**S-N diagram.** Plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test. Data for each curve on an S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The stress axis can represent stress amplitude, maximum stress or minimum stress. A log scale is almost always used for the N scale and sometimes for the S scale.

**St. Joe flexometer test.** Method for measuring compression fatigue characteristics of rubber. Results are reported as time and flexing load required to fail specimen. (ASTM D-623).

**Scleroscope hardness.** Measure of hardness or impact resilience of metals. A diamond-tipped hammer falls freely against specimen from a fixed height and rebound height is measured. Scleroscope hardness is read on an empirical scale where 100 rep resents average rebound from a quenched high carbon steel specimen. ASTM A-427 contains a table that relates scleroscope hardness to diamond pyramid hardness.

**Scratch hardness.** Method for determining comparative hardness of materials by measuring width of a scratch made by a scriber drawn across the surface under specified pressure. Often performed on coatings.

**Secant modulus of elasticity.** Ratio of stress to strain at any point on curve in stress-strain diagram. It is the slope of a line from the origin to any point on stress-strain curve.

**Shear modulus of elasticity. **Tangent or secant modulus of elasticity of a material subjected to shear loading. Alternate terms are modulus of rigidity and modulus of elasticity in shear. Also, shear modulus of elasticity usually is equal to torsional modulus of elasticity. A method for determining shear modulus of elasticity of structural materials by means of a twisting test is given in ASTM E-143. A method for deter mining shear modulus of structural adhesives is given in ASTM E-229.

**Shear strength.** Maximum shear stress that can be sustained by a material before rupture. It is the ultimate strength of a material subjected to shear loading. It can be determined in a torsion test where it is equal to torsional strength. The shear strength of a plastic is the maximum load required to shear a specimen in such a manner that the resulting pieces are completely clear of each other. It is reported in psi based on the area of the sheared edge. (ASTM D-732). The shear strength of a structural adhesive is the maximum shear stress in the adhesive prior to failure under torsional loading. (ASTM E-229). Methods for deter mining shear strength of timber are given in ASTM D-143 and ASTM D-198.

**Softening point.** Temperature at which a uniform fiber of glass elongates under its own weight at a specified rate. (ASTM C-338). The Vicat softening point of plastics is the temperature at which a flat ended needle of 1 sq mm circular or square cross section penetrates a thermoplastic specimen to a depth of 1 mm under load and conditions specified in ASTM D-1525.

**Splitting resistance.** Measure of the ability of felt to withstand tearing. It is the load required to rupture a slit felt specimen by gripping lips of the cut in jaws and pulling them apart. (ASTM D-461). An alternate term is tear resistance.

**Springback.** Degree to which a material returns to its original shape after deformation. In plastics and elastomers it is also called recovery.

**Stiffness.** Measure of resistance of plastics to bending. It includes both plastic and elastic behavior, so it is an apparent value of elastic modulus rather than a true value. (ASTM D-747).

**Strain. **Change per unit length in a linear dimension of a part or specimen, usually expressed in %. Strain as used with most mechanical tests is based on original length of the specimen. True or natural strain is based on instantaneous length and is equal to In l/lo where l is instantaneous length and lo is original length of the specimen. Shear strain is the change in angle between two lines originally at right angles.

**Strain energy.** Measure of energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress strain diagram, and is a measure of the toughness of a material.

**Strain hardening exponent.** Measure of increase in hardness and strength caused by plastic deformation. It is related to true stress and true strain by the equation: O= oOn where O is true stress, Oo is true stress at unit strain, O is true strain and n is strain hardening exponent.

**Strain point. **Temperature at which internal stress in glass is substantially relieved in about 1 hr. (ASTM C 336).

**Strain rate.** Time rate of elongation.

**Strain relaxation.** Alternate term for creep of rubber.

**Strength reduction ratio.** Alternate term for fatigue notch factor.

**Stress.** Load on a specimen divided by the area through which it acts. As used with most mechanical tests, stress is based on original cross section area without taking into account changes in area due to applied load. This sometimes is called conventional or engineering stress. True stress is equal to the load divided by the instantaneous cross section area through which it acts.

**Stress amplitude.** One-half the range of fluctuating stress developed in a specimen in a fatigue test. Stress amplitude often is used to construct an

S-N diagram.

**Stress concentration factor.** Ratio of the greatest stress in the area of a notch or other stress raiser to the corresponding nominal stress. It is a theoretical indication of the effect of stress concentrators on mechanical behavior.

Stress concentration factor usually is higher than the empirical fatigue notch factor or strength reduction ratio because it does not take into account stress relief due to local plastic deformation.

**Stress corrosion cracking.** Failure of a material due to combined effects of corrosion and stress. Generally, stress corrosion cracking refers to the phenomenon by which stress in creases corrosion rate.

**Stress ratio.** Ratio of minimum stress to maximum stress in one cycle of loading in a fatigue test. Tensile stresses are considered positive and compressive stresses negative.

**Stress relaxation. **Decrease in stress in a material subjected to prolonged constant strain at a constant temperature. Stress relaxation behavior is determined in a creep test. Data often is presented in the form of a stress vs time plot. Stress relaxation rate is slope of the curve at any point.

**Stress rupture strength.** Alternate term for creep strength.

**Stress-strain diagram.** Graph of stress as a function of strain. It can be constructed from data obtained in any mechanical test where a load is applied to a material and continuous measurements of stress and strain are made simultaneously. It is constructed for compression, tension and torsion tests.

**Stress-strain ratio.** Stress divided by strain at any load or deflection. Below the elastic limit of a material it is equal to tangent modulus of elasticity. An alternate term is secant modulus of elasticity. Stripping strength. Alternate term for peel strength.

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