Mechanical Characterization of polyamides used in Intercoolers

Transcription

1 Mechanical Characterization of polyamides used in Intercoolers D. Faustino 1, V. Infante 1, L. Neves 2 1 Instituto Superior Técnico, ICEMS, Universidade de Lisboa, Av. Rovisco Pais, Lisboa, Portugal 2 João de Deus & Filhos SA, Estrada dos Arados 5, Samora Correia, Portugal, luis.neves@jdeus.com Abstract The present study characterize the different polymers used in a manufacture of automotive components through injection molding. Combining the results of this characterization to a numerical simulation based on actual injection conditions, it was possible to perform a structural analysis similar to the geometry of an experimental, standardized specimen, as well as an automotive component. For the experimental work, polyamides PA6+PA66 GF30, PA46 GF50 and PA 6T/6I GF50 or PPA GF50 were tested at room temperature and at high temperatures. Tensile tests were performed to obtain the quasi static characterization of materials. For the dynamic characterization of these polymers, fatigue tests were conducted at high temperature. In this regard, the experimental work has concentrated primarily on obtaining different inherent mechanical properties of the material behavior. The numerical analysis focused on the simulation of structural components using the commercial software for purpose finite element, Autodesk Moldflow, DIGIMAT and ANSYS. The capabilities of ANSYS were allied to the Moldflow simulation capability. Simulation models of fiber orientation Moldflow allowed the interaction of DIGIMAT software, and so, through the simulation of ANSYS software to compute distinct component analysis. These analyzes relate specially with the behavior of isotropic or anisotropic polymer. The results obtained in this article led to the capture and interpretation of the fracture phenomenon observed in the component during service and thus, propose to hold future changes. Keywords: Intercooler, Thermoplastics, Polyamide, Orientation of glass fibers, Isotropy, Anisotropy 1 Introduction In the context of CO2 emission reduction, the automotive industry makes an increasing use of plastic materials in order to take advantage of their light weight and their complex mould designs. Glass fiber reinforced thermoplastics exhibit the required stiffness for structural applications due to their properties. The choice of a polyamide matrix also provides a good thermal strength for a moderate cost [1]. One of the complicating factors for injection-molded plastic parts is the change of the plastic properties during the manufacturing process. While this is not a problem in and of itself, problems can arise if the structural analyses are based on generic material data that does not accurately represent the actual properties of the molded part. This can lead to over engineering

2 of components, resulting in increased costs and material usage or under-engineering, which can result in premature failure of components. Based on different authors, which suggest a micro-modeling approach to study the overall response of the composite, the fiber orientation must be determined from a rheological simulation of the injection flow. This rheological simulation is based on Jeffery s (1922) [2] and Folgar-Tucker s (1984) equations [3]. Following Advani and Tucker [4], the use of orientation tensors is very suitable to link the fiber orientation to the macroscopic mechanical behavior. But this simulation still finds some difficulties to describe the orientation tensors, nevertheless, a classic skin (fibers oriented parallel to the molding flow) core (perpendicular fibers) orientation is observed (and predicted) for injected plates. This particular orientation distribution explains the overall anisotropic response of the composite [5] [6]. Due to different possibility of studies, it was decided to model the mechanical response of the composite with a phenomenological approach. Nevertheless, it was also consider the multiphasic nature of the material (the thermoplastic matrix shows crystalline and amorphous phases, short glass fibers account for a third phase, and even fiber/matrix interfaces may consist in a fourth phase) to explain the physical origin of the different mechanical phenomena. This approach has already been successfully developed for semi-crystalline thermoplastics [7] [8] in the large-strain framework: the suggested phenomenological models describe the visco- (hyper)elastic response of the macromolecular amorphous network, combined with the viscoplastic response of the crystalline phase [9] [10]. When glass or carbon fibers are added to plastics, the elastic modulus can increase significantly with a negligible effect on part weight. This combination of low weight and high stiffness makes fiber-filled plastics ideal for high-performance applications. The orientation direction and the degree of orientation of the fibers determine the mechanical properties of the molded part. The material will be relatively weak in direction which is perpendicular (across the fibers), but will have higher strength in the direction in which majority of the fibers are aligned. In areas where fibers are randomly oriented, the material will not achieve maximum strength and the material will exhibit isotropic behavior. One of the main issues for the engineers lies in the prediction of the fatigue life duration under complex loadings. The first objective of this study relied in find the static properties of these polyamides according to ISO 527 [11]. The accurate linear and nonlinear modeling of complex composite structures pushes the limits of finite element analysis software with respect to element formulation, solver performance and phenomenological materials models. The finite element analysis of injection molded structures made of nonlinear and/or time-dependent anisotropic reinforced polymer is increasingly complex. Nevertheless, the accurate modeling of such structures and materials is possible with LS-DYNA using LS-DYNA s usermat subroutine to call the DIGIMAT micromechanical modeling software. In addition to enabling accurate and predictive modeling of such materials and structures, this multi-scale approach provides the FEA analyst and part designer with and explicit link between the parameters describing the microstructure, for instance, fiber orientation by injection molding software and the final part performance predicted by LS-DYNA. DIGIMAT can be linked to LS-DYNA through its user-defined material interface enabling the following two-scale approach: A classical finite element analysis is carried out at macro scale, and for each time/load interval [tn, tn+1] and at each element integration point, DIGIMAT is called to perform an homogenization of the composite material under consideration.

