BEAM32R - Strange internal energy values

1

Hi everyone
I am testing a beam structure under compressione and i am finding some issues in the internal energy extraction. I am performing a NL analysis and the internal energy measured for the elements is in a quite strange format. Something like this:
1 3.833492-313
2 3.699148-313
3 1.197441-312
4 7.086744-313
The other outputs are good (displacements and stresses). I also find issues when i try to extract the volume from the model. Same type of values.
Does anyone know the possible reason for that?

The .inp is the following

*HEADING
Model: PROVABYME1.inp Date:2025-05-06
*NODE,NSET=NAll
1, 0.000000, 0.000000, 0.000000
2, 0.000000, 60.000000, 0.000000
3, 0.000000, 120.000000, 0.000000
4, 0.000000, 180.000000, 0.000000
5, 0.000000, 240.000000, 0.000000
6, 0.000000, 300.000000, 0.000000
7, 60.000000, 0.000000, 0.000000
8, 60.000000, 60.000000, 0.000000
9, 60.000000, 120.000000, 0.000000
10, 60.000000, 180.000000, 0.000000
11, 60.000000, 240.000000, 0.000000
12, 60.000000, 300.000000, 0.000000
13, 120.000000, 0.000000, 0.000000
14, 120.000000, 60.000000, 0.000000
15, 120.000000, 120.000000, 0.000000
16, 120.000000, 180.000000, 0.000000
17, 120.000000, 240.000000, 0.000000
18, 120.000000, 300.000000, 0.000000
19, 180.000000, 0.000000, 0.000000
20, 180.000000, 60.000000, 0.000000
21, 180.000000, 120.000000, 0.000000
22, 180.000000, 180.000000, 0.000000
23, 180.000000, 240.000000, 0.000000
24, 180.000000, 300.000000, 0.000000
25, 240.000000, 0.000000, 0.000000
26, 240.000000, 60.000000, 0.000000
27, 240.000000, 120.000000, 0.000000
28, 240.000000, 180.000000, 0.000000
29, 240.000000, 240.000000, 0.000000
30, 240.000000, 300.000000, 0.000000
31, 300.000000, 0.000000, 0.000000
32, 300.000000, 60.000000, 0.000000
33, 300.000000, 120.000000, 0.000000
34, 300.000000, 180.000000, 0.000000
35, 300.000000, 240.000000, 0.000000
36, 300.000000, 300.000000, 0.000000
37, 0.000000, 0.000000, 150.000000
38, 0.000000, 60.000000, 150.000000
39, 0.000000, 120.000000, 150.000000
40, 0.000000, 180.000000, 150.000000
41, 0.000000, 240.000000, 150.000000
42, 0.000000, 300.000000, 150.000000
43, 60.000000, 0.000000, 150.000000
44, 60.000000, 60.000000, 150.000000
45, 60.000000, 120.000000, 150.000000
46, 60.000000, 180.000000, 150.000000
47, 60.000000, 240.000000, 150.000000
48, 60.000000, 300.000000, 150.000000
49, 120.000000, 0.000000, 150.000000
50, 120.000000, 60.000000, 150.000000
51, 120.000000, 120.000000, 150.000000
52, 120.000000, 180.000000, 150.000000
53, 120.000000, 240.000000, 150.000000
54, 120.000000, 300.000000, 150.000000
55, 180.000000, 0.000000, 150.000000
56, 180.000000, 60.000000, 150.000000
57, 180.000000, 120.000000, 150.000000
58, 180.000000, 180.000000, 150.000000
59, 180.000000, 240.000000, 150.000000
60, 180.000000, 300.000000, 150.000000
61, 240.000000, 0.000000, 150.000000
62, 240.000000, 60.000000, 150.000000
63, 240.000000, 120.000000, 150.000000
