Lecture 6 : Case Study: Materials Selection for Vehicle Body
Case Study continued: Materials Selection for Vehicle Body
The third important issue is the resistance against crack propagation. This is indicated
by the parameter called ‘fracture toughness’ or KIC (MPa-m1/2)
Now, let us look at the values of these parameters for four materials: Carbon Steel,
Stainless Steel, Aluminium Alloy, PMMA and GRFP (Glass Fibre Reinforced Plastic).
Material
E1/3/ρ
(E – GPa, ρ-Mg/M3)
σy
(MPa)
KIC
(MPa-m1/2)
Carbon
Steel
(0.4 $/Kg)
0.75
250
12
Stainless
Steel
(1.2$/Kg)
0.77
170
12
Aluminium
Alloy
(1.3$/Kg)
1.63
30
18
PMMA
(2.4$/Kg)
1.34
72
(max)
1.6
(max)
GFRP
(9$/Kg)
1.55
150
(max)
20
(max)
The Table above clearly explains why Carbon steel was tolerated for a long time in Car-body
construction. It was the cheapest among all available materials having
reasonably good material indices. However, the massive growth of automobile use in
the Europe and USA caused tremendous environmental problems. As a result in 1975
a new rule was established by CAFE (Corporate Average Fuel Economy) regulations,
which set a target of fuel economy up to 27.5 miles per Gallon of fuel). The average
fuel efficiency of steel based Cars at that time was only 14 mpg. The new rules have
shifted the favour towards Aluminium. In terms of specific stiffness it is even placed
better than steel. The Table also shows that from all round consideration GFRP is
even better than Aluminium. However, the cost of GFRP is still much higher than
Aluminum prohibiting its use in mass-scale manufacturing of automobiles.
