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End-to-end mechanical joining of tubular components

Guilherme Batalha Reis da Gama Nunes

ABSTRACT

In this paper the need of a new joining process for connecting tubes by their ends is analysed and a new alternative is proposed that has the potential to replace current solutions. The process is based on a new type of tubular lap joint produced by local plastic instability and compression beading. The process consists of two elementary tube forming operations that are carried out in a single stroke; expansion to produce two adjacent counterfacing surfaces, and compression beading caused by plastic instability to lock the tubes. Two variants of this process are tested and the comparison of the results is shown. The present work includes experimentation and numerical simulation of the process variants in order to identify the modes of deformation and the feasibility windows, and destructive testing to establish the working limits of the joints, under different types of loading conditions. The obtained results allowed to elect the best process variant and to demonstrate the simplicity and effectiveness of the proposed joining process on carbon steel tubes. A preliminary study on the application of this process to obtain a joint with two tubes made from dissimilar materials is also presented. Keywords: Mechanical joint, Thin-walled tubes, Plastic deformation, Experimentation, Finite element method

1. INTRODUCTION

Currently available solutions for the end-to-end joining of tubes are based on fastened, crimped, welded, brazed and adhesive bonded joints (Fig. 1). Each option has its advantages and disadvantages for its use in specific applications. Welded joints (Fig. 1c) are probably the most common in end-to-end joining of tubes applications. There are many different welding technics with their own advantages and disadvantages and their field of application regarding different materials and geometries. Nevertheless, all of them require the components to be joined to reach their melting point, which may introduce heat-affected zones where the material’s microstructure and mechanical properties get altered by the heat. This alteration can be detrimental, causing stresses that reduce the strength of the base material. The difficulty to obtain a decent joint increases in the case of thin-walled tubes, where the distortion and warpage can occur. Brazed joints (Fig. 1d) are a good alternative to welded joints in case of thin-walled tubes. This process allows the joint to be obtained with no need for the materials to reach their melting point thus preventing the formation of heat-affected zones. The connection is due to the diffusion effects and metallurgical reactions on the components surfaces, and the filler material spreads by capillarity action [1]. Brazed joints can be a good alternative for dissimilar materials applications but they are not recommended for high temperature working conditions, mainly because of the differences on the thermal expansion of the components’ materials when compared to the expansion of the filler material, which can lead to fatigue related cracks. This process is also not recommended when the finished product is meant to be used under high working forces [2]. Adhesive joints (Fig. 1d) are an alternative to welded and brazed joints in situations where temperature cannot be applied or in applications involving dissimilar materials. This type of connection is obtained due to the attachment forces generated by a substance that acts as a chemical agent, creating chemical reactions on the materials surfaces. There are no heat-affected zones but the process has a low production rate. The careful and time consuming preparation that is required and

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the immobilization waiting period for the adhesive material to solidify, are the two main causes for the low production rate of this process. The adhesives may even loose their properties in adverse conditions and their mending is practically impossible. In the case of an end-to-end joining of thin-walled tubes, the contact area is small, which substantially reduces the strength of the joint. Fastened joints (Fig. 1a) are obtained by using threads, screws, bolts, rivets and others. Unlike other types of joints, these ones depend mostly on the geometry and surface of the components that are to be joined. They don’t depend on the chemical reactions or the need to melt the materials, preventing all kinds of micro-structure disturbance. This type of joints is very good in cases of dissimilar materials components and permits their replacement when damaged or even its temporary removal to be repaired. They are simple to design, easy to assemble and disassemble and are available in standard sizes [3]. Their main limitations are geometrical, aesthetic and dimensional. The external components required to obtain the joint add more weight to the structure and increase its cost. Other disadvantage is the generation of stress concentration areas that originate failure and accelerate corrosion. Crimped joints (Fig. 1b), obtained with processes like electromagnetic-forming [4] and swaging [5] are not limited by their geometries but, instead, by their low mechanical strength and weak tightness. To obtain a crimped joint by electromagnetic-forming, the components must be good electrical conductors, limiting its application to aluminium, coper and its alloys. The study of existing technical solutions for the end-to-end joining of thin-walled tubes allow us to conclude that there is a need to develop a new simple and effective process that is capable to obtain such joint, especially in the case of dissimilar materials components.

Fig. 1 - Different types of joints currently used for end-to-end joining of tubes: (a) fastened joints, (b) crimped joints, (c) welded joints, (d) brazed or adhesive joints. In a recent paper [6], the authors proposed a new process to join two tubes by their ends by plastic deformation. The process consists of two elementary tube forming operations that are carried out in a single stroke; expansion to produce two adjacent counterfacing surfaces, and compression beading caused by plastic instability to lock the tubes. This thesis proposes a new variant of the process which is referred to as “new”, while the previous one is referred to as “conventional”. Schematic representations of the two process variants to join two thin-walled tubes with a reference radius ro=16mm, length lo (upper tube) and li (lower tube), thickness to=1mm and angle of the chamfer α can be observed on Fig. 2.

