Practical 4
Selection of material for chosen machine part – project
Overview of practical
Selection of a correct material for a design is a key step in the process because it is the crucial
decision that links computer calculations and lines on an engineering drawing with a working
design. Materials and manufacturing processes that convert the material into a useful part
underpin all of engineering design. The enormity of the decision task in materials selection is
given by the fact that there are well over 100,000 engineering materials from which to choose.
On a more practical level, the typical design engineer should have ready access to information
on 50 to 80 materials, depending on the range of applications.
An incorrectly chosen material can lead not only to failure of the part but also to unnecessary
cost. Selecting the best material for a part involves more than selecting a material that has the
properties to provide the necessary performance in service; it is also intimately connected
with the processing of the material into the finished part.
In a more detailed approach to engineering design, Dixon and Poli suggest a four-level
approach to materials selection:
Level I. Based on critical properties, determine whether the part will be made from
metal, plastic, ceramic or composite.
Level II. Determine whether metal parts will be produced by a deformation process
(wrought) or a casting process; for plastics, determine whether they will be
thermoplastic or thermosetting polymers.
Level III. Narrow options to a broad category of material. Metals can be subdivided
into categories such as carbon steel, stainless steel and copper alloys. Plastics can be
subdivided into specific classes of thermoplastics and thermosets such as
polycarbonates and polyesters.
Level IV. Select a specific material according to a specific grade or specification.
A materials selection problem usually involves one of the two situations:
Selection of the materials and the processes for a new product or design.
Evaluation of alternative materials or manufacturing routes for an existing product or
design. Such a redesign effort usually is taken to reduce cost, increase reliability, or
improve performance. It generally is not possible to realize the full potential of
substituting one material for another without fully considering its manufacturing
characteristics. In other words, simple substitution of a new material without changing
the design rarely provides optimum utilization of the material.
Methods of Materials Selection
Materials selection is a progressive process of narrowing from a large universe of possibilities
to a specific material selection. With the enormous combination of materials to choose from,
the task can be done only by introducing simplification and systemization. Clearly design
involves decision making. Many analytical methods have been introduced into engineering
design to aid in the evaluation process, which results in a rational decision:
Cost per Unit Property Method
Weighted Property Index Method
Limits on Properties Method
A competent job of material selection should include consideration and documentation of the
following:
The problem, design or redesign objective
The underlying design criteria (primary, secondary or supporting criteria;
manufacturing method considerations; codes and standards)
Analysis (preliminary concepts, modelling or simulation, optimization and trade-offs,
design reliability, economics and cost)
Alternatives considered, selection criterion and decision methodology
Reasons for selection of the final material and manufacturing process.
Objective of practical
The objective of the practical is to select a correct material for the given metal component.
This practical has a form of a project.
Procedure
Define the functions that the design must perform and translate these into required material
properties such as stiffness, strength and corrosion resistance, and such business factors as
cost and availability of the material.
Define manufacturing requirements in terms of such parameters as the number of parts
required, the size and complexity of the part, its required tolerance and surface finish, general
quality level, and overall fabricability of the material.
Compare the needed properties and parameters with a large materials property data base (most
likely computerized) to select a few materials that look promising for the application. In this
initial screening process, it is helpful to establish several screening properties. A screening
property is any material property for which an absolute lower (or upper) limit can be
established. No trade-off beyond this limit is allowable. It is a go-no-go situation. The idea of
the screening phase in materials selection is to ask the question: "Should this material be
evaluated further for this application?"
Investigate the candidate materials in more details, particularly in terms of trade-offs in
product performance, cost, fabricability and availability in the grades and sizes needed for the
application. Material property tests and other testing are often done at this stage. This detailed
evaluation will be made by weighted property index method.
Develop design data and/or a design specification. Step 4 results in selection of a single
material for the design and a suggested process for manufacturing the part. In most cases, this
results in establishing the minimum properties through defining the material with a generic
material standard such as those issued by the American Society for Testing and Materials
(ASTM), the Society of Automotive Engineers (SAE), the American National Standards
Institute (ANSI) and the United States military (MIL specs). For critical parts in sensitive
applications, for example in aerospace and nuclear areas, it may be necessary to conduct an
extensive testing program to develop design data that are statistically reliable.
Methodology of Materials Selection
At the initial screening stage, use the Ashby method.
At the detail (parametric) stage, use the weighted property index method.
Potential components to “new design”
Engine parts:
The piston / connecting rod / crankshaft system.
The cylinder / piston system - including the piston rings.
The gearbox assembly.
The engine housings.
The various types of bearings.
The cylinder head, camshaft, valve and guide assembly.
Structure of the project
Definition of the functions that the component must perform, and translating them into
required materials properties
Definition of the manufacturing requirements
Comparison of the needed properties and parameters with a large materials property data base
– initial screening selection of materials
Detail selection of materials by use of weighted property index method
Choice of optimal material
