Sheet Molding Compound Research Articles

Coupled effects of hygrothermal degradation and fatigue damage of sheet molding compound (SMC) composite

Industrial sheet molding compound (SMC) composite structures are susceptible to environmental degradation, primarily from moisture and temperature. Furthermore, these materials are subjected to fatigue loading. It is therefore necessary to generate Wohler curves for a range of service conditions, taking into account exposure time and temperature. Given the time-consuming nature of these preliminary characterizations, this paper presents an innovative approach to rapid fatigue life prediction using both monotonic and fatigue tests. The core concept of the proposed model is to establish an equation of state that correlates first-cycle macroscopic damage to fatigue life. By coupling this relationship with micromechanical modelling of quasi-static damage, we can rapidly determine SN curves for any considered aged state. The methodology also integrates the microstructure as an input, significantly reducing the need for extensive experimental characterization. A comparison between experimental and simulated Wöhler curves shows excellent agreement over different ageing conditions for SMC composites.

Multiscale image-based modeling for failure prediction of sheet molding compound composite under uniaxial tension

Carbon fiber reinforced polymer (CFRP) composites are extensively utilized as primary load-bearing components in various engineering applications due to their superior strength-to-weight ratio and excellent mechanical properties. However, their intricate microstructural interactions within composite present a significant challenge for failure analysis of CFRP. Although finite element (FE) simulations have been proven feasible to conduct the failure analysis, the classical FE models are developed based on homogeneous fiber characteristics, ignoring the influence of internal structures on the damage evolution process. This paper presents a multiscale image-based modeling approach to predict the tensile failure procedure of chopped carbon fiber sheet molding compound (SMC) composite. To accurately reconstruct the representative volume element (RVE) model of the SMC composite, synchrotron micro-X-ray computed tomography (μXCT) was adapted to explore the SMC internal microstructure. Then microscale RVE models with different fiber volume fractions were constructed to predict the corresponding microcosmic mechanical properties, which were used as the inputs for mesoscale RVE models to determine the constitutive parameters of fiber chips having varied fiber volume and orientations. Finally, the YOLOv5_Seg algorithm was employed to extract the geometric feature parameters of the fiber chips for mesoscale RVE modeling and then the failure location and sequence under uniaxial tension were predicted. It is found that the final simulated failure behaviors were consistent with the experimental observations, confirming the feasibility of this approach for understanding the failure mechanisms of CFRP composites. Thus, once the internal microstructure is determined using experimental techniques or predicted by simulating the composite manufacturing process, this approach can also be utilized for design optimization and performance evaluation for CFRP composites.

Predictions of sheet molding compound composite structures using probabilistic finite element simulations with bimodal Weibull strength distributions

Due to the large scatters in Sheet Molding Compound (SMC) mechanical properties, the uncertainty in the prediction of mechanical responses of SMC composite structures is significantly high as compared to that for continuous fiber composite structures. Another challenge is the size effect resulting from the scatter in mechanical strengths, i.e., the measured material flexural strength is much higher than its tensile strength. As a result, classical approaches that rely on the mean tensile strength can substantially underpredict the flexural responses in Finite Element Analysis (FEA). This work investigates the use of a material model with bimodal Weibull distributions for its strength properties in Probabilistic FE (PFE) simulations. A Random Search-based optimization procedure was developed to obtain model parameters which are difficult to determine. The material model was verified by PFE simulations of standard experiments, and then examined in simulations of open-hole tension and disk bending tests.

In-situ experimental characterization and numerical investigation of the crack initiation of SMC composite under uniaxial tension

This paper explored the crack initiation of sheet molding compound (SMC) composite under uniaxial tensile loads by integrating in-situ experimental characterization and subsequent numerical analysis. Firstly, a synchrotron micro-X-ray computed tomography was utilized to character the morphology and evolution of the microstructure and fracture features of the SMC sample with different loading conditions. Meanwhile, the digital volume correlation technology was employed to determine the internal three-dimensional deformation. Then, a numerical analysis was conducted to describe the relationships among the microstructure, deformation distribution, and position of the crack initiation. Finally, a predictive model was developed based on the internal microstructure and the deformation distribution and then utilized to predict the location and sequence of crack initiation. The good agreement between the predicted results with the experimental ones reveals the feasibility of the proposed model in exploring the fracture behaviors of carbon fiber reinforced polymer composites with complex internal microstructure.

