Process-induced Distortions Research Articles

Experimentally validated modeling of the temperature distribution and the distortion during the Fused Filament Fabrication process

This paper presents the numerical approach to compute process-induced temperature distribution and distortion in Polylactic Acid (PLA) parts manufactured using the Fused Filament Fabrication (FFF) process. Thermal simulations are performed first to compute the time-dependent temperature distribution which is then input as solution-independent boundary condition into a mechanical simulation to compute warpage of the manufactured part. The developed procedure used experimentally measured process parameters and was experimentally verified at each step of the simulation. The thermal step in the simulation workflow was experimentally validated using thermography yielding quantitative predictions for 25 out of 27 considered points with the highest accuracy of <1°C. To verify mechanical step, the computed process-induced distortions were compared against those measured with an optical 3D scanner. The developed procedure yielded qualitative and, in some cases, quantitative predictions of process-induced geometrical distortions.

Experimental and numerical investigation of tool-part interaction on the process-induced distortions in composite structures

The tool-part interaction was experimentally carried out using FBG sensors adhesively bonded to the thin tool in a tool-part assembly, and corresponding interfacial properties parameters were theoretically quantified according to the acquired data and shear-lag theory. Subsequently, these derived parameters were adjusted and fed into FE model of C-shaped composite structure cured on the aluminium tool to numerically predict its spring-in, in company with experimental validation. Meanwhile, two comparative simulations neglecting tangential friction and tool expansion were provided to clarify the significance of tool-part interaction. The results from the assembly show that the sticking condition mainly exists until interfacial shear stress reaches a critical value at a certain time in the cool-down stage, after which the sliding friction dominates. The good agreement of spring-in between experiment and simulation provides a favourable support for the numerical simulation scheme, and comparative simulations reveal that tangential friction contributes to spring-in but the tool expansion alleviates it.

Analytical solutions for process-induced spring-in of U-shaped composite parts

Analytical solutions for predicting spring-in angles of U-shaped composite parts generating in rubbery and glassy states are proposed. In the newly developed analytical solutions, the process-induced distortions of web and flange are mathematically converted to spring-in angles within the scope of small deformation theory, and then corresponding analytical expressions of web, corner, flange and total spring-in are separately derived. These new solutions can quantitatively give the correlation of web, corner, flange and total spring-in with material properties-related and geometrical configuration-dependent parameters, and evaluate the spring-in angles of multi-angle ply composite parts just using lamina properties. The reliability of newly proposed analytical solutions is checked by the comparison between the analytically calculated and experimentally measured spring-in angles for unidirectional and quasi-isotropic U-shaped composite parts. Meanwhile, numerical simulations on the U-shaped composite parts with various geometries and layups also provide a favourable agreement with analytical results.

A simulation framework for predicting process-induced distortions for precise manufacturing of aerospace thermoset composites

The choice of a suitable material model to simulate process induced distortions is important in order to achieve a right first time approach in the design of new moulds. This work investigates the ability of the Cure Hardening Instantaneously Linear Elastic (CHILE) model and a linear viscoelastic material model to predict process induced distortions of an aerospace composite frame. The material models were coupled with a cure kinetics model and a chemical shrinkage model in order to capture the multi-physics phenomena that take place during the curing process. The modelling of the viscoelastic behaviour of the resin is performed with the use of a generalized Maxwell model. A novel methodology is applied for shifting the relaxation times of the composite based on its temperature and degree of cure. It is shown that the viscoelastic model more accurately predicts the measured distortions due to its ability to account for stress relaxation.

Prediction and compensation of process-induced distortions for L-shaped 3D woven composites

A physics-based numerical model for prediction and compensation of the process-induced distortion of 3D woven composite (3DWC) is presented. A new tool-part interaction modelling method for the Resin Transfer Molding process is developed, in which the nonlinear material properties and non-thermoelastic distortion are taken into account. Flat and L-shaped 3DWC parts were fabricated and the process-induced distortions were evaluated. Adopting the input values determined by the warpages of flat parts, the spring-in of L-shaped parts predicted by the model is consistent with the experimental measurements. Based on the model, it is found that taking the warpage/spring-in of the uncompensated parts as the compensation value will result in a deviation between the manufactured parts and the nominal shape. An improved comprehensive compensation procedure is proposed and applied to an L-shaped 3DWC part. The result shows that the dimensional precision of the part can be significantly improved by the compensation process.

