Complex Deformation Mechanisms Research Articles

Adiabatic Shear Localization in Metallic Materials: Review.

In advanced engineering applications, there has been an increasing demand for the service performance of materials under high-strain-rate conditions where a key phenomenon of adiabatic shear instability is inevitably involved. The presence of adiabatic shear instability is typically associated with large shear strains, high strain rates, and elevated temperatures. Significant plastic deformation that concentrates within a adiabatic shear band (ASB) often results in catastrophic failure, and it is necessary to avoid the occurrence of such a phenomenon in most areas. However, in certain areas, such as high-speed machining and self-sharpening projectile penetration, this phenomenon can be exploited. The thermal softening effect and microstructural softening effect are widely recognized as the foundational theories for the formation of ASB. Thus, elucidating various complex deformation mechanisms under thermomechanical coupling along with changes in temperatures in the shear instability process has become a focal point of research. This review highlights these two important aspects and examines the development of relevant theories and experimental results, identifying key challenges faced in this field of study. Furthermore, advancements in modern experimental characterization and computational technologies, which lead to a deeper understanding of the adiabatic shear instability phenomenon, have also been summarized.

Bending fatigue behavior of metastable and stable austenitic stainless steels with different surface morphologies

AbstractThe surface morphology has a significant influence on the fatigue behavior of components. For austenitic stainless steels (ASSs), this issue is even more pronounced due to their metastability. Based on the complex deformation mechanisms of metastable ASSs, which include dislocation slip, deformation twinning, and deformation‐induced martensitic phase transformation, the metastable stainless steel AISI 347 was investigated in this study together with the stable AISI 904L as a reference material. Four‐point bending fatigue tests with load ratio R = 0.1 and testing frequency f = 10 Hz at ambient temperature were carried out on specimens with five technically relevant surface morphologies: mechanical polished, milled, microshot peened, laser shock‐peened, and ultrasonic modified. Systematic material characterizations were carried out to elucidate the crucial role of surface roughness and deformation‐induced α′‐martensite in the fatigue behavior of both metastable and stable materials. While surface roughness is a well‐known key factor in conventional fatigue cases, deformation‐induced martensite layers implemented by various surface modification methods were proven to improve the fatigue life in metastable austenitic steels, opening new perspectives to extend the lifetime of ASS components.

Deformation behavior, constitutive modeling and microstructure evolution of 2195 Al-Li alloy deformed at high strain rates and cryogenic temperatures

High-speed and cryogenic-temperature forming technologies have exhibited great potential for fabricating advanced Al-Li alloy components for aircraft due to their ability to enhance materials’ formability. However, there remains an urgent need to study the high-accuracy constitutive model and complex deformation mechanism of Al-Li alloys deformed at high strain rates and cryogenic temperatures. In this study, the flow stress behavior and microstructure evolution of 2195 Al-Li alloy were investigated over a wide stain rate range (from 2000 s−1 to 5000 s−1) and temperature range (from 298 K to 123 K). The flow stress increases by ∼ 32% at 2000 s−1 and ∼ 37% at 5000 s−1 as the deformation temperature drops from 298 K to 123 K, but only increases by ∼ 4% at 298 K and ∼ 8% at 123 K as the strain rate increases from 2000 s−1 to 5000 s−1. At the high strain rate of 5000 s−1, the T1 phases exhibit bending (at 298 K) and fracturing (at 123 K), with increased lattice distortion and substructure generation around the intersections of T1 phases. A new constitutive model (g-Johnson-Cook model) is developed, which shows a higher accuracy than both the Arrhenius model and Johnson-Cook model in describing the deformation behavior.

Quantitative evaluation of orientation effects in wear and friction characteristics of pure nickel using experiments and atomistic simulations

Nanoscale tribological behavior of Ni for its applications such as MEMS is an important area of study. The present study investigated the nanoscale friction and wear behavior of Ni on different crystallographic orientations using experiments and molecular dynamics simulations. Results showed an anisotropic friction response, with (100) [001] case showing high COF followed by (111) [1¯10] and (110) [1¯10] cases. Analysis showed that such a trend in COF is observed due to variation in activation of the number of slip systems in each case as determined by the Schmid factor calculations. Dislocation analysis showed that (100) [001] case developed the highest number of glissile dislocations which is attributed to high COF value. The wear volume followed an opposite trend of COF order, and the spatial distribution of displaced material is found to be different in each case, indicating an anisotropic ploughing mechanism. Such behavior is observed due to variations in the availability of the number of slip directions, their orientation on the scratch plane, and dislocation evolution during nanoscratch. Overall, this study provides insights into the complex deformation mechanism and wear behavior associated with scratch tests and has implications for the development of wear-resistant materials.

