Mayorkinos 1
Structural integrity
Applied loads can be:
- Static
- Cyclic
- Stochastic (random)
- Impact
Deterioration
Stochastic loads can be difficult to predict
Structural integrity management
Operate until failure then repair/replace
Run for period of time then carry out maintenance
Operate for period of time and then inspect
Monitor continuously and diagnose problems
Predict damage initiation and evolution and plan maintenance based on this
Maintenance Strategies
Corrective
Preventive
Condition-based
Risk-based or predictive
Data, model or both driven
Higher uncertainty and variability, particularly for stochastic loading
Carried out at pre-determined intervals
Fault is identified and then repaired/replaced
Maintanence is conducted after condition has deteriorated beyond an acceptable level
Reliability-centred maintenance
Based on a combination of different maintanence strategies
May employ preventative measures if catastrophic failure is unnaceptable
Good understanding of failure mechanisms is required
Uses risk-based inspection
Part of overall plant management
Various NDT methodologies may be employed
Requires wide range of information
Reasons for failure
Lack of understanding
Loading conditions are not understood or are underestimated
Additional factors having bigger effect than considered
Innaccurate modelling
Presence of defects or material used outside of specifications
Incorrect installation or maintanence
Normal accident theory
Accidents are unavoidable in complex systems therefore called normal accidents
System is susceptible to normal accidents if it is complex, tightly coupled and has catastophic potential
E.g. Chernobyl
Swiss cheese model
Gaps in defence can align and failure can occur
Safety order in complex systems
- Design for minimum risk - eliminate risks in design
- Incorporate safety into design - use safety features or devices
- Provide warning devices - detect faults
- Develop procedures, standards and training
Failure patterns
A. Bathtub, B. Wear out, C. Fatigue, D. Initial break-in, E. Random, F. Infant mortality
Fault tolerant design
Design considerations:
-Estimated flaws
-Tolerance limit set
-Initial flaw size is assumed
-Time or cycles for crack growth defined
-Inspection requirements defined
Fail safe and safe life
Safe life: no crack initiation, or cracks do not grow to critical dimension to cause failure during operational lifetime
Fail safe: Cracks are not allowed to grow to point of causing failure. Partial failure should not compromise overall safety
Equations
Factor of safety
Structural capacity of a system to sustain loads beyond those predicted
FS = UTS/DS
Ratio of yield stress (YS) or ultimate tensile stress (UTS) to max. design stress (DS)
Margin of safety
Excess capacity of a structure to sustain the max stresses it could ever sustain
MS= (Failure load/Design load)-1
Paris-Erdogan Law
Empirical law used to estimate crack growth rates within load range
a is crack length (mm), n is number of load cycles, C and m are material constants, deltaK is stress intensity factor range (MPa √m )
Stress, strain etc
Fatigue limit
Lab testing shows steels have a safe stress at which failure will not occur, regardless of cycles
Only found in certain materials
Affected by:
-Periodic overloads
-Corrosion
-High temperatures