Sheet Molding Compound Research Articles

A probabilistic virtual process chain to quantify process-induced uncertainties in Sheet Molding Compounds

The manufacturing process of Sheet Molding Compound (SMC) influences the properties of a component in a non-deterministic fashion. To predict this influence on the mechanical performance, we develop a virtual process chain acting as a digital twin for SMC specimens from compounding to failure. More specifically, we inform a structural simulation with individual fields for orientation and volume fraction computed from a direct bundle simulation of the manufacturing process. The structural simulation employs an interpolated direct deep material network to upscale a tailored SMC damage model. We evaluate hundreds of virtual specimens and conduct a probabilistic analysis of the mechanical performance. We estimate the contribution to uncertainty originating from the process-induced inherent random microstructure and from varying initial SMC stack configurations. Our predicted results are in good agreement with experimental tensile tests and thermogravimetric analysis.

Micromechanical transverse tensile crack propagation of unidirectional fiber reinforced epoxy SMC slice imbedded in a TDCB specimen

A mode-I type of crack propagation and branching on account of fracture energy failure mechanisms methodology is investigated in unidirectional (UD) fiber reinforced epoxy sheet molding compounds (SMC). In order to eliminate the influence of structural boundary conditions with various crack lengths on tip K (stress intensity factor) value to a greatest extent, a tapered double cantilever beam (TDCB) tensile sample is selected as the far-field boundary, the SMC slices with pre-cracks are embedded into TDCB samples, which are referred to as the embedded TDCB (E-TDCB) geometries. By applying the transvers tensile load to E-TDCB, embedded SMC is provided with far-field driving force for crack propagation, so as to obtained a great agreement between stable crack propagation and minor tip K oscillation. Both E-TDCB and dummy TDCB (d-TDCB) samples were selected for experimental test with comparation. The dynamic crack propagation parameters (work components of far-field driving force as well as crack length and Crack tip open displacement et al.) were measured and analyzed with progressive damage criterion. Although in disparate principle, the numerical prediction with proposed constitutive law correlates well with experimental results.

Experimental and numerical characterisation of fibre orientation distributions in compression moulded carbon fibre SMC

ABSTRACT A main challenge in design with long discontinuous fibre-based sheet moulding compound (SMC) is to understand flow-induced fibre orientation in the part. There are very few experimental methods for characterising the fibre orientation distribution in carbon fibre SMC. Consequently, even though process simulation models for predicting fibre orientation have been developed, the confidence in using these models remains low. This research aims to study the fibre orientation distribution in compression moulded carbon fibre SMC using a combined experimental and numerical approach. A recently developed micro-CT scanning-based method will be adopted to study carbon fibre SMC samples of various sizes. The data collected from the experimental study will be used to assess the predictive validity of selected commercial process simulation packages.

Towards a Holistic Approach for an Efficient Determination of the Rheological Behavior of Sheet Molding Compounds for Process Simulation

Sheet Molding Compound (SMC) materials offer attractive specific strength and stiffness properties. With an addition of short cycle times, the possibility for complex geometries and a high recycling potential, this material is a promising solution for the manufacture of lightweight components of numerous industrial sectors. Accurate and reliable simulation can contribute to a fast and competitive development process and effectively reduce the time-to-market. Especially, the accuracy of the simulative filling process is crucial to be able to predict the mechanical properties and hence, pave the way to structural parts. An appropriate characterization of the rheological behavior is therefore, crucial for accurate simulation results. To this purpose bar flow tests are performed and the experimental results are compared to the numerical results. A sensitivity analysis is performed to investigate the influence of each parameter on the simulation outcome. Furthermore, guidelines to improve the simulation accuracy and the testing equipment are derived. In the testing method some improvement potential was identified and the necessity for a testing method closer to the compression process could be established.

Flow Behaviour of Carbon Fibre Sheet Moulding Compound

The growing popularity of SMC compression moulding in the automotive industry has led to great interests in process simulation model development. Existing process simulation models are commonly adapted from models originally developed for short fibre composites and lacking experimental validation data. A novel material model specifically developed for SMC flow simulation is proposed in this paper, where the experimental material characterisation is performed using the squeeze flow testing method. The model is validated through simulation of the squeeze flow testing and the accuracy of the model is assessed by comparing the predicted compression forces against experimental data collected from the squeeze flow testing. The proposed new model has demonstrated significant improvement in comparison to existing commercial models in term of compression force prediction, but the predicted compressive forces diverted away from the experimental values towards the end of the test. The predicted cavity pressure distribution has also been investigated and compared to observations reported in the literature where good correlation between predicted pressure distribution and the literature have been achieved.

