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Typical FEA activity is focused on analyzing structural stresses, deflections, and natural frequencies. The analysis begins with a discretized representation of a structure known as a mesh. The mesh is composed of nodes and elements and is often created with geometry from a CAD system. The nodes represent points on the structure where displacements are calculated. The elements are bounded by sets of nodes and enclose areas or volumes. They define the local mass, stiffness, and damping properties of the structure. Equations relating these quantities to the nodal displacements are automatically developed by the software codes. Other inputs, such as boundary conditions, applied loads, and material properties, must be defined by the user. Each of these quantities requires careful judgement for meaningful results to be achieved. Results post-processing includes images of deformed structures under load, coloured stress contours, and mode shape animations.
MSS – Multi-body system simulation is used to study the motion of components and assemblies and is often used to study a vehicle suspension or a vehicle’s handling and ride response. A typical MSS model of a full vehicle will be composed of rigid bodies (wheels, axles,frame , engine, cab, and trailer) connected by idealized joints and force elements. The MSS code automatically develops the non-linear differential and algebraic equations that define the motion of the bodies in the model. The equations are numerically integrated to produce time histories of rigid body displacements, velocities, accelerations, and forces. Results are viewed as graphs and animations of the system motion. As with FEA, CAD data is often used to develop a MSS model. Geometry data from a CAD assembly is used to establish the layout of the MSS model such as the location of joints and force elements. CAD solid model data is also used to estimate the location of the center-of-mass and the inertial properties of each rigid body. Forces acting on a rigid body from a MSS can be used as input loads to a finite element analysis to determine the structural stresses in that rigid body.
The CAE tools discussed in this paper include Pro/Engineer for CAD, ANSYS for FEA, and ADAMS for MSS. The following discussion references the specific capabilities of these codes in developing a customized environment for the engineering analysis of truck frames.
CAE CUSTOMIZATION FOR HEAVY TRUCK
MODELLING
As described above, the current offering of CAD and CAE tools provide a great deal of integration. Nonetheless,these tools are very general in scope and a significant customization effort is required for the analysis of heavy duty trucks and truck frames. To fully understand how changes to the truck frame impact vehicle handling, roll stability, ride, and durability requires a detailed MSS model that can simulate all these effects. Using the ADAMS software code such a model was developed at Western Star Trucks. Refer to Figure 2 for a view of the model in the ADAMS environment.
Figure 2. ADAMS MSS Model
The model includes the following characteristics:
• 100 rigid bodies
• 180 force elements
• 45 joint elements
• 415 degrees-of-freedom
The rigid bodies include the frame, cab, axles, wheels ,engine, hood, radiator, leaf springs, suspension arms, drive shafts, and the trailer. Mass properties for many of these bodies were estimated using simplified solid models in Pro/Engineer. The force elements include linear and non-linear bush ielements that model rubber isolators, such as the cab and engine mounts. Non-linear single component forces are used to model air springs and shock absorbers. Property data for these elements are derived from tests performed by component suppliers. Revolute joints and spherical joints are used to model connection points, such as wheel bearings and torque rod pivots, respectively. Pro/Engineer assemblies are used to determine the geometric location of these elements.