Full‐field effect of flow‐induced fiber orientation during compression molding of carbon fiber sheet molding compounds

AbstractAn image‐based fiber orientation analysis technique has been employed to study the effect of initial charge coverage and preferential fiber alignment on the flow‐induced fiber orientations in carbon fiber sheet molding compounds (SMCs). The initial charge coverage area exhibited minimal fiber reorientation while significant fiber alignment in the flow direction was observed in the regions of greatest flow. Novel subsurface analysis using the same full‐field orientation analysis technique also showed the presence of a skin‐core structure with greater flow‐induced fiber alignment at the mold surfaces, while the core retained more of the initial fiber orientation distribution. Edge effects were also revealed by this analysis at the mold walls. Tensile testing from two orthogonal directions showed that panels molded from lower charge coverage resulted in better mechanical performance and lower anisotropy. Moreover, by encouraging charge flow in the preform's preferential alignment direction, high tensile stiffness and strength in the preferred direction were achieved (56.7 GPa and 238.9 MPa, respectively) at 30% fiber volume fraction. This provides a practical demonstration of the value in controlling fiber orientation, with respect to charge positioning and mold flow, to maximize and tailor the composite performance of SMCs typically used for automotive applications.Highlights Low‐cost scanning technique is extended to assess internal fiber orientations Full‐field orientations are measured before and after molding Investigation of how charge size and shape can affect orientations Combining effects of flow and initial orientation to tailor SMC performance Circular statistics provides a thorough analysis of orientation distributions

Low-velocity impact response of hybrid sheet moulding compound composite laminates

This work presents a comprehensive study on the impact damage tolerance of Sheet Moulding Compounds (SMCs). The performance of glass, carbon and hybrid glass/carbon SMCs are compared by means of tensile, compression, low-velocity impact and compression after impact experiments. Damage analysis of the impacted laminates was performed by ultrasonic and X-ray methodologies. The glass SMC exhibited the highest damage tolerance in low-velocity impact with the smallest damaged area, crack density and loss in compression after impact (CAI) strength. On the other hand, the carbon SMC demonstrated superior in-plane stiffness and strength, but exhibited a large damaged area and crack density under impact. The hybrid SMC displayed an optimal compromise, exhibiting intermediate tensile in-plane performance and excellent damage tolerance at lower impact energy levels, but suffered from extensive delamination at the highest impact energy. Overall, the findings highlight the suitability of hybrid SMCs for structural applications with potential impact risks.

Increasing the prediction quality of artificial neural network–based optimisation approaches for the sheet moulding compound compression moulding process by means of targeted expansion of training data

The resulting fibre orientation when pressing Sheet Moulding Compound (SMC) is essentially dependent on the position and geometry of the resin mat in the mould. It is desirable to adapt the fibre orientation as best as possible to best withstand the applied load. The relationship between the position and geometry of the resin mat and the deflection of the final part under a specific load can be mapped using a numerical simulation; nevertheless, an optimisation is not feasible based on the simulation in an economically reasonable time frame due to the required computing time. For this reason, a metamodel based on an ensemble of artificial neural networks (ANN) was created on the basis of the simulation data, and an optimisation based on these ANN was set up. However, the investigations show that the minimal deflections found on the basis of the metamodel through optimisation have large deviations compared to the simulation. This leads to the hypothesis that the applied optimisation finds weak points of the metamodel instead of actual minima. From this hypothesis, an extension approach of the data set on which the metamodel is based by the weak points found is derived and implemented. This is done in an iterative process. The results show that the goal of increasing the prediction quality of the optimisation approach is achieved by the developed approach. The root mean square error (RMSE) could be reduced from the original deflection of 25 to 7 mm, which corresponds to a reduction of 72%. It is shown that the presented approach is successful in increasing prediction quality; nevertheless, further measures are needed to reach decent prediction quality.

