Updated: Jan 28, 2022
"And step by step, since time began, I see the steady gain of man."
-John Green Leaf Whittier
The history of Homo Sapience narrates stories of the Stone Age Man carving out sharp tools from stone. As time swept, came the Bronze Age, Iron Age and a lot more followed with the discovery of new materials and their profound usage. With a plethora of materials, their choice for different objects became a matter of study. The process of selection of the best material, the most optimized one entered a different branch of Material Science.
The document contains a summary of the webinar on Material Selection for Mechanical Designs organized by SAE India.
The webinar started with sharpening the basic concepts of strength, stiffness, toughness, endurance limit and a host of parameters, upon which the choice of a material depends. The clarity in the different terms lays the foundation for selection of materials.
The objectives of design, the correlation of the function of the object with its shape and material were touched upon. Design is essentially the bridge that connects the specifications for requirement and the final solution.
The design of a system is essentially a blueprint or a model of the final system that can be produced from it.
Steps of Designing:
The design flow chart highlights some major steps that are required to identify the market need and clarify it as a set of design requirements, as concept, embodiment and detailed analysis leading to product specification.
The first step in ideating any solution lies in
identifying the problem statement. If you are a
businessman, you need to know the customer pain
points, if you are solving real cases, you need to know what your client wants. Not much different is the work of a design engineer. Identify the needs of the world, get to the core of the design requirement.
Once you know what is expected out of the final idea, track the constraints. Convert the information that you can abstract into objective, functions, constraints and free variables. Translate the design requirements into a tabulated data.
Start conceptualizing. You get the concept of the design by considering the material data, working principles, and functional requirements. The next step involves creating the layout and scales. Create and assemble the model, and analyse it for defects. This step is called Embodiment. Analyse each component in detail. Optimize their performance and cost, and move on to the final selection of material. The design objectives are a matter of primary concern. It directly influences the choice of material and the geometry of the structure.
The performance of a structural element depends on the functional requirements (F), geometrical requirements (G) and material Properties (M).
Mathematically, P= f(F,G,M)
Relation of Material Data with the Design Methodology:
⮚ Concept- You need all the material data at this point with low precision and
⮚ Embodiment- Narrow down to a group of materials with higher precision.
⮚ Details- Choose the best material with highest precision and generate the
Index and Charts:
Based on the objectives, constraints and function, the material index is specified. The maximization or minimization of this index guides in choosing the right material.
These material indices are derived based on the performance criteria of a material for a given mechanical design. In this method, a pair of material properties is plotted against each other.
The most interesting part of the webinar was dealing with Ashby charts. The properties of the material that influence the functional requirements are chosen. The corresponding Ashby Chart and the appropriate selection guideline is selected to further choose the best material.
This comes under screening and ranking of materials, whereby the top performers are considered and the subsequent step involves in-depth research and documentation to choose the best material.
Given alongside is an Ashby Chart plotting strength vs density.
The selection guidelines are shown, along with the families of materials. For choosing a material which is strong as well as heavy, those in the top right will be selected.
Tungsten alloy is the one with highest density and maximum strength.
The webinar concluded with an interactive assignment on choosing the correct material for bicycle frame, considering its stiffness, strength, light weight and cost. A practical application of translation of design requirements, screening of materials to eliminate those that did not suit the requirements, ranking of top materials using Ashby Charts and finally documentation of the top choices, culminating to the final choice, uplifted the takeaways from the session.
Bear tensile and compressive axial load
Strong and Stiff to prevent torsional deformation
Ability to be welded
● Function- Bear axial tensile and compressive load like a tie rod
● Constraints- Low density and light weight, High stiffness (Elastic modulus), High Strength
● Objective- Maximize the ratio of strength to density and stiffness to density
● Free variables- Choice of material, Cross sectional area of tubing
Screening & Ranking
Appropriate material property charts-
● Strength vs Density,
● Stiffness (Young’s Modulus) vs Density
Selection of Guidelines-
If density of the material is ρ and the tubes have a uniform cross section A, then mass of the material is m= ρAl, where l is the total length.
Strength (σ) =F/A
F is the axial load applied.
We can write- A=m/l ρ
σ = Flρ /m
m=Fl(ρ /σ )
Our objective is to reduce the mass and get a light weight frame.
So, the (ρ /σ) should decrease or (ρ /σ) should increase.
Similarly, E/ ρ should increase. Thus the selection guidelines are chosen.
The possible materials can be-
CFRP, Steels, Al alloys
The ceramics are rejected because they are brittle, cannot be welded, etc.
The possible material can be-
CFRP, Aluminium alloys, Steel
Diamond is rejected because it is not form able and highly expensive
Complex processes to manufacture
Earlier used for bicycle frames
Not as good as CFRP
Easy to manufacture
Earlier used for bicycle frames