Tham khảo tài liệu 'designing capable and reliable products episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Variables in probabilistic design 165 more so when process shift is taken into account as shown by the term Ờ. Anticipation of the shift or drift of the tolerances set on a design is therefore an important factor when predicting reliability. It can be argued that the dimensional variability estimates from CA apply to t he early pa rt of it he product s 1 ife-cycle and may be overconservative when applied to the useful life of the product. However variability driven failure occurs throughout all life-cycle phases as discussed in Chapter 1. From the iil ovc arguments it can be seen that anticipation of the process capab i 1 i ty levels set on the important design characteristics is an important factor when predicting reliability Murty and Naikan 1997 . If design tolerances are assigned which have large dimensional variations the effect on the reliability predicted must be assessed. Sensitivity analysis is useful in this respect. Stress concentration factors and dimensional variability Geometric discontinuities increase the stress level beyond the nominal stresses Shigley and Mischke 1996 . The ratio of this increased stress to the nominal stress in the component is termed the stress concentration factor Kt. Due to the nature of manufacturing processes geometric dimensions and therefore stress concentrations vary randomly Haugen 1980 . The stress concentration factor values however are typically based on nominal dimensional values in tables and handbooks. For example a comprehensive discussion and source of reference for the various stress concentration factors both theoretical and empirical for various component configurations and loading conditions can be found in Pilkey 1997 . Underestimating the effects of the component tolerances in conditions of high dimensional variation could be catastrophic. Stress concentration factors based on nominal dimensions are not sufficient and should in addition include estimates based on the dimensional variation. This is .
