Multi-Ridged Fasteners - An alternative joining tech for the auto industry

As a graduate student at Ohio State University, my research primarily dealt with the design and testing of novel multi-ridged fasteners (MRF). I got the opportunity to present my research at the SAE World Congress, Detroit during April 9-11, 2019. I am writing this article to share details about my study with the LI community and the broader context of how it fits in the big picture.

Before we dive any further, let's talk about joining in the auto industry. Welding is the primary joining method used in the auto industry for sheet metal components.

Welding is well understood, assembly lines are highly optimized with automation systems which have increased productivity and reduced cost over the years. Esoteric welding methods have been developed and tuned over the years: electron beam welding, friction welding, plasma arc welding, friction stir welding, explosion welding and laser beam welding. These processes have increased the range of materials and components that can be welded. This begs the question as to why different technologies are being investigated when we have established welding practices and techniques.

Dissimilar material joining.

To join dissimilar materials, welding becomes less attractive. Possible options are to use laser welding, electron beam welding or friction stir welding. However, two dissimilar materials cannot be chosen at will and welded together. The reason: welding operates by melting the materials at their interface and fusing them together.

Mechanical joining methods are being considered to join dissimilar materials. MRFs are one among the many technologies in contention. The others being adhesives, flow driven screws, self-piercing rivets.

Okay, but why are auto companies trying to incorporate different materials in their vehicle body?

Per the EPA, a 10% reduction in weight can increase fuel efficiency by 8%. Because of stringent fuel economy laws all around the world, such as CAFE* in the US (now on hold by the current administration), automakers have been investigating methods to reduce vehicle weight to further this goal.

*Corporate Average Fuel Economy - Standards created by NHTSA that dictates requirements for fuel economy to be met by an auto manufacturer's fleet of vehicles

A multi-material car body incorporating materials such as carbon fiber, aluminum and steel are used as a means to this end. An example is the 2017 Audi A8 body and the associated joining processes that it uses.

My study dealt with the design, prototype and testing of new MRFs. The main premise of the designs proposed in the study was based on improving worker safety by creating blunt-ended fasteners.

What is a multi-ridged fastener?

Similar to threads on a regular bolt, multi-ridged fasteners possess ridges along their circumference. The joining process occurs through material filling in the gaps between consecutive ridges on the fastener.

MRFs are typically inserted at a high speed, through which the joining process is completed.

Proposed designs

A series of blunt ended fasteners were considered to get an idea of how the strength varies with the change in geometry.

Based on varying degrees of flatness, four different designs were created. The sharp non-modified design was considered for baseline strength comparison. A fifth design based on a fully flat nail with a slight raised lip was created, called the raised edge fastener, named after the lip/edge running along the circumference of the fastener end.

Typically, joints are tested in two different conditions: 1. force is applied perpendicular to the fastener's axis (shear) and 2. force is applied along the axis of the fastener (tension). The forces are applied till failure and the strength response of the joints are recorded.

Based on the results, a curve relating the fastener geometry and its strength was created, which led to some interesting conclusions. In the plots shown, the x-axis shows the diameter of the blunt end of the fastener as a percentage of the fastener's diameter. (0 representing a sharp end and 100% representing a fully blunt fastener).

We observe a slight dip in performance as the 'bluntness' of the fastener increases from F0 to F66P (~ 5% decrease). When the fastener is made fully flat or with a 'raised edge', however, the decrease in strength compared to a sharp fastener is approximately 18%.

Hence, a blunt ended fastener is a strong contender for improving factory worker safety but it should not be made fully blunt lest passenger safety is put in jeopardy!

This interesting study was supported by the Center for Design and Manufacturing Excellence of The Ohio State University.

Thanks for reading, I hope you learned something new today!

I have not dived into great detail in this article, but if you are intrigued and would like to know more, please check out my thesis. The publication can be found here.

References

1. https://energy.gov/eere/articles/timeline-path-lightweight- materials-cars-and-trucks
2. I. Gotlib, “An Analysis of High-Speed Impact Nailing for Lightweight Automotive Structures”, Master’s thesis, The OhioState University, 2014.
3. Bollhoff GmbH.