Hexacor compared with other composites?

[kengrome]

http://www.hexacor.com/images/product_1.jpg http://www.hexacor.com/images/product_9.jpg

I've been discussing Hexacor core material with the manufacturers, but I do not know how to compare it with other composite core materials, nor do I know if it will work as a hull material. I welcome input from anyone who can evaluate the technical info they provide, or just compare it with other products on the market. Their web site is http://www.hexacor.com/ (http://www.hexacor.com/) and here's an excerpt that provides the basics:

The Hexacor Polypropylene Honeycomb is the next-generation polypropylene honeycomb that is the strongest on the market today. With its all-round superior properties and competitive pricing, it is fast becoming a popular composite core material for increasing strength and stiffness at reduced weight.

Hexacor is the polypropylene honeycomb of choice for the following reasons:
Strongest in the market
Thinnest in the market
Most technologically advanced:
Honeycomb panels are manufactured as a single sheet and not by joining smaller sheets together
Honeycomb panel surfaces are completely flat and without trapped air, which facilitates lay-up
Low resin wastage
Skin facings are strongly fused onto the honeycomb core, hence no known delamination issues
Available in different grades of strength Unique Cell Matrix

Hexacor polypropylene honeycomb are made up of cell structures that are round rather than hexagonal, more closely resembling honeycomb structures that exist in nature. The round cell structures give the resulting cell matrix 3 orientations vs. the 2 orientations in hexagonal matrices. As a result, Hexacor polypropylene honeycomb properties are more uniform in all directions across the entire sheet of honeycomb.

Superior Quality

Hexacor polypropylene honeycomb cells are fused together rather than glued. Without the presence of glue as the "weakest link" in the honeycomb matrix, Hexacor polypropylene honeycomb sheets are more resilient.

Every cell in a Hexacor polypropylene honeycomb is very stable. Hexacor's manufacturing technology allows it to produce honeycomb cells that are extremely consistent regardless of any honeycomb sheet size.

[Alik]

You should look at shear strength of hexacor.

We are using hexacor for sides, cabins, bulkheads, acommodation bulkheads and also for bottoms of displacement boats. For hull, we prefer to give some extra thickness to skins to ensure the properties. We also have experinece of vacum bagging hexacor honeycomb - very positive.

[masalai]

I am not sure which it is but something similar can be "shaped" with the heat from a hair-dryer to soften it - this may be a disadvantage in areas where heat is an issue - engine rooms, on deck in the sun etc

Just a wild guess and some lateral thinking in relation to lay-up & forming ready to lay-up. I would be happy to be howled down by the knowledgable if I am wrong... All a learning experience ... Thanks.

[juiceclark]

Sorry to pop-in with a rookie question: but how much lighter can it be than the highest grade plywood after all the little holes are full of resin?

Tony in Sw FL

[masalai]

Not so Juicy. Air... The little sample I have is as light as a feather. in grams/square meter - I forget... My sample has fine woven glass on each side. At ambient (26 deg Celsius or less) is quite strong - good capacity to carry people when placed between 2 saw horses... (sorry three times I tried to correct that and typed "sore whores", now I can correct it:D :o:) Again no numbers - I think my sample is called "nidaplast"

[allan white]

Alik,

How to you stop the resin from filling the cores while vac infusing?

A

[kengrome]

Alik, how to you stop the resin from filling the cores while vac infusing?It appears that you can get it with a PP skin over the holes, which means the resin cannot get inside. Look on this page:

http://www.hexacor.com/products/codes.htm

... and you'll find this description of one of their products:

PP-8H-2 would be polypropylene honeycomb, 8mm cell diameter, high strength grade, with polyester veil and polypropylene film barrier

[kengrome]

You should look at shear strength of hexacor.Thanks Alik. Unfortunately my problem is that I don't know how much shear strength is required ...

I can compare this stuff to plywood for example, but if I see that Hexacor's shear strength is half that of plywood, does that make it "strong enough" for the same boat hull I could otherwise build in plywood?

This is the kind of question I run into when I compare materials I've never used before with materials I have used extensively.

[Alik]

2allan white: I have never saild we infuse this core. We vacum bag it. The fabric on both sides of honeycomb prevents resin form filling the cells.

2kengrome:
When one calculates sandwich structures the shear strength is critcal. With plywood it is different story.

[kengrome]

When one calculates sandwich structures the shear strength is critcal. With plywood it is different story.This makes it even more difficult for me then, doesn't it? Is there a rule of thumb I can use that would answer a question like this:

"What is the minimum sheer strength that I should accept when using Hexacor as the core hull material for a moderate speed planing power boat?"

[Alik]

Do not use hexacor on the bottom of planning powerboat. Other areas have to be claculated considering particular structure, panel size, boat size, etc. Please refer to ISO12215-5 as guideline. There is no easy universal answer... we do it for every boat.