3 Load [kn] Load [kn] D. Faustino, Mechanical Characterization of an inlet tank used in Intercoolers Based on the macroscopic strain tensor ε given by LS-DYNA, DIGIMAT computes and return, amongst other, the macroscopic stress tensor at the end of the time increment. The microstructure is not seen by LS-DYNA but only by DIGIMAT, which considers each integration as the center of a representative volume element of the composite material [12]. 2 Material and experimental procedure For this study, three different polyamides used in intercoolers were tested. Specimens extracted from injection moulded plates of PA6+PA66 GF30, PA46 GF50 and PA 6T/6I GF50 or PPA GF50 were conducted in thermal tensile and fatigue tests carried out at high temperatures. 3 Experimental Results 3.1 Quasi-static analysis. The tensile tests were carried out at room temperature, 190ºC, 210ºC and 230ºC. Nevertheless the PA6+PA66 GF30 was tested at room temperature, 150ºC, 180ºC and 210ºC, according to the industrial application. In order to avoid dispersion of results were tested 3 specimens of each polymer at different temperatures Displacement [mm] Test 1-230ºC Test 2-230ºC Test 3-230ºC Test 4-210ºC Test 5-210ºC Test 6-210ºC Test 7-190ºC Test 8-190ºC Test 9-190ºC Figure 1 Tensile test results of PA6T/6I GF50 at 230ºC, 210ºC and 190ºC Displacement [mm] Test 1-230ºC Test 2-230ºC Test 3-230ºC Test 4-210ºC Test 5-210ºC Test 6-210ºC Test 7-190ºC Test 8-190ºC Test 9-190ºC Figure 2 - Tensile test results of PA46 GF50 at 230ºC, 210ºC and 190ºC

