S. Karditsas's research while affiliated with Aristotle University of Thessaloniki and other places

Heat and surface treatments of spring steels can greatly affect fatigue life by increasing the number of cycles to failure. Heating, quenching and tempering of spring steel will increase its fatigue limit, as tempered martensite formation with an appropriate surface hardness is achieved from an initial ferritic/perlitic microstructure. This study deals with the parameters of the manufacturing process to transforming the initial microstructure of 56SiCr7 steel to tempered martensite, and increasing the surface hardness and fatigue resistance. Microscopy helps determine the decarburized surface layer thickness. Macro- and micro hardness measurements distinguish between core and surface microstructure hardness of the heat-treated steel. Shot-peening induces compressive residual stresses to the surface of the steel, therefore increasing its surface hardness and consequently the number of cycles to failure. This study shows significant hardness increase on the surface of the steel, and verifies how heat- treatment and shot-peening affect surface hardness. Experimental and analytically calculated (FKM guideline) S-N curves at 4- point cyclic bending quantify the individual effects resulting from the applied heat treatment and shot peening on fatigue life.

The paper deals with the fatigue and failure analysis of serial shot-peened leaf springs of heavy trucks emphasizing on the influence of thermal treatment and shot peening on fatigue life. Experimental stress–life curves are determined by investigating smooth specimens subjected to fully reversed rotating bending conditions. These test results are compared to corresponding ones determined from cyclic three-point bend tests on shot-peened serial leaf springs in order to reveal the influence of the applied thermal treatment and shot peening process on the fatigue life of the high-strength steel used for leaf spring manufacturing, dependent on the load level. Microstructure, macro- and micro-hardness analyses are performed to support the analyses and explain the effects resulting from the certain shot peening process on the surface properties of the high-strength spring steel under investigation. The assessment of the fatigue results reveals nearly no life improvement due to the manufacturing, emphasizing the necessity for mutual adjustment of shot peening and thermal treatment parameters to take account for life improvement.

The present paper deals with the influence of leaf-spring design parameters on axle kinematics and stress behavior. Parametrical studies on asymmetrical parabolic front mono-leaf springs for heavy duty vehicles were exemplarily performed considering the most significant driving maneuvers using the Finite Elements Method. Kinematic curves of the wheel joint were determined and their influence on the steering kinematics was assessed. Additionally, the developed stress distributions along the leaf-spring's arms were determined. The results reveal the influence of each design parameter on the kinematic and stress behavior of leaf springs both qualitatively and quantitatively.

Taking high-performance parabolic steel leaves from serial springs used for the suspension of front axles of trucks as an example, the present paper focuses on the influence of the material, the heat treatment and the stress peening processes on the microstrucure, the surface properties (residual stresses, micro- and macro-hardness, roughness, decarburization zone and percentage) and, especially, the fatigue life of parabolic leaf springs. Hardnessess of the leaf core and surface, roughness profiles and residual stress profiles of both the stress peened and untreated leaf surfaces were determined to quantify the influence of the applied heat treatment and stress peening. Analytical calculations based on the FKM guideline were performed to evaluate every above mentioned fatigue life influencing factor. Comparison of the analytical results with the corresponding experimentally determined ones reveal the accuracy of the current theoretical tools.

Experimental results regarding the fatigue behaviour of metal foam based exhaust aftertreatment systems for passenger cars are discussed. Different prototypes were tested under fully reversed cyclic loading with variable acceleration amplitudes at room temperature and a constant frequency of 180 Hz. The cyclic tests exhibited the overall mechanical response of the specimens to fatigue loading, especially the rela- tions between the drive acceleration and the accelerations acting at various posi- tions of the specimens, and identified the failure critical elements.