Rapid prediction and inverse design of distortion behaviors of composite materials using artificial neural networks

If the process‐induced distortions (PIDs) of asymmetrical laminates can be predicted accurately and tailored at the early design stage, the production of curved panels from flat molds could be an attractive technique in a cost‐driven production environment. A data‐driven computational methodology which integrates the finite element method (FEM) and artificial neural network (ANN) is presented to rapidly predict the maximum PID and to perform high‐throughput screening of thermosetting‐matrix composites of an asymmetrical laminate for a targeted maximum PID. We performed a grid search on ANN architectures and hyper‐parameters using cross‐validation and obtained a well‐trained ANN model with high generalization performance. For the forward problem, the ANN model was adopted to predict the maximum PIDs of CYCOM X850 and CYCOM 977‐2 prepregs, which were subsequently verified experimentally. For inverse design, a large‐scale screening method based on the ANN model was utilized to determine the candidates for a targeted maximum PID, with an experimental demonstration using one of these candidates. The well‐trained ANN model provides an alternative approach to faster computation with high accuracy for the maximum PID prediction and further guides the discovery of materials with desired distortion behaviors.

Process-Induced Distortions Characterization of MBWK Fabric Reinforced Composite Helmet Shell.

In order to characterize the process-induced distortions of 3D thin shell composites with complex shape, the multilayered biaxial weft knitted (MBWK) fabric reinforced high-performance composite helmet was selected as the research object, and the 3D laser scanning machine was used to scan the helmet surface, then the 3D scanning data was compared with the CAD model to evaluate the deformation. The results and discussion indicated that the conventional method was workable, but the speed of convergence was slow and the calculation results were easy to drop into local optimization. According to detailed analysis, a measurement method focusing on the principle of “Feature Distance” was developed. The measurement results shown that this method can not only give accurate results, but also reduce working procedure and greatly save the computing resources, which is proved to be a feasible approach for the deformation measurement foundation of 3D thin shell textile composites.

Understanding process-induced spring-in of L-shaped composite parts using analytical solution

This paper aims to understand formation mechanism of the process-induced distortions in the L-shaped composite parts by analytical solutions. To this end, an analytical solution for the total spring-in of L-shaped composite parts is derived by defining the total spring-in as the integrated process-induced distortions in the flange and corner components in combination with shear-lag theory. The new solution, together with previously proposed analytical solution for the corner spring-in of L-shaped parts, not only quantifies the correlation of total spring-in with material and geometrical properties, but also calculates the specific contribution of flange and corner to the total spring-in. The close consistency and trend between analytical predictions and experimental data on the spring-in of L-shaped composites parts show the validity of new analytical solution. Further comparison between numerical simulations and analytical predictions for total spring-in of L-shaped composite parts with various configurations also gives favourable validation for the newly proposed analytical solution.

On abnormal thermal-expansion properties of more orthotropic M21E/IMA carbon-fiber-epoxy laminates

The present paper reports on an anomaly of laminates’ coefficients of thermal expansion (CTE), which has been observed for more-orthotropic laminates of a series-quality M21E/IMA prepreg-made composite frame. It was found that measured CTEs increasingly deviate from theoretical CTE values, derived from unidirectional ply data, when the degree of laminate orthotropy increases. The paper comprises experimental CTE measurements for the unidirectional M21E/IMA ply and six specific laminates of a full-scale frame. Analyses suggest that the observed discrepancy correlates with the laminates' degree of orthotropy, which is quantified by the in-plane Young's modulus ratio Ex/Ey in this paper. The observed effect is considered critical as inhomogeneous CTEs lead to process-induced distortions of curved composite structures. Thus, they play a key role when carbon-fiber-epoxy composite frames shall be made in tight dimensional tolerances. Wrong CTE calculation leads to erroneous shape predictions, inaccurate tool compensation and finally to assembly problems.

Modeling the influence of interleaf layers in composite materials on elastic properties, thermal expansion, and chemical shrinkage

Within this paper, it is shown that the assumption of transversal isotropy usually applied to unidirectional layers is not valid, if interleaf layers are introduced to increase the impact performance of the material. An approach is suggested to consider the meso-structure of composite materials with interleaf layers in the calculation of the elastic properties, the thermal expansion coefficients, and the chemical shrinkage coefficients. The approach uses known rules of mixture in a first step, to determine the properties of the carbon fiber–reinforced polymer layer as well as those of the interleaf layer and combine them in a second step by new meso-scale rules of mixture. The approach is discussed using known material data and shows to provide plausible results. The effects of the material properties influenced by the meso-scale rules of mixture on the process induced distortions are then discussed exemplary considering the thermally induced spring-in. It is shown that the meso-structure has a considerable influence on the process-induced distortions and cannot be neglected in the prediction of it.

