Papers by Keyword: Residual Stress

Abstract: The six independent axes available for free-form bending enable the production of complex three-dimensional bent tube and profile geometries. In industrial environments, only tangential bending strategies are currently used, which means that the bending head is always positioned parallel to the cross-section of the tube in the current bending section. Therefore, the individually controllable axes make it possible to apply other, non-tangential bending strategies. In so-called overbending, the bending head is rotated more in comparison to tangential bending. However, in order to ensure that the bending radius does not change compared to tangential bending, the translational deflection of the bending head must be reduced at the same time. In contrast, the bending head is rotated less during underbending and the translational deflection is increased. Overbending and underbending offer the possibility of improving the mechanical properties while maintaining the same bending geometry. These strategies allow the components to be optimized for individual load cases. As part of this work, a structural component was produced multiple times using free-form bending. Both conventional tangential bending strategies as well as innovative overbending and underbending strategies were applied. The mechanical stiffness of the bent components was then examined on a test bench. The influence of the bending strategy on the cross-sectional change in the bent area was investigated by using a tactile coordinate measuring machine. Furthermore, residual stress measurements were performed on the bent tubes, which allowed the different mechanical behavior of the tangentially bent, overbent and underbent tubes to be explained.

Abstract: Various heat treatments were applied to reduce quench-induced residual stresses while improving the mechanical properties of particle-reinforced aluminum composite. The residual stress distribution of samples quenched in water with different cooling media was measured. The results showed that quenching with the 30% polyethylene glycol quenchant (PAG) yields up to an 86.8% reduction in residual stress magnitude compared with cold-water quenching (CWQ). Still, the tensile properties of samples quenched in 30% polyethylene glycol quenchant were low, with a 12.5% reduction in yield strength. The experimental results show that the uphill quench (UQ) method is an effective means of reducing residual stresses induced by quenching. At the same time, the effect on tensile properties is negligible. Moreover, the study found that combining uphill quenching with short-aging treatment can further improve the residual stress, strength, and fracture toughness of SiCp/Al-Cu-Mg composites.

Abstract: This study investigates the effects of various cutting technologies on a 0.25 mm thick ferritic steel, a material widely used in packaging and other lightweight applications. The study provides a comprehensive comparison of four distinct cutting technologies: Laser Cutting, Milling, Electrical Discharge Machining (EDM), and Water Jet Cutting. The research focuses on the impact of these cutting processes on the material’s properties and its performance under uniaxial tension. X-ray diffraction is used to precisely measure the magnitude and distribution of residual stresses along the cut edge in order to correlate them with changes in the material's flow curve, which is critical for accurate mechanical characterization. Furthermore, a laser-scanning microscope was used for detailed morphological analysis of the cut edge and for roughness measurement. To quantify mechanical property changes, microindentation hardness testing was used to assess the degree of work hardening induced by each cutting method. Finally, Digital Imaging Correlation (DIC) was employed to track strain distribution and observe strain field variations.

Abstract: Fibre tension is an important process parameter during filament winding. It strongly affects the void content and fibre volume fraction, which in turn determine the mechanical performance of the part. Fibre tension also contributes to the residual stresses that develop in the filament-wound material. This study focuses on how the fibre tension shapes the stress distribution in filament-wound composites. To this end, a numerical predictive model was developed. Experimental validation was conducted using a force sensor, and good agreement was observed between the model predictions and the experimental measurements. These findings provide deeper insight into the role of fibre tension in filament winding and offer practical guidance for optimising the process to enhance the performance of composite pressure vessels.

Abstract: In order to achieve a long service life for highly stressed parts such as shafts for drivetrains, a combination of a bainitic microstructure with compressive surface residual stresses is beneficial. While a bainitic microstructure offers a good balance between strength and toughness, compressive residual stresses especially near the surface have a positive impact on service life. Research has shown that this is due to a shift of the crack initiation towards the core and a reduced crack growth . These properties can be achieved by hot forming as an established method for manufacturing highly stressed parts followed by an adapted cooling strategy. As this general approach was demonstrated for a simplified process in a prior study , the present article is dedicated to the functionalisation for hot forming processes. In detail, a customised spray cooling is presented for a hot impact extrusion process whereby shafts made of AISI 4140 are cooled down from the forming heat in a single step with adjusted cooling rates. In a finite element-based process design, different cooling strategies were investigated and adequate heat treatments to achieve the combined properties were identified. Following this process design, shafts are formed via hot impact extrusion and spray cooled according to the cooling strategy for experimental validation of the numerical model. Additionally, shafts with air cooling are produced as a reference. During forming, force-displacement curves are measured, which are used for the validation of the numerical hot impact extrusion simulation. The resulting plastic strain and temperature distribution significantly influence the following cooling simulation. The final microstructure as well as hardness values of the produced shafts are determined and compared for the varying cooling strategies.

