Investigation of the strength of puncture holes

Аuthors

Fedotov D. A.^*, Skvortsov K. G.^**

JSC “Avangard”,

*e-mail: fedotov.da.88@mail.ru
**e-mail: kostyan-grinders@mail.ru

Abstract

The object of the present study is holes obtained by piercing the uncured polymer-fiber material of the structure. Puncture is carried out with the help of a pointed rod – indenter. The force field of the reinforcing fibers is not interrupted. The wall of the hole is thickened due to the increased content of fibers.

The aim of the study is to develop a hole punching technology, thread cutting by puncture, and also experimental study of the holes obtained.

Tensile tests carried out on multi-layered woven glass fiber-reinforced plastic samples with a hole of 8 mm showed a reduction in tensile strength in comparison with solid samples by 31.7% and 4.2%, respectively, for drilled and molded holes.

A special device was developed to pierce holes of various diameters in GRP plates up to 30 mm thick, as well as piercing elements for smooth holes and threaded holes.

Initially, the tensile test was carried out. Tests were made on samples cut from prefabricated plates in the size of 350×350 mm. To fabricate plates, T-13P cloth (100) was chosen, impregnated with binder EP-5122. The plates were made on a special device. The layers of fabric were stacked by alternating warp and weft. Such a technique made it possible to obtain characteristics of plate material, which are the same in all reinforcement directions. In the center of each plate we punctured holes.

18 plates with punctured holes in the center with a diameter of holes 6; 9; 13; 16; 18 and 20 mm were made. We made 3 plates for each diameter. We cut out of them the following samples: 1 sample with a punctured hole, 2 samples with holes drilled and 2 samples without holes. The width of the samples was equal to 3 hole diameters. Samples with holes of 20 mm were 50 mm wide. All samples were stretched to failure.

Based on the results of the tests, the following conclusions were drawn:

1. For a T-13P fabric (100), a 20 mm hole is the limiting size when it is punctured. This required a significant load for the puncture. There was a snacking of the tissue between the piercing element and the edge of the hole at the bushing. This situation is caused by the impossibility of further compression of the filaments perpendicular to them and it becomes impossible to further expand the hole. When puncturing a hole 18 mm in diameter, all these troubles were absent. Thus, a hole with a diameter of 20 mm was prohibitive, therefore, we can confirm that the limiting value of the hole for puncturing is in the range of 18 to 20 mm.

2. For a T-13P (100) fabric, holes of 6 mm or less do not create a stress concentration near the holes in the fiberglass material.

3. The structures with punctured holes are about one and a half times stronger than those with holes drilled.

4.  As the diameter of the pierced hole increases, the stress concentration increases.

Further, the samples were subjected to cut tests. For the testing, 5 plates with punctured holes were made. Samples for testing were cut from the same slabs. In this case we had one sample with a punctured hole, and two with drilled holes. In the holes the bushings were inserted, through which the sample was loaded. On the one hand, the load was applied to the sample, and on the other hand it was applied to the pin. In such tests, the pin cuts the sample material along two planes.

Analysis of the test results showed that the breaking stresses on the cut in samples with punctured holes are 2-3 times larger than in the samples with drilled holes. This result can be explained by the fact that in the first case the threads of the reinforcing material near the hole are not cut, and the drilling cuts them.

Also we carried out tests to cut the thread obtained in the prepreg by the puncture method. For the production of plates with threaded holes, we made piercing elements which had a thread cut on the cylindrical part. We produced five such plates with different hole diameters. To test the thread in the holes we screwed rods with a corresponding thread into these holes. A tearing force was applied to the bolt until the thread was broken.

As a result of the tests, the following conclusion was drawn: the stresses on the cutting edge of the GRP thread have a value equal to the cutting strength of the GRP material. Thus puncture of the threaded hole allows to achieve the maximum value of the strength characteristics.

Keywords:

composite product, prepreg, puncture hole, test, adaptation, thread, cloth

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