Conception, Design and Development of a Test Bench for the Automotive Industry Intercoolers

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1 Conception, Design and Development of a Test Bench for the Automotive Industry Intercoolers Nelson Miguel Marques Carvalho Instituto Superior Técnico Universidade de Lisboa Portugal May 2014 nelson.carvalho@ist.utl.pt Secção de Tecnologia Mecânica, Instituto Superior Técnico, Avenida Rovisco Pais, Lisboa, Portugal Abstract The objective of the present work, conducted in partnership with J.Deus Corporation, was the study of the mechanical response of Porsche 997 intercooler. This objective was achieved following two methodologies; the numerical analysis of the intercooler mechanical response and the design of a test bench, by adapting an existing press machine, to allow the experimental analysis. For the computational analysis it was necessary to perform uniaxial tensile tests in order to characterize the mechanical behavior of the intercooler materials (polyamide and aluminum). Polyamide strain rate dependence and the influence of brazing in the aluminum alloys properties were also analyzed. For the experimental study, a test bench was developed to experiment the intercooler under uniaxial traction conditions. This test bench was designed to be performed in a press machine existent in the company. The numerical simulations of the intercooler behavior under traction conditions allowed to study stress and strain distributions, to identify the location of the maximum values and to obtain the force evolution. Due to the time requirements to manufacture the tool parts for the test bench, the experimental study was not completed. Therefore, it has not been possible to compare the numerical and the experimental results. KEY- WORDS: intercooler, aluminum alloy, thermoplastic, numerical simulation, test bench 1. Introduction An intercooler in service suffers mechanical stresses originated from the oscillation of the car body that consequently are transmitted to its components. Aware of the problems, JDeus Corporation intended to evaluate the loading conditions that promote intercooler failures, namely finding a way to experimentally test the components on a test bench under tensile forces until fracture, with force and displacement records. In this context, this paper begins with a literature review of intercoolers, addressing its needs, characteristics and working 1

2 environment. A comprehensive study of the mechanical behavior of the intercooler materials is presented. This material characterization was executed through standard uniaxial tensile tests. The intercooler mechanical response will be then studied using two different methodologies: experimentally by designing a tool to perform a test bench, and numerically by means of the finite element method (FEM) analysis. At the end of the study, the results obtained by both methods will be compared to validate the use of the numerical model. This validation will allow to study the intercooler behavior under more complex and difficult loading conditions using the numerical analysis. Although an intercooler is subjected to different loading conditions in service, which might result in more or less complex deformation modes, the focus of this work is essentially based on a simple mode of deformation, traction. 1. Intercooler An intercooler, represented in figure 1.1, is used to cool the air compressed by the turbocharger before the cylinder engine. The materials used in intercooler in study are restricted to two large families, the polymers and nonferrous metals (aluminum). The aluminum components are: two top plates, two sides, tubes and fins, while the polymers are present in the two boxes and rubber gasket. The intercooler is fixed to the chassis of the car directly through six pins, three in the outbox and three in the inbox, and indirectly through the pipes of indoor air circulation (figure 1.2). Figure 1.2 Pin fixation of the intercooler. The axis of the fixing pin is not vertically oriented with the tubes. 2. Material Analysis Standard tensile tests were carried out to characterize the mechanical properties of materials and these values were then used for the numerical analysis. With the aim of reproducing the numerical simulation as accurate as possible, the mechanical characterization of the polymer was carried out by analyzing the influence of strain rate on the mechanical behavior of the polymer. During operation the box of the intercooler also experiences different loading speeds. In the case of aluminum, it was studied how the material properties are influenced by brazing and by the time elapsed after this joining process. To address this effect, specimens were tested before, one day after and a month after brazing. 2.1 Thermoplastic The polymer used in the manufacture of intercooler boxes is an engineering thermoplastic known as PA6+PA66, commonly named polyamide. The standard used in this study to determine the mechanical properties of the polymer is BS EN ISO 527 (Plastics - Determination of Figure 1.1 Intercooler applied in Porsche

