When we ask Dave Archer, president, Archetype Joint (www.archetypejoint.com; Orion, MI), about joining, his initial reaction is that: Joining is permanent-as in welding-and while fastening can be permanent, but likely to be something that can be taken apart. "The only reason you would want to use a bolted joint is (1) when you have to take things apart and (2) it is a structural joint." But we're using the "joining" term in a more-encompassing sense, as in "putting things together."
Which then leads us to a question about "structural" joints. He points out, for example, that on vehicle interiors you'll find a number of fastening devices, from snaps to clips. They're not structure. In a structural joint, he says, "The joint needs to behave as a single part." That's because if the forces acting upon that joint cause the two pieces to slip, move or separate, chances are something bad will happen. Consider, he suggests, what happens if the head bolted to the block has slippage or movement: the cylinder pressure won't be contained and there will be poor performance. Consider, he suggests, if there is lateral movement in a joint in a suspension system: this could lead to metal fatigue and assembly failure.
There is something that comes down in the middle between the weld and the threaded fastener: adhesives. "It's more of a weld competitor," Archer says, "more of a permanent joint."
Regardless of the type of method used, Archer says that the number-one challenge that needs to be addressed is that of structures being produced with new or at least different materials, such as aluminum, magnesium, and composites. What,s more, these materials may be thinner. Archer notes that as regards welding, there are different processes that are being deployed, from friction-stir welding to laser welding, to accommodate these material changes. On the bolted side of the equation, there are changes as regards the material of the fasteners. For example, he points out that if a standard alloy steel bolt is used in a magnesium assembly, there could be problems associated with the difference in thermal expansion between the two materials. That, in turn, causes a difference in the relative clamp load . . . and lead to the relaxation of the joint. Which then can lead to additional problems, especially in the powertrain area.
And as for composites: Well, a challenge as regards bolts is that there tends to be a concentration of stresses at the point of assembly, and composites don't like that concentration. What's more, often to assemble composites with threaded fasteners it is likely that there is a threaded insert put in the composite so that the threaded fastener is actually being threaded into metal, not composite. Archer puts it rather simply: "Composites like adhesives more than bolts."
Archer admits, "Most of the work I do is with bolted joints." He further admits that bolts "create special problems." That is, he says that because they are used for future disassembly at some point but have to be structural in application before that disassembly occurs, you have to deploy them in just the right way. One of the challenges that needs to be addressed is the fact that bolts are tightened by applying a torque, yet "they are there for a clamp load." To which you might react with a "So?"
Archer explains: "The big disconnect in bolted joints is that the engineer designs a joint around a certain clamp load. Then in the factory they tighten these bolts to a torque in Newton-meters. The two have nothing to do with one another." Yes, the bolt may do its job, but he says the thing is that what is generally happening is that the amount of torque applied in order to achieve the tension tends to be the result of experimentation.
In Archer's lab they use ultrasonic equipment to measure the elongation of bolts in a joint in order to determine the amount of clamp load: "Bolt tension is created by elongating the bolt. The elongation may be a couple thicknesses of a human hair." And adding to this is that the bolt is possibly in a block such that it can't be physically measured. You need to be able to measure elongations in the bolt. The bolt is stuck into a block. Compare that with, say, deploying a simple beam torque wrench, which he describes as a "tool so simple that I could make one in my garage." Archer says that work is being done at the USCAR consortium to try to develop direct tension measurement for factory applications. He suggests that the ideal scenario would be to tighten the tension and to forget about torque-which would necessitate developing measuring equipment that could do it within the production environment at line rates.
Looking ahead, Archer thinks that compared to the use of fasteners and welding, adhesives may gain ground. He ticks off a number of reasons, such as that unlike welding, there is no change in the appearance of the materials being joined or heat-related distortions; unlike fasteners, holes aren't required. He acknowledges that there has been concern regarding the levels of strength that are required, but goes on to say that the chemistries are being improved, and that cure rates, another challenge, are becoming shorter.