Surface effects 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 • Hydrostatic bulging test. In this test a circular disc is clamped around the edge and bulged to a domed shape by fluid pressure. From measurement of pressure, curvature and membrane or thickness strain at the pole, a true stress–strain curve under equal biaxial tension can be obtained. The advantage of this test is that for materials that have little strain-hardening, it is possible to obtain stress–strain data over a much larger strain range than is possible in the tensile test. • Simulative tests. A number of tests have been devised in which sheet is deformed in a particular process using standard tooling. Examples include drawing a cup, stretching over a punch and expanding a punched hole. The principles of these tests are covered in later chapters.

1.4 Exercises

Ex. 1.1 A tensile specimen is cut from a sheet of steel of 1 mm thickness. The initial width is 12.5 mm and the gauge length is 50 mm. a The initial yield load is 2.89 kN and the extension at this point is 0.0563 mm. Determine the initial yield stress and the elastic modulus. b When the extension is 15, the width of the test-piece is 11.41. Determine the R -value. [Ans: a: 231 MPa, 205 GPa; b 1.88] Ex. 1.2 At 4 and 8 elongation, the loads on a tensile test-piece of half-hard aluminium alloy are 1.59 kN and 1.66 kN respectively. The test-piece has an initial width of 10 mm, thickness of 1.4 mm and gauge length of 50 mm. Determine the K and n values. [Ans: 174 MPa, 0.12] Ex. 1.3 The K, n and m values for a stainless steel sheet are 1140 MPa, 0.35 and 0.01 respectively. A test-piece has initial width, thickness and gauge length of 12.5, 0.45 and 50 mm respectively. Determine the increase in load when the extension is 10 and the extension rate of the gauge length is increased from 0.5 to 50 mmminute. [Ans: 0.27 kN] Ex. 1.4 The following data pairs load kN; extension mm were obtained from the plastic part of a load-extension file for a tensile test on an extra deep drawing quality steel sheet of 0.8 mm thickness. The initial test-piece width was 12.5 mm and the gauge length 50 mm. 1.57, 0.080; 1.90, 0.760; 2.24, 1.85; 2.57, 3.66; 2.78, 5.84; 2.90, 8.92 2.93,11.06; 2.94,13.49; 2.92, 16.59; 2.86, 19.48; 2.61, 21.82; 2.18, 22.69 Obtain engineering stress–strain, true stress, strain and log stress, log strain curves. From these determine; initial yield stress, ultimate tensile strength, true strain at maximum load, total elongation and the strength coefficient, K, and strain-hardening index, n. [Ans: 156 MPa, 294 MPa, 0.24, 45, 530 MPa, 0.24] Material properties 13