Martin Neumann 
email: martin.neumann@bam.de
  
Jürgen Herter 
  
Bernhard O Droste 
  
Sylvius Hartwig 
  
Compressive behaviour of axially loaded

Keywords: Stoßdämpfende Bauteile von Transportbehältern für radioaktive Stoffe sind oft als holzgefüllte abhängig von seitlicher Dehnungsbehinderung unterschiedliche Kompressionsmechanismen für axial beanspruchtes Versuche mit Fichtenholzproben (Picea abies) wurden durchgeführt, um mit Hilfe statistischer Methoden den Einfluss der Dehnrate von statisch bis 30s -1 auf Druckfestigkeiten, Druckspannungen und Energieabsorptionen bei 50% Stauchung. Die seitliche Dehnungsbehinderung hatte keinen Einfluss auf die Druckfestigkeit, mit seitlicher

come    

Introduction

The BAM Federal Institute for Materials Research and Testing is the German government institute responsible for the mechanical and thermal safety assessment of transport casks for radioactive material as required by the IAEA Regulations (IAEA 2005(IAEA , 2008)). The mechanical tests to be performed according to IAEA Regulations include, among others, a 9 m free drop test onto an unyielding target in order to demonstrate safety under hypothetical accident conditions. Energy absorbing cask components are used to transform kinetic energy to plastic deformation energy over an appropriate long drop impact time and deformation path. Longer impact time and higher deformation lead to a reduction of the maximum impact load. In most cases impact limiting components are wood-filled steel casing constructions, which are bolted to the cask body. By applying large volumes of spruce-wood, the necessary energy absorption capacity is allocated for existing masses and dimensions of transport packages. In the process of energy absorption the wood, mostly aligned in axial direction, is crushed up to large strains. Since it is important to specify the energy absorption capacity of impact limiting devices in analysis of transport casks, compressive behaviour and energy absorbing capacity of axially loaded wood under large deformations are of major interest.

Wood behaviour under mechanical stress

Wood on a macroscopic level can be divided into bark, cambium, sapwood, heartwood, pith and wood rays (Forest Products Laboratory 1999). Year rings are visible for most woods in temperate climates. They are zones formed by earlyand latewood within a growing season. Wood is macroscopically mainly considered an orthotropic material with three main directions "axial" along the stem (a), "radial" perpendicular to the year rings (r) and tangential along the year rings (t) [START_REF] Dinwoodie | Timber: its nature and behaviour[END_REF]Forest Products Laboratory 1999;[START_REF] Gibson | Cellular solids[END_REF][START_REF] Niemz | Physik des Holzes und der Holzwerkstoffe[END_REF]. On a microscopic level, wood has a cellular structure. The wood cells, called tracheids, are the structural elements of wood tissue. They are elongated and towards the ends often pitched cells that are mostly aligned with the stem [START_REF] Fengel | Wood -Cemistry, Ultrastructure, Reactions[END_REF]. On even smaller scale wood can be understood as fibre reinforced composite, where lignin is the matrix, and cellulose can be understood as the fibre [START_REF] Bodig | Mechanics of wood and wood composites[END_REF][START_REF] Niemz | Physik des Holzes und der Holzwerkstoffe[END_REF].

Regarding mechanical properties, published work focuses on the elastic region or small plastic deformations until reaching compressive strength. A wide range of studies concerning the elastic properties and elastic compression mechanisms of wood has been published [START_REF] Dinwoodie | Timber: its nature and behaviour[END_REF][START_REF] Grimsel | Mechanisches Verhalten von Holz[END_REF][START_REF] Grosse | Zur numerischen Simulation des physikalisch nichtlinearen Kurzzeittragverhaltens von Nadelholz am Beispiel von Holz-Beton-Verbundkonstruktionen[END_REF][START_REF] Hermanson | The triaxial behaviour of redwood using a new confined compression device[END_REF][START_REF] Kollmann | Technologie des Holzes und der Holzwerkstoffe[END_REF][START_REF] Niemz | Physik des Holzes und der Holzwerkstoffe[END_REF][START_REF] Poulsen | Compression in clear wood[END_REF].

