Why don’t branches snap? The mechanics of bending failure in three temperate angiosperm trees

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 compressive strength, particularly tangentially and was crushed by transverse stresses. Though the other species yielded in longitudinal compression on the concave side, they ultimately 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.

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“…We have shown that these branches are significantly smaller, weaker, and more flexible than those used in the main structure of the nest. Compliant living branches hardly ever break completely across (13,14), and to detach them takes a degree of skill. Observations of nests revealed that the broken ends of detached branches often had a tail of bark or wood material (Fig.…”

“…3A), breaking branches only halfway across and not detaching them. Greenstick fracture is the natural fracture mode expected from the denser wood types typical of canopy tree species (13,14) and by a considerable margin was the most prevalent fracture type recorded within the nest structures. It could be argued that the predominance of greenstick fracture within the nest structures is because orangutans are not able to completely break and detach the larger, stronger branches used in the structural base of the nest.…”

“…Branches, when loaded under bending, do not exhibit a uniform mode of fracture and rarely simply break completely across and detach (13,14). The low-density woods of fast-growing pioneer trees tend to buckle and fail without fracturing (14). Such buckling and failing occurs as a consequence of the low lateral strength of the wood material, which is crushed by the transverse stresses generated during bending.…”

“…This phenomenon is called "greenstick" fracture. In this process, although fracture has occurred, detachment does not (13,14), and it can prove extremely hard to completely break off green twigs. Knowledge of the mechanics of branch fracture therefore leads to the consideration that orangutans may be using some experience of the way in which branches break during bending in the construction of their nest structures.…”

“…However, breaking living, and hence compliant, branches is not as simple as one might think. Branches, when loaded under bending, do not exhibit a uniform mode of fracture and rarely simply break completely across and detach (13,14). The low-density woods of fast-growing pioneer trees tend to buckle and fail without fracturing (14).…”