64, 240.000000, 180.000000, 150.000000
65, 240.000000, 240.000000, 150.000000
66, 240.000000, 300.000000, 150.000000
67, 300.000000, 0.000000, 150.000000
68, 300.000000, 60.000000, 150.000000
69, 300.000000, 120.000000, 150.000000
70, 300.000000, 180.000000, 150.000000
71, 300.000000, 240.000000, 150.000000
72, 300.000000, 300.000000, 150.000000
73, 125.106601, 216.097348, 0.017156
74, 90.699772, 44.026767, 13.850789
75, 55.878063, 103.668218, 59.515121
76, 161.645020, 125.758354, 102.782925
77, 61.335675, 263.435231, 4.108139
78, 201.140253, 125.191441, 83.803474
79, 42.116082, 59.430447, 120.111685
80, 290.478473, 94.027253, 103.848392
81, 262.916746, 268.381999, 12.756632
82, 11.716435, 50.949126, 131.721376
83, 21.058041, 29.715223, 60.055843
84, 5.858217, 25.474563, 65.860688
85, 27.939032, 81.834109, 29.757561
86, 21.058041, 59.715223, 60.055843
87, 5.858217, 55.474563, 65.860688
88, 5.858217, 85.474563, 65.860688
89, 27.939032, 111.834109, 29.757561
90, 27.939032, 141.834109, 29.757561
91, 30.667837, 251.717615, 2.054069
92, 30.667837, 281.717615, 2.054069
93, 75.349886, 22.013384, 6.925395
94, 51.058041, 29.715223, 60.055843
95, 57.939032, 81.834109, 29.757561
96, 51.058041, 59.715223, 60.055843
97, 75.349886, 52.013384, 6.925395
98, 75.349886, 82.013384, 6.925395
99, 57.939032, 111.834109, 29.757561
100, 92.553301, 198.048674, 0.008578
101, 57.939032, 141.834109, 29.757561
102, 92.553301, 228.048674, 0.008578
103, 60.667837, 251.717615, 2.054069
104, 60.667837, 281.717615, 2.054069
105, 105.349886, 22.013384, 6.925395
106, 105.349886, 52.013384, 6.925395
107, 140.822510, 92.879177, 51.391463
108, 87.939032, 81.834109, 29.757561
109, 87.939032, 111.834109, 29.757561
110, 105.349886, 82.013384, 6.925395
111, 140.822510, 122.879177, 51.391463
112, 140.822510, 152.879177, 51.391463
113, 122.553301, 198.048674, 0.008578
114, 87.939032, 141.834109, 29.757561
115, 122.553301, 228.048674, 0.008578
116, 90.667837, 251.717615, 2.054069
117, 90.667837, 281.717615, 2.054069
118, 170.822510, 92.879177, 51.391463
119, 190.570127, 92.595720, 41.901737
120, 170.822510, 122.879177, 51.391463
121, 190.570127, 122.595720, 41.901737
122, 170.822510, 152.879177, 51.391463
123, 190.570127, 152.595720, 41.901737
124, 152.553301, 198.048674, 0.008578
125, 221.458373, 254.191000, 6.378316
126, 190.570127, 182.595720, 41.901737
127, 152.553301, 228.048674, 0.008578
128, 221.458373, 284.191000, 6.378316
129, 265.239236, 47.013627, 51.924196
130, 220.570127, 92.595720, 41.901737
131, 265.239236, 77.013627, 51.924196
132, 220.570127, 122.595720, 41.901737
133, 265.239236, 107.013627, 51.924196
134, 220.570127, 152.595720, 41.901737
135, 265.239236, 137.013627, 51.924196
136, 251.458373, 224.191000, 6.378316
137, 251.458373, 254.191000, 6.378316
138, 220.570127, 182.595720, 41.901737
139, 251.458373, 284.191000, 6.378316
140, 295.239236, 47.013627, 51.924196
141, 295.239236, 77.013627, 51.924196
142, 295.239236, 107.013627, 51.924196
