Impact of repeated overload testing

[BrianBT]

I have been allowed to make an argument to remove frequent (6 monthly) overload testing on a class of Lifting equipment. This would have a very significant impact ( I believe positive) on a large industry sector.

As a lead into this I am looking for examples where such overload testing lead to equipment damage. I am also looking for personnal opions, research papers etc

I am aware that in the mobile crane industry the need for such testing has been removed replaced by targetted inspections.

 

[GregLocock]

I'm not a huge fan of annual testing to some multiple of SWL. If you test to 4 times SWL (common in Oz for man rated gear) you are doing roughly 250 times as much fatigue damage as one cycle at SWL. Having said that it is obviously horses for courses.



Cheers

Greg Locock


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[Artisi]

"impact of repeated overload testing" = failure.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[BrianBT]

many thanks for responses.

It is the accumulative load/fatigue issues that is of concern as some of the systems being tested are 30 Years old plus.
The tests are at 1.25 every 6 months and 1.5 every 12 months SWL.

 

[GregLocock]

What is the typical usage of the system? If it is loaded to SWL once a day or more then those overload tests are only a small hit on fatigue life.

Cheers

Greg Locock


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[BrianBT]

It carries a fixed load which may be considered the SWL. The design load of the system utiilises this fixed load and incorporates a factor of safety based upon dynamic and hydrodymanic effects on the load

I would assumme four deployement per day, 200 days per annum

 

[Walterke]

The point of the test is to make sure your machine won't break during normal work, so if they are getting old, you WANT them to break on the test.
If you're only overloading it 25% the structure should be able to handle it. If it can't, I don't think it's safe to continue using it at 100%.

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[BrianBT]

The dynamic and hydrodynamic effects are substantially greater than the overload rating.

An example, for around a 15 tonne lift the various effects ( including passing the load through a moonpool and thereby having significant water column issues, can lead to a proof load testing( 5 year repeated) of significantly greater than 30 tonne.

Thus if it was a case of detecting failure we would be proof testing every 6 months and not the 1.25

I would suggest the overload tests I am describing are more a function demonstration with phsycological benefits for such as our clients.

My argument will most likely not take the form of any perceived cost benefit or demonstration of gross physical harm by carrying out these tests.

More along of the lines of that I do not see the point. I will argue there are other process we could have in place some of which I have elluded to but also acoustic emission, deflection etc that could providefor something more relevant. I will try to argue that incorrect lift planning and additional load application rigging arrangement can see loads reacted into structures that were not inherently designed for .

Testing of the brakes should be through a dynamic rather than static load test as is currently being required

 

[rb1957]

overload testing is sometimes used in aerospace as a means of validating structure.

the idea is that the load proves a maximum crack size isn't there, and any crack that is there won't grow to critical size before the next test. in the case of the F111 they acually used a cold specimen (cold soaked the plane) so that the toughness under test conditions was lower than under in-service conditions so this reduced the test load level (or reduced the demonstrated crack size).

there is a fatigue impact, however it is generally beneficial ! an occassional overload sets up compression at the crack tip which retards future crack growth. It is a bunch of work to figure out what crack size you're proving, and crack growth under service conditions, proven material fracture toughness, etc; so much that i'd hazard a guess that it hasn't been done. i think the present rule is along the lines of "from past experience, if we do this testing we haven't had any in-service failures". a question might be have you had any failures under test ?

two words of caution ...

1) if the fatigue life of the specimen assumes repetitive testing, i think you'll be unconservative in removing this testing; that you'll be removing the beneficial compression.

2) what are the regulatory requirements ?