SIMULIA India Regional Users Meeting „11 Page 1 of 16 Buckling of beverage cans under axial loading Vishwanath Hegadekatte 1 and Yihai Shi 2 1 Novelis Inc. c/o Aditya Birla Science and Technology Company Ltd., Plot No. 1 & 1-A/1, MIDC, Taloja, Navi Mumbai – 410208, India 2 Novelis Global Technology Centre, 945 Princess Street, Kingston, ON Canada K7L 5L9 Abstract: Wrinkling is one of the major defects in sheet metal forming processes. It may become a serious obstacle to implementing forming process and assembling the parts, and may also play a significant role in the wear of the tool. Wrinkling is a local buckling phenomenon that results from compressive stresses (compressive instability) e.g., in the hoop direction for axisymmetric systems such as beverage cans. In the present work, we have studied the buckling of ideal (no imposed imperfections like dents) beverage cans under axial loading both by laboratory testing and finite element analysis. Our laboratory test showed that 2 out of 11 cans fail by sidewall buckling. We have developed finite element models to study the effect of a couple of manufacturing parameters on the buckling of beverage cans. Further we have studied the buckling of dented beverage cans under axial compression through both laboratory testing and finite element analysis using Abaqus and LS-Dyna. Our results show that Abaqus did not predict sidewall buckling during axial compression of beverage cans while LS-Dyna predicted buckling in a few cases. Keywords: Buckling, Experimental Verification, Forming, Plasticity, Shell Structures, Springback. 1. Introduction Wrinkling is a local buckling phenomenon that results from compressive stresses (compressive instability) in the hoop direction for axisymmetric systems. Wrinkling is one of the major defects in sheet metal forming processes. It may be a serious obstacle to implementing the forming process and assembling the parts, and may also play a significant role in the wear of the tool. Therefore a good understanding of the buckling/wrinkling phenomenon is needed to effectively overcome the issues arising from such defects in sheet metal forming. This article summarizes the work done within the Novelis global R&D organization on prediction of wrinkling in axisymmetric (in terms of geometry) systems with material parameters and friction being assumed to be isotropic. This work was initiated after Mao & Santamaria (2009) published a paper at the 2009 Simulia Customer Conference where the authors simulated wrinkling using a 3D model of the deep drawing of a cup and used it to optimize the tooling and forming parameters. Making use of symmetry, Mao & Santamaria (2009) modeled a 15 o segment of the cup drawing
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SIMULIA India Regional Users Meeting „11 Page 1 of 16
Buckling of beverage cans under axial loading
Vishwanath Hegadekatte1 and Yihai Shi
2
1Novelis Inc. c/o Aditya Birla Science and Technology Company Ltd., Plot No. 1 & 1-A/1, MIDC,
Taloja, Navi Mumbai – 410208, India 2 Novelis Global Technology Centre, 945 Princess Street, Kingston, ON Canada K7L 5L9
Abstract: Wrinkling is one of the major defects in sheet metal forming processes. It may become a
serious obstacle to implementing forming process and assembling the parts, and may also play a
significant role in the wear of the tool. Wrinkling is a local buckling phenomenon that results from
compressive stresses (compressive instability) e.g., in the hoop direction for axisymmetric systems
such as beverage cans. In the present work, we have studied the buckling of ideal (no imposed
imperfections like dents) beverage cans under axial loading both by laboratory testing and finite
element analysis. Our laboratory test showed that 2 out of 11 cans fail by sidewall buckling. We
have developed finite element models to study the effect of a couple of manufacturing parameters
on the buckling of beverage cans. Further we have studied the buckling of dented beverage cans
under axial compression through both laboratory testing and finite element analysis using Abaqus
and LS-Dyna. Our results show that Abaqus did not predict sidewall buckling during axial
compression of beverage cans while LS-Dyna predicted buckling in a few cases.
SIMULIA India Regional Users Meeting „11 Page 5 of 16
flatness of sheet, presence of hard particles. The thickness variation along the axial direction of the
can was measured and is shown in Figure 4 (a). It should be noted that this variation in the
(a)
(b)
Figure 4: (a) Variation of wall thickness along the (a) axial direction (in terms of absolute value) and (b) circumferential direction (in terms of percentage values).
thickness along the length of the can is by design. However, the thickness variation in the
circumferential (hoop) direction may be due to imperfect tool geometry during the forming
process. In Figure 4 (b) we present the measured thickness variation in the circumferential
direction. It can be seen from this graph that the maximum circumferential wall thickness variation
is approximately 10% which in absolute terms is approximately 8 – 13 micron.
