Process-induced Residual Stresses Research Articles

Optimization of process-induced residual stresses in automated manufacturing of thermoset composites

The present study investigates the effect of automated fiber placement process on the induced residual stresses in polymer-matrix composites. The robotic fiber placement is utilized to fabricate a total of 150 flat specimens. The specimens are then cured inside an autoclave. The incremental hole-drilling method is applied to estimate the maximum stresses induced in the laminates. Further, an optimization process is conducted based on the response surface method to minimize the generated stresses, while maintaining the highest production rate. A mathematical modeling of the relationship between the process parameters and the residual stresses is generated. The statistical analysis (ANOVA test) and sensitivity analysis are conducted to evaluate the generated model. The results show considerable effect on the stresses when changing the manufacturing process parameters due to the existence of in-situ curing and the change in material porosity. The optimization strategy could significantly decrease the stresses with maximizing the fabrication speed.

Interrelation between Microstructure and Residual Stresses for Low-Pressure Carburizing of Steel AISI 5120 under Defined Process Parameter Variation

AbstractLow-pressure carburizing (LPC) is a recipe-controlled process for surface layer hardening. These recipes are mainly based on experience and contain the process parameters used to achieve the desired hardening result. The process parameters influence the chemical gradients which have set in the boundary layer, the local microstructure and the depth distribution of the process-induced residual stresses. Within the scope of this work, a systematic parameter study and advanced characterization was carried out to quantify the influence of these process parameters on the resulting material state. The varied parameters include the carburizing temperature, the hardening temperature, the quenching rate as well as the number of repetitions and durations of the carburizing cycles’ steps. The results obtained should help to extend the fundamental process understanding of the LPC process. The analyses showed that the retained austenite content and its depth profile change significantly for certain process parameter variations, reaching contents of up to 45 vol% in the near-surface region. The differences regarding the residual stress states of the case-hardened samples can first and foremost be related to the formation of varying depth distributions of the retained austenite.

Study of the Automated Process-Induced Residual Stresses in Multi-Axis Laminates

The integrity of the composite structures is affected by the manufacturing conditions. Controlling the ply orientation during the manufacturing process plays an important role in defining the induced residual stresses inside the laminate. This work evaluates the generated residual stresses inside multi-axis laminates of carbon fiber reinforced polymers. The samples are manufactured using the robotic fiber placement technique. The residual stresses are measured through the incremental hole-drilling method. Finite element modeling is used to provide the calibration process of the strain measurements. Additionally, microstructure analyses have been performed to show the configuration of the manufactured samples. The effect of the laminate configurations and plies orientations are investigated and compared. The layers’ orientations have shown a considerable effect on the generated stresses.

Residual stress constrained self-support topology optimization for metal additive manufacturing

This paper proposes a topology optimization method to design self-support structures for metal additive manufacturing. Constraining the process-induced residual stresses is considered with this method to avoid part failures of cracking, delamination, or warpage. Specifically, an inherent strain method-based finite element analysis model is presented to simulate the complex mechanical behavior during the additive manufacturing process. Then, according to the inherent strain-based fast simulation model, the residual stress constrained self-support topology optimization algorithm is formulated, and more importantly, the critical sensitivity information is derived through the adjoint analysis and validated with the finite difference method. Finally, the proposed method is applied to several 2D benchmark examples to demonstrate the effectiveness on residual stress and distortion control. The influences of different RAMP interpolation parameters, residual stress upper limits, and printing directions on the residual stress-constrained design effect are thoroughly explored. A comprehensive discussion is presented at the end to summarize the numerical phenomena.

Understanding the Origin of the Low Cure Shrinkage of Polybenzoxazine Resin by Computational Simulation

Thermoset resin-based composite materials are widely used in the aerospace industry, mainly due to their high stiffness-to-weight and strength-to-weight ratios. A major issue with the use of thermoset resins in fiber composites is the process-induced residual stresses that are formed from resin chemical shrinkage during the curing process. These residual stresses within the composite material ultimately result in reduced durability and residual deformations of the final product. Polybenzoxazine (PBZ) polymer resins have demonstrated near-zero volumetric shrinkage during the curing process. Although the low shrinkage of PBZ is promising in terms of reduced process-induced residual stresses, little is known about the physical causes. In this work, Molecular Dynamics (MD) simulations are performed with a reactive force field to predict physical properties (gelation point, evolution of network, mass density, volumetric shrinkage) and mechanical properties (Bulk modulus, Shear modulus, Young's modulus, Poisson's ratio, Yield strength) as a function of crosslinking density. The MD modeling procedure is validated herein using experimental measurements of the modeled PBZ resin. The results of this study are used to provide a physical understanding of the zero-shrinkage phenomenon of PBZ. This information is also a critical input to future process modeling efforts for PBZ composites.

