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Typical methods of replicating aerodynamic environments in the laboratory are generally poor. A structure which flies “freely” in its normal operating environment, excited over its entire external surface by aerodynamic forces and in all directions simultaneously, is then subjected to a vibration test in the laboratory whilst rigidly attached to a high impedance shaker and excited by forces applied through a few attachment points and in one direction only. The two environments could hardly be more different. The majority of vibration testing is carried out at commercial establishments and it is understandable that little has been published which demonstrates the limitations with the status quo. The primary objective of this research is to do just that with a view to identifying significant improvements in vibration testing in light of modern technology. In this paper, case studies are presented which highlight some of the limitations with typical vibration tests showing that they can lead to significant overtests, sometimes by many orders of magnitude, with the level of overtest varying considerably across a wide range of frequencies. This research shows that substantial benefits can be gained by “freely” suspending the structure in the laboratory and exciting it with a relatively small number of electrodynamic shakers using Multi-Input–Multi-Output (MIMO) control technology. The shaker configuration can be designed to excite the modes within the bandwidth utilising the inherent amplification of the resonances to achieve the desired response levels. This free-free MIMO vibration test approach is shown to result in substantial benefits that include extremely good replication of the aerodynamic environment and significant savings in time as all axes are excited simultaneously instead of the sequential X, Y and Z testing required with traditional vibration tests. In addition, substantial cost savings can be achieved by replacing some expensive large shaker systems with a few relatively small shaker systems.

The Impedance-Matched Multi-Axis Test (IMMAT) technique has been shown to offer significant benefits when compared with the conventional single-shaker vibration test. Thus far, IMMAT has only been demonstrated on relatively small and lightweight structures and to moderate vibration severity. This paper describes the activities undertaken to scale-up the IMMAT technique to larger structures and to severe vibration levels.

This paper describes a radically new approach to the vibration testing of structures in order to demonstrate their endurance under simulated service conditions. The excitation mechanisms of structures in-service typically fall into one of three configurations; (i) excitation from a parent structure through mechanical connections (e.g. during transportation), (ii) excitation from aerodynamic forces distributed over the outer surface of the structure (e.g. aircraft and rockets in flight), or (iii) A combination of (i) and (ii). In nearly all cases, the in-service excitation is multi-directional, yet it is standard practice to replicate these environments with three orthogonal single-axis vibration tests. In addition, a considerable mismatch of the boundary conditions between the in-service and laboratory configurations is common, especially when replicating aerodynamic environments. This paper presents quantitative evidence of limitations with the status quo and demonstrates a superior method; Impedance Matched Multi-Axis Testing (IMMAT). Three noteworthy improvements of the new method are; (i) enhanced replication of the in-service environment, (ii) much shorter test durations, and (iii) a significant reduction in costs associated with random vibration tests.

Many structures experience random vibration caused by the rapid flow of air over the external surface of the structure. One example of this “aerodynamic excitation” occurs when missiles fly through the atmosphere en-route to target in powered flight, or slung to the undercarriage of an aircraft. In most cases, it is necessary to carry out laboratory testing in order to qualify the design of the structure and to assess the pedigree of the manufacturing and assembly process. The laboratory test should replicate, as closely as possible, the damage potential of the in-flight environment. The traditional method of replicating the aerodynamic induced vibration in the laboratory is to rigidly attach the structure to a large electrodynamic shaker and to subject the structure to random vibration. This testing methodology is inadequate and non-representative for two main reasons: (1) the excitation mechanisms are very different, i.e. through a distinct region when attached to a shaker compared to distributed excitation over the entire outer surface in-flight, (2) The boundary conditions are very dissimilar, i.e. attachment to a very high mechanical impedance shaker compared to “free” flight. There is much evidence to show that this testing methodology often leads to overly severe tests. Furthermore, the test program can be costly and time-consuming as tests are often carried out sequentially in three orthogonal axes. In addition, tests have to be repeated with different response control accelerometers as it is impossible to maintain in-flight responses all over the structure simultaneously due to the two reasons given previously. This paper details research carried out to replace the traditional rigid shaker approach to one with the structure “freely” supported and excited at multiple locations simultaneously using Multi Input Multi Output (MIMO) vibration control. The research focusses on analytical models in Matlab and Ansys to carry out “virtual tests” in order to demonstrate issues with the current testing methodology and to highlight the benefits of a new approach. Results from the analytical models show significant improvements in degree-of-replication and would result in faster and more cost effective laboratory testing.