2.2 Finite element modeling of the axial compression of an ideal can
In this section, we discuss the possible reasons for the 2 ideal cans that failed due to side wall
buckle during physical testing as presented in Figure 3. For our study, we consider two probable
scenarios where we tested the circumferential thickness variation and a slightly slant trim edge.
Generally, three types of elements are employed in sheet metal forming simulations, i.e.
membrane element, continuum element and shell element. Membrane elements have been widely
used to model the forming processes, due to its simplicity and lower computation time, especially
in the inverse and optimization analysis where many iterations of forming are required. However,
it does not include bending stiffness, therefore, it may not be appropriate in cases where one has to
SIMULIA India Regional Users Meeting „11 Page 6 of 16
deal with the buckling phenomena. In general, the bending-dominant processes are simulated by
the continuum or shell elements. In continuum analysis, the bending effect can be taken into
(a)
(b)
Figure 5: (a) Schematic of continuum shell element and its usage in can modeling. (b) Comparison of load-displacement curves obtained using continuum (CS3D)
and conventional axi-symmetric continuum elements (CAX).
account by having multiple layers of elements through the thickness. However, this leads to
extremely large computation time especially for three-dimensional problems. Shell elements may
be considered as the compromise between the continuum and membrane elements. It is possible to
take into account the effect of bending with much less computation time than continuum analysis
although integration in the thickness section is still needed (Wang & Cao, 2000). Therefore, using
shell elements in an explicit code is an attractive proposition for studying wrinkling.
Abaqus provides a new type of shell element which it calls the “continuum shell element” that can
be used to discretize an entire three-dimensional body as shown in Figure 5 (a). The thickness is
determined from the element nodal geometry. Continuum shell elements have only displacement
degrees of freedom. From a modeling point of view continuum shell elements look like three-
dimensional continuum solid elements, but their kinematic and constitutive behavior is similar to
conventional shell elements. User has to take care of properly defining the thickness direction
when using continuum shell elements.
SIMULIA India Regional Users Meeting „11 Page 7 of 16
In our study we used continuum shell elements to model the axial compression of beverage cans.
Since this is a fairly new element, we bench marked this element against the conventional axi-
symmetric continuum element. It should be noted that with the continuum axi-symmetric element
the beverage can was modeled in 2D while with the continuum shell element and the conventional
shell element it was modeled in 3D. It can be seen from Figure 5 (b) that with continuum axi-
symmetric element and continuum shell element model we got a peak load (critical load for base
squat) of approximately 350 lbf (1.58 kN). Further we also modeled the axial compression of the
beverage can with conventional shell elements in 3D. The load-displacement response from the
model showed that the critical load predicted by the conventional shell element is close to 400 lbf
(1.75 kN) It should be noted that the critical load predicted by the model (350 - 400 lbf) is quite
higher than the measured value of approximately 270 lbf presented in Figure 1 (b). It should be
noted that the geometry used for all the three element types is exactly the same. In order to speed
up the simulation we used the time scaling technique by trying different loading rates of 0.25 in/s,
25 in/s and 250 in/s. Time scaling can be appropriate for this case as the material model is not rate
dependent. We found that we got the same predicted response for loading rates of 0.25 and 25 in/s
while with 250 in/s loading rate we obtained a wave propagation response due to increased inertial
forces. Therefore we chose a loading rate of 25 in/s which are approximately 6000 times higher
than the loading rate used in the laboratory test. This loading rate enabled us to speed-up the
calculations while giving a satisfactory response. To further reduce the computational time we
have used one half of the beverage can making use of symmetry.
2.2.1 Circumferential thickness variation
In this section the model prediction for the onset of buckling for different circumferential
thickness variations is presented. The circumferential thickness variation in the model was
achieved by shifting the internal surface of the can in relation to the outer surface as shown in
Figure 6 (a). As a result there is a uniform variation in the thickness around the circumference of
the can where the maximum percentage thickness variation is calculated using the relation in
Figure 6 (a).
When such a can with varying circumferential thickness is compressed in the axial direction under
displacement control, the response is as shown in Figure 6 (b) and Figure 7. It can be
SIMULIA India Regional Users Meeting „11 Page 8 of 16
(a)
(b)
Figure 6: (a) A schematic showing the calculation of the circumferential thickness variation and (b) the predicted load-displacement curves for various thickness
variations.
seen from these two figures that atleast a 50% variation in the circumferential thickness is needed
for the can sidewall to buckle before base squat. The measured circumferential thickness variation
Figure 7: Predicted response of the can under compression for various circumferential thicknesses.