143, 250.570127, 152.595720, 41.901737
144, 295.239236, 137.013627, 51.924196
145, 281.458373, 224.191000, 6.378316
146, 281.458373, 254.191000, 6.378316
147, 281.458373, 284.191000, 6.378316
148, 21.058041, 29.715223, 135.055843
149, 5.858217, 25.474563, 140.860688
150, 5.858217, 55.474563, 140.860688
151, 21.058041, 59.715223, 135.055843
152, 5.858217, 85.474563, 140.860688
153, 27.939032, 111.834109, 104.757561
154, 21.058041, 89.715223, 135.055843
155, 27.939032, 141.834109, 104.757561
156, 30.667837, 251.717615, 77.054069
157, 30.667837, 281.717615, 77.054069
158, 35.858217, 25.474563, 140.860688
159, 51.058041, 29.715223, 135.055843
160, 75.349886, 22.013384, 81.925395
161, 51.058041, 59.715223, 135.055843
162, 35.858217, 55.474563, 140.860688
163, 57.939032, 111.834109, 104.757561
164, 51.058041, 89.715223, 135.055843
165, 57.939032, 141.834109, 104.757561
166, 92.553301, 198.048674, 75.008578
167, 92.553301, 228.048674, 75.008578
168, 60.667837, 251.717615, 77.054069
169, 60.667837, 281.717615, 77.054069
170, 81.058041, 29.715223, 135.055843
171, 105.349886, 22.013384, 81.925395
172, 81.058041, 59.715223, 135.055843
173, 105.349886, 52.013384, 81.925395
174, 140.822510, 92.879177, 126.391463
175, 87.939032, 81.834109, 104.757561
176, 87.939032, 111.834109, 104.757561
177, 81.058041, 89.715223, 135.055843
178, 140.822510, 122.879177, 126.391463
179, 140.822510, 152.879177, 126.391463
180, 122.553301, 198.048674, 75.008578
181, 87.939032, 141.834109, 104.757561
182, 122.553301, 228.048674, 75.008578
183, 90.667837, 251.717615, 77.054069
184, 90.667837, 281.717615, 77.054069
185, 170.822510, 92.879177, 126.391463
186, 190.570127, 92.595720, 116.901737
187, 170.822510, 122.879177, 126.391463
188, 190.570127, 122.595720, 116.901737
189, 170.822510, 152.879177, 126.391463
190, 190.570127, 152.595720, 116.901737
191, 152.553301, 198.048674, 75.008578
192, 152.553301, 228.048674, 75.008578
193, 221.458373, 254.191000, 81.378316
194, 190.570127, 182.595720, 116.901737
195, 221.458373, 284.191000, 81.378316
196, 265.239236, 47.013627, 126.924196
197, 220.570127, 92.595720, 116.901737
198, 265.239236, 77.013627, 126.924196
199, 220.570127, 122.595720, 116.901737
200, 265.239236, 107.013627, 126.924196
201, 220.570127, 152.595720, 116.901737
202, 265.239236, 137.013627, 126.924196
203, 251.458373, 224.191000, 81.378316
204, 251.458373, 254.191000, 81.378316
205, 220.570127, 182.595720, 116.901737
206, 251.458373, 284.191000, 81.378316
207, 295.239236, 47.013627, 126.924196
208, 295.239236, 77.013627, 126.924196
209, 295.239236, 107.013627, 126.924196
210, 281.458373, 224.191000, 81.378316
211, 250.570127, 152.595720, 116.901737
212, 295.239236, 137.013627, 126.924196
213, 281.458373, 254.191000, 81.378316
214, 281.458373, 284.191000, 81.378316
215, 143.375811, 170.927851, 51.400041
216, 73.288918, 73.847493, 36.682955
217, 66.407927, 51.728607, 66.981237
218, 126.172396, 84.892561, 58.316857
219, 108.761542, 114.713286, 81.149023
