Lecture 4.pdf

Design and Materials Selection Materials Selection The Basics

Lecture 4 Dr. Muslim Muhsin

This Lecture sets out the basic procedure for selection, establishing the link between material and function Figure 1

Figure 2

The universe of materials is divided into families, classes, subclasses, and members; each member is characterized by a set of attributes: its properties

Selection Strategies

Figure 3

Required features are constraints; they are used to screen out unsuitable cars. The survivors are ranked by cost of ownership

Choosing a Material

Figure 4

Design requirements are first expressed as constraints and objectives. The constraints are used for screening. The survivors are ranked by the objective, expressed as a material index.

Strategy for Materials Selection The four main steps: Translation Screening Ranking Documentation

Figure 5

Translating Design Requirements

Figure 4

Material Indices Constraints set property limits. Objectives define material indices, for which we seek extreme values.

Minimizing Mass: A light, strong tie

Objective Function: equation describing the quantity to be maximized or minimized.

We can reduce the mass by reducing the cross-section, but there is a constraint: A must be sufficient to carry F*, requiring that:

Eliminating A between these two equations gives:

Material indices are generally expressed so that a maximum value is sought, so the material index for a light, strong tie is:

Minimizing Mass A light, stiff panel

Objective Function Constraint on Stiffness

Second Moment of Area

Material index for a light, stiff panel Material index with a constraint of strength rather than stiffness

Minimizing Mass A light, stiff beam

Objective Function Constraint on Stiffness Second Moment of Area

Material index for a light, stiff beam Material index with a constraint of strength rather than stiffness

Performance Equation

The performance of a structural element is determined by three things: the functional requirements, the geometry, and the properties of the material of which it is made.

Figure 7

The specification of function, objective, and constraint leads to a materials index. The combination in the highlighted boxes leads to the index E1/2/ρ.

• An engineering component has: (boundary condition for Materials Selection)

1. Function: to carry load, transmit heat, contain a pressure, etc.. What does the component do?

2. Objectives: as cheap as possible, light, safe, strong, etc… What is to be Maximized or Minimized?

3. Constraints: subject to constraints such as carry load without failure, certain dimensions are fixed, cost is within limits etc…  What non-negotiable conditions are to be met? (Rigid)  What negotiable but desirable conditions? (Soft) 4. Free Variables: materials choice, cross-section area, thickness, and length are free Which design variables are free? (variables which can be changed)

Figure 8

A schematic E-ρ chart showing a lower limit for E and an upper limit for ρ.

Ranking: Indices on Charts

Figure 9

A schematic E-ρ chart showing guide lines for the three material indices for stiff, lightweight design.

Figure 10

A schematic E-ρ chart showing a grid of lines for the material index M=E1/3/ρ.

Figure 11

A selection based on the index M=E1/3/ρ > 2(GPa)1/3 (Mg/m3) together with the property limit E > 50 GPa. The materials contained in the search region become the candidates for the next stage of the selection process

Figure 12

Computer-aided selection using the CES software. The schematic shows the three types of selection window. They can be used in any order and any combination. The selection engine isolates the subset of materials that passes all the selection stages.