Hi Anders

I believe there are studies that show that wood can fatigue. After all, if it can break, then it should be able to fatigue as well. That said, there may well be a fatigue limit involved, ie a threshold below which fatigue does not occur or, below which fatigue mechanisms differ.

This seems to be the case for metals where high cycle fatigue appear to initiate at grain boundaries situated below the surface. Once started and with a crack tip on the surface, propagation appears to differ.

Add to the above the complexity of anisotropic properties, a Statistical material and so on.

Sincerely

Claes

Thank you Dr Durrani,

I guess vibration damage to buildings from e.g. earthquakes or construction work has a quite clear and well established connection. (By the way I have recently collected data from construction equipment like compactors, hand held hammers, machine held hammers and pile and sheet pile driving hammers etc, but only in norwegian text here). I also do some measurements from weight drops in gyms, where 1-10mm/s can occur on concrete floors or floating floors of different construction from a noisy deadlift.

I see the typical limits used for construction work may be around 2mm/s and they plan to use such a limit in the viking ship museum too with allowance for up to 3mm/s in seldom occations.

3mm/s is about what a violin top do experience while playing, and nothing constructionwise with the instrument happens over centuries. Wood is more elastic in some way than bricks. A 200mm sample of wood with no knots in it can be bent up to several cm before we hear any crack formation happens. Enventually it makes noise and cracks. Older wood might be more brittle, but still it can be bent at rather extreme amplitudes in comparison to bricks and concrete. A new wooden boat could survive 10cm of amplitude vibrations, or at least 1cm I would believe.

Anyway, there seem to be some literature on earhquakes and wooden houses and museums out there. And I have seen an article with the S-N curve also for wood.

I think the vibration limits in construction work now for buildings in Norway is removed and that they look only at types of ground and state of the building regarding what to do with the construction work and vibrations. There are difficulties with the limits and predictions I believe and who to take responsibility. A failure have so expensive consequences.

Hi Anders

Note that simple rules of thumb as vibration limits apply only at resonance and that they ignore any effects of prestressing.

Sincerely

Claes

As exemplified here, depending on assumptions, the vibration criteria can differ quite a bit.

https://qringtech.com/2014/05/27/piping-vibration-fatigue-risk-assessment/

Claes

Hi Claes! Thanks for valuable input!

I think that moisture driven stress is an important factor. The ships are also cracked and the boards broken as they were found. Some parts of them are new bords and possibly nails. Some boards are more detoriated than the other. Some look somwehat like "dried mud" an effect maybe coming from moisture drying faster on the outside than inside.

I understand that the coupled effects of moisture movement and stress and vibrations, as well as cracks already there, may be more suceptible to changes. But according to the violin makers (and retired engineers) the moisture stress is more risky and a much more important driver for damages than vibration. This point is not even mentioned in our literature on this and not in the references either.

The museum people are aware of the moisture creep and swelling and measure the shape of the ship every now and then with lidar (laser). They also know in detail how he Oseberg ship react to humidity variations as they have weighed it with load cells over a year I believe. Maybe it still are on those cells. It can be seen in the 3D display at the museum site. A corona time offer I guess. Details in the wood can be seen.

It is often a problem with subjects "crossing subject borders". The vibration specailits knows nothing about wood and humidity cykling. The wood expert does not know much about vibrations. The secret is in between these I guess.

I know they have done some work on shape measurements on the Wasa ship. There are some literature out there. And of course on the violin, but not much. The Cannon del Gesu has been monitored with very sensitive MC meters and weights over time, but within very small limits. The large natural variation is more interesting. It is even for our new CLT buildings, but no one pay attention to it in regard to the acoustics, except me I think.

Here is a link to 3D tour of the Oseberg ship: https://my.matterport.com/show/?m=x4xgpD8hUBj

It has been under ground for 900 years and above ground the last 115 years. It has been vibrating for half, to one third the time of our oldest played violins.

This is what it looked like just after excavation i 1904: https://www.khm.uio.no/english/visit-us/viking-ship-museum/exhibitions/oseberg/the-story-begins/index.html

I am waiting for this book to arrive, Fracture and Fatigue in Wood: https://books.google.no/books/about/Fracture_and_Fatigue_in_Wood.html?id=QEppx1J3BTUC&redir_esc=y

