ssc sr 1464 - ship structure · 2011. 2. 17. · this version is ptc confidential- an abridged...

THIS VERSION IS PTC CONFIDENTIAL- AN ABRIDGED VERSION WILL BE MADE AVAILABLE FOR THE WEBSITE

SSC SR-1464 1st Teleconference Nov.18.2010 Page 1 of 13

SSC SR 1464 Teleconference 18 November,

Hosted via ABS Conference Call Center

Duration 1hr: 45mins

Able to attend

Eric Greene Royale Underhill James Huang

Raymond Kramer David Howarth Jeannette Grant

Stephan Billian Daniel Woods Elli Lembessis

Unable to Attend

Robert Dow Paul Miller Paul Cojeen

Pramod Dash Nicholas Koreisha Roger Basu

Fai Cheng Ron Caputo

*Meeting was not recorded, so Notes should not be taken as complete and the quotation marks will provide only the gist of what was said and by

whom, rather than a precise record.

Timeline

08:30 EST Lines open for informal conversation & troubleshooting connection problems.

09:00 SR 1464 Teleconference Starts

09:00- 09:20 Review of Action points from Kick-off meeting. Agenda determined by E.Greene

‘Telecon Meeting #1’ Notes transmitted on 11/16/10

09:20 -10:30 Presentation delivered by Eric Greene

NB. The detail in the minutes is only intended to inform the PTC members unable to attend, the shaded

segments will be deleted for open publication.

Review of Action Items from kick-off meeting

Eric Greene > RE: Item 3; the literature search, the majority of the results are covered by the two

presentations (kick-off meeting and presentation provided with 1st teleconference). The information is

not yet available in report format, but the majority of the work has been concluded.

RE: The Eurocode and British Standards.

I have pursued contacts through Germany and contacted the NDE engineer for Sweden’s Visby Corvette.

Much of the earlier efforts undertaken in Europe were reviewed by DNV, but as they are not

represented within this project, I will try to contact them directly. Visby is a good indication of activities

in Europe, and I understand they conduct 100% laser shearography during construction, which provides

a good baseline. Despite this the literature search is essentially complete.

David Howarth> RNLI in the UK is also using 100% shearography.



Eric Greene> I do have an RNLI document -written at the early stages when they were just starting.

RE: Paul Miller possibility of providing panels,

Eric Greene> I accompanied Paul Miller and Paul Cojeen to the Naval Academy the day after the Kick-off

Meeting and examined the panels discussed. In addition a further 10 panels are available for use. These

panels are approximately 1m2, E-glass/ vinylester, vacuum infused, and in good condition to work with.

We do have the option to build the panels if we need to make something special, but for the first

iteration we need to see if we can induce realistic damage into the panels that we have readily available

to us.

RE: Carderock contacts.

There is a difference between the building programs, the manufacturers have very specific detailed

e.g.ultrasonic NDE technique, versus the in-service inspection where it is comparatively haphazard once

in-the-field where. In speaking with Gene Campneschi when trying to find out if there is an allowable

defect size, he said it’s very structure specific; basically when we are at finite element analysis level, it is

standard practice to conduct a flaw criticality analysis study, whereby different flaws are induced into

the model in order to determine the maximum allowable size. I’ve tried specify allowable defects in my

tables.

RE: Bruce Bandos

A Level 3 Ultrasonic NDE practitioner I have experience working with in Philadelphia. Bruce tells me that

Northrop Grummon conducts all NDE in-house with, in his opinion, the most sophisticated NDE set-up

for marine composites in the country. I have e-mailed their NDE contact, but it is unlikely I will receive a

response.I plan to visit Bruce Bandos as he has ultrasonic, thermography and laser shearography set ups.

The shearography is developed by the Materials Science Corporation (MSC), which is a company I also

intend to visit.

RE: Planned Tests. The first test is to be visual. After consulting with Surveyors and Practitioners, the

most realistic method used is visual, the importance of this will be discussed further in the presentation.

RE: Equipment manufacturers

I have an understanding of the necessary manufacturers to approach, but I think it important to perform

some preliminary work in order ascertain the size of defect the equipment needs to detect.

RE: PTC members to suggest critical defects

These are covered in the table towards the back of the presentation. The table is a flexible first draft but

it is focused on in-the-field techniques and in-service defects, as opposed to manufacturing defects,

however, both are considered.