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Fig. 2 – Schematic representation of the operations of the two process variants: (a) Process to obtain the conventional joint, (b) process to obtain the new joint.

The present work aims to: (i) identify the different modes of deformation in order to set-up the feasibility window for each of the two processes variants’ major operating parameters, (ii) perform destructive testing to establish the working limits of the joints under different types of loading conditions, (iii) compare the two process variants and (iv) analyse the technology’s potential to join components made from other types of materials, as well as dissimilar materials. In order to obtain the results, the present work includes experimentation and numerical simulation of the process and destructive tests.

2. EXPERIMENTAL METHODS AND PROCEDURES 2.1. Feasibility windows set-up

In order to set-up the feasibility window for each variant of the process, several joints were performed while varying the parameter lgap (lgap=lo+li – dimensions represented at Fig. 2). Firstly, this study evaluated the joints obtained from the conventional variant (cases 1-7) where the lgap parameter took the values 10, 20, 30, 40 and 50 millimetres (cases 1-5, respectively), and the value of lo-li was made different from zero, assuming the values 1 and 2 millimetres (cases 6 and 7, respectively). Afterwards, the joints obtained from the new variant of the process were studied, evaluating the joints corresponding to values of lgap of 31, 41, 51, 61 and 71 millimetres (cases 8-12, respectively). More details of each case are shown at Table 1.

2.2. Destructive Testing The tests were chosen with the intention to submit the joints to types of loading conditions that were thought to reflect the future working conditions of the final components. Therefore, the following tests were performed: (i) tensile test, (ii) torsion test, (iii) bending test and (iv) water tightness test. The purpose of the tensile test (Fig. 4a), was to evaluate the joints to tensile loading conditions, that can be found in automotive frames, truss structures and daily objects structures, and other applications. It was performed by a regular tensile test machine and some tools were manufactured to permit the machine to grab the joined tubes (Fig. 3 a and b). The torsion test (Fig. 4b) intended to evaluate the joints’ potential in applications such as drive shafts, steering column, among others. It was performed by a regular torsion testing machine and some tools were manufactured to permit the machine to grab the joined tubes (Fig. 3 a and c).

(a) (b)

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The bending test (Fig. 4c) intended to evaluate the joints’ potential in applications such as automotive frames, structural frames and others. It was performed on a press machine, where the experimental apparatus was mounted, in order to perform a 4 points bending test. The water tightness test (Fig. 4d) was performed because the evaluation of the potential of the joints to be used in plumbing and pressure systems tubing applications, among others, was considered to be important. Two PVC piston-like tools were manufactured in order to permit to seal the ends of the tube and raise the interior pressure until the water put in the inside of the joint exits through the voids of the joints (Fig. 3d). A joint obtained from the new variant of the process with the placement of approximately 20 layers of teflon tape in the inside of the joint (case 21) was also tested.

Fig. 3 – Manufactured tools for the destructive tests: (a) components of the grabbing system for the tensile test, (b) components of the grabbing system for the tensile and torsion tests, (c) components of the grabbing system for the torsion test and (d) PVC pistons to seal the ends of the tube and raise the interior pressure.

Fig. 4 – Experimental apparatus of the destructive tests: (a) tensile test, (b) torsion test, (c) bending test and (d) water tightness test.

(a) (b) (c) (d)

(a) (b)

(c) (d)

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2.3. Experimental work plan

Case Phase Study/ Test

Process variant Material ol

(mm) il

(mm) gapl

(mm)

α (degrees)

����

��

1 F

easi

bilit

y w

indo

ws

set-

up

gapl study

Conv.

S460MC

5 5 10 25 0.312

2 10 10 20 25 0.625

3 15 15 30 25 0.938

4 20 20 40 25 1.25

5 25 25 50 25 1.562

6 ol - il

study

16 15 31 25 0.969

7 17 15 32 25 1

8

gapl study New

16 15 31 25 0.969

9 21 20 41 25 1.281

10 26 25 51 25 1.594

11 31 30 61 25 1.906

12 36 35 71 25 2.219

13

Cha

ract

eriz

atio

n

Tensile Conv. 16 15 31 25 0.969

14 New 26 25 51 25 1.594

15 Torsion

Conv. 16 15 31 25 0.969

16 New 26 25 51 25 1.594

17 Bending

Conv. 16 15 31 25 0.969

18 New 26 25 51 25 1.594

19 Water

Tightness

Conv. 16 15 31 25 0.969

20 New

26 25 51 25 1.594

21 26 25 51 25 1.594 Table 1- Experimental work plan.

