Testing of mechanical properties
Tensile Test (Part 2)
If the true stress based on actual cross section area of specimen, it is found that the stress-strain curve increases continually up to fracture.
From engineering stress-strain curve, it can be observe yield point which identifies yield strength and limit of deformation form elastic deformation to be plastic deformation. The stress of elastic deformation region is linearly proportional to strain which is called “proportional limit”. There is a greatest stress the material can withstand without any measurable permanent strain remaining on the complete release of load while the plastic deformation is permanent deformation after load releasing.
Yield stress could be finding in two ways depending on graph characteristic. First, the yield point could be observed clearly, the value on the stress axial is equal with yield stress. In case of unclearly yield point such as in carbon steel with annealing or skinpass rolling, it uses 0.2 percent strain off set by parallel lining with graph at proportional limit region, 0.2 percent strain. The point which is intersection between line and stress-strain curve is yield point or proof stress at 0.2 percent strain offset.
If apply load continuously, it will reach the maximum load or ultimate tensile strength which appear as the peak of curve. After that, necking will occur on some areas of material that effect to stress decreasing rapidly while the strain or elongation is increased until fracture occurs. Total change in length of gage length is used for calculation of percent elongation as mentioned above.
Tensile Test (Part 1)
The Tensile Test is used to identify mechanical properties or strength of materials by milling specimen as any testing standards. In this test, an increasing uni-axial load is continuously applied to the specimen by testing machine until it fractures. During testing, elongation of specimen will be measured continually and plotted as a load versus elongation diagram (engineering stress-strain curve) which shows relationship between the applied load and corresponding elongation. After that it will calculate the engineering values that are yield strength, ultimate tensile strength and percent elongation.
The engineering stress is calculated from applied load divided by the original cross sectional area of specimen that show in the unit of N/mm2, MPa, kgf/mm2, psi and ksi. The change in length of gage length divided by the original gage length, expressed as a percent, is the engineering strain or percent elongation.
True stress use actual cross section area which is reduced at any time for calculation instead original cross section of specimen in stress-stain curve. Although during testing, it has to change the dimension or cross section area of specimen. Especially in ductile material, the cross section area of specimen is decreasing rapidly in the test. This effect to the load required continuing deformation falls off. The average stress based on original cross section area likewise decreases, and this produces the fall off in the stress-strain curve beyond the point of maximum load. In fact, the metal will generate strain-hardening continually all the way up to fracture. So the stress requires to deform should be increased.
What is spring back?
In general, all materials have properties of elastic deformation (shape after deform is same as before deform) and plastic deformation (shape after deform isn’t same as before deform). Final deformation is elastic deformation or plastic deformation will depend on applied force and elastic recovery. In case of applied force is greater than elastic recovery, plastic deformation will occur. If applied force is smaller than elastic recovery, material will recover back to pre-deform shape, this is elastic deformation.
In bending process, this recovery phenomenon is known as spring back. Spring back will make final bend angle smaller and final bend radius larger than before spring back had affected. This is normal phenomenon that could take place not only in flat sheets or plates, but also in bending bars, rod and wire of any cross section. Factors that effected to spring back are bend radius and material thickness. The greater the bend radius, the greater the spring back effect. While the more material thickness is, the less spring back effect.
Resource : Kalpakjian, S., and S.R. Schmid, Manufacturing Process for Engineering Materials, Prentice Hall, 2003
The bending test is used for determining the bending ability of material by bending material to specific bending radius or angle. The cross section of testing sample might be any shape such as square, sphere etc.
The bending test is necessary to apply load with stable direction and slowly to prevent side slipping of material. After testing as specification, tester should check cracking at outer surface of testing sample which is received tension stress during testing by using visual check or microscope (not over than 20X, normally use by visual check). In case of testing material has width/thickness ratio greater than 8 times, if we found cracking at testing material edge, we can eliminating this crack and test it again.
Minimum bend radius is minimum radius applied to material without cracking on surface of testing sample. Normally it will be ratio with thickness then bending ability is flavor to present as ratio of thickness i.e. 3t; it means that this material can be bended with minimum bend radius at 3 times of thickness without cracking.
In case of hot rolled strip, bending test will refer to testing standard of each grade which is difference in dimension and rolling direction of sample including bending radius.
Resource: Iron and Steel Institute of Thailand