[rambat]

Its not good to put open cell core of any kind in a hull bottom, The core face laminate is too thin when calculated as needed to prevent water migrating into those cells. If you make the outer laminate thick enough to prevent migration it won't require a core. We used it once in a 50' offshore race boat, temperature differentials and a dry as possible lamination allowed those cores to suck up water like combs full of honey. With less expensive, lighter and stronger foam cores I don't know why they are still promoted in any cored lamination, but be warned about using them in bottoms. Corecell can be heated and formed as well as a hexcell core

[Alik]

2rambat:
This is correct, but for displacement boats we use honeycomb panels to shape the sharp-chine bottom, then we use extra layers on outside skin to prevent water penetration. Rest of boat is a real sandwich.

The primary concern here is that Hexacor is cheap, but foams are damned expensive.

[kengrome]

Its not good to put open cell core of any kind in a hull bottom, The core face laminate is too thin when calculated as needed to prevent water migrating into those cells. If you make the outer laminate thick enough to prevent migration it won't require a core.What kind of resin did you use?

We used it once in a 50' offshore race boat, temperature differentials and a dry as possible lamination allowed those cores to suck up water like combs full of honey. A "dry as possible lamination" seems to suggest that not enough resin was used to properly seal the bottom, which would lead to water infiltration -- especially in a polyester build. I suspect that epoxy might be substantially more waterproof, am I wrong about this?

With less expensive, lighter and stronger foam cores I don't know why they are still promoted in any cored lamination, but be warned about using them in bottoms.I don't know enough about these synthetics to know the answer to this one either, but my gut feeling is that the Hexacor won't fail like some foams might by separating within the core itself.

[Trimariner]

With less expensive, lighter and stronger foam cores I don't know why they are still promoted in any cored lamination, but be warned about using them in bottoms.


IT appears from your statements that you have very little or no knowledge of PP honeycombs! Hexacor PP High strength has a shear of .8 I have subjected the bare core to submersion for months and have experienced no water intrusion into the core,and as to foams being cheaper I suggest you check your facts regarding this also!
On a site named www.austrol.com.au there is an interesting impact video of PP honeycomb, listed under polycore in the menu column! The melt point for polypropylene is 160 c and the skin on the outside is Polyester allowing any resin to be used for laminating, it can be thermoformed by preheating, it has to reach 80 to 85 deg c to soften if a boat was to suffer a sun that was that hot I would certainly not like to be in it!
There are a number of PP honeycombs on the market Hexacor, Polycore,Nidaplast, Plascore, Tubus are a few that are popular Polycore and Hexacor have the Highest shear and compressive strengths so go check the out.

[kach22i]

I have subjected the bare core to submersion for months and have experienced no water intrusion into the core
1. Have you built a 50' offshore race boat using it?

2. What temperature differentials were the test samples exposed to?

3. What water/slamming force pressures were the samples exposed to?

4. What is a "dry as possible lamination" mean? (perhaps this question is directed to rambat).

NOTE: The homebuilt hovercraft built using plastic hex material (not Hexacore) sometimes see the air pockets heated by the sun, which results in the air expanding and or puckering like little quilt pillows.

On the up side, there has been no failure or delamination of the fiberglass coating from the core material, nor any reported water absorbtion. Then again hovercraft are stored on land (covered), and not left floating in the water for very long.

EDIT: Cool slamming picture for ref;
http://www.safehavenmarine.com/GENESIS%2037%20development%20page.htm
http://www.safehavenmarine.com/IMG_8452.jpgp.jpg

[Trimariner]

1. Have you built a 50' offshore race boat using it?

No I haven't, but there have been a number of power cats built here in Australia one 15.6 meters, several 11 meter power cats, numerous 6 and 7 meter power cats, and numerous sailing cats these boats have been subjected to attrocious conditions, wave pounding, violent bar crossings and groundings and have suffered no reported damage, there is also a tunnel hull race boat that has been racing for just over twelve months with no signs of failure!

2. What temperature differentials were the test samples exposed to?

The samples have been subjected to -30c to +125c without any negative effect!

3. What water/slamming force pressures were the samples exposed to?

The test samples were placed in an agitator and pounded for 80 hours and apart from very minor bruising were still intact and had not induce any water intrusion into the core, as to what water/slamming pressures the core has been subjected to I believe that has been answered in my answer to question one!

4. What is a "dry as possible lamination" mean? (perhaps this question is directed to rambat).

To try to explain as simple as possible a dry as possible lamination would be where the resin content is reduced as far as it can without reducing the structural strength, some would consider 40% resin 60% resin a dry as possible lamination, some have different ratios but all would be subject to good engineering!

NOTE: The homebuilt hovercraft built using plastic hex material (not Hexacore) sometimes see the air pockets heated by the sun, which results in the air expanding and or puckering like little quilt pillows.

On the up side, there has been no failure or delamination of the fiberglass coating from the core material, nor any reported water absorbtion. Then again hovercraft are stored on land (covered), and not left floating in the water for very long.