Rapid prediction of fibre-dominant tensile failure in randomly oriented strands

A stochastic morphological modelling framework has been established to predict the tensile behaviour of Randomly Oriented Strands made of ultra-thin Carbon Fibre Reinforced Thermoplastic prepreg tapes. The tape properties from their distributions are generated in a Monte Carlo simulation. The Young’s modulus of a laminate is accurately predicted using classical laminate theory. Fibre-dominant tensile failure is also accurately predicted in Carbon Fibre Reinforced Thermoplastics Sheet Moulding Compounds using Weibull theory. Material discontinuity is accounted for via the introduction of a stress concentration factor, as a result of tape overlaps. The predicted tensile strength values and scatter were found to increase with increasing tape length, which agrees well with literature data, and thus demonstrates the reliability of the proposed modelling framework. The rapid modelling framework is well-suited for application in structures.

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.

Dielectric Analysis of Single Walled Carbon Nanotubes Loaded SMC Composites Depending on Resin Type

This paper investigates effect of different industrial resins including vinyl ester and unsaturated polyester produced by sheet moulding compound (SMC) method on conductivity and dielectric properties of single walled carbon nanotubes (SWCNT) modified glass fiber reinforced polymer composites (GFRP), comparatively. Alternating current (AC) conductivities, complex dielectric permittivity, dissipation factor (DF), electric modulus of unsaturated polyester and vinyl ester resin-based composites was examined by impedance analyzer depending on frequency range from 10-2 Hz to 107 Hz at room temperature. Cole–Cole approach was used to describe the impedance characteristics of tested samples. In order to clearly understand the effect of resin type, the glass fiber and carbon nanotube filling ratios in the produced materials were kept constant. AC conductivity results showed that VE composite exhibited better performance than UP composite in all frequency. The theoretical models including Power Law and Linear Regression (LR) algorithm also confirmed the experimental results about the conductivity of the composites. A comparison of the dielectric characteristics between SWCNT/VE and SWCNT/UP binary composites indicated that VE based composite exhibited superior dielectric properties compared to the UP based sample. In addition, scanning electron microscopy (SEM) images have been taken to observe distribution and organization of fillers in polymer composites.

On the dependence of orientation averaging mean field homogenization on planar fourth-order fiber orientation tensors

A comprehensive study on the influence of planar fourth-order fiber orientation tensors on effective linear elastic stiffnesses predicted by orientation averaging mean field homogenization is given. Fiber orientation states of sheet molding compound (SMC) are identified to be in most cases approximately planar. In the planar case, all possible fourth-order fiber orientation tensors are given by a minimal invariant set of structurally differing planar fourth-order fiber orientation tensors. This set defines a three-dimensional body and forms the basis for a comprehensive study on the influence of a fiber orientation distribution in terms of a fourth-order tensor on homogenized stiffnesses. The methodology of this study is the main contribution of this work and can be adopted to analyze the orientation dependence of any quantity which is a function of a planar fourth-order fiber orientation tensor. At specific points inside the set of planar fiber orientation tensors, effective stiffnesses are calculated with selected mean field homogenization schemes. These schemes are based on orientation averaging of transversely isotropic elasticity tensors following Advani and Tucker (1987), which is explicitly recast as linear invariant composition in the fiber orientation tensors of second and fourth order of Kanatani third kind. A maximum entropy reconstruction of a fiber orientation distribution function based on leading fiber orientation tensors, enables a new numerical formulation of the Advani and Tucker average for the special planar case. Polar plots of Young’s modulus and generalized bulk modulus obtained by selected homogenization schemes are arranged on two-dimensional slices within the body of admissible fiber orientation tensors, visualizing the influence of the orientation tensor on the stiffness tensor. The orientation-dependence of the generalized bulk modulus differs significantly between selected homogenizations. Restrictions on the effective anisotropic material response caused by orthotropy of closure approximations are discussed.

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.

Radar sensor integration and quality testing of sheet moulding compound parts in the automotive frequency range

Sheet moulding compounds (SMC), a type of fiber-reinforced polymers with discontinuous fibers, are a highly used lightweight material due to their cost efficiency and processability. Since the use of radar sensors behind structural skin body parts for advanced drivers assistant systems in the automobile industry is increasing, the usability of this kind of material as a radar cover is examined. Additionally, a method for the analysis of SMC parts with radar measurement methods in the Gigahertz area is introduced. The hereby considered parameters are the fiber orientation, fiber volume content, and quality failures like welding lines as they are crucial for the mechanical properties of the material. The methods presented are already in use in the automotive industry for quality assurance of radar covers and show potential as a fast and non-destructive method to analyse those important material parameters of SMC components.

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.

Experimental and numerical analyses of the tensile strength and mixed-mode fracture behavior of sheet molding compound plates

Polymer matrix composites are the most prevalent ones among all the composite materials because they offer high specific strength to weight ratio, toughness, and ease of processing. These composites own most of their properties to the high-strength reinforcement materials as carbon, glass, and aramid fibers. The mass production method of such composites is basically the sheet molding compound (SMC) compression molding technology. However, regarding the inherent defects of SMCs, such as porosities and internal cracks, the current trend of improving their performance requires a thorough understanding of mechanical and fracture properties of such products. Therefore, in this work, the commercially available chopped glass-fiber reinforced polyester SMC has been subjected to uniaxial tensile and mixed-mode fracture experiments, along and perpendicular to the rolling direction. The results of these experiments were employed as input data in a finite element model to determine the fracture toughness (KIC and KIIC) and critical strain energy release rate (GIC and GIIC) of material under mixed-mode loading conditions. Overall, although the reinforcing material were in the form of strands of glass fiber, SMC specimens exhibited lower mechanical properties and fracture toughness in transverse direction as compare with that of the longitudinal (rolling) direction under all modes of loadings.

