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Mechanical Reliability Prediction: 
A Different Appro ach
Abstract 
Market trend 
Exploring alternative approaches 
Introduction to Case Study: Hydraulic Accumulator (HYDAC) Soluti...
Mechanical Reliability Prediction: A Different Approach | 3 
Abstract 
Reliability Prediction is a practice of predicting ...
Mechanical Reliability Prediction: A Different Approach | 4 
Exploring alternative approaches 
A combination of PoF, NSWC ...
Mechanical Reliability Prediction: A Different Approach | 5 
The accumulator Barrel will be subjected to 1,951,000 pressur...
Mechanical Reliability Prediction: A Different Approach | 6 
Step 3: Convert the spectrum of stress into an equivalent str...
Mechanical Reliability Prediction: A Different Approach | 7 
a. In Table 2 Cumulative damage of 8.25E-02 % damage is cause...
Mechanical Reliability Prediction: A Different Approach
Mechanical Reliability Prediction: A Different Approach
Mechanical Reliability Prediction: A Different Approach
Mechanical Reliability Prediction: A Different Approach
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Mechanical Reliability Prediction: A Different Approach

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This paper critically analyses the current industry practices for making reliability prediction prevalent among the aircraft manufacturers and further explores the more accurate and cost effective methods for predicting the failure rate of a component or subsystem during the early design phase of the product development cycle namely NSWC method , PoF approach and SSI theory. It elucidates the effectiveness of these alternative approaches with the help of a case study on Hydraulic Accumulator (HYDAC).

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Mechanical Reliability Prediction: A Different Approach

