Wood as a bioinspiring material

Materials Science and Engineering C 31 (2011) 1174–1183

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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c

Wood as a bioinspiring material Stefanie E. Stanzl-Tschegg ⁎ University of Natural Resources and Life Sciences, Vienna, Institute of Physics and Materials Science, Peter Jordan Straße 82, 1190 Vienna, Austria

a r t i c l e

i n f o

Article history: Received 31 May 2010 Received in revised form 15 November 2010 Accepted 6 December 2010 Available online 10 December 2010 Keywords: Wood Bioinspiration Structure–property relationship Mechanical optimization Fracture tolerance

a b s t r a c t Wood is one of the natural materials that inspired mankind very early to imitate some of its features to develop new “intelligent” artificial materials. The properties of wood are optimized in various respects and in various areas. It exhibits, for example, excellent mechanical properties together with low density, and both are determined by its structural design. In the present paper, the effects of structural features of wood on selected mechanical properties and especially fracture properties are discussed. Some examples of how manmade materials are tailored by mimicking wood's intriguing optimization principles are provided. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Bioinspired materials play an increasing role in many fields of practical application today, and optimization requirements exist in different directions. During the last decade, the structural features of natural materials, such as wood, bone, shells and spider silk, were characterized more and more in detail by new physical and chemical techniques; this significantly improved the understanding of the correlation between structure and the extraordinary properties of several natural materials. Nature optimized its materials for multiple purposes. Efforts were undertaken to use the underlying principles for the design of new optimized synthetic materials and/or structures or to use natural materials as prototypes and models themselves. Sophisticated mechanical, physical and chemical techniques serve to develop and fully characterize new bioinspired materials with specific optimized properties. Cell structure, layered structure and hierarchical structure copied of wood, for example, are applied by architects or by the transportation industry to achieve optimized mechanical stability, flexibility and weight reduction. Similar optimized structures are also copied from other biological materials, such as other plant stems, bone, glass sponge skeletons, bird beaks etc., although their chemical composition partly is quite different. The importance of structural design is considered when new synthetic materials or surfaces are

⁎ Tel.: +43 1 47654 5160; fax: +43 1 47654 5159. E-mail address: [email protected]. 0928-4931/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2010.12.001

designed according to the principles of natural materials. In the field of composites and reinforced polymers, for example, special synthetic multi-compartmented fibers offer a promising new tool to mimic the natural healing processes in wood and wood like structures. In order to optimize the mechanical and fracture properties of new materials or components, several features of wood as a natural material may serve as inspiration, which is meant by “bioinspiring material” in the following. Some properties of wood, however, are considered as disadvantages, especially if special applications are envisaged. As these, of course, cannot be seen as “inspiring” (rather on the contrary), they will not be discussed in the context of bioinspiring materials in the following. An example of such disadvantages of wood would be e.g. the lower absolute strength as compared to metallic materials contrary to the relative strength (which is normalized with density). It could cause problems if constructions with a high load bearing capacity are required. Also, the high anisotropy of the mechanical properties and the different kinds of flaws in wood, such as knots, may be considered as drawbacks, if homogeneous properties are needed or expected. Likewise, the natural decay of wood has to be considered as disadvantage in constructions which are aimed to keep their mechanical properties and an unchanged (stable) structure for long times. However, trying to understand the principles of optimization strategies used by nature makes it possible to improve related properties of man-made materials. In this sense, the present study tries to give a limited overview of the influence of some structural features on the mechanical and fracture properties of wood and of their implications for the structural design of man-made materials. Furthermore, recent results on the relationship between structure and fracture tolerance of wood are reported.

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2. Mechanical properties, structure of wood and implications for optimizing man-made materials 2.1. Wood – a complex, hierarchically structured composite material One of the special features of wood, which contribute to its manifold optimized properties, is its hierarchical architecture [1]. The basic structural units of this architecture are elongated cells [2], which form layers, which finally result in a complex anisotropic construction. On a macroscopic scale, year rings form the layer structure, and on a microscopic level, the cells of early and late wood with systematically varying wall thicknesses become visible as the elements of the layers. On a sub-microscopic scale, the cell walls show another layer structure, which, in addition, has different inclinations of the micro (or elementary) fibrils in each layer. On the nanometer level, molecular chains of carbon hydrogen and oxygen are the chemical constituents of the microfibrils. The complex structure of wood leads to an anisotropy of several properties. Nevertheless, the anisotropy is not complete, and special properties, such as strength or ductility, are similar in the two transverse (radial, tangential) directions and very different in the axial (longitudinal) direction). This is caused by the elongated shape of the cells and by their arrangement in layers. Speaking in technical terms, the cells and layers of wood form a composite material. It is even a “graded” material, where a gradual increase of density caused by increasing cell wall thicknesses from early wood to late wood cells may be seen. Metal-metal or metal-polymer composites for special requirements are designed in a similar way today.