The role of phase fraction on the deformation behavior and tensile properties of dual-phase polycrystalline Fe–Ni alloy: A molecular dynamics simulation study

An alloy containing two or more phases with significantly distinct properties may produce a unique combination of high strength and ductility. Fe–Ni alloys processed through the powder metallurgy route form dual-phase structures containing both body- and face-centered cubic phases. The detailed atomic scale microstructural evolution and deformation mechanism of such alloys have not been performed as dual-phase structure materials generate more complex deformation mechanisms than single-phase structure materials. The present study utilizes molecular dynamics simulation to compute the mechanical properties and investigate the deformation mechanism in a dual-phase Fe–Ni alloy with a variable phase fraction of body-centered cubic during a uniaxial tensile test. The results show that phase fraction affects the mechanical properties as well as the deformation behavior. Shear strain distribution and dislocation density provided an explanation for the change in the average flow stress and strengthening mechanism of the dual-phase Fe–Ni alloys. Additionally, the influence of temperature on deformation behavior is investigated. The deformation mechanism at higher temperatures is found to be dislocation slip and grain boundary sliding.

Remnants of shifting early Cenozoic Pacific lower mantle flow imaged beneath the Philippine Sea Plate

Seismic anisotropy could provide vital information about the evolution and internal convection of the deep Earth interior. Although previous seismological studies have revealed a wide distribution of seismic anisotropy in the upper portion of the lower mantle beneath many subduction zones, the existence of anisotropy at these depths away from subducted slabs remains debated. Here we use P-wave azimuthal anisotropy tomography to image the crust and mantle down to 1,600-km depth. We find prominent anisotropic patterns in the upper portion of the lower mantle beneath the Philippine Sea Plate. Substantial azimuthal anisotropy with N–S fast-velocity directions occurs at 700–900-km depths. We interpret this azimuthal anisotropy as a remnant of the Pacific lower mantle flow field about 50 million years ago. Two isolated high-velocity anomalies at 700–1,600-km depths may be vestigial pieces of the subducted Izanagi slab with seismic velocity features suggesting a shift in the Pacific lower mantle flow field by about 40 million years ago. Our findings provide seismic evidence for the existence of complex lower mantle flows and deformation mechanisms away from subduction zones.

Tailoring nanoprecipitates to achieve ultrahigh strength (CoCrNi)94.5W3Ta2.5 medium-entropy alloys

Metallic materials with high mechanical performance are in constant demand by various engineering applications. However, most conventional strengthening mechanisms inevitably suffer from fiercely sacrificing ductility, which severely limits service safety of metallic alloys, especially under various harsh circumstances. In this work, a novel ultra-strong (CoCrNi)94.5W3Ta2.5 medium entropy alloy (MEA) with dual nanoprecipitates is successfully developed. By conducting multiple thermomechanical processes, not only quantities of coherent γ" nanoprecipitates with D022 superlattice are introduced, but also massive semi-coherent η nanoprecipitates with hexagonal D024 superlattice are reconstructed with appropriate sizes and distributions, meanwhile ultrafine grains are achieved, together enabling superior strength-ductility synergies. This MEA exhibits not only high tensile strength of about 1.7 GPa and ultimate elongation of 23.8 % at room temperature, but also ultra-high tensile strength of about 2.2 GPa and still sufficient ultimate elongation of 13.2 % at cryogenic temperature. In-depth microstructure characterization indicates that the nanoprecipitates are effective barriers to dislocation motion at the early stages of plastic deformation. Intriguingly, the slip and deformation twinning could be transmitted across the phase boundaries with increasing applied strain, ensuring both strength enhancement and plasticity maintenance. At cryogenic temperature, much more complex deformation mechanisms are activated, e.g., multiple slip systems and high-density deformation twins, facilitating even better mechanical properties. The design concept of strengthening materials via tailoring dual coherent nanoprecipitates may afford a paradigm to develop advanced metallic materials with ultrahigh strength.

