Tensile test under wedge load – Mr. Caliper will explain how it works!

Mr. Caliper, what exactly is tensile test under wedge load?

In a tensile test under wedge load on finished screws, a screw is subjected to axial tensile stress until it breaks. A wedge with a defined angle is placed under the bearing surface of the screw. During the tensile test, the transition between the head and the non-threaded shaft is altered by the wedge. The advantage of this test is that the tensile strength as well as the intactness of the under-head radius can be checked.

Why is it important?

The worse thing that can happen to a screw connection is that the head of a tightened screw breaks off. The tensile strength as well as the under-head radius can be checked relatively easily through this test. If the radius is not shaped properly, the screw head either breaks or cracks are formed.

Which types of tensile tests under wedge load can be performed for screws?

Our tensile test machine can apply a force of 600kN. This is sufficient for tensile test under wedge and axial tensile test up to M30 for property class 8.8 or M24 for the property class 12.9. Additionally, can we check machined samples, perform proof load tests on nuts up to M33 or test eyebolts.

Test sequence

Mr. Caliper is ready for angular tensile test on products. Learn about the individual steps in detail!

Suitability test (using a screw)
angular tensile test-suitability test

The screw is checked as it is delivered. Before starting the test, the screw length should be checked. Here, the rule of thumb of minimum 2.5 x diameter (d) is applicable. The wedge angles are defined in DIN EN ISO 898-1.

Set up of test machine
angular tensile test-set up-test machine

To begin with, all important data regarding the product (item number, test lot, etc.) should be updated in the system. Additionally, details such as nominal diameter, screw length and minimum tensile strength should be entered for subsequent calculations.

The tensile test machine is checked every 3 months with certified reference samples. The results are documented and recorded in a diagram. Additionally, the device is calibrated every year.

Performance of the inspection
angular tensile test-perform-test angular tensile test-perform-test angular tensile test-perform-test

If all the data is maintained, then the screw can be installed on the testing machine. For this purpose, a distance disc is first placed over the threaded sleeve. Subsequently, the screw is inserted through the corresponding chamfer (here 6°) and screwed into the threaded sleeve. It is important to ensure that the sample is screwed min. 1xd in the threaded sleeve. Otherwise, the thread of the screw may strip off during the test.
The chamfer is inserted in the upper bracket, the traverse is inserted and the threaded sleeve on the lower bracket. As soon as the start button is pressed, the traverse moves upward with maximum 25mm/min. This increases the stress on the screw, until it finally breaks. The breakage should occur in the free threaded length. Once the tensile test is complete, the traverse goes back to its original position and the two broken pieces can be removed from the machine.
Subsequently, the transition area between the shaft and the screw head is checked for cracks with a magnifier.

Evaluation of results
angular tensile test-assessment of results

If the required tensile strength is met and the break does not occur in the transition area to the head, but rather in the thread, then the test counts as “passed”. A report can be created for additional documentation with all important test parameters and items. Additionally, the tensile test is shown in a stress-strain-diagram and the measurement results are listed in a table.

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Complete overview of the tests

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