2.2. Wood – a cellular and layered material: Consequences for mechanical and fracture properties and applications

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Fig. 1 shows the fracture surface of a beech specimen, which demonstrates that the cellular structure of wood is much more complex than of a man-made metal foam material [3]. The cross sections of mainly longitudinal tracheids (fibers) of different size are visible, but also numerous fibers oriented perpendicular to these tracheids, the socalled wood rays. They reinforce the tree like radial bracings [4]. Fracturing of the beech specimen shown in Fig. 1 was achieved by loading the specimen with a cyclic strain of 2 × 10−3 during 1 × 107 cycles at a frequency of 20 kHz. Cell wall fractures, fracture along rays, and also deviations of the main crack into other crack planes occurred. Thus, Fig. 2b does not only give the reasons for the good strength properties of wood, but also for its excellent fracture properties in comparison to a simple-structured cellular material. It demonstrates that the complex structure of differently oriented fibers in a “softer” matrix (lignin) has led to a complicated kind of fracturing. As a result, retardation of crack propagation and non-linear elastic behavior is observed (for more details see chapter 2 and [2]). The thicker and thus stronger cell walls of latewood may act as reinforcement and thus may contribute to a higher fracture resistance as compared to those of earlywood. This is especially effective if the loading is in the longitudinal direction and crack propagation transversely to this (LR or LT systems). However, only a slight influence on the fracture process is detected, when loading is in the transverse plane and crack propagation transversally or longitudinally (RL, TL, RT, TR) [32]. Rays exert another interesting effect. As rays are fibers oriented perpendicularly to the longitudinal direction (i.e., they lie in the transverse plane), they lead to a reinforcement of strength and fracture mechanical values in radial direction under RL or TL loading conditions [30]. If crack propagation is in the transverse plane (i.e., RT or TR loading) rays may act as either reinforcement (for RT loading) or may lead to easier crack propagation (for TR loading): In the second case, fracturing along rays was observed by cell wall peeling, creating smooth

The fibers, being hollow cells, cause the low density of wood [2], which is imitated by material scientists today by developing metallic foams in order to reduce the weight of moving parts of automobiles, airplanes, aerospace equipment and machines as well as medical devices. Another “intelligent” strategy of nature is to construct wood cells not only as symmetric cells, but as strongly elongated tubes (tracheids). This leads to high stiffness of wood in longitudinal loading direction, i.e., high modulus of elasticity and higher compressive and tensile strength in longitudinal direction than transversally. These properties give the living tree a high static stability and allow the use of wooden beams for buildings.

Fig. 1. Wood – a cellular composite: Different cell sizes, different orientation of tracheids and rays, longitudinal shape of cells (lengthy rays are visible). Fracture surface was obtained by 20 kHz fatigue loading of beech wood with a cyclic strain amplitude of 2.5 × 10−3 during 1 × 107 cycles.

Fig. 2. Laminate composites with fibers as reinforcement. (a) Fracture of ARALL® laminate containing Al alloy layers and high-strength Aramid fibers in Epoxy layers by 20 kHz fatigue loading with a cyclic stress of 136 MPa (fracture after 7.94 × 107 cycles). Fibers are still intact, although outer Al alloy sheets have broken, bridging the crack and protecting the inner Al alloy from breaking [18]. (b) Fracture of a beech specimen, which was loaded in tangential direction; crack grew in radial direction. Fiber bridging of already separated crack surfaces.