4 Load [kn] D. Faustino, Mechanical Characterization of an inlet tank used in Intercoolers Displacement [mm] Test 1-210ºC Test 2-210ºC Test 3-210ºC Test 4-180ºC Test 5-180ºC Test 6-180ºC Test 7-150ºC Test 8-150ºC Test 9-150ºC Figure 3 - Tensile test results of PA66 PA6 GF30 at 210ºC, 180ºC and 150ºC According to the results of Figures 1, 2 and 3 the PA6T/6I GF50 represents the material with higher mechanical strength and the PA6+PA66 GF30 the weakest at highest temperature. Relatively to displacement, PA6T/6I GF50 represents the polyamide with higher stiffness independently the temperature. This results agree with different analysis related with the plasticization effect on tensile properties of injection-moulded glass-fibre polyamides. According with different authors, the incorporation of glass fibre into polyamides gives rise to a significant improvement in Young modulus and tensile strength, while tensile strain is reduced [13]. Nevertheless, in order to obtain other mechanical properties, namely the Young modulus of this polyamides, tensile tests at room temperature supported by digital sensors were realized. In terms of comparison two different techniques of measurement, video extensometer and DIC (Digital Image Correlation) were applied. According to the Standard D 638 [14] for that material where is no proportionality of stress to strain evident, the Poisson s ratio must be determined based on axial strain range of to mm/mm. In order to avoid dispersion of results due to external factors, the Poisson s ratio was also calculated based on software, Curve Expert, which permit calculate the Poisson s Ratio without such dispersion. Therefore instead the strain ranges of to mm/mm, this method considers only the axial strain at mm/mm and the origin. Table 1, 2 and 3 presents the Young modulus values obtained for the different polyamides. Table 4 represents the values provided by the supplier [15]. Table 1 Values of Young modulus obtained for PA6T/6I GF50 Young Modulus Test 1 - DIC Test 2 - DIC Test 3 - DIC Test Video Extensometer Standard ISO GPa 20 GPa 23 GPa 10 GPa Curve Expert 22 GPa 30 GPa 24 GPa 12 GPa Table 2 - Values of Young modulus obtained for PA46 GF50 Young Modulus Test 1 - DIC Test 2 - DIC Test 3 - DIC Test Video Extensometer Standard ISO GPa 19 GPa 17 GPa 11 GPa Curve Expert 16 GPa 15 GPa 24 GPa 11 GPa

5 σmax (MPa) D. Faustino, Mechanical Characterization of an inlet tank used in Intercoolers Table 3 - Values of Young modulus obtained for PA6+PA66 GF30 Young Modulus Test 1 - DIC Test 2 - DIC Test 3 - DIC Test Video Extensometer Standard ISO GPa 10 GPa 11 GPa 9 GPa Curve Expert 19 GPa 14 GPa 15 GPa 8 GPa Table 4 - Values of Young modulus provided by the supplier Young Modulus PA6T/6I GF50 PA46 GF50 PA6+PA66 GF30 Supplier 18 GPa 16 GPa 10 GPa These results suggest the presence of divergence on values obtained using in both methods, despite the general similarity obtained in PA6+PA66 GF30. In fact, the lack of accuracy on sensors allows to understand this difference. 3.2 Dynamic analysis As described before, due to industrial application, PA6+PA66 GF30 and PA46 GF50 were chosen to the dynamic tests. The specimens were tested until fracture or up to cycles and a constant amplitude loading with stress ratio, R=0.1 and frequency of 6 Hz. In our study all experiments used to plot the S-N curves, were performed at 210ºC. 100 PA66PA6 GF30 PA46 GF E E E E E+07 Fatigue Life, N cycles Figure 4 Stress-number of cycles for PA6+PA66 GF30 and PA46 GF50 fatigue tests The equation (3.1), admits a linear regression between logarithmic function of stress and number of cycles. N r σ m = k 0 (3.1) Where, N r represents the number of cycles until fracture, σ the stress range, and k 0 and m constants related with experimental analysis. Therefore for PA6+PA66 GF30, k 0 =1.85x10 23 and m= and for PA46 GF50, k 0 =3.27x10 28 and m=