Damage Assessment of Sheet Molding Compound Composites

The investigation focuses on the initiation and progression of damage in SMC (Sheet Moulding Compound) composites subjected to tension. These composites consist of a polyester matrix reinforced with randomly oriented short C glass fibers. Damage monitoring is conducted using the acoustic emission technique. To assess the influence of surface density on damage in SMC, three series of composites with surface concentrations of 300 g/m2, 450 g/m2, and 600 g/m2 are tested. Based on stress-strain curves, the main mechanical properties of the different composites are determined. Additionally, the analysis of the collected acoustic emission data enabled us to determine the damage thresholds in the different studied composites. While tensile tests and the acoustic emission technique allowed us to determine important composite parameters, further research is necessary to comprehensively discern the various damage modes.

Effects of temperature on the anisotropic mechanical properties and failure mechanisms of Sheet Molding Compounds

Sheet Molding Compounds (SMC) are widely used in many industries. Due to their high in-mold flow, fibers tend to align during the component processing and generate anisotropic mechanical properties. Besides, being SMCs thermoset composites, their application in high temperature environments often results to be of interest. In this research, the fiber orientation and temperature-dependent mechanical behavior and failure mechanisms were studied. First, materials were compression molded into plates, promoting fiber orientation. Then, the materials’ orientation-dependent viscoelastic behavior was analyzed by Dynamic Mechanical Analysis, and their anisotropy level was assessed at quasi-static tensile, bending and compression modes at 23 °C and 130 °C. Finally, the failure mechanisms were inspected by confocal microscopy to evaluate the influence of temperature and fiber orientation on them. The results demonstrate how the temperature influences the material’s mechanical performance and their anisotropic behavior, especially at 45°. Likewise, temperature alters failure mechanisms, by triggering the fiber-matrix interface failure.

Quantification of the Influence of Charge Variations on the Flow Behavior of Sheet Molding Compounds.

Using a newly developed flow test bench, several charge configurations were analyzed to quantify the influence of the charge configuration in the mold in sheet molding compound (SMC) manufacturing. A test bench was developed to satisfy the industrial needs for the incoming goods inspection as well as the need for the flow characterization of rheological models in the simulation. The test setup has a cylindrical opening for the charge placement, from where the material is pressed into a thin flow channel, forcing the material to reorient. A comparison was performed by juxtaposing the resulting compression pressure recorded during the process. The charge for this test series, placed into the cylindrical opening, has two basal configurations, one consisting of a stack of disks, and the second in a rectangular sheet rolled up into a spiral. Six charge variations were tested in total. The amount of material, the batch, the layering and the production direction of the sheet proved to have a significant influence on the necessary compression pressure. Guidelines about the recommended charge configurations could be derived for optimized production settings, such as a reduction in the compression pressure and modifications to the charge.

Design, manufacturing and experimental validation of additive manufacturing cores for bearing seats in carbon fibre reinforced polymer structures

The integration of carbon-fibre reinforced polymers (CFRP) in structural applications offers significant advantages due to their high strength-to-weight ratio. However, these materials exhibit limitations under out-of-plane loads, particularly in bearing applications. This study explores an innovative approach to enhance the performance of CFRP structures in such scenarios by incorporating annular polyamide inserts manufactured via additive manufacturing (AM).To evaluate the mechanical performance of the AM inserts, a novel ring tensile test is designed to emulate the bearing load conditions. This test also enables the analysis of the impact of several design and manufacturing parameters of the AM inserts, including surface geometry, surface treatment, internal structure, and curing process. These are then compared with specimens made of carbon fibre sheet moulding compounds (CF-SMC), commonly used to support bearing loads. The study reveals that AM inserts provide a viable alternative to state-of-the-art CF-SMC, offering a significant enhancement in mechanical properties under specific bearing loading conditions. The test results indicate a 17.7% improvement in the first failure limit and an 8.6% increase in the ultimate strength for AM inserts compared to CF-SMC.Additionally, the study develops a simplified analytical model to predict stress distributions and potential failure mechanisms, validating its efficacy through experimental data with discrepancies of less than 6%. Economic analysis underscores the cost benefits of AM inserts due to reduced labour and higher repeatability. This research demonstrates the potential of AM inserts to improve the stability and strength of CFRP structures, paving the way for their broader application in demanding load-bearing environments.