Works on wood failure mechanisms under compression include [START_REF] Bariska | On the fracture morphology in wood. Part2: Macroscopical deformations upon ultimate axial compression in wood[END_REF], [START_REF] Bodig | Stress-strain relationship for wood in transverse compression[END_REF][START_REF] Debaise | Morphological and mechanics of wood fracture[END_REF], [START_REF] Easterling | On the mechanics of balsa and other woods[END_REF], [START_REF] Gindl | Comparing mechanical properties of normal and compression wood in Norway spruce: The role of lignin in compression parallel to grain[END_REF], [START_REF] Gong | Failure of softwood under static compression parallel to grain[END_REF], [START_REF] Grosse | Zur numerischen Simulation des physikalisch nichtlinearen Kurzzeittragverhaltens von Nadelholz am Beispiel von Holz-Beton-Verbundkonstruktionen[END_REF], [START_REF] Müller | Effects of wood macro-and micro-structure on selected mechanical properties[END_REF], [START_REF] Poulsen | Compression in clear wood[END_REF], [START_REF] Smith | Fracture behavior of softwood[END_REF], [START_REF] Tabarsa | Stress strain response of wood under radial compression. Part I. Test method and influences of cellular properties[END_REF]. The underlying fracture criteria have been investigated by [START_REF] Daudeville | Fracture in spruce: experiment and numerical analysis by linear and non linear fracture mechanics[END_REF][START_REF] Eberhardsteiner | Mechanisches Verhalten von Fichtenholz, Experimentelle Bestimmung der biaxialen Festigkeitseigenschaften[END_REF] and [START_REF] Mackenzie-Helnwein | A multisurface plasticity model for clear wood and its application to the finite element analysis of structural details[END_REF]. Different angles to the grain have been investigated by [START_REF] Reiterer | Compressive behaviour of softwood under uniaxial loading at different orientations to the grain[END_REF]. Further publications deal with special aspects like influence of temperature and moisture content [START_REF] Bodig | Mechanics of wood and wood composites[END_REF][START_REF] Hamdan | Softening characteristics of wet wood under quasi static loading[END_REF][START_REF] Kollmann | Technologie des Holzes und der Holzwerkstoffe[END_REF][START_REF] Mascia | Effect of moisture content, specific gravity, temperature and number of annual growth rings on redwood elastic constants[END_REF][START_REF] Niemz | Physik des Holzes und der Holzwerkstoffe[END_REF][START_REF] Seborg | The compression of wood[END_REF], size effects [START_REF] Dill-Langer | Size dependency of tension strength in natural fiber composites[END_REF][START_REF] Schneeweiss | Der Groesseneinfluss im Druckversuch quer zur Faser[END_REF]) and creep dependency [START_REF] Keith | The mechanical behaviour of wood in longitudinal compression[END_REF].

Strain rate effects have been investigated by [START_REF] Reid | Dynamic uniaxial crushing of wood[END_REF] and [START_REF] Vural | Dynamic response and energy dissipation characteristics of balsa wood: experiment and analysis[END_REF]. Both studies were performed with Split Hopkinson Bars and deal therefore with large impact velocities and strain rates up to 5000 1/s. The impact velocity for 9 m drop tests with casks for radioactive material equals 13.3 m/s resulting in initial strain rates of about 20-40 s -1 (corresponds to wood thicknesses ranging from 330 mm to 660 mm). The published results do not allow a quantification of the influence for strain rates around 20-40 s -1 against static tests.