6 4 Numerical results Structural analysis capability of ANSYS was used in conjunction with flow simulation capability of Moldflow in the numerical simulations. Moldflow has been used to simulate the manufacturing process (injection molding) and ANSYS has been used to simulate the structural performance of the part. Despite this procedure, an interface between these software has been used, the DIGIMAT software. The polyamide PA6+PA66 GF30 was chosen for all numerical simulations. In order to test this procedure and to compare the experimental with the numerical results, a standardized specimen used in static analysis was simulated. The geometry was provided taking into account the presence of two gates localized on top and bottom (a bi-injected component). Considering the final numerical analysis, a component used on intercoolers was also simulated in order to compare an isotropy and an anisotropy approach. While the first one is related with the experimental results and considers a regular fibers orientation, the second one is related with an analyze through the software DIGIMAT, which predict the performance of a composite structure taking into account the local nonlinear anisotropic behavior of the multiphase material at each integration point of a finite element model of the structure. The distribution of maximum principal stress computed by ANSYS through the simulation of DIGIMAT and Moldflow is reported on Figure 5. Due the highest stress zone localized on center, this zone tends to suffer the fracture. In fact, the necking phenomenon is associated to the weld line provided by the lack of fibers orientation during the injection. Figure 5 Distribution of maximum principal stress on specimen geometry considering anisotropic behavior The maximum principal stress distribution computed by ANSYS and Moldflow (Figure 6) show a regular distribution along the geometry which is related with a regular fibers orientation provided by the isotropic behavior of the material.

7 Figure 6 - Distribution of maximum principal stress on specimen geometry considering anisotropic behavior The total distribution deformation of the component used on intercoolers (Figure 7) show a regular deformation along the material. Comparing with the results provided by the anisotropic behavior (Figure 8) both analyses present a regular distribution. Regarding with the maximum values obtained, while the highest deformation obtained for isotropic behavior is 0.18 mm, for the anisotropic behavior the value is 0.32 mm. Figure 7 - Distribution of deformation on component geometry considering isotropic behavior Figure 8 - Distribution of deformation on component geometry considering anisotropic behavior

8 The behavior of the maximum principal stress of the component, is opposite of the analysis of deformation, concluding that the behavior of material has a strong influence in the numerical results. Observing the results of Figures 9 and 10, despite the presence of a general lower stress along the component, the main critical zones show different results. While the anisotropic analyses show a maximum stress value of 42 MPa, the isotropic analyses provide a highest stress of 26 MPa. Comparing these results with the experimental analysis related with the fatigue behavior of material (Figure 11), for cycles related with the fracture of component, the isotropic analysis provided results under the admissible stress limit of the material. Figure 9 - Distribution of maximum principal stress on component considering isotropic behavior Figure 10 - Distribution of maximum principal stress on component considering anisotropic behavior

9 σmax (MPa) D. Faustino, Mechanical Characterization of an inlet tank used in Intercoolers 100 PA66PA6 GF E E E E E+06 Fatigue Life, N cycles Anisotropic analyses Isotropic analyses Figure 11 Comparison of PA6+PA66 GF30 experimental results with numerical simulations results 5 Conclusions The experimental study focused on the characterization of polyamides used in the manufacturing of intercoolers held based on two distinct goals: to obtain based the experimental properties that could be applied in the numerical simulation performed after; to compare the experimental results with the properties provided by suppliers. The experimental Young modulus values show in general significant differences from those reported by the suppliers. The Young modulus results obtained using a digital image correlation system showed a good correlation. The lack of definition of a proportional limit in tension of these materials was evident, leading the extremely complex behavior presented. However, it was found that the polyamide PA46 GF50 has a higher mechanical strength. The polyamide PA46 GF50 comparing with PA46 GF50 and PA6+PA66 GF30 showed greater strength due to the aforementioned reasons. The stress distribution values supports the previous results obtained through mechanical characterization. According the numerical study of the automotive component, is noted the influence of the geometry of the component in the way the fiber orientation is presented and, consequently, the distribution of the respective stresses. Comparing the analysis based on phenomena isotropy, there was a generally even distribution of stresses, demonstrating the lack of sensitivity to potentially critical areas of the component. It is considered therefore that in certain local zones, the distribution of stresses is slightly over-sized. On the other hand, in the analysis of anisotropy phenomena, such facts are not observed because the stress distribution takes into account the different orientations of fibers. The anisotropic structural analysis provides workable results comparatively to the isotropic analysis, the results demonstrate the impossibility of choosing this material for future production. This was demonstrated by comparing the results obtained in the numerical analysis with the results obtained from experimental tests.

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