Factor of Safety – Theories of Elastic Failure – Strength of Materials


Hello friends here in this video we will see what is Factor of Safety now Factor of safety here I’ll write the definition that factor of safety that is fos is defined as the ratio of ultimate or yield stress to the permissible or working stress so factor of safety is defined as the ratio of ultimate or yield stress to the permissible of walking stress next fos that is factor safety is used to reduce the value of stress so that a material can be stressed below the maximum stress so here fos is used that is factor safety it is used to reduce the value of stress so that a material can be stressed below the maximum stress value because we don’t want the material to operate in the maximum stress region because if the material reaches maximum stress after that even if you reduce the value of stress then the material would be failing at any low value of stress so to avoid the failure of material it is necessary that the material operates below maximum stress so we divide the value of the stress by a factor and that is call this factor of safety to get the permissible or working stress next fos is always greater than one next fos is a number and it does not have a unit so after this I can write the formula of fos so first I’ll write down considering ultimate stress so when we are using the ultimate stress so factor of safety as it is defined as the ratio of I have written here ultimate stress I will consider first ultimate ultimate stress to the permissible stress or we can say it is working stress and walking stress is the that stress to which the object or a material is stressed within its working cycle of working life next considering yield stress factor of safety is given by yield stress upon working stress so when we are considering the yield stress factor of safety is given by yield stress upon working stress and mostly this factor of safety where we are having the yield point yield stress this is used for ductile materials whereas ultimate stress upon working stress this is used for brittle materials so if for example I will just take an example here that if for an example yield stress are denoted as Sigma y is equal to 400 Newton per mm square and assuming the factor of safety is 4 so therefore working stress will be yield stress divided by factor of safety walking stress is Sigma walking that will be yield stress upon FY s which is 400 divided by 4 and that gives the value as 100 Newton per mm square so as we can see by dividing the yield stress with factor of safety we have reduced the stress from four hundred to hundred Newton per mm square and this 100 Newton per mm square would be called as the working stress which would be used when the component or the material is being used during its life means whenever we are using that material it will never cross a value of 100 100 will remain the maximum stress for that material and if it is not reaching the value of 400 then the chances of breaking of that material is very red so in this video we have seen what is meant by a factor of safety

16 Replies to “Factor of Safety – Theories of Elastic Failure – Strength of Materials”

  1. explain the term more clearly , in Hindi if you like , because student should understand it's physical significance also(which type of it actually is because here we in the field of Stresses, Forces etc Repetitive so much that confusion arises), Only writing down will make us to Learn the formula only not to understand the basic concept of that Force or Stress because we have not understood it in the easiest way

  2. We also need to understand the concept more clearly. You are just writing down the definition from the book. Typical.

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