RE: Contact Survey Practitioners,

I have located approximately 200 Thermography practitioners in yacht survey, with varying degrees of

expertise. UT is more challenging and laser shearography practitioners are scarce.



Questions:

Ray Kramer> When working on aluminum fatigue, I was very impressed with the Eurocode, in particular

Eurocode 9, was interesting to see how a codified approach handled fatigue of aluminum- though not

designed specifically for maritime applications there were a number of curves and options which alluded

to a seawater environment.

Eric Greene> There is difficulty in crossing over to different industries – it was only the ABS Guide that gave

an indication of allowable void populations. However, it is difficult to tolerate any kind of core damage,

-it was difficult to obtain information on core damage.

David Howarth> the European Commission have published research on wind turbines and wings such as

Airbus.

Eric Greene> There is currently more honeycomb cores in aeronautics, but they are increasing in the marine

industry so they will be examined in greater depth. Aeronautical plies are typically thinner -20 plies, so

UT yields good results.

Presentation Commentary Eric Greene> There is a spreadsheet associated with the presentation which will be uploaded to the forum

website, but the tables within the presentation are extracts from it, and it provides a comnparison

between the low-tech E-glass/vinylester PVC/balsa cored materials and the high-tech carbon fiber

prepreg honeycombs. The low-tech composites typically have a larger factor of safety and may

incorporate an extra ply to compensate for unknown variables, however, the high-tech structures are

highly engineered to precise thicknesses so one would expect smaller defects in these structures. In the

first phase, I will estimate an allowable defect size, and in the second I will determine that which may be

detected with our equipment. As ‘rule of thumb’ I estimate that the detectable size will be half that of

the allowable.



Slide 2

These images illustrate the defects we aim to detect.

Slide 3

Good resolution of microcracking, with an indication of the shear paths which may progress through the

matrix

Slide 4

This image depicts voids and provides an indication of the ply through-thickness, as opposed to existing in

isolation within a single layer, and emphasizes the need to refer to voids within the context of density.

Slide 5

Edge delamination. The micrograph on the right depicts delamination from a hole drilled through the

laminate. It is very instructive to see the scale.



Slide 6

Lightning damage. These images are revealing. From personal experience, a client will approach me with a

request for a 100% inspection of their yacht following a lightning strike. Invariably one begins inspection

about the visible signs of damage and resin burn-off where the stricke enters and leaves the structure.

However, in the image on the left, the laminate with an incorporated copper mesh for grounding shows resin

burn next to it. The image on the right (without a copper mesh), shows a greater degree of multi-level

delamination between the skins. This is a serious failure, and for more so than the moderate amount of

micro burn-off of resin. This underscores that improvements are required when examining a structure for

post lightning damage. The methods used by the aeronautics industry merits greater attention. In addition,

this will be of greater significance for an all carbon structure. Current practice is simply to create a path via

lightening rods in an attempt to bypass the structure and find an easier path for the lightening to ground. I

will raise this question with Engineers and Surveyors in the field. For the cost of a yacht, this is an

improvement which would gain support across the Industry.



Slide 7

Impact damage. This demonstrates the detail visible with x-ray, but I will not dwell on this because of the

feasibility of application to a large structure.

Slide 8 The start visible impact damage. This will be discussed in greater depth when I show you the slide about our

panels, how they will be damage them, and the selection of blunt or sharp impactors.

Slide 9

The Left illustrates the surface, the right, an internal view of the laminate. Left is resin starved –you can see

the dry spots. Right on the top, are pockets of resin rich areas.



Slide 10

This ubiquitous slide is beneficial to ensure the correct terminology is used. Most of these are included in the

defect table, though little emphasis has been placed upon direct buckling and dimpling.

Slide 11

Testing performed by a Scandinavian agency- highlighting concern about sandwich core failure. These are

foam core failures after the panels were fatigued. Issues I will consider will include core type, density, bond

strength.

Slide 12

Classic core shear failure.



Slide 13

Core compression failures. I was interested in this article mainly for the pictures on the right where they have

displayed the moiré patterns as this area is going to be maximum resolution for looking at defects.

Slide 14

Indentation damage. Detection is more difficult with increasing ductility of the core due to recovery of the

cells which may conceal underlying damage. This is illustrated in the image where the bond area is affected.

Slide 15

The left image depicts a blunt impact, the right a sharp impact profile. The definitions of sharp and blunt in

this case are relative to the aerospace industry. It is likely that their definition of sharp is more acute than

that for a marine application. For our application, we will test our panels with a blunt impactor head to

ensure that damage is not quite visible and less localized. It is unlikely we will be able to relate it to wave

slamming damage.



