Process-induced Residual Stresses Research Articles

Processing-induced residual stresses in TWIP steel weld spots

ABSTRACTThe present study investigates the evolution of the residual stresses in TWIP steels induced by manufacturing chain for the production of automotive body-in-white. Two different manufacturing routes were considered. The first route encompassed a plastic deformation prior to the welding stage, whereas the second involved the spot welding followed by a baking treatment. A convergent approach was adopted to isolate the effects of the first and final manufacturing steps. The findings showed that the plastic deformation prior to the welding stage is not annihilated by the welding thermomechanical cycle. Abrupt hardness gradients along small material fractions are observed. The residual stresses state changes, although its profile is still defined by the welding stage. The post-weld bake treatment showed to promote slight residual stresses relaxation, but it is not effective in inducing the same post-weld residual stresses state for different RSW parameters set.

Phase Transformation-Induced Changes in Microstructure and Residual Stresses in Thermally Sprayed MnCoFeO4 Protective Coatings

The contribution comprises the investigation of the microstructure and residual stresses in thermally sprayed Mn1.0Co1.9Fe0.1O4.0 (MCF) protective coatings for interconnectors of SOFC stacks, deposited on ferritic steel Crofer 22 APU via atmospheric plasma spraying (APS). The coatings are designated to prevent Cr evaporation during high operation temperature of the SOFCs. The local microstructure, pore distributions and pore shapes, phase fractions, micro-hardness, Youngs’ modulus and residual stresses through the coating thickness were characterized in as-sprayed state and compared with longtime (10-100 h) heat-treated samples (700 and 850 °C). The results show that the long-term thermal aging treatment causes a successive high sintering of the coatings characterized by a reduction in pore density, by phase transformation from the metastable rock salt structure that gradually transformed to a spinel structure and by a slight relaxation of the process-induced tensile residual stresses in the coating. For SOFC application of the MCF coating, this indicates an improvement in the coatings integrity. During operation, a self-repair proceeds leading to dense and gas-proof coatings, while the mechanical properties are mainly retained.

Phenomenological Analysis of Thermo-Mechanical-Chemical Properties of GFRP during Curing by Means of Sensor Supported Process Simulation

Inherent process-induced deformations (PID) and residual stresses impede the application of composite parts. PID lead to a geometrical mismatch in assemblies and require subsequent work for tolerance compensation. Unknown residual stresses cause overweighted structures resulting from unnecessary high safety factors. To counteract the deformations, the tool design needs to be modified until the component geometry meets the specifications. This process is mostly carried out empirically and is time and cost intensive. To improve the efficiency of the development process, an in-deep comprehension of the manufacturing processes is mandatory. Therefore, experimental and simulation-based methods are increasingly applied and enhanced. The object of this work is to investigate the development of process-induced strains as well as the material behaviour during the manufacturing for a GFRP plate. The process-induced strains are monitored by optical fiber Bragg grating (FBG) sensors. The change of the material phases is detected by dielectric sensors. Furthermore, a detailed process simulation considering viscoelastic effects and reaction kinetics is performed. Finally, the measurements are correlated with the simulation data to validate the simulation approach. A very good correlation for both the reaction kinetics as well as the process-induced strains is observed.

A micromechanical-based finite element simulation of process-induced residual stresses in metal-CFRP-hybrid structures

Automotive lightweight design is a considerable measure to meet the worldwide need for reducing CO2 emissions. However, the lightweight potential of common materials like high strength steels or aluminium is limited. Hybrid materials allow to combine metals and CFRP in a manner to offset the drawbacks of every single material and reach an optimum of mechanical properties (Wang et al., 2016). Nonetheless, an essential shortcoming of hybrids are thermally induced residual stresses after cooling down from moulding temperature, driven by varying coefficients of thermal expansion and chemical shrinkage of the FRP.For a reliable prediction of the residual stress evolution in hybrid-structures during curing and after cooling down, a numerical homogenization technique of representative unit cells is proposed to calculate effective cure-dependent properties. Starting from a heterogeneous microstructure, a thermo-chemical-mechanical constitutive model for the curing process of the epoxy resin is presented and applied to two representative volume elements (RVE). The transition between the micro- and macro-scale is done by using a numerical homogenization framework. The effective cure-dependent properties predicted by the homogenization are used to simulate the curing-process and analyses the residual stresses of a metal-CFRP plate. The results are compared with experimental data obtained by the incremental hole drilling measurement.

