Doctoral dissertation: Adhesively bonded joint is superior to welding in high-strength structures
Researcher Ahti Oinonen at Aalto University in his dissertation developed design tools for adhesively reinforced friction joint, a competitor for welding.
New high strength steels reduce the weight of machinery, but welding of even thinner materials is challenging.
- When the component walls become thinner, new joining methods are needed, researcher Ahti Oinonen points out.
Crane components and forestry equipment components are continuously subjected to changing loads. A small crack originating from a stiff welded joint advances quickly, as the machinery is operated almost round the clock.
Oinonen's solution for critical joints is adhesively reinforced friction joint. Adhesive combined with high strength bolts as a substitute for a welded joint sounds weak, but that is not so. The glue creates an elastic joint in which loads are distributed more evenly than in a welded joint. Bolts ensure that the joint is secure.
- Adhesive improves the strength of the friction joint 2.5 times, Oinonen says.
Friction joints are bolt joints tightened with high-strength bolts. The operation of these joints is based on the friction between the joined parts. If the joint slips and the load is transferred to the bolts, the joint starts to suffer damage.
- There has been a lot of research on adhesive joints, but almost none on combining high-strength bolts with them.
Gary Marquis, Professor of Strength of Materials at Aalto University, was the inspiring force behind Oinonen's research. The dissertation research started under his supervision at Lappeenranta University of Technology in 2007. When Marquis was nominated as a professor at Aalto University, Oinonen followed him.
In three years, the research progressed from the development of a testing method for friction joints to the construction of a mathematical model describing the damaging process that joints are subjected to. In his dissertation, Oinonen also investigated how the bolts that tighten the glued joint should be positioned.
First, Oinonen did experimental research on the shear strength of glued joints with different surface roughnesses, adhesives and compression loads. Based on the experiments, Oinonen calculated the material parameters that are used in a numeric calculation program running on the screen of the researcher's work station.
Oinonen tightens bolts with mouse clicks. The head of a bolt on the screen penetrates slightly into metal.
Load is gradually added to the virtual joint. Finally, the glued joint fails.
Oinonen zooms on the joint interface where the bonded seam starts to slip.
- The displacement is 0.7 millimetres.
With the modelling method developed by Oinonen, engineers are able to design bonded joints and model any potential damage.
The new joining method is not yet used in mechanical engineering, and it cannot yet be found among standards in the field. There is potential, undoubtedly, especially for critical fatigue-related targets. In the aerospace industry, adhesive-bonded structures are already widely used. We will soon have more accurate research data on the fatigue strength of friction-adhesive joints. The expectation is that the fatigue strength is excellent.
- The joining method would be suitable, for example, for the attachment brackets of the hydraulic cylinders in forestry equipment and cranes. In shipbuilding also the trend is towards lighter structures.
The research was partially funded by FIMECC, a research network for Finnish mechanical engineering industry.
Image caption: Based on the research, the optimal bolt pattern for an eccentrically loaded friction joint is semicircular.
The doctoral dissertation of Ahti Oinonen, M.Sc (Tech), titled ”Damage Modelling Procedure and Fastener Positioning Optimization of Adhesively Reinforced Frictional Interfaces” was examined at the Aalto University School of Engineering on Monday 28th November 2011.
For further information please contact:
Researcher Ahti Oinonen
Aalto University Department of Applied Mechanics
ahti.oinonen [at] aalto [dot] fi
Tel. 040 524 8654
Text: Petja Partanen/Tarinatakomo