Since 2012 we have been collaborating with Professors Georgios Savaidis and Nikolaos Michailidis, of the Aristotle University of Thessaloniki, to study the mechanical behavior, performance and fatigue of our leaf springs. In 2015 we completed a project entitled “Optimization of the Development and Manufacturing of High Performance Leaf Springs”. Our research proposal came first among 174 proposals evaluated by the Greek Secretariat of Research and Technology (GSRT). The goal of the project was the design of technological foundations that will improve the manufacturing process leading to new high quality products according to the European industry standards.
In 2012 our company in collaboration with Associate Professors of the Aristotle University of Thessaloniki, Department of Mechanical Engineering, Assoc. Prof. Georgios Savaidis and Nikolaos Michailidis and the laboratories of:
decided to study the mechanical behavior and fatigue of our leaf spring products, in order to investigate:
a) The optimization of the design and manufacturability of leaf springs.
b) The technological parameters of the heat and surface treatments of steel leaves in order to improve the surface hardness properties and the life of the leaf springs.
The goal of the project is the design of technological foundations that will improve the manufacturing process leading to new high quality products according to the European industry standards, which is the main customer for our products. The main goal of the optimization of the design of leaf springs is a timely and low cost production process focusing on high quality products.
The proposal of the project was submitted within the scope of the General Secretary of Research and Technology (GSRT) sector «Supporting New Companies’ Research and Development Activities». Our project «Optimization of the Development and Manufacturing of High Performance Leaf Springs» was selected first among the 174 submissions and was completed within 27 months.
Our research was divided in two parts. A theoretical/computational part and an experimental part:
1) Determination of leaf spring geometry through the parametric design of single leaf specimens.
2) Computational analysis for the determination of key parameter of heat treatment and cambering of the above specimens.
3) Computational analysis for determining the parameters of specimen surface treatment.
1) Characterization of the microstructure using SEM and optical microscopes and Vickers hardness nanoidentation on cross-sectional surfaces of the specimens.
2) Fatigue tests of specimens in order to define their fatigue life and performance through Wöhler diagrams.
Experiments were conducted on specimens designed based on the results of the theoretical/computational part of the study. These experiments showed that:
1. Heat treated specimens show ~25% higher surface hardness than non-treated ones.
2. Surface treatment following heat treatment will further increase surface hardness by ~40%.
3. Surface hardness by shot-peening quadruples the fatigue life of heat treated specimens.
Fig. 1. Micro-hardness profiles measured on shot peened specimen surface areas (up), raw material specimen surface area (down-right) and only thermally treated specimen surface area (down-left)
Fig. 2. Wöhler (S-N) Diagrams
Fig. 3. Leaf spring specimen with strain gages positioned on test rig
The results and conclusions of the project have been published in peer-reviewed journals and have also been presented in international conferences:
• "Microstructural, Surface and Fatigue Analysis of Stress Peened Leaf Springs," G. Savaidis, S. Karditsas, A. Savaidis and R. Fragoudakis, International Journal of Structural Integrity, to be published in 2015.
• "The Effect of Heat and Surface Treatment on the Fatigue Behavior of 56SiCr7 Spring Steel," R. Fragoudakis, S. Karditsas, G. Savaidis and N. Michailidis, Procedia Engineering, 2014, pp. 309-312 & presented at the XV11 International Colloquium on Mechanical Fatigue of Metals (ICMFM17), Milan, Italy, June 25-27, 2014.
• "Microstructure, Surface Characterization and Fatigue Assessment of 56SiCr7 Spring Steel," R. Fragoudakis, F. Stergioudi, N. Michailidis and G. Savaidis, Periodical of Key Engineering Materials, vol. 605, Proceedings of the 3rd International Conference on Materials and Applications for Sensors and Transducers (IC-MAST 2013), Prague, Czech Republic, September 13-17, 2013.
• "Leaf Springs - Design, Calculation and Testing Requirements," S. Karditsas, G. Savaidis, A. Michailidis, A. Savaidis and R. Fragoudakis, 35th Workshop on Mechanics of Materials, Rhodes, Greece, June 3-7, 2014.