General Discussion
This test method should be used to establish a degree of confidence that the materiel can structurally and functionally withstand a specified dynamic environment that is defined in more than a single-degree-of-freedom (SDOF) motion; i.e., in multiple-degree-of-freedom (MDOF) motion. Specification of the environment may be through a detailed summary of measured field data related to the test materiel that entails more than one degree-of-freedom, or analytical generation of an environment that has been properly characterized in MDOF. In general specification of the environment will include several degrees of freedom in a materiel measurement point configuration, and testing of the materiel in the laboratory in a SDOF mode is considered inadequate to properly distribute vibration energy in the materiel in order to satisfy the specification. As a result of the increased complexity of application of MET over even multiple application of SDOF single-exciter testing (SET), an analyst after careful review of the available data and specification, will need to provide rationale for selection of the MET method. MIL-STD-810G, Methods 514.6, 516.6, 519.6 and 525 provide guidance in developing the rationale and requirement for MET.
Reasons for selection of Multi-Exciter Testing may include:
- MET provides a distribution of vibration or shock energy to the materiel in more than one axis in a controlled manner without relying upon the dynamics of the materiel for such distribution.
- MET may be selected when the physical configuration of the materiel is such that its slenderness ratio is high, and SET must rely upon the dynamics of the materiel to distribute energy.
- For large and heavy test materiel, more than one exciter may be necessary to provide sufficient energy to the test item.
- MET allows more degrees-of-freedom in accounting for both the impedance matches and the in service boundary conditions of the materiel.
An example of a single-exciter test setup follows. This is a single vertical axis test.
An example of a multi-exciter test setup follows. This test will excite the test object in all 3 orthogonal axis simultaneously.
The following sketch shows a concept for a multi-dimensional tester which excites the tested material in 3 orthogonal axis as well as 3 rotational axis.
Whenever practical, ensure test items are active and functioning during vibration tests. Monitor and record achieved performance. Obtain as much data as possible that defines the sensitivity of the materiel to vibration. Where tests are conducted to determine operational capability while exposed to the environment, operate the test item. In other cases, operate the test item where practical. Operation during transportation will not be possible in almost all cases. Also, there are cases where the operational configuration varies with mission phase, or where operation at high levels of vibration may not be required, and may be likely to result in damage.
Test Facility
The specialized nature of MET requires use of a test facility that includes proven MET capability, fixture(s) for mounting the test materiel, and appropriate equipment for recording the response of the test item at the specified control and the monitor locations. In addition, the test facility will have expertise necessary to appropriately configure the test according to the form of test materiel. Ensure the exciter control has appropriately validated vendor supplied MET hardware and software.
Physics of Failure
Analyses of vibration related failures must relate the failure mechanism to the dynamics of the failed item and to the dynamic environment. It is insufficient to determine that something broke due to high cycle fatigue or wear. It is necessary to relate the failure to the dynamic response of the materiel to the dynamic environment. Thus, include in failure analyses a determination of resonant mode shapes, frequencies, damping values, and dynamic strain distributions, in addition to the usual material properties, crack initiation locations, etc.