PRACTICAL ASPECTS OF SHAKER MEASUREMENTS FOR MODAL TESTING
Shaker testing is commonly used as a method for measuring forced input in experimental modal analysis. The setup of conventional shakers, stingers and transducers is often the source of test difficulties and avoidable measurement errors. This paper reviews the basic design of shakers as beneficial to modal testing, and the common problems associated with setup issues and resulting measurement errors. These include shaker selection, shaker alignment, stinger selection, connections, reciprocity assumptions and other common mishaps and misunderstandings.
Advantages of Multiple-Input Multiple-Output ( MIMO ) testing using low level excitation systems
Multiple-Input Multiple-Output (MIMO) measurement techniques are a well-proven and well-established method for collecting FRF data sets. Despite that, the idea of using multi-references and multiple shakers can be intimidating for an inexperienced modal test engineer. MIMO methods offer some distinct advantages for the measurement and extraction of basic modal parameters especially while testing larger structures. Compared to Single-Input, Single-Output (SISO) techniques, MIMO allows for more uniformly distributed energy across the structure, force levels can be kept lower, modal test data can be taken in one single shot, while nonlinearities are less likely to be excited. Experimental data was taken using single and multiple shakers on an aeronautical structure and results were compared to illustrate the basic principles behind the MIMO technique. Practical aspects of shaker set up, reciprocity checks and the capability of MIMO to resolve closely spaced modes and repeated roots are highlighted throughout the example.
Damage initiation and propagation in composite shells subjected to impact
Abstract The paper investigates the impact-induced damage initiation and propagation for laminated composite shells under low-velocity impact. A three-dimensional eight-node non-conforming element is described to analyze interlaminar stress distribution, and progressive failure. The damage analysis is performed by using Tsai–Wu quadratic failure criterion, Tsai’s damage modes and an additional delamination formula at all Gaussian points. The damage modes considered are matrix cracking, fiber breakage and delamination. The progressive failure is expressed by reducing stiffness of the material at all failed Gaussian points. Special attention is centered on curved laminates. Impact energy threshold for the onset of damage and transient damage size are predicted for composite shells. The analyses of the flat and curved laminates are compared for discussing their different damage mechanism. In addition, the influence of the stacking sequence, the thickness and the radius of curvature on damage behavior of composite shells are studied.