Cure-induced Residual Stresses Research Articles

Cure-induced residual stresses and viscoelastic effects in repaired wind turbine blades: Analytical-numerical investigation

During scarf repair of wind turbine blades, the difference in coefficients of thermal expansion and chemical shrinkage between the original part and the repair patch leads to the development of residual stresses. These residual stresses are detrimental when the repaired composite structures are subjected to operational cyclic loads and affect their post-repair lifetime. This paper uses a hybrid analytical-numerical model to evaluate the residual stresses in a scarf-repaired composite panel. A Prony series-based viscoelastic model is used to describe the material behaviour of the composite undergoing cure to replicate real-life effects more closely. Experiments on the repaired composite samples and numerical simulations on a model of the same are performed to study the post-repair mechanical behaviour. It is found that the damage initiates at the adhesive interface between the scarf patch and the base composite. The resulting debonding and damage to the base composite leads to the failure of the repaired section.

Rapid evaluation and prediction of cure-induced residual stress of composites based on cGAN deep learning model

In this piece of present work, we propose a deep learning model driven by conditional generative adversarial network (cGAN) for rapid evaluation and prediction of cure-induced residual stress (CRS) of composites. The CRS is evaluated by solving for the material behaviors of the cure kinetics, viscoelasticity, thermal expansion, and cure shrinkage under heat transfer condition using finite element (FE) method. The geometry and CRS fields are extracted from numerous simulations and subsequently utilized in the proposed cGAN training process. With this end-to-end unsupervised learning, the cGAN model can predict the CRS with high fidelity based on the fiber random distributed microstructure and capture the variation of fiber volume fraction. In qualitative measures, peak signal to noise ratio (PSNR) and structure similarity index (SSIM) are employed for accuracy verification. Moreover, the cGAN model can also significantly reduce the computational cost and provide some insights for optimizing the manufacturing process of composites.

A method for optimization against cure-induced distortion in composite parts

This paper describes a novel method developed for the optimization of composite components against distortion caused by cure-induced residual stresses. A novel ply stack alteration algorithm is described, which is coupled to a parametrized CAD/FE model used for optimization. Elastic strain energy in 1D spring elements, used to constrain the structure during analysis, serves as an objective function incorporating aspects of global/local part stiffness in predicted distortion. Design variables such as the number and stacking sequence of plies, and geometric parameters of the part are used. The optimization problem is solved using commercial software combined with Python scripts. The method is exemplified with a case study of a stiffened panel subjected to buckling loads. Results are presented, and the effectiveness of the method to reduce the effects of cure-induced distortion is discussed.

A novel micromechanical model to study the influence of cure process on the in-plane tensile properties of z-pinned laminates

This paper proposes a novel micromechanical model to study the curing effects on the in-plane tensile properties and failure mechanisms of carbon/epoxy laminates reinforced with z-pins. The microstructures including fiber distortion and resin-rich pockets caused by z-pins are characterized. The results indicate that cure-induced residual stresses greatly exist with increasing z-pin density. Thermal expansion and volume shrinkage mismatch between z-pins and resin-rich regions attribute to the high stress concentration. The existence of interfacial stress concentration and resin-rich channels produce a loss of tensile strength and lead to an initial pathway for crack growth under tension. The failure mode changing from the interfacial cracking to softening and damages of both interface and resin-rich regions is also revealed with increasing z-pin density.

Probabilistic multi-scale design of 2D plain woven composites considering meso-scale uncertainties

This paper presents a novel multi-scale design strategy for 2D plain woven composites considering the manufacturing process. The design methodology integrates the meso-structural geometry of woven fabric composites to maximise the buckling load of composite stiffened panels at the macro-level. A numerical model to simulate the autoclave manufacturing process of woven composites is developed, in which a new geometric modelling method is proposed to construct a more realistic RVE model for 2D plain woven composites accounting for the compaction effects. Deterministic and robust design optimisations are conducted to find solutions with both high performances and high robustness to uncertainties. In particular, this study considers the morphological uncertainties of the meso-scale fabric structure, i.e. yarns’ waviness and porosity incurred during manufacturing and its influence on the structural integrity of the parts. The optimisation results obtained from the deterministic and robust design problems are presented. A parametric analysis is conducted to characterise the sensitivity of the fabric properties to the geometry of yarns and related uncertainties. It is shown that the space between yarns has a significant negative impact on the structural buckling load and negative impact on the cure-induced residual stresses.