Dear Anders, dear all, you should start with the question "what is wood"? As a living material, it does not face fatigue as its fibres are modified by the static and dynamic loads (eg a tree growing adapts to dominant winds, the tree at the periphery of a wood are stronger than those in the middle of the forest...). But used as a material, wood dries and some of its components (providing resilience and cohesion among fibres) degrade or vanish, making it more fragile; it comes with a weight loss and a volume reduction (essentially transversally to the fibres). Wood is physically and chemically a complex fibre-polymer composite. Young wood react to humidity and recover partly the original volume, retracting again when drying, hence the variations you can see in solid wood floorings, shutters, doors etc. Older woods are less sensitive but still react to humidity variations and ultraviolet light (Mediterranean sun is degrading wood faster than rain, my house demonstrate it clearly!). Back to the musical instruments you are referring to, violin makers were very attentive to have the tree cut at the right period, dried carefully before and after sawing the planks, selecting the planks with the right fibre density and pattern to start with a stable and durable basis; then comes the varnish to protect the wood from the daily variations of humidity and exposure to UV, while "breathing" by the internal unvarnished side of the planks, less exposed to light. Still, it ages, but much slower: I guess that properly maintained violins will play right for another millennium at least without showing fatigue fractures!

Back to your question, I believe the only scientific response would require microscopy, analysis of its polymer content and average length of its molecular chains etc etc etc. Resilience to cyclic loads would have to be assessed as a function of this chemical degradation. Should you be able to manage with a publication in French, refer to https://tice.ac-montpellier.fr/ABCDORGA/Famille/CHIMIEDUBOIS.html to capture an idea of what is living wood from a polymer chains perspective; however I could not find a decent paper on how to assess the ageing of a sample from a structural perspective, but for sure the more you lose these chemicals the less resilient it becomes! (no mention of the attacks by insects and fungi)

The viking ships, as far as I know, came out with a very degraded wood structure, most of the original polymers were gone ans substituted by water and various salts allowing them to keep their original shape. Drying would have made them cracking, squeezing shape and falling ultimately into dust, but brilliant chemists suggested to bath them in pure glycol, introducing a conservation solution for all antique wooden artefacts found in humid zones. But while looking as "wood", it is no more wood but a new composite material now combining altered fibre matrices with glycol! So you cannot take any lesson from real wood ageing, someone has to undertake specific cyclic fatigue studies on real artefacts preserved with this technique. (to the contrary of Egyptologists which find "real wood" objects in dry tombs, the viking drakkars or the Vasa or the Marie-Rose are no more "real wood" but a sort of "ghost wood")

As a (retired) expert of anti-vibration devices, I am pleased to suggest you the best/cheapest/ most effective way to protect your fantastic viking ships: a thick layer of sand can provide an absolute protection against both ambient ground floor vibration (trucks and buses on nearby roads, trains...) and visitor's walking impacts. I solved this way the challenge of isolating barrels for ageing very pricey wines from structural roadnoise years ago in a French cellar affected by close-by intense traffic... For the curator, it even adds to the display by suggesting the usual way of grounding these vessels on the beaches, as vikings were not commonly building quays but only breakwater walls! What really matters is a sufficient sand layer (say 0.2 m under the floating display structures) and the total absence of structural liaisons to the museum structures (why I use the word "floating" for the ship craddle and the visitor's gangways), see attached scheme.

If you adopt it, take for free the consultancy, but just please invite me for the Oseberg museum reopening! :)

I think that the effect of fatigue on fixture elements points are more important. That ranging noise on the stairs are due to the fatigue on glue, nails and screws caused not only by vibration of the every day use but also by repeated cycles of humidification and drying, heat and cold.

Dear all

As pointed out by Bernard Garnier , what started out as wood is no longer necessarily wood as we understand it. Therefore, rules of thumb for vibration should be used with caution as they do not necessarily translate well.

Replicating the ageing process may be far from easy. The best material to test would be the material from the actual item. I believe there are methods available to find material and fatigue data from small samples.

As stated by Rogerio Parra positions with stress concentration are the likelier to fail in fatigue. The Oseberg ship has a set of support pins that extend from the floor up to the hull and apply stress concentration and have done so for quite some time.

Bernard Garnier suggestion of letting the ship ride on sand could perhaps be a good way to move it? Apply a large and strong cloth below, add sand as an interface layer, then lift? Perhaps sand or an adsorber can be used for controlled- and as a buffer for- moisture?

Vibration isolation is not that hard to achieve, though one must watch out also for infrasound that may end up rocking the boat. The building will have to insulate from such.

To answer your main question. Wood, old wood and aged (referring to the item specific history) likely fatigue though probably not the same way as metals do.

As moisture is known to change the ship volume, it appears probable as a source of low cycle fatigue. The trick lies in relating moisture to material stress and from there to location in the item while deciding if it matters as a preload component. If it does, then spectral stress methods are in doubt.

