Analysis of the mechanical expansion process of thin-walled tubes for air heat-exchanger production

Finned tube air heat-exchangers are built by joining the tubes to the fins by various techniques. A technique mostly used for large heat-exchangers consists of fitting the tubes into the holes of the fins by expanding the tubes using a mechanical process. The expansion is achieved by inserting an ogive of a larger diameter into the tube. The intimate contact of tubes and fins due to the press fitting ensures the proper thermal connection. This work tries to describe an experimental-numerical procedure useful to study and predict the mechanical process and the process parameters. The procedure, based on material properties obtained from tensile tests and the use of the inverse method to identify the material parameters, is based on bi-dimensional finite element (FE) models used to simulate the expansion process. The FE model is then used for process optimisation regarding such parameters as the ogive shape and ogive sizes, friction coefficient and speed of insertion of the ogive into the tube. Indeed, size and shape uncertainties strongly influence the process parameters and the process quality, as well as the heat-exchanger efficiency. The use of numerical models was proven highly effective in predicting and optimising the process by quickly analysing the influencing factors and optimising the production

Battery Pack and Underbody: Integration in the Structure Design for Battery Electric Vehicles—Challenges and Solutions

The evolution toward electric vehicle nowadays appears to be the main stream in the automotive and transportation industry. In this paper, our attention is focused on the architectural modifications that should be introduced into the car body to give a proper location to the battery pack. The required battery pack is a big, heavy, and expensive component to be located, managed, climatized, maintained, and protected. This paper develops some engineering analyses and shows sketches of some possible solutions that could be adopted. The possible consequences on the position of the vehicle center of gravity, which in turn could affect the vehicle drivability, lead to locate the battery housing below the passenger compartment floor. This solution is also one of the most interesting from the point of view of the battery pack protection in case of a lateral impact and for easy serviceability and maintenance. The integration of the battery pack’s housing structure and the vehicle floor leads to a sort of sandwich structure that could have beneficial effects on the body’s stiffness (both torsional and bending). This paper also proposes some considerations that are related to the impact protection of the battery pack, with particular reference to the side impacts against a fixed obstacle, such as a pole, which are demonstrated to be the most critical. By means of some FE simulation results, the relevance of the interplay among the different parts of the vehicle side structure and battery case structure is pointed out

Experiment based modeling of the mechanical expansion of tubes for the construction of heat exchangers

Tube heat exchangers are made by assembling metals tubes, which the fluid to be refrigerated is passed through, with fins where a refrigerating fluid (usually air) is flown over. The heat exchange between tubes and fins is obtained by exploiting their tight contact. This necessary very tight contact is obtained by means of brazing (typically in smaller equipment) or through the forced expansion of the tubes into the fins holes. The forced expansion can be hydraulic (by some fluid put in pressure in the assembly operation) or mechanic through the insertion of a sphere or an ogive with external diameter slightly larger than the internal diameter of the tube. The sphere or the ogive is pushed along the entire length of the tube so that the tube remains plastically forced into the fins holes. The process is then repeated for all the tubes of the heat exchanger. The present work concentrates on the mechanical expansion: to optimize the construction process it is necessary to have a model able to describe the mechanical phenomenon: that is, to evaluate the stress state in the tube during the insertion of the ogive, the residual stresses after the sphere/ogive passage, and the force required depending on the process and materials parameters (including the geometry of the tube, ogive, and fins, their material properties, friction, insertion speed etc.). The present work will describe an analytical model able to describe the process with a good level of predictability showing the effect of the main parameters involved in the process. The model is based and validated by means of experimental tests and numerical simulations at different levels and in different conditions and materials

Axial crushing of metal-composite hybrid tubes: experimental analysis

In the automotive sector, special attention is paid to the study of the behavior of the structural components that make the car bodies. The continuous demands on the weight saving imply the car bodies to be assembled with components made in different materials and using different manufacturing processes. Considering the making of the sacrificial structures aimed to the energy absorption, composite materials are increasingly used to replace conventional metal materials. However, the use of composites is accompanied with a change in the type of deformation obtained during the impact phenomenon. Usually, with the conventional metal materials, the crushing behavior is a progressive buckling whereas the composite materials are characterized by a brittle fracture. The combination of the traditional metal materials with the composite ones can represent a good solution to obtain high levels of performance. In this context, the structural performance of metal-composite hybrid tubes subjected to quasi-static axial crushing is experimentally evaluated in this work. The specimens, with circular cross section, were obtained with tubes made in a fully thermoplastic composite internally reinforced with aluminum tubes. The composite material used were made in polypropylene both for the matrix and for the reinforcing fibers. This material has a good axial absorption capacity but irregular behavior during crushing. The addition of a conventional material as reinforcement allowed to increase the absorption capacity by ensuring a more progressive and controlled crush. The analysis was carried out by evaluating, for various geometric configurations, different parameters (mean load, average stress, specific energy, efficiency). The results, discussed in the work, showed how the energy absorption performance of a hybrid structure are higher than the sum of the performance of the single materials

