P_2.1.3 proof_grading

Development of a Grading Device based on Stiffness Parameters by the means of the ‘Tensile Proof Loading’ Procedure

In the context of the Project P03 qm_online of holz.bau forschungs gmbh a “tensile proof loading” device was developed and implemented in collaboration with a plant manufacturer (Ledinek company in Slovenia) and an industrial partner (Holzindustrie Leitinger GmbH / Preding, Steiermark) on the principle of the “tensile proof loading“ procedure; the testing plant is used in the field of solid construction timber. The plant enables the removal of firmness-reducing growth properties that are usually difficult to detect, such as local and global angle grain, pre-broken tree tops, bucklimg ruptures, bark pockets, etc. as well as wrong productions of the finger joint connection, such as wrong milling and wrong gluing concerning the tensile test (“proof level“), and to eliminate these from the further production process. The plant therefore represents a quality-assuring and trust-building measure for all rod-like sawn timber products of the trade since the customer can, in this way, expect an engineered product that exhibits a guaranteed minimum firmness. In the context of this research project, an expert opinion was established in a close collaboration with the Institute of Mechanical Engineering in Innsbruck that dealt with the special characteristics of tensile-tested materials. The results showed that in dependence of the charged test stress and the coefficient of the variation of the basic material, the failure probability of tensile-tested elements was able to be reduced and that thus the safety level was able to be increased. This means that, when measuring rod-like tensile-tested building components, a reduced partial safety factor γm is achieved on the material side. All research results obtained in the course of this project helped to shape a solid basis for establishing the Austrian Standard “ÖNORM B 4125” (Timber constructions – requirements concerning tensile test procedures and special characteristics of sawn timber tested by this procedure).

In the course of a follow-up project based on the knowledge gained so far, the existing device shall be adapted in a way that the tensile test described before is applied as early as on the basic material, i.e. the individual sawn pieces of timber. Growth defects that are otherwise hard to detect or flaws in the wood can, by way of this method, be detected early on through breakage of the sawn pieces of timber, and be eliminated. As far as the remaining sawn timber is concerned, the static modulus of elasticity is determined by means of a deformation measuring. This information allows for a subsequent further classification of the graded sawn timber into various stiffness and strength classes.

Additionally, it shall thus be made possible that, once the modulus of elasticity has thus been determined, the tensile test (“proof level“) is varied accordingly so that the sawn timber that shows a higher stiffness and belongs to a higher grading level can be charged with a higher proof level. In this way, minimum strength can be guaranteed for sawn timber graded with the help of this device, and the static modulus of elasticity can be determined for the division into the various strength classes. Another aspect is to make use of this information on stiffness with a view to building continuous lamellas for rod-like products on the basis of their stiffness, and thus to reduce the variation in the final product to a minimum.

Any further development is mostly concerned with the introduction of the tensile forces to the sawn timber to be graded by means of a jamming unit. These forces should only be introduced to the sawn timber over lengths as short as possible in order to limit the untested areas to a minimum. Furthermore, there is the innovation to develop a measuring system together with Joanneum Research, the measuring system being able to grasp any deformation of the sawn timber contact-free during the tensile test. It is from these two bits of information, i.e. force and deformation on the one hand, and tension and extension on the other hand, that the static modulus of elasticity shall be able to be determined online.