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02/09/1

7

Nondestructive Evaluation of Materials

Abstract

A “nondestructive evaluation of materials” method was simulated using an oscilloscope, pulse-receiver, and a piezoelectric transducer to generate a tension-compression and shear waves. The measure of the waves was then used to calculate: Young’s Modulus, shear modulus, and Poisson’s ratio. The study was performed on four specimens: 1020 cold rolled steel, 6061 –T651 aluminum, cartridge brass, sintered polycrystalline aluminum oxide and polycarbonate. The results of the testing show the effectiveness of nondestructive testing by the accuracy of the calculated Young’s Modulus, shear modulus, and Poisson’s ratio to that of the textbook.

Introduction

In many large processes it is important to manage the equipment without affecting its production output. The best way to analyze most fully functioning machines is to use nondestructive testing. In the process of an air compressor there air easy inspections that can be performed while the compressor is still online. Those are vibration analysis of the drive pulley, gearbox, and motor to verify the bearings are still intact. Another example would be to pressurize the system and perform a pressure test on the storage tank. These are only two examples of the many types of nondestructive testing that exist in industry today. This paper will focus directly on the use of another type of testing called ultrasonic testing.

Ultrasonic testing is utilizes the use of ultrasonic waves. The use of the combination of an oscilloscope, pulse-receiver, and a piezoelectric transducer allow the electronic pulse generated by the oscilloscope to be converted to a mechanical pulse or a vibration. The mechanical pulse is then converted back into an electric pulse and displays on the oscilloscope monitor. The pulses displayed are shear waves (s-waves) and tension –compression waves (p-waves). An s-wave travels at a right angle to the propagated wave direction, where a p-wave travels in the direction of the propagated wave direction. The waves are displayed over a time interval or the “time of flight” which is the time it takes the pulse to travel through an object and back to the receiver. The two waves interact with the molecular structure of the material, which based on the material properties dictates the speed of the wave. Therefore if a material were available in both a polycrystalline and single crystal structure the speeds of the p-wave would vary. This is due to the gaps that are in a polycrystalline structure. The gaps don’t allow for as direct of compression of atoms as that of the crystal structure. Which allows for a faster p-wave in the single crystal.

Experimental Procedure

Each of the four materials was first measured to find the thickness that the piezoelectric transducer would sit on. Then the density of each material was looked up in the Handbook of Chemistry and Physics. The test then began with by placing the transducer onto the material and pressing down with a consistent force to record the time of flight for each material. These steps gave the following data presented in Table 1.

Table

1.

Initial Measurements

Material

Density (g/m3)

Thickness (mm)

Time of flight S-wave (μs)

Time of flight P -wave (μs)

Steel

7.85

19.1

11.8

6.0

Aluminum

2.70

19.33

12.0

5.5

Brass

8.53

53.23

48

24

Alumina

3.98

101.06

34

19

Nylon

1.14

3.31

5.6

2.5

The data collected from the oscilloscope (Table 1.) can be converted from s and p waves to find the Young’s Modulus, shear modulus, and Poisson’s ratio of 1020 Cold Rolled Steel (Steel), 6061-T651 Aluminum (Aluminum), Cartridge Brass (Brass),Sintered Polycrystalline Aluminum Oxide (Alumina), and Polycarbonate (Nylon) through the following sequential equations.

Equations

7
1.

2.

3.

4.

5.

6.

