Tensile strength and ultimate elongation, while extremely useful for compound development and control, are of lesser importance to the design engineer. Elastomeric parts are seldom loaded in tension and then only to a small fraction of their ultimate strength. Tensile strength and elongation generally cannot be correlated with performance in service. The relationship of stress to strain is more useful because it shows how an elastomer responds to loading.

Tensile properties are measured by recording axial stress in a standard ASTM dumbbell specimen at a constant rate of strain. Tensile strength and elongation, as applied to rubber, are defined as follows:

Tensile is the force per unit of the original cross-sectional area which is applied at the item of the rupture of a specimen.

Elongation or Strain is the extension between benchmarks produced by a tensile force applied to a specimen and is expressed as a percentage of the original distance between the marks. Ultimate elongation is the elongation at the moment of rupture.

Modulus is the stress in pounds per square inch (of original cross section) required to produce a certain elongation.

If a tensile stress of 1800 psi produces an elongation of 300%, the compound is said to have a 300% modulus of 1800 psi. In rubber, unlike steel, stress and strain in tension are not proportional; and therefore, the term modulus has a different meaning. When applied to steel, modulus means stress at a certain strain - not a ratio and not a constant, merely the coordinates of a point on the stress-strain curve.

Procedures for conducting stress-strain tests are standardized and describe in ASTM D-412. Dumbbell shaped specimens four or five inches long are die-cut from flat sheet and marked in the narrow section with bench marks one or two inches apart (FIG 1). Ends of the specimen are placed in the grips of a testing machine (FIG2). The lower grip is power-driven at 20 inches per minute and stretches the specimen until it breaks. As the distance between bench marks widens, measurement is made between their centers to determine elongation.

FIG 1 - Standard Tensile Test Specimen

FIG 2 - Testing Machine

Stress-strain properties are useful in compound development and for manufacturing control. As control tools, stress-strain properties reveal whether or not the ingredients have been mixed properly or if contaminants are present. Property changes by environmental conditions are easily detected by a change in stress-strain properties. For a product which as been put in production, modulus and elongation measurements can be used as quality control tools. They are sensitive to manufacturing variations and indicate if the product has been properly processed.

Natural rubber must be loaded with carbon black to obtain a modulus approaching that of Urethane with no filler (Figure 3). Tensile stress-strain curves for compounds of urethane are shown in Figure 4. The tensile strength of commercial compositions of urethane will vary from 2000 psi to over 11,000 psi. Elongation will vary from 250% to 800%. Generally, tensile strength increases with an increase in hardness.

Figure 4 also show that urethane elastomers retain extensibility at high hardness. Elongation at break of 75 durometer D, achieved with urethane, is usually 250%.

The response of materials at low moduli is more important to the design engineer than the ultimate tensile strength. As shown in Figure 5, tests from the same material can yield widely varying tensile strengths based on small differences in elongation. In this case, an elongation at break of only 50% can result in a 2200 psi change in tensile strength. The variability in tensile strength can be due to small voids in the specimen or small invisible nick in the sample. Stress at 100% modulus yields very consistent results and are reproducible compared to other tensile properties.

The stress-strain properties of many elastomeric compounds change with the addition of a reinforcing filler. Modulus and tensile values can increase significantly. Amine-cured urethane without filler has a stress-strain curve which closely resembles filler-reinforced elastomers (figure 6). Polyol-cured urethane does not have this change with reinforcement.

In many non-rubber materials, Young's Modulus may be taken in tension or compression, the values being approximately the same; it is the ratio of stress to strain, expressed in psi per unit strain. In rubber, the assumption that tension modulus equals compression modulus is valid only for extremely small deformations and for certain shapes, such as specified in ASTM D-797, "Young's Modulus in Flexure of Natural and Synthetic Elastomer" and ASTM D-1053, (Sec7) "Measuring Low-Temperature Stiffening by the Gehman Torsional Apparatus".
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