Laser-Induced Breakdown Spectroscopy Analysis of Sheet Molding Compound Materials

Sheet Molding Compound (SMC) materials are extensively utilized as high-voltage insulation materials in electrical equipment. SMC materials are prone to aging after long-term operation. Conducting non-destructive testing to assess their electrical and physicochemical properties is crucial for the safe operation of electrical equipment. This study identifies the optimal equipment parameters for testing SMC materials using Laser-Induced Breakdown Spectroscopy (LIBS) technology through experimental investigation and also explores the ablation characteristics of SMC under various laser parameters. The results indicate a significant positive correlation between the ablation depth and laser pulse number, while there is no correlation with single laser pulse energy. However, the ablation area demonstrates a strong positive correlation with both single laser pulse energy and laser pulse number. Additionally, LIBS spectral analysis provides elemental results comparable to Energy Dispersive Spectroscopy (EDS), facilitating the examination of variations in Na, Ti, Fe, Mg, Ca, and C elemental contents with depth. Moreover, an enhanced iterative Boltzmann plot method is suggested for calculating the plasma temperature using 21 Fe I spectral lines and the electron density using the Fe II 422.608 nm line. The variations of these plasma parameters with laser pulse number are documented, and the results show consistent trends, confirming that the laser-induced SMC plasma adheres to local thermodynamic equilibrium.

On the infiltration of cellular solids by sheet molding compound: process simulation and experimental validation

This study examines a numerical method to simulate the production of novel multi-material metal-composite components, where an additive-manufactured cellular solid is infiltrated by a sheet molding compound (SMC) in a single-step compression molding operation. A single-fiber numerical approach is adopted to predict microstructural changes, such as fiber orientation, fiber-matrix separation, and fiber volume content variations during molding. The accuracy of the numerical predictions is confirmed by physical samples using micro-computed tomography and optical microscopy investigations at both the qualitative and quantitative scales. From optical microscopy observations, there emerged a positive correlation between experimental outcomes and simulation results, accurately capturing fiber swirling, wrinkling, and draping that occurred during molding. At a quantitative scale, a 0.6% mismatch was observed when void volume and unfilled areas were compared, as measured by micro-computed tomography and numerical simulation.

Optimization of precharge placement in sheet molding compound process

This study aims to provide precise predictions for the compression of reinforced polymers during the sheet Molding Compound (SMC) process, ensuring the attainment of a predefined structure while preventing material overflow during the process. The primary challenge revolves around identifying the optimal initial shape to prevent material rebound during the process. To confront this issue, a numerical model is utilized, faithfully simulating the SMC process and forming the foundation for our investigations. Furthermore, to optimize the pre-fill stage, a surrogate model is proposed to enhance modeling efficiency, and then an inverse analysis method is applied. This approach of minimizing material rebound during the SMC process results in a reliable metamodel to predict an initial mass shape accurately and at a low computational cost, thus ensuring the squeezed material fits the mold shape.

A stochastic modelling framework for predicting flexural properties of ultra-thin randomly oriented strands

A stochastic modelling framework is developed to predict the flexural properties of high-strength sheet moulding compounds made of randomly oriented ultra-thin carbon fibre-reinforced thermoplastic prepreg tapes. The model enables reliable designs using ultra-thin randomly oriented strands with less testing, leading to potential applications in automotive primary structures. The stochastic model is based on a Monte Carlo simulation. The flexural modulus is predicted using classical laminate theory, while the flexural strength is predicted by following a Weibull distribution. Fibre discontinuities are considered through stress concentrations introduced by tape overlaps. The results are validated against new 3-point bending and previously reported 4-point bending experimental results. A scaling effect on strength and scatter is predicted, and its implications for structural applications are also discussed.

Sustainable Optimization of Mold Heating: A Dual Approach with SWARA and MARCOS Methods

There are many methods and raw materials in the manufacturing of fiber reinforced plastics (FRP) with hot press, such as Sheet Molding Compound (SMC), Bulk Molding Compound (BMC), and Prepreg fabrics. In most applications, it is commonly used to put the new dough into the mold without cooling the mold, press it again and cure it. Putting the mold into the dough without cooling causes the surface of the poured product to cure faster than the inner region, thus causing a structurally discontinuous structure in the product. Therefore, in more professional manufacturings, the mold is lowered to approximately 120 oC and the dough is put into the mold at this stage. However, this increases energy consumption and carbon emissions for the heating and cooling phases. In this study, the energy efficiency in the manufacturing of fiber reinforced plastics (FRP) produced by the hot press method was investigated. At the end of this study, by using alternative energy methods in manufacturing processes, results such as investment cost, depreciation expenses, reduction in invoices, and carbon emissions were achieved. In order to find the best alternative among these results, the criteria weights were determined with SWARA, and the founded alternatives ranked by the MARCOS method. As a result of this ranking, the best alternative was determined as a 40 kWh battery and a 25 kW solar panel option among the solar panel power and battery capacity alternatives.