3 tensile proprieties) [1] at four distinct testing speeds: 2, 10, 50 and 100 mm / min. The figure 2.1 shows experimental engineering stress-strain curves obtained in the uniaxial tensile tests at four testing speeds. Figure 2.1 Engineering stress-strain curves. To analyze the influence of the testing speed on the polymer mechanical properties the following parameters were considered: the elasticity modulus (E), the tensile strength ( R), strain at tensile strength (e R) and the fracture energy per unit volume (U T ). Figure 2.2 summarizes the variation of these parameters with the strain rate. Figure 2.2 Elasticity modulus and fracture energy of the polymer at different strain rates. It can be observed that the modulus of elasticity, fracture energy and tensile strength increase with the strain rate. Regarding the strain at tensile strength the conclusion is not so obvious, but the curve of evolution of the strain at tensile strength appears to have an upward trend with the test speed increase. 2.2 Aluminum The intercooler has two types of aluminum. The aluminum Hogal AA4045/3551 which is present in both top plates and aluminum AA4343/AA3003 present in the tubes, fins and sides. Each aluminum sheet has two types of alloy, core and clad, which allows the brazing process. Figure 2.3 Schematic representation of aluminum sheet for brazing. Brazing is a metal joining process that provides a permanent bond between the pieces to be joined with the help of a filler 3

4 metal (clad) [2]. The temperatures involved in the brazing process are superior to an annealing treatment for either treatable or non-treatable alloys. The annealing process may, if the temperature is high enough, cause recrystallization of the alloy [3]. The mechanical characterization of the aluminum was performed according to the standard ASTM E8-83 (Standard methods of Tension Testing of Metallic Materials) [4]. The tensile tests were executed in different aluminum alloy specimens: before brazing, one day after and one month after brazing. Figure 2.4 Engineering stress-strain curves obtained for aluminum specimens in different brazing conditions. In the same way as for the polymer, to analyze the influence of the brazing process on the mechanical properties of the material, the following characterization parameters were considered: the elasticity modulus (E), the tensile strength ( R), the strain at tensile strength (e R), the fracture energy per unit volume (U T ) and the stress at 0,2% of strain. Figure 2.5 Characterization parameters to the aluminum. Figure 2.5 shows that the time elapsed after brazing just seems to influence the modulus of elasticity in the direction of reducing its value, since the remaining variables have no statistically significant variations. However, to confirm the trend observed for the modulus of elasticity it is required to perform a new set of tests with a more meaningful sample. Moreover, when comparing the results obtained from the condition of non-brazed and brazed, it can be concluded that the brazing process works as a thermal softening treatment of the alloy, resulting in a significantly more ductile behavior of the material. 4

5 3. Numerical Analysis The structural analysis of the intercooler was performed using the commercial finite element program ANSYS, version 14.5 (in workbench mode). The type of analysis chosen was static structural. 3.1 Material proprieties The mechanical properties of the polymer were introduced in the numerical simulation using a pre-defined material model. Thus, to consider the elastic behavior of the material, an isotropic elasticity model is used, while the plastic behavior was defined using a multilinear isotropic hardening model, following the methodology used by other authors for this type of material [5]. 3.2 Analysis Model Due to the size of the numerical model, the analysis was simplified by excluding one of the boxes and its top plate, the two sides, all fins and the rubber gaskets. This simplification reduced the model size, as well as the computation time, without compromising the objectives defined for the problem. Thus, the model was reduced to a box, a top plate and all tubes. The three pins are numbered so that they are easily identified and individualized (figure 3.3). Figure 3.3 Analysis model; a) Fixed support, b) Displacement in pin (direction z). Figure 3.1 True stress-strain curve for polymer. Similarly, the mechanical characteristics of the two aluminum alloys were made with predefined material models. The elastic behavior of the material was characterized by an isotropic elasticity model and plastic behavior by a multilinear isotropic hardening model. Concerning the boundary conditions, the model was fixed using a rigid support on the faces of the free end of the tubes, simulating the brazed connection that exists between them and the top plate. At the other end of the model, a displacement was applied on the three pins in the z direction aligned with the vertical axis of the tubes. The displacement must be high enough to carry the model to reach at least one of the three stresses corresponding to each of three materials failure. 3.3 Mesh generation Figure 3.2 True stress-strain curve for aluminum alloys. The meshing process was performed iteratively until a good quality of the elements in the model was achieved. The process was carried out first, with the adjustment of global mesh controls, and then, according to need, 5

