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

Identificazione di schiume in polipropilene e relazione tra densità e parametri dei modelli

La varietà praticamente infinita di solidi cellulari rende particolarmente ardua la modellazione di questi materiali, per ognuno dei quali è in genere necessaria una specifica identificazione. La ricerca di legami tra i parametri dei modelli e le caratteristiche note della schiuma è utile per ridurre il lavoro di identificazione ed evitare una eccessiva quantità di esperimenti. Prove sperimentali di compressione uniassiale statica su schiume rigide di EPP a cinque densità diverse sono state utilizzate per identificare i modelli di Gibson e Rush. I parametri identificati per il modello di Gibson sono stati confrontati con quelli dati dalle espressioni dello stesso autore in funzione della densità. I parametri identificati per il modello di Rush sono stati studiati per individuare relazioni analitiche simili a quelle di Gibson tra i parametri e la densit

Resistenza statica e dinamica di punti di saldatura in differenti condizioni di sollecitazione

Le saldature a punti sono il principale metodo di giunzione delle lamiere nelle scocche dei veicoli terrestri. Esiste un'ampia letteratura sulla resistenza statica ed a fatica dei punti e sulla loro modellazione statica. Per simulare fedelmente le strutture puntate occorre conoscere le proprietà dei punti in condizioni di sollecitazione dinamica. Su questo problema non esistono molti risultati per cui in questo lavoro si propone un metodo di prova che permette di studiare la resistenza a differenti velocità di impatto ed in diverse condizioni di sollecitazione. I risultati permettono di definire un criterio di rottura del punto in funzione del tipo di caric

Analisi numerica del comportamento a frattura di provini DC(T) monolitici e multistrato

In questo lavoro sono riportati i risultati ottenuti da uno studio, attraverso il metodo degli elementi finiti, del comportamento a frattura di provini DC(T) in materiale monolitico e multistrato. Nella prima parte sono presentati i metodi di calcolo utilizzati per ricavare i parametri fondamentali della meccanica della frattura. Per quanto riguarda i provini in materiale monolitico sono stati realizzati modelli bidimensionali e tridimensionali allo scopo di determinare le soluzioni più opportune da adottare nella costruzione del modello e nell'applicazione dei metodi di calcolo del fattore di intensificazione delle tensioni KI. A tal fine è stata effettuata una serie di confronti con quanto previsto dalla relazione riportata nella norma ASTM E-399. Per quanto riguarda i provini in materiale multistrato sono riportati i risultati ottenuti in termini di fattori KI corrispondenti a ciascuno strato di materiale. Questi risultati sono stati messi a confronto con quanto previsto dalla relazione riportata nella norma ASTM E-399. Tale relazione, come noto, assume un valore del fattore KI costante su tutto lo spessore del provino, in quanto si riferisce a provini in materiale monolitic

Design of an under-bonnet heat exchanger for the improvement of energy efficiency

The present work is aimed at reporting the methodology used to develop an innovative type of heat exchanger to exploit the free space under the upper skin of a car bonnet between the stiffener ribs. The idea is to take advantage of the large surface of the bonnet itself as a radiating area to increase the heat exchange capacity of the cooling system to save energy and improve the overall vehicle efficiency. The heat exchanger can be built with a thin-shell under-bonnet properly shaped that, fixed below the lower surface of the bonnet skin, realizes a closed chamber where the cooling fluid can pass and exchange heat. With the aim of weight reduction, the thin-shell under-bonnet will be made of plastic material and the technology to fix it and create the closed chamber will be, necessarily, structural bonding. To this aim, it has been necessary to develop a methodology to characterize the metal-to-plastic adhesive joint to design and verify the proposed solution. In the work, the experimental method consisting of a C-shaped metal half-specimen (made of steel, and obtained from a sample of a vehicle bonnet) on a plastic plate of the material used for the thin-shell under-bonnet will be illustrated. The adhesion properties of the joint have been then obtained by means of the inverse method with a parametric numerical model reproducing in detail the experimental test in order to identify the parameters of the material model used to describe the adhesive behavior. The necessity of this type of test depends on the type of applied load, mainly direct pull-out, and the type of joined parts and materials. Once the adhesive model parameters obtained, they have been used to virtually study a prototype of the heat exchanger to obtain a suitable solution in terms of thermo-mechanical strength and energetic efficiency

Tribological characterization of modified polymeric blends

The present work reports of a series of experimental tests with two polymeric materials, a thermoplastic polyurethane (TPU) and a polyamide (PA), modified with the inclusion of additives, in terms of their tribological properties of friction and wear. Many thermoplastic materials are in fact used in applications with sliding contact and friction (as in journal bearings, supports\u2026) and, to improve their properties, the polymer is modified with additives having the capacity to change the surface properties. Used additives are of several types: in this work a comparison is made between graphite, polytetrafluoroethylene, a silicone (siloxane), molybdenum disulfide, and carbon nanotubes. For each additive, different percentage in weight have been considered. All these materials can modify the surface properties of the base material exploiting different physical and chemical phenomena. Moreover, the presence of such additives can alter the mechanical properties of the materials sometimes reducing stiffness, strength, and strain limit. The work reports of the experimental methods obtained with a typical tribological test (pin-on-disk method) to measure the tribological properties of the compounds in terms of friction and wear, together with mechanical tests. The analysis will show correlations between the composition, in terms of type and quantity of the additive, on the properties of the compounds

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

A mechanical model of cellular solids for energy absorption

Cellular materials have a variety of applications in the fields of packaging and mitigation in the case of impact of vehicles, due to their ability to protect goods by absorbing energy. To design energy absorption systems, it is necessary to use predictive models of cellular materials. The models must describe the stress-strain behavior then energy absorption characteristics can be evaluated. Moreover, it must consider affecting factors like strain-rate. Modeling the influence of the density helps designer in selecting the best foam solution. In previous works the authors already presented models able to describe the quasistatic stress-strain behavior of several cellular materials. The current paper presents a general model able to describe the mechanical characteristic of a much larger variety of cellular materials including metal foams and considers the influence of strain-rate. Among the considered materials there are the Foaminal\uae foam and APM\uae aluminum foams. The model is fitted to experimental tests with parameters identified based on experimental data. Tests include quasi-static, dynamic, and impact tests in different loading conditions. It will be shown that the proposed model is fundamentally suitable for most materials, virtually any foamed material, and it is a useful tool for designers in the mentioned areas