Materials selection strategies 1. What are principles of materials selection? 2.
ID: 1885202 • Letter: M
Question
Materials selection strategies
1. What are principles of materials selection?
2. What is material index? List material indices for stiffness and strength-limited design for tie, beam and panel.
3. What is the second moment of area? How to improve stiffness and strength by using efficient shapes?
4. Car bumpers (considered as beams) used to be made of steel, but now are to be replaced by CFRP to save weight. Here are the material properties.
Material (kg/m3) y (MPa) Embodied Energy Hm (MJ/kg) Cost Cm ($/kg)
CFRP 1550 800 450 42
Steel 7900 295 33 0.8
a. If the bending strength of the bumpers must remain unchanged. Take the weight of a set of steel bumpers to be 20 kg, what is the maximum potential weight saving that this substitution allows?
b. Work out how much use-energy is saved by changing the bumper set from steel to CFRP, over an assumed life of 200,000 km. The use-energy consumption of this car is 2.1×10-3 MJ/km kg.
c. Calculate whether the switch from steel to CFRP has saved energy over life.
d. The switch from steel to CFRP increases the price of the car. Using a pump price of $1.5/l for gasoline (equivalent to an energy price of 0.03 $/MJ), work out whether, over the 200,000 km life, it is cheaper to have the CFRP bumper or the steel one.
5. It is suggested that an epoxy-matrix composite reinforced with 30% of randomly dispersed, single-walled carbon nanotubes (SWNTs) could have a specific strength that exceeds that of any existing material. Explore this claim, assuming that one third of the nanotubes contribute to strength in any given direction and that the composite strength and density can be estimated by a rule of mixtures, using the data in the following table.
Density (kg/m3) Strength (MPa)
SWNT 1,350 30,000
Epoxy 1,250 60
Explanation / Answer
Material selection is the most important part of material science engineering. Physical properties, requirements. It's the application requirement that defines the selection. The goals must be achieved and at the same time the cost must be brought down for same level of performance. Thus the properties and cost are the basic guidelines for selection. For example a cable bridge must have cables with very large tensile strength. The selection becomes complicated if the same material has to deliver on more than one front, for example a rope has high tensile strength but is prone to burning. Ashby charts are very common to solve this problem of selection. It involves three variables required to be determined.
a) material variable involving density, modulus of elasticity,
b) free variables, the quantities that can change during force application.
C) design variables, including the cross section etc.
All these variables put together give a performance index equation that tells as to how desirable a material is for a particular task.