A hybrid experimental and numerical technique for evaluating residual strains/stresses in bonded lap joints

This paper investigates the processing-induced residual strains developed in the adhesive layer of a single lap joint consisting of glass-fiber reinforced polymer (GFRP) adherends bonded with a thermoplastic adhesive , Acrylonitrile Butadiene Styrene (ABS). High-resolution optical fiber sensors were embedded inside the adhesive for in-situ monitoring of strains and temperature during the processing and cooling cycle. The experimental fiber-optic strains are the total strains experienced by the fiber and do not represent the strains in the adhesive because of interfacial effects between the fiber and the adhesive. Hence, a detailed finite element analysis (FEA) model was developed that included the optical fiber within the bondline . The experimentally measured optical fiber strains were first used to calibrate the FE models at each cooling rate. These FE models were then used to predict residual strains and stresses in the bulk adhesive . The FE models showed that the strains calculated along the optical fiber provided a close approximation to the actual strains experienced by the ABS adhesive around it at the center of the overlap region. The experimental technique, with appropriate post-processing, can be used to get a reliable first-hand approximation of the residual strains induced in the bulk adhesive during curing/processing. A critical conclusion from the FE investigation is that, for all cooling rate cases, the von-Mises residual stresses generated inside the bulk ABS adhesive ranged between 65 and 80% of its tensile strength , which showcases the importance of processing-induced residual stresses and their effects on safety-critical components like bonded joints. The hybrid FE-experimental approach used in this work can be extended to other joint geometries and material configurations.

Experimental and Finite Element Analysis of the Tensile Behavior of Architectured Cu-Al Composite Wires.

The present study investigates, experimentally and numerically, the tensile behavior of copper-clad aluminum composite wires. Two fiber-matrix configurations, the conventional Al-core/Cu-case and a so-called architectured wire with a continuous copper network across the cross-section, were considered. Two different fiber arrangements with 61 or 22 aluminum fibers were employed for the architectured samples. Experimentally, tensile tests on the two types of composites show that the flow stress of architectured configurations is markedly higher than that of the linear rule of mixtures’ prediction. Transverse stress components and processing-induced residual stresses are then studied via numerical simulations to assess their potential effect on this enhanced strength. A set of elastic-domain and elastoplastic simulations were performed to account for the influence of Young’s modulus and volume fraction of each phase on the magnitude of transverse stresses and how theses stresses contribute to the axial stress-strain behavior. Besides, residual stress fields of different magnitude with literature-based distributions expected for cold-drawn wires were defined. The findings suggest that the improved yield strength of architectured Cu-Al wires cannot be attributed to the weak transverse stresses developed during tensile testing, while there are compelling implications regarding the strengthening effect originating from the residual stress profile. Finally, the results are discussed and concluded with a focus on the role of architecture and residual stresses.

Effects of process-induced residual stress and geometric characteristics on pressure-resisting capability of corrugation in primary barriers of liquefied natural gas carriers

The primary barrier of a liquefied natural gas (LNG) carrier generally has a corrugated shape, so as to withstand thermal shrinkage at cryogenic temperatures. Over the years, studies have been actively performed to improve the performance of the primary barrier through structural analysis. However, when analyzing the pressure-resisting capability of the corrugation, previous studies have not considered the residual stress generated during the forming process of the corrugation. This can lead to an overestimation of the pressure-resisting capability of the corrugation. In this work, the pressure-resisting capabilities of the corrugation were evaluated by considering the residual stress generated during the deep drawing process of the corrugation; the initial buckling mode was also considered through a linear buckle analysis based on a finite element analysis. Universal hydraulic test equipment was developed for measuring the pressure-resisting capabilities of corrugations of various sizes. By comparing the analytical results from the experiments, it was confirmed that the pressure-resisting capability of the corrugation decreased by 16% when the residual stress and initial buckling mode were considered simultaneously. We believe that this study can be used as a reference to present guidelines when setting design parameters to improve the pressure-resisting capability of a primary barrier.