Information about compression under large deformations for wood has been published by [START_REF] Adalian | Wood model for the dynamic behavior of wood in multiaxial compression[END_REF], [START_REF] Easterling | On the mechanics of balsa and other woods[END_REF], [START_REF] Gibson | Cellular solids[END_REF]. All studies show a "lock-up" (Fig 1) (Gibson and Ashby 1999) after reaching a certain wood-dependent strain, where the tracheids are fully compressed and compressed wood behaves like cell wall material. Therefore, it hardens significantly. Furthermore, all studies reveal a softening behaviour of axially compressed wood, but plateau stress (stress after reaching compression strength) in stress-strain-curves (Fig 1) is presented as universally valid.

Wood behaviour in impact limiter

An analysis of wood filled impact limiter after drop tests with casks for radioactive material shows different compression characteristics for axially loaded wood depending on the zone in the impact limiter.

At the beginning of compression all zones in the impact limiter show comparable response mechanisms. After the elastic range compression strength is reached and wood fibre bundles buckle after lateral tensile failure (Fig 3). The number of buckle-points is not constant. An apparent transversal elongation is visible.

In subsequent deformation, energy absorption mechanisms differ depending on the position of wood in the impact limiter. Mostly axially compressed wood can be found in the main impact zone of the impact limiter (B Fig 2 ), where wood is crushed directly between cask and target. Wood in this area shows a large number of local buckling spots. Due to a lateral confinement from surrounded wood, it is mainly compressed in axial direction, and therefore it can be assumed that absorbed energy is relatively large (Fig 4).

Wood is laterally little restricted in zones that lie on the brink of the impact limiter (Fig 5). Wood fibres in this region can, after macroscopic lateral tensile failure [START_REF] Grosse | Zur numerischen Simulation des physikalisch nichtlinearen Kurzzeittragverhaltens von Nadelholz am Beispiel von Holz-Beton-Verbundkonstruktionen[END_REF], evade from axial compression. Wood fibres buckle less frequent and are afterwards compressed to a larger extent in transversal directions (Fig 5).

Although zone location in the impact limiter alters with different impact limiter construction, the different principles of energy absorption have been found on all analysed impact limiters [START_REF] Musolff | Results of full scale CONSTOR V/TC prototype 9m horizontal drop test[END_REF]Quercetti et al. 2008).

Work scope

The topic of strain rate dependency of mechanical properties of wood under large deformations has not been addressed yet. Although it is generally understood that higher strain rates lead to higher strengths [START_REF] Reid | Dynamic uniaxial crushing of wood[END_REF] the influence is difficult to estimate due to the -compared to earlier tests [START_REF] Reid | Dynamic uniaxial crushing of wood[END_REF][START_REF] Vural | Dynamic response and energy dissipation characteristics of balsa wood: experiment and analysis[END_REF] -relatively low strain rate of up to 40 s -1 for impact scenarios with casks.

The influence of lateral strain restriction on energy absorption capacity of wood has not been identified yet. Results from analysis of impact limiter wood indicate that lateral strain restriction increases energy absorption capacity for axially loaded wood. The influence is of major importance since the amount of energy which can be absorbed by the impact limiter has to be specified correctly.

Extensive impact limiter compression has to be ruled out since it leads to very high impact loads in the cask.

The following experimental investigation should therefore identify 1. the influence of strain rate on compression strength at strain rates from static to 30 1/s 2. the influence of strain rate on absorbed energy at high deformations at strain rates from static to 30 1/s 3. whether different lateral constraints have an influence on the energy absorption capacity of axially loaded wood.

Materials and Methods

Materials

The material used in this investigation was spruce-wood (Picea abies) of German production. The samples were made from wood of grade S10 (DIN4074-1 2008) with a moisture content of 15±3%. This equals strength class C24 according to DIN EN 338 (2008). The densities varied in a range from 400 till 450 kg/m³. The wood was without major macroscopic defects like knots or cracks. [START_REF] Tabarsa | Stress strain response of wood under radial compression. Part I. Test method and influences of cellular properties[END_REF] proposed cube shaped specimens of 8 mm length, while in DIN 52185 (1976) an ashlar shaped specimen with dimensions of 20 mm x 20 mm x 50 mm is proposed. However, this geometry is proposed for static tests.