A zone-based approach to predict process-induced distortions of composite structures based on a ‘spring-in reference curve’

Manufacturing of composite structures is inevitably impeded by process-induced distortions. In aerospace industries suppliers are often responsible for making parts according to requirements. Thus, the typical ‘supplier scenario’ is characterized by a strong demand for distortion predictions for sake of tool compensation, in order to make parts conform to given tolerances. However, suppliers simultaneously face a lack of relevant information to derive compensation measures, as neither process-simulation models nor corresponding parameters are accessible.A novel approach is presented within this paper which amalgamates latest experimental results on spring-in distortions in form of a ‘spring-in reference curve’ and an earlier presented phenomenological-numerical simulation concept. This combination leads to a zone-based prediction strategy, focusing on process-induced distortions, which is close to engineering practice and applicable within the typical ‘supplier scenario’.

Simulation and Analysis of Process‐Induced Distortions in Hemispherical Thermostamping for Unidirectional Thermoplastic Composites

This work aims at analyzing process‐induced distortions (PIDs) for unidirectional thermoplastic composites in the thermoforming process and evaluating the effects of fiber reorientations and stamping‐induced stresses. Hemispheres with symmetric and asymmetric lay‐ups are manufactured by the thermostamping process. A simulation method of PIDs is developed and a constitutive model is presented to reflect fiber reorientations in thermostamping. Additionally, stamping‐induced stresses are embedded in the simulation. Good correlation is obtained between the prediction and experimental results. Regarding hemispheres with the symmetric lay‐up [0/90/0/90]S, stamping‐induced stresses play a dominant role in PIDs and determine the deformation trend; fiber reorientations contribute approximately 76% to thermal distortions. Stamping‐induced stresses could be neglected in the PID prediction for the investigated three different asymmetrical lay‐ups, and contributions made by fiber reorientations range from 18% to 28%. Both fiber reorientations and stamping‐induced stresses are significant factors for PIDs for unidirectional thermoplastic composites in the thermoforming process. POLYM. COMPOS., 40:1786–1800, 2019. © 2018 Society of Plastics Engineers

Probabilistic process simulation to predict process induced distortions of a composite frame

Within this work, an enhanced simulation strategy for predicting process-induced distortions and residual stresses of composite structures is developed, while taking into account probabilistic variations. The simulation strategy is demonstrated for a carbon fibre-reinforced frame of an aircraft fuselage. Based on a sequentially coupled temperature-displacement analysis, temperature and degree of cure distributions as well as stresses, strains, and distortions are determined. For the description of the reaction kinetic, a phenomenological-based model considering chemical and diffusion-controlled reactions is introduced. The mechanical material behavior is described by a viscoelastic material model depending on temperature, time, and degree of cure. Corresponding process-induced distortions and occurring residual stresses are calculated and analyzed. The results of the process-induced distortions are used to derive a compensated tooling design. To analyze the effect of statistical and systematic deviations of underlying process and material parameters, a procedure for a probabilistic process simulation is created. Based on a first sensitivity analysis, a design of experiment is performed over a specified parameter range. From this surrogate, models are derived to enable a full but efficient probabilistic analysis of spring-in angles and residual stresses at multiple frame positions. The effect of process parameter variations and fluctuation of inherent material properties on process-induced distortions is assessed. Moreover, the influence of different process cycles is shown. This allows to determine the expected range of the final part distortion and to derive the resulting tolerances during assembly. For validation of the probabilistic process simulation, several frames are manufactured at different process conditions, and frame geometries are measured by digital image correlation technique. The resulting part thicknesses, spring-in angles, and global frame distortions are determined. Finally, a comparison of measurements and probabilistic predictions is given, and results are evaluated with respect to main sensitivities.

Compensating process-induced distortions of composite structures: A short communication

Dimensional non-conformance of fabricated parts is an issue in composite manufacturing. Numerous studies have been performed, focusing on simulation-based distortion prediction. In these studies it is implicitly assumed that a simple update of the nominal tool shape leads to parts in nominal shape. However, unsatisfactory little effort has been spent to prove this assumption experimentally.The present paper reports on the experimental validation of a prediction-based tool compensation process for an integral box structure which showed a complex distortion behavior in an earlier study. The nonlinearity of a spring-in compensation is analytically demonstrated and its relevance is discussed.

A review on process-induced distortions of carbon fiber reinforced thermosets for large-scale production

During the manufacturing process of carbon fiber reinforced plastics, residual stresses and shape distortions occur. Analytical or numerical models can be employed to accurately predict these stresses and deformations. For a virtual compensation of a part’s geometry, an understanding of the main driving mechanisms behind process-induced distortions is essential. The present study shows main sources as well as influencing factors on stresses and distortions that arise during large-scale production of composites based on thermoset resin systems. Besides basic information on the thermal, chemical and mechanical behavior of thermoset resins and carbon fibers, state-of-the-art numerical approaches are reviewed with a focus on finite element discretization and constitutive modeling. Current virtual compensation strategies as well as possible influences of the forming operation are discussed. Finally, a conclusion is drawn that includes recommendations for future developments in the field of process-induced distortions.