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surfaces [32] in accordance with Ashby et al. [2], who identified rays as planes of weakness and preferred crack paths. The mechanical properties of wood as a cellular material were modeled conclusively mainly by Gibson and Ashby [2], whereby the relative thickness of the cell walls, and thus the relative density (ratio of wood density and cell wall density) were recognized as main factors in determining the mechanical properties - besides moisture content, strain rate and temperature. Thus, the above-mentioned much higher stiffness, i.e., higher moduli of elasticity in longitudinal direction EL (along the cell axis) than in transversal direction ET and ER (perpendicular to the cell axis) is confirmed. Assuming cell wall extension or compression for longitudinal loading leads to a linear dependence of EL on the density. Cell wall bending is made responsible for deformation under loading in tangential and radial direction, so that ET and ER vary with the square of relative density. The compressive strength values of wood are determined by analogous mechanisms and vary linearly with the square of the density. Similar large differences in longitudinal and radial/ tangential direction are found for the shear moduli, and the shear strengths follow similar patterns as the shear moduli. Modeling the fracture properties of wood on basis of cell structural features by [2] results in a principally same dependence of the fracture toughness on the relative density. KIC values by a factor of about 10 higher for a crack parallel to the grain than perpendicular were reported. This can be explained by the low-energy, peeling mode of fracture along the boundaries between fibers and tracheids. Reinforcement of many man-made structures by fibers (metallic, ceramic, carbon; oriented, non-oriented, short, long) is a well known strategy, which is applied for improving alloys, for designing metalpolymer composites, and also large structure elements like concrete are reinforced by steel or textile fibers [5,6]. Quite often, a composite consisting of different layers is formed. Fig. 2a shows the ARALL laminate [7], which is composed of 1.3 mm thick Al alloy and Epoxy sheets containing Aramid fibers as an example. Such a specimen was fatigue loaded with a cyclic stress amplitude of 136 MPa (testing frequency 20 kHz) in longitudinal direction (parallel to the fibers), and fracture took place after 7.94 × 107 cycles. Cracking of the Al alloy sheet occurred perpendicular to the fiber direction, and delamination between the metal, epoxy and fiber interfaces took place, leading to a redistribution of the stresses ahead of and behind the crack tip. The major part of the Aramid fibers remained intact and kept the compound together, even after the two outer Al alloy layers had broken. By using fibers, fatigue-damage tolerant behavior is achieved, which was the purpose for developing this class of materials as construction materials for airplanes [8]. In wood, the fibers are made mainly of cellulose and the matrix mainly of lignin–hemicellulose. The former may cause fiber-strengthening and fiber-bridging in different orientations to the crack plane. As an example, Fig. 2b shows the crack surfaces of a beech specimen, which was loaded in tangential direction. The crack propagated in radial direction (TR crack growth system), and fibers bridging the already separated crack surfaces are visible. In conclusion – and as described in detail by Gibson and Ashby [2] – several mechanical properties, such as elastic modulus, shear modulus, strength and fracture toughness may be ascribed to, explained and modeled by the pore structure of wood (as effect of porosity, without regarding the hierarchical structure). This means that a characterization on a microscopic level, considering a fiber reinforced composite consisting of single fibers/fiber bundles in an amorphous matrix forming layers, leads to satisfying results. Huang and Gibson [9] developed performance indices, describing the mechanical efficiency of materials by referring their mechanical properties (especially elastic modulus E and strength σ) to their density ρ (or mass), i.e., considering their specific mechanical properties. With this and modeling beams and plates as material with a honeycomb microstructure, they conclude that wood has an optimized structure: “Wood corresponds to the best possible choice for the purpose of

building columns which do not buckle. Even though wood itself is a polymeric material, its mechanical performance for this task is better than typical polymeric materials used in engineering. This advantage in the performance index E/ρ2 results mainly from its low density.” Fig. 3 visualizes this outstanding property of wood in comparison with bone, ceramics and engineering polymers. Summarizing, one of the most important consequences of the pore structure of wood and many other biological materials is the low specific weight (cell structure), which in addition is quite often combined with high stiffness, flexibility and high toughness. Thereby, a great variety of structural means to combine low weight with optimized stiffness and flexural response has been developed by nature. This may be seen in many plant stems, such as bamboo culms with a density gradient in the tube wall [10] or the radial density gradient of palm stems, which, for example, leads to an increase of the flexural stiffness of about a factor of 2 higher than for the stem of equivalent uniform density [10]. The idea of making use of the complicated pore structure of wood for technical applications led to the production of templates by different techniques such as chemical procedures or pyrolysis of wood [11]. With these, “biomorphic” cellular Silicon carbide ceramics are obtained by subsequent liquid infiltration of Si, SiO and CH3SiCl3 into the carbonized template. The resulting material shows a complex mechanical behavior, such as a pronounced anisotropy with respect to the orientation of the cell structure. Stress–strain behavior with less brittle deformation and a higher fracture resistance are obtained, and application of this material as high temperature resistant catalyst and filter support structure is reported. Thermal degradation of the wood polymers and evolution of the atomic/molecular and mesoscopic structure of the new material was studied by wide- and small-angle X-ray scattering and Raman spectroscopy [12]. 2.3. Hierarchical structure of wood, cell wall features and mechanical consequences If one aims to know about the reasons and development of special properties, the hierarchical structure of the cell wall has to be considered. It leads to the capability of adaption, for example, and “optimized” properties. Some examples are reviewed in the following. Wood fibers with a diameter of ~40 μm and a length of several hundred micrometers, themselves, are made up by fibers (diameter~ 2.5 nm, length~11 nm), the microfibrils, which are arranged in several layers, the cell walls, on a sub-microscopic scale. The cell wall is a composite and consists of bundles of cellulose micro (elementary)-fibrils embedded in a polymeric (lignin–hemicellulose) matrix (Fig. 4a) [13]. Man-made ropes are designed in a similar way (Fig. 4b). The elementary fibers have

Fig. 3. Map of elastic modulus E vs. density ρ according to [9], comparing wood with cortical and trabecular bone, as well as bamboo, ceramics and engineering polymers. The two lines indicate slopes of performance indices E/ρ and E/ρ2.