Application of a Three-Level Elastoviscoplastic Model for Describing Complex Loading Processes

It is known that the meso- and microstructures of metals determine the physical, mechanical and operational properties of their final products. Scientific and technological progress of recent decades has given impetus to the elaboration and use of models capable of describing the evolving structure of materials. The most promising are multilevel models that include internal variables and are based on physical theories of elastoplasticity (elastoviscoplasticity). This paper presents the structure and basic relationships of a three-level (macro-, meso-1 and meso-2 levels) elastoviscoplastic model. The developed model operates on such internal variables as dislocation densities on slip systems, barriers on split dislocations and sources of edge dislocations. The model describes the mechanisms of production, annihilation, formation of barriers and sources of dislocations. The law of hardening directly takes into account the densities of dislocations and barriers. The mechanism of inelastic deformation is the gliding of edge dislocations along slip systems. Special emphasis is placed on the influence of split dislocations (prone to forming hard Lomer–Cottrell and Hirth barriers) on the deformation of the material. The model is used to describe the behavior of an elastoviscoplastic polycrystalline aggregate with an FCC lattice. Geometric nonlinearity is taken into account by utilizing decomposition of the crystallite motion into quasi-rigid and deformation components. For this purpose, a rigid moving coordinate system for the crystal lattice is introduced. Examples of the application of the model for analyzing the simple and complex deformation mechanisms of materials with different stacking fault energies and, consequently, with different tendencies toward the decomposition of dislocations and barrier formation are given.

A unified strength criterion for two-dimensional materials via bond failure analysis

The family of two-dimensional (2D) materials, including graphene, hexagonal boron nitride, transition metal dichalcogenides, and others, exhibits a wide range of lattice structures and defect configurations, leading to complex deformation mechanisms and mechanical failure behaviors. However, there is currently no universally accepted criterion that accurately describes the mechanical failures of these materials. In this study, we aim to address this issue by introducing the concept of intrinsic bond strength, which is solely dependent on the local chemical environment of the bond and is independent of loading states, defect types, and fracture bonds. We demonstrate the existence of this intrinsic bond strength and propose a unified strength criterion that considers the balance between the intrinsic bond strength and local stress state for any 2D material. By employing this unified strength criterion, we are able to accurately predict the failure of various 2D materials with different types of defects, including voids, cracks, grain boundaries, and hydrogenation. This work resolves a longstanding issue in predicting the mechanical failure of 2D materials via bond failure analysis, and carries important implications for the design and development of 2D materials with enhanced mechanical properties.

Unraveling size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti by non-local crystal plasticity modeling

Titanium alloy is prominent in manufacturing of high-performance miniature devices, while size-affected plastic heterogeneity and asymmetry in micro-scaled deformation are not well understood due to its complex deformation mechanism. How the interplay among dislocation slip, deformation twinning, and strain gradient affects the size-dependent plastic heterogeneity and asymmetry is an intractable and non-eluded issue that needs to be unraveled. In this research, a non-local crystal plasticity model (NLCPM) considering discrete twins was established and implemented into a finite element framework. The proposed NLCPM successfully represented the size-dependent asymmetric mechanical response, primary and secondary twinning, and texture evolution. The size-dependent fracture behavior, plastic heterogeneity, and T-C asymmetry during micro-scaled deformation of commercial pure titanium (CP-Ti) were explored by coupling micro-tension/compression test and full-field simulation. Results showed that T-C asymmetry in flow stress was enhanced in the coarse-grained and small-diameter samples. For the sample with D≥0.6 mm, the ductility of CP-Ti was improved with the increase of grain size as there are more prevailing twins and the concentrating stress could be relieved by the abundant slip rather than cracking. Grain-level strain is more homogeneous in compression when d was increased from 60 to 110 μm since more non-prismatic slips were activated that accommodated strain along various directions. Meanwhile, plastic strain was allocated within discrete twin bands and thus alleviated strain concentration. Furthermore, the hardening effect of twin boundaries cancelled out the softening effect near the free surface, resulting in a narrower softening layer in the coarse-grained samples compared to its grain size. With the decrease of the sample size, the proportion of the softening layer increased, while the volume fraction of twinning decreased, leading to a reduction in macroscopic stress and fracture strain. This work enhances the in-depth understanding of size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti and supports the forming of high-quality micro components under complex deformation path.