Study on cure-induced residual stresses and spring-in deformation of L-shaped composite laminates using a simplified constitutive model considering stress relaxation

In this paper, a simplified constitutive model of resin in curing process considering stress relaxation was proposed. The simulation of composite curing using the proposed model is accurate as the simulation using viscoelastic model, but the computing efficiency is greatly improved. The proposed model balances the simulation accuracy and efficiency, which is urgently needed in practice. Then it was utilized to predict the spring-in deformations of L-shaped composite laminates with different structural parameters. The results were in good agreement with the corresponding experiments. A new interpretation of the generation mechanisms of residual stresses and spring-in deformation was presented based on the monitoring of changes of strains and stresses. The simulated results show that the residual stresses and spring-in deformation would be overestimated without considering stress relaxation. The quantitative discussions of the influence mechanisms of different structural parameters, including thickness, corner radius, corner angle and lay-up, were also presented.

Visco-thermo-elastic Simulation Approach For Prediction of Cure-induced Residual Stresses in Fiber Reinforced Composites

Liquid composite molding (LCM) has established as a high quality manufacturing process for fiber reinforced composite structures. In order to reduce cycle times significantly, novel fast curing matrix resins are being introduced into series production. These put high requirements on process control and part reproducibility. Problems that may be encountered in this context involve process-induced distortion and surface waviness resulting from anisotropic and cure-dependent material properties. Numerical simulations represent a powerful approach to avoid the use of costly trial-and-error methods. For this reason, a simulation approach is being developed which aims at the prediction of residual stresses and accompanying effects on different length scales. Based on a resin characterization comprising reaction kinetics, cure-dependent relaxation modulus as well as thermal expansion and pressure-dependent chemical shrinkage, a generalized MAXWELL model is selected to describe the process-related mechanical behavior of the thermoset. Taking into account the influence of the process parameters on the resin properties enables a detailed analysis of process-property-relationships. By this, the developed simulation approach offers the possibility of a comprehensive analysis of both local and global process-induced phenomena and hence prevention of flaws.

Microscale viscoplastic analysis of unidirectional CFRP composites under the influence of curing process

Cure-induced residual stresses have a great impact on the mechanical behavior of carbon fiber reinforced polymer (CFRP) composites. This paper develops an integrated analytical framework to study the effect of the cure-induced residual stresses on the mechanical behavior of unidirectional IM7/8552 composites. During the curing process, evolution of the material properties of the resin matrix is captured using the cure hardening instantaneous linear elastic (CHILE) model. In addition to the mechanical strain, thermal and chemical shrinkage strains of the resin are taken into account in the determination of the residual stresses. By introduction of the predicted residual stresses into the representative volume element (RVE), the quasi-static and dynamic compression analyses are performed to demonstrate the local viscoplastic deformation of the matrix and the homogenized stress–strain response of the composites. Simulation results are in good agreement with the experimental data. The comparative analysis indicates that the cure-induced residual stresses lead to an earlier nonlinearity and lower compressive strength. Furthermore, to investigate the effect of the fiber distribution on the residual stresses and compressive response of the composites, a parametric analysis is performed, in which two parameters are considered, i.e., minimal inter-fiber distance and fiber volume fraction.

Cure-induced residual stress buildup and distortions of CFRP laminates with stochastic thermo-chemical and viscoelastic models: Experimental verifications

This paper proposes a stochastically coupled thermo-chemical and viscoelastic model and provides new insights from comparisons of the cure-induced residual stresses and distortions between various ply configurations. Six different laminate configurations were modeled to compare the material response during four manufacturing steps: in-mold curing, cool-down, post-curing, and final cool-down. While homogeneous plates with cross-ply configuration showed the same residual stresses at all the analyzed points, nonhomogeneous cross-ply laminates with random shrinkage showed variations in the stresses. It shows the importance of considering the randomness of the parameters when modeling composite materials’ manufacturing process.

Volumetric effective cure shrinkage measurement of dual curable adhesives by fiber Bragg grating sensor

A robust optical strain sensor, called fiber Bragg grating, is employed to measure the effective cure shrinkage—a part of cure shrinkage that is accumulated only after the gel point—of a dual cure adhesive during each stage of curing—(1) UV-curing stage and (2) thermal-curing stage. Unique experimental setups and procedures are developed and implemented to cope with the technical challenges associated with each stage of curing, including gel point detection and uniform curing. The effective cure shrinkage is measured to be 2.1% and 0.78% for acrylate compound (UV-curing) and two epoxy compounds (thermal-curing), respectively. The measurement accuracy of the proposed approach is corroborated by an additional measurement under the identical curing conditions. The results can be used to predict the cure-induced residual stresses of dual curable adhesives.