The influence of work hardening and residual stresses on the fatigue behavior of high frequency mechanical impact treated surface layers

In the present study, the effect of process-induced residual stress and work hardening was separately analyzed to describe the fatigue life improvement by High Frequency Mechanical Impact (HFMI) treatment. For this purpose, blunt notched specimens made of mild steel S355J2+N and high strength steel S960QL were HFMI-treated under different mechanical pre-stress conditions. In that way, the effect of residual stress and work hardening could be separated. The generated residual stress states were measured by means of X-ray- and neutron-diffraction techniques. Based on the FE post processing the fatigue life was estimated using a damage mechanics evaluation with the consideration of the HFMI-induced residual stresses.

Identification of process-induced residual stresses in 3D woven carbon/epoxy composites by combination of FEA and blind hole drilling

Process-induced residual stresses in 3D woven carbon-epoxy composites are studied by blind hole drilling experiments interpreted with finite element (FE) modeling. It is assumed that residual stresses are primarily caused by the difference in thermal expansion coefficients of the constituents which are modelled as temperature-dependent linear elastic solids. The impact of residual stresses is quantified by drilling blind holes in the composite panels and mapping the resulting in-plane surface displacements by electronic speckle pattern interferometry. Mesoscale finite element models are used to correlate these surface displacements with the volumetric distribution of the residual stresses in the composite. This is done by determining the effective temperature drop ΔTeff that results in the same predictions for the surface displacements as experimentally measured. The effective temperature drop approach allows to use linear elastic models while approximately accounting for various nonlinear effects occurring in the material during processing. The models are also used to establish the sensitivity of the predicted results to the exact location of a hole and its depth.

Design optimization for filament wound cylindrical composite internal pressure vessels considering process-induced residual stresses

The manufacturing process has significant influence on the product quality of filament wound composite parts. The aim of this work is to provide an analytical and optimization tool to find the optimal winding parameters to obtain better mechanical performance of the filament wound composite internal pressure vessels accounting for the process-induced residual stresses. First, the winding process model is established to calculate the winding process-induced residual stresses based on the three-dimensional thick cylinder theory. Then, an analytical model is constructed to predict the cure-induced residual stresses considering the thermo-viscoelastic effects based on linear thermo-viscoelastic constitutive model. Moreover, the optimization model is established to maximum the failure pressure based on proposed winding and curing process model. A novel iterative algorithm is given to determine the failure pressure based on the Tsai-Wu failure criterion and vonmises strength theory. The winding and curing process models are verified by the results of FEA and other work. The simulation results show that it is feasible to find the optimal winding angels and tensions to improve the load capacity of the filament wound cylindrical composite internal pressure vessels with aid of the proposed process analysis and optimization model.

The influence of coverage for high frequency mechanical impact treatment of different steel grades

Abstract The High Frequency Mechanical Impact (HFMI) process is a comparably new, manually-driven machine hammer peening (MHP) process applied to increase the fatigue performance of metallic components. The HFMI-process is characterized by a large number of impacts, also when the tool was used with a high feed rate. This leads to a fast saturation and highly plastic deformation at the surface layer. Experimental investigations of the surface topography, near-surface residual stresses, and work hardening were performed on HFMI-treated, flat specimens made of S355J2, S690QL and S960QL steel grade to investigate the influence of the process parameters on the surface conditions. The residual stress state, experimentally determined by neutron diffraction, was supplemented by the numerical simulation of the HFMI-process. For this, an elasto-viscoplastic material model was implemented and calibrated with experimental test data. As a unified parameter for the process quality, a representative coverage value, defined as the number of impacts per reference surface area, was calculated for different sets of process parameters. This coverage value was also used to compare the numerical simulation and the experimental investigations by neutron diffraction techniques of the HFMI-induced residual stress field. It is shown, that high coverage has a strong effect on the depth of indentation and in the work hardening effect for mild and medium steel grade. Only slight changes of the process-induced residual stresses could be observed with an increasing coverage.

