method is to use a creep-adjusted apparent modulus of elasticity in a second cut structural analysis to determine whether there will be serious creep effects on the product which must be accounted for in the design. With increased sophistication of the computer programs available it will soon be possible to do a direct computer analysis of the creep effects on a plastics component. In fact, there is current work on programs that produce a time model of the plastics component under load that will show how the stresses affect shape and load capabilities with time. 

AST FAAR supplies a torsion test instrument for the measurement of the apparent modulus of elasticity versus temperature of plastics and elastomers (Table 18.2), according to the following specifications ASTM D1043 [21], DIN 53477 [22], ISO 498 [55], and BS 2782 Method 156 [3]. 

It is thus possible to measure accurately the apparent modulus of elasticity of specimens obtained from a wide range of materials (the modulus is, in this case, defined as apparent as it is obtained by measuring the angular torsion of the specimens under test). 

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