Evaluation of Extrusion Web Buckling Strength
January 16, 2007
Victor Guarino
ANL
- Introduction
Long term stability of the assembled detector requires that the horizontal extrusions withstand a significant lateral force. This force will be transferred through the horizontal extrusions by the webs. The extrusion webs will be subjected to a compressive force and the buckling strength needs to be determined to understand the strength of the detector.
- Test Description
Figure 1 to 3 below shows the test setup and the geometry of a typical extrusion that was loaded in compression. Samples were cut from extrusions so that 3 webs were loaded. Each sample was 3.5” x 6” which is a projected surface area of 21 square inches. A compressive load was applied until sudden buckling deformation was observed. The buckling load was found by measuring the applied force and was determined as the maximum applied force. At buckling the applied force would suddenly decline as the extrusion failed.
As seen in Figures 1 to 3 failure occurred at the center of each web. It was observed in the extrusions that the thinnest part of the web occurred at the center where knitting occurring during the extrusion process. The average web thickness for each test sample was recorded.
- Test Results
The table below shows the results from the tests that were performed.
Test Number / Material / Web Thickness (mm) / Failure Force (lbs) / Pressure (psi)1 / N21 / 1.6 / 821 / 39.0
2 / N21 / 1.6 / 973 / 46.3
3 / N21 / 1.65 / 967 / 46.0
4 / N21 / 1.60 / 1004 / 47.8
5 / N21 / 1.60 / 973 / 46.3
6 / N21 / 1.35 / 730 / 34.7
7 / 1029 / 1.05 / 407 / 19.4
8 / 1029 / 1.05 / 414 / 19.7
9 / 1029 / 1.02 / 395 / 18.8
As expected the buckling load is dependent on the thickness of the web, the thinner webs buckling at a much lower load. It should be noted that when failure occurred a “hinge” basically developed at the center of the web where it was the thinnest. The material that was used was also recorded because the initial extrusion runs using these material experienced problems during extruding with all of the knits being completed. Clearly the quality of the knitting that occurs in the center of the web plays a significant role in the compressive strength of the web. Poorly knitted webs will be significantly thinner at the center of the web which will induce premature failure in compression. Also, while the extrusions samples were being cut up for this test a section of the N21 extrusion was found where the web was not continuous with the outside surface of the extrusion.
Figure 1 – Test setup of web buckling test
Figure 2 – Buckled Webs
Figure 3 – Typical Geometry of Test Piece
A simple FEA model was created of the extrusion profile that was tested using a 2.0mm thick web wall and 1,000 lbs applied. The eigenvalue was 1.2 for this geometry and loading which indicates that the buckling loading found in the tests is consistent with the calculated values.
Figure 4 – FEA Model of Buckled Geometry of Web
- Conclusions
This series of initial tests showed that the compressive strength of the extrusions webs is dependent on the thickness of the web and the quality of the web knitting during extrusion. Failure in compression occurred at the thinner portions of the web where knitting occurred.
Extensive testing should be conducted using extrusions that will be produced in the near future which will be using the actual PVC mixture and will have a higher quality profile with webs that are expected thickness. The results of all testing should be compared to predictions from an FEA model.