Experimental process characterisation for high-volume compression moulding of hybrid-architecture composites

Compression moulding of woven fabric prepreg and sheet moulding compound (SMC) in a single-shot process combines the superior mechanical properties of continuous fibre composites and the high design flexibility of discontinuous fibre composites. This experimental study presents insights into the moulding characteristics of prepreg and SMC and explores the interaction between these two materials during manufacturing of hybrid-architecture composites. All tests were performed under typical compression moulding conditions using a squeeze flow testing rig. The response of each material was interpreted through proposed material models applied to the data acquired from single architecture tests. Critical interaction mechanisms were identified from hybrid architecture tests, alongside some additional deformation mechanisms, such as high level of in-plane fibre tow deformation not observed in single-architecture tests. Novel experimental methods were also introduced to quantify the change in meso-scale fibre architecture in the prepreg induced by the flow of SMC. The outcomes of this study not only shed light on the essential interaction mechanisms of hybrid-architecture composites but also provide valuable insights into deformation mechanisms not apparent in isolated material studies. Ultimately, these findings will facilitate the future development of a numerical simulation model for hybrid moulding processes.

Characterization of Embedded and Thinned RF Chips

This work studies the effect of thinning down chips with transmission line structures for flexible embedding technology. Test chips with 90 Ω grounded coplanar waveguide (GCPW) transmission lines have been designed and manufactured to characterize the high-frequency performance before and after embedding in sheet molding compound (SMC), and thinning down from 300 μm to 20 μm. Molding first technology enables single-die or multiple-die thinning with the embedding material while being held in the exact position, where the subsequent fanout copper (Cu) redistribution layer (RDL) structuring realizes the direct contacts to the embedded chip, followed by semiadditive manufacturing of traces. The embedded and thinned chips are characterized by S-parameter measurements in the frequency range from 20 MHz to 65 GHz using a vector network analyzer (VNA). The results show the feasibility of the examined embedding and thinning processes which are suitable for millimeter wave (mmWave) components as shown by the example of a G-CPW transmission line without degrading its RF performance in terms of characteristic impedance and attenuation.

Mechanical Characterizations on biobased Sheet Molding Compound (SMC) developed for battery boxes.

In this work, performed within FENICE project (www.fenice-composites.com), semifinished materials for the mass production of fire-resistant battery box in the automotive. SMC were developed using PFA (polyfurfuryl alcohol) resin as the matrix and both long and short basalt fibers as reinforcement in a SMC and impregnation pilot line. Mechanical characterization is used to compare prepreg based on fabric impregnation and SMC and measure basic data needed for modelling. Short fiber reinforced composites are cheaper and more isotropic than those reinforced with fabrics, making them more interesting for mass market application in the automotive, such as electrical cars. The semifinished material production must be optimised, and basic mechanical data must be measured, to be used as input into FEM of the components to be produced.

The strain-rate-dependent tensile failure and energy-absorbing behavior of sheet molding compounds

Sheet molding compounds (SMCs) as an alternative material for low-cost and lightweight automotive structural components require special attention to their failure behavior and collision safety. This study aims to analyze the energy-absorbing response and variability of unsaturated polyester matrix SMCs under the range of strain rates from 10−3/s to 500/s through quasi-static and dynamic tensile tests. Additionally, the tensile process and fracture morphology were characterized using Digital Image Correlation and Scanning Electron Microscopy to reveal the underlying rate-dependent failure mechanism. The results demonstrate a pronounced positive correlation between the tensile strength and absorbed energy of SMCs with strain rates. Specially, the absorbed energy exhibits substantial variability, primarily attributable to the plastic damage behavior during the nonlinear phase. A transition in the failure mode from debonding to pseudo-delamination, accompanied by a more extensive failure area at high strain rates, serves as the main mechanism for enhancing the material energy absorption capacity.