METHOD FOR TENSILE TEST OF STEEL
IS: 1608/ISO: 6892
The tensile testing of metallic materials involves straining a test piece by tensile force, generally to fracture, for the purpose of determining one or more of the mechanical properties. The test is carried out at ambient temperature between 10°C and 35°C, unless otherwise specified. Tests carried out under controlled conditions shall be made at a temperature of 23°C.
a) Tensile testing machine of suitable capacity. Universal testing machine (UTM) of 1000 kN capacity, with loading accuracy of +1%, provided with attachments for tension testing of shouldered and threaded specimens is recommended.
b) Extensometer for UTM for measuring elongation of the specimen.
3. Test piece
3.1 Shape and dimensions
a) The shape and dimensions of the test pieces depend on the shape and dimensions of the metallic product from which the test pieces are taken. The test piece is usually obtained by machining a sample from the product or a pressed blank or casting.
b) The cross section of the test piece may be circular, square, rectangular, annular or even any other shape.
c) Machined test pieces shall incorporate a transition curve between the gripped ends and the parallel length, if these have different dimensions.
d) The dimensions of this transition radius are important and it is recommended that they may be defined in the material specification if they are not given in the appropriate annexure of IS: 1608.
e) The gripped ends may be of any shape to suit the grips of the testing machine. The axis of the test piece shall coincide with or parallel to the axis of the application of the forces.
f) The parallel length (Lc) or, in the case where the test pieces has no transition curve, the free length between the grips, shall always be greater than the original gauge length Lo.
g) If the test piece consists of an unmachined length of the product or of an unmachined test bar, the free length between the grips shall be sufficient for gauge marks to be at a reasonable distance from the grips.
h) As cast test pieces shall incorporate a transition radius between the gripped ends and the parallel length.
i) The gripped ends may be of any shape to suit the grips of the testing machine. The parallel length (Lc) shall always be greater than the original gauge length Lo.
The main types of test pieces are defined in annexure A to D of IS: 1608 according to the shape and type of product as indicted in table1.
3.3 Preparation of test pieces
The test pieces shall be taken and prepared in accordance with the requirements of the relevant international standards (ISO: 377) for the different materials
|Table 1: Main types of test pieces|
|Sheets & flats with thickness given below||Wires, bars and other sectionswith dia or side in mm||Reference to annexureof IS: 1608.|
|0.1 mm to less than 3 mm||A|
|-||Less than 4 mm||B|
|3 mm and more||4 mm and more||C|
3.4 Determination of original cross-sectional area (So)
The original cross sectional area shall be calculated from the measurements of the appropriate dimensions. The accuracy of this calculation depends on the nature and type of the test piece. It is indicated in annexes of A to D for the different types of test piece.
3.5 Making the original gauge length (Lo)
Each end of the original gauge length shall be marked by means of fine marks or scribed lines, but not by notches which could result in premature fracture. Annex F gives a nomogram for determining the original gauge length corresponding to the dimensions of the test pieces of rectangular cross sections. The original gauge length shall be marked to an accuracy of ±1%. If the parallel length (Lc) is much greater than the original gauge length, as, for instance, with unmachined test pieces, a series of overlapping gauge length is drawn.
4. Conditions of testing
a) Unless otherwise specified in the product standard, the speed of testing within the elastic range and up to the upper yield strength (ReH) shall correspond to the stressing rates given in table 2
Table 2: Rate of stressing
Modulus of Elasticity of
Rate of stressing N/mm2 per second
the material N/mm2
b) If only the lower yield strength (ReL) is to be determined, the rate of straining the parallel length of the test piece shall be between 0.00025/s to 0.0025/s. However the rate of stressing within the elastic range shall not exceed the values in table 2.
c) If both the upper yield strength (ReH) and the lower yield strength (ReL) are to be determined, the conditions for determining lower yield strength (ReL) shall be followed.
d) For determining proof strengths, the rate of stressing shall be as given in table 2. Within the plastic range and up to proof strength, the straining rate shall not exceed 0.0025/s.
e) If the testing machine is not capable of measuring or controlling the strain rate, a cross head separation rate equivalent to the rate of stressing given in table 2 shall be used till the completion of the yield.
f) For determining tensile strength (Rm), the straining rate of the parallel length shall not exceed 0.008/s within the plastic range. If yield stress or proof stress is not required, the maximum rate of the machine permitted in the plastic range may be used.
g) The test pieces shall be held by suitable means such as wedges, screwed grips, shouldered holders etc.
h) Ensure that test pieces are held in such a way that the force is applied as axially as possible.
5.1 Determination of percentage elongation after fracture (A)
a) Increase in length of the gauge after removal of the specified stress expressed as a percentage of the original length (Lo) is called percentage permanent elongation.
b) Permanent elongation of the gauge length after fracture (Lu-Lo) expressed as a percentage of the original length (Lo) is called percentage elongation after fracture.
c) Special precautions shall be taken to ensure proper contact between the broken parts of the test piece while measuring the final gauge length.
d) Elongation after fracture (Lu-Lo) is determined to the nearest 0.25 mm with a measuring device of 0.1 mm resolution.
e) The value of percentage elongation after fracture (A) is rounded off to the nearest 0.5%.
f) It is not necessary to mark gauge lengths if the machine is fitted with extensometer to measure the extension at fracture.
5.2 Determination of percentage total elongation at maximum force (Agt)
a) From the force-extension diagram obtained with an extensor meter, the extension at maximum force (?Lm) is determined.
b) The extensometer gauge length (Le) shall be recorded on the test report.
c) The percentage total elongation at maximum force is calculated by the following formula:
Agt = ?Lm/ Le x 100
5.3 Determination of proof strength, non-proportional extension (Rp)
a) The proof strength (non-proportional extension) is determined from the force-extension diagram as explained below:
(i) Select a point on the X-axis at a distance equivalent to the specified non-proportional percentage (Say 0.2%).