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the MFAs were measured from pith to bark and a decrease of the angles recognized, which means a decrease with the age of a tree [14]. In Fig. 5a, the MFA in spruce (Picea abies [L.] Karst) can be seen to decrease from 35° near the pith to 0° near the bark. In a region between 50 and 130 mm away from the pith, the MFA jumps between 0° in early wood and 20° in latewood in each annual ring. In the outermost part of the stem, the MFA in late wood drops to 0°. Late wood with a MFA of 0° was also found in some annual rings nearer to the pith, but only in the one in which the proportion of early wood was very low. Additionally it was found that a decrease of the fibril angle entails an increase of the modulus of elasticity and a decrease of the fracture strain values (Fig. 5b) [15,16]. Obviously the tree optimizes these features depending on its requirements: The old tree with its higher weight needs a high static strength and stiffness, whereas the young slim tree needs high flexibility. Both properties can be obtained by varying the microfibril angles. An even more sophisticated optimization strategy of strength/ stiffness and flexibility was observed for branches by means of microfibril angle (MFA) measurements of a branch of Picea abies [L] Karst [17]. The change of the MFA was measured for increasing age, size and therefore

Fig. 4. Fiber bundles, wound in spirals around cell (rope) axis. (a) Wood cell wall consisting of several layers of varying thickness and varying angles of the cellulose microfibril angles, which are embedded in a lignin–hemicellulose matrix [13]; a magnified elementary fiber of S2 is shown. (b) Similar structure of man-made rope. Fiber nature and load sensitive angles lead to high strength and flexibility.

different orientations in the different wood cell wall layers, and the angles to the cell axis differ distinctly. The S2 as the thickest cell wall mainly determines the mechanical response of wood and the cellulose microfibrils run approximately parallel to one another there and wind in spirals around the cell axis in a usually steep angle (Fig. 4a). Strength and flexibility depend on this angle, and instead of a solid uniform material, the layers give the tree or rope not only a higher strength but especially a higher flexibility. The angle of the cellulose microfibril bundles in the cell walls is an important structural design feature of wood for optimizing strength and flexibility according to the tree's requirements. Variation of these microfibril angles (MFAs) is obviously used by nature to obtain different flexibilities, depending on the demands of the living tree [14]. High angles, as observed by small-angle X-ray scattering in early wood of spruce, allow for a higher flexibility whereas lower angles give high stiffness in longitudinal direction. Extensive investigations have been performed on measuring MFAs, especially with the smallangle X-ray scattering (SAXS) technique, and several systematic features could be detected. In studies on spruce, pine, oak and beech,

Fig. 5. Correlation of microfibril angle (MFA) and mechanical properties. (a) MFAs as a function of the distance from the pith. Full circles: late wood, open circles: early wood. Error bars indicate the Gaussian width of the angle distribution. Decreasing MFAs from pith to bark parallel to the age of the tree (high angles: juvenile wood, low angles: mature wood) [14]. (b) Mechanical properties as a function of the MFA. Upper diagram: longitudinal elasticity modulus, lower diagram: maximum extensibility of the cell wall [16].

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increasing gravitational load of the branch. It was found that in compression wood, the MFA decreased continuously from the trunk towards the tip in all annual rings. In opposite wood, however, the course of the microfibril angles changed in addition considerably with the age of the branch. In the first annual rings, large MFAs were measured providing high flexibility to the young branch. In the subsequent annual rings, a gradual stiffening of opposite wood with very small microfibril angles was observed. These results indicated that not only the so-called reaction wood on the compression side responds to different loading conditions but also the “normal” wood on the opposite side. It was suggested that the stiff regions in the opposite wood with very small MFAs may act as an effective reinforcement, keeping the branch from too high bending under its own weight and thus protecting the branch from compressive failure. 2.4. Other specific structure–property relationships Numerous other optimization examples of wood may be discussed, for example such which are related to the design of structural (macroscopic) shapes. In a living tree, structural shapes are adapted by strain distributions in a way that stresses are reduced. Mattheck [18] was the first who demonstrated the natural optimization of branch junctions in a tree by means of finite element modeling and fracture mechanical studies. He showed that optimizations result in avoiding or reducing the fracture susceptibility. Resulting consequences for the

design of technical applications in machining, car industry, orthopedics and architecture are manifold and were treated in [18]. Even a special kind of self-repair mechanism takes place in wood (on a molecular level): This is partly based on the unique feature of a natural material being a living material, which of course cannot be imitated by mankind. It could be shown that molecular bonds are restored even after wood has been stressed beyond the yield stress, i.e., after it was irreversibly deformed [19]. This was demonstrated by synchroton insitu stress relaxation experiments on reaction wood (see load drops in Fig. 6a). Simultaneous registration of the microfibril angle (MFA) shows a continuous decrease (Fig. 6b), while the microstrains (Fig. 6c) [1] increase steadily. If the elongation is kept constant temporarily in the second part of the stress–strain curve with lower stiffness, microstrain as well as MFA remain constant, while stress relaxation occurs by viscous flow (load drop). After re-loading, a temporarily high slope (stiffness) being identical with the initial high slope is registered until deformation continues again along the second part of the stress–strain curve (Fig. 6d). The microstrains were modeled [1] assuming that the cellulose fibrils are rigid and carry most of the load, while all deformation takes place by shearing of the (deformable) hemicelluloses– lignin matrix in the cell wall. This is possible via a strong binding between fibrils and matrix. It is assumed that a hemicellulose layer is attached partly to the crystalline parts of the cellulose fibrils, whereas the other part forms a hydrogen-gel like matrix with hydrogen bonds.

