Sheet Molding Compound Composites Research Articles

Fracture analysis of chopped carbon fiber sheet molding compound composite under tensile loading via in-situ μXCT

This paper investigated the crack initiation in a chopped carbon fiber sheet molding compound (SMC) composite under different tensile loads by integrating synchrotron micro-X-ray computed tomography (μXCT) and micromechanics analysis. It was found that the varied distribution of fiber chips in the SMC composite led to distinct fracture propagation behaviors. With the loading increasing, the crack primarily occurred within the fiber chips, and the delamination exited when the crack propagated along the interface between adjacent layers. To understand the location and sequence of crack initiation in SMC composites, the sample were divided into identical cubes. After tracking the microstructure of each cube, the corresponding interfacial debonding strength as well as Young's modulus were predicted. Predictive modeling of crack initiation characteristics was conducted assuming homogeneous material deformation under small strain. The predicted outcomes closely matched with the experimental findings.

Thermal Stability, Flame Retardancy and Flame Retardant Mechanisms of Hollow Glass Microspheres/Montmorillonite/Epoxy Sheet Molding Compound Composites

Epoxy sheet molding compound (ESMC) composites have excellent mechanical properties, dimensional stability and electrical insulation, and are widely used in the automotive industry. In our previous work silane coupling agent modified, hollow glass microspheres (SiHGM) and montmorillonite (MMT) were incorporated to enhance the mechanical properties of the ESMC composites. It was found that the ESMC composites containing MMT (EP/SiHGM/MMT-1) exhibited excellent mechanical strength. However, the flammability of the epoxy resin (EP) limits the use of ESMC composites for applications that require good flame retardancy. In this paper we describe our research in which MMT was used to increase the flame retardancy of ESMC composites. The results showed that the addition of MMT made the ESMC composites have excellent flame retardancy. The limiting oxygen index (LOI) of the ESMC composites containing 1 phr MMT (EP/SiHGM/MMT-1) was 26.8%, which was 8.94% higher than that of the ESMC composites without MMT (EP/SiHGM/MMT-0). In addition, the peak heat release rate (PHRR), average heat release rate (AHRR) and total heat release (THR) of the EP/SiHGM/MMT-1 were 322.8 kW/m2, 95.3 kW/m2 and 38.6 MJ/m2, respectively, which were 22.6%, 8.2% and 8.0% lower than those of EP/SiHGM/MMT-0. The carbon monoxide production (COP) and carbon dioxide production (CO2P) of EP/SiHGM/MMT-1 were 0.0092 g/s and 0.2071 g/s, respectively, which were 34.3% and 22.8% lower than those of EP/SiHGM/MMT-0. The fire growth index (FGI) of EP/SiHGM/MMT-1 was 2.4, which was a decrease by 29.4% compared to EP/SiHGM/MMT-0. In addition, the flame retardant mechanisms were revealed. We suggest that MMT promoted dehydration and charring of the epoxy resin and helped construct an intact physical barrier, which had good blocking effects. The prepared composite has great application prospects for those occasions that require high flame retardancy. Thus, this work provides an innovative strategy for preparing low density epoxy composites with excellent flame retardancy and high mechanical properties.

Hollow Glass Microspheres/Montmorillonite/Epoxy Sheet Molding Compound Composites With Low Density and Excellent Mechanical Properties

Epoxy sheet molding compound (ESMC) composites have good mechanical properties, insulation performance, and corrosion resistance, and are widely used in the automotive industry. However, the density of a typical ESMC composite is high, which limits its wider application. Lightweight fillers can reduce the density of ESMC composites, but the addition of lightweight fillers can lead to a decrease in the mechanical properties of the composite. In this paper, we describe our research in which hollow glass microspheres (HGM) were utilized to decrease the density of the ESMC composites. In addition, surface modification of the HGM was performed to enhance the compatibility between the HGM and the epoxy resin (EP), thus improving the mechanical properties. Further, montmorillonite (MMT) was introduced to further improve the mechanical properties of the ESMC composites. The results showed that the density of the prepared composite was about 1.5 g/cm3, which was significantly lower than the industry standard sheet molding compound (SMC) density of 1.9 g/cm3. The flexural strength and impact strength of the resulting ESMC composites containing the modified HGM and an appropriate amount of MMT (EP/SiHGM/MMT-1) reached the maximum value of 80.0 MPa and 28.6 kJ/m2, respectively, which were 39.9 and 51.3% higher than that of the ESMC composite containing only HGM (EP/HGM/MMT-0). On the one hand, the modified HGM was well dispersed in the EP for the chemical bonding effect. On the other hand, the fiber/matrix interface was further improved after introducing an appropriate amount of MMT. Thus, the resulting ESMC composite exhibited low density and excellent mechanical properties and showed great potential for applications that require lightweight and high strength.