The fibre arrangement and resulting failure mechanisms in the case of this study were to be as close as possible to the application in the impact limiters for tests with large deformations. Due to the available test equipment a minimum sample height of 100 mm was necessary in order to ensure a constant crushing speed.

Therefore cube shaped specimens with 100 mm edge length (Fig 6) were chosen.

The samples were consecutively numbered (Fig 6). They were arranged and named in groups according to impact speed (0.001 m/s, 0.3 m/s and 3.0 m/s, initial strain rate 0.01 s -1 , 3.0 s -1 and 30 s -1 ), loading direction (l for axial and q for transversal oriented wood) and lateral constraint (m with constraint and o without constraint). Hence, e.g. ol3000 is the sample group without lateral strain restriction, loaded axially at 3.0 m/s.

Methods

Test equipment

The servo-hydraulic 1000 kN impact test facility (Fig 7) [START_REF] Bam | BAM Impact testing machine[END_REF] was developed in order to investigate the behaviour of components under impact load, notably e.g. an impact of an aircraft on the reinforced concrete structure of a nuclear power plant containment. The maximum tensile or compressive force varies according to the used cylinder between 1000 kN and 630 kN at 600 mm cylinder travel. The maximum piston-speed equals 4.5 m/s and 8.5 m/s, respectively. The facility can be operated travel-or force-dependent. A forcetime-or travel-time-curve can be programmed. The dimensions are 8 m x 3 m x 2.6 m. For the experiments here the maximum available impact speed was 3.0 m/s, resulting in an initial strain rate of 30 1/s.

During the experiments hydraulic system pressure as well as force acting on the sample through a load cell situated below the wood sample (grey-red in lower right corner of Fig 7 ) were measured. Additionally system piston travel was measured by a magnetic induced system. Signals were recorded using a portable data logging device.

Realisation of lateral strain restriction

Steel sheets surrounding the wood sample were applied as lateral strain restriction (Fig 8). The steel sheets had 1 mm thickness. Tests were also performed with empty steel sheets in order to show that energy absorption by the steel housing is negligible compared to the energy absorbed by wood.

Performance of tests

The tests were performed inside a test building at ambient temperatures between 17-19°C. The samples were photographed and then inserted into the test facility.

After compression they were removed, photographed and documented. The signals were stored and later on processed.

Test plan

Table 1 shows a plan of the performed tests. Ten samples per parameter set were compressed in order to ensure statistical evaluation. Tested were axial grain direction at three different impact speeds and the whole set once with lateral strain restriction and once without any lateral strain restriction except for friction from load inducing surfaces, which were very finely machined. The three impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s) result in initial strain rates of about 0.01 s -1 , 3 s -1 and 30 s -1 .

Signal processing and statistical evaluation

The signals were processed after the tests in order to ensure comparability.

Average curves for every parameter set were calculated. The statistical evaluation was performed using the Sigmaplot-Software (Sigmaplot 2007). With help of paired t-tests for unconnected entities [START_REF] Rice | Mathematical Statistics and Data Analysis[END_REF] [START_REF] Dinwoodie | Timber: its nature and behaviour[END_REF] and [START_REF] Kazumi | Analysis of the Strength Data of wood Structures on Limit States Design. I. The influence of probabilistic and statistic method on specified values[END_REF].

The sometimes proposed Weibull distribution [START_REF] Kufner | Distribution of wood property values within a Norway spruce log[END_REF] is preferred for predicting the lower end of the distribution due to its weakest-link concept [START_REF] Dinwoodie | Timber: its nature and behaviour[END_REF] and therefore, here less suitable.

Results and Discussion

Resulting stress-strain curves 2 states the significance levels for different strain rates and the lateral strain restriction.

The compression strength increased for samples with lateral strain restrictions with higher strain rates from 0.01 s -1 (53 MPa) over 3 s -1 (62 MPa) to 30 s -1

(69 MPa). This corresponds to impact speeds of 0.001 m/s, 0.3 m/s and 3.0 m/s.