Advanced process simulation of compression molded carbon fiber sheet molding compound (C-SMC) parts in automotive series applications

The following paper focuses on the development of a 3D virtual process chain (VPC) for parts made from Carbon Fiber Sheet Molding Compounds (C-SMC). The VPC consists of two modules addressing the compression molding and warpage simulation of parts made from two different high fiber volume fraction C-SMC materials. The press simulation module applies a solid mechanics material modelling approach within the Coupled Eulerian-Lagrangian (CEL) simulation framework available in ABAQUS/Explicit. Through a combination of flow characterization experiments and numerical simulations, local mechanical properties and fiber orientation information can be predicted from numerically simulated local flow strains. A warpage simulation module has also been developed in ABAQUS/Standard together with a data processing/transfer interface implemented in ANSA. With these highly integrated numerical tools, process characteristics such as press forces, flow fronts and weld-line development based on material amount, insert geometry and placement position in the mold can all be predicted and analyzed.

Investigation of the Mechanical Properties of Vinylester-Based Sheet Moulding Compound (SMC) Subject to Particle Recycling

In light of climate change, fiber-reinforced plastics (FRP) are becoming increasingly important due to their lightweight construction potential. This benefit is additionally expanded on with the low cycle times, scrap rates and material costs of Sheet Moulding Compounds (SMC), making it the most often used material on the FRP market. While extensive studies regarding the recycling of SMC with unsaturated polyester-based matrices (UP-SMC) have been conducted in the past, this is not the case for vinylester-based SMC (VE-SMC). In this research, VE-SMC components were subject to a particle-recycling approach. Recyclate in the form of VE-SMC regrind was used to substitute SMC matrices during the compounding process. The dependence of the mechanical properties and failure behavior of VE-SMC containing regrind was investigated by means of microscopic and mechanical test methods for varying mass proportions of SMC regrind. Due to the addition of SMC regrind, a decreased fiber/matrix adhesion was observed. Furthermore, an increase in pore formation was observed with an increasing proportion of SMC regrind. The flexural modulus increased by 20% with a low percentage of regrind (ωrSMC = 10 wt.%) in comparison to virgin SMC. In contrast, the tensile properties decrease (up to 30%) with the addition of SMC regrind independent of the investigated proportions of SMC regrind.

Manufacturing Demonstration of Automotive Seat Backrest Using Sheet Molding Compound and Overmolding with Continuous Reinforcement

Manufacturing Demonstration of Automotive Seat Backrest Using Sheet Molding Compound and Overmolding with Continuous Reinforcement

Experimental and Numerical Analysis of SMC Compression Molding in Confined Regions—A Comparison of Simulation Approaches

The compression molding process of sheet molding compound (SMC) is an economical manufacturing process for lightweight parts. However, molding defects, such as fiber matrix separation, and fiber re-orientation, may develop during the molding process in confined regions, such as ribs and bosses. Hence, the mechanical properties of the composite depend on the local fiber architecture. Consequently, this work compares the predictive capabilities of tensor-based and directly modeled process simulation approaches regarding compression force, fiber volume content and fiber orientation on the example of honeycomb structures molded from SMC. The results are validated by micro-computed tomography and thermal gravimetric analysis. The fiber orientation in the honeycomb varies between individual samples because a sheet molding compound is macroscopically heterogeneous and thus the fiber architecture is strongly influenced by random events. Tensor-based fiber orientation models can not reliably predict fiber volume content and fiber orientation in the part’s thickness direction if there is a lack of scale separation. Therefore, directly modeled process simulations should be preferred in cases in which fiber length and mold dimensions prohibit scale separation. The prediction of fiber volume content is a difficult task and no simulation can predict the severity of fiber matrix separation precisely in all cases.

Use of bio-based and renewable materials for sheet molding compounds (SMC) – Mechanical properties and susceptibility to fungal decay

In Europe, Sheet Molding Compound (SMC) has high market relevance in the field of glass fiber reinforced polymer composites (GFRPC). This is based on their wide range of application abilities. Almost 20 wt.-% of GFRPC are processed of SMC material, e.g. for semi-structural components in various applications from construction industry to automotive components. Based on the ability for the production of lightweight structural components in a cost-efficient manufacturing process with high design freedom and high formability, material and process are well established. Standard glass fiber reinforced SMC semi-finished products (GF-SMC) consist of resin paste, based on unsaturated polyester resin, different application specific additives, reinforcing fibers with a length of one to two inches and mineral filler materials. In this paper, the impact of the replacement of conventional mineral filler material by bio-based and renewable filler material sunflower hull meal to mechanical properties and biological decay was analyzed.

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).