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