Qualification of Large Structures for Shock Loads

Many critical equipments and components have to withstand shock and vibration during its life time. In defence, space and nuclear industries in particular, it is mandatory to test and analyze such components before acceptance. Analytical and experimental qualification of these components is an important exercise to be fulfilled as per the applicable codes and practices. Test for shock qualification is particularly important for components designed for defence and space applications. When the structure is large, generally an analytical approach is followed and a small scale model is experimentally validated for such qualification and the result is extended to the prototype. This approach may or may not be true always. The paper discusses about the phases of qualifying a large component for a certain level of shock. Achieving the required shock level and the pulse duration experimentally with repeatability is a challenging task. Besides, when the size and shape of the structure is odd, the qualification procedure becomes more complicated and at times difficult to achieve the desired objective. The objective was to achieve the required shock pulse by dropping the test structure from a predetermined height on a target made of commercially available rubber material. The test object is a long stainless steel vessel weighing 805 kg and 2.8 m long. The vessel is housed in a rigid cage with arrangement to lift with a magnetic clutch for instantaneous release for dropping. It was required to subject the vessel to both vertical and horizontal shock of 25 g of 35 ms duration. For dropping the vessel horizontally, the housing cage was dropped on more than one target from a predetermined height. The procedure followed in arriving at the drop height, actual drop height and the achieved shock levels are discussed in the paper.