Fracture point

The fracture point (similar to the yield point) describes a solid material’s elastic limit, which is the maximum stress per unit area it can withstand before there is permanent deformation or fracture. In other words, it is the limit of the material’s elasticity, for up to that point, the solid can resume its original shape when the load is removed; after that point, it undergoes permanent (plastic) deformation and will not return to its original shape even after the load (yield load) has been removed. For example where a timber beam might bow under a weight but will return to its original straight shape once the load is removed.

However no structural material exhibits perfect elasticity: depending on the type of structure and the material, permanent deformations are often unavoidable whenever loads exceed certain values. That is why engineers design structures to ensure the materials are being used within their elastic range and the loads involved will not produce permanent deformations. All structural materials behave plastically beyond their elastic range.

However, even if some materials show elastic behaviour, they may – after a long period of service, usually many years – exhibit a degree of plastic flow (or creep). In the example of the timber beam bowing, if the weight remains for sometime and is then removed the beam may remain bowed, showing signs of creep, this may also be the case with timber beams bowing over time from a combination of self weight, angle of cut and drying out.

On a graph showing a stress-strain curve, the point of the limit of elastic behaviour is called the ‘yield point’ and this is where plastic deformation begins – some of this deformation will be plastic and irreversible. In structural engineering, the yield point is regarded as a ‘soft failure’ mode which does not usually cause catastrophic or ultimate failure, it might be described as ductile fracture depending on the material, or a brittle fracture. This is effect the fracture point, where the material starts to break, either gradually or quickly, the behaviour depending on the material and stresses, the breaking point might be where a timber beam goes beyond, the yield, soft or ductile failure and brittle or fracture points to break and collapse.

[edit] Related articles on Designing Buildings

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• Elasticity.
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• Moment.
• Stiffness.
• Structural engineer.
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• Structures at the end of their design life.
• The development of structural membranes.
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