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.
Qualification of products to their vibration and shock requirements in a laboratory setting consists of two basic steps. The first is the quantification of the product's mechanical environment in the field. The second is the process of testing the product in the laboratory to ensure it is robust enough to survive the field environment. The latter part is the subject of the “Boundary Condition for Component Qualification” challenge problem. This paper describes the challenges in determining the appropriate boundary conditions and input stimulus required to qualify the product. This paper also describes the step sand analyses that were taken to design a set of hardware that demonstrates the issue and can be used by round robin challenge participants to investigate the problem.
In the aerospace industry, hail strikes on a structure are an environment that must be considered when qualifying a product. Performing a physical test on a product would require a test setup that would launch a fabricated hail stone at an expensive prototype. This test may be difficult or impossible to execute and destructive to the product. Instead of testing, a finite element model (FEM) may be used to simulate the damage and consequences of a hail strike. In order to use a FEM in this way, an accurate representation of the input force from a hail stone must be known. The purpose of this paper is to calculate the force that a hail stone imparts on an object using the inverse method SWAT-TEEM. This paper discusses the advantages of using SWAT-TEEM over other force identification methods and exercises the algorithm for a test series of hail strikes that include multiple angles of attack and multiple velocities which include speeds that are supersonic.
During an environment, it is desirable to know the forces or inputs on the system of interest. With the inputs, one can directly use a finite element or experimental model to predict responses not measured in a field test. One can attempt to measure point forces using force gauges, however, these gauges are insufficient due to the inability to place a gauge at a forcing interface or to measure a force applied over an area. SWAT (Sum of weighted acceleration technique) is a method that uses mode shapes as a modal filter with measured accelerations and to solve the inverse problem and calculate the forces and moments on the system. This paper will examine an application where the use of a force gauge is impossible due to the external forces being applied over an area. The paper will calculate the sum of the forces and moments imparted on the system and will use a finite element model to check the plausibility of the calculated forces.