Slide 16 &17

Damage to honeycomb the damage mechanisms are very different.

Slide 18

I found the illustration on the top left to be very instructive, simply for the terminology of a honeycomb, and

specifically the references to adhesion vs cohesion. The images show the of bond failures that need to be

addressed if we are to produce a forward looking report, as there are increasing number of honeycomb

structures in the marine industry, and we need to know the failure modes.



Slides 19-21

The next few pages were obtained from DERAKANE, a very useful source of visual surface damage

classification. The more manufacturing defects may not fit into our table but for instance wrinkles do need to

be included as the effects on strengthen reduction can be significant.

Slide 22

The FAA are conducting a parallel effort, and are funding an inspection project specific to composites. This is

their layout for a solid 20 ply carbon laminate. They drill holes of various diameters and to various depths at

the rear face. The dimensions shown are significantly smaller than would expected for a marine laminate,

which may be a quarter inch thick. Here there are fine tolerances; even when they drill 20% into it, it is only

4mm. I have seen a thermography scan which could detect all but the four smallest defects, i.e. , the four in

the bottom right. For our panels we won’t use these numbers, but we will use this arrangement and drill in

from the back.

Ray Kramer > Back towards the beginning of the presentation and the image of fiber fracture, pull-out, and

matrix cracking –are matrix cracks an in-service or fabrication defect? Is there going to be a different

technique to detect each case?

Eric Greene>Ideally you would find a technique that works for both so that you could have a baseline. Resin

cracking can occur from exotetherm or stress curing, and somewhat from handling. It could be present

following fabrication, but fiber breakage is most likely an in-service defect. The wind industry is an

interesting example, because while the blade is on the shop floor, a trolley may run down the full length

to conduct scans in a controlled environment. While this may work in the laboratory, it is not feasible in-

the-field. This is a repeated finding with UT. There is also the consideration of post analysis time.

James Huang> You mentioned X-ray of large composite structures was not feasible. Does that apply to both

the film and digital X-ray?

Eric Greene> More film. This is more anecdotal evidence that I have heard about exposure, the radiation of

the operators and variability – I have not seen it performed on marine composite structures.



James Huang> I noticed that recently Digital X-ray has been adopted within the NDE field and the ASTM

Committee is working on both the technology itself and the kind of defects detected.

Eric Greene> I will look into it further - nothing has come to my attention on the topic yet. I did look at all the

ASTM standards for NDE, the standards detail the training of operators, which should be included in the

report.

Slide 23

I extracted these tables from the major spreadsheet. In these tables I have not included description, just the

name of the defect. There are two categories; E-glass, and carbon fiber. The references are in Slide 34.

Slide 24

The blister also may need a density value - how many blisters per square inch.

Core crushing, no permanent set is allowed,

Slide 25

Core shear failure. The values here is too high -no values are available for the carbon fiber

Slide 26

Delamination – the values I have here are reversed between the carbon and glass fiber. Allowing a greater

degree in the carbon fiber is a typo.

Fiber failure-, none would be a better value, but I do have a reference for less than 1% by volume, not to

exceed 10mm2, which I think is reasonable for a low tech laminate. This will assist in developing a threshold.

Slide 27

The kissing bond may be difficult to detect because while the surfaces may be touching they may not be

transmitting shear.

Local impact damage- The values for the E-glass were reduced by half for the carbon fiber.

Slide 28

No cracking will be allowed in the carbon fiber laminates. Moisture ingress- little guidance is available for

this, I estimate 10% allowable for low tech E-glass laminates , and none for carbon fiber laminate. This

requires more research for me to improve the estimate here.

Slide 29

Ply waviness- This is of increasing concern as the use of infusion methods have escalated.. This is a problem

derived from the dry lay-up of fibers within a vacuum bag which can lead to a risk of wrinkling about convex

shapes. There is a subsequent loss of strength, and especially compressive strength.

In the master spreadsheet which will be made available is reference to work conducted by ‘Josef Barco’ on

the Effects of Defect in composite Materials, in which a quantified strength reduction is offered. For example

a specific ply gap may yield a 9-17% strength reduction; visible impact damage can cause 80% strength loss,

but barely visible damage may cause 65% loss. With respect to fatigue, it is reported that ply waviness will

reduce fatigue life by a factor of 10.