Probabilistic finite element analysis of sheet molding compound composites with an extended strength distribution model

Sheet Molding Compound (SMC) composites exhibit large scatters in mechanical properties and a strong correlation between the strength and stress distribution. For example, SMC demonstrates a much higher strength in the 3-pt bending test compared to that demonstrated in the uniaxial tension test. As a result, finite element (FE) simulations with the mean mechanical properties underpredict the flexural response of the SMC structures. Also, probabilistic finite element (PFE) analysis with a unimodal statistical strength model built from the uniaxial tensile data did not solve this problem. To account for the dependency of strength on stress distribution, an extended strength distribution model (ESDM), in the form of a trimodal Weibull distribution consisting of unimodal Weibull distribution models of tension and flexural strengths, has been proposed. The ESDM was subsequently tested in PFE simulations of SMC tensile and flexural experiments following a randomization algorithm which considers the tensile strength as a random variable and assigns it according to ESDM across finite element models. Simulations with this approach successfully reproduced both the tensile and flexural responses with the predicted mean peak load, post-peak behavior, and energy absorption similar to experimental results. • A computational efficient method to include the scatter and size effect in FEA. • A trimodal Weibull strength model based on both tension and flexure strength data. • Simulations reproduced the tensile and flexural responses in experiments.

Function-oriented defect assessment in hybrid sheet molding compound tensile specimen using surrogate models

Glass fiber (GF) Sheet Molding Compound (SMC) composites are popular lightweight materials due to their good processability. Hybrid SMCs expand the field of operation, combining the high stiffness of unidirectional carbon fibers (CF) with the economic efficiency of GF. Combinations of manufacturing deviations (delamination, varying GF content, CF misorientation) occur during the production of hybrid SMCs and impede the mechanical performance of the part. A function-oriented quality assurance instead of strict tolerances is proposed. Finite element (FE) simulations are computationally too expensive for an assessment within the cycle time. Hence, surrogate models are trained on multiple parameterized FE simulations. The surrogate models shall allow for an individual functional assessment in real-time based on integrated measurement inputs. This work focuses on the generation of parametrized FE simulations for measurement inputs and surrogate modeling. Simulations and surrogate models show acceptable deviations from tensile tests for multiple combinations of manufacturing deviations. The measurement uncertainty of the stiffness prediction is assessed for both the FE simulation and the surrogate models in accordance with the Guide to the Expression of Uncertainty in Measurement (GUM).

Experimental and cohesive zone modelling study on composite joining by co-curing and adhesive bonding for sheet moulding compound or carbon-fibre prepreg laminates

Material selection plays an important part in engineering as the chosen material impacts not only performance but also the final cost of a component. Engineering has evolved to a point in which it allows manipulation of base materials to meet specific design and functional requirements. The present work experimentally and numerically studies the adhesion of both sheet moulding compound composites, and carbon fibre reinforced composites, using the single-lap joint geometry. Two grades of sheet moulding compound were studied, and two bonding methods were used: co-curing and adhesive bonding. Numerical analysis by cohesive zone modelling, after estimating the adhesive and composite interlayer properties, enabled predicting the maximum load and fracture mode. The cohesive zone modelling results were accurate, and the best joining solution involved co-curing the CFRP adherends, while all sheet moulding compound adhesive joints failed to attain the full adhesives’ capacity due to premature substrate tensile failure.

Fatigue behavior of hybrid continuous-discontinuous fiber-reinforced sheet molding compound composites under application-related loading conditions

Hybrid continuous-discontinuous sheet molding compound (SMC) composites are considered suitable candidates for structural automotive applications, due to their high mass-specific mechanical properties combined with high geometrical flexibility and low costs. Since structural automotive parts are subject to repeated loading, profound knowledge of their fatigue behavior is required. This paper presents an experimental study on the bending fatigue behavior of hybrid SMC with discontinuous glass fibers in the core and unidirectional continuous carbon fibers in the face layers. Effects of hybridization on the S-N behavior and stiffness degradation have been analyzed in constant amplitude fatigue tests under 3-point bending load at different temperatures and frequencies. Microscopic investigations on polished specimen edges were used to study the damage behavior. The ultimate flexural strength at quasi-static (UFSS) and fatigue strain rate (UFSF) of the hybrid composite was 54 % and 59 % higher than that of discontinuous SMC, respectively. In contrast, the flexural fatigue strength at 2.6⋅106 cycles increased by 258 %. The relative stiffness degradation of the hybrid composites was smaller during most of their fatigue lives due to the continuous carbon fiber reinforcement. The carbon fiber ply on the compression loaded side was the first ply to fail. Fatigue stress significantly decreased at 80 °C due to early kinking of the continuous carbon fiber-reinforced ply on the compression loaded side. Variation of frequency had no significant effect on the fatigue behavior of both discontinuous and continuous-discontinuous SMC.

