Elsevier

Composite Structures

Volume 230, 15 December 2019, 111461
Composite Structures

Process-induced deformation of L-shaped variable-stiffness composite structures during cure

https://doi.org/10.1016/j.compstruct.2019.111461Get rights and content

Abstract

One of the most key issues for ensuring the accuracy of manufacture and assembly of composite structures reflects on determination of process-induced deformation of composites during cure. In this paper the parameterized investigation on the process-induced spring-in of L-shaped variable-stiffness composites was presented. The Kamal model was used to simulate the cure kinetic for AS4/3501-6 prepregs. A cure hardening instantaneously linear elastic (CHILE) constitutive model was adopted to determine the modulus of the matrix resin. Self-consistent micro-mechanical models were employed to represent the mechanical properties and behaviors of the lamina. The three-dimensional (3D) model of an L-shaped variable-stiffness composite part was established using a linear fiber angle variation. The influence of the corner radius, the fiber orientation, the thickness and the length of flange on the spring-in of the L-shaped variable-stiffness part was evaluated using ABAQUS. The results show that the spring-in angle increases with increases of the corner radius and the length of flange and decreases as the thickness increases; in addition, the layup of 0°±<0°|60°>2S results in the minimum spring-in angle. The present model and method can provide a useful tool for prediction of L-shaped variable-stiffness composite structures.

Introduction

Composite structures have been widely used in aircrafts, marine and new energy automobiles owing to their superior mechanical advantages. The advantages of the composite structures have not yet been completely utilized due to the constant stiffness of the layer. Compared with the conventional straight-fiber laminates, using variable-stiffness laminates can tailor the stiffness distribution and improve the structural performance by means of fiber-angle altering [1]. Process-induced deformation is inevitable during cure of composite structures. The deformation leads to a severe inaccuracy of the resulting shape and the assembly of the composite parts. Various factors such as the difference in coefficients of thermal expansion (CTE) between fiber and matrix, thermal strains of the part and the tool, heat transfer within the part and the tool and at their boundaries, the cure cycle, resin flow, the curing kinetics of the matrix, chemical shrinkage of the matrix, and the tool-part interaction will result in process-deformation of composite structures after demolding.

Various researchers have so far investigated the process-induced stresses and deformations of non-planar structures. The behavior of thin non-planar laminates (L, T, U shapes) is not the same as thin flat panels [2], [3]; even though the structure is balanced and symmetrical, the conditions for obtaining stresses and deformations are much more complicated and cause geometric deviations [4]. Çınar et al. [5] found that fiber wrinkling at the corner sections of L-shaped parts decreases the spring-in angles. Johnston et al. [6] accurately calculated the spring-in angle of the final simple L-shaped components. Yoon et al. [7] investigated the effects of the CTE properties and the resin chemical shrinkage on the spring-in of L-shaped composite components. Kappel [8] concentrated on the scattering of spring-in distortions of L-sectioned parts; the effects of various tooling materials and layups and the part thickness on spring-in of L-parts were also evaluated. Takagaki et al. [9] investigated the influences of the flange length, thickness and shape on process-induced deformation of components. Bellini et al. [10] developed a thermo-chemical-mechanical model to simulate the cure process and calculate spring-in angle of thin laminates. Wiersma et al. [11] investigated three possible influences on process-induced deformation of the L-shaped components: the heterogeneous through-thickness distribution of fiber and resin, the heterogeneous temperature distribution during cure and the difference in CTE between the tool and the part. Nawab et al. [12] studied the spring-in angle of L-shaped carbon/epoxy woven composite considering thermal-kinetics and thermal-chemical-mechanics using COMSOL. Baran et al. [13] developed a numerical simulation to compute the process-induced shape change in a pultruded L-shaped part. Ding et al. [14], [15] incorporated the calculation of the material properties into the path-dependent constitutive law to study the deformation of L-section composite parts. Radford [16] et al. developed a method by which the stacking-sequence of the laminate was modified to counterbalance the environmentally induced shape accuracy. Fernlund [17] proposed an engineering method for predicting the process-induced distortions of 3D composite parts. Li et al. [18] investigated the distortion of a T-shaped part using a thermo-viscoelastic finite element analysis.

Most previous research has mainly focused on evaluation of process-induced deformations of straight-fiber composite laminates. However, far less attention has been devoted to the process-induced deformations of variable stiffness composites during cure. However, variable stiffness composite structures made of curvilinear steered fiber paths have gained increasing attention and applications in many industries due to the flexible design and structural performance. Therefore, the investigation of process-induced stresses and deformation of variable-stiffness composites became imperative for design and manufacture of this novel class of composite structures.

The goal of this paper is to calculate the process-induced spring-in angle of L-shaped variable-stiffness structures. The 3D model of L-shaped variable-stiffness structures were established considering the analysis of thermo-mechanical-chemical. A parameterized investigation on process-induced spring-in angles of L-shaped parts was carried out.

Section snippets

Heat transfer equations

The theoretical model of the heat transfer is evaluated using the following equation:kx2T2x+ky2T2y+kz2T2z+Q=ρCcTtwhere kx, ky and kz represent the thermal conductivities in the x, y, z directions, respectively; T is the kelvin temperature; ρ is the composite density; Cc stands for the specific heat of the laminates; t is time; Q is the instantaneous heat, given by:Q=ρr1-VfHrdαdtwhere ρr is the resin density; Vf represents the fiber volume fraction; dα/dt denotes the instantaneous curing

Finite element analysis of process-induced deformation

The geometry configuration is schematized in Fig. 2. In order to investigate the deformation of the L-shaped variable-stiffness structure with various parameters, the commercial FEA package ABAQUS was employed in this study. Fig. 3 shows the L-shaped variable-stiffness part with a layup of 0°±<0°|45°>2S.

The manufacturer’s recommended cure cycle consists of 2 ramps and 2 dwells. The temperature rose from room temperature to 383 K at 2 K/min and kept constant for about 1 h. Then, the temperature

Results and discussions

The spring-in angle of the 3D finite element model of the L-shaped structure with the flange length L = 100 mm, the laminate thickness tc = 2 mm, the corner radius Rc = 10 mm, a stacking sequence of 0°±<0°|45°>2S, as shown in Fig. 5. It is illustrated that the solidification deformations of the L-shaped structure in the length direction are basically the same. A parametric model was established to investigate the influence of the corner radius, the fiber orientation, the laminate thickness and

Conclusions

In this paper the previously developed model with the fully 3D thermomechanical model was presented to simulate the process-induced deformation of L-shaped variable-stiffness composite structures. The variation in resin modulus, Kamal model, cure shrinkage, CTEs of the composite and the tools during cure were incorporated into the FEM-based model. The mechanical properties of the lamina were directly calculated using self-consistent micro-mechanical models. The FEM-based calculation for

Acknowledgement

This work is supported by the National Natural Science Foundation of China, China (Grant No. 11902231), the China Postdoctoral Science Foundation, China (Grant No. 2017M622537, Grant No. M632933).

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

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