I agree that this has occured with some PP honeycombs, and some have actually delaminated, to the best of my knowledge neither Polycore nor Hexacor have suffered from this problem.

[Trimariner]

My apologies I made a mistake regarding resin glass it should read 40% resin 60% glass.

[kach22i]

Thank you for the answers, this sort of thing is what makes this forum so special for me, I get to learn a lot.

I have a nice 12" x 12" sample from Plascore after I took a factory tour last year. The marble-like factory finish is much more rigid than any of the field applied examples I have seen. I also have several unfinished (non-resin) Nidacore samples which always spur my imagination for possible uses.

I have the Hexacore website bookmarked, I don't ever recall a similar product talking about forming curves with heat before.

Cutting or scoring the material has never appealed to me.

Question:
1. When thermal forming Hexacore, would the inside radius fabric have to be removed to prevent puckering and folds?

2. When thermal forming Hexacore, will the fiberglass fabric on the outside radius be able to stretch without compromising it's bond to the cells?

3. I am in the USA (not China or Australia), would it be possible to have free samples mailed to my office so that I may experiment with the material?

Locations:
http://www.hexacor.com/contact/offices.htm

[masalai]

karch, with Nidaplast, Multihull Haven, up near Cardwell (North Queensland between Townsville & Cairns) were using a female mold and tiewire to hold it whilst heat shaping with a "ladies Hair Dryer" I understand, then glassing the inside, pull it out & when upside down do the outside, roll again for fitout.

I understand the whole operation has moved to China...

[kach22i]

I understand the whole operation has moved to China...
It seems like everything has, how sad for us all.

[juiceclark]

I like the idea of Hexacore and will consider it for the decks and flybridge on our sportfish project. The fabric peeling off while working the material is the obvious concern as mentioned above.

Having some samples would also be of great interest to me. Could you please send 30 to 40 4'x8' sheets so we can determine its usefulness? With the low labor costs in China, that shouldn't be a problem.

Thanks!
Tony in Sw FL

p.s. Have you sold Hexacore to Hampton Yachts there in China yet? If Jeff Chen thinks it's ok...it's ok.

[mikereed100]

While on the Subject of Hexacor....

I am planning on building the cabin and deck of my catamaran with the stuff. Some of the surfaces will require a bend with a 100cm radius. I am told that the way to accomplish this is by glassing the inside surface and then scoring the outside surface, bending to shape and then glassing the outside surface. My concern is that this will create an uneven outside surface that will require much fairing and added weight.

Is this the case or does the outside surface even out as it is bent? Also, at what intervals should I score the outer surface? Does it need to be scored the entire thickness or just the veil/film on the outside? Do the scored cells need to be filled prior to glassing? (I assume not but am not sure).

Lots of questions, I know, but any thoughts would be helpfull.

Thanks,
Mike

[masalai]

Jeees Juiceclark, You want to do your build for free? - I have been told (consequent to similar requests) that there is a tree that does not exist in a deep, dark cave - try barking up that tree - you will be more successful... :D:D:D

[cioko]

can anybody sugest me a literature about kevlar.Or some work published about that?
Thanks.

[masalai]

At the top of this page is a search button, a search on this net should yeild something of value with "kevlar"

[cioko]

Thanks

[keanu916]

I would like to know if any boat manufactuer will be interested in Aluminum Honeycomb Material,as I know Polypropylene is kind of plastic core,and has problem in fireproof,but if you use Aluminum Honeycomb core material,sercurity is higher,am I right?

[cioko]

Well. you can use both materials but the question is whats cheeper.Now, I do not know the exact material properties of dose honeycomb ,aterials but im am sure thet the tensile and shear strenght of the material with aluminim is higer then the other one.

[keanu916]

Talking about the price,has anyone ever considered aluminum honeycomb which is original from China?This is the good solution for price,and if you control good on quality test,maybe you will obtain a better material at the same price you pay for other materials.Will any one give it a shot?

[wjones]

There is a company in the US that manufactures aluminum honeycomb core panels that are IMO, DNV, USCG and SOLAS class-C certified.

www.ayrescom.com

[juiceclark]

WJ,
I had no idea. I've already began figuring what laminates we'll need on their cores for our bulkheads. They owe you a discount! TC

[Edmundo Souto]

PP honey combs are cheaper but foams are easer to use specially when you are infusing a whole hull at once.

In a infusion, you can use vinylester poliester or epoxi resin, but vaccuun bagging you can only use epoxi.

[TeddyDiver]

Does someone have an idea where/how to get some 30m2 in Europe. I sended an info request to Hexacor in China but they just want to mingle..
It's not a big deal but..

[Analyst]

I just wanted to clear up some of the fog on polypropylene honeycomb core materials and what I have found through testing and parametric study for use on high production fishing boats and yachts.

