ME 3501L: Column Buckling Experiment — Transcript

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1. 0:01>> Welcome to ME 3501's buckling lab.  In this lab what we are going to be
2. 0:07doing is we are going to be taking some  steel welding rod. This is welding rod.
3. 0:13The main reason we're using this is because it's a  nice small cross-sectional diameter so that we can
4. 0:20get large slenderness ratios without getting  extraordinarily large specimens. This is a fairly
5. 0:27thin piece of steel over here. What you're going  to be doing in this experiment is you're going
6. 0:31to be using some clippers over here to snip  off lengths of the welding rod, which you're
7. 0:38going to measure the lengths with your tape  measure over here. I'm going to measure about
8. 0:44four inches over here. Then I'm going to  clip off the end of that with the clipper.
9. 0:56Then I'm going to clean off the ends of that  specimen. When you clip it with the clippers,
10. 1:04you get a very sharp wedge-shaped at the end  of it, which is not very good for the test
11. 1:09that we're doing. Just using a file, you clean off  the edges and ends of the specimen so that they're
12. 1:16more rounded. Then I'm not going to continue to do  that all the way with this one. But in real life
13. 1:22you would clean it until the wedge is completely  gone and it's nice and round. This is our testing
14. 1:28machine that we are going to be using for this  experiment. It's just like the test machine that
15. 1:33we used for the tension test, except it's a much  smaller capacity machine. The tension test machine
16. 1:39that we use could go up to 10,000 pounds. This  one can only go up to 2,000 pounds. But that also
17. 1:44means that it's more sensitive to small loads,  which is the loss that we are going to have in
18. 1:48the buckling experiment. We have two kinds of end  caps that we're using in the buckling experiment.
19. 1:58For the experiments where we have a pinned end  as just a little dimples on the end cap and
20. 2:05it's hemispherical so your specimen can rock in  the end cap in whichever direction it wants to,
21. 2:11which is basically what you have with  a pinned end. For the fixed end point,
22. 2:16what we have is in the end cap we have drilled a  little hole. Your specimen is going to actually
23. 2:22drop into the whole, if you notice over  here, it's going to drop into that hole.
24. 2:32It's a pretty snug fit. That's a fair  approximation of a fixed end. Now it
25. 2:37is not exactly a fixed end because it  still has a little bit of slack in it.
26. 2:42You can see that from the tape. But if you were  feeling the specimen, you'd feel a little bit of
27. 2:48wiggle in there. A truly fixed end, it's rigidly  clamped into the fixture and it can't move at all.
28. 2:56This is not truly fixed. That's something  that you might want to take into account
29. 3:01when you are doing your analysis in terms of  the results that you get for specimens that
30. 3:08have at least one fixed end or possibly two fixed  ends. We clamp these end caps into the machine.
31. 3:19Let's say for example, that we are doing  a pinned test. I'm going to take the two
32. 3:25end caps that have the little dimples  in them, the top end cap I'm going to
33. 3:31put in the top over here and then pin it in  place. That's suspended now. The bottom end cap,
34. 3:40there's a little spring in here. I'm just going to  drop the bottom end cap over here. It's floating.
35. 3:49The reason we do it that way is that if I were  to push this all the way down and then pin it
36. 3:55in place, then you have rigid loading  conditions at the top and the bottom.
37. 4:00Then as you lower the cross-head to load  the specimen up, the instant you touch you
38. 4:06would exceed the buckling load. It's very  different because it would be very stiff.
39. 4:10It would be very tricky to get it in position  for your test without ruining your specimen. A
40. 4:18specimen is too long, let's see if this  one will fit. Let's jog it a little bit.
41. 4:48You see I've got the specimen over here.  It's got about one pound of load on it and
42. 4:54it's still floating. That allows me to put it  in position without crushing the specimen. Then
43. 5:05once it's in position, then I can actually  start my test. That's the purpose of that little
44. 5:15spring on the bottom of it. I'm going to first  of all calibrate my load cell with the grips in
45. 5:23it already. Then having cut all of my specimens  to the appropriate length and smooth the edges,
46. 5:30we just put the right n condition on, bring the  jaws down until it just barely makes contact
47. 5:37less than a pound of force between the  specimen and the jaws of the machine or the
48. 5:45trucks on the machine. Then you run the test and  the test will plot displacement versus force.