3. FINITE ELEMENT MODELING

Numerical modelling was performed resorting to the computer program I-FORM, which was initially developed in the 1980s in the department of Mechanical Technology at Instituto Superior Técnico. I-FORM is based on the finite element flow formulation, which is based on the following variational principle (extended to account for contact and friction),

( ) ( )∑∑∫ ∫∫ ∫∫==

++

+−+=Π

cc

f

r

T

N

c

ct

N

c

cn

S

u

rf

V

i

S

iv

V

gQgQdSdudSuTdVKdV1

2

21

1

2

21

||

0

221 µτεεσ && (1)

where σ is the effective stress, ε& is the effective strain, Vε& is the volumetric strain rate, iT and iu are

the superficial tractions and velocities on the surface TS . K is a high value positive constant to impose the incompressibility constrain of the volume V. fτ and ru are the friction shear stresses on the

contact interface fS between tubes and tooling (dies and mandrel). The last two terms in (1) account

for contact between the counterfacing surfaces of the two tubes by penalizing normal gap velocity cng ,

when otherwise leading to penetration, and by constraining tangential gap velocity ctg to account for

frictional effects [7].

Finite element simulations were performed by a computer equipped with a quad-core intel i7 CPU (2.7 GHz). The average computing time for a regular simulation with a 1650 node and 1300 elements mesh was less than 25 minutes. To create the dies and tubes, Autocad and GID software were used. GID was also used to help evaluating the process geometry and the most important parameters evolution.

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4. RESULTS AND DISCUSSION 4.1. Feasibility windows set-up

As it was mentioned before, several joints were obtained by varying the parameter lgap. The evolution of the compressive force during the processes and final geometries (experimental and numerical) are shown in Fig. 5.

Fig. 5 - Evolution of the compressive force during the conventional process to obtain the joint (experimental and numerical), and final geometries (experimental and numerical).

Observing Fig. 5, the joint obtained in case 3 was considered the best, with respect to the required specifications: (i) completely closed joint, (ii) occurrence of only one instability and therefore one expansion, (iii) appealing aesthetics (iv) material/weight in the joint and (v) compression bead formed by both tubes. Cases 1 and 2 were disregarded because they didn’t verify specification (i). Case 5 didn’t verify specification (ii). Case 4, despite verifying specifications (i) and (ii), was considered to be outmatched by case 3 because it uses more material (and subsequently weights more) and is aesthetically less appealing. Considering that the case 3 joint verifies specifications (i) and (ii) and uses less material than in case 4, it was considered that in case 4 there was excess of material. There was also the need to find out what had to be the difference between the two tubes’ lengths. This need is due to the upper tube’s (with the interior chamfer) obligation to expand in order to slide on the exterior of the lower tube, and therefore has to contour it. This implies that, if they have the same length, the two tubes won’t reach the opposite matrix at the same time. The conclusion by experimenting (cases 6 and 7 of the experimental work plan) was that the upper tube had to be 1 millimetre longer than the lower tube (nomenclature according to Fig. 2). A similar study with the aim to choose a value for the parameter lgap was performed for the new variant of the process (cases 8 to 12). Results are shown in the figures below.

Fig. 6 – Joint geometries obtained in cases 8-11 (a) experimentally and (b) numerically.

(a)

(b)

Case 8 Case 9 Case 10 Case 11 Case 12

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Fig. 7 - Results obtained from the joining new variant process for the study of the lgap parameter value. Process force-displacement chart (experimental and numerical) and exterior geometry of the joints obtained experimentally are presented.

The joints obtained in cases 8 and 12 were immediately disregarded. In case 8, the compression bead was not formed by the two tubes, missing the lower tube (specification (v)). In case 12 the expansion to instability occurred two times (specification (ii)). Analysing cases 9, 10 and 11, and despite verifying specifications (i), (ii), (iii) and (v), the joint obtained in case 11 was disregarded because of the excess of material (specification (iv)). The chosen joint was the one obtained in case 10, despite the fact that in case 9 there was less material in excess, considering to be important for the comparison that the new and conventional joints had corresponding lgap values.

4.2. Destructive testing This section of the work is focused on evaluating and comparing the performance of the joints obtained from the two process variants by means of destructive tests mentioned before. The tests were performed on the joints chosen on the chapter 4.1.

Fig. 8 - Torsion test results. The photograph shows a detail of the experimental apparatus.

The results presented in Fig. 8 allowed to conclude that the conventional joint had an inferior performance in this test, unlocking the rotation for a torque higher than 95 Nm, whereas the new joint only experienced minor rotation (below 5 degrees) when the nominal capacity of the testing machine (200 Nm) was reached. The highest performance of the new joint is attributed to the overlapping of the two connected tubes, that significantly increases the friction force that opposes rotation.