:!: GENERAL CORE DESIGN:

(Not directly concerning polypropylene, but core in general) The proper method of design for sandwich structures is to include the effects of transverse shear deformation, or the sliding of the face sheets due to the shearing component of the core during bending. If displacement is of concern, a low value for shear stiffness (as found in PP and many foam cores) will cause larger deflections in reality, usually much larger than isotropic (uniform properties) equations will give you. Some references would be books by Jack R. Vinson or Chandrashekhara or any modern composite plate and shell text. Hexcell also covers this for (oversimplified, but ballpark) semi-composite analysis in their honeycomb design reference which I believe is still free. Diab also discusses it in their online manual.

:!: Generally the ideal sandwich structure has:

* Thin face sheets (you would be surprised at how thin if you use the correct materials and core height) -- (The major weight and cost savings is dramatic for the proper use of sandwich design). (not necessarily core material dependent, the design philosophy is more important) I have also seen engineers use the same solid laminate and just throw in a core somewhere in the middle, and expect a weight or cost reduction. Strange, huh?)
* A core with extremely low density (PP has at around 5 pcf)
* Low cost (PP is the lowest I've found to date)
* High stiffness (PP is low compared to many foams and balsa, but how much do you need? Comparison studies will tell you what is best if the proper equations are used). It is important to note here that by nature of materials under dynamic loading, high stiffness materials become brittle and softer ones become stiff, so a study should be performed for the specific application to determine the proper stiffness and strength requirement of the core.
* High shear strength (PP is also low in this category, but the question is emphasized as to "how much do you need to not fail?" which is answered through rigorous engineering analysis.)
* High strain elongation to prevent catastrophic failure (PP has a very high strain elongation), formability (Honeycomb is inherently the best at this, but the scrim of PP makes it difficult, but can be scored)
* No resin absorption. (PP absorbs very little)
* Little to no print-through at splice joints (PP was excellent compared to balsa for the same resin content)

:!: POLYPROPYLENE HONEYCOMB:

Therefore PP Honeycomb contains most of the important requirements for ideal core, with the exception of the two major ones: strength and stiffness. However, if proper design (designing to what you actually need instead of some arbitrary standard and using the correct equations to carry out the design) is used, the result is exceptional.

Polypropylene is next to impermeable to water. It is also a recyclable thermoplastic with a higher Tg than many thermoset materials and will melt instead of burn eliminating a major energy source for boat fires.

:( CAUTIONS WITH THERMOPLASTICS AND PP HONEYCOMB:

Thermoplastics are awesome. When I was in aerospace, only the best applications got thermoplastics becaues of their processing expense. They give you the so needed ductility and resilient that thermosets lack and are recyclable, heat formable and cheap. However for combining PP core with thermosets such as Polyester, Vinylester and Epoxy, care must be taken when bonding to thermosets in regions where the scrim has been removed because PP is a thermoplastic. It is effectively a release agent and will not allow chemical bonds to most materials. Ingenuity must be present when bonding at tapered regions, where the scrim is removed. There are methods to do so, such as thermofusing the scrim onto the exposed angled surface with back a vibrating tool.

The key to PP core, is that the increase in height to maintain deflection allowables or reduce facesheet stresses is usually at an insignificant cost and weight increase and a major cost savings and improvement in durability, water resistance and surface appeal foam and balsa. In other words I can achieve the same deflections and stresses as the stiffer materials while also achieving a cost and weight savings by increasing the core height to match the flexural properties.

:confused: * Note that I have not discussed plywood. Why? Plywood is technically not a core material. It is closer to a stack of fiberglass than any core material on the market. Although extremely heavy and disastrous in wet and humid environments, it is a very - very efficient material from a specific strength and stiffness standpoint. Its in-plane stiffness jumps the laminate flexural properties through the roof and its shear stiffness properties put even the best balsa to shame, while maintaining in the range of one-third to two-thirds the weight of fiberglass. So by comparison with other core it is too heavy to be used as an efficient core material and by comparison with fiberglass too environmentally unstable to be exposed. As a result, I don't endorse plywood as a good material for structural use in wet environments for dynamically loaded "lightweight" products such as boats.

:confused: THERMAL STABILITY?

An issue that I am concerned with and looking into is thermal degradation effects. It has been observed that while ambient air temperatures do not reach 180 degrees F under UV exposure, insulated surfaces such as FG can, (ever burn you feet on hot sand, or on the pavement or on the boat floor?). The mechanical properties as a function of T could play an important factor in the continued use of PP on UV exposed regions. I have been told that it can be autoclave cure with prepregs, which tells me that it should be mechanically sound at these temperatures, but I wonder if anyone has any information or reference about this topic?

I could say more but as long as this thing already is would just be rude.

Thanks ;) :cool: :eek:

[Edmundo Souto]

Did you noticed DNV DO NOT allow any PP honeycomb on decks and sides...??? Check DNV approval sheet for PP core....

[kengrome]

What is DNV?

[Edmundo Souto]

It´s independent international company of classification,certification and consulting for materials, construtions ....etc
Used as reference speacially in boatbuilding and others...