220, 33.797249, 77.308672, 95.618248
221, 48.997072, 81.549332, 89.813403
222, 181.392637, 125.474898, 93.293200
223, 245.809363, 109.609347, 93.825933
224, 26.916258, 55.189786, 125.916530
*ELEMENT,TYPE=B32R,ELSET=EAll
1, 1, 83, 79
2, 1, 84, 82
3, 2, 85, 75
4, 2, 86, 79
5, 2, 87, 82
6, 3, 88, 82
7, 3, 89, 75
8, 4, 90, 75
9, 5, 91, 77
10, 6, 92, 77
11, 7, 93, 74
12, 7, 94, 79
13, 8, 95, 75
14, 8, 96, 79
15, 8, 97, 74
16, 9, 98, 74
17, 9, 99, 75
18, 10, 100, 73
19, 10, 101, 75
20, 11, 102, 73
21, 11, 103, 77
22, 12, 104, 77
23, 13, 105, 74
24, 14, 106, 74
25, 14, 107, 76
26, 14, 108, 75
27, 15, 109, 75
28, 15, 110, 74
29, 15, 111, 76
30, 16, 112, 76
31, 16, 113, 73
32, 16, 114, 75
33, 17, 115, 73
34, 17, 116, 77
35, 18, 117, 77
36, 20, 118, 76
37, 20, 119, 78
38, 21, 120, 76
39, 21, 121, 78
40, 22, 122, 76
41, 22, 123, 78
42, 22, 124, 73
43, 23, 125, 81
44, 23, 126, 78
45, 23, 127, 73
46, 24, 128, 81
47, 25, 129, 80
48, 26, 130, 78
49, 26, 131, 80
50, 27, 132, 78
51, 27, 133, 80
52, 28, 134, 78
53, 28, 135, 80
54, 28, 136, 81
55, 29, 137, 81
56, 29, 138, 78
57, 30, 139, 81
58, 31, 140, 80
59, 32, 141, 80
60, 33, 142, 80
61, 34, 143, 78
62, 34, 144, 80
63, 34, 145, 81
64, 35, 146, 81
65, 36, 147, 81
66, 37, 148, 79
67, 37, 149, 82
68, 38, 150, 82
69, 38, 151, 79
70, 39, 152, 82
71, 39, 153, 75
72, 39, 154, 79
73, 40, 155, 75
74, 41, 156, 77
75, 42, 157, 77
76, 43, 158, 82
77, 43, 159, 79
78, 43, 160, 74
79, 44, 161, 79
80, 44, 162, 82
81, 45, 163, 75
82, 45, 164, 79
83, 46, 165, 75
84, 46, 166, 73
85, 47, 167, 73
86, 47, 168, 77
87, 48, 169, 77
88, 49, 170, 79
89, 49, 171, 74
90, 50, 172, 79
91, 50, 173, 74
92, 50, 174, 76
93, 50, 175, 75
94, 51, 176, 75
95, 51, 177, 79
96, 51, 178, 76
97, 52, 179, 76
98, 52, 180, 73
99, 52, 181, 75
100, 53, 182, 73
101, 53, 183, 77
102, 54, 184, 77
103, 56, 185, 76
104, 56, 186, 78
105, 57, 187, 76
106, 57, 188, 78
107, 58, 189, 76
108, 58, 190, 78
109, 58, 191, 73
110, 59, 192, 73
111, 59, 193, 81
112, 59, 194, 78
113, 60, 195, 81
114, 61, 196, 80
115, 62, 197, 78
116, 62, 198, 80
117, 63, 199, 78
118, 63, 200, 80
119, 64, 201, 78
120, 64, 202, 80
121, 64, 203, 81
122, 65, 204, 81
123, 65, 205, 78
124, 66, 206, 81
125, 67, 207, 80
126, 68, 208, 80
127, 69, 209, 80
128, 70, 210, 81
129, 70, 211, 78
130, 70, 212, 80
131, 71, 213, 81
132, 72, 214, 81
133, 73, 215, 76
134, 74, 216, 75
135, 74, 217, 79
136, 74, 218, 76
137, 75, 219, 76
138, 75, 220, 82
139, 75, 221, 79
140, 76, 222, 78
141, 78, 223, 80
142, 79, 224, 82
*NSET,NSET=CNODES,GENERATE
1,36
*NSET,NSET=LNODES,GENERATE
37,72
*MATERIAL, NAME=Ti6Al4V
*ELASTIC
110000.,.34
*PLASTIC
880.,0.
950.,0.0035
980.,0.0118
1020.,0.0245
1040.,0.0440
1000.,0.0673
*DENSITY
4.43E-09
*BEAM SECTION,ELSET=EAll,MATERIAL=Ti6Al4V,SECTION=CIRC
5.,5.
1.,0.,0.
*BOUNDARY
CNODES,1,3
LNODES,1,2
*STEP,NLGEOM
*STATIC,DIRECT
.04,1.
*CLOAD
LNODES,3,-10000.
*NODE PRINT,NSET=NAll
U
*EL PRINT,ELSET=EAll,TOTALS=YES
ELSE,EVOL
*NODE FILE
U
*EL FILE
ELSE
*END STEP