  1. 1. Mechanical Reliability Prediction: A Different Appro ach
  2. 2. Abstract Market trend Exploring alternative approaches Introduction to Case Study: Hydraulic Accumulator (HYDAC) Solution 1. Approach to select an appropriate method: 2. PoF, SSI and NSWC methods explained a. A sample prediction for explaining the PoF approach to predict Barrel (Cylinder) failure rate: b. Failure rate calculation using SSI Theory: c. Sample prediction using NSWC for Dynamic Seal 3. Summary of analysis Best Practices in the Industry Common issues with alternative methods CCoonncclluussiioonn Reference 3 3 4 4 4 4 5 7 8 9 9 10 10 1111 Table of Contents © 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.
  3. 3. Mechanical Reliability Prediction: A Different Approach | 3 Abstract Reliability Prediction is a practice of predicting the failure rate of a component or subsystem during the early design phase of the product development cycle. This is carried out in an attempt to ensure product reliability. Primarily, reliability prediction is a design-supportive study intended for the following functionalities Design feasibility evaluation Comparing design alternatives Identifying potential failure areas Tracking reliability improvements In the aerospace industry, the failure rates predicted by this study are used for safety assessment in compliance to FAR 25.1309 and ARP 4761. The predicted failure rates are used in FMEA which in turn, provides input to FTA. The current industry practice uses a standard database approach called NPRD to find failure rates of mechanical components/systems. In recent times, aircraft manufacturers are increasingly rejecting NPRD predictions, as the failure rates predicted using this database is arguable in terms of design closeness. Also, in most cases, they were found to vary with actual failure rates observed in the field. A possible solution for this problem could be devised using a combination of approaches, ssuucchh aass Stress Strength Interference (SSI) theory Physics of Failure (PoF) approach Naval Surface Warfare Centre (NSWC) methods Although prediction with the above methods are accurate, these methods are not yet implemented due to a lack of understanding, increased complexity, requirements of large amounts of design data, and the need for extensive time and effort. In this whitepaper, we have built a case study around Accumulator. This helps explain an effective solution for mechanical predictions using a blend of the SSI theory, PoF approach, and NSWC methods. Market trends In the current scenario, the NPRD method is extensively used for mechanical product reliability prediction as this method is relatively easy to predict the part failure rate. NPRD has remained the preferred method for years. However, the current market trend is moving towards exploring other appropriate methods for mechanical predictions. Aircraft makers feel the need to explore alternative methods due to the following reasons: Need for more accurate, precise and reliable failure data which considers actual usage conditions in its prediction models Predictions should be specific for each manufacturer. Also, the quality of components and manufacturing processes should be accounted for NPRD assumes a constant failure rate, which is not true in all cases In NPRD, failure rates are not application sensitive and have limited accuracy It is doubtful that sizeable design improvements will result from the NPRD prediction process © 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.
  4. 4. Mechanical Reliability Prediction: A Different Approach | 4 Exploring alternative approaches A combination of PoF, NSWC and SSI methods can be used appropriately to arrive at more accurate component and system-level reliability predictions. As field data is rarely available, the use of field data for Reliability Prediction is likely to be ruled out as an alternative. Hydraulic Accumulator (HYDAC) A hydraulic accumulator is a pressure storage reservoir. In this a non-compressible hydraulic fluid is held under pressure by an external source like a spring, a raised weight, or compressed gas. An accumulator enables a hydraulic system to cope with extremes of pressure using a less powerful pump. This enables the hydraulic system to respond more quickly to a temporary demand, and to smooth out pulsations. Even though all 3 alternative methods can help to arrive at the part failure rate, selecting an appropriate method of Reliability Prediction is the key. Figure 1 explains the logic behind finalizing a prediction technique. As seen in Figure 1, a good starting point would be the life-limited items. Aircraft life-limited parts are those ppaarrttss tthhaatt aarree identified by the aircraft manufacturer or production certificate holder as being limited to a total life counted in hours, cycles, landings, or by calendar. Typical life-limited parts are seals, bearings and springs. Figure1. Determination of the prediction technique In case prediction models are not available in NSWC then either the PoF or SSI approach can be selected by following the logic explained in above diagram. The prediction method chosen for each of the accumulator components is shown in Table 3 -“Summary of analysis”. Methods of failure analysis a. Physics of Failure (PoF) is a science-based mathematical approach for reliability predictions that uses modeling and simulation to design-in reliability. This approach models the root causes of failure such as fatigue, fracture, wear, and corrosion. Fatigue and fracture are the two root-causes considered in this case study. Various models were applied for these two causes and the failure rates were predicted. © 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.
  5. 5. Mechanical Reliability Prediction: A Different Approach | 5 The accumulator Barrel will be subjected to 1,951,000 pressure cycles at different pressure levels shown in Table 1. It is also subjected to a static pressure of 3500 psi. As per the logic explained in an earlier section “Approach to select an appropriate method:”, the SSI method would be used for static failure rate estimation caused by 5000psi pressure and fatigue calculations would be used for failures caused by 1,951,000 pressure cycles. A combination of SSI and PoF approach should be used for ‘Barrel’ failure predictions. LUBRICATION PASSAGE Failure rate calculation using the PoF theory 5,000 to 3,950 to 5,000 5,000 to 3,700 to 5,000 5,000 to 3,600 to 5,000 5,000 to 1,900 to 5,000 5,000 to 3,450 to 5,000 5,000 to 2,000 to 5,000 220000 ttoo 55,,000000 ttoo 220000 Step 1: List all types of stress on the Barrel (Cylinder): Types of stresses: Hoop stress, longitudinal stress and stress at the weakest location. In Figure 2, and This Barrel is a thin walled cylinder. (Criteria for thin wall cylinder: Cycles per Aircraft life (ni) 160,000 800,000 500,000 15,000 24,000 350,000 11,,995511,,000000 Step 2: Convert all pressure cycles into stress values: Convert all minimum and maximum pressure levels into both ‘longitudinal’ and ‘hoop stress values’. The ‘stress at the weakest location’ test would be calculated by Finite Element Analysis. Hope Stress, Longitudinal Stress, Whereas, ) AIRPORT PISTON HYDRAULIC FLUID PORT END CAP END CAP PACKING & BACKUP RING BARREL ASSEMBLY Pressure Cycle (psig) Total Cycles 1,951,000 Table 1: Pressure cycles at different Pressure Levels Figure 2. Barrel showing all the stresses © 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.
  6. 6. Mechanical Reliability Prediction: A Different Approach | 6 Step 3: Convert the spectrum of stress into an equivalent stress: Equivalent stress equation for Barrel material 13‐8Mo CRES, as taken from MMPDS database is Seq = Smax(1-R)0.11 Where, Seq = Equivalent Stress, Smax = Maximum Stress (Stress induced by maximum pressure in a pressure cycle) R = Stress Ratio = Minimum Stress (Stress Induced by minimum pressure in a pressure cycle) Maximum Stress Repeat the above step for all three stresses (hoop, longitudinal and stress at the wweeaakkeesstt llooccaattiioonn)).. Step 4: Calculate number of cycles before failure: The empirical relationship for S-N curve as given in MMPRD for 13‐8Mo CRES. Log Nf = 18.12 - 6.54 lod (Seq) Where Nf = Number cycles to failure at an Equivalent stress of Seq Step 5: Use Minor’s rule to calculate cumulative damage caused by each set of pressure cycles Cumulative damage caused by each Pressure cycles Number of cycles over the design life, ni Life to failure corresponding Table 2: Cumulative Damage Calculation to stress, Nf Damage due to each set of pressure cycle Cumulative Damage ni Nf © 2014, HCL Technologies. Reproduction Prohibited. This document is protected under Copyright by the Author, all rights reserved.
  7. 7. Mechanical Reliability Prediction: A Different Approach | 7 a. In Table 2 Cumulative damage of 8.25E-02 % damage is caused by 1,951,000 cycles b. 1,951,000 cycles would take 102,000 hours to complete c. This means 8.25E-02 % damage is caused in 102,000 hours d. Time for first failure (MTBF) = Time for 100% damage = 102000 / 8.25E-02 = 1.2E6 hours e. Failure rate = 8.09E - 09 failure per flight hour The above values 8.09E-09 are the failure rate due to longitudinal stress . But in Hoop stress , the equivalent stress is less than the Endurance Limit of the 13‐8Mo CRES (AMS 5629). This means that failure rates should be almost zero under such stress levels. Then the failure rate caused by stress at the weakest location is calculated as, = 9.27E-03 and as zero. The weakest locations and levels of stress at those locations are calculated from Finite Element Analysis. b. SSI theory: Fracture and deformation analysis was performed using Stress/Strength Interference theory. Stress/Strength Interference analysis is a practical engineering tool used for designing and quantitatively predicting the reliability of mechanical components subjected to mechanical loading. This method treats both stress and strength as random variables subject to natural scatter. If failure is defined by Stress > Strength, then the failure probability would be equivalent to the interference of stress and strength distribution. Failure rate calculation using SSI Theory In case of ‘Barrel’, the SSI theory was used for calculating failure rate resulting from a static pressure of 5000psi. In this method, the corresponding Hoop stress, Longitudinal stress, Stress at the weakest location on the air and fluid side of the ‘Barrel’ are calculated at a pressure level of 5000 psi. The interference when stress exceeds strength is calculated using a. Using the standard Normal distribution table, the Interference probability was calculated for each stress based on ‘Z’ value b. Reliability = 1- Interference Probability c. Failure rate was calculated using the relationship: , where R = Reliability,

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