Which material should be selected for fatigue critical structures

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Which material should be selected to improve the fatigue behaviour of your structure. Is it low strength or high strength material? Is it steel or aluminium? Very often materials are selected based on their static properties (and, obviously, cost). The stronger the material, the better. This may be the case if static strength is critical, but for fatigue critical components this kind of material selection is at the least too simplified, and in many cases just incorrect.

Material Groups

At first, let’s have a look at the fatigue strength (fatigue limit) of different groups of materials. The figure below shows the relation between the fatigue strength and ultimate strength for steel, titanium and aluminium for un-notched specimens. From the figure it can be seen that the fatigue strength increases with increasing ultimate strength, but not proportional. For strong materials, the fatigue strength is relatively less good than for not so strong materials.

The figure also shows that there is a difference between titanium, steel and aluminium, with titanium performing the best. Roughly, following fatigue strengths can be found:

• Steel: S_f ≈ 0.45 R_m
• Titanium: S_f ≈ 0.5 R_m
• Aluminium: S_f ≈ 0.3 R_m

However, be aware that these values are prone to a very large amount of scatter.

Notch Sensitivity

The above figure is valid for un-notched specimens with good quality surface. A real structure however does have notches, and certainly does not always have good quality surface. As a consequence, the fatigue strength of the structure will be reduced compared to the un-notched situation. Further, the notch sensitivity of a material should be taken into account as well. The stronger the material, the more sensitive it is for notches (and also for surface roughness and for mean stress level), which is disadvantageous for fatigue (larger reduction of the fatigue limit).

An increasing ultimate tensile strength and increasing yield strength leads in general to a lower ductility (higher ratio yield strength/ultimate strength and lower strain to failure). So with equal ratio between operational loads and ultimate strength, ductile materials (lower strength) are always better in fatigue than brittle materials (higher strength).

Material Selection in Aerospace – Why Aluminium is better

The ultimate strength vs. fatigue strength diagram shown earlier suggest that aluminium is less good in fatigue than steel and titanium. Still aluminium is the most applied metal for fatigue critical structures like aircraft. So why as that? It has to do with the density of the material. The specific fatigue strength (i.e. fatigue strength divided by density) of aluminium is much better than that of steel and titanium. So if weight is an important design criterium, aluminium is a much better choice than steel and titanium.

This is true for fatigue initiation, but in aerospace also crack growth and fracture toughness are important material properties. Again, aluminium performs better, it has a superior fracture toughness/density ratio and much better specific crack growth properties.

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