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Properties & Definitions




Breaking Stress ( or ultimate tensile stress)


This is the maximum tensile stress that produces fracture.





Brittle materials cannot be permanently stretched. They break after the elastic limit is exceeded.





Creep occurs when a material is under constant stress over time. Strain gradually increases, causing eventual fracture.



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A dislocation is the movement of a broken bond through a crystal. The broken bond creates a small void between molecules. When a force is applied to the crystal, the void moves across lattices in the opposite direction to the force.





This is the property a material has whereby it can be permanently stretched.



Elastic Limit


This is the maximum force a body can endure and still regain its original shape and size, when the force is removed.



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Elasticity is the property a material has whereby it regains its original size and shape when a deforming force is removed.





Fatigue is the weakness induced in a material by it repeatedly being stressed in opposite directions. It can also occur when a material is repeatedly stressed and the stress being removed.



Hooke's Law


The extension of a stretched spring(or wire) is directly proportional to the extending force, provided the limit of proportionality is not exceeded.



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The stiffness of a body is the a measure of its resistance to changes in size and/or shape.





Strain is a the ratio of change in length to original length. It can also be considered as the extension per unit length. Strain has no dimensions.





Strength is the maximum force applied to a body before it breaks.



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Stress is the applied force divided by the cross-sectional area it operates over. It is the force per unit area of cross-section. Units of stress are Nm-2 (also Pascals Pa).



Yield Point


For a body being extended by an applied force, the yield point is when a large extension is produced for a very small increase in force. The material of the body is said to show 'plastic' behaviour.




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