Investigating Failure Mechanisms, Defect Tolerance and Repair of Plant Stems for Practical Applications

Abstract

Plants are complex in their biological composition and can provide inspiration for man-made engineering products. Designing bio-inspired products involves understanding the intricacies of plants? mechanical properties and response mechanisms to damage. There has been considerable work executed on investigating fully lignified trees, but little research has been performed on less lignified bushy species or young stems which have not yet lignified. This work presents four studies investigating mechanical properties and responses of plant stems when subject to three-point bending and complimented by a variety of staining and imaging techniques.

The research focused on fuchsia (Fuchsia magellanica var. gracilis) though tests were also carried out on elder (Sambucus nigra) and ash (Fraxinus excelsior). It was demonstrated that fuchsia could fail in one of two ways when subjected to three-point bending; a greenstick fracture or a plastic hinge. Phloroglucinol staining of the stems revealed that a greenstick failure is likely to occur in stems with relatively high density and stiffness. Further reasons for these complex failure mechanisms include the fact that the tensile strength of the stems is greater than the compressive and transverse strength. Mathematical and Finite Element modelling of these stems during three-point bending provided a method of quantifying the mechanical properties responsible. During three-point bending, MicroCT scans revealed that in failed stems an internal crack occurs before any external damage was visible. This internal crack provided inspiration for the development of a bio-mimetic self-healing structure. A common issue with modern self-healing structures is their inability to restrict healing agents from leaking out of the crack once it occurs. This research has demonstrated that developing a structure that intentionally fails from the inside out has the potential to mitigate this obstacle. The result was near perfect self-healing. Finite Element Analysis of the part of the structure under compression successfully predicted the early-stage deformation and the initiation of damage.

Defect tolerance is a prominent concern when developing modern engineering structures made of complex material such as carbon fibre and similar composite materials. Investigating how the mechanical properties of plants are impacted by damage can provide inspiration for the design of defect tolerant materials and structures. This study compared the defect tolerance of three species: fuchsia, ash and elder. Computational modelling of idealised engineering materials provided a framework in which to contrast the species against such materials. The modelling further revealed that fuchsia was 39% more defect tolerant than a plastic, tough material such as steel. Self-repair is a ubiquitous process found in plant stems and has a magnitude of applications if replicated in modern materials. A pilot study on the repair of fuchsia revealed the self-healing capacity of a stem damaged from overloading in bending rather than cutting. Testing was made possible by the development of a portable three-point bending instrument which could be used on living plants in their natural environment. Healing mechanisms, including development of a callus and woundwood, were revealed using a combination of imaging techniques. Further data need to be gathered in the growing seasons of spring and summer before submitting the findings for publication, but an experimental protocol has been developed for future studies on plants of all species.