Tearing of sheet materials during forming
Sheet materials, particularly those made from metals, are used in a wide range of industries, from the automotive and electrical industries to the aerospace and construction industries. This is mainly due to their versatility in being formed into different shapes and sizes by a variety of processes such as stamping, bending, and drawing, to name a few.
When a sheet material is subjected to a mechanical force during forming, the integrity of the material can be compromised and brittle fracture can occur. This is known as tearing and it can interfere with the necessary functionality of the part or product. It is thus paramount that steps are taken to limit the risk of fracture due to tearing in forming operations.
Tearing of a sheet material is mainly a consequence of the presence of notches, sharp corners, and stress concentrators. These structural features can led to an increase in local stress at the feature and can cause brittle fracture in the material. While the exact mechanism of tearing and subsequent fracture is not well understood, it is thought to be related to plastic flow ahead of the crack, microstructural inhomogeneity and retardation of crack coalescence, and strain hardening that occurs in the material.
The amount of tearing present in a material can be assessed using mechanical tests. These tests are mainly done on samples of the material in order to prove that the material is stable and free from defects. The most common test for measuring tearing is the Charpy impact test. In this test, a sample of the material is subjected to an impact force from a swinging pendulum. The energy absorbed by the sample is a measure of its fracture toughness. The test is relatively simple to perform and cost-effective, thus making it very popular.
In order to reduce the risk of tearing in sheet materials, it is important that the design be free from sharp corners, notches and other stress concentrators. Whenever possible, sheet materials should be formed using a process that causes minimal stress to the material. Additionally, it is important to choose a suitable material for the application that is capable of withstanding the mechanical forces imposed on it during forming.
In summary, tearing of sheet materials during forming can cause problems with the usability and functionality of the part or product. It is thus important to pay attention to the design of the part, choose the right material, and use the appropriate forming procedure in order to ensure that the sheet material does not tear during forming. Additionally, mechanical tests can be used to check for tearing and to ensure that the material is free from defects.
