Roselita Fragoudakis

Microstructural, surface and fatigue analysis of stress peened leaf springs

Purpose – The purpose of this paper is to investigate the fatigue and failure of commercial vehicle serial stress-peened leaf springs, emphasizing the technological impact of the material, the thermal treatment and the stress-peening process on the microstructure, the mechanical properties and the fatigue life. Theoretical fatigue analysis determines the influence of each individual technological parameter. Design engineers can assess the effectiveness of each manufacturing process step qualitatively and quantitatively, and derive conclusions regarding its improvement in terms of mechanical properties and fatigue life. Design/methodology/approach – Two different batches of 51CrV4 were examined to account for potential batch influences. Both specimen batches were subjected to the same heat treatment and stress-peening process. Investigations of their microstructure, hardness and residual stress state on the surface’ areas show the effect of the manufacturing process on the mechanical properties. Wohler cur...

Microstructure, Surface Characterization and Fatigue Assessment of 56SiCr7 Spring Steel

A high number of cycles sustained before failure, under cyclic loading application, is one of the key performance requirements of springs. Heat and surface treating of the spring steel can have a significant influence on its fatigue life. The main purpose of heat treating is to achieve a tempered-martensitic microstructure with appropriate surface hardness, in order to increase the fatigue limit of the spring. The heat treatment aims to an end martensitic formation from an initial ferritic/perlitic microstructure through the steps of heating, quenching and tempering. This study shows that the correct parameters chosen for each of these steps for 56SiCr7 steel can result to an appropriate microstructure, and therefore increase the steel surface hardness up to approximately 550-600 HV. With the aid of optical microscopy, the thickness of the decarburized surface layer is determined, in order to distinguish between the core and surface microstructure hardness of heat treated steel. Surface treatment, through shot-peening, induces compressive residual stresses on the surface of the steel, thus increasing the hardness by at least 204 HV, compared to the raw material, and doubling the number of cycles to failure. Vickers micro-hardness measurements conducted on a cross-section, at different depths from the surface of the steel, show the trend of hardness increasing towards its core, and verify the dependence of the surface hardness of the steel on heat treatment.

Cantilever Box-Beam Application of Composite Stacking Sequence Optimization Using Adaptive Genetic Algorithm

Composite materials due to their high stiffness to mass ratio as well as their anisotropic properties are the material of choice for many different weight critical structures such as wind turbine blades. Wind turbine blades have been modeled as composite cantilever box-beams for optimization purposes. The use of Genetic Algorithms (GA) has become a fairly common practice for optimization of composite laminates, where the objective is to find a laminate stacking sequence that optimizes the composite for a given condition. The purpose of this work is to study further adaptations to the GA search technique for use in the composite laminate stacking sequence optimization problem. In this work an Adaptive Genetic Algorithm (AGA) is studied for the stacking sequence optimization of a composite box-beam.

Low-weight: high-stiffness glass fiber reinforced polymer beams with embedded piezoelectric fibers

This work presents a theoretical study of the effects on stiffness and deflection of embedding piezoelectric fibers within glass fiber reinforced polymer beams. Through this study, enhancements to the beam stiffness and flexural capabilities are analyzed as a result of the piezoelectric effect of the embedded piezoelectric fibers. Fiber orientation of glass fiber reinforced polymer laminated beams is optimized based on stiffness requirements following classical lamination theory. The piezoelectric effect on the glass fiber reinforced polymer beam is analyzed for simply-supported mechanical boundary conditions. The symmetric unidirectional general stacking sequence laminates are shown to have optimal stiffness and deflection behavior. The addition of piezoelectric fibers with d333 piezoelectric actuation mode further increases stiffness and reduces deflection. This enables tuning of the mechanical properties of the laminate beam. Introducing piezoelectric fibers to the reinforcing phase further optimizes the deflection range under bending while additionally minimizing the weight of the structure. The strengthening effect of the piezoelectric fibers can reduce the required number of laminate layers while maintaining optimal behavior.

Quantifying the Directionality of Liquid Crystalline Polymers in Extrusion Processes Using an Order Parameter

Liquid crystalline polymers (LCPs) are among a high-performance class of materials, which derive unique mechanical, chemical, and electrical characteristics from their long-range molecular order. The evolution of anisotropic orientation in the LCP microstructure during processing, however, can adversely affect the macroscopic polymer behavior. Simulation of this anisotropy is crucial to the design of manufacturing processes producing the desired material properties, and the ability to quantify the polymer directionality is a necessary metric of the model. Using a Monte-Carlo approach introduced by Goldbeck-Wood et al., a practical method for simulating LCP orientation is used to model the polymer flow, and the directionality results are then used to calculate a quantitative molecular degree of order. This metric, known as the order parameter, is an ideal candidate for measuring the LCP orientation, ranging from zero to unity between the isotropic and perfectly aligned states, respectively, as it is sensitive to both the direction of the average molecular orientation, as well as to the distribution of crystals around the average orientation. The effects of varying process parameters in the directionality model on the order parameter are shown. Understanding of these relationships will ultimately drive the design of manufacturing processes for more isotropic materials.Copyright

Application of a Ag Ductile Layer in Minimizing Si Die Stresses in LDMOS Packages

Lateral Diffused Metal Oxide Semiconductors (LDMOS) normally have a Cu-W flange, whose CTE is matched to Si. Low cost Cu substrate material provides 2X high thermal conductivity, and along with a AuSi eutectic solder is recommended for optimal thermal performance. However, the CTE mismatch between Cu and Si can lead to failure of the semiconductor as a result of die fracture, due to thermal stresses developed during the soldering step of the manufacturing process. Introducing a Ag ductile layer is very important in minimizing such thermal stresses and preventing catastrophic failure of the semiconductor. Ag is a ductile material electroplated on the Cu substrate to absorb stresses developed during manufacturing due to the CTE mismatch between Si and Cu. The Ag layer thickness affects the magnitude of the resulting thermal stresses. This study attempts to measure the yield strength of the Ag layer, and examines the optimal layer thickness to minimize die stresses and prevent failure. The yield stress of the ductile layer deposited on a Cu flange was measured by nanoindentation. The Oliver and Pharr method was applied to obtain modulus of elasticity and yield depth of Ag. A finite element analysis of the package was performed in order to map die stress distribution for various ductile layer thicknesses. The analysis showed that increasing the ductile layer thickness up to 0.01 - 0.02 mm, decreases the Si die stresses.

Effect of Aluminum Foam and Foam Density on the Energy Absorption Capacity of 3D “S” Space Frames

Passenger safety during front collision depends to a great extent on the capability of the car’s front space frame structures to absorb energy generated during the crash, while deforming the least possible, Han and Yamazaki [1], and Cheon and Meguid [2].