DNV - Services for Managing Risk

www.dnv.com/industry/maritime/

[Analyst]

Edmundo Souto:

Yes, I have heard (through a PP HC core supplier) that DNV has created a restriction on PP materials on decks and sides.

To that, I ask you, what is the motivation of a certification and standards organization in restricting a material type for any reason other than environmental factors, human health factors, and/or processing hazards?
PP has none of the problems mentioned above.

Unless the material has one of the three problems stated above, there is no intelligent means of restricting a material type.

Otherwise, the motivation for the restriction could only be 1. political (ties with companies that make competitive materials intended for the same application), 2. empirical (equations in standards and testing methods are insufficient in defining the behavior of the material (common problem)) 3. Prejudice (individuals creating the standard have a personal preference in what kind of materials that they want to see for that application and apply a God-like rule over the design for structures requiring their certification).

As stated before, material selection must be done on an application basis. Good certification organizations and self-checking companies apply specifications on the mechanical or physical properties of the material (not the material type itself) on an application based method (hull/liner/door/spar/wing skin/etc.). To go even further, especially with composite structures, laminate and/or sandwich properties are a means of specification, ignoring completely (within the spec) the properties of the plies/or core making up the laminate, although they must be known to create the laminate/sandwich properties.

(i.e. for a hull bottom panel of a ship of X length and Y beam with a panel aspect ratio of a/b constrained by BC1, BC2, ... BCn, at a max speed of ZZ knots, in sea conditions ABC or D which creates a panel pressure zone of C psi or N/mm^2 at that station location, the cross section of the structure/panel must have an overall flexural stiffness of XXXX to maintain a deflection of W.WWW inches or mm and critical buckling load higher than XXXX lb/in or N/mm and must have a stress/strain allowables of YYYYY psi or N/mm^2 or must meet a factor of safety of EEE.)

Note that in the above example, the material type would never enter the calculation.

Also additional constraints that may be more material dependent could be peel stress/strain of a bond, or energy release rate of an cut-out, thermal/moisture expansion rate, thermal/moisture degradation rate, impact energy absorption, surface hardness for debree, degree of chemical resistance, etc. These are all still defined by their performance in the specific application, not by the name of the material.

Material type just does not come into the picture. The requirement is only that the mechanical properties of the materials within the structure meet these requirements. Hence, Foam versus PP versus Balsa are be examined on an equal playing field.

If the motivation for the restriction is test based, what may be likely is that the DNV performed tests on panels with identical core thickness and did not account for the specific properties. You cannot expect a core material of the same thickness but with a lower transverse shear modulus (Gc or Gxz, Gyz or G13, G23 or GL, Gw) to achieve the same deflection as a material with a higher transverse shear modulus. This still cannot be grounds for material-type restriction. It should be designer’s preference. (If the designer wants to save money and eliminate weight and wood by replacing the 0.5 inch Balsa with 0.75 inch PP Honeycomb, it should be allowed to do so. If a designer wants to use 10 inches of HDPE on the sides and bottom of his/her boat and can prove that structural integrity and stability is achieved then this should be his/her choice to do so. It may not be a good idea, but if it works, it works. A certification company cannot limit the ingenuity of the marketplace to materials that they see fit.)

It is also likely (and common) that the wrong test for Honeycomb sandwich panels was performed. C393-62 is acceptable, whereas the common 1 inch wide strip 3-point bending method is inaccurate for HC because of cell size.

Nevertheless, material selection MUST BE APPLICATION BASED NOT MATERIAL TYPE BASED.

Sorry about the length. I will try and keep it short next time.

[kengrome]

Hi Analyst,

There's no need to keep it short the next time, I appreciate everything you've posted and more. Please continue to post as much new and additional information as you wish -- the more detailed the better!

:)

Here's where I was coming from when I started this thread ...

I suspect that 9mm thick Hexacor is not thick enough to replace 9mm plywood as the core in a composite sandwich if I want a sheer strength that's high enough for a particular application -- a small, extreme shallow water planing powerboat. I was thinking of using 25mm Hexacor instead, not only for light weight but also to eliminate wood as the core material so buyers won't be thinking of 'rot problems'.

Unfortunately I do not have the ability to do the calculations to determine whether or not 25mm Hexacor will be strong enough to be a good replacement for the 9mm plywood core, and this is why I've been trying to learn more from people who actually have more experience using this material than I do.

[Analyst]

Kengrome:

You are correct in assuming that you will need a thicker PP Honeycomb core to achieve the same stiffness and strength properties as plywood.

To assist you in your analysis, I wanted to give you some of the basic simplified equations to use for comparison.

What you will want to know and compare for complete structural analysis are the differences between each construction for:

1. The maximum deflection of the panel.
2. The maximum stresses of the core and the facesheets.
3. The critical buckling load of the panel(s) and facesheets.
4. The natural vibration frequency of the panel.