49. 5:52What you're looking for is the peak of the force  curve so the displacement doesn't even really
50. 5:58matter all that much. All you're looking for is  the peak of the force because that's the point
51. 6:03at which the specimen undergoes buckling. What I'm  going to do is first I'm going to come over here.
52. 6:10I'm going to zero out my stroke and  calibrate my load. If I right-click on the
53. 6:19force window over here and push calibration,  they will do an electronic calibration now.
54. 6:41Now it's reading 0.004 pounds, which is a tiny  miniscule power load. Then the only thing that
55. 6:51you have to watch out for when you're cutting the  specimens to the sizes that you want is for the
56. 6:59pinned ends, the specimen is pretty much exactly  the length that you want it to be because it's
57. 7:04is just like ten thousandths of an inch deep  or so. It doesn't really change the length of
58. 7:09your specimen. But for the fixed end, your  specimen actually drops down into the hole
59. 7:16a half an inch. If you wanted a  nut specimen length of four inches
60. 7:24and one side of your specimen is fixed, you'd have  to cut the specimen to four-and-a-half inches.
61. 7:29If you wanted a foreign specimen and it's  fixed on both sides then you have to cut your
62. 7:34specimen to five inches to compensate for the  portion of the specimen that's inside the grip.
63. 7:41Let's go ahead and do one experiment  here. I'm going to mount my specimen here.
64. 7:58Then I'm going to run the test. On the graph what  you will notice is you will notice the load going
65. 8:05up and then it'll reach a peak, and then  it will drop down again. This is actually
66. 8:14pinned panes, so it probably won't be very much  load. Let's put the maximum of the y-axis at 50
67. 8:21and then run the test. It says check your  limit switches, limits switches are good.
68. 8:28Limit switches are these switches over here  and above, which basically are there to prevent
69. 8:36you from destroying the tests machinery. My  limits, which is an emergency switch, are good.
70. 8:42I start the test. What you'll notice  is there's the load. Load is going up
71. 8:521.6 pounds, three pounds, 10 pounds, 12  pounds and I just reached the maximum.
72. 9:01You notice, it just went up to about maybe  12.513 pounds or so, reached a maximum and now
73. 9:06it's coming down and you can see the buckled shape  on the specimen. I'm going to stop the test now.
74. 9:17You can see the classic sine wave buckled  shape to the pin-pin specimen. I'm going to
75. 9:24jog the cross-head up, and remove that specimen.  Then for this one, and I'm going to do fixed
76. 9:40pinned. On the lower end we're  going to have a fixed end condition.
77. 9:47On the upper end, we'll have a pinned end  condition. Let's drop this central position.
78. 10:02Then let's roll it down.
79. 10:08When I get close to my specimen, I'm going to  change from the high-speed jog to the low-speed
80. 10:13jog. You can move this cross-head first for  positioning purposes. This is high speed jog.
81. 10:21When you want to make gentle adjustments, you push  this button and then it moves a whole lot slower.
82. 10:51There we are. It's in  position now. Now I'm going to
83. 10:58save the information for this  test, save raw data to CSV.
84. 11:06I'll call it
85. 11:18pinned-pinned three inches. Then now I'm going  to run a new test. I'm going to close this one.
86. 11:35We're ready to roll on our next specimen.  Now this one has a fixed lower boundary
87. 11:40condition and an upper boundary  condition. Let's start the test
88. 11:50and see what happens.
89. 11:58This gentle rise over here is because what's  going on is the spring is getting compressed.
90. 12:04When the spring bottoms out, that's when the  load goes up really rapidly. You see it went up
91. 12:10to about 17, 18 pounds or so and then buckled.  I'll stop the test. You will notice that the
92. 12:21shape of the buckle is different than it was.  In the pinned-pinned case, it was symmetric, it
93. 12:27was a half sine wave. In this case, at the fixed  end, it's coming straight up out of the bottom.
94. 12:33At the pendant, it's got an angle to it, so the  shape is different. Let's unload the specimen
95. 12:43and extract it. Then you can see that  the maximally bent buckled portion is not
96. 12:52symmetric. That will give you the data  for a number of different end conditions
97. 12:59and lengths of specimen and go for it. Be  aware that in this particular experiment,
98. 13:06it is not unusual to have very large errors. Think  about why it is that you might be getting large
99. 13:14errors. Think about which kinds of specimens  give you bigger errors and smaller errors
100. 13:21and consider that in your analysis. But  don't be shocked if you have errors of 50
101. 13:27percent in your experiments. Some of them are  wildly off. Good luck with the experiment.