Effect of Manufacturing on the Transverse Response of Polymer Matrix Composites.

The effect of residual stress build-up on the transverse properties of thermoset composites is studied through direct and inverse process modeling approaches. Progressive damage analysis is implemented to characterize composite stiffness and strength of cured composites microstructures. A size effect study is proposed to define the appropriate dimensions of Representative Volume Elements (RVEs). A comparison between periodic (PBCs) and flat (FBCs) boundary conditions during curing is performed on converged RVEs to establish computationally efficient methodologies. Transverse properties are analyzed as a function of the fiber packing through the nearest fiber distance statistical descriptor. A reasonable mechanical equivalence is achieved for RVEs consisting of 40 fibers. It has been found that process-induced residual stresses and fiber packing significantly contribute to the scatter in composites transverse strength. Variation of ±5% in average strength and 18% in standard deviation are observed with respect to ideally cured RVEs that neglect residual stresses. It is established that process modeling is needed to optimize the residual stress state and improve composite performance.

Mesoscale Process Modeling of a Thick Pultruded Composite with Variability in Fiber Volume Fraction.

Pultruded fiber-reinforced polymer composites are susceptible to microstructural nonuniformity such as variability in fiber volume fraction (), which can have a profound effect on process-induced residual stress. Until now, this effect of non-uniform distribution has been hardly addressed in the process models. In the present study, we characterized the distribution and accompanying nonuniformity in a unidirectional fiber-reinforced pultruded profile using optical light microscopy. The identified nonuniformity in was subsequently implemented in a mesoscale thermal–chemical–mechanical process model, developed explicitly for the pultrusion process. In our process model, the constitutive material behavior was defined locally with respect to the corresponding fiber volume fraction value in different-sized representative volume elements. The effect of nonuniformity on the temperature and cure degree evolution, and residual stress was analyzed in depth. The results show that the nonuniformity in fiber volume fraction across the cross-section increased the absolute magnitude of the predicted residual stress, leading to a more scattered residual stress distribution. The observed gradient promotes tensile residual stress at the core and compressive residual stress at the outer regions. Consequently, it is concluded that it is essential to take the effects of nonuniformity in fiber distribution into account for residual stress estimations, and the proposed numerical framework was found to be an efficient tool to study this aspect.

Numerical model development to predict the process-induced residual stresses in fibre metal laminates

The thermomechanical interactions between the different materials constituting a hybrid laminate make residual stresses arise during the manufacturing process of such laminate. In turn, the residual stresses make the laminate deform as it is extracted from the mould. Cure induced deformation represent a problem for the assembly step, since additional operations are required for allowing the joining of the different components. Therefore, a tool suitable for the calculation of residual stresses and deformation is needed in order to predict them and to implement the necessary remedies. In the present work, a numerical model for the determination of residual stresses is presented and validated through experimental tests. The case study was a notched laminate, in order to emphasize the deformation without altering the thermal cycle experienced in the material. Both numerical and experimental runs highlighted a higher deformation near the vertex of the laminate.

Adjusting residual stresses by flexible stress superposition in incremental sheet metal forming

Process-induced residual stresses significantly influence the mechanical properties of a formed component. A polymer pad is used as a flexible die in two-point incremental forming to induce compressive residual stresses in the component during the forming process. Experimental and numerical results illustrate the influence of compressive stress superposition on the component properties. It is shown that the active support, using a geometry-independent polyurethane die, causes beneficial compressive residual stresses on the tool side compared to the tensile residual stresses induced by the single-point incremental forming process without such a supporting die.

Reducing processing-induced residual stresses in SAE 4140 steels laser welded using modulated power emission

Residual stresses are a concern in weld beads since they increase the susceptibility to cracks and fatigue failure. Conventional methods to reduce these stresses are based on post-processing or the application of additional tools in the process, which increases the processing cost and time. In this context, this study aimed to measure the effect on residual stresses in welds of modulating the laser power emission applying two different wave shapes. The modulation method allows the energy delivery of the beam to be controlled by adding power steps, which changes the heat absorption and distribution by the molten pool. Square and step-shaped power modulation waves were used, through a YLS 10,000 fiber laser source (IPG Photonics) with a maximum power of 10 kW, in the welding of SAE 4140 steel of high hardenability. To measure residual stress fields in a specific critical point near the weld bead, the measuring technique combines the hole drilling method and speckle interferometry (instead of traditional strain gauges) for the identification of the displacements around the hole. The results showed that a reduction in the stress magnitudes was obtained for the step-shaped modulation wave, in the order of 77 MPa, which represents a reduction of 16% in comparison with the continuous wave with an increase of 37% in penetration depth, which is appreciable for applications in thicker plates.

