The Stress-Strain Relationships of Vulcanized India-Rubber. Part I. The Inflection Point

Abstract Definition .—The relation between applied load and the resulting extension of vulcanized or unvulcanized rubber may be represented graphically as a curve which is “s” shaped, and there is a point of inflection in the curve. If a load W grams be applied to the rubber, and the resulting length be L , the point of inflection is defined as the point on the curve where the second differential becomes zero and changes sign. More simply, the point of inflection is where the tangent to the curve crosses the curve. It is well known that the point of inflection exists, but there appears to be no reference in the literature to any attempts to determine precisely its position. It is also apparent from the many stress-strain curves of rubber which have been published that the point of inflection varies with both the period of cure and the compounding of the rubber. What follows is an account of some experiments made to investigate the position of the point of inflection in the stress-strain curve of vulcanized rubber.