An Introductory Review of Mechanical Reliability Testing

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland An Introductory Review of Mechanical Reliability Testing Zongzhi Zhang 1,a, Junqi Qin 1, Changchun Di 1 and Dong Wang 2 1 Artillery Department of Ordnance Engineering College, Shijiazhuang , China 2 The 32th Experimental Base, Huayin , China a zzz_0712@163.com Keywords: Mechanical Reliability Testing; Reliability Enhancement Testing; Accelerated Life Testing. Abstract. Mechanical reliability testing is the focus in the research of reliability testing at present. The basic concepts of reliability testing are introduced. Names of reliability testing are sorted. Characteristics of mechanical reliability testing are discussed. The key issues of mechanical reliability testing and its development are then reviewed. Finally, some possible fields in the research are summarized. Introduction According to national standards[1], the definition of reliability: Products abilities to complete the required functionality under the conditions and within the time specified requirements. And reliability testing is a general term for the various tests for the analysis of the reliability and product evaluation[2]. Broadly speaking, any test related to failure of products can be considered as a reliability testing. On the one hand, it aims to find products of various defects in design, materials and workmanship, and to provide information to improve the integrity of the product and improve mission success rate and reduce maintenance and security costs, to verify compliance with quantitative reliability requirements; The other is to obtain the data required for evaluation of the level of reliability. When the device has just been produced, it should ideally meet the reliability requirements of the contract or its mission statement, the reality is far from the case. According to the U.S. government reports, the average mean time between failures(mtbf) of large electronic-mechanical products only meet one-tenth of the standard required, so the product must undergo a series of reliability tests, and found to ascertain the presence of defects and to improve the inherent reliability. According to different purposes of testing, reliability testing can be divided into engineering testing and statistical testing The purpose of engineering testing is to expose the defects of products in design, technology, components and materials, so measures can be taken to improve the reliability of products. The purpose of statistical testing is to verify whether the product reaches the reliability or longevity requirements. At present, the research and application of reliability testing is in the ascendant, the name of the kind of tests at home and abroad have not been reunified, which is not conducive to the smooth conduct research work in this field, so it is necessary to standardize the names and concepts of the meanings of such tests. In fact, as shown in Figure 1, reliability engineering testing includes reliability enhancement testing(ret) and accelerated reliability growth testing(argt). Highly accelerated life testing(halt) and highly accelerated stress screening(hass) belong to RET. Statistical reliability testing includes accelerated life testing(alt) and accelerated degradation test(adt). This paper describes the engineering testing in RET and statistical testing of ALT. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,11:49:23)

2 Applied Mechanics and Materials Vols Figure 1 Features of mechanical reliability testing Reliability theory and testing methods come from the field of electronics, so most theories are based on accidental failure and electronic product life assumption of exponential. Over the years, with the improvement of electronic technology and electronics reliability theory, reliability testing technology of electronic products has been relatively mature; due to mechanical reliability has its own characteristics, electronic reliability theory does not fully applicable to mechanical products. Mechanical reliability testing techniques developed slowly because of some theoretical problems. Mechanical reliability testing techniques are still in the infancy, which is mainly caused by the special nature of mechanical products. Mechanical reliability testing s specialty is mainly manifested in the following aspects: (1) Mechanical reliability testing is difficult to get a larger sub-sample capacity, and has long testing cycle. (2) Diversity of mechanical failure mechanisms and dependence on environmental factors are decided not only by stress factors, but also by environmental factors. And the actual working environment of mechanical products is worse than the testing environment. (3) Mechanical reliability theory is more complex. Life of most electronic products is exponentially distributed, but most failures of mechanical products are depletion type, their life distributions are not limited to a single form. Most issued reliability design, test and analysis methods or standards are based on the failure of electronic products, which are not fully applicable for mechanical products. (4) Parts of mechanical products are generally designed for a specific purpose, the versatility is not strong, and it s not easy to accumulate data for sharing. So it s difficult to assess the reliability of non-electric products because of the lack of data. (5) Since there is not a standard reliability screening testing, early failures of mechanical products can not be ruled out by environmental stress screening testing which is used for electronic products, so it increases the difficulty of describing reliability growth model[3]. The key technology research status of mechanical reliability testing at home and abroad Research status of RET at home and abroad Boeing first proposed the concept of RET, and its purpose is to find the products design defects quickly with higher environmental stress and working load than actual use, so they can improve the inherent reliability level of products. RET is mainly for module-level and unit-level electronic, mechanical and electrical products, and is required to carry out early in the product development phase. RET belongs to the category of provocation test, the earliest provocation test is aging tests in the 1950s and developed into a broader environmental stress screening(ess) in the 1970s. ESS can not really improve the inherent reliability of the product, and the product inherently reliable plays a decisive role in the reliability of

