Implementation of surface tension force in fluid flow during reactive rotational molding

During Reactive Rotational Molding (RRM), it is very important to predict the fluid flow in order to obtain the piece with homogeneous shape and high quality. This prediction may be possible by simulation the fluid flow during rotational molding. In this study we have used a mixture of isocyanate and polyol as reactive system. The kinetic rheological behaviors of thermoset polyurethane are investigated in anisothermal conditions. Thanks to these, rheokinetik model of polyurethane was identified. Then, to simulate the RRM, we have applied Smoothed Particles Hydrodynamics (SPH) method which is suited method to simulate the fluid flow with free surface such as occurs at RRM. Modelling and simulating reactive system flow depend on different parameters; one of them is the surface tension of reactive fluid. To implement force tension surface, the interface between polymer and air is dynamically tracked by finding the particles on this border. First, the boundary particles are detected by free-surface detection algorithm developed by Barecasco, Terissa and NAA [1, 2] in two and three dimension. Then, analytical and geometrical algorithms have been used for interface reconstructions. The aim of this work is the implementation of surface tension force in the SPH solver applied to RRM. To illustrate that, we used novel and simple geometric algorithm fitting circle and fitting sphere, in two and three dimensional configurations, respectively. The model has been validated using a well-known dam break test case which covered the experimental data

Characterization and modeling of sintering of polymer particles

An experimental study of simultaneous sintering of several particles has been carried out using spherical polymer grains. Considering rotational molding condition, coalescence of several grains in contact, happens simultaneously on internal surface of the mould. Theoretical model based on the effect of surface tension and viscosity can accurately predict the coalescence of a pairs of grains. However, it was observed in this study that coalescence rate changes with presence of neighboring grains and their position and the theoretical model proposed for two grains, is not able to predict the coalescence rate of mutli-grains. Based on this finding, we have modified this model with taking into account the effect of neighboring particles in the sintering rate of multi-grains. Obtained modified model is capable of predicting the multi-grains sintering rate observed in this study

Coupled effect of loading frequency and amplitude on the fatigue behavior of advanced sheet molding compound (A-SMC)

This paper presents the experimental results of tension-tension stress-controlled fatigue tests performed on advanced sheet molding compound (A-SMC). It aims at analyzing the effect of fiber orientation, loading amplitude, and frequency on the fatigue response and the related temperature evolution due to the self-heating phenomenon. Two types of A-SMC have been analyzed: randomly oriented (RO) and highly oriented (HO). The coupled effect of the loading amplitude and the frequency has been studied. It has been shown that the couple frequency-amplitude affects the nature of the fatigue overall response which can be governed by the damage mechanisms accumulation (mechanical fatigue) and/or by the self-heating (induced thermal fatigue). For fatigue loading at 100 Hz, self-heating has been observed and yielded to a temperature rise up to 70 C. The latter causes a decrease of the storage modulus related to the b-transition of the vinylester. It has been demonstrated that the self-heating produced a material softening and decreased the fatigue life. SEM observations revealed that the samples tested at 100 Hz, exhibit smooth debonding surfaces due to the induced thermal softening of the matrix whereas more brittle fracture of the matrix surrounding fibers is observed during the fatigue tests achieved at 10 Hz

Thermal Aging Effect on Mechanical Properties of Polyurethane

This study concerns the effect of thermal aging on mechanical properties of polyurethane. Polyurethane samples were exposed at 85° and 120°C under inert atmosphere. Mechanical tests were carried out on these samples the aging period. Tensile tests were performed to see the effect of aging on elastic modulus (E), stress (σr), and strain (ϵr) at break. It was shown that there are two distinct periods. Due to aging, E and σr increase in the first period, then they decrease in the second period. ϵr decreases first and then increases. Fatigue tests were performed on unaged and aged samples. It was shown that the fatigue behavior of polyurethane (PU) is improved the same way during the first stage of aging. In the second step, the number of cycles to failure increases due to aging. The results show that aging has an important effect on mechanical properties of PU. The strain at break decreases during the first step of aging due to post-cross-linking and then increases due to chain scission in the network. Based on these results, the effect of cross-linking and chain scission on the mechanical properties of PU was discussed

Sheet Molding Compound Automotive Component Reliability Using a Micromechanical Damage Approach

The mastering of product reliability is essential for industrial competitiveness. If for metallic materials the topic is well-known, especially in automotive industry, Original Equipment Manufacturers are expecting strong support of their suppliers to full-fill the lack data. This paper presents a new original approach, using a micromechanical based on damage model to address the problem of reliability of Sheet Molding Compound (SMC) components. The first part demonstrates the inadequacy of the standard method of reliability on SMC material through its application on the new Peugeot 3008. In fact, the very flat S-N curve of SMC, and in general, composite materials is not appropriate for acceleration effect. The proposed model correlates the stress, damage and strength with both cycle number and slamming velocity. It emphasizes the relation between the effective distribution with the slamming velocity effect. Then, a new reliability approach based on a micromechanical fatigue/damage model was developed. The definition of new probability distributions based on damage was necessary to apply properly the stress-resistance approach. It allows taking into account the velocity effect by switching in damage space. Finally, applying this new methodology on the Peugeot 3008, leads to the definition of the optimal validation laboratory tests to ensure the reliability. Indeed, the required number of cycles to ensure reliability has been reduced significantly. Micromechanical damage reliability approach could be an efficient way to ensure the reliability of short fiber reinforcement composite components used in industrial context.Authors address a strong acknowledgment to E. FEIGE and Y. HAMOY, from PSA, for the data provided. Their comments and advices were also very useful.We are grateful to Mr. OZOUF for teaching and advices on the general topic of reliability