6 small local changes in the mesh. As the tubes have a simple shape, a method was used, allowing them to remain mostly hexahedral, which is the desired shape for this element. The model has elements and nodes, where the largest number of elements is in the box. About 90% of elements have good quality parameters, namely, skewness, orthogonal quality and aspect ratio. Figure 3.5 State of deformation in the box for a displacement of 0,2 mm; a) Stress b) Strain. By analyzing the stress and strain, it was observed that for the displacement of 0.2 mm none of the pins reached critical deformation values, so, the analysis proceeded to a new increment of displacement State of Deformation in the Box for a Displacement of 0,7 mm 3.4 Results Figure 3.4 Meshed model. It was extracted information about the true stresses and true strains distribution in the model, as well as the loads involved in the analysis. These results were used as a baseline for the design of the test bench tool. Analysing the values of stress and strain, corresponding to the analysis of materials one can conclude, given the magnitudes of values that the polymer is likely to be the first material to reach failure. Thus, the polymer will be examined more closely in the first displacement increments imposed to the model State of Deformation in the Box for a Displacement of 0,2 mm Figure 3.5 a) shows the distribution of the true equivalent stress (von Mises) in the box. Where, pin 1 appears to be the intercooler part with the highest value of MPa. Regarding the true strain in the box, the figure 3.5 b) shows a detail of the distribution in the surroundings of the critical area. The most requested element has a maximum value of At the displacement of 0.7 mm, the most requested element of the box remains in the same pin, as identified in the displacement of 0.2 mm. However, it has now a value of true stress of MPa (please refer to figure 3.6 a)). Concerning the maximum true strain, its value was , nearly equal to the strain at tensile strength of the polymer, as it can be seen in Figure 3.6 b). Figure 3.6 State of deformation in the box for a displacement of 0,7 mm; a) Stress b) Strain. At this step the deformation of the box seems to approach the critical conditions obtained for the polymer in the experimental tensile tests. At this stage become relevant to analyze the maximum deformation of the other intercooler elements, in order to check if the regions that have been analyzed actually are those that might fracture. 6

7 3.4.3 State of Deformation in Other Components for a Displacement of 0,7 mm The maximum strain in the top plate reaches a value of 0.04 in an element located on the inside of the castle and a maximum true stress of MPa. It appears that this state corresponds to a deformation starting in plastic deformation zone of the material of the top plate, but far enough from the critical values of the failure of the material; ε R = and σ R = MPa. Thus, it can be concluded that these loading conditions are far from promoting the destruction of the top plate in the connection region of the box. plastic and are therefore, away from the fracture conditions State of Deformation in All Components for a Displacement of 1,2 mm Although the conditions leading to the fracture of the box were analyzed, it would be interesting to investigate how the deformation progresses beyond that point. Figure 3.9 a) shows the stress distribution in the box with a maximum stress of MPa, while figure 3.9 b) shows a detail of pin 1 with the maximum true strain. It is observed a strain with a value of , far exceeding the strain limit of the polymer. Figure 3.7 State of deformation in the top plate for a displacement of 0,7 mm; a) Stress b) Strain. Figure 3.9 State of deformation in the box for a displacement of 1,2 mm; a) Stress b) Strain. Other component that is interesting to analyze is the set of tubes constituting the nest of the intercooler. Figure 3.8 State of deformation in the tubes for a displacement of 0,7 mm; a) Stress b) Strain. In this component, the most requested element is on the side of the pin 3 (figure 3.8 a) and 3.8 b)) and shows the maximum true strain with a value of and maximum true stress of MPa. Comparing these values with the critical values of the material obtained in the uniaxial tensile test (ε R = and σ R = MPa), it can also be concluded that in this region the tube is in a transition zone between the elastic and It can be noted that the strain determined by the finite element software goes far beyond the strain at tensile strength of the polymer, while the saturated stress seems to be very close to the tensile strength. This is due to the fact that the program, in order to maintain the value of the tensile strength of the material, has to do extrapolations for values greater than the strain at the tensile strength of the material. This conclusion is visible on the graph in figure 3.10 for pins 1 to 3, where there is a saturation stress for a strain beyond their critical value. Figure 3.10 Stress and strain evolution in pins 1, 2 and 3 7

