Tensile strength at yield

Table 13.12 gives the numerical values of the strength properties of a series of polymers. The data on the tensile strength are graphically reproduced in Fig. 13.62, where crmax (i.e. the tensile strength at break of brittle (linear) polymers and the tensile strength at yield of ductile (linear) polymers) is plotted as a function of E, the tensile modulus. As an approximation the following empirical expression may be used (drawn line) ... 

Polymer Tensile strength at yield Elongation at yield (%) Tensile strength at break Elongation at break (%) Tensile modulus Flexural strength Flexural modulus Compressi strength... 

Table III. Tensile Strength at Yield and Kail as Function of Temperature ...

UV exposed samples were tested on an Instron tensile machine and tensile strength at yield was recorded. Failure time for exposed samples was loss of 50% of original tensile strength. 

For those copolymers where other physical property data was available, an attempt was made to correlate the run number with the flexural modulus and the tensile strength at yield. Results are given in Figures 9 and 10 for a limited number of points. This data should be obtained from carefully annealed samples or ones with similar thermal histories however, a clear trend is seen in both figures. The run number does relate to crystallinity and to attendant physical properties such as flexural modulus and tensile strength. 

Figure 10 Run Number Versus Tensile Strength at Yield for a Few Ethylene-l-Hexene and Ethylene-l-Butene Copolymers...

Tensile strength can be measnred at yield or at break. In plastics, when a curve of stress versus strain is determined, a continued increase in deformation without an increase in load is observed rather often. In other words, it is the first point on the stress-strain enrve at which an increase in strain occurs without an increase in stress. The point at which a curve first shows such a behavior is defined as the yield point. Yield point is also defined as the first point on the stress-strain curve when it shows zero slope. At this point the tensile strength at yield is measured. Otherwise, if the specimen rnptnres at a continuons increase of load, the tensile strength at break is measnred and reported. 

Precision of flexnral modulus measurements is usually fair. ASTM D 638-03 lists examples for several different plastics, including polypropylene, tested by eight laboratories nsing the Type I specimen, of nominal 0.125-in. thickness. Each test result was based on five individual determinations. Each laboratory obtained two test results for each material. For polyethylene, tensile modnlns of elasticity was 210,000+8900 psi for within-laboratory tests and 210,000+71,000 psi for between-laboratory tests. Tensile strength at yield for polypropylene was 3630+22 psi for within-laboratory tests and 3630+161 psi for between-laboratory tests. Elongation at yield was 8.79+0.45 and 8.8+5.9% for within- and between-laboratory tests, respectively. 

TABLE 8.1 Tensile strength at yield for polyethylene plastics, obtained by ten different laboratories (ASTM D 638)... 

For polyethylene plastics, the ASTM procedure lists the following values for tensile strength at yield, obtained by 10 different laboratories (Table 8.1). 

Elongation at break, 71 Secant modulus, 71 Tensile strength at yield, 71 Branched MDPE, 71 Elongation at break, 71 Secant modulus, 71 Tensile strength at yield, 71 Branched polyethylene plastics, 52, 54, 67 Break load, 16, 231, 241, 243, 252, 255, 287, 289, 311... 

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