3 1416 Engineering and Manufacturing Technologies the product. To solve this problem, Dr. Hobbs presented HALT and HASS in The former is for product design stage, the purpose is to expose design flaws quickly to promptly improve the design and inherent reliability of product; The latter is for stage of production, the purpose is to expose a variety of manufacturing defects to provide users with high-reliability products. The theoretical basis of RET is Physics of failure. It views the malfunction or failure as the main object of study, and achieve the purpose of improving reliability through discovery, research and cure failure. Currently, there are already a variety of testing techniques such as step-stress testing, HALT, HASS and so on. In abroad, after the proposing of RET, it has been popularized in various industrial sectors, so products reliability are greatly improved. According to statistics of QualMark company from May 1995 to March 1996, they have provided RES services for 47 kinds of products(involving electronic products and non-electrical products) from 19 different industrial sectors of 33 companies[4], which all have gained remarkable achievements. The major foreign institutions engaging in RET are QualMark company, Otis Elevator company, Hobbs Engineering company. In China, the RET technology and its application are still in their infancy, mainly limited to follow-up study. Currently, the reliability laboratory of National Defense University has undertaken a lot of study in RET technology and applications. The reliability engineering center of Beijing University of Aeronautics and Astronautics has also made some achievements in the analysis of RET. Research status of ALT at home and abroad In 1967, American Roma Airline Center gave a unified definition of ALT: In the basis of scientific engineering and statistical assumptions, it uses a statistical model related to laws of physical failures to matrix the reliability information of product which is obtained under accelerated stress level[5]. According to different ways of applying stress, ALT can be divided into constant stress testing, step stress testing and progressive stress testing. (1)Constant stress ALT. In Figure 2, t represents testing time, s represents the stress, x represents a sample of failure. During the test, the selected set s1<s2< <sk is higher than the normal stress level, the samples are divided into k sets, each sample is under each stress level independently for life testing. When each group has a number of failures, the testing can be stop. Foreign statistical analysis of ALT began in the 1960s, the statistical analysis method of constant stress testing first developed. At present, the constant stress testing mainly focuses on how to improve the statistical analysis of accuracy and other issues. Mazzuchi and Hirose[6,7] engaged in the perspective of statistical analysis model, Watkions and Bugaighis engaged in the perspective of parameter estimation method[8,9], they all achieved a lot in improving the accuracy of statistical analysis. Figure 2 (2)Step stress ALT. As is shown in Figure 3. Due to the lowest stress levels of constant stress tests are close to normal stress levels, it s longer and inefficient. So the research and application of step stress tests are increasingly important. The initial step stress test method is mechanical durability test in step load method. In 1961, Bell Labs scientists Dodson and Howard carried out

4 Applied Mechanics and Materials Vols solutions of stepping temperature stress tests for semiconductor devices. Step stress test reduces the number of samples required, and has higher acceleration efficiency, but the key issue of step stress test is how to separate out the complete life information from the data step stress practical results. Figure 3 (3)Progressive stress ALT. As is shown in Figure 4, the stress level changes continuously with time. In 1958, Kimmel first tried progressive stress test in electronic products. Because its statistical analysis is very complex and requires specialized stress control device, progressive stress test s application has been greatly restricted. Figure 4 Each of these three types of test has its own characteristics, the main difference is shown in Table 1. Table 1 Method Advantages Shortcomings Remark Constant stress ALT Mature model, Test is simple, Easy to succeed Test requires much longer time The most widely used Test requires a relatively small Step stress Statistical analysis of the sample, The acceleration ALT experimental is difficult efficiency is relatively high Progressive stress ALT Maximum test efficiency Specialized required equipment Few studies and applications

5 1418 Engineering and Manufacturing Technologies Development and Prospect Compared with single failure modes of electronic products, failures of mechanical products have much more modes. And mechanical reliability testing has problems such as long period testing time, large costs and small sub-sample. Therefore, the following problems need to be studied in the technology of mechanical reliability testing. (1)Most of the standards and specifications are mainly for reliability testing of electronic products. There is not a perfect solution for mechanical products. (2)Mechanical products mostly work under complex stress environment. In developing the test model, we must consider the environmental stress s influence on the material aging and failure, and the relationship between environmental stress and the mechanical material s failure mechanism. (3)The samples in mechanical ptoducts reliability testing can not be great, and sometimes even none failure occurs in a small sample. Therefore, the identification and evaluation of ending time of testing is also an important direction. In summary, the mechanical reliability testing techniques are faced with a large number of problems to be solved. With the progress of research, the technology also has broad application prospects in the areas of mechanical products. Reference [1] Reliability Terminology and definitions. (GB ). [2] People's Liberation Army General Armament Department of Military Training Textbook Committee: Reliability test technology. Defence Industry Press, Beijing [3] Sineanth R M. Case history of a mechanical system reliability development test Annual Reliability and Maintainability Symposium, Philadelphia, Pennsylvania USA, [4] Silverman M: Summary of HALT and HASS Results at an Accelerated Reliability Test Test Center Proceeding Annual Reliability and Maintainability Symposium, [5] Zhang Chunhua, Wen Xisen, Chen cycle: Accelerated life test Survey. Ordnance, 2004, 24 (4) : [6] Mazzuchi TA, Soyer R: Dynamic Models for Ststistical Inference from Accelerated Life Tests. IEEE Proceedings of Annual Reliability and Maintainability Symposium, [7] Hirose H: Estimation of T hreshold Stress in Accelerated Life Testing. IEEE Transactions on Reliability, 42 (4), [8] Watkins A J. Review: Likelihood Method for Fitting Weibull Log Linear Models to Accelerated Life Test Data. IEEE Transactions on Reliability, 43 (3), [9] Bugaigh M M: Exchange of Censorship Types and its Impact on the Estimation of Parameter of a Weibull Regression Model. IEEE Transactions on Reliability, 44 (3),

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