Materials selection in mechanical design

A novel materials-selection procedure has been developed and implemented in software. The procedure makes use of Materials Selection Charts: a new way of displaying material property data; and performance indices: combinations of material properties which govern performance. Optimisation methods are employed for simultaneous selection of both material and shape. INTRODUCTION The performance of an engineering component is limited by the properties of the material of which it is made, and by the shapes to which this material can be formed. Under some circumstances a material can be selected satisfactorily by specifying ranges for individual properties. More often, however, performance depends on a combination of properties, and then the best material is selected by maximising one or more 'performance indices'. An example is the specific stiffness E/p (E is Youngs modulus and p is the density). Performance indices are governed by the design objectives. One is derived later in this paper and many others are tabulated elsewhere [ I , 21. Component shape is also an important consideration. Hollow tubular beams are lighter than solid ones for the same bending stiffness and I-section beams may be better still. Information about section shape can be included in the performance index to enable simultaneous selection of material and shape. THE PROCEDURE Performance Indices A performance index is a group of material properties which governs some aspect of the performance of a component [I, 21. They are derived from simple models of the function of the component, as illustrated by the following example. A material is required for a light, stiff beam. The aim is to achieve a specified bending stiffness at minimum weight. The beam has a length L and a square, solid, cross-section as shown in Figure la. The mass of the beam is where A is the area of the cross-section and p is the density of the material of which the beam is made. The stiffness S of a simply-su ported beam with modulus E, second moment of area I, central load F, and central deflection [ is Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993701 2 JOURNAL DE PHYSIQUE IV with C1 = 48 for 3-point bending. Other supports, or other distributions of load, change C1, but nothing else. Assume that the beam has a square section, of side b. The second moment of area is I = b4/12 = ~ ~ 1 1 2 (3) The stiffness S and the length L are constrained by the design. The area A is a 'free' variable that we wish to choose so as to minimise the mass, while meeting the constraints.