Rapid prototyping of aircraft canopy based on the incremental forming process

Today, the incremental forming process is widely used as a rapid prototyping method. One of the strategic polymer parts in the aviation industry is the canopy of various aircrafts. Polycarbonate sheets with suitable mechanical, thermal, chemical, and optical properties are used in the manufacturing of the latest integral canopies. Conventional processes are not cost-effective for low-volume production of polymer products, particularly for spare parts. In this paper, rapid prototyping of a geometry similar to integral canopies is investigated using the incremental forming process of transparent polycarbonate sheets. To this aim, the apparent transparency and the geometric accuracy of the final parts were considered as the main concerns. In single-point incremental forming experiments, the effect of tool rotational velocity on apparent transparency and the effect of toolpath strategy on geometric accuracy of the samples were investigated. Three toolpath strategies including raster, spiral from outside, and spiral from inside were applied. According to the FE results, applied toolpath strategies did not noticeably affect the thickness distribution for the critical section. Post-heating treatment was also used to release process-induced residual stresses and reduce the spring back. The use of a non-rotating tool as well as a mechanical–chemical surface polishing improved the final finishing and transparency of samples. In addition, by the employment of the raster strategy, higher geometric accuracy was achieved compared with the other two strategies. The forming of samples with appropriate apparent transparency and geometric accuracy promises to apply the rapid prototyping of aircrafts canopy using the incremental forming processes.

Parameter Identification for Thermo-Mechanical Constitutive Modeling to Describe Process-Induced Residual Stresses and Phase Transformations in Low-Carbon Steels

Reinforcing steel bars (rebars) are widely manufactured using the Tempcore™ process. Several studies have been conducted analyzing the effect of the heat treatment route on the strength and corrosion resistance of rebars, but knowledge of its effects on the residual stresses of the finished product are largely lacking. This paper presents experimental investigations to identify the material parameters necessary to simulate the Tempcore™ process using thermo-elasto-plastic constitutive modeling in order to study the generation of residual stresses during the manufacturing process. Mechanical parameters such as yield strength at elevated temperatures and elastic constants were determined experimentally. A continuous cooling transformation diagram needed to model the phase transformations was also identified and is presented here. Residual stress distributions in the surface region of the rebar were characterized using X-ray diffraction. Further characterizations of microstructure, chemical composition, and hardness were carried out. The constitutive modeling approach for the numerical simulation is briefly described for which the experimentally determined parameters are required as input.

Residual stress analysis of gray cast iron manufacturing processes

ABSTRACT Much research has focused on residual stress detection, analysis and elimination for alloy materials. There is however limited work considering residual stress in gray cast iron materials (HT200, 250, 300, 350) which are commonly used in heavy machine tool components. Herein an experimental program is undertaken to quantify the influence of typical machine tool production processes (casting, treatments, machining) on component residual stress. In addition, for a select component element (a guideway mounting surface), an extended measurement program tracks residual stress and surface deviations over a two-year period, quantifying the time-dependent behavior of production-induced stresses during initial in-service conditions. The experimental program establishes the relationship between material carbon content and component residual stress, with greater carbon content resulting in less residual stress. For the aging treatments (heating, vibration, natural aging), the program quantifies and ranks the achievable stress reductions (68.74%, 36.83%, 29.1%, respectively). Finally, the extended measurement program demonstrates that within 2 months of production, greater than 99% of process-induced residual stress has been released and surface deviation is less than 0.5 μm per month.

