Optimizing the life cycle of aerospace products using accelerated life testing.
The reliability appeared as a result of the significant importance that the problems of operating safety of industrial equipment, devices and components have gained, representing today a top technique indispensable to engineers (Martinescu, 2005). Figure 1 presents a product's life cycle; accelerated life testing is part of the phase of internal verification (Ireson et al., 1995).
[FIGURE 1 OMITTED]
2. BACKGROUND THEORY
Increasing demand on product reliability and shortening of product development cycle have motivated research in accelerated life testing which aims alleviating such pressure (Nelson, 2004).
The accelerated tests have been introduced in the lifecycle of a product from the necessity of ascertaining its behavior in conditions of overstressing in the stages of qualification and production (Guo & Pann 2007). The principle of accelerated tests is to intensify the stress parameters with the purpose of obtaining data regarding the product's reliability in conditions of economic efficiency and to shorten the duration of the test (fig. 2). The accelerated tests don't need to be realized by increasing the restrain in relation to the conditions of exploitation, unless the acceleration factor is known and controlled. The accelerated life tests have as objective the determination of reliability indicators (usually the failure rate or the mean life), and are achieved by increasing the stress over the nominal margins, with the condition of maintaining the mechanisms responsible for the occurrence of failures. The following can be used as stress parameters: temperature, tension, humidity, mechanical stress (shocks, vibrations).
[FIGURE 2 OMITTED]
Accelerated life testing (Nelson, 2004) is used in electronics, (resistors, lasers, liquid crystal displays, electronic bounds, switches, circuit breakers, relays, cells and batteries) in the study of metals and composite materials, but also for certain components and mechanical assemblies (automobile parts, hydraulic components, tools, bearings, gears). In aerospace industry, the accelerated life testing is used to test certain components: the engine, the oil pumps, fuel pumps, the landing gear, onboard electronic equipment and stiffening components (strips, spear).
3. CASE STUDY
This paper describes the accelerated life tests on the Puma IAR 330 helicopter blades with the purpose of determining the mean life in normal operating conditions. These components were chosen for testing because of the extended testing time for the blades that can reach up to 1.6 million cycles in normal operating conditions. The major advantages of introducing accelerated life testing in the aviation industry are as follows (Zaharia et al., 2007):
* It reduces the testing time, resulting in faster marketing time;
* Lower product development costs;
* Lower warranty costs;
* It improves product design and manufacturing.
To conduct the accelerated life tests a test plan has been made using the ALTA 7 software (it is the first, and still the only, commercially available software package designed expressly for quantitative accelerated life testing data analysis). The parameter that intensifies in order to determine the mean life is the bending moment; in normal operating conditions the bending moment has the value of 200 Nm (Newton meter) with a maximum value of 300 Nm.
The mechanism of blade degradation is fatigue; the Weibull distribution and Inverse Power Law model are the best suited for mechanical stress. The test plan is described in figure 3.
[FIGURE 3 OMITTED]
Using this dialogue box two level of acceleration will result: the first level of acceleration will be 230 Nm and the second level 300 Nm. The tests have been made on a Brequet test bench at acceleration levels calculated with the software ALTA7 and the following results occurred (Table 1):
The data resulted from the tests were introduced in the ALTA7 software, with which the data analysis will be made and the mean life of the blades at 200 Nm in normal conditions will be determined. In figure 4 we can see the two distributions of failure time at the accelerated stress levels (230 and 300 Nm). The plot representation Life vs. Stress is frequently used for the Inverse Power Law model to ascertain the mean life. From figure 4 results that for the normal testing level the mean life for the blades is of 1.414.619 cycles. At the tests for the approval of the blades made by the producer in normal testing lavel a mean life of 1.422.022 cycles has resulted. We can see that the mean life obtained in accelerated level and presented above (Table 1) and that obtained by the producer is very similar (in accelerated testing level 1.414.691 cycles and in normal testing level 1.422.022 cycles). By using the accelerated tests in the life cycle of the tail rotor blades the testing time was reduced 4.34 times (the total testing time in accelerated testing level was of 3.929.035 cycles and the total testing time in normal level obtained by the producer was of 17.064.270 cycles).
[FIGURE 4 OMITTED]
The testing method described in this paper presents significant interest in the lifecycle of the helicopter blades because of the decrease in testing time and of the development costs. In this field's fierce competition accelerated life tests will represent in the future indispensable method to all the companies that wish to introduce products with a higher reliability on the market.
As further research I will expand the accelerated tests on other components from aviation and engineering products.
This research has been supported by CNCSIS Project TD_189 with title: Theoretical and experimental research regarding reliability testing.
Guo, H. & Pan, R. (2007). D--Optimal Reliability Test Design for Two--Stress Accelerated Life Tests, Available from: http://www.reliasoft.com/pubs/2007IEEEPID469965.pdf Accessed: 2008-07-19
Ireson, W. G., Clyde F. C. & Richard Y. M. (1996). Handbook of Reliability Engineering and Management, 2nd Edition, McGraw-Hill, ISBN 0-07-012750-6, New York, USA
Martinescu, I. (2005). Fiabilitatea si Securitatea Sistemelor (The Reliability and Security Systems), Editura Lux Libris, ISBN 673-9458-42-4, Brasov, Romania
Nelson, W. (2004). Accelerated Testing; Statistical Methods, Test Plans, and Data Analyses", Wiley, ISBN 0-47169736-2, New Jersey, USA
Zaharia, S.M., Buican, G. & Martinescu, I. (2007). A study about reliability tests with suspended elements, Proceedings of the Fifth International Conference on Challenges in Higher Education and Research in the 21-st Century, Kolev .N., Dimitrov, L., (Ed.), pp. 161-164, Heron Press, ISBN 978-954-580-227-0, Sofia, Bulgaria
Table 1. Results of accelerated tests. Time to Bending Crt. failure Moment No. [cycles] [Nm] 1. 387.234 230 2. 456.723 230 3. 560.819 230 4. 589.090 230 5. 623.120 230 6. 732.109 230 7. 78.902 300 8. 80.132 300 9. 89.672 300 10. 98.098 300 11. 109.458 300 12. 123.678 300
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|Author:||Zaharia, Sebastian Marian; Martinescu, Ionel|
|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2008|
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