2

Can you try to expand the model to 3D and check? Many times these 1D elements give erroneous values due to the internal expansion. It may be a bug that needs full reporting.

4

Since i know and I can see from the NL results the elements are automatically 3D espanded for probably avoiding shear locking issues. For the volume I can actually avoid the problem by just calculating the distances between the nodes of the beams and then use density and section but for the energy i don’t know.
I encountered this problem only because i was trying to set up an automatic convergence process for the results where the criteria were 2: a Mechanical one (using displacements) and ah energetical one (using int energy)

5

Understood. I will have to check the source code to make sure this a bug. But if you can test an already expanded 3D model, then we know that is the case.

6

@cellini I tried your input deck and I’m getting regular values. What version of ccx are you running? Which solver? Compiler?
I’m running latest 2.22 + Intel One API 2025 with pardiso turned on from MKL. Values are 3.18E+2 on the first element at last converged increment 0.6… solution is actually diverging.

7

With ccx 2.22 using PaStiX on Windows, I observe non-convergence of this analysis (the values appear normal). With Abaqus, it converges, but I have to change the element type to B32 (Abaqus doesn’t have B32R elements).

8

On my calculix command says: CalculiX for Windows version: 2.10 build 002 (64 bit) - so i suppose it should be the 2.10 (?)
The fact that the solution is diverging is actually what i expected since this is a reticular structure under strong compression. Run kill indirectly tells me that buckling is happening and thanks to the step and the load set i should be able to find the buckling stress.
However if you check the values obtained from the analysis stack in the .dat file you should be able to find for a certain step (let’s say the third one) the internal energy of the structure. What i meant when i wrote the first message was about those values. In those values i found the issue.

9

This version is ancient. Considering the amount of bugs that have been fixed since then, you definitely should switch to 2.22. You can get it from here: https://www.dhondt.de/

1 Like
10

I will try and i’ll let you know if the issue is still persisting

11

I would try with just one member under compresion first to see if your aproach and the element is buckling as expected. Buckling load can be compared with hand calculation. (No plastic card yet).

1 Like
12

I think there is a bug on CIRC element. Frequency analisys shows that constraining Rz (longuitudinal axis) also constrains Rx and RY. One can compare bottom and top response of the beam.

** Generated by Mecway 28
*NODE
1,0,0,0
2,0,0,1
3,0,0,0.5
4,0,0,0.1666666666667
5,0,0,0.3333333333333
6,0,0,0.6666666666667
7,0,0,0.8333333333333
*ELEMENT,TYPE=B32R
1,6,7,2
2,1,4,5
3,5,3,6
*NSET,NSET=NALL
1
2
*NSET,NSET=CNODES
1
2
*ELSET,ELSET=EALL
1
2
3
*BOUNDARY
1,1,,0
1,2,,0
1,3,,0
1,6,,0
2,1,,0
2,2,,0
*MATERIAL, NAME=Ti6Al4V
*ELASTIC
110000.0E6,.0
*DENSITY
44300
*BEAM SECTION,ELSET=EAll,MATERIAL=Ti6Al4V,SECTION=CIRC
5.0E-3,5.0E-3
1.,0.,0.
*STEP
*FREQUENCY
5
*NODE FILE,GLOBAL=YES
U
*END STEP

@cellini

Your buckling load will not be accurate if you only use one element per beam.

13

I ran the same input as @cellini provided with 2.22 and pardiso, values seemed to be reasonable. I would say that we need more tests before diagnosing the circ cross-section.

14

Sorry about the terminology. It can be keept as unexpected behaviour until the source of discrepancy is more clear.
It is better seen if we only constrain Rz in one node .