Cure-induced residual stresses for warpage reduction in thermoset laminates

The paper addresses the role played by the cure stage of a vacuum assisted resin transfer molding process in residual stresses generation. The Airstone 780E epoxy resin and Hardener 785H system broadly used in the wind turbine blade industry has been used in this study. The viscous–elastic properties of the resin have been characterized and implemented in a thermo-mechanical FE model. The model has been validated against manufactured [0/90]4 asymmetric laminates. Analysis of residual stresses generation highlighted that compressive stresses generation occurs when the cure is shrinkage dominated and tensile stresses when expansion dominated in the 0° plies. The finding points out that 10% reduction in warpage and 33% reduction in process time can be obtained by selecting cure cycle parameters that allow tensile stresses development during the cure process in the 0° plies.

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.

Residual stresses created during curing of a polymer matrix composite using a viscoelastic model

A modified viscoelastic constitutive model is proposed to predict cure-induced residual stresses in polymer matrix composites. The modifications rely on using the inverse of the Deborah number to describe regimes corresponding to a fully relaxed state, a viscoelastic state and an elastic state. The composite is only in a viscoelastic state for limited ranges of the Deborah number. By considering the evolution of the Deborah number during curing of the AS4/3501-6 composite, the composite is in a fully relaxed state when it is cured at high temperature and the degree of cure is lower than 0.73, and no further changes in the viscoelastic characteristics when the degree of cure is higher than 0.8. A 3D simulation of a composite laminate plate is used to predict the evolution of the residual stresses. The analysis reveals that the accurate simulation on the cure-induced residual stress should include the last two states of the entire cure process, and consider the stress relaxation, thermal deformation and chemical shrinkage.

Multiscale Modeling of Residual Stress Development in Continuous Fiber-Reinforced Unidirectional Thick Thermoset Composites

The primary objective of this research is to develop a multiscale simulation framework to arrive at more realistic estimates of cure-induced residual stresses in the vicinity of the fiber-matrix interface in thick thermoset composites. The methodology involves simulations at the part level—employing homogenized rendering of the composite using micromechanics approach—within a finite element framework to obtain part-level temperature and degree-of-cure gradients and strains, and imposition of this information as boundary conditions at the mesoscale simulations, employing microstructural representative volume elements (RVE). A simple implementation of the multiscale framework, involving simulations at the part as well as the RVE levels, is demonstrated in the context of a thick, unidirectional continuous-glass-fiber-reinforced thermoset composite. The trends in the mesoscale residual stresses estimated by employing different RVE-level thermal and thermomechanical boundary conditions—displaying different degrees of coupling between the global and part-level simulations—are then examined. Significant differences are observed in the estimates of mesolevel cure-induced residual stress evolution obtained from simulations with a conventional symmetric RVE and those obtained by employing the multiscale approach involving detailed boundary conditions that realistically account for global thermal and mechanical strain histories.

Development of residual stresses during electron beam processing

In order to further advance the state of the art in the electron beam processing of polymers and composites, it is necessary to better understand the development of cure-induced residual stresses. In situ measurement of electron beam cure–induced stress development is challenging due to the bombardment of specimens with intense beams of very high-voltage electrons. In this study, a custom fixture was designed to measure the deformation of an electron beam–cured specimen during processing and thereby to assess specimen stress state during the curing process. The unbalanced composite specimen consisted of two cured unidirectional carbon–epoxy laminates separated by a thin woven fibreglass carrier, which was infused by a layer of electron beam curable epoxy resin systems (Tactix 123 and CAT B). The out-of-plane specimen deformations were monitored during various electron beam irradiation schedules and subsequent thermal post-cure. The preliminary results confirmed that the apparatus was not affected by the irradiation and was capable of accurately measuring the specimen warpage during the cure. It was also shown that the electron beam–cured specimens exhibit a reduced residual stress state compared to equivalent thermally cured specimens. It was observed that the irradiation dose rate applied to a specimen is the main factor in the difference in the way residual stresses develop during electron beam curing. Furthermore, the results suggest that there is a direct relation between the stress-free temperature ( TSF) and the temperature of the specimen at gelation ( TGEL).

