Causes of weight reduction effects of material substitution on constant stiffness components

The substitution of lightweight materials, such as aluminum or magnesium alloys, to produce lightweight car bodies, has been the subject of intensive research in recent years. It has been established that an aluminum body is lighter than a steel body for constant stiffness. The causes of this weight reduction have not been established. In particular, since the specific modulus (modulus of elasticity/density) of steel, aluminum and magnesium are nearly identical, there is no easy answer for their ability to reduce weight.In this paper, it is shown that there are thin-walled effects that are dependent on the thickness of the material. These thin-walled effects appear in joints and other parts with complex geometry and loading conditions and can significantly weaken the part. For example, the flanges on a curved beam in flexure have an effective (load-bearing) width that increases as the material is thickened. A part made of thicker material uses its material more efficiently (has a higher percentage of load-bearing material) than a thinner part. This thickness dependence is important because thin-walled parts made from lightweight materials, such as aluminum, are thicker than their steel counterparts; hence the parts made from lightweight materials use their material more efficiently. It is further shown that reducing these thin-walled effects through better reinforcements produces significant weight reduction effects and also reduces the weight reduction effects of substituting aluminum for steel. Thus a strong, lightweight body can be achieved using steel and proper design of joints and other complex geometry parts. This is a more cost-effective way of achieving a lightweight body.The concept of a structural index, λ, is developed and applied to the joints of a truck cab. It is assumed that when performing material substitution, only the thickness, t, is changed. It is shown that the stiffness, K, of the beam can be defined as a function of t^λ, that 1≤λ≤3, and that λ can be used to predict the weight savings from material substitution where stiffness is held constant. It is then demonstrated that λ can be used to predict the weight savings from material substitution in the more complex cases of the joints of a light truck cab.