The increase of compression strength for samples without lateral strain restriction was comparable to samples with restriction (50 MPa, 62 MPa, 71 MPa). The influence of strain rate was statistically relevant for all impact speeds and lateral strain restrictions (Table 2). The lateral strain restriction did not have a significant influence on the compression strength at any impact speed.

Dependency of compression strength on the lateral strain restriction was not expected since Poisson factors in elastic compression are negligible for a-r (axialradial) and a-t (axial-tangential) direction [START_REF] Foster | Damping and poisson factor behavior in timber considered as an orthotropic material, part 1, the loss factor[END_REF]) and therefore no large lateral dilatations occur. However, absence of an increase in compression strength proofs that the strain energy of the metal sheet is negligible compared to the influence of strain rate and lateral strain restriction. If the steel sheet led to a significantly increased load the influence would have to be significant here.

Therefore, it can be concluded that lateral strain restriction did not affect the compression strength significantly.

Stress at strain level of 50% Fig. 11 shows box plots of stresses at strain levels of 50% under different strain rates and lateral strain restrictions. Table 3 states the significance levels for the influence of different strain rates and lateral strain restriction.

The stress at strain level of 50% increased with strain rate for samples with lateral strain restriction from static (30 MPa) to 3 s -1 (38 MPa), results were not significant for 3 -30 s -1 (40 MPa, compare Table 3). The difference in strain rate

(3 s -1 -30 s -1 ) might not be large enough to produce a result; more tests should be performed in order to clarify the result. The lateral strain restriction had significant influence on stress at a strain level of 50%. The average stress in static mode with lateral strain restriction was about 29.6 MPa, without lateral strain restriction about 13.3 MPa. This resulted in ca. 2.2 times higher stress levels for samples with lateral strain restriction than without, for the boundary conditions in these tests.

Samples without lateral strain restriction flew partly out of the test facility during the compression process with speeds of 0.3 m/s and 3.0 m/s. The faster the test velocity, the more fibres left the impact pad and were no more available for energy absorption. Therefore, results without lateral strain restriction at impact velocities of 0.3 m/s and 3.0 m/s (3 s -1 and 30 s -1 ) were physically not useful and disregarded here.

For two to three samples at each impact speed lateral strain restriction failed during the compression process (Fig. 12). Results from these tests were not taken into account for the statistical evaluation of stress and energy absorption at strain level of 50%.

Specific energy absorption at 50% deformation

Box plots of the strain energy levels at 50% deformation under different strain rates and lateral strain restrictions are shown in Fig. 13. Table 4 states the significance levels for different strain rates and the lateral strain restriction.

The absorbed strain energy at 50% deformation increased with strain rate for samples with lateral strain restriction from static (15.3 MJ/m³) to 0.3 m/s (18.9 MJ/m³). However, results were not significant for 3 s -1 -30 s -1 (Table 4).

The resulting strain rates might not have been unequal enough; more tests should be performed in order to clarify the result. The lateral strain restriction had significant influence on absorbed strain energy at 50% deformation. The average absorbed strain energy in static mode with lateral strain restriction was about 15.3 MJ/m³, without lateral strain restriction about 11.9 MJ/m³. This resulted in approximately 30% higher specific energy absorption for samples with lateral strain restriction than without, for the boundary conditions in these tests.

The decrease of energy absorption with higher strain rates for samples without lateral strain restriction can be explained according to 3.3.

Summary of results

Mean values and standard deviations of compressive strength, stress at a strain level of 50% and absorbed specific energy (strain energy) are shown in Tables 5 -

Conclusion

The influence of strain rate on the compression strength of spruce wood was quantified and proofed to be statistically significant. Compared to static tests ain the scope of packages for radioactive materials interesting -strain rate of 30 s -1 led to approximately 30% larger compression strength. The influence of strain rate between static and 30 s -1 on specific energy absorption and stress at strain level of 50% was significant, a larger strain rate led to larger energy absorption. The increase in stress at a strain level of 50% was about 36%; the increase of absorbed energy was 23%.