Fig. 6. Mechanical and structural response of spruce wood in an in situ synchrotron experiment [1,19]. (a) Stress–strain curve with initial high stiffness and four relaxations by interruptions of macroscopic straining in the post yield phase. (b) Decrease of microfibril angle MFA with increasing strain. (c) Increase of microstrain with increasing macroscopic strain and prediction assuming stiff fibrils and shearing of matrix [1]. (d) Stress relief and momentarily stiff response of “damaged” material after interrupting strain application and subsequent more “ductile” response according to slope of post yield curve. (e) Molecular bonds between cellulose fibrils and initial MFA μ. (f) Reformed molecular bonds and reduction of MFA to μ’ above yield point.

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This enables shearing of the hemicellulose matrix upon tension or compression load application and tilting of the fibrils (see decrease of MFA μ in Fig. 6e to μ′ in Fig. 6f). The almost perfect coincidence of the microstrains and the applied macrostrains, leads to the conclusion that the fibrils indeed do not deform, but that deformation takes place by shearing of the hemicelluloses–lignin matrix. The immediate recovery after stress release is attributed to the re-forming of unspecific bonds in a new position of the fibrils (Fig. 6f). This mechanism is, in principle, similar to the macroscopic mankind made Velcro connection as known from burdocks and has been imitated, especially by the clothing industry, for about 20 years. 2.5. Similarity in structure of wood with other hierarchical biological systems Besides wood, numerous other biological materials, such as bone, cartilage, tendon, dentin, muscles, nacre, glass sponge skeletons, bird beaks, shells of mussels and beetles, cork, plant stems [1,2,20,21], etc., are structured in a hierarchical way over many length scales, and each structural feature serves/optimizes a certain property. Many features are directed towards attainment of a certain mechanical response, and high stiffness in spite of low weight often plays an important role. Therefore, a cellular construction with various kinds of stiffening means, based on a hierarchical architecture, was developed by nature. Certain bones, such as femur and tibia, for example, consist of a hard and stiff outer tubular bone shell, which is partly filled with a spongy trabecular bone of high (approximately 80%) porosity [22–24]. Wood cell wall, bone or tendon structure is similar in the sense that fibrils (cellulose or collagen) are embedded in an amorphous polymeric matrix. Skeletal muscles are also reported to form a hierarchical structure with thousands of force-producing muscle fibers arranged within a tissue network [25]. Although many biological materials have partly similar structure plans (though several variations of structural features may be observed), which lead to a similar mechanical response, their compositions may be entirely different. The wood cell wall and also muscles, for example, are almost purely polymeric composites (cellulose, collagen), bone and nacre are organic-inorganic composites of polymer and mineral portions and the glass sponge skeleton consists of almost pure silica mineral. The deformation mechanism found in bone shows many similarities with that of wood (see 1.4). A model for the deformation of bone was constructed for external tensile loading at different levels of structural hierarchy by Gupta el al. [22]. The high toughness of bone is considered to be a result of its ability to dissipate deformation energy without crack propagation. Different mechanisms, such as microcracking ahead of the crack tip, crack deflection and blunting at interlamellar interfaces, and crack bridging in the wake of the crack are made responsible for this on a microscopic level. At a sub-microscopic (fibril arrays) and nanometer (mineralized collagen) level, the mechanical response of the stiff, long and thin mineralized collagen fibrils, lying parallel to each other and being separated by a 1–2 μm thin matrix, was investigated by in situ tensile testing with synchrotron X-ray diffraction and scattering. Besides tensile deformation of the fibrils, application of a load leads to shear deformation of the inter-fibrillar matrix by successive breaking of a series of “sacrificial bonds.” A mechanical model with a staggered arrangement of mineral particles on collagen fibrils [26] was assumed. Thus, it is concluded that deformation of bone could be associated with plastic deformation in a thin glue-like layer between the fibrils. Tendon is treated [1] as hierarchically structured material similar to bone, with the difference that tendon normally is not mineralized. The similar mechanical properties of wood and bone and also the antiseptic properties of wood soon inspired mankind to apply wood as bone-implant material [27]. Reports on wood implants in human orthopedy go back to the Inka period, and resinous softwood was used since that time to stabilize bone fractures [28]. The similar strength of cortical bone and wood as well as the similar density of trabecular

