Paddy Fields
Well-Known Member
For critical structural components made from GRP (e.g. keel), is NDT common at all in the recreational yachting industry?
NDT (nondestructive testing) of GRP (glass reinforced plastic)
- Thread starter Paddy Fields
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D
Deleted member 36384
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Moisture levels in GRP is common, usually for prepurchase survey; ultrasonic has been used to determine lamination defects, but it is not common. By far the most common method is visual examination for surface indications such as cracks, crazed, deformities, blisters or other irregularities.
Paddy Fields
Well-Known Member
If external impacts, such a grounding, causes stress related damage to the surface of the grp (at the point of high stress), hence allowing moisture to ingress and create a visible flaw, then that might make sense.
grumpy_o_g
Well-Known Member
It's very difficult to do compared to a lump of steel of similar. GRP could have stress fractures in it and still retain adequate strength so crack testing is difficult (though not impossible) always assuming you can get at the bit you need. The gel coat is totally cosmetic but can be a handy indicator that the structure has flexed a bit more than normal. Having said that the benefit of GRP is that is possible to hack it open to check for internal damage in the case of a grounding or similar as it can be repaired to the strength as the original. I'm not aware of x-raying being used outside of the aerospace where it is sometime used for composite structures. Crack testing is sometimes done on carbon-fibre too but these are for very controlled and precise critical components.
The real problem is knowing what's not OK - a stainless steel frames simply shouldn't have any cracks or corrosion - any thing detected warrants investigation and careful analysis to make sure it's both with limits and that it doesn't propagate any further. The same is true of a precise composite mounding. Trying to find whether or not the hull of your Nick 35 is still OK after a hard grounding is much trickier - you've got a probably over-engineered and very thick piece of fibreglass to true and even if you could work out what's going on inside the rovings then you'd still have to know whether or not you found made it unsafe or not.
The real problem is knowing what's not OK - a stainless steel frames simply shouldn't have any cracks or corrosion - any thing detected warrants investigation and careful analysis to make sure it's both with limits and that it doesn't propagate any further. The same is true of a precise composite mounding. Trying to find whether or not the hull of your Nick 35 is still OK after a hard grounding is much trickier - you've got a probably over-engineered and very thick piece of fibreglass to true and even if you could work out what's going on inside the rovings then you'd still have to know whether or not you found made it unsafe or not.
PeterV
Well-Known Member
I spent a lot of time investigating NDT methods for GRP in the defence industry and didn't find a single one where the results could be relied upon. There are some which are useful in very specific applications during manufacture but the changes in properties during service mean that interpreting changes is pretty much impossible. I concluded that good design and following detailed manufacturing processes were far more important.
Paddy Fields
Well-Known Member
I only have a passing familiarity with the fundamentals of NDT.
Metal is a homogeneous material. When it begins to fails, it deforms plasticity, meaning cracks stay open, and I’m guessing that makes it easier to detect damage.
I’d guess the main problem is that composites are by definition not homogeneous, and also possibly contains small air pockets when manufactured.
I’m also guessing that when there are delaminations, the void created by a delamination may “spring closed” if not under load, so I’m guessing you only get to detect old damage that have been there for a while, and had a chance to opens up more.
I was looking at how the wind turbine industry joins the grp blades to the steel hubs, and it makes interesting reading. The design life is 20 years, under almost constant load (ie whenever the wind is blowing).
Metal is a homogeneous material. When it begins to fails, it deforms plasticity, meaning cracks stay open, and I’m guessing that makes it easier to detect damage.
I’d guess the main problem is that composites are by definition not homogeneous, and also possibly contains small air pockets when manufactured.
I’m also guessing that when there are delaminations, the void created by a delamination may “spring closed” if not under load, so I’m guessing you only get to detect old damage that have been there for a while, and had a chance to opens up more.
I was looking at how the wind turbine industry joins the grp blades to the steel hubs, and it makes interesting reading. The design life is 20 years, under almost constant load (ie whenever the wind is blowing).
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D
Deleted member 36384
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Google thermography surveys of GRP yachts. There are people doing this. A crack does not spring shut such that the material properties are reinstated to the pre crack condition, ultrasonic might be able to detect such changes. NDT methods could be used to detect changes over time from a base line survey although it would take a degree of skill to interpret results. I guess it’s just too much off a faff with a degree of uncertainty.
Paddy Fields
Well-Known Member
Since it’s the NDT of the structure surrounding the keel joint that is of primary interest, I’ll go off topic a bit and describe other grp/steel joints.
Wind turbine grp blades are attached to a steel sleeve “interface”. The sleeve is then bolted onto the metal structure of the wind turbine.
The blade is either bonded to the interface sleeve, or compressed between a sandwiched of the sleeve and a metal collar that are bolted together. There may be other methods.
I guess the key is that the interface sleeve is assembled under no load, in factory conditions, and which allows for more scope for designing and assembling a “good” interface. Field assembly involves the simple process of bolting two metal parts together. Would such a joint be easier to test?
Although boat keels are attached in factories, faffing around with keel bolts after the boat has been lifted in a marina counts as a field conditions.
Wind turbine grp blades are attached to a steel sleeve “interface”. The sleeve is then bolted onto the metal structure of the wind turbine.
The blade is either bonded to the interface sleeve, or compressed between a sandwiched of the sleeve and a metal collar that are bolted together. There may be other methods.
I guess the key is that the interface sleeve is assembled under no load, in factory conditions, and which allows for more scope for designing and assembling a “good” interface. Field assembly involves the simple process of bolting two metal parts together. Would such a joint be easier to test?
Although boat keels are attached in factories, faffing around with keel bolts after the boat has been lifted in a marina counts as a field conditions.
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Tomahawk
Well-Known Member
A small hammer and an experienced ear. The same as a wheel tapper.
Solid grp with no cracks makes a sharp sounds. Cracks deadened the sound to a dull thud.
Solid grp with no cracks makes a sharp sounds. Cracks deadened the sound to a dull thud.
