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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.

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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

Bending Test

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

What is the hardening process?

Hardening is a kind of heat treatment process that can improve the plain-carbon steel properties in hardness and abrasive resistance. This process starting with steel is heated to the austenitizing temperature, held in furnace to acquire homogeneous austenitic structure and then quenched at such a rate that martensitic structure is produced or critical cooling rate. In order to obtain fully martensitic structure in steel, the related factors are recommended as following ;

1. Carbon content : The higher percentages of carbon in steel, the more occasion that martensitc structure is produced. Moreover,the alloying elements such as Nickel, Chromium and Molybdenum will also increase hardenability by decreasing the critical cooling rate.

2. Cooling rate : Cooling rate that required to provide martensite should not be lower than the critical cooling rate and also depends on severals factors as follwing ;
- The surface always cools faster than the center of the part. In addition, as the size of the part increases, the cooling rate at any location is slower. Consequently, the smaller part has more possibility to produce fully martensitic structure than the bigger one in the same condition.
- The different quenching media provide different cooling rate. For example, water and brine (water plus various percentages of Sodium choride or Calcium choride) provide a faster cooling rate than oil. In addition, agitation of the quenching media is one method which also increases the cooling rate

Reference: 1.Donald R. Askeland.,The Science and Engineering of Materials, 3rd editionn,PWS Publishing company (1994)

What is the heat treatment of steel and what is the purpos

Heat treatment of steel is a process to improve steel properties especially mechanical properties, by means of thermal process to obtain the appropriate properties for each application. Steels which have been undertaken this process will have the better mechanical properties than those as rolled such as increasing hardness and also abrasive,wear resistance.

What is a Hot-Dip Galvanizing?

Hot-Dip Galvanizing

The galvanizing process consists of three basic steps;

1. Surface preparation

1.1 Caustic cleaning: A hot alkali solution often is used to remove organic contaminants such as dirt, paint marking, grease and oil from the metal surface. Epoxies, vinyl, asphalt or welding slag must be removed by grit/ sand-blasting or other mechanical means.
1.2 Pickling: Scale or rust normally is removed by pickling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid.
1.3 Fluxing: Fluxing removes oxides and prevents further oxides from forming on surface of metal prior to galvanizing.
- Dry galvanizing process: The steel or iron is dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. Then material will be dried before immersing in molten zinc.
- Wet galvanizing process: A blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The steel or iron being galvanized passes through the flux on its way into the molten zinc.

2. Galvanizing

In this step, the material is immersed in a bath (at least 98% pure molten zinc, maintaining at 449°C) until it reach bath temperature. The zinc metal reacts with the iron on the steel surface to form a zinc/iron inter-metallic alloy. The products are withdrawn from zinc bath; the excess zinc is removed by draining, vibrating and/or centrifuging and they are cooled in either water or ambient air immediately.

3. Inspection

The two properties with closed scrutinizing are thickness and appearance of coating. The physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance.

Reference: American Galvanizers Association, Hot-Dip Galvanizing for Corrosion Protection of Steel Products.

Shearing and Blanking

Shearing is the separation of metal by two blades moving. A narrow strip of the metal is severely plastically deformed to the point where it fractures at the surfaces in contact with the blades. The fracture then propagates inward to provide complete separation.

The clearance between the blades is an important variable in shearing operations. With the proper clearance the cracks that initiate at the edge of the blades will propagate through the metal and meet near the center of the thickness to provide a clean fracture surface.

Insufficient clearance will produce a ragged fracture and also will require more energy to shear the metal than when there is proper clearance. With the excessive clearance there is a greater distortion of the edge and more energy is required because more metal must plastically deform before it fractures.

Too large clearance, burr or sharp projections are likely to form on the sheared edge. A dull cutting edge also increases the tendency for the formation of burrs.

The height of burr increases with increasing clearance and increasing ductility of the metal.

Clearances generally range between 2-10% of the thickness of the sheet; the thicker the sheet the larger the clearance.

Neglecting friction, the force require to shear a metal sheet is the product of the length cut, the sheet thickness, and the shearing strength of the metal.

Blanking: the same concept of shearing
When the metal inside the contour is desired part: call blanking.

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