Pits or pinholes at the surface are not critical, but it is worthwhile to provide a value.

Slide 30

Porosity is similar to voids, but again there will be different criteria.

Slide 31

Skin to core disbond. As a first estimate, I used the same criteria as kissing bonds.

Slide 32

Surface cracking - not real critical but a 1/4inch is allowed.

Thermal damage-data is from pressure vessels so these will need to be aimed for marine structures and the

language changed to ‘mm’ per square ‘m’. Residual strength is difficult to ascertain.

Slide 33

The most guidance was available from ABS. The most detail is in the carbon fiber column. These values will

be revisited as they are structure specific

Slide 35

Telephone Discussions. These have already been discussed. I am using an article written by ‘Ginger

Gardner’(?) for ‘Professional Boat Builder’ as a reference in which she interviews Mike Bergen from

Carderock. Bergen comments that thermography was immediately dismissed as a technique because it

required 1 man to hold a heat source and a second the detector, while UT required only 1 man. The

decision was based on the logistics of 1 man day versus 2 man days. However thermographic techniques

have now advanced beyond this stage. My point is that we need to discuss equipment costs and the

training needed. RE: Paul Miller. I will discuss the panels in further detail in the next slide.

Slide 36

1. Drill different diameter holes at varying depths from back side. NB. These will be sandwich plates and will

entail drilling through the rear skin and then through the core to various depths into the top skin.

2. Impact damage. Paul Miller has an Instron impactor that we can be used to induce impact damage on

the inspection side at various energy levels.

3. Cut panel into beams. To obtain the shear failure modes in the core, it is necessary to cut the panel into

beams, but keep the beams wide enough so edge effects will not be introduced from the NDE

equipment. The beams will be positioned in either 3 or 4 point bending, with eth loading spans adjusted

to obtain the desired failure modes. I intend to develop skin to core separation and the classic shear

fracture of the core.

4. Water injection. This challenging, the first approach will be to drill directly from the back and insert a

pressure tap in the back. Water will then be injected at various pressures on a trial and error basis. It will

not be easier to build the panels with water in them. Additionally, this is a more realistic pathway of

water into the structure – rather than surface absorption it will ingress predominantly through holes.

5. Delamination. Bending will induce delamination



What is missing is an equivalent source of panels for a high-tech carbon/honeycomb equivalent. It is problematic

to obtain the same panels as the Navy but I may be able to obtain some comparable ones. The E-glass /

Sandwich panels are a good start.

Slide 37 In addition to these notes, the plan for the next reporting period is to

start contacting vendors,

refine and focus on specific technologies,

refine the allowable table

Insert defects into the sandwich panes and start to take some measurements and document the

measurements with video.

Questions

James Huang > RE: NDE vendors –there is a new Digital UT inspection technique from ‘Imperium’ which is

operated in a similar manner to an optical digital camera. The image sensor is a miniature array of 112 x 112 UT

probes, collectively the size of a stamp. I’m not endorsing the technique but I am aware the Navy is trying out

the technique.

Eric Greene > Imperium are on my list of contacts. I would also like an idea I’d of how many sq meters can be

inspected by each technique per hour. In addition , there is another device currently available which

incorporates Thermography and UT within the same portable unit. With respect to shearography, the most

common method of stressing the structure is vacuum on the skin.

Ray Kramer.> The values provide in your table for bond failures seem too high –if a Survey reveals 25% of the

area is unbounded would you call that acceptable?

Eric Greene> I had a similar reaction, so I am tracing the source. On the other hand if you think of the sloppy

bond lines in many boats, I think it probably does come out as 25-30%. It is not a design value, but it may be

typical of production. Additionally I have images from the wind turbine industry, where of course after a failure,

the bonding is visible, but the adhesives are tenacious. I expect this debond area would be visible with

thermography.

Ray Kramer> It must be similar to welding, where a full weld length is unnecessary from a strength perspective,

but without it there is an increased risk of fatigue cracking.

Eric Greene> This may be drastically different between the high and low tech composites.

Action Items Summary

□ Eric Greene visits vendors and practitioners incl. Bruce Bandos and MSC,

□ Eric Greene wishes to pursue contacts performing NDE for Visby, and related DNV personnel.

□ Refinement of Allowables Table and pursuit of moisture ingress, and lightning dissipation effects and

approaches to damage limitation.

□ February 17th teleconference

ssc sr 1464 - ship structure · 2011. 2. 17. · this version is ptc confidential- an abridged...

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