PROPERTIES OF MATERIALS ENF 150 Chapter 10: Properties of Materials

What happens when an axial load is applied to a specimen ? L δ P P

Tensile Test Apparatus

Tensile Test Test specimen

Tensile Test

Specimen close to break

Tensile Test

Computations Measures of Ductility: % Elongation = (increase in gauge length ) / (original gauge length ) * 100 % Reduction in area = (original area – final area) / (original area) *100 Measures of Strength: Strength = Ultimate Load (P) / A o Yield Strength = Yield Load / A o

Strain - Stress Diagram

Stress-strain diagram for ductile materials strain stress Low carbon steel Aluminum alloy Breaking point Ultimate stress

Estimation of Yield Strength strain stress Yield strength 0.2% offset

Stress-strain relationship for Brittle Materials stress strain Rupture (Breaking) Strength

Stress-strain diagrams for iron and different grades of steel strain stress Pure iron Carbon steel Alloy steel

Elasticity and Plasticity If the strain disappears when the stress is removed, the material is said to behave elastically. The largest stress for which this occurs is called the elastic limit. When the strain does not return to zero after the stress is removed, the material is said to behave plastically. Strain is not zero after removing the load. strain stress

Mechanical Properties of Materials Stiffness – Property that material to withstand high stress without great strain. It is a function of modulus of elasticity. Material having high Elasticity also has high stiffness. Strength – Property determined by the greatest stress that the material can withstand prior to failure. Elasticity – Property of material enabling it to regain its original dimensions after removal of the deforming load. Ductility – Property of material enabling it to undergo considerable plastic deformation before actual rupture. High percent elongation indicates that the material is highly ductile.

Mechanical Properties of Materials Brittleness – brittle materials fail without any plastic deformation. Brittle materials are typically weak in tension and strong in compression. Malleability – Property of a material enabling it to undergo considerable plastic deformation under compressive load before actual rupture. Toughness – Property of a material enabling it to endure high-impact loads or shock loads. The measure of toughness is equal to the area under the stress-strain.

Factor of Safety (FS) The ratio of failure stress to the allowable stress is called the Factor of Safety. Failure stress is the material yield strength (for ductile materials like steel, aluminum). FS is typically around 1.5. Failure stress is the material ultimate strength (for brittle materials like cast iron, wood). FS is typically around 20.

ELASTIC AND IN-ELASTIC BEHAVIOUR Two types of failures are defined (a)In-elastic deformation for ductile materials (yield design) Failure is assumed to start at the yield point due to undesirable deformation. A B C P L 0.75L L All 3 members have the same elongation (δ) So the middle member has the largest strain (ε) and reaches the yield point first. Then the structure is assumed as failed.

ELASTIC AND INELASTIC BEHAVIOUR (b) Ultimate strength design An alternate design approach holds that failure is not considered to have occurred until all the bars have been stressed to the yield point. In other words in this approach we assume materials have infinite ductility and this approach is called the ultimate strength design strain stress Idealized stress-strain curve εyεy When strain is less than ε y stress is proportional to strain. When strain is greater than ε y stress is equal to yield stress s y sysy