1. OBJECTIVES

-To determine the mechanical properties of steel specimen.

-To perform the tensile test of mild steel.

-To observe the tensile strength of different steel grades.

-To study the failure pattern of different steel grades.

-To compare the performances different steel grades.

2. ASTM REFERENCE

ASTM E 8 Standard Test Methods for Tension Testing of Metallic Materials

3. SIGNIFICANCE

This experiment provides fundamental knowledge on tension behaviour of materials specially mild steel, test procedure, universal testing machine and its working principal, tension specimens, failure patterns etc.

4. APPARATUS AND MACHINE

UTM, stop watch, digital slide calipers and computer.

5. SPECIMEN

Mild steel specimens (40, 60, and 72.5 grades) of 16 mm diameter.

6. THEORY

Elasticity & Plasticity: When external forces are applied on a body, made of engineering materials, the external forces tend to deform the body while the molecular forces acting between the molecules offer resistance against deformation or displacement of the particles continues till full resistance to the external forces is setup. If the forces are now gradually diminished, the body will return, wholly or partly to its original shape. Elasticity is the property by virtue of which a material deformed under the load is enabled to return to its original dimension when the load is removed. If a body regains completely its original shape, it is said to perfectly elastic.

Plasticity is the converse of elasticity. A material in plastic state is permanently deformed by the application of load, and it has no tendency to recover. Every elastic material possesses the property of plasticity. Under the action of large forces, most engineering materials become plastic and behave in a manner similar to a viscous liquid. The characteristic of the material by which it undergoes inelastic strains beyond those at the elastic limit is known as plasticity. When large deformations occur in a ductile material loaded in the plastic region, the material is aid to undergo plastic flow

Proportional Limit (Point A): It is the limiting value of the stress upto which stress is proportional to strain.

Elastic Limit (Point B): This is the limiting value of stress upto which if the material is stressed and then released (unloaded), strain disappears completely the original length is regained. Its determination, experimentally, is extremely difficult, and therefore its exact location on the stress-strain diagram is usually not known, even though it is generally higher than the proportional limit.

Permanent set/permanent deformation: If the load exceeds the elastic limit before it is removed, the material does not fully regain its initial dimensions. In such a case the material is said to experience a permanent deformation.

Elastic Recovery: The recovered deformation after removal of load.

Yield stress (Point C and D): Soon after the stress the elastic limit, low carbon steel attains it yield point stress. The yield point of a material is defined as that unit stress that will cause an increase in deformation without an increase in load. Upon the arrival of yield point, a ductile material such as low carbon steel stretches an almost unbelievable amount, frequently 10%of the original length. When the yield stress is reached elongation takes place more rapidly as

plastic flow takes place over and atoms move into new positions and a return to the original shape of the test piece is impossible.

Upper Yield Point (Point C): This is the stress at which the load starts reducing and the extension.

Lower Yield Point (Point D): At this stage the stress remains same but strain increases for some time.

The upper yield point is influenced considerably by the shape of the test specimen, speed of testing, accuracy of alignment , the condition of the test piece (especially the presence of residual stresses in a test on the full cross section) and by the testing machines itself and is sometimes completely suppressed. The lower yield points much less sensitive and is considered to be more representative.

Yield Strength by Offset Method: For materials having a stress-strain diagram such as shown in figure (those that do not exhibit a well-defined yield point) a value of stress, known as the yield strength for the material, is defined as one producing a certain amount of permanent strain.

Ultimate Strength/Tensile Strength (Point E): This is the maximum stress the material can resist. The ultimate strength represents the ordinate to the highest point in the stress-strain diagram and is equal to the maximum load carried by the specimen divided by the original cross-sectional area.

Breaking Strength/Fracture Strength/Rupture Strength (Point F): The stress at which finally the specimen fails is called breaking point. It is the engineering stress at which specimen fracture and complete separation of the specimen parts occurs.

Strain Hardening/Work Hardening: If a ductile material can be stressed considerable beyond the yield point without failure, it is said to strain harden (When a material deformed plasticity, it work hardens, that is, the stress has to be increased to give further deformation).

Necking: After reducing the maximum stress, a localized reduction in area, called necking, begins, and elongation continues with diminishing load until the specimen breaks.

Modulus of Rigidity (G): It is defined as the ratio of shearing stress to shearing strain within elastic limit.

Modulus of Resilience: The work done on a unit volume of material, as a simple tensile force is gradually increased from zero to such a value that the proportional limit of the material is reached, is defined as the modulus of resilience.

Modulus of Rupture/ Modulus of Toughness: The work done on a unit volume of material as a simple tensile force is gradually increased from zero to the value causing rupture is defined as the modulus of toughness.

