Originally Posted by Cheechako
Well.... failure is always part of design. Design specifies material also. Many designs use a safety
factor of 3. That is... if your allowable stress is say 30,000 psi, then you build for 90,000 psi.
Engineering is an imperfect science... why? because not all conditions or stresses are predictable. Remember the Americas Cup boat that broke in half and sank in light airs in San Diego
? There were millions spent on engineering on that boat.
Choosing design factors
Appropriate design factors are based on several considerations, such as the accuracy
of predictions on the imposed loads
, strength, wear
estimates, and the environmental
effects to which the product will be exposed in service
; the consequences of engineering failure; and the cost of over-engineering the component to achieve that factor of safety. For example, components whose failure
could result in substantial financial loss, serious injury, or death may use a safety factor of four or higher (often ten). Non-critical components generally might have a design factor of two. Risk analysis
, failure mode and effects analysis
, and other tools are commonly used. Design factors for specific applications are often mandated by law, policy, or industry standards.
Buildings commonly use a factor of safety of 2.0 for each structural member
. The value for buildings is relatively low because the loads are well understood and most structures are redundant
. Pressure vessels
use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.2 to 3.0 depending on the application and materials. Ductile
, metallic materials tend to use the lower value while brittle
materials use the higher values. The field of aerospace engineering
uses generally lower design factors because the costs associated with structural weight are high (i.e. an aircraft with an overall safety factor of 5 would probably be too heavy to get off the ground). This low design factor is why aerospace parts
and materials are subject to very stringent quality control
and strict preventative maintenance
schedules to help ensure reliability
. A usually applied Safety Factor is 1.5, but for pressurized fuselage it is 2.0, and for main landing gear
structures it is often 1.25.
In some cases it is impractical or impossible for a part to meet the "standard" design factor. The penalties (mass or otherwise) for meeting the requirement would prevent the system from being viable (such as in the case of aircraft or spacecraft). In these cases, it is sometimes determined to allow a component to meet a lower than normal safety factor, often referred to as "waiving" the requirement. Doing this often brings with it extra detailed analysis or quality control verifications to assure the part will perform as desired, as it will be loaded closer to its limits.
For loading that is cyclical, repetitive, or fluctuating, it is important to consider the possibility of metal fatigue
when choosing factor of safety. A cyclic load well below a material's yield strength can cause failure if it is repeated through enough cycles.