Compression strength was not dependent on lateral strain restriction. Stresses at a strain level of 50% were significantly affected by lateral strain restriction. With lateral strain restriction, consisting here of a 1 mm steel sheet lateral encapsulation, the stress at a strain level of 50% was by a factor of 2.2 higher than without lateral strain restriction. Absorbed specific energy at 50% deformation (strain energy) was increased by 30% for samples with lateral strain restriction of 1 mm steel sheets.

Therefore, it can be concluded that for axially loaded wood under large deformations a generally valid yield curve can not be defined. The dependency of stress and therewith energy absorption capacity on lateral strain restriction leads to a set of yield curves. The choice of yield curve is then dependent on the lateral strain restriction.

This work was co-sponsored by German Federal Ministry for Education and Research under contract 02S8274. The authors thank Gesellschaft für Nuklear-Service mbH (GNS) and Mitsubishi Heavy Industries (MHI) for the permission to publish pictures of their drop test objects taken at the BAM drop test facility. for samples with lateral strain restriction 1 mm steel sheet (ml) and samples without lateral strain restriction (ol) for sample cross sections of 0.01 m² and sample heights of 0.1 m.

Figures English

Abb deutsch

Abb 3.0 m/s, initial strain rates 0.01 s -1 , 3.0 s -1 and 30 s -1 ) and strain restriction on stress at strain level of 50% for samples with lateral strain restriction 1 mm steel sheet (ml) and samples without lateral strain restriction (ol) for sample cross sections of 0.01 m² and sample heights of 0.1m on stress at strain level of 50% 4 Significance levels (p) for influence of three different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, initial strain rates 0.01 s -1 , 3.0 s -1 and 30 s -1 ) and strain restriction on strain energy at 50% deformation for samples with lateral strain restriction 1 mm steel sheet (ml) and samples without lateral strain restriction (ol) for sample cross sections of 0.01 m² and sample heights of 0.1m on stress at strain level of 50% 

Fig 9

 9 Fig 9 presents average stress strain curves for samples with lateral strain restriction (left) and without (right). Since the cross section is 0.01 m² and sample height is 0.1 m, the earlier force deformation relationships can be obtained by multiplying abscissa values with 0.1 m and ordinate values with 0.01 m². The often described lock up or densification at high levels of strain (Gibson and Ashby 1997) has not been reached in the test, strain levels remain to small. However, the beginning of densification is clearly visible in Fig 9.

Fig 1 Fig 2 Fig 3 Fig 4 Fig 5

 12345 Fig 1 Stress-strain relationship for axially loaded balsa-wood from Gibson and Ashby (1997) Fig 2 Cask with attached impact limiters (white) at both ends after 9 m horizontal drop test, including schematic sketch of left-end impact limiter cross section with zones of different energy absorption (A, B) Fig 3 Piece of spruce wood from deformed impact limiter; beginning of buckling after lateral tensile failure Fig 4 Impact limiter zone of mainly axial wood compression and large energy absorption (comparable to zone B in Fig 2) Fig 5 Impact limiter zone with deformation of laterally little restricted pieces of wood (comparable to zone A in Fig 2) Fig 6 Cube-shaped sample of spruce wood in dimensions of 100 mm x 100 mm x 100 mm Fig 7 BAM 1000 kN impact test facility Fig 8 Lateral strain restriction of axially loaded wood by partial (lateral) encapsulation with 1 mm welded steel sheets before (left) and after (right) compression test Fig 9 Average stress strain curves of axially loaded spruce wood for different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, initial strain rates 0.01 s -1 , 3.0 s -1 and 30 s -1 ) for sample cross sections of 0.01 m² and sample heights of 0.1 m; left: with lateral strain restriction (1 mm steel sheet); right: without lateral strain restriction Fig 10 Box plots of compressive strength of axially loaded spruce wood for three different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, initial strain rates 0.01 s -1 , 3.0 s -1 and 30 s -1 ) for samples with lateral strain restriction 1 mm steel sheet (ml) and samples without lateral strain restriction (ol) for sample cross sections of 0.01 m² and sample heights of 0.1 m Fig 11 Box plots of stress at a strain level of 50% of axially loaded spruce wood for three different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, initial strain rates 0.01 s -1 , 3.0 s -1 and 30 s -1 ) for samples with lateral strain restriction 1 mm steel sheet (ml) and samples without lateral strain restriction (ol) for sample cross sections of 0.01 m² and sample heights of 0.1 m. Fig 12 Failed lateral strain restriction Fig 13 Box plots of strain energy at 50% deformation of axially loaded spruce wood for three different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, initial strain rates 0.01 s -1 , 3.0 s -1 and 30 s -1 )

