The Box Assembly with Removable Component (BARC) structure was developed as a challenge problem for those investigating boundary conditions and their effect on structural dynamic tests. To investigate the effects of boundary conditions on the dynamic response of the Removable Component, it was tested in three configurations, each with a different fixture and thus a different boundary condition. A “truth” configuration test with the component attached to its next-level assembly (the Box) was first performed to provide data that multi-axis tests of the component would aim to replicate. The following two tests aimed to reproduce the component responses of the first test through multi-axis testing. The first of these tests is a more “traditional” vibration test with the removable component attached to a “rigid” plate fixture. A second set of these tests replaces the fixture plate with flexible fixtures designed using topology optimization and created using additive manufacturing. These two test approaches are compared back to the truth test to determine how much improvement can be obtained in a laboratory test by using a fixture that is more representative of the compliance of the component’s assembly.
Across many industries and engineering disciplines, physical components and systems of components are designed and deployed into their environment of intended use. It is the desire of the design agency to be able to predict whether their component or system will survive its physical environment or if it will fail due to mechanical stresses. One method to determine if the component will survive the environment is to expose the component to a simulation of the environment in a laboratory. One difficulty in doing this is that the component may not have the same boundary condition in the laboratory as is in the field configuration. This paper presents a novel method of quantifying the error in the modal domain that occurs from the impedance difference between the laboratory test fixture and the next level of assembly in the field configuration. The error is calculated from the projection from one mode shape space to the other, and the error is in terms of each mode of the field configuration. This provides insight into the effectiveness of the test fixture with respect to the ability to recreate the mode shapes of the field configuration. A case study is presented to show that the error in the modal projection between two configurations is a lower limit for the error that can be achieved by a laboratory test.
Engineering designers are responsible for designing parts, components, and systems that perform required functions in their intended field environment. To determine if their design will meet its requirements, the engineer must run a qualification test. For shock and vibration environments, the component or unit under test is connected to a shaker table or shock apparatus and is imparted with a load to simulate the mechanical stress from vibration. A difficulty in this approach is when the stresses in the unit under test cannot be generated by a fixed base boundary condition. A fixed base boundary condition is the approximate boundary condition when the unit under test is affixed to a stiff test fixture and shaker table. To aid in correcting for this error, a flexible fixture needs to be designed to account for the stresses that the unit under test will experience in the field. This paper will use topology optimization to design a test fixture that will minimize the difference between the mechanical impedance of the next level of assembly and the test fixture. The optimized fixture will be compared to the rigid fixture with respect to the test’s ability to produce the field stresses.
The ability to measure full-field strains is desirable for analytical model validation or characterization of test articles for which there is no model. Of further interest is the ability to determine if a given environmental test’s boundary conditions are suitable to replicate the strain fields the test article undergoes in service. In this work, full-field strain shapes are estimated using a 3D scanning laser Doppler vibrometer and several post-processing methods. The processing methods are categorized in two groups: direct or transformation. Direct methods compute strain fields with only spatial filtering applied to the measurements. Transformation methods utilize SEREP shape expansion/smoothing of the measurements in conjunction with a finite element model. Both methods are used with mode shapes as well as operational deflection shapes. A comparison of each method is presented. It was found that performing a SEREP expansion of the mode shapes and post-processing to estimate strain fields was very effective, while directly measuring strains from ODS or modes was highly subject to noise and filtering effects.
Multi-degree of freedom testing is growing in popularity and in practice. This is largely due to its inherent benefits in producing realistic stresses that the test article observes in its working environment and the efficiency of testing all axes at one time instead of individually. However, deriving and applying the “correct” inputs to a test has been a challenge. This paper explores a recently developed theory into deriving rigid body accelerations as an input to a test article through sub-structuring techniques. The theory develops a transformation matrix that separates the complete system dynamics into two sub-structures, the test article and next level assembly. The transformation does this by segregating the test article’s fixed base modal coordinates and the next level assembly’s free modal coordinates. This transformation provides insight into the damage that the test article acquires from its excited fixed base shapes and how to properly excite the test article by observing the next level assembly’s rigid body motion. This paper examines using next level assembly’s rigid body motion as a direct input in a multi-degree of freedom test to excite the test article’s fixed base shapes in the same way as the working environment.
Structural dynamic testing is a common method for determining if the design of a component of a system will mechanically fail when deployed into its field environment. To satisfy the test's goal, the mechanical stresses must be replicated. Structural dynamic testing is commonly executed on a shaker table or a shock apparatus such as a drop table or a resonant plate. These apparatus impart a force or load on the component through a test fixture that connects the unit under test to the apparatus. Because the test fixture is directly connected to the unit under test, the fixture modifies the structural dynamics of the system, thus varying the locations and relative levels of stress on the unit under test. This may lead to a false positive or negative indication if the unit under test will fail in its field environment depending on the environment and the test fixture. This body of research utilizes topology optimization using the Plato software to design a test fixture that attaches to the unit under test that matches the dynamic impedance of the next level of assembly. The optimization's objective function is the difference between the field configuration and the laboratory configuration's frequency response functions. It was found that this objective function had many local minima and posed difficulties in converging to an acceptable solution. A case study is presented that uses this objective function and although the results are not perfect, they are quantifiably better than the current method of using a sufficiently stiff fixture.