8 In the top plate, figure 3.11, maximum values were located on the same elements, as in the case of 0.7 mm of displacement, with values of for the true strain and MPa for the true stress, sufficiently distant from the material fracture values obtained in the tensile tests. Figure 3.13 Force [N] vs Displacement [mm] evolution in order of displacement. Figure 3.11 State of deformation in the top plate for a displacement of 1,2 mm; a) Stress b) Strain. For the set of tubes, these also maintain the element that is considered when the displacement was 0.7 mm. This element now has a true stress of MPa and a value of the true strain of , as it can be seen in Figures 3.12 a) and 3.12 b), respectively. Also in this case significantly lower values than the failure point of the material of the tubes. Figure 3.12 State of deformation in the tubes for a displacement of 1,2 mm; a) Stress b) Strain Force - displacement Evolution The knowledge of the force involved in the test is of particular importance from the perspective of the design of the test bench and the project of the test tool. Thus, and considering the offset range (0.7 to 1.2 mm), within which it is expected to occur the intercooler destruction, the numerical results suggest that the intercooler failure occur for a traction force between 10,6 KN and 14 KN. 4. Test Bench As a result of the the numerical simulation, it was found that the intercooler might reaches failure, when subjected to traction conditions in its pins, for a displacement of the numerical model between 0.7 mm and 1.2 mm. These displacement values indicate that there cannot be gaps of this magnitude in the press and traction tool. However, this mechanism, with the modification of a small number of parts, can be used to perform other deformation modes involving higher values, for example in the case of analyzing de-spiking. The design of a test bench to test an intercooler under traction conditions was made to be performed on a press machine that exists in JDEUS company. The tool was designed with the intent to be adaptable, so that by changing a small number of tool parts, it could be possible to test other intercoolers. The press machine in study is a hydraulic press KPD 50 (Mega) and is in the prototype section of JDEUS. This press has the limitation applying force only in the vertical direction and only downwards. The hydraulic pump is manually operated by two levers, being one a fast forward and the other a slow forward. Figure 4.1 shows tool designed for the bench test. 8

9 (figure 2.2) increase with increased testing speed. In what concerns to the aluminum alloy, the time elapsed after brazing do not influence its mechanical proprieties, but there is a big difference between brazed and not brazed aluminum states in terms of mechanical proprieties (figure 2.5). Figure 4.1 Tool designed for the bench test. The assembly steps are the following: 1. Put the intercooler in place. 2. Fix the intercooler with two plates that are supported on the press table and fixing the intercooler by the top plate. 3. Pass the rods by the plates placed above and pin them through eight nuts to a plate with the load cell (green). 4. Put in place the two bottom plates previously coupled with the aid of the bushings and screws. 5. Adjust the bottom plate so that it stands exactly horizontally through the nuts, the slope of the other plate is given by the inclination of the bushings. 6. Pins will fix the sloping plate with the aid of three brakes, one for each pin. 7. Place the load cell, adjusting for such cylinder of the press. 8. Load the intercooler. During the intercooler test displacement and force must measured and recorded using specific hardware and software. 5. Conclusions It was verified that, in the case of the polymer, its mechanical behavior depends on the strain rate. The four parameters analyzed Regarding the numerical simulation, it was necessary to simplify the model in terms of components to reduce the computational resources without compromising the objectives defined for the problem. It can be concluded that intercooler failure might occur, under traction conditions, for a displacement in model between 0,7 mm and 1,2 mm in the box pins. The force necessary to get this failure is between 10,6 e 14 KN. Regarding the test bench, the objectives outlined earlier pointed to the conception, design and manufacturing. However, it was not possible to meet the manufacture due to time limits. However, the results obtained in the numerical simulation, predicting the destruction of the intercooler for a significantly reduced displacement (of the order of 1 mm) indicate that the required accuracy to the test bench, including machine tool, has to be significantly increased. The draft test bench which has been developed is inevitably associated with the characteristics of the press machine available for the project. However, it is recognized that due to the equipment stiffness, including gaps and alignment, the system may not be the most appropriate to the test the destruction of intercoolers under traction conditions. 6. References [1] Plastics Determination of Tensile Properties (all parts), BS EN ISO 527, [2] R. Mundt, Hoogovens, Koblenz, Introduction to Brazing of Aluminum Alloys, Lecture 4601,

10 [3] Mel M. Schwartz, Brazing, ASM International, 2 nd edition, [4] Standard Methods for Tension Testing of Metallic Materials, E8-83, ASTM Standard, [5] A. Arriaga, et al., Finite-element analysis of quasi-static characterization tests in thermoplastic materials: Experimental and numerical analysis results,