A numerical prediction of residual stress for a thin-walled part with geometrical features fabricated by GMA-based additive manufacturing

PurposeAn accurate prediction of process-induced residual stress is necessary to prevent large distortion and cracks in gas metal arc (GMA)-based additive manufactured parts, especially thin-walled parts. The purpose of this study is to present an investigation into predicting the residual stress distributions of a thin-walled component with geometrical features.Design/methodology/approachA coupled thermo-mechanical finite element model considering a general Goldak double ellipsoidal heat source is built for a thin-walled component with geometrical features. To confirm the accuracy of the model, corresponding experiments are performed using a positional deposition method in which the torch is tilted from the normal direction of the substrate. During the experiment, the thermal cycle curves of locations on the substrate are obtained by thermocouples. The residual stresses on the substrate and part are measured using X-ray diffraction. The validated model is used to investigate the thermal stress evolution and residual stress distributions of the substrate and part.FindingsDecent agreements are achieved after comparing the experimental and simulated results. It is shown that the geometrical feature of the part gives rise to an asymmetrical transversal residual stress distribution on the substrate surface, while it has a minimal influence on the longitudinal residual stress distribution. The residual stress distributions of the part are spatially uneven. The longitudinal tensile residual stress is the prominent residual stress in the central area of the component. Large wall-growth tensile residual stresses, which may cause delamination, appear at both ends of the component and the substrate–component interfaces.Originality/valueThe predicted residual stress distributions of the thin-walled part with geometrical features are helpful to understand the influence of geometry on the thermo-mechanical behavior in GMA-based additive manufacturing.

Analytical model for the winding process-induced residual stresses of the multilayered filament wound cylindrical composite parts

The residual stresses due to winding tensions have significant influence on the product quality of the filament wound composite parts. In this paper, an analytical model is proposed to calculate the winding process-induced residual stresses for the multilayered filament wound composite parts with metal linear based on the three-dimensional thick-walled cylinder theory. In the modeling of winding process, the contribution of the current outmost winding tension to the residual stresses of the previously wound composite layers is first calculated and then the total residual stresses is obtained based on the elastic superposition principle. Moreover, the winding tensions are modified for equal residual stress in fibers based on the inverse iteration principle. Compared with the results in other work and obtained by finite element method, it shows that the proposed analytical model is of high accuracy and can be used to calculate the residual stresses due to winding tensions for the multilayered filament wound composite parts. The expected winding tensions for the equal residual fiber stresses can be quickly obtained after several iterations. Moreover, the effects of thickness of metal linear on the winding tensions distributions are further investigated based on the proposed model and the iterative algorithm.

On a simultaneous use of fiber-Bragg-gratings and strain-gages to determine the stress-free temperature [formula omitted] during GLARE manufacturing

Process-induced residual stresses in fiber-metal laminates depend on the stress-free temperature Tsf, which refers to the moment during the manufacturing process where a resilient mechanical connection between metal and fiber-reinforced epoxy layers is established. Tsf is often determined reversely by evaluating distortions of FML specimens with asymmetric layup.The present paper focusses on an in situ strain-measurement-based procedure, which allows for the direct determination of the stress-free temperature during manufacturing. The paper reports on a combined application of strain-gage and fiber-Bragg sensors, during manufacturing of a GLARE laminate in an oven environment. The experiment setup allowed for the simultaneous measuring of strains in both constituents along the entire fabrication process. The in situ technique gives insight into mechanical interactions of the GLARE constituents during processing. As will be shown, Tsf can be identified from the monitored strains and therefrom, the material’s residual stress state is quantified.

On the optimal design of manufacturing-induced residual stresses in filament wound carbon fiber composite cylindrical shells reinforced with carbon nanotubes

Process-induced residual stresses occur in composite structures composed of dissimilar materials. As these residual stresses could result in fracture, their consideration when designing composite parts is necessary. In this research, the residual stresses behavior is characterized in terms of three manufacturing features including ‘environmental’, ‘materialistic’ and ‘dimensional’ features. In experimental part, after the fabrication of carbon nanotubes (CNTs) reinforced composite shells, the slitting technique is employed to measure the released strains. Then, the elements of the stiffness matrix are obtained accomplishing an extensive computational modeling in ANSYS commercial software. The obtained results of experimental and computational parts are correlated using a MATLAB programming code, and residual stresses are calculated by calibration factors. Additionally, a statistical analysis is carried out to analyze the influence of mentioned parameters in overall scale. The experimental results had a good agreement with the analytical ones. Finally, this study highlights the complexity and the multifaceted characteristic of residual stresses development in terms of mentioned parameters in CNT-reinforced carbon fiber composite shells.

