Tensile Test The most common static test is the uniaxial tensile test, which provides information about a variety of properties. As a load is applied to a test specimen, the elongation is recorded. The ordered pairs are graphed in a stress/strain curve.

Specimen mounting Universal Testing Machine

Schematics of the Process

Units of Strength: Stress Strength of materials are recorded in units of Force/Area, and are referred to as various stresses. The symbol for stress is the Greek letter sigma: Stress is synonymous with strength 

Examples of Normal Stress Cables on suspension bridges. Legs of chairs: dent the pine floors? (compressive strength of pine vs oak) Skyboxes at the ball park. Cutting tool operations. Forming and fabricating operations.

Units of Deformation: Strain Tension equates to elongation Units of elongation are Strain: Note that strain is dimensionless: merely a ratio 

The Stress-Strain Diagram (fig A.5)

Region I The modulus of Elasticity (E) is calculated in stress/strain up to the proportional limit. Designers live here

Young’s Modulus (E) Is in the Elastic region: strain is directly proportional to stress. (Hooke’s Law) Units are in psi or MPa Yield stress is the strength of the material when permanent deformation takes place. The larger value of E, the more resistant a material is to deformation. This is a measure of material stiffness in response to an applied load: a quantified property: E

Elastic Deformation Airplane wings: sway with fracture 1950’s accident: B17 into Empire State building: swayed 12’? Bridge swaying glass vs steel Design safety factors

Region II: Plastic Deformation Ductility: the ability to plastically deform in tension without breaking Malleability: the ability to plastically deform in compression without breaking Fabrication processes: forming

Region III: Failure Material thins from the stress, work-hardening leads to fracture Cutting processes occur in this region

Specific Tensile Properties Modulus of Elasticity E Yield stress (strength) Ultimate Tensile strength (stress)(UTS) Ductility: elongation percentage

Strength of Materials Strength is the property determined by the greatest stress the material can withstand prior to failure, as failure is determined by the designer. It may be defined by the proportional limit, the yield point, or ultimate strength. No single value is adequate to define strength, since the behavior under load differs with the kind of stress, and the nature of the loading.

Stiffness The property that enables a material to withstand high stress without great strain (elongation). It is a resistance to any form of deformation, and is a function of the modulus of elasticity.

Elasticity & Ductility Elasticity is the property of material enabling it to regain its original dimensions after removal of a deforming load. Ductile materials can undergo considerable plastic deformation under tension, before breaking. % elongation is the measurement. Wires, extrusions as examples Malleability is material ability to deform plastically in compression

Brittleness Brittleness implies the absence of any plastic deformation prior to sudden failure. This means the breaking strength (UTS) is usually equal to the yield stress. In general, poor in tension, and tested in compression: concrete, cast iron, ceramics.

Toughness & Resilience Toughness is that property enabling it to endure high impact loads or shock loads. A body that can be both highly stressed, and greatly deformed without failure is tough. Resilience is that property enabling it to endure high impact loads without inducing a stress in excess of the elastic limit (yield).

Creep Permanent deformation at stress less than yield. Materials with higher resilience are more susceptible to creep. Temperature is the most contributing environmental factor. For example: Creep limit for LEXAN at 73oF is 2000psi. (GE, p.5) Easiest to beat by adding reinforcing materials.

Comparison of Materials

The Nature of the Load Determine the stress on the 1.0” Ø rod. If, by design, the stress on the rod cannot exceed 10,000psi determine the minimum diameter Shear Loads Shear Stresses Shear strengths can be approximated from tensile strengths

Torsion Stress

Brazing Application L 0.75 6 ton load Determine L