List of Tables
Chapter 2 Joining of a tube to a sheet through end curling
2.2 Results and discussion
2.2.8 Load-displacement behaviour and energy absorbed during pull-out tests During Pull-out tests of the end formed joints and welded joints, it can be
Chapter 2 technology like joining through welding, joining by adhesive bonding and joining through
fasteners.
Alves et al. (2011b) proposed a different method of joining sheet to a tube using end forming of the tube. This process basically consists of two stages – first stage is compression beading and the second stage is tube inversion. When the proposed method is compared with the existing method, some advantages and limitations of the proposed method can be revealed. In the proposed method, the joining is performed in one downward vertical stroke of the punch and no tool is in contact with the joint location during the joining process, producing a better and compact joint. The preparation of sheet before the actual joining process and regular removal of punches and dies after the process completion are the major limitations of the proposed method.
2.2.8 Load-displacement behaviour and energy absorbed during pull-out tests
Chapter 2 larger displacement and hence a larger maximum load is required to unlock the tube-sheet
joint. Hence it can be said that since complete unlocking needs larger displacement in case of end formed joints, larger load is required as compared to welded structure in case of flat die.
Fig. 2.34 shows the pull-out data of joints tested for die inclination of 10°. The welded tube-sheet joint takes the largest fracture load among all type of joints. Among the end formed joints, joint made under Case 3 takes the largest load to fail. The fracture load reached for any case in end formed joints is a function of displacement on angular plane.
In Case 3, the displacement to complete unlocking is larger and hence fracture load reached is also larger. For end formed joints no physical fracture or crack formation is observed in the joint region, rather an unlocking phenomenon takes place in the tube- sheet joint region. This unlocking phenomenon is considered as fracture in the end formed joint. When welded structure is compared with end formed joints, it is seen that it needs larger load as compared to end formed joints, despite the fact that it needs lesser displacement as compared to Case 1 and Case 3. Since welded structure undergoes physical fracture, it needs larger load as compared to end formed joints which exhibits joint unlocking.
Fig. 2.32 Tested samples of (a) end formed joint, (b) welded joint, (c) sectioned view of end formed joints before and after pull-out tests
Chapter 2
Fig. 2.33 Load-displacement behaviour of joints made under different conditions for flat die
Fig. 2.34 Load-displacement behaviour of joints made under different conditions for die with inclination of 10°
Fig. 2.35 shows the pull-out test results of joints fabricated for die with inclination of 15°. The welded tube-sheet structure takes the largest load to fail. For end formed joints, as the displacement to failure increases, the maximum load required for failure also increases. In Case 3, larger displacement is observed for complete unlocking and hence larger load is required for failure in this case too. Since the welded structure fails near the weld zone, larger fracture load is observed for welded structure among all cases.
Chapter 2
Fig. 2.35 Load-displacement behaviour of joints made under different conditions for die with inclination of 15°
In summary, it can be said that for end formed joints on inclined planes, lesser load is observed as compared to flat die. Here inclination helps in easy removal of the tube from the sheet. For welded structure, the fracture load increases for inclined dies as compared to flat dies. In this case, larger displacement to failure is needed when inclined planes are used and this holds responsible for larger peak load.
Fig. 2.36 shows the summary of variation of fracture load for welded and end formed joints. Though end formed joints perform better than welded joints in case of flat die, the welded joints performance is slightly better in other cases (like 10º and 15º die inclination). There is about 4 to 6 kN difference between the joints for inclined dies.
Within end formed joints, Case 3 better performed. The joint performance in case of welded joints depend on metallurgical changes in the weld region as the raw tube is of mild steel category. On the other hand, the end formed joints depend only on mechanical interlocking and strain hardening undergone in the joint region. Though metallurgical changes during joint formation aides in joint strength, the end formed joints are advantageous in few ways.
Chapter 2
Fig. 2.36 Fracture load for joints made for different joint cases using different dies
There is no fume generation, consumable wastages, health hazards in the case of fabricated end formed joints as compared to arc welded joints or the fusion welding processes. In the case of welded joints, physical failure occurs at lower displacement during pull-out tests. But in end formed joints, since physical failure is not observed, even a slight pull-out displacement will keep the joint intact without quality loss. Complete unlocking takes larger displacement in this case.
