Abstract:

This is the first issue of the year 2014 and the International Journal of Performability Engineering (IJPE) enters the 10^th year of its publication. We are sometimes receiving queries from students, teachers and researchers expecting promotions, and also from some librarians, particularly from India about the impact factor of IJPE. Let us not forget that IJPE is a highly specialized technical journal. In this issue, for the benefit of our readers, we have explained on page 94 of this issue what does impact factor with its limitations and applicability signify. We also bring to our readers a note, on pages 119-120, on the visibility factor of IJPE in the scientific world which would inform our readers how IJPE has fared since its inception.in 2005. I must say here that we are not in competition with any other journal but we will continue to provide the best of literature to the profession we are engaged in and I leave it to our readers to judge after reading these two notes to assess the value and usefulness of contributions that this journal has made to the scientific world over years.

As usual, we present to our readers a section on the Accelerated Life Testing, in this issue. We all know that the greatest contribution of reliability engineering is not only to be able to quantify and design reliability in products but also to demonstrate it through tests. The objective of reliability testing is to ferret out potential problems with the design as early as possible and provide confidence that the system meets its reliability requirements.

Accelerated life testing of a product is often resorted to by subjecting the product to stresses in excess of its normal conditions of use in order to uncover faults and potential modes of failure in a short amount of time in the laboratory. The stresses can be any of the environmental stresses of use such as temperature, vibration, voltage, pressure, humidity, cycling rate etc. This kind of testing is particularly necessitated in case of high reliability products since it may take a very long time before a failure is encountered in normal environmental conditions of use. The main objective of an accelerated test is either of the following:

(i) To discover failure modes

(ii) To predict the normal field life from the high stress laboratory life

By analyzing the product's response to such high stress level tests in the laboratory, one can make prediction about the service life and maintenance intervals of a product.

The following steps are usually followed while conducting accelerated life tests:

• Objective and scope of the test is defined
• Stresses to be considered are identified and their levels are determined
• The accelerated test is conducted and collected data is analyzed.

Three types of Accelerated life tests (ALT) are often used in practice, viz., constant-stress ALT, progressive-stress ALT and step-stress ALT. In constant-stress ALT, the stress is kept at a constant level throughout the test whereas in progressive-stress ALT, the stress is continuously increased over time. However, in Step-stress ALT, starting with some level of stress, the stress is increased in steps in such a way that it will change at specified times. If the product does not fail at the specified time, stress level is raised and held over a fixed time. This procedure is repeatedly until the product fails or the censoring time is reached. Usually, to determine a life-stress relationship, the models that are generally used are: Arrhenius Model, Eyring Model, Inverse Power Law Model Temperature-Humidity Model and Temperature Non-thermal Model.

Once an accelerated life experiment has been modelled, testing the fit of the proposed model can be done. I am happy to note that all relevant aspects related to accelerated life testing have been presented in this special section.

I am grateful to Dr. Jean-Fran?ois Dupuy of France, who very kindly accepted our invitation to bring out this special section on accelerated life testing and to make this issue possible. My sincere thanks are also due to all those authors, who have contributed to this issue. We will continue to present various aspects of performability engineering and new research to our readers through this journal in the years to come.