(ii) Through this point, draw a line parallel to the straight portion of the curve.
(iii) The point at which the line intersects the curve gives the force corresponding to proof strength (non-proportional extension).
(iv) This force divided by the cross sectional area (So) of the test piece gives the desired proof strength
5.4 Determination of proof strength, total extension (Rt)
a) The procedure is similar to 5.3 except that the line is drawn at the point of specified total extension.
b) If automatic devices are fitted with the testing machine, force-extension plot is directly obtained.
c) The test piece is subjected to a force corresponding to the specified stress for 10 to 12 seconds. After removing the force, the permanent set extension or elongation is checked and verified that it does not exceed the limit specified for the original gauge length
5.5 Determination of percentage reduction of area (Z)
a) The maximum change in cross sectional area (So-Su) occurred during the test is expressed as a percentage of the original area to determine the percentage reduction in area (Z). Z = (So-Su)/ So x 100
b) Two broken fractions of the tested piece is carefully fitted together and the minimum cross-sectional area after fracture is determined (Su). Knowing the original area of cross section (So), Z can be calculated.
METHOD FOR BEND TEST OF STEEL
Bend test is conducted for determining the ability of metallic materials to undergo plastic deformation in bending. The above standard is not applicable to certain materials such as tubes in full section or welded joints, for which other standards exist. The bend test consists of subjecting a test piece of round, square, rectangular or polygonal cross section to plastic deformation by bending without changing the direction of loading until a specified angle of bend is reached.
The bend test shall be carried out in testing machines or presses equipped with the following devices:
a) Bending device with two supports and a mandrel (Fig.1):
The length of the supports and the width of the mandrel shall be greater than the width or diameter (d) of the test piece. The diameter of the mandrel (D) is determined by the material standard. The test piece supports shall be rounded to a radius between 1 and 10 times the thickness of the test piece and shall be sufficiently hard. Unless or otherwise specified, the distance between the supports (l) shall be taken as approximately, l = D + 3d, and shall not change during the bend test.
b) Bending device with a V-block and a mandrel (Fig.2):
The tapered surface of the V-block shall form an angle of 180°- α, where α is the angle of the bend. The edges of the V-block shall be rounded to a radius between 1 and 10 times the thickness of the test piece (d) and shall be sufficiently hard.
c) Bending device with a clamp
The device consists of a clamp and a mandrel of sufficient hardness it may be equipped with a lever for applying force to the test piece.
3. Test piece
a) Round square, rectangular or polygonal cross section test pieces are used in the test. The edges of rectangular test pieces shall be rounded to a radius not exceeding 1/10th of the thickness of test pieces.
b) Any areas of the material affected by shearing or flame cutting and similar operation during cutting of test pieces shall be removed.
c) The rounding shall be made in such a way that no transverse burrs, scratches or marks are formed which might adversely affect the test result.
d) The width of the test piece should be same as its width, when the width of the product is equal to or less than 20 mm.
e) When the width of the product is more than 20 mm, the width of test piece shall be:
(i) 20±5 mm for products of thickness less than 3 mm.
(ii) Between 20 mm and 50 mm, for products of thickness equal to or greater than 3 mm.
f) The thickness of the test piece from sheets, strips and sections shall be equal to the thickness of the product to be tested.
g) If the thickness of the product is greater than 25 mm, it may be reduced by machining one surface to not less than 25 mm. During bending, the un-machined side shall be the tension-side surface of the test piece.
h) The round or polygonal cross section test piece is subjected to the bend test in the cross section equal to that of the product.
i) When the diameter or the inscribed circle diameter of the test piece exceed 30 mm up to and including 50 mm, it may be reduced to not less than 25 mm.
j) The length of a test piece depends on the thickness of the test piece and the test equipment used.
k) The test is carried out at ambient temperature between 10 and 35°C.Tests carried out under controlled conditions shall be made at a temperature of 23±5°C.
a) The bend test is carried out using one of the following methods specified in the relevant standard.
(i) That a specified angle of bend is achieved under the force and for the given conditions.
(ii) The legs of the test piece are parallel to each other at a specified distance apart while under force.
(iii) The legs of the test piece are in direct contact while under the force.
b) The test piece is placed over the supports (Fig.1) or V-block (Fig.2) and the bending force applied slowly to permit free plastic flow of the material.
c) If it is not possible to bend the test piece to the specified angle in the above manner, complete the bend by pressing directly on the ends of the legs of the test piece.
d) In the bend test to parallelism of the legs, the test piece may be sent first by pressing directly on the ends of the legs of the test piece and then placed between the parallel plates of the press where it is further formed by the application of a continuously increasing force to obtain parallelism of the legs. The test may be carried with or without the insert. The thickness of the insert shall be defined in the relevant standard or by agreement.
d) If specified, the test piece, after its preliminary bending, is further bent between the parallel plates of the press, by application of a continuously increasing force to obtain direct contact between the legs of the test piece.
a) If the requirements of test materials are not specified, absence of cracks visible without the use of magnifying aids is considered as the evidence that the test piece has withstood the bend test.
b) The angle of bend, specified in material standards, is always considered as a minimum requirement.
c) If the internal radius of bend is specified, it is considered to be the maximum.
6. Test report The test report shall include the following information:
(i) Reference to the relevant standard.
(ii) Identification of the test piece (i.e., type of material, cast number, direction of test piece axis relative to a product etc.).
(iii) Shape and dimension of the test piece.
(iv) Test method.
(v) Test result.