Blade manufacturing story
Resin technology has progressed a great deal since the early days of composites in the 70s. Much of the experience that the older crowd has with carbon composites is with older fibers and first generation resin systems - combined made for some pretty spectacular failures due to the stiffer and lower strain capability of those materials.
Nowadays, most carbon composites leverage elastomer toughened resin systems that makes the matrix more compliant in concert with the high and ultra high modulus fibers - yielding (no pun intended) composite structures are are very strong and lightweight without being as "brittle" and flaw sensitive as the old days.
NDI technology is also rather impressive today, ranging from handheld trandcucer based pulse echo to full water tables, x-ray, computer tomography, laser shearography to name a few. That said, mapping voids in thin laminates or skin to core is still most easily and directly done with a tap hammer and a well trained inspector.
Nowadays, most carbon composites leverage elastomer toughened resin systems that makes the matrix more compliant in concert with the high and ultra high modulus fibers - yielding (no pun intended) composite structures are are very strong and lightweight without being as "brittle" and flaw sensitive as the old days.
I wonder about the state of the art in NDT
NDI technology is also rather impressive today, ranging from handheld trandcucer based pulse echo to full water tables, x-ray, computer tomography, laser shearography to name a few. That said, mapping voids in thin laminates or skin to core is still most easily and directly done with a tap hammer and a well trained inspector.
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
The major issue with bonding in blades is with the methodology that establishes the tolerable defect limits for bonded joints. Typically a damage tolerance analysis is undertaken and/or testing is undertaken to demonstrate that the structure can sustain the required loads in the presence of a defect of a certain size, and then NDT is used to demonstrate that all defects are smaller than that size. This valid for PRODUCTION defects, but may not be valid for defects that occur in service.
Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.
Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.
The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.
The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.
Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.
Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.
The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.
The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.
Radiography, either film type or real time fluoroscopy, has been known to find bond voids in a bonded structure, provided the adhesive is of sufficient radiopacity. That one was quite a surprise!
Automated vision systems are used to optically verify the fiber orientation of individual plies during layup.
Rap tests, or some variation thereof, which determine the natural frequency of a structure, will likely be used at some point in production and repair, if not being done now. The information could be used to verify the correct assembly was performed, the correct stiffness and mass distribution was achieved (for balance purposes), and to assess possible degradation in blades being repaired.
DS
The major issue with bonding in blades is with the methodology that establishes the tolerable defect limits for bonded joints. Typically a damage tolerance analysis is undertaken and/or testing is undertaken to demonstrate that the structure can sustain the required loads in the presence of a defect of a certain size, and then NDT is used to demonstrate that all defects are smaller than that size. This valid for PRODUCTION defects, but may not be valid for defects that occur in service.
Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.
Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.
The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.
The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.
Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.
Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.
The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.
The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.
1. Blade designs always include bond test coupon areas, typically within the molded assembly but outside the final machined EOP (as well as prior in the constituent detail parts), that contain all representative adhesive interfaces and laminate structures. Short beam shear, lap shear, block tension, and myriad other destructive tests are performed on these coupons to verify the bonding prep, process, and other specifications were met. It is at this point that defects like porosity and slick bonds (which you mention are undetectable by NDI, which is mostly true) are detected as a requirement prior to acceptance.
2. With respect to moisture susceptibility and "hydration" affecting bond lines, and the inability to reliably detect this, you comments on design allowables omits the fact that responsible manufacturers of composite bonded blades typically use a set of more stringent hot/wet allowables that are derived from destructive testing for precisely this sort of concern.
2. With respect to moisture susceptibility and "hydration" affecting bond lines, and the inability to reliably detect this, you comments on design allowables omits the fact that responsible manufacturers of composite bonded blades typically use a set of more stringent hot/wet allowables that are derived from destructive testing for precisely this sort of concern.
The failure mode that blakmax describes is not addressed by the hot/wet environmental qualification testing you reference. The hydration mode is an adhesive failure mode at the metal/ adhesive primer interface, initiating at an edge and propagating along a line, creating a disbond. It is driven by exposure to moisture, calendar time, and to a lesser extent, loads.
As you know, the environmental qual tests that establish hot/wet design allowables for structural adhesives strive for a cohesive failure mode, to develop the full strength of the adhesive, with the presumption that the interfaces and adherends will always be stronger than the adhesive. If an adhesive mode failure mode is encountered in such tests, the data would be suspect and the failure surfaces carefully scrutinized. Of course, a failure could also indicate a fundamental problem with the adhesive/primer system, at least in a particular application.
Wedge tests can sometimes reveal the hydration mode, but the exposure time for the test, say up to a month, is orders of magnitude shorter than the years required to create a hydration mode in typical field usage .
To address your concern, the degradation does not occur in a bulk or volumetric sense. It requires an exposed edge that allows moisture ingress, so it is more of a localized and linear initiation. The resulting disbond is detectable by traditional NDI methods, presuming of course it is accessible and is of sufficient size. Propagation is slow, so it can be managed by periodic maintenance.
DS
I hate to admit that the last time I opened a Physics textbook was in college - one half-century ago! I have about five hours right seat time in a Bell 206 LR and was always fascinated at the exquisite design as well as inestimable strength of helicopter blades. Too lazy to look up the necessary equations to calculate load distributions, I found the following on Copters.com: "The rotating blades of a helicopter produce very high centrifugal loads on the rotor head and blade attachment assemblies. As a matter of interest, centrifugal loads may be from 6 to 12 tons at the blade root of two to four passenger helicopters. Larger helicopters may develop up to 40 tons of centrifugal load on each blade root. In rotary-wing aircraft, centrifugal force is the dominant force affecting the rotor system. All other forces act to modify this force."
No wonder these magnificent machines are known as "frantic palm trees"! I am astounded that they hold together as well as their fixed-wing cousins, which, as we all know, are simply "groups of rivets flying in close formation"!
- Ed
For the Geeks among us:
If you know the velocity of the object, simply use the following formula:
where:
, you can recalculate it to normal velocity by simply multiplying it by the circumference of the circular path. Use the following equation:
in case your ω is in Hz (1/s).
Or the formula:
for ω in rad/s
Time to grab a beer....
No wonder these magnificent machines are known as "frantic palm trees"! I am astounded that they hold together as well as their fixed-wing cousins, which, as we all know, are simply "groups of rivets flying in close formation"!
- Ed
For the Geeks among us:
If you know the velocity of the object, simply use the following formula:
Code:
F = mv²/r
- is the force (expressed in newtons);Code:
F
- is the mass of the object;Code:
m
- is the velocity; andCode:
v
- is the radius.Code:
r
Code:
ω
Code:
v = ω2πr
Or the formula:
Code:
v = ωr
Time to grab a beer....
Join Date: Aug 2006
Location: California
Posts: 78
Likes: 0
Received 0 Likes
on
0 Posts
It is interesting that centrifugal force is what gives the blades the strength to lift the aircraft. If you went to a stationary chopper and tried to lift it via the blades, they would bend right outa shape, but spinning around gives them strength. The vectors sort themselves out in beautiful style - the lift vector acting at 90 degrees from the blade, making it cone up, and the centrifugal force, depending on RRPM, acting straight out from the blade, trying to flatten the cone - the resultant is right along the blade.
A bit like a piece of nylon fishing line cutting through grass on a whipper snipper. Magic.
A bit like a piece of nylon fishing line cutting through grass on a whipper snipper. Magic.