Kinematic and hydrodynamic modeling of a wire-driven robotic fishtail: An experimental study

Robotic fish have been extensively developed for scientific research and engineering applications, with the robotic fishtail serving as a crucial component. Due to the complex deformation mechanism and hydrodynamic interaction, it remains a significant challenge to establish a real-time precise kinematic and hydrodynamic model for flapping robotic fishtails. This paper presents a kinematic and hydrodynamic model for a wire-driven robotic fishtail, incorporating experimental investigation. The wire-driven mechanism featuring a continuously compliant backbone and position-variable vertebrae is developed, enabling more flexible fishtail flapping and adjustable fish morphology. The kinematic and hydrodynamic models are established through the quasi-static method with proposed corrections accounting for flapping velocity and nonlinear backbone deformation during a flapping stroke, precisely assessing real-time hydrodynamic responses. The proposed models have been validated through physical tests in an open-water environment, and exhibit superior response predictive ability compared to traditional methods. A significant force peak is observed at the beginning of a flapping stroke, while small amount of reversed thrust occurs at its end. In a high-frequency flapping motion, the soft caudal fin generates greater thrust than a hard fin. Comprehensively, this study presents a systematic research methodology for modeling and experimental investigation of novel robotic fishtails and biomimetic propellers.

Insights into fracture behavior of Ni-based superalloy single crystals: An atomistic investigation

Comprehending the plastic deformation and failure mechanisms in Ni-based superalloys is vital to enhance their intrinsic ductility. In view of this, the present work intends to illustrate the complex deformation mechanisms ensued during tensile deformation of pre-cracked Ni-Ni3Al single crystals using atomistic simulations. Simulations of fracture, thermodynamic continuum treatments of crack initiation and detailed calculations of crack propagation toughness are performed synergistically to analyze the competition between dislocation emission and cleavage. This competition is evaluated under Mode-I loading for 18 different pre-cracked configurations (three different crack lengths, three phase orientations and two phases) in totality. Our results demonstrate that dislocation as well as twin plasticity eventuate and the competition between the two plastic modes depends on the crystallographic orientation of the abutting phases. Higher initial crack lengths can otherwise lead to cleavage like propagation in the material; however this does not imply inferior ductility of the system as long as the phase boundary lies in the path of advancing crack. Changes in configurations in terms of pre-cracked phase or initial crack lengths can engender appreciable differences in the elementary crack tip phenomena. The numerical computations of crack propagation toughness are compared favorably well with the available experiments in the literature. The calculated variations in overall toughness of the material are discussed and analyzed in light of the observed deformation landscapes.

Identifying the effect of coherent precipitates on the deformation mechanisms by in situ neutron diffraction in an extruded magnesium alloy under low-cycle fatigue conditions

As a new class of heat-treatable magnesium alloys, the low-alloyed Mg-Al-Ca-Mn alloy has great engineering potential because of its excellent extrudability and high strength by the dispersion of Guinier-Preston (G.P.) zones. The complex deformation mechanisms associated with hexagonal crystals, the interactions between defects with such G.P. zones, as well as their impacts on fatigue lifetime, are all critical issues before the actual deployment of such materials. In this study, this type of Mg alloy with and without G.P. zone dispersion during cyclic deformation was investigated by in situ neutron diffraction measurements. The relationship between the macroscopic cyclic deformation behavior and the microscopic response (particularly twinning and detwinning) at the grain level was established. The general deformation mechanism evolution in samples under the solution-treated (S.T.) state was generally similar to that in samples under the peak-aged (P.A.) state. Both samples plastically deformed by extension twinning during compression, and by a sequential process of detwinning and dislocation motion under reverse tension. Results suggest that the precipitates provided the greatest strengthening against the prismatic slip (∼ 45% increase), the moderate to the {101¯2} extension twinning (∼ 27 %), and the least to the basal slip (∼ 11% increase). The P.A. samples have shorter fatigue lifetime primarily due to the excessive dislocation pileups and the higher backstress that promote crack initiation.

Hybrid interatomic potential for Sn

To design materials for extreme applications, it is important to understand and predict phase transitions and their influence on material properties under high pressures and temperatures. Atomistic modeling can be a useful tool to assess these behaviors. However, this can be difficult due to the lack of fidelity of the interatomic potentials in reproducing this high pressure and temperature extreme behavior. Here, a hybrid EAM-R---which is the combination of embedded atom method (EAM) and rapid artificial neural network potential---for Tin (Sn) is described which is capable of accurately modeling the complex sequence of phase transitions between different metallic polymorphs as a function of pressure. This hybrid approach ensures that a basic empirical potential like EAM is used as a lower energy bound. By using the final activation function, the neural network contribution to energy must be positive, assuring stability over the whole configuration space. This implementation has the capacity to reproduce density functional theory results at 6 orders of magnitude slower than a pair potential for molecular dynamics simulation, including elastic and plastic characteristics and relative energies of each phase. Using calculations of the Gibbs free energy, it is demonstrated that the potential precisely predicts the experimentally observed phase changes at temperatures and pressures across the whole phase diagram. At 10.2 GPa, the present potential predicts a first-order phase transition between body-centered tetragonal (BCT) $\ensuremath{\beta}$-Sn and another polymorph of BCT-Sn. This structure transforms into body-centered cubic near the experimentally reported value at 33 GPa. Thus, the Sn potential developed in this paper can be used to study complex deformation mechanisms under extreme conditions of high pressure and strain rates unlike existing potentials. Moreover, the framework developed in this paper can be extended for different material systems with complex phase diagrams.

