Modeling the capture of a Mars sample return (MSR) orbital sample (OS) involves understanding the complex dynamic behavior, including the contact of the OS with the interior of the capture enclosure. The MSR program required numerical verification of contact dynamics predictions made using their commercial software tools. This commercial software used “free” parameters to establish contact modeling. Free parameters (also called free variables) are not based on contact physics. The commercial contact model is used by MSR.
Seven free parameters are required including Hertzian contact stiffness, surface penetration, stiffness expression, penetration velocity, contact damping, maximum penetration depth for contact damping value, and a smoothing function. An example of a parameter that is not independent is the coefficient of friction, which is a physics-based parameter. Consider the free parameter, contact stiffness. Contact stiffness is already present in the finite element model (FEM) stiffness matrix where the bodies come into contact, and surface penetration is not allowed in the physically realizable contact model, because FEM meshes have a The other must not be penetrated (ie, zero. Contact boundary access barrier condition).
Thus, with each set of free parameters chosen producing a different contact force signature, additional numerical verification is needed to guide the tuning of these parameters. Contact modeling is nonlinear. This means that the stiffness measurements of the contacting entities are constantly updated as the bodies come into contact, possibly re-contact (due to vibration) and disengage. Model properties of contacting bodies change continuously with state transitions (eg stick-to-slip). Some contact models have been proposed and incorporated into commercial finite element analysis solvers, and most include static loading. A relatively small number of dynamics are involved, which has historically proven challenging.
In 2005, NASA conducted a study testing several commercial contact solvers to predict contact forces in transient dynamic environments. It was required by the Space Shuttle Program (SSP) – following the Columbia accident of February 2003 – to include contact dynamics in the transient coupled loads analysis (CLA) of the Space Shuttle to capture the effects of contact nonlinearity. What was the decision? This made the whole CLA nonlinear. The study found major difficulties in performing nonlinear CLAs in commercial software. A nonlinear solver developed by NESC and Applied Structural Dynamics (ASD) that was able to produce physically feasible results was verified numerically by NASA and later experimentally. went This nonlinear solver was subsequently used to execute all NASA SSP CLAs (i.e., crewed space flights) from 2005 until the final flight in 2011, as well as currently supporting the SLS program.
The goal of the MSR contact validation work was to provide data that could be used by the MSR team to help define the free parameters listed above for the commercial tool contact model. The NESC/ASD solver was used to model the contact between the simple cantilever and the free beam, to derive the contact forces and the corresponding displacements. This resulting data can be used to determine parameter values for more complex structures. Two of the configurations modeled, one for axial contact (Figure 1) and the other for stick/friction (Figure 2), and sample results of NESC nonlinear dynamic analyzes are presented in Figures 1 and 2. .
For information contact:
Dr. Dexter Johnson dexter.johnson@nasa.gov
Dr. Arya Majeed arya.majed@nasa.gov