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FACULTY OF MECHANICAL ENGINEERING UiTM Terengganu, Bukit Besi Campus.
Program
:
Diploma in Mechanical Engineering
Course
:
Mechanics and Materials Lab
Course Code : Lecturer
MEC 291
:
Laboratory Report
Experiment Title
TENSILE TEST
No.
1. 2.
Name
Student ID No.
Putera Ezzarif Bin Ahmat Yatim Nur Syahira Binti Zawawi
Date of Experiment : ________________ ________________
2013426418
Received by:-
Date of Submission : ________________ ________________
Lecturer
Signature
Objective i.
To obtain a general understanding of how different materials and cross sections behave under uniaxial tensile loading.
ii.
To determine the stress-strain relationship and compare mechanical / material properties of various materials and cross section.
iii.
To obtain the mechanical properties : the modulus of elastic ity , the yield stress , the ultimate stress , the fracture stress and the ductility ratio.
Apparatus 1. Universal testing machine 2. Vernier calliper 3. Steel ruler 4. Two or three specimens (steel,aluminium and brass)
(Universal testing machine)
(Vernier calliper)
Theory Mechanical testing play an important role in evaluating fundamental properties of engineering materials (i.e : modulus of elasticity, Poisson’s, ultimate strength, yield strength, fracture strength, resilience, toughness, % reduction in area and elongation) as well as in developing new materials and controlling the quality of materials for use in design and construction. Most of these engineering values are found by graphing the stress and strain values from testing. A number of experimental techniques are developed for mechanical testing of engineering materials subjected to tension, compression,bending and torsion loading. Ductile materials will neck down through the plastic range before rupture (Figure 1a). Brittle materials do not neck significantly (Figure 1b). Instead the y fail sharply and abrutly at maximum load because brittle materials do not exhibit much plasticity.
a) Failure of ductile material
b) Failure of brittle material
Figrure 1 : Typical of failure of materials When specimen is loaded so that the resultant force passes through the centroid of the specimen cross section, the loading is called as axial and can be either tensile or compressive. The test measures force and change of length of the specimen which are used to calculated nominal stress and nominal strain. The term nominal (or engineering) is used to indicate that the stress is based on the original test specimen cross section area and the strain is based on the original gage length as shown as the force P per unit area A :
Stress, σ p =
Strain is measure of the deformation that has occured in a material. In the case where the magnitude of deformation is the same over the entire le ngth of a body, strain is defined as :
Strain, ɛ =
−
where : Lo = the initial length Lf = final length
A typical stress-strain diagram from a tensile test for structural steel is shown in Figure 2. The particular properties are designated on the Figure 2 and are described as below :
1. Young’s modulus (Modulus of elasticity), E
Young ‘s modulus is the ratio of stress to strain for the initial straight line portion of the stress-strain curve (slope of the straight line). Determined by :
E=
where : σ p = proportional limit stress ɛ p = proportional limit strain 2. Proportional limit
Proportional limit is the value of engineering stress (the load is divided by the initial crosssectional area) at the point where the straight-line portion of the stress-strain curves ends. 3. Yield point
Yield point is a point on the stress-strain curve, after which there is a significant increase in strain with little or no increase in stress. The corresponding stress is called the Yield strength/stress of the material. For materials that do not posses well-defined yield point, “offset method” is used to determine it.
4. Elastic limit
Elastic limit is the value of stress on the stress-strain curve after which the material deforms plastically (maximum stress for which stress will be directly proportional to strain). 5. Ultimate strength
Ultimate strength is the highest value of apparent stress on the stress-strain curve. It is also known as the tensile (or compressive) strength. 6. Fracture strength
Fracture strength is the value of stress at the point of final fracture on the stress-strain curve. 7. Percent elongation
Percent elongation is the measure of the deformation at the point of final fracture. Determined by : % elongtion =
−
× 100
8. Percent reduction of area
Percent reduction of area is the measure of the fracture ductility. Determined by : % RA =
−
× 100
where : Af = the final cross-sectional area at the location of fracture Ao = the initial cross-sectional area
9. Ductility
Ductility is the characteristic of a material where the material can undergo large plastic deformations before fracture, especially in tension. Ductility of materials is measured by ductility ratio ;
ductility , µ =
where : ɛu = the ultimate strain ɛy = the yield strain
Figure A : A typical stress-strain diagram for a ductile material
Procedure 1. The dimensions of the each test specimen before a nd after test is measured and is filled in the table 1. The gauge length is marked on the t est specimen. 2. The machine is switched on. 3. The test specimen in the grips of the machine is mounted. 4. Load and the corresponding deformation is applied and recorded. 5. Steps (1) to (4) is repeated for various type of the test specimen.
Result Table 1 Material : Steel/copper/aluminium
Type : Rectangular/round
Initial (unit : mm) Material
Lo
Ao
bo
Final (unit : mm) ho
(mm2)
Lf
Af
df
bf
(mm2)
Steel Copper Aluminium d=diameter ; b=width ; h=height(thickness) ; L=length ; A=are a Table 2 No