• design of a small number of critical spot welds can be modified to improve their performance characteristics, • subparts joined by the critical spot welds can be redesigned to absorb their adverse impact, • most critical spot welds may be quality controlled to ensure their effective presence. 3.3 Effect of quality control on robustness As noted that after optimization, there still remains few spot welds if missing can cause large variations in the performance criteria. Hence, to take an advantage of the information obtained by robustness curves, we propose a quality control of limited number of spot welds in order to guarantee the robustness of the population of identical structures within acceptance level due to missing of remaining uncontrolled spot welds. Let us assume that first 20 spot welds identified while obtaining the robustness curves for optimized design are quality controlled and thus effectively present on the structure. The procedure was applied again to obtain the robustness curves for the remaining uncontrolled spot welds. New robustness curves along with the initial curves without quality control of spot welds are shown in fig 8 for both modes. See the remarkable improvement in robustness as up to 3 times lower worst case variations are observed now for mode 1 up to missing of 100 most influential spot welds. 20 40 60 80 100 −8 −7 −6 −5 −4 −3 −2 −1 0 1 α − Number of RSWs Relative Eigenfrequency Shift (%) Without QC With QC (a) 20 40 60 80 100 −1.6 −1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 α − Number of RSWs Relative Eigenfrequency Shift (%) Without QC With QC (b) Fig. 8 Robustness curves for two modes after quality control of 20 RSWs (a)mode 1. (b)mode 2 Q. I. Bhatti and M. Ouisse and S. Cogan 306
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