High-Throughput Investigation of Multiscale Deformation Mechanism in Additively Manufactured Ni Superalloy

In this paper, Inconel 718 (IN718) superalloy was processed by laser powder-bed fusion additive manufacturing (L-PBFAM), followed by heat treatment. High-resolution nanoindentation was used to investigate the complex deformation mechanisms that occurred at various length scales in both conditions. The nanoindentation elastoplastic maps show a strong crystal orientation dependency of modulus and hardness, which is attributed to the high mechanical anisotropy of IN718. The hardness map effectively resolves complex microscale strength variation imparted due to the hierarchical heat distribution associated with the thermal cycles of L-PBFAM. The disproportionately high hardening effect of Nb, Mo-rich chemical segregations and Laves phases in dendritic structures is also observed. The heat treatment resulted in a 67% increase in yield strength (from 731 MPa in the L-PBFAM condition to 1217 MPa in the heat-treated condition) due to the activation of multiple precipitation-strengthening mechanisms. The nanoindentation mapping of a heat-treated sample delineates the orientation-dependent hardness distribution, which apart from high mechanical anisotropy of the alloy, is also contributed to by a high degree of coherency strengthening of the D022 γ″-precipitates oriented parallel to the <001> crystal plane of the γ-matrix. The mean hardness of the sample increased from 13.3 GPa to 14.8 GPa after heat treatment. Evidence of extensive deformation of twin networks and dislocation cells was revealed by transmission electron microscopy of the deformed region under the nanoindentation tip.

Rapid assessment of out-of-plane nonlinear shear stress–strain response for thick-section composites using artificial neural networks and DIC

Thick-section fiber-reinforced polymer matrix composites (TSCs) have been increasingly used in primary and secondary load-bearing structures. Due to complex deformation mechanisms, accurate out-of-plane constitutive relations, including nonlinear shear stress–strain response, are required for structural analysis and failure prediction of TSC structures. A plate torsion test is proposed to assess the nonlinear out-of-plane shear stress–strain response using an artificial neural network (ANN) and digital image correlation (DIC). The ANN model was developed using three-dimensional finite element numerical simulations, and the out-of-plane shear stress was obtained with good accuracy. The efficiency of the shear stress calculation has been improved. The usability of the test method has been refined substantially. A comparison of the shear stress with the FEM-calculated result showed that the average error of the ANN-predicted shear stress was less than 0.5%. Accordingly, the constitutive parameters of a 15-ply thick S6C10/AC318 glass/epoxy unidirectional tape panel have been extracted from the minimization of the normalized error between ANN-calculated shear stress and DIC-measured shear strain iteratively. The efficiency of stress updating for the plate-torsion specimen has been improved by 105 times to extract the material constitutive parameters. Not limited to plate-torsion specimens, the proposed ANN-DIC method can be extended further for rapid assessment of material constitutive parameters for other anisotropic polymer-based composite materials using complex, statically indeterminate tests.

Complex Crustal Deformation Beneath the Cathaysia Block in South China: Evidence From Receiver Function Analysis

AbstractBy harmonically fitting the arrival times of the P‐to‐S converted phases from the Moho, we systematically investigate the crustal anisotropy beneath the Cathaysia Block (CaB) and the surrounding areas in South China for the first time. The measured anisotropy parameters show strong lateral variations with an average splitting time of 0.35 ± 0.12 s. The resulting fast directions exhibit clear geographic variations with two possible boundaries that separate the CaB into three parts. The inconsistent fast direction of crustal anisotropy among each part, and the comparisons with teleseismic XKS splitting measurements indicate that the mechanisms of crustal deformation vary between distinct regions. The crust deforms coherently with the upper mantle in the western CaB, which is possibly driven by mantle flow. The fast directions of crustal anisotropy in the central CaB exhibit a radial pattern and show large discrepancies with XKS splitting measurements. We suggest that the crust has a complex deformation mechanism in this region. We propose a new scenario that emphasizes the interaction of the crust and upper mantle for interpreting the radial‐patterned crustal anisotropy, which involves the processes of subduction plate rollback, asthenospheric upwelling, crustal melting and lithospheric mantle intrusion. The fast directions of crustal anisotropy in the eastern CaB are dominantly parallel to the continental margin, suggesting subduction‐controlled crustal deformation in this region.

