Notching during random vibration test based on interface forces - The JWST NIRSpec experience
Publication date: 18 March 2010
Authors: Salvignol, J.-C., laine, B., Ngan, I., Honen, K., Kommer, A.
Force limited vibration was used during the sine and random qualification tests of the NIRSpec instrument, to limit stresses in the brittle structure while demonstrating adequate qualification with regard to the environmental flight conditions. First, NASA provided a force limit curve based on their internal 'Semi-Empirical Method'. Then, strain gages were mounted on the legs of the kinematic mounts to recover interface forces during the vibration test. Two different methods were then used to determine the notches: one called the 'Apparent Mass' method that is based on sine sweep signatures and another one based on direct force measurement in the time domain during random test. The second method resulted in the most effective notch determination, allowing the justification of the notches in real time with high accuracy. The resulting RMS forces are well below the forces corresponding to static design loads that is a more conventional method.Link to publication