A Simplified Computational Strategy Focused on Resin Damage to Study Matrix Cracking of The Cross-Ply Laminates Under Uniaxial Tension Load.

Transverse cracking is probably the first and most dominant mode of damage in composite materials. In this paper, transverse cracking of cross-ply [02/90n]s (n = 2,3,4) laminates under uniaxial tension load was studied by means of experimental and numerical methods. In the numerical simulations, a simplified computational strategy only focusing on the damage of the resin was proposed and the mechanical response of the cracking cross-ply laminates was studied by finite element analysis of multi-scale representative volume elements (RVEs). In the RVEs, the longitudinal 0° plies were represented by macro-scale, homogeneous, orthotropic elastic solids while the 90° plies were modeled by the discrete fibers and the surrounding matrix resin in micro-scale. Based on researching the critical longitudinal mechanical strain which initiates the cracks, the in-situ transverse ply strength and the stiffness degradation of the transverse plies, the simplified computational strategy proposed was proven correct. In addition, the crack initiation is sensitive to residual stress. Higher process-induced residual stress levels are dangerous to laminates, leading to early crack initiation.

Self-propagating combustion of sputter-deposited Al/CuO nanolaminates

The complex Al/CuO self-propagating reaction involving multi-phase and multi-species dynamics was studied in order to investigate the very high flame temperature around the vaporization temperature of alumina, even under a neutral environment. Experiments were performed on different sputter-deposited Al/CuO multilayers coupling optical spectroscopy with high speed camera measurements. The clear presence of both AlO and Al signatures in gas phase suggests that the redox reaction starts in the bulk nanolaminate, which then rapidly tear off the substrate to continue burning in a heterogeneous (condensed and gas) phase in the environment. The flame temperature increases with the stoichiometry but is independent of the bilayer thickness. In addition to the confirmation of the effects of stoichiometry and the bilayer thickness on the characteristics of the self-propagating reaction, the predominant role of process-induced residual stress was highlighted for the first time; it can lead to an early disruption of the multilayer long before the completion of the redox reaction.

An experimental study of residual stress and direction-dependence of fatigue crack growth behaviour in as-built and stress-relieved selective-laser-melted Ti6Al4V

Selective-laser-melting (SLM) is a powder-bed fusion additive-manufacturing process that has the potential to deliver three-dimensional complex parts with mechanical properties comparable or superior to parts produced via traditional manufacturing using cast and wrought alloys. Concerns for metallic parts built via SLM are the process-induced residual stresses, and anisotropic mechanical properties. This paper investigates the effect of residual stresses on the fatigue crack growth rate of SLM Ti6Al4V in as-built and stress-relieved conditions. Neutron diffraction and the contour method are employed to measure residual stresses in compact-tension samples. Neutron diffraction results are in good agreement with the contour method. It was found that tensile stresses are present at the notch root and the free edge areas, and compressive stress is seen in the middle of the sample. The tensile stresses in the as-built condition resulted in a higher fatigue crack growth rate. After stress relieving by heat treatment, the tensile residual stress diminished by around 90%, resulting in decreased crack growth rate. The build direction was seen to affect the crack growth rate, although the trend was different between the as-built and stress-relieved conditions.

An Improved Analytical Solution for Process-Induced Residual Stresses and Deformations in Flat Composite Laminates Considering Thermo-Viscoelastic Effects.

Dimensional control can be a major concern in the processing of composite structures. Compared to numerical models based on finite element methods, the analytical method can provide a faster prediction of process-induced residual stresses and deformations with a certain level of accuracy. It can explain the underlying mechanisms. In this paper, an improved analytical solution is proposed to consider thermo-viscoelastic effects on residual stresses and deformations of flat composite laminates during curing. First, an incremental differential equation is derived to describe the viscoelastic behavior of composite materials during curing. Afterward, the analytical solution is developed to solve the differential equation by assuming the solution at the current time, which is a linear combination of the corresponding Laplace equation solutions of all time. Moreover, the analytical solution is extended to investigate cure behavior of multilayer composite laminates during manufacturing. Good agreement between the analytical solution results and the experimental and finite element analysis (FEA) results validates the accuracy and effectiveness of the proposed method. Furthermore, the mechanism generating residual stresses and deformations for unsymmetrical composite laminates is investigated based on the proposed analytical solution.