Impact behavior of a fully thermoplastic composite

Composites are materials of choice for lightweight structures due to their excellent strength/weight/and stiffness/weight/properties. For several years, the application of composite materials with continuous fiber was limited to those with thermosetting matrix. Recently, interest in composites with thermoplastic matrix is growing thanks to their considerable advantages also in terms of recyclability. The thermoplastic composites appear to be the right alternative to the materials currently used, replacing not only the non-structural parts, but also the structural components located in areas potentially subject to impacts. This paper presents the results of an experimental campaign made on a fully thermoplastic composite, where both the reinforcement and the matrix are made in polypropylene. The target is to analyze its behavior under different impact loading conditions using a drop weight testing machine. The influence of the impact mass and of the velocity on the energy absorption capability of the material have been analyzed and discussed. During the tests, the material showed a ductile behavior and developed extended plasticity without a crack tip. The main observed damage mechanisms were the yarn sliding

experimental and numerical analysis of a thermoplastic lamina for composite material

Abstract Thermoplastic composites nowadays belong to an interesting class of materials for different type of industries. Those materials present a considerable number of advantages compared to the thermosetting composites. Their low density, low production cost, recyclability, are some of the positive aspects encouraging their usage. The numerical simulation is still an open research field due to the peculiar behaviour of the thermoplastic composites. According to that, this works starts a detailed numerical study in which the main deformation mechanisms are simulated using different modelling approaches. In this overview, the object of this study is a fully polypropylene composites made up of woven polypropylene laminas. These laminas are stacked and hot pressed. Previous research showed a ductile and plastic crush behaviour of this material, in which the main failure mode is governed by the delamination. Firstly, an experimental campaign is carried out, in order to define the constitutive properties of the single lamina. Woven laminas are orthotropic composites responding differently according to the direction of the load. Yarn test was executed to capture the tensile modulus and the strength, whereas the Bias-Extension test was carried out to examine the in-plane shear properties. The definition of those properties required several considerations and a detailed analysis because the load applied in the test is directed neither along the weft nor along the wrap direction. For this reason, geometrical approximations and hypothesis about the boundary condition are necessary to evaluate the shear stress and the shear angle parameters. Hence, three different FE models were developed in LS-DYNA and results were validated against the experimental tests. Two geometry discretization method and three material models were implemented. The first numerical model was developed for fabric materials and it represents a macro-mechanics approach with low computationally cost. The second model instead accounts for the fabric architecture, allowing to evaluate the weave geometry and the reorientation effect of the yarns during the deformation. The third model represents the discrete architecture of the fabric, modelling the specimen at the tape level. The numerical results showed a good approximation of the experimental evidence, especially considering the second numerical model. This material model confirmed the geometrical assumptions used to define the mechanical properties. This work gives a first important step for the simulation of components made of thermoplastic composites and with more complex geometry using a mesoscopic approach

Injury criteria for vehicles safety assessment: a review with a focus using Human Body Models

This paper aims at providing an overview of the most used injury criteria (IC) and injury metrics for the study of the passive safety of vehicles. In particular, the work is focused on the injury criteria that can be adopted when finite element simulations and Human Body Models (HBMs) are used. The HBMs will result a fundamental instrument studying the occupant’s safety of the Autonomous Vehicles (AVs), since they allow to analyze a larger variety of configurations compared to the limitations related to the traditional experimental dummies. In this work, the most relevant IC are reported and classified basing on the body segments. In particular, the head, the torso, the spine, the internal organs, and the lower limbs are here considered. The applicability of the injury metrics to the analyses carried out with the HBMs is also discussed. The paper offers a global overview on the injury assessment useful to choose the injury criteria for the study of the vehicle passive safety. To this aim, tables resuming the presented criteria are also reported to provide the available metrics for the considered body damage

Investigation of creep phenomenon on composite material for bolt connections

One of the main target in the automotive design is the weight reduction. This reduction leads to the reduction of the gas emissions. The designers tend to use innovative materials for the automotive field such as plastics and composites. To use the right material for the right application, multi material solutions are increasingly adopted. To join dissimilar materials, solutions like adhesive, bolt and nuts, riveting are necessary. It is necessary to know the behaviour of the materials to be joined, under different loading conditions to ensure the joint. In this work, a bolt connection between composite and aluminium plates has been considered. The behaviour of a carbon fibre reinforced material under compression load, taking into account creep is studied. A specific experimental equipment has been design and built. A series of experimental compressive tests, in the laminate thickness direction, have been done on carbon fibre reinforced material specimens. Different set-up in terms of temperature, compression load and surface roughness have been investigated. The obtained results are presented and discussed. A mathematical model will be proposed for interpolation of the obtained results. Finally, a possible strategy for reducing the tight loss in the initial phase of the joint life is propose

Mechanical properties and impact behavior of a microcellular structural foam

Structural foams are a relatively new class of materials with peculiar characteristics that make them very attractive in some energy absorption applications. They are currently used for packaging to protect goods from damage during transportation in the case of accidental impacts. Structural foams, in fact, have sufficient mechanical strength even with reduced weight: the balance between the two antagonist requirements demonstrates that these materials are profitable. Structural foams are generally made of microcellular materials, obtained by polymers where voids at the microscopic level are created. Although the processing technologies and some of the material properties, including mechanical, are well known, very little is established for what concerns dynamic impact properties, for the design of energy absorbing components made of microcellular foams. The paper reports a number of experimental results, in different loading conditions and loading speed, which will be a basis for the structural modeling