  Abb, 1 Spannungs-Dehnungs-Zusammenhang für axial beanspruchtes Balsaholz nach Gibson und Ashby (1997) Abb, 2 Behälter mit befestigten stoßdämpfenden Bauteilen an beiden Enden nach einem 9m Horizontalfallversuch inklusive einer Skizze des Querschnitts des linken Stoßdämpfers mit verschiedenen Energieabsorptionszonen Abb, 3 Fichtenholzstück aus verformten Stoßdämpfer, beginnendes Ausknicken nach Lateralzugversagen Abb, 4 Stoßdämpferbereich mit hauptsächlich axialer Holzkomprimierung und großer Energieabsorption (vergleichbar mit Zone B in Abb. 2) Abb, 5 Stoßdämpferbereich mit Holzkomprimierung unter geringer seitlicher Dehnungsbehinderung (vergleichbar mit Zone A in Abb. 2) Abb, 6 Würfelförmige Fichtenholzprobe mit den Abmessungen 100 mm x 100 mm x 100 mm Abb, 7 BAM 1000 kN Stoßprüfstand Abb, 8 Partielle seitliche Dehnungsbehinderung mit 1 mm starkem verschweißtem Stahlblech vor (links) und nach (rechts) dem Versuch Abb, 9 Mittlere Spannungs-Dehnungs-Verläufe von axial beanspruchtem Fichtenholz für verschiedene Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 and 30 s -1 ) für eine Probenquerschnittsfläche von 0.01m² und einer Probenhöhe von 0.1m; links: mit lateraler Dehnungsbehinderung (1mm Stahlblech); rechts: ohne seitliche Dehnungsbehinderung Abb, 10 Box Plots der Druckfestigkeit von axial beanspruchtem Fichtenholz für verschiedene Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) für Proben mit seitlicher Dehnungsbehinderung (ml) und Proben ohne seitliche Dehnungsbehinderung (ol) für eine Probenquerschnittsfläche von 0.01m² und einer Probenhöhe von 0.1m Abb, 11 Box Plots der Druckspannung bei 50% Stauchung von axial beanspruchtem Fichtenholz für verschiedene Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) für Proben mit seitlicher Dehnungsbehinderung (ml) und Proben ohne seitliche Dehnungsbehinderung (ol) für eine Probenquerschnittsfläche von 0.01m² und einer Probenhöhe von 0.1m Abb. 12 Probe, bei der seitliche Dehnungsbehinderung versagt hat Abb. 13 Box Plots der Verformungsenergien bei 50% Stauchung von axial beanspruchtem Fichtenholz für verschiedene Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) für Proben mit seitlicher Dehnungsbehinderung (ml) und Proben ohne seitliche Dehnungsbehinderung (ol) für eine Probenquerschnittsfläche von 0.01m² und einer Probenhöhe von 0.1m

  Fig 01

  