0

40

80

120

160

200

0 10 20 30 40 50 60

To

rqu

e (N

m)

Degrees (o)

New joint

Conventional joint

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Fig. 9 - Tensile test results: (a) evolution of the compressive force during the process, in the different cases, (b) and (c) pictures of the conventional joint after the tensile test, (d) picture of the new joint after the tensile test, (e) comparison of the final geometries obtained experimentally and numerically with critical damage (FEM2), (f) representation of the geometry obtained by numerical simulation without critical damage (FEM1), (g) representation of the initial geometry for numerical simulation and (h) representation of the final geometry obtained by numerical simulation with critical damage.

The tensile test of the new joint (case 14) was also simulated using the software I-FORM, and results are presented in Fig. 9. Two simulations were performed: (i) FEM1 was performed without the definition of a critical damage value, so that the geometrical results could be compared with the experimental final geometry in order to determine the critical damage value, (ii) FEM2 was performed with the critical damage value defined (0.6) and therefore the rupture was obtained. Observing the chart presented in Fig. 9a, the new joint is capable of withstanding 60 kN of tensile force before failing suddenly by cracking along the compression bead of the outer tube, with no prior signs of damage. The numerical simulation results support the experimental results. In contrast, the conventional joint is only capable of withstanding 40 kN (33% less) of tensile force before failing by cracking of the inner compression bead and subsequently unfolding what remains of the connection between the two tubes.

Fig. 10 – Four point bending test results: (a) experimental bending moment-displacement curves with a schematic representation of the points of application of the forces and mounts and two photographs of an instant during the tests, (b) final geometry of the conventional joint after the bending test, (c) final geometry of the new joint after the bending test.

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Observing Fig. 10, the new joint was capable of withstanding a bending moment of roughly 1100 Nm before failing by cracking of the two connected tubes, whereas the conventional joint was only capable of resisting to bending moment of 840 Nm (24% less) before failing by cracking of the interior tube (lower tube of Fig. 2a). The higher performance of the new joint is attributed to the fact that, because it has an extensive overlap above and below the compression bead, the external tube (upper tube on Fig. 2) did not expand and had to fracture for the joint to open. The cracking of the two tubes lead to a higher performance of this process variant.

Fig. 11 – Water tightness test results: (a) experimental evolution of interior pressure with displacement of the upper piston with two photographs of the joints leaking water and (b) photograph of the conventional joint in the beginning of the test.

Because of the low forces applied in this test, the friction concerning the movement of the upper component of the press and concerning the contact between the upper piston and the tube were taken into account and removed from the force evolution on the press during these tests. The tests were carried out by increasing the water pressure inside the joints by means of two pistons up to a value at which leakage occurs. The pressure then remained constant as the water leaked during the descent of the upper piston. Observing Fig. 11, the new joint had a lower performance (70 atm) when compared to the conventional joint (190 atm). Only when the teflon tape was placed, the new joint improved its performance (500 atm).

4.3. Preliminary study on different materials In order to demonstrate the technology’s potential in cases of components made from non-carbon steel or dissimilar materials, several joints connecting tubes made from other materials, as well as pairs of tubes made from dissimilar materials were obtained using the conventional process variant. The materials chosen for this study were AISI304 stainless steel and AA6060-T6 aluminium. Firstly the lgap value was varied to join two tubes made from the same material. Afterwards, hybrid joints were made, joining tubes made from carbon steel to tubes made from stainless steel and aluminium.

Fig. 12 – Hybrid joints: (a) carbon steel and stainless steel tubes joints, (b) carbon steel and aluminium tubes fractured joints and (c) carbon steel and aluminium tubes joint with the pre-shaped carbon steel tube.

(a) (b) (c)

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5. CONCLUSIONS

In this work a new process with two variants to connect thin-walled tubes by their ends was presented and a feasibility window for each variant was defined. The joints obtained from the two process variants were tested by means of destructive testing. I-FORM software allowed a numerical validation of some destructive tests, as well as the study of the primary parameters of the process, therefore being of extreme help. The feasibility window of each process variant was set-up, especially the ratio lgap/ro. The admissible values for this ratio were between 0.939 and 1.25 in the case of the conventional process variant and between 1.281 and 1.906 in the case of the new process variant. It was also demonstrated that the relation lo-li must take a value of approximately 1 millimetre in both cases. The results verified in the destructive tests performed on joints obtained by the two variants of the process showed that the new joint has a higher performance facing all of the loading conditions, except on the water tightness test. Nevertheless, by simply applying teflon tape in the interior of the joint, the new joint was able to surpass the conventional joint performance. The preliminary study on different materials demonstrated that joints of dissimilar materials are attainable with this process, when subjected to some changes.

ACKNOWLEGEMENTS

The author would like to acknowledge the support and precious guidance given by Prof. Luís Alves and Prof. Paulo Martins.

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