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bone and wood, but also the similar structural hierarchies and chemical composition are considered as important for the integration of wood. In more recent times, Gross and Ezerietis [29] even demonstrated the similar mechanical properties of wood and bone by placing them in an Ashby map (similar to Fig. 3) and performing experiments on Juniper wood with the goal of using it as possible implant material. 3. Structural design and fracture tolerance of wood The layered and cellular composite structure of wood is essentially effective in leading not only to optimized mechanical and physical properties, such as strength, strain and density, but also especially to a “fracture-tolerant” behavior of wood. Some more recent studies on design principles contributing to fracture tolerance are described in the following. 3.1. Experimental procedure and evaluation A straight forward experimental procedure is possible by performing in-situ ESEM loading tests. Load-displacement diagrams are recorded simultaneously with observation of the specimen surfaces on the screen of an environmental scanning microscope (ESEM). A microloading device, as described in more detail in [30] is operated inside the chamber of a Philips XL 30 ESEM, which was operated in the so-called “low pressure mode” at a chamber pressure of approximately 5.7–6.1 Torr and a temperature of 5 °C. This resulted in a relative humidity (RH) of about 90%. Two spindle driven crossheads carry the fixtures for clamping the specimens. The specimens have a rectangular shape (30 mm× 36 mm, thickness 5 mm) and contain a groove with a notch (1 mm deep), into which a wedge is driven by the microloading device. By applying this wedge splitting technique according to Tschegg [31], a tensile load is transmitted to the notch tip. A load cell with a capacity of 1 kN is placed between one crosshead and a jaw and registers the tensile forces. The machine is driven by a DC motor and a gear which enables deformation rates from 0.3 to 10 μm/s. For the experiments in this study, the deformation rate was 0.5 μm/s and the corresponding deformation was determined by the rotations of the motor. Load and deformation data were recorded digitally with an amplifier connected to the PC, and several images of the crack path were taken for each specimen in order to evaluate the interrelation between load F, crack mouth opening displacement (CMOD) d and crack length. The loads are plotted versus the displacements, and the resulting load-displacement diagrams are used to determine the specific fracture energy necessary to break the specimen, i.e., to completely separate the fracture surfaces. The product of load and displacement is integrated and divided by the fracture area A. Thus the specific fracture energy Gf is obtained [31]:

Gf =

∫ Fd d A

:

3.2. Fracture tolerance of spruce for LR (R-) loading–non-LEFM Owing to the high anisotropy of wood, the crack growth velocities differ, depending on whether the load is applied in longitudinal or transversal direction relative to the wood (fibers). Loading in longitudinal direction (static or dynamic tension, compression load) usually leads to crack formation in radial direction first in a notched specimen. As an example, an experiment on spruce wood (Picea abies [L.] Karst) is shown in Fig. 7. A load was applied with the wedge splitting device, as described above [30,31], leading to a tensile load in longitudinal (L) direction on 200-μm-thick and 3.5-mm-wide sheets of wood, containing an initial notch along the R(-) direction: Thus crack propagation was radial from bark to pith. Fig. 7a shows the load-displacement diagram (LDD). The long arrows indicate displacements where the images in

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Fig. 7. In situ ESEM splitting of spruce (Picea abies [L.] Karst) wood in LR (R-) system (loading in longitudinal direction, radial crack propagation). (a) Load-displacement diagram (LDD). Arrows indicate interruptions where images were taken. (b) ESEM image of crack at the specimen surface perpendicular to loading direction (F ~ 10 N), starting to branch. (c) At F ~ 25 N crack propagates in longitudinal direction along late wood border. (d) Extensive brittle – though not final – fracturing at F ~ 35 N [30].

Fig. 7b, c and d of the propagating crack were taken. It is obvious and amazing that a crack is formed in a very early stage of the LDD (Fig. 7a) and the load bearing capacity is still increasing, although the crack obtains a considerable size. In Fig. 7b, the crack starts to change its direction by approx. 45° and subdivides into two branches and becomes wider. In Fig. 7c, it has approached the late wood boarder, changes its direction by 90° and propagates along the cell walls. Nonetheless, the load is not reduced, but increases until approximately 35 N, where an immediate load drop occurs and the crack becomes wider and propagates along the latewood boarder. Nevertheless, the load bearing capacity is still not completely lost, and another, smaller load increase is recorded (after Fig. 7d), before final fracturing takes place. In summary, after long periods of slow crack propagation, caused by changes of the crack growth direction and branching, the last bridging fibers broke, leading to final sudden fracture. This sequence of figures together with information on the acting load values allows identifying the relevant mechanisms: (a) change of fracture plane and crack branching, (b) fiber-reinforcement (by thicker late wood fibers) inducing change of fracture plane and (c) fiber bridging of already separated crack flanks. All these mechanisms lead to higher energy consumption for crack propagation than for a more brittle material. In other words, failure is avoided or delayed, and spruce wood loaded in longitudinal direction may be considered as a failure tolerant material. This means that nonlinear elastic fracture mechanic's principles have to be applied for this loading condition. Evaluation of the recorded load-displacement diagrams resulted in specific fracture energy values Gf of approximately 0.3 N/mm. Another – usually minor – influence on crack resistance may be due to wood rays, which are fibers oriented perpendicularly to the longitudinal direction (i.e., they lie in the transverse plane). They are