SIMULIA India Regional Users Meeting „11 Page 9 of 16
had a maximum value of 10% which is clearly quite low compared to what is required as per the
model predictions. It should be noted that the stress-strain curve used for the entire model is that
for the work hardened side wall obtained from a tension test on a specimen cut from the side wall.
But a non work hardened material model for the can bottom would have increased the propensity
for the can to fail from base squat. Therefore it can be safely concluded that the circumferential
thickness variation was not the reason for the 2 cans that failed by side wall buckling during the
physical testing.
2.2.2 Trim edge flatness variation
In this section we present the modeling predictions for the onset of buckling under axial
compression of beverage cans with slant trim edges. In Figure 8 (a), the schematic of the axial
load test of a can with a slant trim edge is shown. The slope, m of the trim edge is written in terms
of the thin wall thickness, t and the diameter of the can, D as m=nt/D where n=1, 2, 3, …. The
load-displacement response from the model for such a can with a slant trim edge undergoing axial
compression is presented in Figure 8 (b). It can be seen from this graph and Figure 9 that for trim
(a)
(b)
Figure 8: (a) Schematic showing the can with a slant trim edge and the set up for the axial load test. (b) The predicted load-displacement curves for various slopes
of the trim edge.
SIMULIA India Regional Users Meeting „11 Page 10 of 16
Figure 9: Predicted response of the can under compression for various slopes of the trim edge.
edge slopes of, m>3t/D, the can begins to buckle at the side wall. 3t is approximately 300 microns
which is quite small compared to the typical diameter of a can of 65 mm which means that if the
slope of the trim edge is greater than 1 in 250, an ideal beverage can will buckle at the side wall
under axial compression. This imperfection seems to be a more likely cause (compared to the
circumferential thickness variation) for the 2 cans that failed due to side wall buckling during the
physical testing.
However, it should be noted that in our study we only modeled two of the several possible causes
for such buckling failures. Other notable causes include material anisotropy, surface roughness,
initial off flatness of the sheet etc. which were not selected for our modeling exercise.
SIMULIA India Regional Users Meeting „11 Page 11 of 16
2.3 Laboratory testing of axial compression of dented cans
Axial compression of dented cans was conducted at Novelis Global Technology Centre. For
creating the dent on the beverage can sidewall, a spherical tipped pin with a radius of 1 mm was
used. Dents were created at 1, 2, 3 and 4 inch from the shoulder as shown in Figure 10 (a). No die
(a)
(b)
(c)
Figure 10: (a) Cross section profile of the can as measured using …. along with the spherical tipped pin to create the dent on the side wall. Buckled cans with a side wall dent at 1 inch (b) and 4 inch (c) from the shoulder after the axial load test.
was used from the inside of the can to locate the denting pin and create a perfect spherical dent,
instead from the inside a manual support for creating the dents was adopted. This means that the
created dents were not perfect but we can still use this imposed imperfection to get a trend for the
buckling behavior. In Figure 10 (b), we can see that the buckling initiates at the dent when the dent
location is 1 inch from the shoulder. The video recording of the test also confirm that buckling
initiated at the dent locations for dents at 1, 2 and 3 inches from the shoulder. However, when the
dent was at 4 inches from the shoulder, the buckling initiated at the middle of the can, away from
the dent as shown in Figure 10 (c).
SIMULIA India Regional Users Meeting „11 Page 12 of 16
Figure 11: The measured load-displacement curves for various locations of the dent.
In Figure 11, the measured load-displacement curve for various locations of the dent is shown.
The graph clearly shows that with a dent on the sidewall (imposed imperfection), the can fails
from buckling (drop in the slope of the load-displacement curve) well before the critical load for
base squat is reached indicated by the peak in the load-displacement curve for the ideal can (
“Normal Can”) in the above graph.
2.4 Finite element modeling of the axial load test of dented cans
Load-displacement response from LS Dyna and Abaqus model of the axial compression test of
dented cans will be presented in this section.
In Figure 12 we present the load-displacement response predicted by LS-Dyna for dented cans
with dents at 1, 2 and 3 inches from the shoulder of the can bottom. For comparison the load-
displacement curve predicted for an ideal can (NoDent) is also presented in the same graph. It can
be seen from Figure 12 that LS-Dyna predicts sidewall buckling for dents at 1 in and 2 in from the
shoulder which is qualitatively in agreement with experimental data presented in Figure 11.