Gradient optimization of transversely graded Ti-TiB structures for enhanced fracture resistance

Metal-ceramic functionally graded materials (FGM) represent a class of composites in which the variation of ceramic (or metallic) content over a definable geometrical length can lead to a combination of desirable physical, mechanical, and thermal properties. These materials are often processed in high-temperature conditions. The asymmetric and variable thermo-physical properties along gradient directions in metal-ceramic FGMs lead to the development of process-induced residual stresses. Since these materials are primarily used as load-bearing components, the processing-induced residual stresses can be superimposed on in-service stresses and lead to premature failure. The present study aims to analyze the concurrent effects of gradation on processing-induced residual stresses and the fracture response of cracked transversely graded structures, that are FGMs with gradients parallel to the crack front. The analytical approach proposed in this work facilitates the identification of optimal gradients that lead to enhanced strength and fracture resistance.

Effect of Fiber Orientation and Residual Stresses on the Structural Performance of Injection Molded Short-Fiber-Reinforced Components

To optimize the strength design of short-fiber injection molding components, a three-dimensional flow and structural analysis simulations were employed. In particular, these Finite Element Analyses (FEA) were based on orthotropic, linear and elastic models including process-induced residual stresses. Through an appropriate interface, the results of injection molding analysis of 35 wt.% fiber reinforced polyphthalamides (PPA) manifold block used in hot-water fluid engineering application are transferred to the structural analysis software. In particular, Autodesk Moldlfow software was used to predict the fiber orientation and the in-cavity residual stresses considering the flow kinetics and moulding parameters. The Abaqus interface for Moldflow was used to translate this data into a form that can be used for the structural analysis. Experimental tests were carried out on the injection moulded component in order to evaluate the internal burst pressure values. The burst strength of the component was then compared with those predicted by the numerical analysis. The computations are found in good agreement with the experimental results.

Numerical investigation on the influence of the electro-resistance welding pipe manufacturing process on the local variation of the yield strength of the pipe material

In the present research work, a finite element model of the electro-resistance welding pipe forming process chain is developed using the ABAQUS/Explicit software. The forming process, which is composed of 22 tandem roll stations, has been fully modeled in the developed finite element simulation. In order to account for the Bauschinger effect on the pipe material properties as a consequence of the loading and the unloading during the process, a non-linear kinematic hardening model has been utilized in all the proposed finite element simulation models. The constants for the non-linear kinematic hardening model were estimated by means of cyclic experiments on the K55 steel pipe material. In order to properly simulate the electric arc welding (electro-resistance welding) operation, the ABAQUS welding interface has been utilized to account for the joining between the two edges of the formed pipe as well as to assess the influence of the welding-induced temperature field on the residual stresses on the pipe material. The sizing operation, which is the final station of the electro-resistance welding process, has been also accounted in the developed finite element method model and is composed of six tandem rolls. To export and import the results between two different modules, a mapping strategy has been utilized and allowed exporting the element results, in terms of stress, strain, and temperature, and importing them into the following simulation module. Finally, in order to estimate the influence of each process station on the yield strength of the material, a finite element simple tension test simulation has been implemented in ABAQUS/Static, mapping the results of each station on the tensile specimen. This mapping operation allowed to estimate the yield stress of the material after each of the three process stations, a consequence of the residual stresses present in the material, and has been carried out on eight circumferential locations around the pipe, evenly spaced with a 22.5° angle. The model has been validated by comparing the geometrical results, in terms of average pipe diameter and thickness, obtained from the finite element model with those of the relevant industrial production, showing deviations equal to 1.25% and 1.35% (forming) and 1.29% and 1.43% (sizing), respectively, proving the reliability of the proposed process chain analysis simulation. The results will show how the process-induced residual stresses arising on the pipe material make the material yield strength to vary from station to station as well as having different values along the circumferential direction of the pipe.

Multiscale numerical and experimental investigation into the evolution of process-induced residual strain/stress in 3D woven composite

The process-induced residual strain and stress have a significant impact on the forming quality and service performance of 3D woven composites (3DWC). A multiscale model of 3DWC is developed to predict the residual strain and stress after the manufacturing process. Representative volume element at fiber scale and yarn scale are developed according to the geometric characteristics of 3DWC, and the modulus-development model is developed with respect to finite element based micromechanics method. An equivalent temperature load method is proposed to develop the cure shrinkage strain model. A thermal-chemical-mechanical coupling analysis of 3DWC curing process is carried out by integrating the abovementioned models, and the evolutions of temperature, degree of cure and residual strain/stress are obtained. Fiber Bragg Grating sensors are hybridized into the fabrics before Resin Transfer Molding (RTM) processing. The signal from sensors during RTM shows that the residual strain evolution is in good agreement with our modeling prediction.