What do you mean by factor of safety?
Design load
When you design a component the load that component has to carry during its service is calculated taking in to consideration of all the possible loads that might act on it. This calculated load is known as design load.
Why use a factor of safety?
When this design load acts, we expect the component to be safe. That means the stress induced in the member when this design load acts on it has to be less than yield point stress. This is because if the stress reaches yield point stress, the component is considered to be failed (since yielding can't be permitted in most of the applications)
Hence we need to keep the stress during its service well below the yield point stress. By what factor you reduce this stress is left to the design engineer. He select a suitable number (Factor of safety-FoS) by which he divide the yield stress to get the safe value of stress which is known as design stress or working stress. The selection of factor of safety depends on the design engineers experience, cost involved and various other safety parameters. We use this working stress or design stress to find the size of the component required.
Hence the component will be subjected to a maximum stress of design stress when the design load acts on it. Any accidental overloading may increase the stress above design stress, still the stress may fall below yield stress due to the usage of factor of safety.
For ductile materials which has a well defined yield point factor of safety is defined as,
Factor of safety (FoS) or Safety factor (SF)
It is defined as the ratio of yield stress to working stress for ductile materials.
FoS = yield stress/ working stress
or
Design stress = yield stress / FoS
For brittle materials which has no well defined yield point the factor of safety is calculated based on.ultimate stress.
FoS = Ultimate stress/ working stress
When factor of safety is defined based on the ultimate stress, higher values are generally used.
· practically Factor of safety indicates the safety of a component during its working period.
taking an example of lift
if the actual capacity of lift can carry up to 1000kg.
but on the sign board it recommend only to fill up 500kg then going as per the design formulae (factor of safety) = ultimate tensile stress(actual capacity)/allowable stress(recommended)
hence in our case the ans is 2
now if we suppose to recommend weight up to 200kg then the factor of safety would be 5 moreover to sum up it tells that with a factor of safety of 5 the life and smoothness of the lift will be maintained more for a longer time. so to conclude higher the factor of safety lower the chances of failure of a component/material as their are many unknown stresses, cyclic loads, non cyclic loads which are acting on the material/component but a designer don’t take all in its consideration so they provide a high f.s to the stuff to decline any uncertainty.
· Factor of safety is a ratio of maximum stress withstand by an object to applied stress.
Whenever a Factor of safety is greater than or equal to 1,then the applied stress is less than or equal to the maximum stress so the object can withstand load.But when the ratio is equal to 1 ,the object tough enough to withstand load.
Whenever a Factor of safety is less than 1 , the applied stress is greater than maximum stress then the object can’t withstand the stress applied it leds to failure.
Ex:
Suppose the maximum and applied stress of the beam is 50k pa and 10k pa
Then the Factor of safety(FOS)
FOS=maximum stress/applied stress
FOS=50k/10k
FOS=5
the applied stress is 1/5th of the maximum stress,so the beam can withstand the applied stress.
· there are two ways of defining it…
1. FOS = (Ultimate Stress / Working Stress).
2. it gives us a value which tells as to how much extra load which any structure can bear without any failure, actually FOS is a boon to civil enginers coz….if structure were designed according to actual load then in future if the load acting on the structure increases then the structure won’t be able to sustain that extra load and it will fail, leading to lot of damage, but if its designed according to FOS then even if the applied load increases beyond the specified limit, then also the structure can withstand that extra load and will be safe for a few morw years.
· Factor of safety is define as the ratio of yield stress to working Strees. According to designing point of view, none of the metarial is 100 % reliable, we degin the structure member for given working Strees and keep the Strees in such a way that they can not reached or exceed there permissible values because ones the material of the members reaches to there permissible values then yielding get start and the structure is treated as failed structure. We design the structure member with in the proportional limit and If the materials subjected to the Strees which is equal to permissible values and more than this , Hence structural member get fail.
So as a designer we degin the structure including some factor of safety. And design the member in such a way that material of members keeps with in its proportional limit by the help of factors of safety
For example, In case of RCC design the factor of safety for concrete is taken as 1.5 and for steel it is 1.15.
· FOS is just a number but play a key role in ever design by seeing your FOS the can judge your design is safe or not
FOS=matrial stress/design Stress
FOS=Utimate stress/allowable Stress (for brittle material)
FOS=yield stress/allowable Stress (for ductile material)
And it always more than 1 it is because of the definition of FOS and basic on your part, component or assembly. The FOS is varied for the part to part if we consider more FOS then over a design if FOS is less then it is under design. Base on the requirement you have to fix them FOS.