SIMULIA India Regional Users Meeting „11 Page 13 of 16
Figure 12: The predicted load-displacement curves from LS-Dyna with dents as 1 in (Pos1), 2 in (Pos2), 3 in (Pos3) from the shoulder of the can bottom
However, when the dent is at 3 in from the shoulder of the can bottom, LS-Dyna does not predict
sidewall buckling. Further the critical load for base squat (peak load in the load-displacement
curve for the ideal can) predicted by LS-Dyna is approximately 0.98 kN which is quite lower than
the experimental value of approximately of 1.2 kN as shown in Figure 11. We have developed a
(a)
(b)
Figure 13: The predicted load-displacement curves from Abaqus for (a) ideal can and (b) for a dented can with the dent at 1 in from the shoulder of the beverage
can.
SIMULIA India Regional Users Meeting „11 Page 14 of 16
python code to translate the LS-Dyna input deck to an Abaqus input file. In Figure 13 (a), the
load-displacement curve using Abaqus for an ideal beverage can under axial compression is
presented. This load-displacement curve compares favorably with the results from LS-Dyna.
(a)
(b)
Figure 14: (a) Finite element model showing the forming of the dent on the sidewall (b) the predicted load-displacement curves for an ideal and dented can.
However, Abaqus did not predict sidewall buckling (no drop in the slope of the load-displacement
curve) for dented beverage can for dents located at any position along the axial direction. It should
be noted that identical mesh, material property, boundary conditions, loading cases and default
solver settings for both Abaqus and LS-Dyna model were used in our studies.
In order to understand the reason for Abaqus not predicting sidewall buckling, we developed a half
symmetry 3D model of the beverage can used in the laboratory tests. The geometry of the
beverage can used in the laboratory test was measured using a CMM co-ordinate measuring
machine. The 3D model was built using continuum shell elements and in order to make the model
more accurate we modeled the denting of the sidewall (see Figure 14 (a)). After extracting the
geometry of the dented can from this model it was then used in the axial compression model. The
predicted load-displacement curve from the axial compression model is shown in Figure 14 (b). It
can be seen in Figure 14 (b) that Abaqus does not predict sidewall buckling even when there is a
dent in the sidewall. The load-displacement curve is infact identical to the corresponding curve
predicted for an idea beverage can. Analyzing the dent in the model we noticed that the perimeter
of the dent
SIMULIA India Regional Users Meeting „11 Page 15 of 16
(a)
(b)
Figure 15: (a) Finite element model incorporating a sharp perimeter for the side wall dent and (b) the predicted load-displacement curves for an ideal and dented
can.
was “smooth”, therefore we decided to change this to a “sharp” perimeter as shown in Figure 15
(a). With this type of dent, when we axially compressed the beverage can, Abaqus predicted side
wall buckling as seen by the drop in the slope of the load-displacement curve presented in Figure
15 (b). This indicates that the predictions from Abaqus are not mesh independent. Since we did
not conduct such tests with LS-Dyna, we do not have anything to comment on LS-Dyna‟
performance in this respect.
3. Conclusion
In this article, we have shown that for ideal beverage cans, a thickness variation of at least 50 %
(w. r. t. thinwall ) is needed for the can to fail from sidewall buckling which is much more than the
measured wall thickness variation in the circumferential direction. However, a slope of slightly in
excess of 1 in 250 for the tilt in the trim edge is sufficient for the can to fail from buckling. We
believe that the latter is a more likely cause for the two out of eleven ideal cans that failed due to
sidewall buckling in our laboratory tests.
SIMULIA India Regional Users Meeting „11 Page 16 of 16
Abaqus did not predict sidewall buckling during axial compression of dented cans while LS-Dyna
predicted buckling in at least a few cases. The load-displacement curve predicted by Abaqus when
tried with different element types yielded different results even when everything else remained
constant. Abaqus consistently predicted higher critical load compared to LS-Dyna for the same
mesh, material property, boundary conditions and loading cases. We are not sure if this is because
of any difference in the default solver settings that we used for the two FE packages.
4. References
1. Mao, K. & Santamaria, A. (2009). Aluminum bottle forming simulation with Abaqus. 2009
Simulia Customer conference.
2. Wang, X., Cao, J. (2000). On the prediction of side wall wrinkling in sheet metal forming