Residual stresses in parts manufactured by near-net-shape-blanking

Near-net-shape-blanking processes enable to manufacture functional surfaces in a single manufacturing step, which offers great potential in the production of gears, for example. Among these processes are fineblanking, blanking with a small die clearance, blanking with a v-ring, precision blanking with and without a blank holder. All of these are able to produce almost identical cut-surfaces, this means a high proportion of clean-shear almost without a fracture zone, but plastic deformation in the shearing zone varies due to the different tool setups. Therefore, the residual stresses in the produced parts, which are highly relevant for the fatigue behavior of shear-cut gears, vary as well. These process-induced residual stresses are addressed in this paper. First, sheet metal strips made of S355MC (1.0976) were subjected to a stress relief heat treatment to get a defined and low initial residual stress state. Afterwards, a circular hole was cut in the strips by the five different blanking processes. The cut surfaces of the thus produced parts were measured as well as the hardness. The residual stresses in tangential and axial direction were measured for a depth of up to 0.34 mm by X-ray diffraction. Finally, a comparison between the different manufacturing techniques regarding the residual stresses is carried out.

Utilizing numerical models to identify process-induced residual stresses in 3D woven carbon/epoxy composites

A combined experimental-numerical procedure is proposed to evaluate residual stresses in 3D woven carbon/epoxy composites. It is assumed that residual stresses are caused by the difference in thermal expansion coefficients of the constituents. The impact of residual stresses is quantified by drilling blind holes in the composite panels and mapping the resulting in-plane surface displacements by digital image correlation and electronic speckle pattern interferometry. Then mesoscale finite element models of the composite are used to determine the effective temperature drop ΔTeff that results in the same predictions for hole drilling. The local distributions of residual stress are given by the finite element simulations of the cooling by this temperature drop ΔTeff

Process-induced residual stress of variable-stiffness composite laminates during cure

One of the most important issues for designing variable-stiffness composite structures reflects on determination of the steered fiber paths. In this paper a linear variation of steered fiber curves and the change rule of the fiber angles were presented to create the mathematical model of variable-stiffness composite laminates. Compared to the straight-fiber laminate, the reference path with linearly changed angles leads to higher mechanical strength and more design freedoms. A novel methodology was developed to predict the distributions of process-induced residual stresses during cure. A three-dimensional (3D) thermochemical model of the curing process was established and the mechanical responses during cure were evaluated coupled with the results of thermochemical analyses. The distributions of the temperature and the degree of cure were obtained. The process-induced residual stresses were calculated using ABAQUS. The resin modulus was determined using the cure hardening instantaneous linear elastic (CHILE) model. The cure kinetic process was simulated using Kamal model for AS4/3501-6 prepregs. The results show that the process-induced residual stresses of variable-stiffness composite panels are reduced with increasing the end angle of the present fiber path during cure.

A heat-balance method for autoclave process of composite manufacturing

In composite manufacturing, large composite parts usually exhibit high heating gradients during the autoclave process, which may intensify the process-induced residual stresses and deformations. As the thermal behavior of molds is of crucial importance to the curing performance of composites, a heat-balance method is presented to reduce the heating rate on overheated areas of molds, thus providing a more homogeneous curing process. The method is based on a local-isolation structure installed under the mold plate, which is used to change the local heat transfer coefficient of the mold. In the local-isolation structure application, an optimization process combining numerical simulations with a greedy genetic algorithm is developed to find the optimal layout and geometry of local-isolation structure in molds. The optimization results suggest that more uniform heating condition and more synchronous curing process can be achieved with the optimal design of local-isolation structure. In the case of a typical mold for C spar component, the maximum temperature difference in the composite part is reduce by 45.69%, while the maximum difference in degree of cure is decreased at a rate of 40.16%.