Various machine and structure components are subjected to tensile loading in numerous applications. For safe design of these components, their ultimate tensile strength and ductilityto be determined before actual use. A material when subjected to a tensile load resists the applied load by developing internal resisting force. These resistances come due to atomic bonding between atoms of the material. The resisting force for unit normal cross-section area is known as stress.

The value of stress in material goes on increasing with an increase in applied tensile load, but it has a certain maximum (finite) limit too. The minimum stress, at which a material fails, is called ultimate tensile strength.

The end of elastic limit is indicated by the yield point (load). This can be seen during experiment as explained later in procedure with increase in loading beyond elastic limit, initial

cross-section area (Ai) goes on decreasing and finally reduces to its minimum value when the specimen breaks. Some typical mechanical properties of mild steel are as follows:

Proportional Limit, p = 30~65 ksi (larger for stronger specimens) Yield Strength, y = 35~75 ksi (larger for stronger specimens) Ultimate Strength, ult = 60~100 ksi (larger for stronger specimens)

Modulus of Elasticity, E = 29000~30000 ksi (almost uniform for all types of specimens)

Poisson’s Ratio, ν = 0.20~0.30 ksi (larger for stronger specimens) Modulus of Resilience = 0.02~0.07 ksi (larger for stronger specimens) Modulus of Toughness = 7~15 ksi (smaller for stronger specimens) Ductility = 10~35% (smaller for stronger specimens)

Reduction of Area = 20~60% (smaller for stronger specimens)

1. PROCEDURE

i) Measure the diameter of the specimen by slide calipers. Record gage length.

ii) Fix the specimen in proper position and apply the load

iii) Record the maximum load and apply load till the breakage.

iv) Remove the broken specimen and measure the smallest cross-sectional area and the final length between the gage marks by fitting the two ends of the broken pieces together.

v) Note the characteristics of the fractured surface.

2. SAMPLE CALCULATIONS

Initial length of specimen, hi = Final length of specimen, hf = Initial diameter of specimen, di= Final diameter of specimen, df= Initial cross-section area, Ai = Final cross-section area, Af=

1. Draw stress-strain curve in tension.

Determine Modulus of Elasticity,

and Modulus of Resilience in tension

3. Determine ultimate (max.) tensile strength from graph

4. Determine yield stress from graph

5. Determine percentage elong~~ation i~~n length (or height) of the specimen

6. Determine EMF (elongation at maximum force) from graph

7. Also determine proportional limit (σ_p), elastic limit ( σ_E), yield point ( σ_y), ultimate load (

σ_u), breaking strength ( σb), etc.

10. DATA TABLE

Deformation rate=		mm/min, Grade=			ksi, Brand=
Time (s)	Load (N)	Time (s)	Load (N)	Time (s)	Load (N)	Time (s)	Load (N)
0		470		560		740
10		480		570		750
20		490		580		760
30		500		590		770
40		510		600		780
50		520		610		790
60		530		620		800
70		540		630		810
80		550		640		820
90		560		650		830
100		570		660		840
110		580		670		850
120		590		680		860
130		600		690		870
140		610		700		880
150		620		710		890
160		630		720		900
170		640		730		910
180		650		740		920
190		660		750		930
200		670		760		940
210		680		770		950
220		690		780		960
230		700		790		970
240		710		800		980
250		720		810		990
260		730		820		740
270		740		830		750
280		750		840		760
290		760		560		770
300		770		570		780
310		780		580		790
320		790		590		800
330		800		600		810
340		810		610		820
350		820		620		830
360		830		630		840
370		840		640		850
380		470		650		860
390		480		660		870
400		490		670		880
410		500		680		890
420		510		690		900
430		520		700		910
440		530		710		920
450		540		720		930
460		550		730		940

11. GRAPH

1. Tensile stress vs. strain curve of 40 Grade bar.

2. Tensile stress vs. strain curve of 60 Grade bar.

3. Tensile stress vs. strain curve of 72.5 Grade bar.

4. Combined Tensile stress vs. strain curve of 40, 60 and 72.5 grade bar.

5. Show all the points on the graphs 1, 2, and 3.

12. RESULT

(Students will fill up this section with their individual outcome/result about the test. Write the stress values in psi and MPa as shown in Table)

Properties	40 grade steel	60 grade steel	72.5 grade steel
Properties	40 grade steel	60 grade steel	(500W grade)
E, psi (MPa)
p, psi (MPa)
p, in/in (mm/mm)
E, psi(MPa)
E, in/in (mm/mm)
y, psi(MPa)
y, in/in (mm/mm)
u, psi(MPa)
u, in/in (mm/mm)
b, psi(MPa)
b, in/in (mm/mm)
Ductility ratio, u / y
% Elongation
TS/YS
EMF, in/in (mm/mm)
Failure pattern
Failure type

13. DISCUSSION

(Discuss on the results found, graphs, and failure patterns and also compare the results found, graphs and failure patterns.) Point out the discussion