Effects of hybridization on the tension–tension fatigue behavior of continuous-discontinuous fiber-reinforced sheet molding compound composites

This study is focused on the fatigue of sheet molding compounds (SMC) with a discontinuous (DiCo) glass fiber-reinforced core and continuous (Co) unidirectional ([0/DiCo/0]) or cross-ply ([0/90/DiCo/90/0]) carbon fiber-reinforced face layers. Tension-tension tests were conducted on the constituents (i.e., DiCo and Co SMC) and hybrid specimens. Hybridization effects were larger under cyclic than under monotonic loading. Constraining effects resulted in enhanced high fatigue strength of the Co plies in [0/DiCo/0]. [0/90/DiCo/90/0] was more sensitive to fatigue than [0/DiCo/0] due to localized damage starting from 90°-ply cracks. The S-N behavior of the hybrid was approximated from the behavior of DiCo SMC.

Fatigue behavior and modeling of chopped carbon fiber reinforced sheet molding compound composites

AbstractThe quasi‐static and fatigue behavior of chopped carbon fiber reinforced sheet molding compound (SMC) composite was investigated. The microstructure of material was inspected by X‐ray Computerized Tomography (XCT) and the fiber orientation distribution was analyzed. Quasi‐static tensile test was performed on specimens with different gauge length to study the size effect on mechanical properties and digital image correlation system was applied to record the strain distribution. The fatigue behavior of material was evaluated under both tension‐tension (R = 0.1) and tension‐compression (R = −1) loadings, in which XCT scanned specimens were tested to characterize the microstructure at failure location. Interrupted fatigue test was performed to observe the damage evolution process on the side surface of specimen by optical microscope and the dominant fatigue failure modes were identified. A representative volume element based modeling methodology was modified and applied to establish the relationship between heterogenous microstructure and macroscopic mechanical behaviors. The simulation results show good consistency with experimental observations which demonstrates the effect of local fiber orientation on durability performance of material.

A computational multiscale model for anisotropic failure of sheet molding compound composites

We present a holistic multiscale approach for constructing anisotropic criteria describing the macroscopic failure of sheet molding compound composites based on full-field simulations of microscale damage evolution. We use an anisotropic damage model on the microscale that directly operates on the compliance tensor, captures matrix and bundle damage via dedicated stress-based damage criteria and allows for a comparison of simulation and experimental results. To identify the damage material-parameters used in the non-linear and time-consuming full-field simulations, we utilize Bayesian optimization with Gaussian processes. We validate the full-field predictions on the microscale and subsequently identify macroscopic failure criteria based on distributions taken from experimental findings. We propose failure surfaces in stress space and stiffness-reduction triggered failure surfaces to cover both a structural analysis and a design process perspective.

Static and fatigue behavior of a carbon‐fiber SMC‐R composite under combined tensile and shear stresses

Randomly oriented short fiber reinforced sheet molding compound composites (SMC-R) are among the most commonly used composites for automotive and many non-automotive structural applications. Many studies have reported the static and fatigue properties of SMC-R composites under uniaxial stress conditions. However, there are many applications in which they may be subjected to biaxial stress conditions and their biaxial properties have not been reported in literature. The current study considers the static and fatigue characteristics of a randomly oriented carbon fiber SMC-R under both uniaxial and biaxial loading conditions with various combinations of tensile and shear stresses. A butterfly shaped Arcan specimen was used to conduct the biaxial stress tests. Since both static strength and fatigue life data exhibited significant variability, statistical analysis was conducted using Weibull distribution to analyse the variations in these properties of the material. In biaxial conditions in which a shear stress is imposed along with a tensile stress, the tensile strength of the material decreases with increasing shear stress. A failure envelope drawn based on von Mises equation shows reasonable agreement with the experimental results. As a percentage of the static strength, the fatigue strengths at 1 million cycles in tension and combined equal tension and shear are significantly higher than the fatigue strength in pure shear.

Multiscale failure and damage analysis of sheet molding compound (SMC) composites using Micro-CT image-based reconstruction model

This paper presents a multiscale failure and damage analysis methodology for sheet molding compound (SMC) composites through a new bundle packing reconstruction algorithm and complete failure and damage constitutive modeling. Due to the complex flow pattern during the compression molding process, the SMC composites are featured by a spatially varying distribution of fiber bundles. The novel bundle reconstruction algorithm can generate representative volume element (RVE) mesostructure models considering the statistical fiber bundle orientation and dispersion based on image analysis of actual SMC composites. In particular, statistical bundle dispersions are investigated in-depth, which has not yet been considered in damage analysis of SMC composites. Moreover, constitutive modeling of constituents is implemented for simulating various local damage such as bundle failure, cohesive damage, and matrix damage observed in SMC composites. The effects of the statistical mesostructure heterogeneities on the failure and damage behavior are discussed through a series of failure analyses. Finally, the simulation results are experimentally validated through a tensile test and actual inspection results by X-rays, 3D imaging, and scanning electron microscope (SEM).