  

  the statistical relevance of the differences resulting from different test conditions was assessed. Results were regarded as statistically relevant when their difference was distinct with a level of

	significance of	p	<	α	with	α	=	0	.	05	. Paired t-tests require a Gaussian normal

distribution, which is approximately given for mechanical properties of wood as compression strength according to

Table 3

 3 Signifikanzniveaus (p) des Einflusses dreier Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) auf die Druckspannung bei 50% Stauchung für Proben mit seitlicher Dehnungsbehinderung (ml) und

	Proben ohne seitliche Dehnungsbehinderung (ol) für eine Probenquerschnittsfläche von 0.01m²
	und einer Probenhöhe von 0.1m			
		ml1	ml300	ml3000	ol1	ol300	ol3000
	ml1	-	0.0249	9.6e-3	2.3e-4	
	ml300		-	0.371		0.0
	ml3000			-			3.3e-9
	ol1				-	0.0185	5.1e-3
	ol300					-	0.159
	ol3000						-
	Table					

Table 5

 5 Mean values and standard deviations of compression strength for axially loaded spruce wood for different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, resulting initial strain rates 0.01 s -1 , 3 s -1 , 30 s -1 ) for samples with (ml) and without (ol) lateral strain restriction Tab 5 Mittelwerte und Standardabweichungen der Druckfestigkeiten von axial beanspruchtem Fichtenholz für drei Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) für Proben mit (ml) und ohne (ol) seitliche

	Tab 4 Signifikanzniveaus (p) des Einflusses dreier Beanspruchungsgeschwindigkeiten (0.001 m/s,
	0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) auf die
	Energieabsorption bei 50% Stauchung für Proben mit seitlicher Dehnungsbehinderung (ml) und
	Proben ohne seitliche Dehnungsbehinderung (ol) für eine Probenquerschnittsfläche von 0.01m²
	und einer Probenhöhe von 0.1m			
		ml1	ml300	ml3000	ol1	ol300	ol3000
	ml1	-	1.3e-3	1.3e-3	0.037	
	ml300		-	0.90		4.0e-10
	ml3000			-			3.1e-7
	ol1				-	8.2e-3	0.0101
	ol300					-	0.57
	ol3000						-

Table 6

 6 Mean values and standard deviations of stress at a strain level of 50% for axially loaded spruce wood for different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, resulting initial strain rates 0.01 s -1 , 3 s -1 , 30 s -1 ) for samples with (ml) and without (ol) lateral strain restriction Tab 6 Mittelwerte und Standardabweichungen der Spannung bei 50% Stauchung von axial beanspruchtem Fichtenholz für drei Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) für Proben mit (ml) und ohne (ol)

	seitliche Dehnungsbehinderung		
	Test set	Mean [MPa]	Standard deviation [MPa]
	ml1	29.6	7.01
	ml300	38.0	5.31
	ml3000	40.4	2.22
	ol1	13.3	6.05
	ol300	7.9	2.54
	ol3000	5.2	5.31

Table 7

 7 Mean values and standard deviations of specific energy (strain energy) at 50% deformation for axially loaded spruce wood for different impact speeds (0.001 m/s, 0.3 m/s, 3.0 m/s, resulting initial strain rates 0.01 s -1 , 3 s -1 , 30 s -1 ) for samples with (ml) and without (ol) lateral strain restriction Tab 7 Mittelwerte und Standardabweichungen der Verformungsenergie bei 50% Stauchung von axial beanspruchtem Fichtenholz für drei Beanspruchungsgeschwindigkeiten (0.001 m/s, 0.3 m/s, 3.0 m/s, entspricht initialer Dehnrate von 0.01 s -1 , 3.0 s -1 und 30 s -1 ) für Proben mit (ml) und ohne

	(ol) seitliche Dehnungsbehinderung	
	Test set	Mean [MJ/m³]	Standard deviation [MJ/m³]
	ml1	15.3	1.40
	ml300	18.9	1.69
	ml3000	18.8	1.49
	ol1	12.0	3.22
	ol300	8.4	1.55
	ol3000	7.0	2.97