Study on Cure-Induced Residual Stresses for Fibre Metal Laminate

During autoclave curing process, because of the mismatch between coefficients of thermal expansion of fibre layers and metal layers, residual stresses are developed in Fibre Metal Laminates (FML). In this paper, a three-dimensional finite element model was developed to predict cure-induced residual stresses for FML. And then FML-2/1 and FML-3/2 structures (plate and L-shape) were studied with the finite element model. The results show that residual tensile stress is in metal layers and residual compressive stress is in fibre layers. Residual stresses in Fibre Metal Laminates are bigger enough to affect the deformation and fatigue performance.

Modeling of Cure-Induced Residual Stresses in 3D Woven Composites of Different Reinforcement Architectures

This paper presents finite element modeling effort to predict possible microcracking of the matrix in 3D woven composites during curing. Three different reinforcement architectures are considered: a ply-to-ply weave, a one-by-one and a two-by-two orthogonal through-thickness reinforcement. To realistically reproduce the as-woven geometry of the fabric, the data from the Digital Fabric Mechanics Analyzer software is used as input for finite element modeling. The curing processed is modeled in a simplified way as a uniform drop in temperature from the resin curing to room temperature. The simulations show that the amount of residual stress is strongly influenced by the presence of through-thickness reinforcement.

Evolution of cure, mechanical properties, and residual stress during electron beam curing of a polymer composite

Evolution of cure, mechanical properties, and residual stress during E-beam (Electron Beam) processing was studied to evaluate the influence of process parameters – dose and dose per pass – using an epoxy reinforced with IM7 carbon fibers. The composite prepreg was also cured thermally to various cure levels and compared with the E-beam cured composite. Cure evolution changed substantially with irradiation condition; lower dose/pass and wider scanning width of the beam (for the same dose/pass) resulted in rapid curing. For a given degree of cure, the longitudinal ( E 11) and transverse ( E 22) moduli of the E-beam cured composite varied with dose/pass and were less than that of the thermally cured composite. Transverse strength and failure strain of the composite cured at 20 kGy/pass were higher than that of composites cured thermally and at other dose/pass conditions. E-beam curing resulted in lower residual stresses than thermal curing and lower dose per pass resulted in lower cure-induced residual stress than higher dose/pass.

Feature Article: Process-Induced Residual Stresses in Polymer-Based Composites

Abstract The consolidation of a polymer-based composite part is usually accompanied by the build-up of the stresses and deformations that are caused mainly by the polymer matrix shrinkage, the resin rheological and mechanical properties changes associated with the advancement of the cure reaction and by the material properties mismatch of the system constituents, mold, fibers reinforcement, and thermoset resin. These process-induced residual stresses and strains are obviously undesired phenomena that should be avoided leading to low performance, shape distortions, warpage, matrix cracks, and delaminations. Therefore, the optimization of the manufacturing cycle coupled to the capability to predict and measure the processing residual stress and strains represent important issues to be addressed. In this work, after a preliminary introduction on the most relevant mechanisms inducing the formation of stresses and strains in polymer-based composite parts, the most recent advances both in the modeling of the cure-induced residual stresses and strains build-up and in the experimental techniques for their measurement are presented.

Micromechanical Modeling of Stress Evolution Induced During Cure in a Particle-Filled Electronic Packaging Polymer

Particle-filled polymers are widely used in electronic industries. From microscale view, cure-induced residual stress can be generated not only by the external constraints but also by the constraint effect among the particles. In this paper, a three-dimensional micromechanical finite element method (FEM) model has been setup for a silica particle filled epoxy. In the micromechanical model, the epoxy matrix is modeled with a previously developed cure-dependent viscoelastic constitutive model, whereas the silica particles are modeled as elastic with high stiffness. Cure shrinkage is applied to the matrix as an initial strain for each time increment. The cure-dependent viscoelastic properties were obtained from shear and tension-compression dynamical mechanical analysis measurements. Cure shrinkage and reaction kinetics were characterized with online density measurement and differential scanning calorimeter measurements, respectively. In order to simulate a partly constrained object, the micromechanical model is coupled with a macromodel FEM analysis. The displacements from the macromodel are used as boundary conditions for the micromodel. The effect of external constraints on the generation of the micro stresses is studied by using the boundary conditions related to different external constrained states.