Testing reliability

Abstract: We consider the stress-strength problem in which a unit of strength X is subjected to environmental stress Y. An important problem in stress-strength reliability concerns testing hypotheses about the reliability parameter R = P[X > yl. In this article, we consider situations in which X and Y are independent and have normal distributions or can be transformed to normality. We do not require the two population variances to be equal. Our approach leads to test statistics which are exact p values that are represented as one-dimensional integrals. On the basis of the p value, one can also construct approximate confidence intervals for the parameter of interest. We also present an extension of the testing procedure to the case in which both strength and stress depend on covariates. For comparative purposes, the Bayesian solution to the problem is also presented. We use data from a rocket-motor experiment to illustrate the procedure.

Abstract: This study examines the reliability and validity of a measure to evaluate student field performance. Results demonstrated a consistent factor structure with excellent internal consistency, however, there was inadequate consistency between ratings of individual students in their first and second field education experiences. The measure had some predictive validity in that it could differentiate between students identified as having difficulty in Year 1 of the program, but not in Year 2. Scores were significantly associated with academic grades. Implications for future instrument development and the process of evaluation are considered.

Abstract: Applying life estimation approaches to determine in-service life of structures and plan the inspection schedules accordingly are becoming acceptable safety design procedures in aerospace. However, these design systems shall be fed with reliable parameters related to material properties, loading conditions and defect characteristics. In this context, the role of non-destructive (NDT) testing reliability is of high importance in detecting and sizing defects. Eddy current test (ECT) is an electromagnetic NDT method frequently used to inspect tiny surface fatigue cracks in sensitive industries. Owing to the new advances in robotic technologies, there is a trend to integrate the ECT into automated systems to perform NDT inspections more efficiently. In fact, ECT can be effectively automated as to increase the coverage, repeatability and scanning speed. The reliability of ECT scanning, however, should be thoroughly investigated and compared to conventional modes of applications to obtain a better understanding of the advantages and shortcomings related to this technique. In this contribution, a series of manual and automated ECT tests are carried out on a set of samples using a split-D reflection differential surface probe. The study investigates the level of noise recorded in each technique and discuss its dependency on different parameters, such as surface roughness and frequency. Afterwards, a description of the effect of crack orientation on ECT signal amplitude is provided through experimental tests and finite element simulations. Finally, the reliability of each ECT technique is investigated by means of probability of detection (POD) curves. POD parameters are then extracted and compared to examine the effect of scanning index, frequency and automation on detection reliability.