ORIGINAL PAPER Why don’t branches snap? The mechanics of bending failure in three temperate angiosperm trees A. van Casteren • W. I. Sellers • S. K. S. Thorpe • S. Coward • R. H. Crompton • A. R. Ennos Received: 30 March 2011 / Revised: 2 November 2011 / Accepted: 5 November 2011 Ó Springer-Verlag 2011 Abstract Living tree branches are almost impossible to snap. Some show ‘‘greenstick fracture’’, breaking halfway across before splitting along their length, while others simply buckle. In this study we investigated the bending failure of coppice branches of three temperate angiosperm trees: ash, Fraxinus excelsior; hazel, Corylus avellana; and white willow, Salix alba. We carried out bending tests, and made a series of observations on the structure, density and tensile and compressive strength of their wood to understand the pattern of failure. The three species showed contrasting behaviour; willow buckled whereas ash showed clean greenstick fracture and hazel a more diffuse greenstick fracture. These differences could be related to their wood properties. Willow buckled because its light wood had very low transverse compres- sive strength, particularly tangentially and was crushed by transverse stresses. Though the other species yielded in longitudinal compression on the concave side, they ulti- mately failed in tension on the convex side when bent because their higher density wood resisted transverse compression better. However, the crack was diverted down the midline because of the low tangential tensile strength of their wood. Differences in fracture between ash and hazel are related to fine-scale differences in their wood anatomy and mechanics. Keywords Greenstick fracture  Buckling  Wood  Branch  Bending Introduction It has long been noted that branches of different tree species fail in different ways when subjected to bending (Ennos and van Casteren 2010). Many fail by the typical ‘‘greenstick fracture’’ (Fig. 1a) in which tensile fracture occurs on the convex side but is unable to propagate throughout the branch to cause a complete break. In these branches, the crack extends to approximately the mid point of the branch where it then diverts into the longitudinal direction. Branches of some other species show quite different failure. They buckle, failing in transverse compression (Fig. 1b) and fracture does not occur at all. In a recent paper, Ennos and van Casteren (2010) suggested that these differences are due to the pattern of transverse stresses set up in beams when they are bent and the contrasting transverse mechanical properties of the wood. As well as the well-known longitu- dinal stresses that are set up when a beam is bent, transverse compressive stresses are also set up, though they can be ignored in most isotropic materials since these are much smaller than the longitudinal stresses (Ennos and van Cast- eren 2010). However, they can become important in wood, which is highly anisotropic. Buckling should occur in light wood, because the transverse compressive stresses can overcome its extremely low tangential compressive strength Communicated by R. Matyssek. A. van Casteren  W. I. Sellers  A. R. Ennos (&) Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, UK e-mail: roland.ennos@manchester.ac.uk S. K. S. Thorpe  S. Coward School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK R. H. Crompton Musculoskeletal Science Research Group, Institute of Aging and Chronic Disease, Sherrington Buildings, The University of Liverpool, Liverpool L69 3GE, UK 123 Trees DOI 10.1007/s00468-011-0650-y