Modal Correlation Is Required to Reduce Uncertainty in Shock Analysis and Testing

The average aerospace structural analyst is very familiar with performing various types of dynamics analysis (sine vibration, random vibration, transient) but much less familiar with dealing with shock analysis defined by SRS (shock response spectrum) dealing with very high acceleration levels (1000–2000 g’s) and high frequency content (10 kHz). Likewise, the average aerospace engineer performs a lot of sine and random vibration testing at the system and subsystem levels and feedback from those tests are readily compared to analysis predictions. In contrast, typically very little SRS-related analysis and subsequent shock testing is performed either at the system or subsystem level and rarely would shock analysis predictions be compared with shock test results. Typically going into sine and random vibration test, a structural analyst will know expected responses at accelerometer locations. For a typical shock test, many times a dynamics engineer is a bystander and watches the test lab use their shock test method to achieve the SRS tolerance levels in each axis and typically no shock analysis predictions of the shock test setup are performed. Sine and random vibration tests at the system level are common, but shock testing at the system level is much rarer (sometimes the customer will waive the system shock test requirement – that cannot happen!) as it is much more difficult test to perform. Shock loads are much more complex and more difficult to analyze than sine and random vibration loads and thus more engineering is required to get more accurate predictions. Thus, to reduce uncertainty in shock analysis predictions: