Christian Brauner

Microlevel manufacturing process simulation of carbon fiber/epoxy composites to analyze the effect of chemical and thermal induced residual stresses

A micromechanical implicit finite element analysis using Samcef/Mecano was performed to analyze the effect of process-induced deformations and stresses on a unidirectional carbon fiber–epoxy composite material (G1157/RTM6). During the manufacturing process, chemical and thermal shrinkage deformations occur and lead to internal stresses. In this article, several effects on the level of stress on the microscale will be discussed, which address the nonlinear behavior of the polymer matrix material. A parametric study was done on the influence of these effects on residual stresses including temperature dependency of the young’s modulus, thermal expansion coefficient, nonlinear thermomechanical stress–strain behavior, microyielding, microdegradation, and viscoelasticity. Several experiments have been carried out to investigate the thermomechanical behavior of the matrix material and to derivate constitutive equations. The derived equations and the discussed effects are integrated into an analysis model of a squared unit cell and used to perform a coupled curing thermomechanical simulation. As a result, the development of process-induced stresses is presented with the integration of nonlinear local material effects. Assessment of these effects is one of the key aspects for interpretation of process-induced stresses on the macroscale.

Non-damage-related influences on Lamb wave–based structural health monitoring of carbon fiber–reinforced plastic structures

Structural health monitoring with actively excited Lamb waves is a promising technology for health monitoring of aerospace carbon fiber–reinforced plastic structures with reasonable effort, as it has the potential to cover large areas with few sensors and a high sensitivity to damage. The high signal complexity inherent to wave propagation in complex, finite, and anisotropic structures, especially in combination with the sensitivity toward environmental and operational loads, presents the main challenge on the road toward the utilization of this potential. In this study, the effects that real structural features and damage, in combination with environmental conditioning, have on Lamb wave propagation and measurement are investigated. For this, the composite-specific behavior is discussed. Based on the local temporal coherence method, changes of sensor responses containing reflections and interaction with stiffness discontinuities, both unrelated and related to damage, are identified in anisotropic composite materials. The amount of signal changes unrelated to damage, as well as their high dependency on the specific conditions of the measurements, is an indicator for the complex issues faced by compensation. While damage of a sufficient size will be detectable even in the most complex, finite, and anisotropic structures, the establishment of a sufficiently reliable damage detection with an acceptable low detection threshold will require even more careful consideration of the monitored structure and its environmental and operational loads than for metallic structures.

Meso-level manufacturing process simulation of sandwich structures to analyze viscoelastic-dependent residual stresses

This article presents a simulation method to analyze process-induced stresses of a meso-level unit cell of a pin-reinforced sandwich structure (carbon fiber-reinforced plastics/foam). During the manufacturing process, different mechanisms lead to process-induced deformations and stresses. These mechanisms depend on thermal expansion, shrinkage, nonlinear viscoelastic properties of the material, and variation in local temperatures. In critical cases, these residual stresses can lead to initial degradation and up to failure of the material. This article demonstrates a method to perform a sequential coupled thermo-mechanical analysis of the manufacturing process with focus on the correct determination of stresses and description of the viscoelastic relaxation effect. A novel approach to take into account anisotropy of characteristic relaxation times dependent on the cure process is presented.

Analysing process-induced deformation and stresses using a simulated manufacturing process for composite multispar flaps

Although composite materials have numerous advantages, some disadvantages, including high manufacturing costs, are relevant. In particular, if the material is applied to large structural components, such as the wings, flaps or fuselage of an airplane, efficient manufacturing processes are required to generate products that are both high quality and cost effective. Therefore, monolithic designs often become integral due to the lower overall part count and simplified designs (e.g. reducing the number of joints and fasteners significantly). For highly integrated monolithic structures, developing a robust manufacturing process to produce high quality structures is a major challenge. An integral structure must conform to the tolerance requirements because those requirements may change. Process-induced deformations may be an important risk factor for these types of structures in the context of the required tolerances, manufacturing costs and process time. Manufacturing process simulations are essential when predicting distortion and residual stresses. This study presents a simulation method for analysing process-induced deformations on the structure of a composite multispar flap. The warpage depends on the thermal expansion and shrinkage of the resin. In this study, a sequentially coupled thermo-mechanical analysis of the process will be used to analyse temperature distribution, curing evolution, distortion and residual stresses of 7.5 m long composite part.

Embedding Piezoresistive Pressure Sensors to Obtain Online Pressure Profiles Inside Fiber Composite Laminates

The production of large and complex parts using fiber composite materials is costly due to the frequent formation of voids, porosity and waste products. By embedding different types of sensors and monitoring the process in real time, the amount of wastage can be significantly reduced. This work focuses on developing a knowledge-based method to improve and ensure complete impregnation of the fibers before initiation of the resin cure. Piezoresistive and capacitive pressure sensors were embedded in fiber composite laminates to measure the real-time the pressure values inside the laminate. A change of pressure indicates resin infusion. The sensors were placed in the laminate and the resin was infused by vacuum. The embedded piezoresistive pressure sensors were able to track the vacuum pressure in the fiber composite laminate setup, as well as the arrival of the resin at the sensor. The pressure increase due to closing the resin inlet was also measured. In contrast, the capacitive type of sensor was found to be inappropriate for measuring these quantities. The following study demonstrates real-time monitoring of pressure changes inside the fiber composite laminate, which validate the use of Darcy’s law in porous media to control the resin flow during infusion.

Analysis of process-induced deformations in thermoplastic composite materials

The thermoforming process is a manufacturing method to produce fibre-reinforced thermoplastic components within short cycle times (<2 min). During this process, the anisotropic material behaviour provokes residual stresses which furthermore induce unwanted deformations. Thereby, at the beginning, newly produced geometries have a quite high reject rate and the process parameters have to be adjusted iteratively. Thus, an analysis of the process-induced deformations has been carried out to investigate the connections between process parameters and final geometry. In this case, an L-angle bracket has been observed which shows a spring-in effect after the thermoforming process. For the experimental approach, the semi-crystalline polyphenylenesulphide was used as thermoplastic matrix material. In particular, the crystallisation kinetics of this polymer is described by adjusting Nakamura’s crystallisation model to different cooling rates. And furthermore, a simulation strategy has been developed to include the crystallisation behaviour in a thermal and mechanical analysis. The results of these analyses have been compared and evaluated with the outcomes of the experimental approach. Finally, some opportunities for future studies will be introduced to provide a way for improving the simulation analysis.

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.

Cure-dependent thermo-chemical modelling and analysis of the manufacturing process of an aircraft composite frame

This study contributes to the understanding of the mechanism behind thermo chemical aspects related to the resin transfer moulding manufacturing process of a composite part. The aim is to comprehend the phenomena, to identify related parameters and to get knowledge-based methods for the process development. Therefore, the first part of this study is an experimental study about the behaviour of material properties during the manufacturing process of the single component and the composite. It concludes with constitutive equations for single-process parameters and their associated homogenisation approach for the composite properties. During the manufacturing process, material values of the matrix are changing and influenced by a high number of effects. In the second part, a simulation strategy is been derived. This developed material model integrates a dependency of the time–temperature–polymerisation and fibre volume content. The model is validated in a test case of a manufacturing process of an aircraft component, a fuselage frame.

Effects of fabric-based unbalances on process-induced distortions of composite materials

In the present study, process-induced residual stresses of composite materials have been analysed on the mesolevel of a fabric unit cell. The given fabric was the satin-weave G0926, which is used for example in high-lift application of aerospace structures. In the study, a developed viscoelastic cure-dependent material model is applied to analyse the process-induced deformations of the fabric. In fact of the weave structure, this type of fabric tends to be out-of-plane displacement based on chemical and thermal shrinkage. This effect is studied experimentally and numerically, and the results are compared. The knowledge of the sensitivity of this fabric for out-of-plane displacement is an important factor to create real symmetric laminates. Disrespecting can be the source for large scattering warpage on the component level, and therefore, this awareness of the effect of textile structures on process-induced deformations is one key factor to develop robust processes.

Analysis of the non-linear load and temperature-dependent creep behaviour of thermoplastic composite materials

Fibre-reinforced thermoplastic composite materials can be manufactured rapidly using a thermoforming process. The assortment of thermoplastic matrix systems is manifold and starts from bulk plastic like polypropylene (PP) up to high-performance systems like polyether ether ketone. High-performance thermoplastic polymers have durable properties but relatively high raw material costs. For structural application, engineering methods are needed to ensure the availability for use over the full range of the life cycle of parts. This equates to at least 15 years under exposure to varying climatic conditions for an automobile component. Bulk plastics have complex viscoelastic behaviour, which means that advanced methods are needed to ensure the long-term behaviour of both the pure plastic or fibre-reinforced materials with such a matrix system. In the following study, the creep behaviour of a glass fibre-reinforced PP material is investigated using different uniaxially loaded creep tests at different load and temperature levels. Starting from this empirical base, two characteristic creep functions are derived using a modified Burgers approach. To transfer the results of uniaxial creep situations to a three-dimensional multiaxial stress state, a method to interpolate the experimental creep curves is presented. This developed creep model is integrated into the implicit non-linear finite element program SAMCEF/Mecano and used to predict the creep behaviour of a complex laminate. The results are then validated against the performed experiments.