Just my 2 cents

Claes

´Thanks Claes Richard Fredö

I am not part of the viking ship project, just a couple of measurement sessions with Ståle (when Kjetil could not attend). I wish I was part of the proiject because I have been a muesum guide in my student years 9 sommers in the local museum at home. I also have some wood knowledge and literature insight from years of violin and Hardanger fiddle acoustics research, including experiments with humidity effects.

I think a too strong emphasis is put on vibrations in the project and the effects of humidity cylcling is not dmentioned nor discussed in our material. Maybe because htere is not much knowledge in our comany on that exept mine I guess.

There has been done tests at the UIO on old wooden samples And I think our collegues conference papers on the project state that the oak retain some 60% of fresh wood stiffness. I have seen the original work and I think that tere really is a mix like the age of the wood in the ships. Some wood is new, some is more weakened by stress or humidity, soil chemics, other parts are pretty much like new wood, I think.

What they plan to do is to mount the ships in huge stabilizing and vibration insulating rigs on wheels. Those wheels better be big because the floor is a very irregular slate stone and many of them are loose. Trolleys over that floor is noisy like .. One idea of mine would be to start off making that floor as smooth as possible using self leveling screed

Sand might be a good idea, increasing the damping and the impedance. Bernard Garnier . The ideas given by Rogerio Parra fits with the input I got from a retired Jet Propulsion lab engineer now making violins using his way of accelerating aging effects Torrefication. Steam and pressure cooked wood. It becomes less suceptible to humidity uptake for a while, and have a lower inner damping maybe caused by the humidity effect or more. He was concerned about the iron nails, (he looked at the 3D pictures of the ship) and I agree those supports are a concern. Deformation Maps show movement between these. By the way, the retired JPL guy was responsible for the engines on the Mars rovers that survived longer than expected.

The ships are not the most fragiole objects there. I think the sladges and wagon are among these, possibly also the textiles. The ships are not treated, while the rest of the objects are alum treated, and partly or largely in bits. I think the ships can take much more than those alum treated objects. Nevertheless the main fucus has been on the ships, as it seems. More prestige with them Maybe? There are a lot of objects as this was graves and everything they thought was needed for the trip over to the other side was included. Some of these continers e.g. Look much like the wooden objects, like buckts in wood we have at our local museum at my home willage. These are maybe 800 yrs a part In time, same techiques, and materials. An excavation like that is somewhat like a time travel Which is the most interesting part I think. Besides the extraordinarily fine workmanship on the slighs and wagons.

Coming from a handicraft family it feels natural to realte to the obejcts there.

Moreover it is a grave, and I always remember that while visiting this museum. Always with deep respect. I hope the objects and ships are treated the best way possible during the construction work and moving all over to the new climate controlled building and after that.

It is a very competent group working on this. But the vibrations is only a part of the story and risk.

One of your respondents suggested that infrasound could be a cause of long-term fatigue. Interesting concept, but I've got nothing to confirm/deny such a theory. Infrasound within a museum could be caused by a number of factors, both natural, such as wind-induced motion of a building, and manmade, such as wind turbines. I've got some past measurement data down to about 0.1 Hz and the variations from hour to hour and day to day are quite substantial at such low frequencies, on the order of 30-40 dB. More analysis of this data might occur to correlate the SPL at different frequencies with measured wind speed which could create fluctuations not only in natural infrasound but increase wind-turbine emissions as well.

Yes. The project group consists of NGI, Brekke & Strand Akusikk AS (us), Dr Ing Florian Kosche, and a free standing consultant, Morten Gjestvang. Our company do the measurements of the ships and building construction, and part of the outdoor experiments with drop masses (I have participated in one of these), with a passing truck over a hump and with a drill rig (NGI alone I think). We do the measurements on the building, NGI on the outside, propagation etc. I also think they are responsible for calculations of the vibration levels from the construction work. Koshe and his comany does the FEA analyses on the ships and special fragile objects, I think, altough I do not have full overview. The FEA i think includes calculation of creep and tension from humidity changes as well, which probably is the main risk factor. Not the vibrations. But the most fragile alum treated objects are at risk also from vibrations.

I am interested in this subject not only because I have some knowledge on wood, hygroscopy, coupling of acoustics and hygroscopy from violin research. I have also planned to do FEA in this subbject on instruments. Recently I have done investigations into impact force, noise and vibration generation in buildings from dropped weights. I want to extend this to outdoor so we can do predictions not only of construction noise but also some on structural noise and vibrations.

I got the book "Fracture and Fatigue in Wood", by Smith et. al. now, and there is no word of vibrations in it. There are emphasis on stress and load cycling, e.g. The Palmgren Miner rule. I haven't read the book yet, though.