Question

Alloy Name	T	SG	S?y MPa	SE GPa	SKIC   MPa(m)0.5
Al-7075	T6	2.81	179	25.5	9.25
4340 steel	OQT	7.85	148	26.1	7.64
Mg-ZK61A	T6	1.83	106	24.6	8.19
Ti-8Al-1Mo-1V	STA	4.37	130	14.6	18.76

Problem 9

In a design where weight is a critical factor, such as in a space shuttle frame, and specific yield strength, specific elastic modulus, and specific fracture toughness are of equal importance, using the above table, Calculate to show which of the materials Inconel Al-7075, 4340 steel, Mg-ZK61A, or Ti-8Al-1Mo-1V in the above table, is the best material choice?

Answer 1

As mentioned in the question that weight is a critical factor and other properties are having equal importance.

Hence the material having the least specific gravity should be the best one.

Now as shown in table:

Mg-ZK61A  is having the minimum specific gravity = 1.83 but its yield strength is also lowest (106 MPa)

Second light material is Al-7075 which having specific gravity 2.81 but its yield strength is maximum (179 MPa).

Other two parameters are almost similar.

If we take the ratio of both parameters:

Specific gravity of Al-7075/ Specific gravity of Mg-ZK61A = 1.54

Yield strength of  Al-7075/ Specific gravity of Mg-ZK61A = 1.69

From above calculation, it is obvious that Al-7075 id 1.54 times havier than Al-7075 but its strenghth is 1.69 times it means the factor of Yield strength is dominating. Hence Al-7075 is the best suited material for application.