thicker walled (with a round instead of rectangular shape) and stronger than normal wood fibers and may act as reinforcement. This effect is especially effective in wood with a higher proportion of rays, typical for hard woods (volume fraction 12% in alder, 16% ash, 20% oak) and yew [34] in contrast to spruce, where only about 5% fibers are rays. It also plays a bigger role if loading is in the longitudinal direction and crack propagation transversely (LR or LT systems) than if loading is in the transverse plane [32]. As rays are fibers oriented radially, they may lead to easier crack propagation under TR loading than under RT loading. For this, fracturing along rays was identified as cell wall peeling thus creating smooth surfaces, which act as planes of weakness and preferred crack paths. 3.3. Lower fracture tolerance – brittle fracture in TR (R-) direction of spruce If spruce is loaded in transverse direction, mainly brittle behavior is observed. For the experiments, “normal” (not reaction) wood of Picea abies [L.] Karst was used. In Fig. 9, loading was tangential, and the crack propagated in R- direction, i.e., from bark to pith. Fig. 8a demonstrates almost perfect linear elastic behavior during the increase of the load (the first drop originates from interrupting loading in order to take pictures and does not indicate crack formation). At a maximum load of about 25 N, brittle fracturing occurs along a wood ray (Fig. 8b), but interestingly enough, final fracturing is avoided, and a change of the fracture mode is observed (Fig. 8a and c). Cell wall fracturing and branching of the crack is visible, which is typical for the lower acting load (below about 10 N). It could be shown in earlier studies [2,32] that cell wall fracturing may lead to crack retardation by the action of the cell holes as “crack stoppers.” As a result of this mechanism, branching and subsequent bridging of several local cracks is possible and thus failure

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Fig. 8. In situ ESEM splitting of spruce wood in TR (R-) system. (a) Load-displacement diagram (LDD), showing a linear elastic increase of the load up to 20 N. (b) At 20 N, brittle fracturing along a wood ray occurs. (c) After the load drop to about 5 N, the crack continues as an intercellular crack, which is branching. (d) The load does not drop to zero until final fracturing takes place.

tolerance in a very late stage of crack propagation is made possible. Final fracturing is always a combination of ray and cellular fracture in different planes, as visible in Fig. 8d. Crack propagation through the late wood area occurs either along a ray or intercellularly. Therefore, crack arrest or retardation by cell wall fracturing and subsequent branching (as shown for early wood in Fig. 8c) does not take place there usually.

3.4. Fracture tolerance of compression wood of yew (TR loading) Even more impressive is nature's strategy to form reaction wood (consisting of thicker walled and usually roundly shaped cells). As an example, fracturing of yew reaction wood in TR orientation is shown in Fig. 9. Yew (normal) wood (Taxus baccata L.) has a high raw density; the width of the year rings is smaller than those of spruce. Almost nothing was known about the mechanical properties in transverse loading directions until tests were performed by Keunecke et al. [33]. The modulus of elasticity (and stiffness) in longitudinal direction is relatively low, but strength and hardness are high, as well as the axial fracture strain, which is obviously caused by large microfibril angles [33]. Fracture mechanical measurements with the microwedge splitting technique [34] showed relatively small specific fracture energy Gf values (0.31 N/mm) in the RT system, which are almost the same as those of spruce (0.29 N/mm). In the TR crack propagation system, brittle fracturing occurred, so that the Gf values could not be determined.

Reaction (compression) wood of yew, however, is different. The load-displacement diagram in Fig. 9a shows a rather high-strength value (notch tensile load 50 N). After the formation of a first small crack (Fig. 9b) at the highest load, numerous small load drops occur, which are always followed by small increases of the load. The load drops are caused by several hundred micrometer long cracks along rays (Fig. 9c), indicating brittle events. They stop after intercellular cracks appear approximately at the same time or shortly afterwards (Fig. 9d), which causes the cracks within the rays not to propagate along the whole length of the ray any longer. Although the loading response is almost perfectly linear elastic during the increasing loaddisplacement curve and the load drops after obtaining the maximum load all are “brittle” events, crack propagation as a whole is non-brittle owing to the change of the fracture mode, i.e., the formation of numerous intercellular cracks, which are always formed together with, and probably as a consequence of the brittle ray fractures. The most important mechanism causing the fracture-tolerant behavior of yew compression wood is pronounced fiber bridging as reported in [34]. It is probably mainly caused by the simultaneous formation of intercellular and ray cracks. As a consequence, non-linear fracture mechanics has to be applied for a quantitative characterization. Thus, the specific fracture energies, as determined as described above, may serve to quantitatively characterize the fracture tolerance of wood in different loading directions. Additionally, influences of loading mode and environment (humidity and temperature) play important roles [35].

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Fig. 9. In situ ESEM splitting of yew wood in TR (R-) system. (a) Load-displacement diagram (LDD), showing a linear elastic increase of the load up to 50 N. (b) An initial small crack appears, which leads to (c) a several 100 μm long ray crack. Numerous small load increases occur as obvious in (a) and (d) intercellular cracks are always formed together with ray cracks.

