Archetype Joint in Machine Design – Securing Sheet Metal Panels

The April 1, 2005 issue of Machine Design, the annual Fastening and Joining Reference Guide, contained a feature article written by Dave Archer, President of Archetype Joint. The article, titled Joint Decisions, examines the integration of joint design within new product development. Subjects covered include; the impact of joint design on product cost and quality, selection of fastening and joining method, and actual examples of the importance of testing and use of ultrasonic measurement. As length restrictions of the article made it difficult to go into the details that some would like, the following is a more complete coverage of one topic in the article.

Securing Sheet Metal Panels

We were recently contracted by a transportation equipment manufacturer to recommend alternatives to their current methods of securing sheet metal panels. The most common fastening method they currently employed was bucking solid rivets, a very popular approach with large sheet-skinned structures from aircraft to tractor trailers. Those examples illustrate two primary reasons solid rivets are popular. At less than a penny a piece, and with minimal equipment investment required, solid rivets make an engineer look like a hero on a bill of materials. And although access to both sides of the joint is required, access is easier in wider range of situations with an operator and a bucking bar than with the joining equipment required of welding or crimping. Finally, if installed correctly, the rivet shank is expanded to fill the hole, reducing the potential for fatigue failure. However, these benefits come at the considerable cost of necessitating operations that are very skill-sensitive, require ear protection, and are prone to repetitive motion injuries. Moreover, because hole clearance is critical to performance, rivet holes are often match drilled at assembly, or pre-punched pilot holes enlarged at assembly. This is a source of increased span time, significant labor cost and performance variation. In summary, this is a process that is difficult to replace with other methods through typical ROI calculations, yet is just as clearly more costly that the elements captured in the payback calculation make it appear to be.

When considering alternative fastening and joining methods, it is always valuable to classify the joint requirements, which in turn determine the potential fastening and joining options. By utilizing the Selection of Fastening and Joining Options application utility available at archetypejoint.com, the following methods are shown as available for a permanent stationary joint from metal components where the fastening method can be visible.

Part or Consumable Added Nothing Added
Threaded Fastener
Push Fastener
Hook & Loop
Rivet
Drive/Roll Pin
Arc Weld/Braze/Solder
Adhesives
Interference Fit
Snap Fit
Heading/Staking
Skiving
Deform/ Interlock
Resistance/Laser Weld

 

Viewing these alternatives from an application perspective, they could be characterized as:

Permanent Joint Schematic

An additional category can be created by combining securing methods to create a hybrid joint.

Continuous Securing Method Schematic

In most cases hybrid joints add a discontinuous fastening or joining method to a continuous joining method, often an adhesive. A production benefit of this joint is the auxiliary discontinuous method provides a means of fixturing and stabilizing the assembly while the adhesive cures. If typical fasteners are employed, their insertion provides locating features, which are generally not available in bonding large panels. However, most fasteners require holes thru both panels, which is generally not desirable. From the structural aspect, proper selection of securing methods in a hybrid joint provides complementary benefit. For example, strategically located fasteners or welds can prevent an adhesive joint from being subjected to peel loading while the primary load-bearing is borne by the adhesive. Another key consideration in selecting panel securing methods is the need for back-side access and back-side support. It is desirable to have the capability to perform the securing operation from one side without the restriction that the back side of the joint be supported by tooling. However, once a panel is large enough that some locations requires securing far enough from an edge that it requires impractical throat depths of tooling such as a spot weld gun or any “C” shaped head, options are greatly reduced. The client applications involved securing low carbon steel sheet metal panels, generally pre-primed, from 14 to 20 gage (0.075″ – 0.036″) in thickness. The following securing methods were chosen for consideration.

  • Solid Rivets
  • Blind Rivets
  • Resistance (Spot) Welding
  • Adhesive (2 part epoxy)
  • Drill Screws
  • Crimping
  • Self Pierce Riveting (SPR)

The result of one of the several tests performed is shown in Fig.ure 1 below. As important a consideration as the average strength values is the relative variation of those values. The error bars in Fig. 1 represent the max/min values of the samples tested. Note the relatively high degree of variability of the blind rivet. As discussed previously, proponents of solid rivets note their superior structural properties relative to blind rivets. When installed to specifications this is true but the results show that performance is much more greatly operator influenced than with blind rivets. Another point that should be kept in mind is that, with the exception of the bonded joint, tearing of the metal around the securing method led to joint failure, not failure of the fastener or weld. Therefore, calculating joint strength based on the shear or tensile strength of the fastener or weld can be a gross over-estimation in joints where the secured components are the weak link.

Figure 1 - Lap Shear Strength LB

The Fig. 2 below shows some variations on the basic methods above, and illustrate a couple of principles that are not often considered. First, a single lap shear test is not a pure shear test. The offset caused by the material overlap produces an eccentric load on the joint, subjecting it to peel as well as shear forces. Under peel loading, the strength of an adhesive joint is dependant on the width of the bond line rather than the area of the bond. Therefore doubling the bond area of the tested joint (while maintaining the same bond line width) produced only a 7% improvement in test results instead of the 100% that in theory should have been achieved. Next, if one compares the strength of a riveted joint to the associated hybrid joint in Fig. 2 it is not apparent how the combination of processes improved joint strength. By looking at typical plots taken from those tests (Fig. 3), more detailed information can be gathered. The hybrid plot shows the joint being progressively loaded until the coupon begins to yield at about 1700 lb. Loading increases until sudden joint failure at approximately 2300 lb. Load stabilizes at a bit over1000 lb briefly, and then the joint load capability falls off and finally fails completely. Comparing this plot to the independent plots of adhesive and rivet also shown in Fig 3, it can be seen that the rivet enhances the adhesive strength rather that the other way around. In both cases the rivet fails at the same 1000+ lb, while the sudden failure mode shows that it is the adhesive that provides the hybrid joint’s ultimate strength. That strength is improved by about 50% over adhesive alone because the rivet provides some resistance against the peel forces that are the joint’s failure mode.

It should also be pointed out that use of hybrid joints can alter the relative strengths and weaknesses of the complementary securing methods. It was mentioned earlier that, when both are installed properly, solid rivets have a greater fatigue limit that blind rivets because the forming process more completely fills the rivet hole, thus reducing the micro-movement that can lead to fatigue failure. However when combined with an adhesive in a hybrid joint, as long as the adhesive bond remains intact that bond line will react the majority of loading, thus improving the rivet’s fatigue limit.

Figure 2 - Lap Shear Strength LB

Figure 3 - Lap Shear

But joint design is far more than evaluating test results. Production considerations many times result in selecting securing methods that did not necessarily produce the best test results. Even more important is the understanding that fastening and joining decisions should not drive the product development process, but instead reflect, earlier product architecture decisions. For example, when inputting joint requirements of a permanent stationary joint of the same materials, the Selection of Fastening and Joining Options utility generates the suggestion that the joint be eliminated entirely, as same material is being permanently joined. The 61% cost reduction that resulted from this engagement would not have been possible without incorporating fundamental design considerations, as a significant portion of those savings were achieved through the reduction of total joint length, rather than from the change in securing methods. The securing methods were also revised, and the combination of those changes and the revisions to the fundamental design architecture more than halved the total span time required to complete the structure.

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