Computer-aided strength calculation system

An important stage of the designing process in the machine-building industry is the optimization of the strength characteristics of a new product--one of the aims of the said optimization being the reduction of material content of machines and mechanisms. However, carrying out strength investigations via fuU-scale testing of prototypes and models calls for considerable material, labour and time expenditure and, besides, is not always possible. That is why the number of design modifications studied in this way proves, as a rule, to be insufficient for selecting an optimum solution. Carrying out the said research with the help of computer-aided systems for strength simulation allows one to evaluate within a fairly short period of time a considerable number of variants and select the most suitable one. There is quite a number of such systems which are based upon the finite element method, the essence of the latter being the approximation of the product under investigation by a finite number of simple geometry elements (triangles, tetragons, pyramids, etc.). Consequently it becomes possible to replace equations in partial derivatives, which describe the behaviour of the investigated product, with finite equations, describing simple elements of the product and ties thereof. This set of equations, predominantly linear, can be then solved on up-to-date computers. The practical application of strength simulation programs, especially in designing new products, has been restricted to the experimental level. This is primarily due to labour-intensive processes of data preparation and results analysis. Thus, for instance, to build up a 2-dimensional finite element model ofa ZIL-130 truck chassis frame about 4 man-months are necessary, while making a 3-dimensional model of a ZIL-169 diesel engine piston requires 6 man-months. The experience has shown that these operations become highly labour-intensive due to such reasons as building from a product drawing a finite dement model containing several thousand simple geometric elements as well as manual coding and decoding of great scopes of graphic data for inputting a finite element model and outputting computation results. The coding, for example, of a medium complexity model gives about 20 thousand numbers (coordinates of all finite elements nodes, values of initial stresses, characteristics of materials, etc.), which consequently must be entered into a computer via punched cards or an alphanumeric terminal. The scope of strength computation results can now expand up to 25 through 60 thousand numbers (components of translation vectors and stress tensors, temperature values, etc.), all of which are to be consequently analysed by a designer. A designer's labour productivity in solving these problems can be raised thanks to utilization of computer-aided interactive graphic data processing systems. These systems should provide an effective user-computer graphic interface, thus minimizing the scope of manual coding/decoding of graphic data and numerical input into a computer. The work aimed at creating such systems is being conducted by specialists of the joint laboratory of the Academy of Sciences of the USSR and the industrial complex "AutoZIL" as well as co-workers of the computing mathematics chair and the laboratory of computation methods of the mechanics and mathematics department of the Moscow State University. The "AutoZIL" has put into operation the first phase of the interactive graphics system for computer-aided strength calculations (Fig. 1). The main components of this system are the preprocessor for data preparation and the postprocessor for results analysis. A designer's work with the system comprises two steps. As the first step, the designer uses a technical drawing to create in a graphic dialogue with a computer a supermodel of the product, i.e. a basic model divided into elements of rather complex geometry (superelements) to which belong fragments of planar, spherical, cylindrical, spline and other surfaces as well as of volumes bounded by the above surfaces. The superelements are quite complex; the number of them to approximate the basic model will be much less than that of finite elements. The scope of numerical data to be input by a designer could therefore be significantly reduced. As the second step, the designer proceeds from the supermodel to create interactively a macroelement model of the product by subdividing superelements into macroelements, the latter being geometrically similar to polygons. Due to simple geometry of macroelements, it becomes possible to realize sufficiently effective algorithms for automatic/semiautomatic subdivision of macroelements into finite elements. The currently realized first version of the interactive computer-aided system of strength calculations, when compared to the project (see Fig. 1), has the following restrictions: