Sheet Molding Compound Research Articles

Unfolding the mechanical properties of buckypaper composites: nano- to macro-scale coupled atomistic-continuum simulations

Carbon-based nanostructures are receiving increasing attention over the past two decades due to their unprecedented multi-functional features. However, the macro-scale structural applications of these nanostructures have not yet come to full fruition due to the involvement of complex multi-scale computations and manufacturing. Recently, the research community has started investigating buckypaper, which can be described as a sheet or membrane developed using a network of bundles of single-wall carbon nanotubes, multi-wall carbon nanotubes, or a mixture of both. This article aims to focus on the computational bridging of different length scales involving six levels in the range of nano- to macro-scale behaviour concerning buckypaper composites. The sequential derivatives of carbon at six levels, as analyzed in this paper, involve graphene, CNT, CNT bundle, buckypaper, and buckypaper composite automotive components. Here, we adopt a coupled atomistic-continuum modelling approach for the multi-level simulations. Graphene, CNTs, and CNT bundles are modelled using atomistic simulations, while the buckypaper and its composites are modelled using equivalent beam representations for the bundles and continuum solid representation for resin. At the macro-scale, an industry-relevant multi-material composite automotive component has been investigated, wherein the buckypaper is proposed to be embedded involving sheet moulding compound and carbon prepreg. The current simulations have led to the determination of mechanical properties at each level of the carbon-based materials and their mutual dependence. The numerical results demonstrate that a buckypaper composite can enhance the natural frequency and stiffness up to 25 and 37% with respect to conventional monolithic metallic designs, while reducing the weight by 57%. Such outcomes lead to the realization that carbon-based nanostructural derivative in the form of buckypaper can significantly improve the mechanical properties of advanced lightweight structural components as reinforcements for the next generation of aerospace and automotive structures.Graphical abstract

Fibre alignment and mechanical performance of carbon fibre Sheet Moulding Compounds under different preform compaction

This work investigates the effect of preform compaction on the mechanical performance and flow-induced fibre alignment of carbon fibre reinforced Sheet Moulding Compounds (SMCs). Two groups of panels have been compression moulded from reclaimed carbon fibre tows in vinyl-ester resin with low (0.5 MPa) and high (10 MPa) preform compaction pressure Additionally, a low-cost fibre orientation analysis method has been further improved in terms of reliability, and a novel flow assessment method has been developed for carbon fibre SMCs. This approach revealed greater fibre alignment with the flow direction in the lower faces of panels as a result of greater contact time with the heated mould and a lower charge viscosity at the time of pressing. As expected, greater fibre alignment in the flow direction was observed outside the initial charge coverage area in both types of panels, where the flow was greatest. Due to additional fibre flow during the high-pressure compaction stage, the mean degree of flow alignment in the high compaction panel was 47% higher than that of the low compaction panel. Improvements in the tensile stiffness (8%) and strength (32%) were also observed as a result of the high-pressure compaction stage and associated flow alignment.

Effect of low-profile additives on the durability of adhesively bonded Sheet Moulding Compound (SMC)

The mechanical bonding properties and the failure behaviour of adhesively bonded SMC substrates were investigated under the aspect of the influence of low-profile additives on the durability of joints. For this purpose, three SMC materials with different amounts of polyvinyl acetate (PVAc) and saturated polyester were bonded with a structural and semi-structural adhesive and tested under cleavage peel test in reference condition and after thermal and hygrothermal ageing. In addition characterisations of the individual materials and their ageing effects were made. In principle, destructive testing of bonded SMC substrates showed a dependence on the used adhesive and the appearance of a type of fibre bridging. Depending on the PVAc content, different fibre tearing rates occurred. Thermal ageing led to post-crosslinking of the materials and increasing stiffness. Moreover, strong fibre tear-out appeared. Hygrothermal ageing in the SMC results in physical reversible plasticizing effects and chemical ageing by hydrolysis of the low-profile additives, presumed by performed DMTA tests. A weakened interface and disbonding failure phenomena occurred with increased PVAc content of semi-structural bonded specimens. Thus, in terms of adhesive bonding of SMC structures, a focus should be put on the type and quantity of low-profile additives used. The reduction of PVAc content led to the required quality of the joint with regard to the durability.

Influence of fungal decay on mechanical properties of bio-based sheet molding compounds (SMC)

Sheet Molding Compound (SMC) was initially developed in the 1960s and enabled for the first time the production of glass fiber reinforced polymer composite in mass scale production and at an advantageous price. Today, material and process are well established for the production of semi-structural and structural components in various applications such as construction industry, aircraft applications and automotive components. Based on increasing ecological and economical requirements for construction materials a newly and largely bio-based SMC semi-finished material was developed. This paper shows the ability using bio-based components to replace crude oil and mineral based components in SMC. Therefore, test methods for shelf life and fungal decay tests were developed to prove that there is no influence to mechanical properties of the bio-based specimens. The different test series showed that there is a bigger influence of sample preparation for shelf life tests than from fungal decay to the specimens.

Hybridization Process of Carbon Fibre Sheet Moulding Compound with Carbon Fibre Prepregs: a Case Study

The main objective of this work was to show the technical viability of a traditional Sheet Moulding Compound process hybridized with Carbon fibre prepreg reinforcements, in a demonstrative car seat structure. The new reinforced seat is lighter, has better mechanical resistance to impact and the typical carbon fibre visual aspect in the reinforced areas. Currently, the seat structure is composed by the hybrid solution with the matrixes of both materials being epoxy, therefore compatible which promotes a good adhesion under the correct processing conditions. To ensure the viability of this hybridization, an extensive laboratorial test campaign was conducted in a laboratorial hot plate press to study the Sheet Moulding Compound flow dynamics during the moulding process and how it interacts with the prepreg preforms. The cure process of both materials was also analysed to establish the optimal temperature profiles. Finally, mechanical tests were performed according to ASTM D 5868-01 to evaluate the mechanical properties of the bonding between the two materials. The reinforced car seat prototype was validated according to the ECE-R17 and FMVSS202a impact tests on the head rest and a 25% reduction in overall weight was achieved. The life-cycle cost and the visual aspect benefits are expected to compensate the slightly higher manufacturing cost in relation to the original part due to the cost of the prepreg material. Further research will be needed to in terms of process automation to ensure the required process quality.

Novel Test Cell for Overall and Spatial Mechanical Analysis of Sheet Molding Compound Composite Part in view of Heterogeneity

Novel Test Cell for Overall and Spatial Mechanical Analysis of Sheet Molding Compound Composite Part in view of Heterogeneity

Rheological response of compressible SMCs under various deformation kinematics: Experimental aspects and simple modelling approach

Flow conditions during compression moulding of Sheet Moulding Compounds (SMCs) govern the microstructure and the resulting mechanical properties of composite parts. Low-density and high-fibre content SMCs were subjected to simple compression, œdometric compression and through-thickness shear loadings, using dedicated rheometers. Simple compression and œdometric compression experiments revealed the compaction behaviour of both types of SMCs. During simple compression experiments, the compaction of low-density SMCs was accompanied by in-plane elongational flow phenomena, whereas the high-fibre content SMCs exhibited compaction below a characteristic axial strain and elongation and shear above. Shear tests showed that shear stress did not vary over a wide range of shear strain while being accompanied by axial compression stress. Both SMCs exhibited a shear-thinning behaviour with compression viscosities that depended on the fibre volume fraction. Finally, a transversely isotropic tensorial model for SMCs seen as compressible materials was proposed. Its predictions are consistent with the experiments.

Mitsui collaborates on microwave recycling

The Japanese firms Mitsui Chemicals and Microwave Chemical plan to develop microwave depolymerization technology. The companies are targeting automotive shredder residue (ASR) and thermosetting sheet molding compounds (SMC). ASR typically contains polypropylene and polyethylene, which Microwave says the technology can break down into propylene, ethylene, and other chemicals. SMC is made of styrenics and unsaturated polyester. Microwave says the process yields C2–C5 molecules, rather than the C6–C20 molecules that result from pyrolysis-based depolymerization.

Characterization and modeling of fatigue behavior of chopped glass fiber reinforced sheet molding compound (SMC) composite

In the present study, the quasi-static and fatigue behavior of chopped glass fiber reinforced sheet molding compound composite was investigated. Considering the stochastic meso-structure induced by processing, fiber orientation within specimens was analyzed based on the sectional images obtained by X-ray Computerized Tomography (XCT). Quasi-static tensile and compressive tests were conducted, in which the strain distribution was recorded by Digital Image Correlation (DIC) system. The fatigue behavior of material was evaluated under tension–tension (R = 0.1) and tension–compression (R = -1) loading. Interrupted fatigue test was conducted to investigate the damage evolution on the side surface of specimen using optical microscopy. A modeling method based on Representative Volume Element (RVE) was modified and applied to predict the fatigue behavior of glass fiber SMC. The simulation result showed a good consistency with experimental results on both prediction of the fatigue life and the damage mechanisms under cyclic loading.

Effect of geometrical parameters and tool pattern of multi-tooth sawing on cutting of sheet molding compound composite: FE study

Short glass fiber composites, particularly sheet molding compound (SMC) materials, are becoming increasingly important alternative in various contemporary aerospace, automotive, and electronic applications. For these manufacturing industries, the quality of the machined SMC composite is still a challenging target. The article proposes a new tool design with an offset between teeth to minimize friction, limit damage and promote chip removal when drilling composite materials. The effects of the tool’s geometric parameters, especially the rake, the inclination and the complementary side cutting edge angles on the material removal process, as well as the cutting and thrust forces, are investigated. A 3D finite element model of a representative multi-tooth tool is developed using the ABAQUS\\Explicit code. The results show that fine-tuning the geometric parameters of the tool reduces the induced machining damage and enhances the chip removal and the flow evolution. The rake angle has a significant influence on the cutting and thrust forces. However, both forces are insensitive to the inclination angle. The complementary side cutting edge angle influences only the thrust force. The presented outcomes not only give insights into the cutting process, but also improve the SMC machinability.

On the mechanical properties and damage mechanisms of short fibers reinforced composite submitted to hydrothermal aging: Application to sheet molding compound composite

Sheet Molding compound (SMC) composites were subjected to water immersion tests in order to study their durability since such composites are of interest in automotive applications. Water sorption tests were conducted by immersing specimens in distilled water at 25-90°C for different time durations. In order to investigate the combined action of water and temperature over time on composite mechanical behavior, tensile tests and quasi-static loading were conducted. The mechanical properties of water immersed specimens were evaluated and compared alongside to dry composite behaviour. The tensile tests and quasi-static properties of the studied composite were found to decrease with the increase in moisture uptake. This decrease was attributed toinner structure dégradations by means of osmosis phenomenon. It was shown that hydrothermal aging affects mainly the fiber/matrix interfacial zone while a good adhesion between the reinforcement and the matrix was observed for the virgin samples. In order to well understand the damage mechanisms, scanning electron microspy (in-situ three point bending) tests were performed on aged and non aged specimens. Damage mechanisms were identified for different material states. Results display clearly that damage evolution always begins at the interface regions. Furthermore, a quantitave analysis was performed at a local scale in a representative zone of the tensile area.

Calibrating a fiber–matrix interface failure model to single fiber push-out tests and numerical simulations

To characterize the fiber–matrix interface of a glass-fiber reinforced sheet molding compound (SMC), single-fiber push-out tests are performed and simulated numerically. The parameters of a cohesive zone model for the interface are calibrated on the single-fiber push-out tests. The fracture-toughness/energy release rate therein is determined from cyclic (loading–unloading) experiments. The matrix model, consisting of the nonlinear-elastic Neo-Hooke law with a Prony series to model viscoelastic behavior, is calibrated with data from nanoindentation tests by adjusting simulation curves to their experimental counterparts. Using the calibrated model of the single-fiber push-out, the influence of neighboring fibers and thermally induced residual stresses is shown. The interface damage initiates in the single-fiber push-out test at the indented fiber at positions closest to other fibers under the surface. In addition this is the position where the radially largest fiber expansion due to the Poisson effect is found. The results reveal that although the push-out test is simple to perform, the interpretation of its results might be a complicated task.

Experimental and computational analysis of bending fatigue failure in chopped carbon fiber chip reinforced composites

With a better balance among good mechanical performance, high freedom of design, and low material and manufacturing cost, chopped carbon fiber chip reinforced sheet molding compound (SMC) composites show great potential in different engineering applications. In this paper, bending fatigue behaviors of SMC composites considering the heterogeneous fiber orientation distributions have been thoroughly investigated utilizing both experimental and computational methods. First, four-point bending fatigue tests are performed with designed SMC composites, and the local modulus is adopted as a metric to represent the local fiber orientation of two opposing sides. Interestingly, SMC composites with and without large discrepancy in local modulus of opposing sides show different fatigue behaviors. Interrupted tests are conducted to explore the bending fatigue failure mechanism, and the damage processes of valid specimens are also closely examined. We find that the fatigue failure of SMC composites under four-point bending is governed by crack propagation instead of crack initiation. Because of this, the heterogeneous local fiber orientations of both sides of the specimen influence fatigue life. The microstructure of the lower side shows a direct influence while that of the upper side also exhibiting influence which becomes more prominent for high cycle fatigue cases. Furthermore, a hybrid micro–macro computational model is proposed to efficiently study the cyclic bending behavior of SMC composites. The region of interest is reconstructed with a modified random sequential absorption algorithm to conserve all the microstructural details including the heterogeneous fiber orientation, while the rest of the regions are modeled as homogenized macro-scale continua. Combined with a framework to capture the progressive fatigue damage under cyclic bending, the bending fatigue behaviors of SMC composites are accurately captured by the hybrid computational model comparing with our experimental analysis.

Lattice material infiltration for hybrid metal-composite joints: Manufacturing and static strenght

An innovative technological solution for the manufacturing of high strength and high toughness metal-composite interface is here investigated, in which a 3DP printed cellular structure at the metal side is infiltrated by Advanced Sheet Molding Compound composite, in order to understand the effect of main geometric and processing parameters. Some hybrid metal-composite joints were produced, in which the effect of fiber orientation and cell dimension were considered as representative of the main processing issues. The outcome of the infiltration process was first checked on the hybrid joints, then samples were extracted for mechanical characterization of the bond, in comparison with cellular structure free joints.

Review of Manufacturing Process of Natural Fiber Reinforced Polymer Composites

Lately, natural fibers have been gaining consideration in development of composites due to its biodegradability, availability and low-cost compared to synthetic fibers. Natural fibers are renewable sources, biodegradable, environmentally friendly and effortless in machinery. In this paper, the manufacturing of production for natural fiber reinforced polymer composite (NFRC) will be reviewed. These including injection molding, resin transfer molding (RTM), pultrusion, sheet molding compound (SMC) and compression molding processing technique. Injection molding and compression molding are mostly used in industry. This is due to the machines give a good finishing surface of the products and low cost.

Dynamic-mechanical-thermal analysis of hybrid continuous–discontinuous sheet molding compounds

Sheet molding compounds (SMC) are very promising for the production of lightweight structural components, due to the specific mechanical properties combined with the suitability for a large-scale manufacturing process. Automotive industry already implements SMC materials for structural components in their vehicle concepts. Polymeric materials, hence also fiber reinforced polymers, show a viscoelastic behavior and dynamic-mechanical-thermal analysis (DMTA) is an important method of determining the influence of temperature and loading speed of this material class. In this work, SMC which based on a novel hybrid resin system were examined under bending loads using a electric-dynamic test system to realize high-force dynamic-mechanical-thermal analysis. The examined SMC materials were either discontinuously (Dico) or continuously (Co) reinforced. In addition a hybrid continuous–discontinuous reinforcement was realized by stacking different SMC materials. The mechanical characterization aimed to investigate the influence of the reinforcement architecture and the effect of hybridization on the temperature- and frequency-dependent material properties. Glass transition temperature of the hybrid SMC was comparable to glass transition temperature of the discontinuous glass fiber reinforced component. Compared to the continuous carbon fiber SMC, the decrease of storage modulus of the hybrid SMC could be shifted to higher temperatures and damping was also significantly increased due to hybridization.

Application of the stress interaction failure criterion in platelet composite compression molded parts

AbstractThe stress interaction failure criterion was applied to three compression‐molded sheet molding compound (SMC) materials. Two are epoxy‐based compounds with randomly oriented carbon fiber tows, and the third material consists of glass fiber bundles randomly oriented in a vinyl ester resin. The failure surface was built using tensile test results at different orientations with respect to the flow direction, resulting in combined loads in the principal stress coordinate system. Three unknown variables arise from using only tensile strength values. Solving a system of three equations derived from the criterion function and using the failure values obtained from destructively testing at three different orientations, the unknown variables as well as the interaction strength components were determined. Results show good agreement for all three materials. Overprediction was found in some instances, attributed to the effect of fiber orientation distribution in the part, and to the complex interactions of the filled structures. To allow a more direct comparison between predicted and experimental data, the resulting failure surface was translated to the global coordinate system (σxx) and compared to the data derived from destructive tensile tests. Good agreement for all materials was observed, with underprediction noted for some orientations, attributed in part to the equation solving method and the nature of the mesostructures of the composites, where interactions between the tows, resin, mold walls, and fibers are present. The results of this work show that the methodology described permits the development of a failure surface for SMC materials when limited data is available.

Modeling of Short Fiber Reinforced Polymer Composites Subjected to Multi‐block Loading

Short Fiber Reinforced Composite (SFRC) structures exhibit multiple microstructures (due to material flow during the process). They are generally subjected to variable amplitude loadings. In this context, a robust model is needed to predict fatigue life as a function of microstructure. In this paper, we propose a predictive micromechanical damage-based model allowing fatigue life prediction in the case of SFRC submitted to variable amplitude cyclic loading. An experimental study was firstly performed on Sheet Molding Compound (SMC) composite involving different microstructure configurations. Specimens were subjected to stress-controlled block loading. The influence of the order of the sequences was evaluated through Low-High amplitude (L-H) and High-Low amplitude (H-L) schemes. Damage accumulation is computed at the local scale to describe the evolution of the fiber-matrix interface damage until failure. A local failure criterion based on a critical damage state allowed predicting variable amplitude fatigue life as a function of microstructure. A good correlation was found between experimental and numerical results. Once the approach was validated, it has been used to model different useful variable amplitude loading schemes to emphasize the role of the loading sequence parameters and order.

A Comprehensive Assessment of Commercial Process Simulation Software for Compression Moulding of Sheet Moulding Compound

With a growing interest in the application of carbon fibre Sheet Moulding Compound (SMC), a number of commercial software packages have been developed for the simulation of compression moulding of SMC. While these packages adopt different algorithms and meshing strategies, the constitutive material model and processing control are usually adapted from injection moulding process simulation. Little has been done in the literature for assessing the capabilities of these software as design tools, and more importantly, validating the process simulation results using experimental data. This paper aims to provide an independent and comprehensive assessment of existing well-known process simulation software for SMC compression moulding. The selected software will be compared in terms of material models, and available processing settings in order to determine their robustness as a compression moulding design tool. The predictive accuracy of the software will also be assessed by comparing the compression force and filling patterns against the experimental data.

Characterisation of tape-based carbon fibre thermoplastic discontinuous composites for energy absorption

ABSTRACT Tape-based discontinuous composite is a relatively new type of composite material that offers improved mechanical properties for similar process-ability compared to Sheet Moulding Compound or Bulk Moulding Compound. This makes it potentially attractive for the automotive industry. In this paper, a thin-ply carbon fibre reinforced polypropylene-based discontinuous composite is studied. Mechanical tests are performed to obtain the tensile, compression and shear behaviour of the material. The energy absorption via tearing is also studied to assess the suitability of the material for energy absorption applications, such as crash-boxes. The tearing test results show a large degree of plastic deformation and an advancing damage front leading to higher specific energy absorption via tearing compared to conventional composite materials.