Experimental assessment of stress state in timber including cracks

Different experimental methods exploiting full-field imaging capabilities are becoming widely employed and appreciated by researchers and practitioners both for on-site diagnostic applications and for laboratory testing of material samples and structural elements. Among non-destructive testing techniques, in addition to acoustic and electromagnetic tomography, are GPR radar, infrared thermography, sonics, ultrasound, X-ray, drill penetration resistance and others which, when applied according to appropriate methodology, permit area visualisation of surface and sub-surface material condition. Optical techniques have also a long tradition in inspection, survey and testing but only recently, thanks to digitalization progress, they have reached the capabilities for easy set-up and use and have the resolution and sensitivity for high-definition survey and monitoring. Particularly interesting are contactless proofs allowing to give evidence of, for example, variations of material parameters as consequence of mechanical loading. Displacement and deformation as well as tension state of the surface under examination can thus be displayed in a global or local (punctual, linear) manner. These visualization possibilities are acting as a break-through in better understanding the behaviour under loading of traditional and new materials and composites. Only numerical modelling has so far accomplished this task of full-field assessment of mechanical performance, but at times demanding high computational requirements, appropriate constitutive laws and extensive information input together with specialized user know-how. More and more often, highly-complex computational mechanical modelling output is lacking the adequate verification by experimental validation. This contribution presents a novel, simple exercise of monitoring by digital image correlation a mechanical test by in-plane eccentric loading on a slice of timber from a historic beam. The beam cross-section presents fissures and shrinkage cracks in addition to limited bio-damage. Loading was conducted in the elastic field at low stress state. The parameter maps obtained from the slice’s front allow assessing in a reliable manner the distribution evolution of mechanical characteristics. Hence, it was possible i.e. to identify weak points, to visualize opening/closure of cracks, to quantify stress fields between cracks, along cracks and at their apex. Moreover, it was most interesting to be able to observe the local mechanical behaviour of this non-isotropic natural material. The test outcome experience can be transferred to other materials, to multi-scale materials modelling and simulation, to control of material performance and to understand failure mechanisms.