Suitability of material

Now with the calculations completed I can compare the forces which the material will be under with its yield stress.

To recap the values for Aluminium 7075:

Specific Density: 2810 Kg/cb.m

Youngs Modulus: 72 Gpa
Yield Stress: 145 x10^6 pa
USD/LB: 0.97

The calculated stresses &forces:

Max stress experienced: 69 MPa

Max deflection: 0.015 m

As it can be seen the 69 Mpa is well below the materials yield stress of 145 MPa therefore this material will not deform when it is fully loaded which is very desirable. It will also not deform when the beam is at its max deflection as aluminium 7075 can deflect 9-10% before plastic deformation.

Therefore Aluminium 7075 is perfect for this crane as it is light, strong and reasonably priced.

Coating

Aluminium 7075 can also be easily anodized which increases the thickness of the naturally occurring oxidized layer. This layer increases corrosion and wear resistance, as well as creating a good base layer for paint primers and glues than bare metal. This is useful as the crane has to be transported, carried and repeatedly put up and taken down. This would mean that there would be a lot of wear on the crane and it might also be bumped by debris in the disaster zone.

Nuts and Bolts Material

The nuts, blots, pins and fixings used in this crane would not be made of aluminium as this would cause many problems. Aluminium is too expensive to make small components out of, which means that if any were lost or misplaced (which would be very likely in the chaotic environment of a disaster zone) it would be very expensive to replace them. Also there will be more shear stresses and torsion stresses on these fixings so it is better to air on the side of caution and make them of a stronger metal. Finally the weight of the fixings wouldn't matter as they are only small components, so heavier metals such as steel can be considered.

The majority of readily avalible fixings are made of stainless steel. If the crane was a permanent structure it would be important to state that stainless steel will corrode faster than aluminium alloy and will therefore need replacing frequently. However as the crane is to be put up then taken down for a job at a time, a quick inspection of the fixings before they are used to check for corrosion and spares to replace fixings that are is all that is needed.

Chosen Material

The chosen material for the crane is Aluminium alloy 7075. It is commonly used in bikes and aerospace as a light metal with high strength and moderate toughness.

It is also important to note: The yield stress has been used to compare the calculated values not the ultimate tensile stress as by the time the stresses had built up so much to reach the ultimate tensile stress the material will have completely plasticly deformed and the crane would be unusable. The yield stress is the point at which the material begins to plasticly and permanently deform which is very undesirable.

For untreated 7075 aluminium:

Specific Density: 2810 Kg/cb.m
Youngs Modulus: 72 Gpa
Yield Stress: 145 x10^6 pa
USD/LB: 0.97

Compared to steel:

Specific Density: 7850 Kg/cb.m
Youngs Modulus: 210Gpa
Yield Stress: 300x10^6 pa
USD/LB: 0.55

It isn't as strong with a Youngs modulus of 75Gpa whereas steel has 200Gpa. However it is much lighter with a specific gravity of 2810 compared with 7850. Its yield strength is 145 x10^6 pa which is below steel but should be more than enough for our crane.

Specific Density: 2640 Kg/cb.m
Youngs Modulus: 68 Gpa
Yield Stress: 60 x10^6 pa
USD/LB: 1.04

Compared with pure aluminium:
It is stronger than pure aluminium, as aluminium's value is 68Gpa but it is the yield stress that is greatly different as pure aluminium is only 60 x10^6 pa. It is slightly heavier by around 200kg/cu.m but this shouldn't make that much difference for the small amounts we'll be using.


To conclude Aluminium alloy 7075 is well suited for our crane as it is light (lighter than steel), it is strong so it should easily be able to withstand the loads and it is reasonably priced. As with all metals prices do fluctuate with the market but at current values it is 0.97 USD/LB.