Predicting the Creep Rate in Archaeological Wood From the Vasa Ship. I. Bjurhager., E. K. Gamstedt

Transcription

1 Predicting the Creep Rate in Archaeological Wood From the Vasa Ship I. Bjurhager., E. K. Gamstedt

2 Background Sweden anno 1628 A warfare nation under reign of Gustav Adolfus II, the lion of the North His dream: an inland sea The navy needed improvement and reinforcement Four larger ships were ordered

3 Background Vasa: too narrow and top heavy Sinks on the maiden voyage 1628 outside Old town in Stockholm Good conditions at the finding site. Wreck relocated in 1959 and salvaged in 1961 Impregnated with polyethylene glycol (PEG) solution for 17 years Put on display in the Vasa museum in 1990 Alarm in 2000: large salt precipitations and low ph

4 Vasa: present condition Surface (0-10 mm): Rich in compounds (sulfur, iron, PEG), high acidity, high bacterial degradation Low stiffness, low strength Sub-surface (10-40 mm): Rich in compounds, low bacterial/chemical degradation Low stiffness, high strength Interior (>40 mm): Poor in sulfur, PEG, medium iron content, high acidity, high chemical degradation. High stiffness, low strength Photo: Gunnar Almkvist

5 Vasa: present condition Vasa undergoes time-dependent deformation Warping, compression (1-3 mm per year). Creep rate in wood Strongly related to modulus High moisture content High PEG content Low modulus Non-linear relation! High creep rate

6 Modelling the creep of Vasa oak Experiments for investigation of creep behavior, PEG-rich Vasa oak A) Radial/Tangential compression cubes lab, stable climate σ 0 30% and 50% of σ yield (ρ) B) Bending (axial compression) (planks museum, varying climate σ 0 <10% of σ yield ) Purpose, modelling: 1) Creep rate at different stress 2) Stress limits for avoiding permanent deformation 3) Possibility of creep limit Deformation predictions Acceptable stress levels

7 Modelling the creep of Vasa oak Constant climate Establish relation between time-dependent strain and applied stress ε(t) = S(t) σ 0 Standard Linear Solid Model Includes both elastic response (spring) and viscous (dashpot) behavior

8 Results and Predictions cubes (R,T) Constant climate Creep rate PEG-rich Vasa oak.: 50 times higher compared to recent oak (no PEG) Young s modulus 50% lower in Vasa Ref Vasa

9 Results and Predictions cubes (R,T) Constant climate Stress limit for avoiding the yield strain limit (i.e. permanent deformation) σ 0 5% of yield strength (σ yield, ε yield from quasi-static tests) Vasa

10 Results and Predictions planks (L) Museum, varying climate Blue plank: creep limit (others: yet no limit, >5 years) σ 0 3% of longitudinal yield strength (σ yield, ε yield from quasi-static tests)

11 Results and Predictions planks (L) Varying climate (museum) Planks: Variation in deflection correlates with seasonal variation in climate (Climate variations )

12 Results and Predictions planks (L) Varying climate (museum) Planks: Estimated mc variation: 1-2% hard to predict mc in PEG-rich archaeological material (Climate variations ) (Wood Handbook Centennial ed.)

13 Modelling the creep in Vasa oak Varying climate Total creep = visco-elastic + swell/shrink + mechano-sorptive visco-elastic

14 Comparison of models Mechano-sorptive model vs. Visco-elastic model Better fit with visco-elastic Creep limit reached after years model model data data

15 Summary Vasa PEG rich material R, T compression, stable climate: 50 times higher creep rate (due to 50% stiffness reduction) Stress limit for avoiding the yield strain limit (i.e. permanent deformation): σ0 5% σyield Bending (à L compression), varying climate: Stress limit for reaching a creep limit within 1-2 years: σ0 3% σyield Model: visco-elastic Future work PEG rich material: temperature dependence moisture uptake dependence

16 Acknowledgements The Vasa museum: Emma Hocker, Malin Sahlstedt, Ove Olsen, Anders Ahlgren, Magnus Olofsson Former colleagues: Jonas Ljungdahl Magnus Burman Funding: SMM Uppsala Universitet Thank you for your attention!