Abstract: Residual internal stresses arise during thermal processing of thermoplastic composites due to differential shrinkage of stacked orthotropic plies and can lead to defects such as shape distortion, microcracking or delamination. In the current study, a comprehensive thermomechanical model for predicting residual stresses and strains in semicrystalline thermoplastic composites is presented with specific application to unidirectional Carbon fiber reinforced LM-PAEK composite laminates. The model is based on an incremental Classical Laminate Theory (CLT) framework that incorporates temperature-dependent material properties and accounts for both thermal and crystallization-induced shrinkage effects. Material characterization is performed to measure key temperature-dependent properties: thermomechanical analysis (TMA) is used to measure the transverse thermal expansion coefficient (CTE) and crystallization shrinkage upon cooling from melt state, dynamic mechanical analysis (DMA) to obtain the transverse modulus. The stress-free temperature, the temperature at which residual stresses begin to develop, is identified through curvature evolution measurements of unsymmetric laminates using image analysis. The model validation is performed via curvature measurements of unsymmetric cross-ply laminates using laser scanning techniques, demonstrating good agreement between model predictions and experimental measurements.

Abstract: Welding distortion remains a significant challenge in vehicle structures incorporating aluminum components. This study investigates the application of pre-tension to mitigate welding distortion in an aluminum T-joint structure. Through finite element (FE) simulation, the mechanism by which pre-tension alters the residual stress state and reduces distortion is analyzed and comprehensively explained. Based on the findings, an optimized pre-tension condition is proposed to minimize welding distortion in the T-joint configuration.

Abstract: Conventionally, the sin²ψ method has been used as X-ray stress measurement. However, in recent years, the XRD² method and the cos α method have been put into practical use and spreading. In addition, the Fourier analysis method that shares the same measurement principle of the cos α method has been developed and is attracting attention. Therefore, in this paper, the Fourier analysis method is examined from the measurement theory and the measurement accuracy is investigated. It is reviewed that the basic equation is a finite Fourier series, and that stress can be determined from the Fourier coefficients by using coordinate transformations. Then, while comparing it with the multiple regression analysis, the accuracy of the Fourier analysis method is discussed by using numerical calculations.

Abstract: This work investigates the influence of initial residual stresses after additive manufacturing, specifically directed energy deposition, in 5xxx aluminum alloys on the fatigue crack propagation behavior. For this purpose, initial plane stress states (compressive as well as tensile) are introduced along the crack path on a C(T)50 specimen via eigenstrains, mimicking possible residual stress states after both directed energy deposition and possible post-processing. The evolution of the stress intensity factor difference is determined and used to calculate the crack propagation rate via Walker’s equation. The stress state of the vicinity of the crack tip dictates the crack behavior: Compressive stresses perpendicular to the crack path exhibit crack closure, resulting in slower propagation rates. Finally, the influence of a more local distribution of the residual stresses on the fatigue crack propagation is investigated, highlighting the importance of the position of compressive stresses relative to the crack tip for effective crack growth retardation.

Abstract: To produce functional cups by press forming, clad cups with a corrugated structure with voids like the cross section of corrugated cardboard were formed. Deep drawing, which is one type of press forming, is a plastic processing technology that forms thin sheets into three-dimensional containers. In the experiment, pure titanium TP270 and ultra-low carbon steel SPCC were used as test materials. The blank sheet thickness was 0.3 mm and the diameter was 80 mm to 90 mm. To form the corrugated cup, the roller ball die with steel balls installed on the shoulder of the die was prototyped. The steel balls were made of bearing steel JIS-SUJ2 and had diameters of 6.4 mm and 7.5 mm. The corrugated clad cup was formed by the composite die combined with a conventional die. Three conventional dies and two roller ball dies were used to obtain two corrugated layers with voids. The lubricant was a tool oil containing molybdenum disulfide powder. The sheet thickness strain distribution and residual stress distribution of the cup were evaluated. No destruction of the cup occurred during deep drawing. A regular wavy structure was observed in the cross section of the cup. The maximum reduction in the cup thickness was approximately 10 %. The residual stress on the outside of the cup was tensile stress from the bottom to the opening of the cup. The composite die made it possible to form a functional cup.