*NODE
1,0,0,0
2,0,0,1
3,0,0,0.5
4,0,0,0.1666666666667
5,0,0,0.3333333333333
6,0,0,0.6666666666667
7,0,0,0.8333333333333
*ELEMENT,TYPE=B32R
1,6,7,2
2,1,4,5
3,5,3,6
*NSET,NSET=NALL
1
2
*NSET,NSET=CNODES
1
2
*ELSET,ELSET=EALL
1
2
3
*BOUNDARY
1,6,,0
*MATERIAL, NAME=Ti6Al4V
*ELASTIC
110000.0E6,.0
**PLASTIC
**880.0E6,0.
**950.0E6,0.0035
**980.0E6,0.0118
**1020.0E6,0.0245
**1040.0E6,0.0440
**1000.0E6,0.0673
*DENSITY
4300
*BEAM SECTION,ELSET=EAll,MATERIAL=Ti6Al4V,SECTION=CIRC
5.0E-3,5.0E-3
1.,0.,0.
*STEP,PERTURBATION
*FREQUENCY
6
*NODE FILE,GLOBAL=YES,OUTPUT=2D
U
*EL FILE
S,NOE,ENER
*END STEP

There should be 5 rigid body motions and there are only three.

imagen
imagen766×613 31.8 KB

It can also be seen in the buckling load.

Expected is 33N and result is 81N.

E 110000 N/mm2
L 1000 mm
Leff 1000 mm
Circular tube
Radious A I
2.5 19.63 30.680 mm4 Euler Buckling Load 33 N

imagen
imagen567×383 23.6 KB

*NODE
1,0,0,0
2,0,0,1
3,0,0,0.5
4,0,0,0.1666666666667
5,0,0,0.3333333333333
6,0,0,0.6666666666667
7,0,0,0.8333333333333
*ELEMENT,TYPE=B32R
1,6,7,2
2,1,4,5
3,5,3,6
*NSET,NSET=NALL
1
2
*NSET,NSET=CNODES
1
2
*ELSET,ELSET=EALL
1
2
3
*BOUNDARY
1,1,,0
1,2,,0
1,3,,0
1,6,,0
2,1,,0
2,2,,0
*MATERIAL, NAME=Ti6Al4V
*ELASTIC
110000.0E6,.0
*DENSITY
4300
*BEAM SECTION,ELSET=EAll,MATERIAL=Ti6Al4V,SECTION=CIRC
5.0E-3,5.0E-3
1.,0.,0.
*STEP
*BUCKLE,SOLVER=PARDISO
6
*CLOAD
2,3,-1
*NODE FILE,GLOBAL=YES,OUTPUT=2D
U
*END STEP
15

In my project i actually increase the number of elements when convergence for displacements is not reached. I start from a 1 beam - 1 element mesh and then i increase the number of elements for the same beam. After each analysis i take the disp results for a specific step (at about 75% of the total step number) and i confront them with the previous analysis. First analysis with less elements produce a larger number of steps, so quite often the difference is quite important, but it’s good because i want to find the minimum number of elements which does not change the number of steps. After 4-5-6 elements it usually reach the asinthotic number of steps and the convergence kick in well.
Last analysis saved is the one with numerical convergence and last step i think indirectly tells when buckling is occuring (since it is a reticular structure).

A made a small video to explain better the process at this wetranfer link : Unique Download Link | WeTransfer
If something looks like it can lead to an erroneous results please tell me because i am quite a newbie in calculix

About the RZ blocking it actually seems very strange type of behaviour.

16

Use PIPE instead of CIRC.

Convergence failure could have other reason than a Buckling failure. A DIRECT timestep itself has some limitations. Plastic collapse could be another failure mode.

17

Thank you very much
I am trying to perform a simple NL analysis of a beam. Here the source .inp code
*HEADING
Model: TESTCASE.inp Date:2025-05-06
*NODE,NSET=NAll
1, 0.000000, 0.000000, 0.000000
2, 0.000000, 0.000000, 150.000000
3, 0.000000, 0.000000, 300.000000
4, 0.000000, 0.000000, 450.000000
5, 0.000000, 0.000000, 600.000000
6, 0.000000, 0.000000, 750.000000
7, 0.000000, 0.000000, 900.000000
8, 0.000000, 0.000000, 1050.000000
9, 0.000000, 0.000000, 1200.000000
10, 0.000000, 0.000000, 1350.000000
11, 0.000000, 0.000000, 1500.000000
*ELEMENT,TYPE=B32R,ELSET=EAll
1, 1, 2, 3
2, 3, 4, 5
3, 5, 6, 7
4, 7, 8, 9
5, 9, 10, 11
*NSET,NSET=CNODES
1
*NSET,NSET=LNODES
11
*MATERIAL, NAME=Ti6Al4V
*ELASTIC
110000.,.34
*PLASTIC
880.,0.
950.,0.0035
980.,0.0118
1020.,0.0245
1040.,0.0440
1000.,0.0673
*DENSITY
4.43E-09
*BEAM SECTION,ELSET=EAll,MATERIAL=Ti6Al4V,SECTION=CIRC
5.,5.
1.,0.,0.
*BOUNDARY
CNODES,1,6
LNODES,1,2
*STEP,NLGEOM
*STATIC,DIRECT
.04,1.
*CLOAD
LNODES,3,-10000.
*NODE PRINT,NSET=NAll
U
*EL PRINT,ELSET=EAll,TOTALS=YES
ELSE,EVOL
*NODE FILE
U
*EL FILE
ELSE
*END STEP

