T.C. İZMİR INSTITUTE OF TECHNOLOGY FACULTY OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING
ME 409 Mechanical Engineering Laboratory
“ TENSION TEST ”
Özge A.
2009, December 14
İZMİR
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Objective The objectives of this lab are:
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to perform tension tests on aluminum/steel to gain an appreciation of tensile testing equipment and procedures to examine the resulting stress-strain curve to gain an appreciation of the tensile behavior of the tested material and to identify/calculate the significant mechanical properties of the tested material to compare the physical tensile-failure characteristics of the metal
Apparatus
A 150 kN capacity electro-mechanically operated universal tension/compression load frame will be used to test the tensile specimens. The applied load on the specimen is determined indirectly from a tensile load cell. • •
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A caliper will be used to measure the dimensions of the test specimens. The elongation of the loaded test specimen will be determined indirectly by using an extensometer. A computer data-acquisition system will be used to generate load and displacement data.
Materials 6063 Aluminum or 304 stainless steel.
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Analysis of Results EXCEL TABLES
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CALCULATIONS
Table 1
Sample
Aluminum
Code
Gauge Length (Go)
al-2
41,15
mean values
Reduced section(f,mm) 100
Wo (mm) 12,43 12,45 12,5 12,46 Gauge Length (Gf,mm) 50,56
to(mm)
Wf (mm)
tf (mm)
Lf(mm)
3,98 3,99 3,97 3,98
10,58 10,78 10,41 10,59
1,25 1,29 1,23 1,25667
214,5
Ao (mm2) 49,5908
Af (mm2) 13,3081
e,max
92,9324
a. Determination of the tensile strength ( σu ) The ultimate tensile strength (UTS, σ u) is the maximum load sustained by the specimen divided by the original specimen cross-sectional area. As can be easily seen in Figure 1, the maximum point of the Engineering stress-strain curve for Al corresponds to σ u =92,9 MPa and this is the ultimate tensile strength of Al.
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Figure 1 Engineering stress-strain curve of Al
Figure 2 Engineering stress-strain curve and stoke versus engineering strain
b. Calculation of the maximum load (Pmax). σu=PmaxA0 92,9 MPa=Pmax49,5908 mm2 Pmax=4608,59 N
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c. Calculation of the Modulus of Elasticity (E). In the early (low strain) portion of the curve, many materials obey Hooke’s law to a reasonable approximation, so that stress is proportional to strain with the constant of proportionality being the modulus of elasticity or Young’s modulus, denoted E:
σe=E.ϵe
Figure 3 calculation of modulus elasticity,E.
As can be seen in , I specified two points on the elastic region which are not at either the top or bottom and these points are (0,001215; 50,4677) and (0,0003797; 16,1950). Also I use excel to draw a linear line of these specified data region, the line equation is y = 42159x - 0,1198, y is the engineering stress and x is engineering strain. The slope of this line is ⁄dx , its value gives us the Modulus of Elasticity, E.
dydx=42159 E=42159 MPa Furthermore, Modulus of Elasticity, E can be calculated from the two specified points from the relation given below,
E=σ1-σ2ϵ1-ϵ2 With this relation we find a closer value of E that was calculated from the slope of the line.
E=σ1-σ2ϵ1-ϵ2=50,4677-16,19500,001215-0,0003797 E=41030 MPa I think the first method is more reliable because the line includes much more than two data points. “0-strain” location on the strain axis is the x value when y=0 in the equation of line, y = 42159x - 0,1198. If y=0 0 = 42159x - 0,1198 x=2,84*10-6 is the “0-strain” location.
d. Determination/calculation of the yield strength, σ Y , for Al For most engineering materials, the curve will have an initial linear elastic region as in Figure 4 in which deformation is reversible and time independent. The slope in this region is Young’s modulus E. Unloading the specimen at point X in Figure 4 the portion XX ‘ is linear and is essentially parallel to the original line OX ”. 4
The horizontal distance OX ‘ is called the permanent set corresponding to the stress at X. This is the basis for the construction of the arbitrary yield strength. To determine the yield strength, a straight line XX “ is drawn parallel to the initial elastic line OX ’ but displaced from it by an arbitrary value of permanent strain. The permanent strain commonly used is 0.20 percent of the original gage length. The intersection of this line with the curve determines the stress value called the yield strength. In reporting the yield strength, the amount of permanent set should be specified. The arbitrary yield strength is used especially for those materials not exhibiting a natural yield point such as nonferrous metals; but it is not limited to these. Plastic behavior is somewhat time-dependent, particularly at high temperatures. Also at high temperatures, a small amount of time-dependent reversible strain may be detectable, indicative of anelastic behavior.
Figure 4 General Stress- Strain Diagram
0.2%
OFFSET METHOD
Figure 5 Yield Strength
As can be easily seen from Figure 5 the Yield Strength, σy =74 MPa . The stress-strain curve does not remain linear all the way to the yield point. The proportional elastic limit (PEL) shown in Figure 4 is the point where the curve starts to deviate from a straight line. The elastic limit (frequently indistinguishable from PEL) can be seen in Figure 4 is the point on the curve beyond which plastic deformation is present after release of the load. If the stress is increased further, the stress-strain curve departs more and more from the straight line. This curve is typical of that of many ductile metals like Al that we used in our experiment.
e. Calculation of the percent reduction of area, %RA The %RA is given by
%RA=100.A0-AfA0 5
%RA=100.49,5908-13,308149,5908 %RA=73,7%
f. Sketch of the fracture surfaces of ductile materials As can be seen in Figure 6 ,it shows the macroscopic differences between two ductile specimens (a,b) and the brittle specimen (c).
Figure 6 fracture mechanisms
Figure 7 sequence and events in necking and fracture of a tensile test specimen: (a) early stage of necking; (b) small voids begin to form within the necked region; (c) voids coalesce, producing an internal crack; (d) rest of cross section begins fail at the periphery by shearing; (e) final fracture surfaces, known cup and cone fracture.
On the microscopic level, ductile fracture surfaces also appear rough and irregular. The surface consists of many microvoids and dimples. Figure 8 and Figure 9 demonstrate the microscopic qualities of ductile fracture surfaces.
Figure 8 ductile fracture surfaces
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Figure 9 ductile fracture surfaces
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