Anisotropy Effect of properties on forming

1.2.6 Damage

During tensile plastic deformation, many materials suffer damage at the microstructural level. The rate at which this damage progresses varies greatly with different materials. It may be indicated by a diminution in strain-hardening in the tensile test, but as the rate of damage accumulation depends on the stress state in the process, tensile data may not be indicative of damage in other stress states.

1.2.7 Rate sensitivity

As mentioned, the rate sensitivity of most sheet is small at room temperature; for steel it is slightly positive and for aluminium, zero or slightly negative. Positive rate sensitivity usually improves forming and has an effect similar to strain-hardening. As well as being indicated by the exponent m, it is also shown by the amount of extension in the tensile test-piece after maximum load and necking and before failure, i.e. E Total − E u , increases with increasing rate sensitivity.

1.2.8 Comment

It will be seen that the properties that affect material performance are not limited to those that can be measured in the tensile test or characterized by a single value. Measurement of homogeneity and defects may require information on population, orientation and spatial distribution. Many industrial forming operations run very close to a critical limit so that small changes in material behaviour give large changes in failure rates. When one sample of material will run in a press and another will not, it is frequently observed that the materials cannot be distinguished in terms of tensile test properties. This may mean that one or two tensile tests are insufficient to characterize the sheet or that the properties governing the performance are only indicated by some other test.

1.3 Other mechanical tests

As mentioned, the tensile test is the most widely used mechanical test, but there are many other mechanical tests in use. For example, in the study of bulk forming processes such as forging and extrusion, compression tests are common, but these are not suitable for sheet. Some tests appropriate for sheet are briefly mentioned below: • Springback. The elastic properties of sheet are not easily measured in routine tensile tests, but they do affect springback in parts. For this reason a variety of springback tests have been devised where the sheet is bent over a former and then released. • Hardness tests. An indenter is pressed into the sheet under a controlled load and the size of the impression measured. This will give an approximate measure of the hardness of the sheet – the smaller the impression, the greater the hardness. Empirical rela- tions allow hardness readings to be converted to ‘yield strength’. For strain-hardening materials, this yield strength will be roughly the average of initial yield and ultimate tensile strength. The correlation is only approximate, but hardness tests can usefully distinguish one grade of sheet from another. 12 Mechanics of Sheet Metal Forming