FEA Goes Deeper into Vehicular Development

MSC Marc has two new models (Bergstrom-Boyce, and Marlow) for analyzing the time- and frequency-dependent damping characteristics of assemblies using elastomers, such as the seal shown here.

Here’s a truck cab with 96,793 nodes, 580,758 degrees of freedom, and 91,783 elements. Using MSC Nastran Embedded Fatigue (NEF), fatigue analysis is faster (the conventional method, with fatigue analysis done in post-processing, required 118.79 minutes to solve; with NEF, 24.55 minutes—4.8 times faster), requires less disk space (the conventional method requires 282,660 MB of disk space; NEF, 31 MB—a 99% reduction), and is both simpler and easier (the conventional method requires 191 files; NEF, 2 files—a 99% reduction in number of files).

Adams/Machinery is a finite element analysis (FEA) software program from MSC Software Corp. (mscsoftware.com/product/adamsmachinery). Analysts use it to build virtual prototypes from common machinery components to determine the dynamic responses related to motion, structures, actuation, and controls. The latest version, 2013.2, has plenty for automotive. The gears module analyzes the design and behavior of gear pairs, including backlash, contract force, gear friction, and gear rattle. Three new gear types are now supported—hypoid, worm, and rack and pinion—in addition to the current spur, helical, and bevel types. A new wizard automates creating planetary gear sets.

The belts module focuses on transmission ratio, tension and load prediction, belt slippage and dynamics, and other characteristics of various pulley-belt systems. This module includes a new 3D belt method for non-planar pulley-belt simulation, a wizard for applying force or motion to any pulley in the prototype, and the ability to apply a tensioner pulley to take up the extra slack and to control the belt’s routing in the prototype.

The chains module covers the behavior of chain systems, such as drive ratio, tension, chain dynamics, and vibration from preloads. The module has a new 3D method for simulating non-planar sprocket chains and roller chains. It also has a new wizard for applying force or motion to any sprocket in the chain system.

The bearings module helps predict rolling-element bearing loads and service life given bearing stiffness, internal dimensions, offsets, misalignments, and clearances. The module covers 14 different bearing types, provides access to a library of over 24,000 off-the-shelf bearings, and offers over 120 oil- and grease-based bearing lubricants for selection.

The cable module computes the vibration and tension of cables in cable-based transmissions. This helps analyze cable slippage against load performance, cable compliance against system output speed, and winching effects when changing cable lengths.

Last, there’s the new electric motors module. The module covers DDC (shunt or series), DC brushless, stepper, and AC synchronous motors. Motor torque can be defined either by Easy5 controls and systems simulation from MSC Software or by MATLAB and Simulink from MathWorks (mathworks.com). The results help in motor sizing, predicting motor torque, analyzing position control, and producing a realistic drive signal for the rest of the machine components.

Damping and fracturing; two banes in automotive design. 

Among the many improvements in MSC Marc (mscsoftware.com/product/marc), a general-purpose, nonlinear FEA program, are new tools for analyzing elastomers (linear elastic and hyperelastic materials), such as tires, seals, and elastomeric bearings. The analysis focuses on the damping characteristics of elastomers in products that create and transmit mechanical vibrations and other types of noise. Specifically, Marc now includes the Bergstrom-Boyce model for analyzing time-dependent and large strain viscoelastic behaviors, and the Marlow model for analyzing rubbers and frequency-dependent material behaviors.

Fracture analysis, says Ted Wertheimer, MSC Software director, product management, is “really the birthplace of nonlinear FE because nobody wants a failure.” Sure enough, Marc gives fracture and durability analysis plenty of attention. By using adaptive meshing capabilities, analysts can now put cracks anywhere in a solids model without worrying about splitting element faces and having to regenerate the mesh. (For instance, analysts can rotate surface cracks without having to regenerate the surrounding mesh.) As the cracks grow, analysts can specify the delamination along element faces: stay on the material interface, on the element set interface, or on automatic interface. This is all possible using higher order solid elements, particularly tetrahedral elements, which leads to better and more accurate simulations of crack propagation.

Another enhancement involves interference fit. “The real motivation for this was where mesh was significantly overlapping one another,” says Wertheimer. This overlap could be by design or because of flaws in the model or the mesh. “If the overlap was over the contact tolerance, there was nothing we could do,” continues Wertheimer. Now, a pop-up menu in Marc lets analysts specify how to resolve these interferences between parts: move parts relative to each other or scale them to a particular point or coordinate system.

Fatigue, another bane
New to MSC Nastran, a general-purpose FEA program, is Nastran Embedded Fatigue (NEF; mscsoftware.com/product/msc-nastran-embedded-fatigue). “What was once a post-processing exercise is now an integral part of the analysis process,” says Wertheimer. NEF yields benefits way beyond eliminating the need to export results to a separate fatigue-analysis program. For instance, analysts can optimize the tradeoffs of part weight and fatigue life; one example from MSC led to a 24% mass reduction, while increasing fatigue life by 14%. Second, durability calculations are now significantly faster and easier to perform; for example, a fatigue analysis that typically took 8 hours now only took 38 minutes, and only two files were processed instead of 200. NEF is comple-mentary to MSC Patran pre/post-processing software. NEF can use material data for either stress life (S-N) or strain life (E-N), crack initiation, and safety factors.

This release of Nastran also analyzes the poroelastic behavior of trim components, which are used to dampen vibrations and improve passenger cabin comfort. The analysis inspects the vibroacoustic performance (that is, frequency response) of a vehicle by simulating the fluid-solid interaction (air and the trim components) within the passenger cabin (the acoustic cavity). In short, the analysis leads to improved noise, vibration, and harshness properties of components and the entire vehicle. This analysis combines Nastran body-in-white with Patran, then adds the trim component material and the acoustic cavity to get the frequency response. Nastran can handle various trim material coupling connections: glued, sliding, open, or closed.