FOR STARTERS AND TIME CONSTRAINTS, THIS REPLY WILL ONLY BE CONCERNED WITH MAXIMUM DEFLECTION OF A QUASI-ISOTROPIC SANDWICH BEAM WITH TRANSVERSE SHEAR DEFORMATION.

*Note that for simplicity we will assume a linearly static analysis which means that dynamic (time dependent) loads and properties (which are generally not available) are ignored and non-linear terms, such as deformed shape dependent deflection (deflections based on the deformed shape instead of initially undeflected shape) are ignored. For a very detailed analysis, these assumptions should be validated or these terms must be included, but this is seldom the case, if ever, in the marine industry.

*Further, to keep from overloading you with plate and shell theory, which would require more time and room than is available in this format (entire textbooks are devoted to this single subject and many only consider the most simple of cases because of complexity), we will restrict our view to short beam analysis (considering the shorter dimension of the panel as the beam length).

Because I mentioned it before, I will use the equations from the ASTM Standard C393-62 because they include core material and the effect of transverse shear deformation. This method was created using derivations from the principle of minimum potential energy using Reissner's Method.

The maximum deflection can be determined by the following equations, which can then be correlated with an experiment to determine the true values of the properties:

3-Point Bending:

wmax = {(P*L^3)/(48*D11)}+{(P*L)/[4*Gc*((tf+hc)^2/hc))*b]} (1)

4-Point Bending:

wmax = {(11*P*L^3)/(768*D11)}+{(P*L)/[8*Gc*((tf+hc)^2/hc))*b)]} (2)

Where:

* D11 = Flexural stiffness coefficient defined by:

In General:

Dij = (1/3)*(Qbar)ij*(z(k)-z(k-1)) which for isotropic or quasi-isotropic facesheets and core is:

D11 ~ (1/3)*{(2*E11)/(1-v12^2)*[0.75*hc^2*tf+1.5*hc*tf^2+tf^3]+(Ec/(1-vc^2))*(0.25*hc^3)}

for simplified analysis of a sandwich beam with isotropic or quasi-isotropic facesheets.

Where:

-E11 = effective elastic modulus of the facesheets (assuming isotropic), also referred to in data sheets as Ex or Exx. Could also be Ef for Efiber not to be confused with Ef for Eflexural.
-Ec = inplane elastic modulus of the core material, usually neglected because of its small magnitude relative to E11.
- v12, vc are Poisson's Ratios for the facesheet material and core, respectively. v12 is apprx. 0.25-0.35 for woven fiberglass. but be careful because I have seen it at 0.08-0.11. Unidirectional tapes generally have different Poisson Ratios, one being lower and the other higher. Poisson's for most ductile metals is around 0.3. Ceramics see very low Poisson's 0.05-0.1 and rubber is very high 0.4-0.5, with 0.5 being the maximum theoretical value for any material.
-hc = Core thickness
-tf = single facesheet thickness (will be better defined later)

* L = Length of the beam (shorter span length of the panel)

* P = Load in force (lbs or Newtons) (Note that for the purposes of test correlation we are using force instead of a distributed pressure, however we can also go that route later.

* hc = Core height

*tf = facesheet thickness of one side (note that a simplification and a limitation is implied here. 1. Isotropy is being assumed by stating tf as the facesheet thickness. Generally the thickness of each ply must be accompanied by the stiffness of the ply for accurate analysis. However, if your facesheet ply stiffnesses are not too far from each other, the assumption is reasonable. 2. Bending-Stretching coupling effects are being neglected. In lamination theory, or general elasticity theory, a system of equations ( also referred to as the [B] matrix for people familiar with composite analysis) is formed that is based on the fact that eccentricity of a laminate stack causes deformations about the midplane to loads not applied in the direction of deformation. i.e. for a bending load stretching and shearing will occur, or for a stretching load bending and twisting will occur. This is a major problem between manufacturing and design in terms of warpage and residual stresses and can be alleviated by eliminating the [B] matrix. To do this one must make the facesheets of the stack symmetric and balanced about the geometric midplane. What this means is that the you must have the same plies and ply orientations at every incremental distance from the midplane. If you were to cut out a square section of the laminate, a symmetric and balanced design would allow you to flip it over and see the exact same layup in the same order on each facesheet.

With all of that being said (such a seemingly assumption that has a major implication), coupling effects are less important for sandwich laminate with large flexural stiffness terms, however a proper analysis will include them. Once again, this is out of the scope of the format so we will just neglect this for now.

* b = beam width (in the case of this test, the beam width should be taken as: b = 2*(hc + 2*tf). This will eliminate any problems pertaining to core cell size. This is one of the reasons why this test is accurate and the others are not.

* Gc = Transverse (out-of-plane) shear modulus of the core (also referred to as Gxz, Gyz or G13, G23 or GL, Gw in technical data sheets). Beware that this is not the in-plane shear modulus Gxy or G12 unless the material is isotropic.


