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What is a factor of safety? |
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Material properties are variable |
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Often, it is difficult to know exact loadings |
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A factor of safety enables the engineer to
acknowledge these uncertainties |
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F.O.S. can be used to evaluate the quality of a
design |
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F.O.S. can be used to measure the likelihood of
failure |
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First of all, what is stress: |
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F/A =s (Force/Area) |
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Design Stress, sd |
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Used to account for uncertainties in material
properties and inexact information about loadings |
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If loading is static, or material is ductile then
sd = N’*sc (maximum stress calculated * a design
factor) |
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Engineer should select a material with a yield
strength,Sy, of at sd. |
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The safe stress, or working stress is: |
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sw = Sy/N |
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N = factor of safety |
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Where elastic behavior ends, and plastic
behavior begins |
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the proportional limit |
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The proportional limit is difficult to identify
in many cases |
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Use the 2% off-set rule |
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Ductility is a measure of the degree of plastic
deformation that is sustained at fracture |
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ductility is measured using % elongation |
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%EL = 100*(lf – lo)/lo |
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if %EL >5%, material is considered ductile |
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lf = fracture length of test specimen |
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lo = original gauge length of test specimen |
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If a material does not experience much plastic
deformation at fracture, the material is called brittle |
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A brittle material |
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A ductile material |
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Notes