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.

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.

Study on preparation and properties of bentonite-modified epoxy sheet molding compound

Abstract In this study, bentonite/epoxy sheet molding compound composites (BS/ESMC) were prepared with different bentonite contents (0.5, 1, 1.5, 2, 2.5, 5, and 10 wt%) by hot compression molding. The effects of BS content on the mechanical properties, thermal stability, and fire-retardant properties of samples were investigated. When the BS addition amount is 1.5%, the tensile strength, flexural strength, and impact strength reach the maximum, increasing by 24.15%, 26.56%, and 51.33%, respectively. The measurement of mechanical properties showed that the fracture toughness of BS/ESMC composite has been greatly improved from 71.41 to 108.07 MPa. As the content of the bentonite increases, the heat resistance of the sample increases, and the residual carbon content of the system increases by 61.54% when the amount of the bentonite added is 10%. In addition, the value of LOI increased from 25.6 to 27.9 with the addition of the bentonite.

Microstructure investigation of hydrothermal damage of aged SMC composites using Micro-computed tomography and scanning electron microscopy

This paper presents an investigation on the effects of hydrothermal aging on Sheet Molding Compound (SMC) composite. Two different techniques were carried out to study the inner structure of aged SMC composite. Firstly, X-ray micro computed tomography (XµCT) was used to evaluate the changes using 3D images. The results showed cracks in all the composite structure with different shapes and volume in response to the hydrothermal conditions. The cracks results from the build up of an osmotic pressure in microcavities, which is proportional to water concentration. However, it was not possible to quantify separately the hydrothermal induced damage in the studied SMC composite material. Therefore, the XµCT analyzes were supplemented by a microscopic study. This step has been studied in terms of crack density evolution and crack propagation rate. The results obtained by XµCT technique and SEM observations show that the damage increases continuously with time and temperature during aging. The damage was found to be located in the voids contained in the matrix at early stage of aging. Then it is mostly developed into the fiber interface in the form of fiber/matrix interfacial debonding.

On the effective elastic properties based on mean-field homogenization of sheet molding compound composites

Fiber reinforced polymers, for example sheet molding compounds (SMC), have gained significant importance as lightweight materials. In this work, a SMC composite based on unsaturated polyester-polyurethane hybrid resin (UPPH) reinforced with discontinuous long glass fibers is considered. The mechanical behavior of this composite is highly governed by its microstructure. Based on samples extracted from compression molded sheets, the underlying microstructure is characterized by means of X-ray computed tomography (µCT) scans. Tensile tests are performed to investigate and characterize the elastic properties of this composite. A mean-field method is presented to approximate the effective elastic behavior. In this context, the tensile tests performed serve as validation data for the simulation results. The effective Young’s modulus is in good agreement with the experimentally obtained data with a deviation less than 15%.

Hollow glass spheres in sheet molding compound composites: Limitations and potential

AbstractSheet molding compounds (SMC) are commonly used in automotive applications as they lead to increased fuel efficiency of vehicles due to their high strength to weight ratios. This study focuses on understanding the effect of replacing calcium carbonate (CaCO3) with hollow glass spheres (HGS) in an unsaturated polyester resin matrix SMC composite with the ultimate goal to further reduce density without compromising performance. The resulting glass fiber (GF)‐reinforced syntactic foam laminate composites with GF content ranging from 10 to 15 vol% have densities of 1.2 and 1 g/cm3, respectively. This is significantly lower than the industry standard SMC density of 1.9 g/cm3. The HGS parameters of interest are primarily HGS type (S28HS and S32HS), loading level (33–44 vol%), and surface functionalization (methacrylsilane). The syntactic foam composites were characterized in terms of tensile, flexural, and impact properties, and their properties were compared to those of standard density SMC. As expected, the flexural, tensile, and impact properties were compromised with a decrease of density. However, they were higher than the corresponding properties of low‐ and ultra‐low density composites reported in the literature, as we were able to add higher GF content in these low‐density SMC formulations. It was also concluded that methacrylsilanized surface treated HGS showed promise in maintaining the mechanical properties, especially at high sphere loadings. The impact energy of the low‐density formulations was relatively unaffected by HGS type or loading.