Based on equation 2.1, the energy absorbed during pull-out tests of end formed joints and welded joints has been calculated. For this purpose, the load-displacement curves have been fit into a third order polynomial equation of the form A + B. x + C. x2 + D. x3. It is assumed that higher the order of fitting curve better would be the accuracy.
The adjusting R-square value has also been calculated for different cases. The adjusting R-square value is more than 0.96 for different cases as shown in Table 2.11 which indicates that a better fit between load curve and fit polynomial curve has been obtained.
The value of constant A and coefficients, B, C and D are given in Table 2.10 for different cases. Table 2.11 provides a summary of important data from pull-out tests.
Table 2.10 Values of A, B, C and D in the polynomial equation for different cases
Die conditions Joining conditions A B C D
Flat die
Case 1(S)* 0.49843 1.34912 -0.02472 0.00354
Case 2(S)* 0.35015 1.22723 0.08069 -0.0059
Case 3(US)* -0.75755 3.99625 -0.25876 0.00666
Welded structure 0.62939 5.70865 -0.83953 0.04907
Chapter 2
Die inclination 10°
Case 1(S) 0.07203 0.3552 0.15771 -0.00736
Case 2(S) 0.45333 5.0773 -0.91827 0.06316
Case 3(US) -0.06013 0.06928 0.08156 -0.00209
Welded structure 0.61269 2.41905 -0.06427 0.00175 Die
Inclination 15°
Case 1(S) 0.7742 0.0498 0.16702 -0.00668
Case 2(S) 0.3363 0.7465 0.1654 -0.00974
Case 3(US) 0.57989 -0.40649 0.2027 -0.00729
Welded structure 1.61041 -1.2201 0.36009 -0.0127
Table 2.11 Summary of important results from the joint pull-out tests
Die conditions
Joining conditions
Energy absorbed (J)
Adj. R- square
Maximum displacement at
fracture load (mm)
Fracture load (kN)
Flat die
Case 1(S)* 107.23 0.9659 12 18.24
Case 2(S)* 125.21 0.9993 13 16.96
Case 3(US)* 164.12 0.9758 12 21.18
Welded structure
81.65 0.9824 7 15.93
Die inclination
10°
Case 1(S) 125.32 0.99801 15 16.3
Case 2(S) 74.05 0.98265 8 14.34
Case 3(US) 201.50 0.99853 23 18.98
Welded structure
153.57 0.99794 12 23.15
Die Inclination
15°
Case 1(S) 120.94 0.9945 15 16.18
Case 2(S) 86.59 0.99106 11 15.5
Case 3(US) 147.30 0.99707 18 16.73
Welded structure
153.15 0.98681 16 21.83
*Cases 1,2,3 belong to end formed joint category
From Table 2.11, it is observed that the fracture load obtained and energy absorbed is minimum for welded structure in case of flat die. In case of die with inclination of 10°, the fracture load attained for welded structure is maximum amongst all joints, while energy absorbed for welded structure is intermediate as compared to end formed joints. The die inclination helps the tube to come out from the sheet due to slipping at tube-sheet interacting surface and also physical failure is not seen here. So a lesser load is required to unlock the tube from sheet despite the fact that Case 1 and Case 3 takes larger displacement to attain the peak load as compared to the welded structure.
Within end formed joints, Case 3 takes largest load because it needs largest displacement to fracture. Case 3 absorbs more energy because the displacement attained by Case 3 is larger as compared to other cases (Case 1, Case 2 and welded structure). In case of die with inclination of 15°, the peak load attained and energy absorbed are larger for welded structure as compared to end formed joints. In this case, displacement to failure for
Chapter 2 welded structure is quite high as compared to flat die and die with inclination of 10º. As a
result, energy absorbed increases for welded structure.
2.2.9 FE simulation of pull-out tests of end formed joint