From what i can see in the results (animation) there is no divergence and it looks strange since buckling is expected. Can it be due to imperfections (that aren’t present) but are essential to trigger the buckling?

I ask that because i don’t know at this point if this approach can be correct for proper results

18

For a straight beam most probably yes.
Inspect the full load path. Buckling might pass unnoticed if you just look at the final picture.
Mine doesn’t have imperfections but NLBA can correctly anounce where the buckling load is.

screenshot.19
screenshot.191615×866 26 KB

imagen
imagen523×672 11.1 KB

¿Are you able to compute the expected buckling load on a beam with plasticity?

19

I am trying on this model

*HEADING
Model: TESTCASE.inp Date:2025-05-06
*NODE,NSET=NAll
1, 0.000000, 0.000000, 0.000000
2, 0.000000, 0.000000, 20.000000
3, 0.000000, 0.000000, 40.000000
4, 0.000000, 0.000000, 60.000000
5, 0.000000, 0.000000, 80.000000
6, 0.000000, 0.000000, 100.000000
7, 0.000000, 0.000000, 120.000000
8, 0.000000, 0.000000, 140.000000
9, 0.000000, 0.000000, 160.000000
10, 0.000000, 0.000000, 180.000000
11, 0.000000, 0.000000, 200.000000
12, 0.000000, 0.000000, 220.000000
13, 0.000000, 0.000000, 240.000000
*ELEMENT,TYPE=B32R,ELSET=EAll
1, 1, 2, 3
2, 3, 4, 5
3, 5, 6, 7
4, 7, 8, 9
5, 9, 10, 11
6, 11, 12, 13
*NSET,NSET=CNODES
1
*NSET,NSET=LNODES
13
*MATERIAL, NAME=Ti6Al4V
*ELASTIC
110000.,.34
*PLASTIC
880.,0.
950.,0.0035
980.,0.0118
1020.,0.0245
1040.,0.0440
1000.,0.0673
*DENSITY
4.43E-09
*BEAM SECTION,ELSET=EAll,MATERIAL=Ti6Al4V,SECTION=CIRC
5.,5.
1.,0.,0.
*BOUNDARY
CNODES,1,6
*STEP,NLGEOM
*STATIC,DIRECT
.01,1.
*CLOAD
LNODES,3,-20.
*NODE PRINT,NSET=NAll
U
*EL PRINT,ELSET=EAll,TOTALS=YES
S
*NODE FILE
U
*EL FILE
S
*END STEP

From Euler buck is expected at 18.8 N and i am currently trying 20 N.
Except for the first step, i always get disp in xy direction that looks like the ones i attached. The hotspot are however a result of the number of elements used since i tried to change them and i visualized differences.
Am i missing something in my .inp file ?

Immagine 2025-05-09 112517
Immagine 2025-05-09 1125171625×671 59.6 KB

20

Expected Eurler is 144.5N

imagen
imagen590×712 22.4 KB

Form that value you can expect some small deviation due to Poisson Ratio and Plasticity.

21

Thank you
I found the following results on the middle node.
I put 160 N for upper bound and at around 0.9 load factor it experiences displacement peaks.
160 * 0.9 is around the Euler value so i think this proves the validity of the approach for the circ element.

Only thing i wanted to know is why K = 2? for a fixed node i thought it would be just 1 and the effective length equal to the length of the beam.

Immagine 2025-05-09 145703
Immagine 2025-05-09 145703692×331 21.3 KB