Modelling and experiment of process-induced distortions in unsymmetrical laminate plates

Process-induced distortions in composite structures lead to problems with dimensional control, which can be a major concern in the manufacturing. This paper focuses on the prediction of the warpages and curvatures in unsymmetrical laminate plates using closed-form solution and finite element method. A new characterization method that the shape of the deformed composite specimen is conducted in virtue of the coordinate measuring machine through contact method is adopted to experimentally measure the process-induced distortions in unsymmetrical laminate plates. The comparison of numerical and analytical prediction with experimental results for warpages in [06/906] and [09/903] unsymmetrical laminates indicates that shear layer between tool and part has a significant effect on the process-induced distortions, and PTFE release film and release agent between tool and part cannot eliminate effect of the tool-part interaction on the process-induced distortions in unsymmetrical laminate plates. Moreover, the comparison of analytically and numerically predicted development of curvature with experimental results presented by previous literature for unsymmetrical laminates further reveals the effect of tool-part interaction on the process-induced distortions and the accuracy of closed-form solution.

Cure-dependent thermomechanical modelling of the stress relaxation behaviour of composite materials during manufacturing

This study contributes to the understanding of the mechanism behind process-induced distortions and stresses related to the Resin Transfer Moulding manufacturing process. The objective is to comprehend the phenomena and to identify related parameters. During the manufacturing process, engineering constants of the matrix are changing and are influenced by the existence of a large number of effects. A viscoelastic material model has been derived. This developed material model integrates a dependency of the time–temperature–polymerisation and fibre volume content on the relaxation behaviour of residual stresses in a transversally isotropic reinforced material. The model is validated using a test case on the coupon level and results / limitations are discussed.

Flow-compacted deformations coupled with thermo-chemically induced distortions in manufacturing of thick unidirectional carbon fiber reinforced plastics composites

A novel technique to predict the manufacturing process-induced distortions and deformations in thick unidirectional carbon fiber reinforced plastics laminates is detailed in this article. An integrated numerical model is developed to account for non-isothermal resin flow, related compaction, transient resin cure, and resin shrinkage effects to predict the final shape of the autoclave cured thick prepreg laminate. The associated physics are mathematically coupled to solve for the process variables interactively. The results illustrate reduction in the thickness of the laminate prior to the start of curing when initial fiber volume fractions are pre-set. This confirms that the initial transverse deformation of B-stage prepreg is due to the applied vacuum and/or pressure. Once curing initiates, deformation of the laminate due to compaction increases, proportionally, with the increase in fiber volume fraction. Furthermore, the thermo-chemical residual strains contribute to additional compaction. The final distorted shape observed in simulation of the originally flat laminate matches with the shape of the fabricated laminate with 6.6 mm thickness. A solution to minimize the distortion is discussed in detail. This procedure is extended to simulate a curved laminate’s processing; where, the shear moduli were observed to influence the final shape of the laminate. The findings are presented and deliberated in this article.

Strength Analysis and Process Simulation of Subway Contact Rail Support Bracket of Composite Materials

An application of composites for construction of subway brackets is a very effective approach to extend their lifetime. However, this approach involves the necessity to prevent process-induced distortions of the bracket due to thermal deformation and chemical shrinkage. At present study, a process simulation has been carried out to support the design of the production tooling. The simulation was based on the application of viscoelastic model for the resin. Simulation results were verified by comparison with results of manufacturing experiments. To optimize the bracket structure the strength analysis was carried out as well.

Prediction of Process-Induced Distortions in L-Shaped Composite Profiles Using Path-Dependent Constitutive Law

In this paper, the corner spring-in angles of AS4/8552 L-shaped composite profiles with different thicknesses are predicted using path-dependent constitutive law with the consideration of material properties variation due to phase change during curing. The prediction accuracy mainly depends on the properties in the rubbery and glassy states obtained by homogenization method rather than experimental measurements. Both analytical and finite element (FE) homogenization methods are applied to predict the overall properties of AS4/8552 composite. The effect of fiber volume fraction on the properties is investigated for both rubbery and glassy states using both methods. And the predicted results are compared with experimental measurements for the glassy state. Good agreement is achieved between the predicted results and available experimental data, showing the reliability of the homogenization method. Furthermore, the corner spring-in angles of L-shaped composite profiles are measured experimentally and the reliability of path-dependent constitutive law is validated as well as the properties prediction by FE homogenization method.