Some notes on equations (1) and (2):

1. It should be seen that the last term on the right hand side is the deflection due transverse shear deformation and is dominated by the core shear stiffness Gc and core height hc in the denominator. The first term on the right hand side represents the deflection due to flexural stiffness with respect to face sheet modulus and distance from the midplane, dominated by D11. Many equations neglect core shear stiffness in error by the use of only terms of this type.

2. Because Gc is in the denominator, an increase in Gc will reduce the overall deflection and a decrease in Gc will increase the overall deflection. The same can be said for the hc term. The same can be said as well for the D11 term, in which hc has the largest effect.

3. Therefore, if you have a lower Gc and you want to match deflections for the same load you can do 1 of 3 things or a combination of any or all of the 3 possible choices:

1. You can increase the facesheet stiffness E11 to increase D11 (worst possible choice because you are now working on orders of X^3 instead of just X and you will need alot of facesheet stiffness to make of for the loss in Gc.) This also requires a change in material type (i.e. from fiberglass to carbon fiber) and you still may not meet the same deflection constraint because the impact is small.

2. Increase the facesheet thickness tf to simultaneously increase flexural stiffness D11 and shear stiffness. (also a bad choice because now you are using your heaviest and most expensive material to increase bending inertia and adding extra material, weight, processing steps and cost that you did not need before for in-plane (membrane stresses and facesheet buckling).

3. Increase your core height hc. (Best possible choice because you are again simultaneously improving your flexural stiffness D11 and shear stiffness, using your least expensive, lightest weight and easiest to apply material.)

WARNING: There is a limitation to the benefit of increasing core height in that most core material properties decline with increasing core height. It is usually unnoticeable until an critical value of hc is reached. Core manufacturers always (or at least should always) state the range of core height that the values are valid for and if there is a noticeable drop in value at a specific height, then new properties should be provided for that height range. Hexcel does an excellent job of this (best I've ever seen) and in making sure that all the data is available before releasing a product (there's nothing worse than analyzing structures with sparsely fill out data sheets).

NOTE ON PLYWOOD:

As stated before, plywood is more fundamentally a light stack of semi-stiff plies, and more than a core material because of its high in-plane stiffness values. Its density and weight compared to other core materials is an equal motivation for reclassification. It is far too heavy to be considered side-by-side with core and too light for side-by-side comparison with fiberglass. It is interesting that much of the theory and development of modern laminated composite materials and structures comes from the invention and validation of plywood at the Forest Products Laboratory, and now I am having trouble fitting it into the picture.

At any rate, for plywood, the D11 term is greatly affected by Ec which is non-negligible, whereas for Balsa, Foam, PP etc. Ec is negligible compared to the facesheet materials. While a lot of weight is added this also adds a considerable contribution to bending stiffness. To add Gc of plywood is rather large as well contributing to shear stiffness. Check APA website or Forest Product Laboratory for technical manuals and data sheets on plywood.
However, you end up with a heavy, relatively expensive, water absorbent and degradation susceptible core material.

It would be wise to cross reference equations (1) and (2). I am getting it right now out of my memory, because the actual standard is in a reference manual at my office.

[naturewaterboy]

Very interesting discussion! I have a simple question: Why not use balsa for decks and everything but hulls? As long as water doesn't get to it (easy to prevent) it has hi strength, low weight and is not affected by temperature. So why use anything else? I'm not being a smartass, just trying to understand more....:confused: :D :confused:

[masalai]

Naturewaterboy, "horses for courses", and because some fools do not know the difference, then leglislation happens in its very heavy handed way making a total mess of everything. PP honeycombe panels are far lighter and cheaper than balsa core - if done right excellent for "light point impact load roofing" on cabin tops etc and cupboard/shelving on boats but has a disadvantage in some applications which can be partially addresses by adding appropriate cloth/resin skins.... It is easy to "heat mold" and then add more "glass" to hold it in place, is a good thermal & sound insulator. Likewise balsa core is a bit soft for bottoms BUT, when laminated with appropriate thin layer of plywood and glassed over is OK for lightweight boat bottoms.... ONE MUST KNOW the appropriate materials to achieve the desired result and do good engineering testing and analysis before using anything then quality control in construction and manufacture is the final critical phase in building .....

To put it all very briefly... You still gotta do your engineering - - Don't just take my word for it....

[naturewaterboy]

I see! :idea: (said the blind man to his deaf wife as he picked up his hammer and saw)...:D

[masalai]

Pity you are not over here, I would like a steel drum (no musical talent whatsoever, but I like them & would live in hope that I would find people who could and would play it :D)

[mikereed100]

Hi Analyst,

There's no need to keep it short the next time, I appreciate everything you've posted and more. Please continue to post as much new and additional information as you wish -- the more detailed the better!

:)

Here's where I was coming from when I started this thread ...

I suspect that 9mm thick Hexacor is not thick enough to replace 9mm plywood as the core in a composite sandwich if I want a sheer strength that's high enough for a particular application -- a small, extreme shallow water planing powerboat. I was thinking of using 25mm Hexacor instead, not only for light weight but also to eliminate wood as the core material so buyers won't be thinking of 'rot problems'.