Where:

·

·
·
·
·
·
·
·
·

Table 2. Calculated Data

Material

Cs (m/s)

Cp (m/s)

G (GPa)

λ (GPa)

E (GPa)

v

Steel

3237.28

6366.7

82.27

1.53 x 1011

160.5

0.325

Aluminum

3221.67

7029.09

28.02

7.73 x 1010

7.66 x 1010

0.49

Brass

2217.91

4435.83

4.19 x 1010

8.40 x 1010

1.67 x 1011

0.3335

Alumina

3944.70

10637.89

6.09 x 1012

1.17 x 1013

1.56 x 1012

1.042

Nylon

1180.14

2648

1.59 x 109

4.81 x 109

5.95 x 109

0.3757

Table 3. -Published Data

Material

Cs (m/s)

Cp (m/s)

E (Gpa)

v

Steel

3235.00

5960.00

211.4

0.292

Aluminum

3040.00

6420.00

73.6

0.333

Brass

2110.00

4700.00

104.5

0.342

Alumina

6360.00

10840.00

355.3

0.273

Nylon

1070.00

2620.00

4.8

0.344

Results and Discussion

The extrapolated data shows us that the steel, aluminum, and brass all calculated with in close proximity of the published data. It also shows how the alumina and nylon are off by significant amounts. The large difference between those two can be attributed to their structure and the amount of atomic free space in the material.

In the case of steel, the reason for the consistent data is because of the carbon that is trapped inside the steel as it is cooled. The carbon atoms are trapped as the iron atoms try to form a body centered cubic (BCC) structure. The result is a body centered tetragonal structure, which reduces the gap in the crystalline geometry. [1]

Aluminum exists as a face centered cubic structure (FCC). It similar to steel has little gap space because of its structure. Aluminum’s low elasticity proves true to its accurate numbers because the structure is more flexible or compactable then steel when a vibration occurs the waves transition the aluminum reducing free space and acting like a spring increase the speed of the p-waves giving it the fastest p –wave time of flight.

Brass’s composition of zinc and copper allows for a similar recrystallization process to that of steel. Copper, an FCC structure, during the curing process has zinc atoms become trapped inside the FCC structure, but not all of the zinc become trapped in the structure, it will form alternate lattice structures. Similar to aluminum the brass is malleable from the Zn in it, which allows the structure to deform but it doesn’t act like a spring because of the soft spots from the Zn. [2]

Alumina’s has the highest modulus of elasticity as well as the fastest s and p wave velocity. This is true due to its sintering characteristic. Sintering which takes the FCC structure of aluminum and its mixed composition to merge into each other and delete the gap as it is pressed together. The slow time of flight is a representative of the effects of mixture without a consistent lattice formation when compared to a single crystal formation. This will always be the case when comparing a ceramic to a metal.

Nylon as a polymer forms a hexagonal chain. The polymers connect to one another by the tails off of the hexagonal carbon ring. The resulting configuration forms a zigzag like chain that leaves large gaps between molecules in comparison to that of metallic bonding. In this manner polymers are dissimilar to ceramics and metals. It should also be noted that an error occurred when the study of Nylon’s time of flight was being performed.

Conclusion

In conclusion, the use of “nondestructive evaluation of materials” is an accurate source to identify a material and displays its material properties. The evaluation of 1020 cold rolled steel, 6061 –T651 aluminum, cartridge brass, sintered polycrystalline aluminum oxide and polycarbonate proved to match in relative proximity of the published data points. Deviations in the data can be accounted towards to inconsistent testers operating the oscilloscope, pulse-receiver, and a piezoelectric transducer as well as to the degree of accuracy the system provides on the screen. The samples should all be retested multiple times by the same operator to generate an average of the time of flight in an effort to get more precise and accurate data.

In a separate study performed liquid dye was applied to an aluminum bracket. The dye under ultraviolet light illuminates. After the initial application of the dye and a wipe down of the bracket the light was put over the bracket. The dye’s residual still remained in a crevice that is not identifiable by eye easily. The crevice was a stress fracture at the base of the spindle on the bracket. This is another example of “nondestructive evaluation of materials” as the dye did not affect the chemical or physical of the bracket.

References

1. “Metallurgy of Copper-Base Alloys.” Standards & Properties: Metallurgy of Copper-Base Alloys. N.p., n.d. Web. 09 Feb. 2017.

2. Nov 15, 2002 Machine Design. “Crystal Structure.” Basics of Design Content from Machine Design. N.p., n.d. Web. 09 Feb. 2017.