Design Criteria Flashcards
(22 cards)
What is a mechanical failure?
Any change in size, shape or mechanical properties of a structure, machine or component that renders it incapable of satisfactory performing its function.
What are the activation parameters?
Also called the failure mechanisms, these are the parameters that could trigger a mechanical failure in a machine, component or structure. The classification considers mainly 4: Time, Force, Environment, Temperature.
Which are the main failure modes for design?
- Elastic deformation: Displacement due to elastic deformation could be a failure if not controlled adequately. 2. Yielding: Increase of the displacement field, typical for static, less dangerous than breakage. 3. Brittle fracture: Failure with very little deformation, typical of glass or ceramic. 4. Ductile fracture: Larger deformation than brittle, causing ‘necking’ of the material. 5. Fatigue: Related to constant application of loads leading to failure over time.
Which are the three types of failure modes to be addressed?
- Fracture: Originated from static load application or fatigue. 2. Yielding: Static application of loads, typical for ductile. 3. Elastic deformation.
What is the static assessment for machine design?
The analysis of the static behavior of the mechanical system to guarantee its functioning under the application of ultimate and limit loads, aiming to avoid static fracture and high deformation.
Do fatigue and static conditions produce different failure conditions?
For static, it is necessary to verify the yielding condition. For fatigue, it is necessary to verify the fracture condition.
Which are the loading conditions in static assessments?
- Limit loads: Maximum load during life, ensuring no failure occurs, typical safety factor 1.5. 2. Ultimate loads: Exceptional loads, safety factor lower than 1.5.
What is the importance of the definition of safety factor?
It is important for design as it involves decision making of yielding, such as cost or weight in exchange for higher resistance or durability.
What are the three approaches of fatigue analysis?
- Safe life. 2. Fail safe. 3. Damage tolerance.
What is the safe life approach?
The component or structure is designed to sustain loads for its entire lifetime (previously defined) and then replaced at the end of it.
What are the main features of the safe life approach?
- High thickness. 2. Low stresses. 3. Structural integrity during lifetime (no damage allowed).
In the safe life approach, what are the main two sub-approaches?
- Stress based: Identification of high stress zones, High Cycle Fatigue (HCF). 2. Strain based: Identification of high strain zones.
What is the fail safe approach?
Design approach where components are designed to ensure structural integrity even in case of partial failures, accomplished with multiple load paths or special materials/components.
What are the main features of the fail safe approach?
- Lower safety factors. 2. Regular maintenance scheduling. 3. Alternative load paths.
What is the damage tolerance approach?
Approach where damage of the component is allowed, provided it does not affect the function or integrity of the system. The damaged component is not replaced until minimal strength is reached.
What is the acceptability of defects?
Identification of the maximum defects that could be adverse to the system.
What are the main features of the damage tolerance approach?
- Advance maintenance scheduling. 2. Non-destructive testing. 3. Structural health monitoring. 4. Estimation of damage evolution.
What are some types of damage in the damage tolerance approach?
- Flaws due to technology process. 2. Accidental damages. 3. Damages due to service loads.
What are the two sub-approaches in the damage tolerance approach?
- Fracture mechanics analysis. 2. Fatigue crack growth (FCG).
What are the three important tools for structural design?
- Analytical methods. 2. Numerical methods. 3. Experimental activities.
Are the three tools applied individually?
No, a robust design involves the three tools for the system, machine or component to be designed.