3.5. Influence of humidity It has been known for thousands of years that moisture influences the mechanical properties of wood strongly; in general, stiffness and strength decrease and the ductility increases with increasing moisture content [35]. Formerly, fracture properties were primarily characterized by the linear elastic fracture (LEFM) parameter of the fracture toughness KIC, and its increase by moisture was attributed to the viscoelastic properties of wood [36]. Later, the non-linear fracture properties of spruce were determined in wedge splitting experiments in RL orientation with different moisture contents (MC) between 7% up to values beyond the saturation point at 55% MC [37]. From the resulting decreasing stiffness, maximum stress, and decreasing ductilities with increasing moisture, higher specific fracture energy values were derived. The increase of ductility and thus higher dissipated energy was detected not only in the crack initiation phase but also during crack propagation. It obviously overcompensates the reduction of the stiffness and the maximum stress, leading to an increased specific fracture energy. In addition, microstructure observations were performed on the green state of four different wood species, namely Norway spruce (Picea abies (L.) Karst), pine (Pinus sylvestris), beech (Fagus sylvatica L.) and oak (Quercus alba), in an ESEM [35]. Green wood is usually defined as freshly sawn wood, in which the cell walls are completely saturated with water. It also contains water in the cell lumina, named free water, as opposed to bound water in cell walls. No liquid water is present in wood below 84% RH [38]. The experiments were performed with specimens in RL orientation vs. the load axis with a microwedge splitting device at different relative humidity (RH) levels in the ESEM

chamber at a strain rate of 0.1 μm/s. Therefore the cooling temperature was kept constant at 5 °C and the pressure (6.5 Torr at the begin) was gradually decreased. A change of the RH humidity from 98% to 80% resulted in the evaporation of free water from the cell lumina, with a beginning diffusion of bound water and a small amount of shrinkage. The resulting averaged (from at least five experiments) loaddisplacement curves for oak, beech, spruce and pine wood in the green state in RL orientation showed a more ductile response of spruce and pine, while oak was the strongest with highest initial stiffness (elasticity modulus). Evaluation of the curves led to total fracture energy values Gf as shown in Fig. 10a. They increase steadily with moisture content (MC) for spruce and pine, whereas for oak and beech a slight decrease from 7% to 12% MC and an increase from 12% to the saturation point at 30% MC is recognized. The increase of the total fracture energy Gf with increasing moisture content obviously reflects the moistureinduced higher energy dissipation better than fracture toughness KIC or notch tensile stress σt (not shown in this paper), since KIC and σt account only for crack initiation (evaluation only of first part of LDD) and not for the whole fracture process (Gf − evaluation of total LDD). Fig. 10b shows the cells of oak in the green state at an initial RH of 95% in TR orientation. Some lumina are filled with free water, and the swelled cell walls indicate complete saturation with bound water. The lumina which are partly free of water result from the evaporation due to the change of them ESEM chamber humidity from 95% to 48% RH. The swelling of the cell walls may be modeled considering changes of the microfibril angle in the S2 cell wall, taking into account the interaction of matrix polymers with water and effective force transfer between cellulose fibers and matrix.

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• Fiber bridges: fibers, which are stronger than the surrounding matrix keep already broken parts together. In wood, all these structural features and mechanisms are present, whereas most man-made materials concentrate on only one or a few of these design strategies. Information on the moisture content and its effect on the mechanical properties and fracture behaviour is provided.

Acknowledgement Language help provided by Dipl.-Ing. Veronika Doblhoff-Dier is gratefully acknowledged.

References

Fig. 10. Humidity effects on fracture and structure. (a) Increase of specific fracture energy Gf of different wood species with humidity [35]. (b) Swelled cell walls of oak (Quercus alba) which are completely saturated with bound water.

4. Summary In the first part of this paper, the main structural features of wood, such as elongated cells (fibers), a layered structure on different length scales and complex anisotropic features, as well as its hierarchical structure, are reviewed. The consequences for some of its mechanical properties are reported and application of the observed structure– property relationships on the development of some man-made materials is discussed shortly. Similarity in structure with other hierarchical systems found in biological materials and the mechanisms of similar structure–property relationships are treated. New investigations on the fracture-tolerant performance of wood are presented in the second part of this study. The experiments were performed with a microwedge splitting device in-situ ESEM [29,30], allowing measurement of load-displacement diagrams and simultaneous observation of the propagating crack. It could be shown that several mechanisms are activated which avoid or retard fracturing by increasing the fracture resistance. These mechanisms are possible due to the complex and “intelligent” structural design of wood. They are often imitated and applied in man-made structures, leading to a more fracture-tolerant or fail-safe response. Following main crack growth retarding mechanisms are identified: • Cell structure – holes: act like crack stoppers. • Fibers: may act as reinforcements due to their higher strength, causing, for example, crack deviation and crack branching. • Layers: form a composite consisting of layers with different ability of plastic deformation (toughness) and load bearing capacity, respectively. This may lead to crack deviation, retardation and even arrest.

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