Mechanical Event Simulation: Kinematics, Kinetics and Stress Analysis Combined

Michael L. Bussler
President
Algor, Inc.
Pittsburgh, PA

Engineers have a variety of Virtual Product Development (VPD) software tools, including CAD, finite element analysis (FEA) and kinematic and simulation software, to aid them in developing product designs from conception to production. However, only the integration of VPD tools into a single What-You-See-Is-What-You-Get environment will enable an engineer to fully understand dynamic mechanical systems within shorter time frames and reduce the need for physical prototyping. Mechanical Event Simulation (MES), the latest development in VPD products, expands upon traditional stand-alone kinematic and finite element analysis software by combining the capabilities to replicate motion (kinematics), dynamic loading (kinetics), and flexing (stresses) of parts of an assembly of interconnected components (mechanisms) during a virtual "event."

This suspension model shows Algor’s ability to analyze mechanical linkages and dashpot damping. Algor’s kinematic elements reduce run times for the MES of this detailed CAD solid assembly.

The concept of combining stress analysis with dynamics (kinematics and kinetics) in a single process was pioneered because the skills required to manipulate stand-alone solutions were difficult and error prone. Traditional mechanism programs used for load determination show the motion of mechanisms but do not calculate stresses or show how components flex as a result of motion. Because they require the engineer to assume that all linkages in the assembly are rigid, one must laboriously transfer the dynamic loads to a stress analysis program, such as FEA (this process is typically referred to as "motion load transfer"). Once the loads are transferred to a static program, the FEA software calculates stresses but only at a single instant in time. The motion load transfer methodology requires the engineer to build separate FEA and mechanism models. Often, each model must be analyzed with a different software package, making the data transfer process from one program to another a difficult task subject to errors.

MES, on the other hand, allows engineers to create only one model and work in only one analysis package. MES products eliminate the two-step process by providing one software tool that replicates motion, flexing and stresses in a single process, thus providing an analysis of the complete event on the computer as it may occur in the real world. MES intrinsically calculates the loads and stresses as motion takes place at each instant in time throughout the event, facilitating a more efficient design process because the need to estimate and specify forces is eliminated.

With the ability of MES software to replicate motion, dynamic loading and flexing, the number of design scenarios available is virtually unlimited. MES has successfully been used for many applications including impact and drop testing, contact between multiple objects, mechanical linkages, damping, in-plane motion, elastic deformation, hydrodynamic effects on a structure, local buckling, snap-through buckling and fracture. With recent improvements in desktop computer hard drive space and computer speed, the average engineer has enough horsepower in his desktop computer to run and study comprehensive MES scenarios in a practical amount of time.

MES enables engineers to understand how their designs will function during real-world operation and develop safe, efficient, cost-effective designs and to reduce expensive and time-consuming prototype testing. Reduction in prototype testing enables companies to develop products faster without compromising safety. Shorter design cycle times provide progressive agile companies with the ability to fulfill consumer needs and desires in a more competitive and profitable business environment.

Most VPD software vendors offer interfacing between software packages to enable engineers to use two or more packages together, a CAD system with FEA software, for instance, or kinematic system with FEA software. Algor software is unique in offering the combination of stress analysis with dynamics (kinematics and kinetics) in a single process. In the Release 12 version of Algor software, MES capabilities are available in two packages in the Accupak family of products: Accupak/VE and Accupak/MES. While Accupak/MES is suited for scenarios in which material nonlinearity is not a concern, Accupak/VE can additionally take into account material nonlinearities.

Algor also offers unique kinematic elements in an add-on package to any of the products in the Accupak family to accelerate MES processing time. Kinematic elements are especially useful in large CAD models and assemblies. Kinematic elements are rigid elements that move and dynamically behave like finite elements but for which stresses are not calculated. Engineers could insert these elements in areas of the model in which dynamic effects are essential but for which stresses are of secondary importance. For example, an engineer would first construct a total kinematic model to assess the kinematic functionality of the event and then selectively insert regular finite elements in areas of stress concern. This approach can dramatically reduce computation time. The final proof of the design concept would then be analyzed with a model comprised entirely of regular finite elements.

This automotive fastener was analyzed using kinematic elements for all of the rigid areas (shown in gray). Regular flexible elements show flexing and stresses at the hinges.

In addition, a wider range of physical phenomena is considered by Algor's VPD software. Stress caused by motion is not the only way that failure can occur. For example, pressures, significant temperature gradients or the flow of fluids such as water or air against an object can also induce forces, which can result in motion and stress. Algor offers a range of FEA capabilities including linear and nonlinear stress, vibration and natural frequencies, heat transfer, electrostatics, fluid flow, piping design and composite materials so that multiple physical phenomena can be considered on the same model – a process known as multiphysics analysis. In the future, multiphysics simulations will be even more tightly integrated within the Algor environment.