Designers intro to FEA, Part II

FEA classes spend a little time on theory and hours on commands and features. A tensile load on the plate produces displacements and von Mises stresses. A modal analysis produced the first four natural frequencies of vibration and their associated modes in an unsupported valve plate. Deforming a beam supported by immovable hinges (top) may significantly change its stiffness due to membrane stresses, thereby requiring nonlinear analysis even when displacements are small. If one hinge is movable (bottom), membrane stresses won’t develop and linear analysis may suffice if displacements are small. That's OK, but users work more confidently when they have a firm grip on the science behind the software. Hence, this second part of A designers intro to FEA . (The first appeared in the Aug. 10, 2006 issue.) It's useful to review correct terminology, examine how developers have organized the software, and learn what it can and cannot do. "Structural analysis is a good place to start this review because it divides into different simulations based on whether loads and supports are time dependent or not," says Paul Kurowski, president of ). "If loads and supports do not change with time, it is static analysis. And it's dynamic or, more properly called vibration analysis, when loads, or supports, or both are time dependent. Vibration analysis is further divided into several types based on the nature of the time dependency. The most frequently encountered types include time response, frequency response, and random-vibration analyses." Thermal analysis is also classified as to whether or not thermal loads are time dependent. "Unchanging thermal loads and boundary conditions point to a steady-state thermal analysis. Changing thermal loads and boundary conditions require a transient thermal analysis," he adds. Structural and thermal analyses can also be linear or nonlinear. "Linear structural analysis assumes model stiffness, as