ULTRASONIC EVALUATION OF PAINT CANISTERS

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Introduction
Canisters are cylindrical containers used to store entities like fragile items, chemicals, or liquids etc. [1].Most of the canisters are made either of metal or plastic materials.Usually, these canisters are opaque in nature.Once the canister is filled with the required quantity, it is sealed.Hence, it is difficult to check or visualise the condition of the entities inside a canister.To use or check the condition of the inside material, one needs to break the seal.*Corresponding author: amireddykiran@gmail.comThis work focuses on how to examine the state of objects inside sealed containers without breaking their seals.This will ensure a rapid quality check without causing any damage to the canister [2][3][4][5][6][7][8].

Background
In this work, we analysed the data from a commercially available (Asian paints [9]) paint canister with the following dimensions: height: 160 mm, diameter: 105 mm, and the thickness of the canister is 5 mm.The image of the paint canister is shown in figure 1.

Figure 1. An image of the paint canister
Different NDT Techniques like, Ultrasonic and Thermal imaging are used for the examination of bad (dried) paint canister and the corresponding results are shown in figure 2. But these two techniques are not revealed any information as the inspected canisters are good or bad.This is due to the storage and loss modulus of the paint is almost same (very negligible difference) at room temperature [10][11][12][13] and hence the images of the good and bad canisters are looking same.This paper seeks to develop an NDT technique to evaluate a paint canister.This paper is organised as follows: it starts with an introduction to canisters and their applications, followed by background, usage of different NDT techniques for paint canister evaluation.Then it explains the experimental investigation of ultrasonic time of flight (TOF) measurements in two different configurations, followed by its results and discussion.Finally, this paper concluded with a scope for future inspection and modifications.
Once the ultrasonic imaging technique was used, TOF measurements were performed on the paint canisters.The TOF measurements clearly differentiated the good and bad (weather- reacted) paint.Thirty samples were considered for the experimental analysis.Out of which 10 are in very good condition (fresh samples in which the paint is in liquid form), 10 are semi-weather reacted (in which the paint is in semisolid form) and the remaining 10 are fully weather reacted (i.e., fully damaged or dried, in which the paint is in solid form) as shown in figure 3.

Figure 3. Images of the good and bad paints
All the three categories (good, semi or moderately reacted, and fully damaged canisters) were examined with an ultrasonic test (through transmission mode) and TOF was noted for the same at three different locations (top, middle, and bottom) of the canisters.

Experimental investigation
To examine the paint canisters, ultrasonic pulse velocity measurement equipment (Proceq Pundit Lab (+), a commercially available device on the market, in transmission mode was used.The probe used for the experiments along with the setup is shown in figure 4. The central frequency of the probes used for the experiments is about 82 kHz.

Results and Discussions
The TOF from the paint canisters is monitored for all the above cases and recorded in tables 1 and 2 below.As seen in table-1 for case-1, the average TOF for fresh canisters is about 44±0.6 µs.The average TOF for semi weather reacted is 41±0.8µs and the average TOF for fully reacted canisters is observed at 38±0.5 µs.The TOF of different canisters is different, which clearly differentiates the fresh and reacted canisters.However, the TOF difference between fresh and completely dried canisters is about 6 microseconds only.In order to clearly differentiate these paint canisters and optimise the time of inspection, the case-2 configuration is used for experimental inspection.
In case 2, the TOF for each canister at the top and bottom places is different, but the TOF at the middle is almost the same (provided the canister is uniformly reacted).Hence, the middle reading of each canister is neglected for analysis and an average of the top and bottom readings is taken for consideration.From the results inferred from table-2, the average TOF of a fresh canister is about 54±0.3 µs.The average TOF for semi weather reacted is 49±0.5 µs and the average TOF for fully reacted canisters is observed at 40±0.4 µs.The TOF of different canisters is different, which clearly differentiates the fresh and reacted canisters effectively compared to that of case-1.For fresh and fully weathered canisters in case-2, the TOF difference is about 14 microseconds, which is only 6 microseconds in case-1.The TOF measurements in the case-2 can be very useful in finding the quality of paint canisters.The inspection time for case-2 is much less than that of case-1 because the transmitter position is fixed and only the receiver is moving from top to bottom.So, the cross configuration of inspection (case 2) is the best way to (optimum) test method for any kind of paint canister.

Conclusions
The following conclusions were drawn from the experimental investigation of paint canisters: 1. Demonstrated an NDT test method for quality inspection of paint canisters without breaking their seal.
2. Ultrasonic pulse velocity testing is used for finding the time of flight of a given canister, and this can be used for its quality inspection.
3. For fresh and fully weathered canisters in case-2, the TOF difference is 14 microseconds, which is only 6 microseconds in case-1.So, it was demonstrated that the case-2 configuration i.e., Cross configuration, is ideal for canister inspection.
4. The TOF values determined from Cross configuration are 54 µs for fresh samples, 49 µs for semi weather reacted, and 40 µs for expired paint canisters.
5. Based on TOF values, it can be concluded that, if TOF is less than or equal to 49 µs, it's reasonable to assume that the canister's contents are defective.
The results proved that this ultrasonic (TOF) testing method can be a potential testing method to evaluate the quality of paint canisters without opening their seals.The Cross configuration test method can also be implemented for adhesives and liquid canisters and is also used in industries for bulk inspection of canisters to know the status of the contents of contents in less time.

Figure 4 .
Figure 4. Experimental setup along with the probe used for the inspection.

Figure 5 .
Figure 5. an illustration of the different configurations used for the experimental investigation of a paint canister.

Table 1 .
Time of flight variation of different canisters for probes opposite position (case-1) configuration

Table 2 .
Time of flight variation of different canisters for probes cross position (case-2) configuration