Microsoft Word - Lab-01

Ryan M. Sullenberger Engineering Research Center Colorado State University 1320 Campus Delivery Fort Collins, CO 80523-1320 Attn: Subject: Re: Date:

John McWilliams Tensile Tests for 3 Metals and Hardness Testing Test Results February 12, 2009

Dear John McWilliams, We have received your request to test three metal specimens including hot-rolled steel, cold-rolled steel and aluminum that you obtained from your local extruding company. We performed the hardness tests and the tensile tests using our Instron 4400R. Our results have been included in tabular and graphical form. Axial testing works by applying an increasing uniaxial tensile load to a material of known cross sectional area and length. Tensile stress is calculated by dividing the applied load by the area of the specimen. The strain of the material is calculated by dividing the elongation of the test piece by its original length. These two quantities are then graphed together as something that is referred to as a stress vs. strain curve. The stress vs. strain curve of a material can tell us many things: the modulus of elasticity is the slope of the initial linear portion of the curve, ultimate stress can be chosen as the maximum stress shown, and the yield stress is found by observing the highest stress region at the end of the initial linear portion. For our experimental results dealing with Modulus of Elasticity, we used strain data provided by a strain gage. This was used to more accurately measure the slope to obtain the Elastic Modulus. Hardness testing is done by making an indentation into a material. Rockwell Hardness Testing is done by comparing the depth of penetration of a large load to that of a preload. We performed Rockwell B testing on the materials you sent us, which uses a 1/16 inch diameter sphere as the indenter. The RHB value obtained for the materials were used to find tensile strengths. The stress vs. strain curves we obtained for the three metals have been included in this letter and can be found in Figures 1-3. Please refer to Table 2 for a list of our numerical experimental results for the three samples you provided us. Table 1 contains all the expected values for the metals, and Table 3 shows the percent error between expected and experimental data. For all three metals our experimental values yielded higher ultimate stresses and yield stresses compared to the expected values. There are two major deficiencies that occurred during testing that I would like to point out. When we tested the cold-rolled steel the sample failed very close to the jaws of the machine. This may have happened out of chance, but mostly happens because the jaws apply a compressive force to the sample, so where the material meets the jaws the sample is not in pure tensile stress. We believe there is little to worry about in this case because the ultimate load was reached before the sample fractured.

1

Table 1: Expected Values Elastic Modulus (psi)

Yield Stress(ksi)

Ultimate Stress (ksi)

3.00E+07 3.00E+07 1.00E+07

36 65 21

58 80 30

Hot-Rolled Steel Cold-Rolled Steel Aluminum

Table 2: Experimental Values Elastic Modulus (psi)

Yield Stress(ksi)

Ultimate Stress (ksi)

3.12E+07 2.87E+07 8.80E+06

47.4 100.5 33.1

64 115.6 38.6

Hot-Rolled Steel Cold-Rolled Steel Aluminum

Table 3: Percent Error Elastic Modulus (psi)

Yield Stress(ksi)

Ultimate Stress (ksi)

4.05% -4.23% -12.05%

31.67% 54.62% 57.62%

10.34% 44.50% 28.67%

Hot-Rolled Steel Cold-Rolled Steel Aluminum

Table 4: Rockwell Hardness & Ultimate Tensile Strength

Hot-Rolled Steel Cold-Rolled Steel Aluminum

Rockwell B 50.1 111.8 84.3

2

Ultimate Strength (ksi) <56 >116 81

140000

120000

Stress (psi)

100000

80000 Aluminum 60000

CRS HRS

40000

20000

0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Strain (in/in)

Figure 1: Stress vs Strain for Al, CRS, HRS

50000 y = 3.12E+07x - 5.99E+03 45000 40000 y = 2.87E+07x - 4.25E+03

Stress (psi)

35000 Aluminum

30000

CRS

25000 y = 8.80E+06x + 5.74E+02 20000

HRS Linear (Aluminum)

15000

Linear (CRS)

10000

Linear (HRS)

5000 0 0

0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 Strain (in/in)

Figure 2: Stress vs Strain for Al, CRS, HRS, linear region

3

The experimental results were not exactly in line with the expected values, but this is not unheard of. Variations in results can be caused by factors dealing with the manufacturing processes of the metals, where the expected values are given as a standard for that specific material. Due to this, it is not uncommon to come across samples of a material that have greater ultimate stress and yield stress than advertised. We recommend if you are using these materials in high sensitive or high stress environments that you obtain another three samples to be tested, just for good measure. It has been a pleasure serving you and we hope you consider us in the future. Sincerely,

Ryan M. Sullenberger Project Engineer (Student)

4