- SPECIAL REPORTS
Carbon steel is often the material of the parts being replaced and of the bolts that fasten them. As more engineers explore the benefits of using lightweight materials, it will affect the way that bolted joints are designed. Engineers must consider factors such as joint capacity, joint behavior under load, joint behavior at temperature, creep and corrosion.
Joint Capacity. Because lightweight materials generally have much lower compressive and shear strength than the fasteners used to join them, greater under head bearing areas and thread engagement may be needed to reduce stress if clamp load is to be maintained. In some cases, this reduces the potential weight savings of the material substitution.
Joint Behavior Under Load. A fundamental design objective of most bolted joints is to have the stiffness ratio of the clamped members relative to the bolt be as large as possible. This makes the joint less susceptible to relaxation and will cause a smaller percentage of tensile loads to be “felt” by the bolt. Stiffness is affected by geometry and modulus of elasticity, so for a given geometry, it would be desirable if the modulus of the joint members were greater than the modulus of the bolt. Many lightweight materials are, at best, half the modulus of steel.
Joint Behavior at Temperature. When the temperature of a bolted joint differs from the temperature at which the bolt was tightened, there is the potential for the clamp load to change due to differential thermal expansion. While clamp load can increase or decrease based on the direction of temperature change and the choice of materials for the joint and fastener, the most common result when using lightweight components is that the clamp load is increased. One reason is that an engine or mechanism in which the joints are contained generates waste heat and the clamped components expand at a greater rate than the steel bolts.
The impact on joint reliability varies with application, but a common outcome is that the increased clamp load at elevated temperature causes permanent deformation in the joint, leading to reduced clamp load when the mechanism operates in a start-up (cold) condition. This can be caused by permanent elongation of the bolt, compressive yield of the joint members or a combination of both.
Creep. Creep is defined as plastic deformation that takes place over time at stresses below a material’s yield strength. As this effect is greatly amplified by increased temperature, it is difficult to separate discussion of creep from operation at elevated temperature. However, engineering thermoplastics have such a low melt point that a tendency for clamp load loss exists even at room temperature, due to the stress levels present in highly loaded joints common in structural applications. This often leads to the addition of metal compression bushings.
Corrosion. Replacing steel components with aluminum or magnesium increases the potential for galvanic corrosion with steel bolts. Particular attention needs to be paid to using barrier coatings when magnesium components are used in uncontrolled environments, as Mg alloys are even more sacrificial than zinc.
One might notice that the common thread in these comments is the bolt-they are always assumed to be steel. What if the bolt material was changed to decrease the impact of dissimilar materials? In fact, there are instances of this, such as the use of aluminum bolts with magnesium castings.
However this raises other challenges and compromises. The greater the penalty for added weight, the greater the material cost premium the manufacturer is willing to pay to reduce it.
For example, in addition to game-changing use of composites, Boeing increased titanium content significantly on its new 787 Dreamliner, including bolts (to the extent that bolt shortages caused development delays). While it is possible that aircraft design represents a look into the future for ground vehicles, it is certain that the drive toward greater efficiencies will require solving a series of challenges brought on by the use of alternative materials, including how to join and fasten them. A