What is Material selection in mechanical design?
Mufaddal Rasheed A short summary on the domain of material selection which is key knowledge area for design engineers in product development.
Material selection decision process
The performance, function and cost of the product depends directly on its material specifications.
Selecting wrong material for a critical application can cause catastrophic consequences, also selecting a costly material over a cheap one in a non-critical application would be a waste of money. Hence, the material selection stage in mechanical design is a very important step to ensure the apt material is chosen for the application.
The choice of material cannot be done made independently of the choice of process. The design engineer generally selects the material and the required process simultaneously unless there are multiple processes for the same material. Yet, material properties depend a lot on the processing stages it has gone through.
To select the right material for the design the engineer should have the basic knowledge of
- The variety of materials available
- The manufacturing processes associated with them
- The Material properties associated with them
- Availability of material in market
Relationship diagram
For any physical product the relationship between Function, process, material and shape is as below.
Changing the material can directly affect the Shape, process as well as the function,
Similarly changing the design can affect the process, material and function. These interdependencies are always to be taken care of while selecting material and process for a design.
Types of Materials
There are more than 40000 varieties of different materials for the designer to choose from. They are divided in the general hierarchy as below.
Each Material has their own pros and cons. Choice of material for the application is based on study of their material properties and the knowledge of which property is most crucial for the application and which is not.
Material properties:
Mechanical Properties:
Structural applications require materials to have excellent mechanical properties. Study of materials undergoing mechanical loading is very important for products in terms of reliability and durability.
Ductility and Brittleness
Ductility is a measure of a material's ability to undergo significant plastic deformation before rupture.
A material is brittle if, when subjected to stress, it breaks without significant plastic deformation.
Since the mode of failure is yielding, ductile materials are generally safer when used in structural applications where tensile and bending loads are applied.
Modulus
the tendency of an object to deform along an axis when opposing forces are applied along that axis. Youngs modulus is the term is used for tensile and compression loading. A material with higher modulus will undergo lower deflection for the same load compared to a material with a lower modulus.
Strength
The maximum capacity of a material to take loads without plastic deformation or rupture. Yield strength for ductile materials and Ultimate strength for brittle. One of the main criteria for material selection in structural design.
Toughness
Toughness is the ability of a material to absorb energy and plastically deform without fracturing. Applications where there are shock loads and impact loads toughness plays a vital role for the materials
Fatigue endurance limit
Fatigue failure is the most common failure mode in designs which undergo continuous variable loading. Hence, the maximum magnitude of the cyclic stress that can be applied to the material without causing fatigue failure called the endurance limit is an important criterion for dynamic loaded applications.
Damping
A damping coefficient is a material property that indicates whether a material will bounce back or return energy to a system after undergoing a load. Like a shock absorber in a vehicle, every material has a natural tendency to absorb energy and expend heat.
Thermal properties
Thermal properties and changes in properties due to temperature are important properties in applications where the temperature gradient can be high and operating temperatures reach extremes.
Thermal conductivity
The amount of heat per unit time per unit area that can be conducted through a plate of unit thickness of a given material. The measure of transfer of heat within the material from one end to another.
Specific heat
It is the amount of heat required to raise the temperature of the material by 1 degree Celsius.
Glass transition temperature
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semi crystalline materials), from a hard and relatively brittle "glassy" state into a viscous or rubbery state as the temperature is increased. This property is very important for polymers and elastomers since it may affect the performance in applications where thee is a lot of variation in operating temperatures.
Wear
Gradual removal or deformation of material at the surfaces due to mechanical means. Tendency of wear varies from material to material. Some materials have much more wear resistance than others and are used in applications where wear cannot be avoided. One application is Brake disc pad.
Corrosion
It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Applications in which materials are exposed to the elements endure corrosion. Resistance to corrosion becomes critical in these applications.
There are many more types of properties which play a role in deciding the performance of the material in certain use cases.
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Materials and their peculiar properties:
Metals
Probably, the most used engineering materials exhibiting high modulus and strength.
Good ductility, fracture strength and also toughness are also plus points for metals in demanding applications. Downsides are high density and higher costs for stronger metals.
Polymers
Polymers have a huge family of materials and also wide range of properties for each variety of plastic. They generally have a lower modulus than metals but are much less dense and hence lighter. Generally, possess good chemical-resistance properties and can be processed in multiple ways to form intricate geometry.
Elastomers
Rubber is a good example of elastomers, they have low moduli and low density but have higher ability to damp vibrations and shocks hence they have high damping qualities used for mounting vibrating equipment and even on vehicles for suspension mounts and engine mounts.
Ceramics
These materials are generally hard and brittle but with excellent thermal and chemical resistances. Examples are diamonds, silica silicon nitride and are even used as cutting tools. There main drawback is they have low fracture toughness which means they develop cracks easily and fail in cyclic loading.
Composites
Material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components.
Examples: carbon fiber reinforced polymer, glass reinforced polymers
Considerations before deciding on materials
Before deciding on the materials for critical applications it is always better to have the knowledge of proper use cases of the materials under consideration.
Due to its superior modulus, strength and fracture toughness, metals are most preferred in the applications of high stress, and high cyclic loading. Composites are an alternative and emerging as a replacement of metal in few areas.
Composites face the issue of difficult processing and design compatibility which metals don’t.
Also, metals have considerable wear resistance, resistance to temperature and chemicals which make them the ideal choice.
Recent developments in materials have made composites and polymers and even ceramics to be viable in the mechanical design process. But use and availability of materials is still limited.
Categories: Materials