Constitutive Model of TNM Alloy Using Arrhenius-Type Model and Artificial Neural Network Model

TNM alloys are frequently employed in automotive and aeronautical applications. Hot compression experiments on a Gleeble-3800 testing apparatus were conducted at a range of temperatures (1150°C–1250°C) and strain rates (0.001s-1 - 1s-1) to investigate the high temperature deformation behavior of TNM alloys. The complex deformation mechanisms of TNM alloys at various temperatures and strain rates were studied using the experimentally discovered true stress-true strain curves. The constitutive relationships between deformation parameters and flow stresses were constructed using two methods, Arrhenius model and neural network model respectively. The results demonstrated that the correlation coefficient R and root mean square error (RSME) achieved by BPNN are, respectively, 0.9982 and 4.7784, and are notably better than those anticipated by the Arrhenius-type model. In terms of the distribution of relative errors, the BPNN findings are similarly more concentrated, and the bulk of them fall inside the 10% range. Therefore, the BP neural network is a useful tool for forecasting the elevated temperature flow behavior of TNM alloys.

Temperature-dependent mechanical properties and elastocaloric effects of multiphase nanocrystalline NiTi alloys

We investigate the temperature-dependent mechanical properties and elastocaloric effects of different multiphase nanocrystalline NiTi alloys with grain sizes of 10 nm, 15 nm, 20 nm, 27 nm, 34 nm and 55 nm. These multiphase nanocrystalline NiTi alloys mainly consist of B2, B19′ and R phases and can undergo the thermally induced phase transition over a wide temperature range from 208 K to 353 K. Their superelastic deformation (loading to a strain of 4%) is contributed by combinations of the elastic deformation of B2 phase, the R → B2 phase transition, the reversible martensite reorientation and the B2 → R and B2/R → B19' phase transitions. Such complex deformation mechanism together with the microstructure evolution with temperature brings strong temperature-dependences of the mechanical properties and elastocaloric effects. As results, at the grain sizes of 15 nm ∼ 20 nm, the stress under a constant superelastic strain can show a significant V-shaped variation with temperature. At the grain size of 55 nm, the adiabatic temperature drop can even change from −8.5 K to −29 K ∼ −35.8 K as the ambient temperature rises from 293 K to 333 K. By comparison of the mechanical properties and elastocaloric effects of the different multiphase nanocrystalline NiTi alloys, it is found that the one with the grain size of 20 nm exhibits the best comprehensive cooling performances, which has a soft near-linear superelasticity (a moderate driving force), a very wide refrigeration temperature window (> 245 K) with considerable adiabatic temperature drops (∆Tmax= −17.1 K), and a good functional stability. This work provides an experimental basis for using the multiphase nanocrystalline NiTi alloys as solid-state refrigerants.

Non-isothermal direct bundle simulation of SMC compression molding with a non-Newtonian compressible matrix

Compression molding of Sheet Molding Compounds (SMC) is a manufacturing process in which a stack of discontinuous fiber-reinforced thermoset sheets is formed in a hot mold. The reorientation of fibers during this molding process can be either described by macroscale models based on Jeffery's equation or by direct mesoscale simulations of individual fiber bundles. In complex geometries and for long fibers, direct bundle simulations outperform the accuracy of state-of-the-art macroscale approaches in terms of fiber orientation and fiber volume fraction. However, it remains to be shown that they are able to predict the necessary compression forces considering non-isothermal, non-Newtonian and compaction behavior. In this contribution, both approaches are applied to the elongational flow in a press rheometer and compared to experiments with 23% glass fiber volume fraction. The results show that both models predict contributions to the total compression force and orientation reasonably well for short flow paths. For long flow paths and thick stacks, complex deformation mechanisms arise and potential origins for deviation between simulations models and experimental observations are discussed. Furthermore, Jeffery's basic model is able to predict orientations similar to the high-fidelity mesoscale model. For planar SMC flow, this basic model appears to be even better suited than the more advanced orientation models with diffusion terms developed for injection molding.