ISO/TR 16194:2017 pdf download – Pneumatic fluid power — Assessment of component reliability by accelerated life testing — General guidelines and procedures.
5 Concepts of reliability and accelerated life testing Reliability is the probability (a percentage) that a component does not fail (for example, exceed the threshold level or experience catastrophic failure) for a specified interval of time or number of cycles when it operates under stated conditions. This reliability can be assessed by test methods described in the ISO 19973 series. Generally, reliability analysis involves analysing time to failure of a component, obtained under normal use conditions in order to quantify its life characteristics. Obtaining such life data is often difficult. The reasons for this difficulty can include the typically long life times of components, the small time period between design and product release, and the necessity for testing components under normal use conditions. Given this difficulty and the need to observe failures of components to better understand their life characteristics, procedures have been devised to accelerate their failures by overstress, thus forcing components to fail more quickly than they would under normal use conditions. The term accelerated life testing (ALT) is used to describe such procedures. However, a relationship between the reliability of a component determined by ALT, and its reliability at normal use conditions, is necessary. This can be assessed by extrapolating the test results obtained from an accelerated life test and comparing it to that obtained from testing at normal use conditions. Figure 1 shows the graphical concept for this relationship.
6 Failure mechanism and mode The failure mechanism is the physical or chemical process that produces instantaneous or cumulative damage to the materials from which the component is made. The failure mode is the manifestation of the failure mechanism resulting from component failure or degradation. The failure mode is the symptom of the aggressive activity of the failure mechanism in areas of component weakness where the stress exceeds the strength. It is necessary that the failure modes observed in accelerated life test conditions are identical to those defined for normal use conditions. 7 Strategy of conducting accelerated life testing Before starting an accelerated life test, it is important to identify the types of failures that might occur in service; especially any feedback from the field. Several methods are available to assist in this effort: design analysis and review using the quality function deployment (QFD), fault tree analysis (FTA), and failure modes and effect analysis (FMEA). Another method is a qualitative test like highly accelerated life testing (HALT). Qualitative tests are used primarily to reveal probable failure modes, but they do not quantify the life (or reliability) of the component under normal use conditions.