Unfortunately I do not have the ability to do the calculations to determine whether or not 25mm Hexacor will be strong enough to be a good replacement for the 9mm plywood core, and this is why I've been trying to learn more from people who actually have more experience using this material than I do.


Ken,

You may already be aware of these folks as they are in your neighborhood and are distributors for Hexacor. They are building boats that sound similar to what you are planning and may be of help.

Mike

http://www.bakricono.com/

[Analyst]

Very interesting discussion! I have a simple question: Why not use balsa for decks and everything but hulls? As long as water doesn't get to it (easy to prevent) it has hi strength, low weight and is not affected by temperature. So why use anything else? I'm not being a smartass, just trying to understand more....:confused: :D :confused:

Naturewaterboy:

I agree that by the numbers, balsa is an exceptional core material with high transverse shear stiffness and strength values at low densities, however it is my opinion (only an opinion) that there are some inherent disadvantages that I have seen through comparison. The following are:

1. Resin absorption for wet layup application. I have seen balsa cross-sections from carefully manufactured structure that absorbed a considerable amount of resin. This is a major problem for two reasons. One: the obvious weight penalty for weight critical structure. Two: Print through due to resin shrinkage during cure from the access exotherm caused by the added resin volume. The second is avoided by staging the lay-up process to allow cure states or Barcol states, which will in return cost in time and more weight and money from the added secondary bond layer that may be required during each stage. The alternative is to apply more fiberglass to absorb the excess resin, At any rate, there is a pattern here in which more non-structural weight is added to produce a finish instead of a part.

2. Cost variability. The cost of balsa is very dependent on its availability and distribution location because it is a natural resource. The roller coaster of wood prices from a cost and demand standpoint in world markets is well-known to the construction industry and Balsa is no exception. Well seasoned boat manufacturers have been off and on the balsa price-coaster for a while. On the other hand, in the U.S. with oil barrel prices being much higher than they were a few years ago, the cost of gel-coat, resin and other oil-based products have driven through the roof. As far as PP Honeycomb, I am slightly unfamiliar with the details of this aspect, but a Nida-Core distributor said that the material base for their PP Honeycomb was not oil, but instead a material that I commonly associated with its gaseous form, but cannot remember exactly what it was so won't say what I am thinking. All I remember is that it was not oil based, which should stabilize its cost in todays marketplace.

3. Water absorption. Although it may be a solvable issue from a build standpoint, Balsa is inherently a thirsty material. It likes to drink while just sitting around a humid shop, getting weaker and heavier as it is waiting to be used. Expense and care must be taken with respect to storage and climate. Wate intrusion is also a factor. A colleague of mine has found that a tiny crack in one side of a boat can feed Balsa in the other side as long as their is a path - no matter how small, even though the balsa in the dry side is still "locked" out of the external environment. The water also reduces bond quality.

4. Processing. Balsa is not as easy to cut and taper as other materials. For example a box-knife can be used to cut PP Honeycomb.

NOTE ON WATER ABSORPTION:

It would not be fair to balsa if I did not mention the extensive use of aramid paper Honeycomb cores in aircraft, spacecraft/satellite structure and in racing marine craft. The lightest weight core with the highest stiffness and strength properties (for weight) that I am aware of is aramid paper honeycomb core. It is extremely attractive by numbers, processing (easily cut and bonded) and formability (overexpanded non-scrimmed core can wrap around a softball!) and cost. However, aramid (kevlar) is very hygroscopic and in paper form is like a sponge. The stiffness properties, as you would expect are reduced by this hygroscopic nature. But you still see it in these advanced applications. How do they get away with it? The answer is in their processing techniques. I would not recommend a structural component using aramid paper as a key structural component that did not already consist of prepreg facesheets with film adhesive, vacuum bagged and autoclave cured. In general I don't believe the majority of the marine industry has reached that standard or requirement yet.

Balsa seems to have the same negative problems as aramid honeycomb core, except no justification or promise can be made with respect to the processing methods common to the marine industry, that the material will not absorb resin, get wet (or already be wet), cost the same or less every time and machined or milled easily to the desired contour. It is like the aramid paper in its worst case scenario.

This is, however, as stated before, only an opinion.

[naturewaterboy]

Analyst,
Thanks for the info. I did cut open my 1978 flybridge floor,and was amazed to find some of the balsa was completely rotted into mud, but some was good as new. I did notice that the original scrimmed balsa did not have any resin between the blocks - i guess that this would have slowed the water down, but would have added some weight. My guess is that Silverton just took the easy, least expensive way out, as it appears they relied soley on caulking to seal the deck mounted hardware.

[AndrewK]

Analyst

I tried to correlate your 3 point bending equation with measured deflections for some foam cored panel off cuts. Measurements did not correlate, can you get in touch with me off line via email at ansu_ko@bigpond.net.au ?

Cheers Andrew