Stress Directed Maintenance Daniel Sillivant sillivd@uah.edu

Reliability Centered Maintenance (RCM) Maintenance Programs Reliability Centered Maintenance (RCM) Condition Based Maintenance (CBM) Time Directed Maintenance (TDM) Run-to-Failure (RTF) No Maintenance Solution (NoM) Stress Directed Maintenance (SDM) Reliability Centered Maintenance (RCM) is a process used to determine the maintenance requirements of any physical asset in its operating context. Preserve system functionality. Reliability Centered Maintenance (RCM) analysis provides a structured framework for analyzing the functions and potential failures for a physical asset (such as an airplane, a manufacturing production line, etc.) with a focus on preserving system functions, rather than preserving equipment. RCM is used to develop scheduled maintenance plans that will provide an acceptable level of operability, with an acceptable level of risk, in an efficient and cost-effective manner.

Stress Directed Maintenance What is it? Stress Profiles Strength Profiles Stress–Strength Reliability Models PDF Stress & Strength Profile Strength – Destruct Limit Test Paring of stress and strength limit PDF’s

Interference Theory n Maximum Operating Destruct Limit

s-n Curve – Fatigue Analysis Smax Smed Smin nmax_S nmed_S nmin_S S Fatigue analysis characterizes the cycles-to-failure for new parts at designed stress loads. The results of the fatigue failure analysis are used to develop the s-n curve for the part. A drawback to the s-n curve is that it does not address how different stress levels affect the consumed life for the part. Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. Fatigue Failures: Fatigue failures are failures caused in components under the action of fluctuating loads. Fatigue failures occur when components are subjected to a large number of cycles of the applied stress. With fatigue, components fail under stress values much below the ultimate strength of the material and often even below the yield strength. What makes fatigue failures even more dangerous is the fact that they occur suddenly, without warning.

Reliability Analysis Reliability failure analysis fits a stress-based failure math model for new parts from static tests for applied stress loads acting on the structure. Reliability failure models are limited to a single stress load cycle. The hazard function provides the instantaneous failure rate for the part at increasing stress levels.

Reliability Failure Model Origin of Theory s-n Curve Design Best Practice Reliability Failure Model Modeling Best Practice Bivariate Hazard Function Theory Sn Curve - Only for new parts, Doesn’t address consumed life RFM - Doesn’t address fatigue Theory - Addresses consumed life based upon risk & stress Neither of the above analyses characterize the consumed life of the part. In theory the combination of the fatigue and reliability failure analyses will create a response surface, known as the bivariate hazard function. A risk threshold applied to the discrete bivariate hazard function will characterize the risk based consumed fatigue life of the part as it ages.

Hazard Function Equation β is the shape parameter, also known as the Weibull slope η is the scale parameter γ is the location parameter

Hazard Function Instantaneous failure rate at a point in time. the hazard function is a measure of risk:

Hazard Function Level 1 Level 2 Level 3 Level 4

Hazard Function Level 1 Level 2 Level 3 Level 4

Discrete Bivariate Hazard Function Stress Level – can denote which ever stress level you desire Risk Level - -1 = 0.1, -2 = 0.01, -3 = 0.001

Discrete Bivariate Hazard Function Below the plane – Operating at a level less than the allowable risk level

Discrete Bivariate Hazard Function Fatigue Analysis Stresses h(s,n) smin smed smax h(s,n=1) r s sr … for n = 1 h(smax,nsmax) h(smed,nsmed) h(smin,nsmin) nr n Operating Stress Range

Further Research Basic Research Applied Research Discrete Bivariate Hazard Function Can we use data to fit a continuous bivariate function? Continuous Bivariate Hazard Function Applied Research Put in system for use to measure consumed life Where is this useful? Parts – Critical Parts where failure is participated by stress loading fatigue. Fatigue Failure

Questions Daniel Sillivant University of Alabama in Huntsville Reliability and Failure Analysis Lab sillivd@uah.edu