Study of Bonding Formation between the Filaments of PLA in FFF Process

Fused filament fabrication (FFF) is an additive manufacturing (AM) process that provides physical objects commonly used for modeling, prototyping and production applications. The major drawback of this process is poor mechanical property due to the porous structure of final parts. This process requires careful management of coalescence phenomenon. In this paper, the major influencing factors during the FFF processing of poly(lactic acid) (PLA) were investigated experimentally and with a numerical model. It has been shown that the polymer temperature has a significant effect on the rheological behavior of PLA, especially on the adhesion of the filaments. An experimental set-up has been placed in the machine to have the cyclic temperature of the filament. A variation of the polymer temperature influences process parameters such as feed rate, temperature of the nozzle and temperature of the platform. The results showed that the amount of polymeric coalescence (neck growth) rises when increasing the feed rate, the nozzle temperature, and the platform temperature. A model to predict the neck growth is proposed. It predicts a lower amount of neck growth value than obtained experimentally. This difference has been explained as the effect of other phenomena, such as polymer relaxation time, pressure of the nozzle and especially cyclic temperature which is not taken into account in the model

Rotational Molding of Polyamide-12 Nanocomposites: Modeling of the Viscoelastic Behavior

Nowadays, polyamide 12 (PA-12) is considered as an interesting polymer in the rotomolding process to manufacture different pieces like the liner part in the storage hydrogen tank (type IV). In this study, the pure polyamide-12 and PA12 pieces, incorporated with 0.5%, 1% and 3% wt Nano Carbon Black (NCB), were manufactured by the rotomolding process. Different rotomolding parameters such as heating temperature, time of heating, and cooling rate have been optimized to obtain the ideal piece. The effect of volume fraction of NCB in terms of physicochemical and mechanical properties has been studied. Afterward, the optimal volume fraction of NCB is revealed using different characterization methods. The tensile results specified the addition of NCBs until 0.5% improved the tensile behavior. The addition of NCBs more than 0.5% decreases the mechanical properties in terms of failure stress and strain, while it has no significant effect on the elastic modulus of PA-12. The bi-parabolic the Perez model has been used to study the viscoelastic behavior of PA-12 using the Cole-Cole method. The constants of the Perez model indicate a good correlation between viscoelastic experimental results and the model used

Fast fatigue life prediction of short fiber reinforced composites using a new hybrid damage approach: Application to SMC

Industrial design of Short Fiber Reinforced Composites (SFRC) structures is subject to several compounding and processing steps of optimization. Moreover, these structures are often submitted to fatigue loading. Therefore, SN curves have to be established for each new composite formulation and for several type of microstructure involved in the real component due to processing. While these preliminary characterizations are time and money consuming, this paper propose a new hybrid methodology for fast fatigue life prediction. Moreover, both monotonic and fatigue behavior of SMC composites is essentially determined by local damage propagation. Therefore, the key idea of the proposed approach is to use a Mori and Tanaka based micromechanical model in order to establish an equation of state relating local damage rate to macroscopic residual stiffness rate. The generalization of this relation to fatigue damage multi-scale description leads to the SN curve fast determination of each considered microstructure. Very limited experimental characterization is required in such a way that SN curve could be established in just one day. Comparison between experimental and simulated Whöler curves highlights a very good agreement for several microstructure configurations in the case of a SMC composite material

Modelling of sintering during rotational moulding of the thermoplastic polymers

This paper concerns the study of sintering phenomenon during rotational molding of polypropylene(PP),Polyvinylidenefluoride (PVDF) and Polymethyl methacrylate (PMMA). First, the coalescence (first step of sintering) of two grains has been followed. Bellehumeur’s model has been tested as a model to explain this phenomenon. In order to study the effect of neighboring grains on coalescence of two grains, a third grain has been put in contact with these two grains. For modeling the phenomenon in this case, Bellehumeur’s model has been modified by a geometric parameter called Farz Factor (FF), being this model validated by experimental test. Concerning densification, two different stages have been observed. In the first stage, before welding of the grains and formation of interphases between them, the grains are not stuck yet. The air trapped between the grains escapes through free ways between grains. This first step of densification is directly related to the coalescence where the density of the polymer varies very quickly. A new tridimensional model, based on a Body Centered Tetragonal (BCT) configuration, has been proposed to explain the densification during this first stage. In the second stage, the migration of air is controlled by diffusion

Multi‐scale analysis of short glass fiber‐reinforced polypropylene under monotonic and fatigue loading

Short fiber-reinforced polypropylene is largely used in the automotive industry. Fatigue failure is one of the most failure modes observed in this class of materials. In order to better understand the damage mechanisms and plasticity evolution, this article provides an overall experimental investigation of the mechanical properties of a PPGF40 composite (polypropylene matrix reinforced by a 40% weight content of short glass fibers) including monotonic and cyclic loading. The effect of various parameters such as the loading direction, the strain rate, the temperature, and the fatigue are analyzed. The evolutions of the loss of stiffness and plastic strain during monotonic and fatigue tests are analyzed. Self-heating during cyclic loading is also studied. Moreover, the coupling effect of damage and plasticity is analyzed by plotting the evolution of the relative loss of stiffness vs the plastic strain increment for monotonic and cyclic loadings. For quasi-static loading, the results emphasize an intrinsic curve independent of the loading direction. Moreover, a sharp increase in the damage and plasticity levels due to the local effect occurring during cyclic loading is observed and correlated to SEM fracture surface analysis.This work is carried out as part of a Phd thesis ( CIFRE number 2016/1520) in collaboration between Arts et Metiers Institute of Technologie and Flex N gate Exteriors Europe