Sustained Load Cracking & the VisualPlus 3
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Some of the aluminum tank manufacturers, such as Luxfer, produced tanks at one time with an alloy that sometimes allowed cracks to develop in and around the threads of the tank. These cracks are called Sustained Load Cracking (SLC). Many of these cracks are invisible to the inspector's eye. They will be detected by a device known as the VisualPlus 3.
This is a view from inside the tank looking up at the threads. Notice at the 12 o'clock position there is a neck crack that is starting to grow up through the threads.
The 2 pictures above are the threads in the same tank. This is a view from the top outside of the tank. This tank had 2 neck cracks. One of those cracks had actually migrated through the tank wall and produced a leak of air at that spot!
In the center of the above photograph is the exterior view of the neck crack's exit on the outside of the tank! One can see where the black paint was blown away leaving a small crack visible.
Deep-Six's VisualPlus 3 was used to analyze this tank. Here are the results:
The left graph above shows threads 2, 3, 4, and 5. (The threads are measured from the bottom to the top of the threads.) The purple is thread 2, green 3, blue 4, and red 5. Notice each of those graphs clearly shows the 2 neck cracks passing through each thread.
The above graph shows all the threads from the bottom to the top. Each thread is separated by a small tick mark on the horizontal lines running from left to right. The center top two are the 2 cracks in thread 2. Notice that the magnitude of the neck crack diminishes as the upper threads are approached.
This is a graph of a scuba tank neck threads that had no Sustained Load Cracking. Notice there are no spikes in any of the threads.
VisualPlus 3 is able to show the user various views of the neck:
The picture above shows a tank that had no SLC.
The picture above shows one of the 2 neck cracks as projections from the center threads. The projections (crack) gets smaller from the inner most threads of the tank upward. We will now rotate the view so you will be able to look at the threads from the top down:
In the above photograph the center green area represents the threads. Notice the 2 projections, each representing one neck crack.
Above is a depiction of each thread showing the extent and position of the cracks. Notice as you approach the threads near the top of the tank the size of the cracks diminish.
The top photograph is another analysis of the threads looking down. VisualPlus 3 presents many views of the results to allow the operator to visually support the findings.
VisualPlus 3 will generate a complete report on the condition of a tank's threads. This is the report on the black tank with the 2 neck cracks:
In December 2004 Deep-Six tested 21 aluminum scuba cylinders. Six of those were found to have neck cracks using the VisualPlus 3. Not wanting to condemn cylinders unnecessarily, we would take the failed ones, clean the threads with brushes, detergent, and hydrochloric acid. No matter what was tried, the VisualPlus 3 consistently located the sustained neck cracks every time. When the cylinders were visually inspected most (but not all!) of those cracks became evident to our trained VTI personnel. All the failed tanks were well within the hydro due dates.
An analysis of the tested tanks in December 2004 (Serial Number followed by date of manufacture and disposition of VisualPlus 3):
Deep-Six does not sell VisualPlus Inspection devices nor are we, in any way, connected to the company. We use the VisualPlus 3 for our aluminum tank VTI's and have found it to be quite satisfactory. We will provide you with the company's location in case you wish to obtain additional information:
Advanced Inspection Technology
14020 Eagle View Road
Lake Matthews, CA 92570
800-611-9077
www.visualplus.net
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Statements from Luxfer about their cylinders:
Luxfer scuba cylinders were manufactured from 6351 aluminum alloy during the following periods:
Luxfer requires that every Luxfer 6351-alloy all-aluminum scuba cylinder be visually inspected at least every 2.5 years by a properly trained inspector. As part of this inspection, Luxfer further requires that the cylinder neck be tested with an eddy-current device such as Visual Plus™, Visual Plus 2™, Visual Plus 3™,Visual Eddy™ or equivalent non-destructive testing equipment. When properly used, eddy-current devices contribute significantly to early detection of difficult-to-observe sustained-load cracks in the necks of 6351-alloy cylinders. If the cylinder passes the inspection, the inspector will document that fact. If the cylinder fails the inspection, it must be removed from service immediately. Do not use Luxfer 6351-alloy scuba cylinders that have not been both visually inspected and eddy-current tested and then properly documented.
This Luxfer-required visual and eddy-current inspection is in addition to periodic requalifications (including retesting and inspections) required or recommended by various regulatory agencies around the world. The intervals between retests and inspections vary from country to country.
In addition to the required inspection and testing described above and in keeping with U.S. scuba industry standards, Luxfer recommends that all Luxfer 6351-alloy scuba cylinders be visually inspected at least once each year by a properly trained inspector.
For cylinders in heavy use (for example, those filled five or more times a week), Luxfer recommends visual inspection every four months. For more information, refer to Luxfer’s Scuba Cylinder Inspection Guide, which is available by calling Luxfer at 800-764-0366 or by visiting the Luxfer web site at www.luxfercylinders.com.
Inspection and Testing of Luxfer Scuba Cylinders Manufactured from 6061 Aluminum Alloy
Luxfer began manufacturing scuba cylinders from a proprietary 6061 aluminum alloy in mid-1988 in the United States , in 1995 in England and in 1990 in Australia . The majority of Luxfer scuba cylinders currently in service are made from this proprietary 6061 alloy, which Luxfer is still using.
Cylinders made from Luxfer’s proprietary 6061 alloy are not susceptible to sustained-load cracks. Therefore, Luxfer does not recommend the use of earlier-generation eddy-current devices—e.g., Visual Plus, Visual Plus 2, and Visual Eddy—with Luxfer 6061-alloy cylinders because field experience has shown that using these devices can result in misleading “false-positive” readings and erroneously condemned cylinders.
However, Luxfer does approve the use of the Visual Plus 3™ eddy-current device with Luxfer 6061-alloy cylinders. After laboratory testing, Luxfer has determined that properly calibrated Visual Plus 3 equipment eliminates the “false positive” problem, accurately measures features in the neck area of aluminum cylinders, and generally provides correct pass or fail assessments with cylinders made from Luxfer’s 6061 alloy. As of the date of this Luxfer Policy Statement, Visual Plus 3 is the only eddy-current testing device suitable for use with Luxfer 6061-alloy cylinders.
Luxfer emphasizes that the use of eddy-current devices to improve the quality and accuracy of inspections does not eliminate the need for a thorough visual inspection by a properly trained and qualified inspector.
In keeping with U.S. scuba industry standards, Luxfer recommends annual visual inspection of Luxfer 6061-alloy cylinders by a properly trained inspector. For cylinders in heavy use (for example, those filled five or more times a week), Luxfer recommends visual inspection every four months. These recommended inspections are in addition to periodic requalifications (including retesting and inspections) required or recommended by various regulatory agencies around the world. The intervals between retests and inspections vary from country to country.
For more information, refer to Luxfer’s Scuba Cylinder Inspection Guide, which is available by calling Luxfer at 800-764-0366 or visiting the Luxfer web site at www.luxfercylinders.com.
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