Experimental analysis on small-scale finger-jointed specimens at elevated temperatures
Finger joints have to meet strength performance requirements for the use in structural engineered wood products such as glued-laminated timber beams (glulam). Among these, the thermal stability of the adhesive used in the finger joint is an important criterion to determine the suitability of the connection. The influence of adhesives on the load-carrying capacity of glulam beams and finger-jointed members can be assessed by large-scale fire tests; however, there are obvious benefits in using small-scale specimens tested at elevated temperatures as an alternative. In Europe, there is currently no small-scale test available to test the fire performance of adhesives in structural timber members. The work presented in this paper addresses this issue and was supported by members of a European standardisation committee to develop such a small-scale test. This paper presents, as a first step, the results of a series of tensile tests on small-scale finger-jointed specimens tested at elevated temperatures. The tests were performed with different types of adhesives which are currently also tested in large-scale fire tests. In the small-scale tests here, structural adhesives showed nearly no weaknesses with increasing temperature up to 140 °C. Further, MUF and PRF adhesives showed mostly wood failure even up to temperatures of 220 °C. In the next step of this investigation, the results obtained from the small-scale tests at elevated temperature will be compared to large-scale fire test results in order to find a possible link between both testing methods.
Comparative creep characteristics of structural glulam wood adhesives
Materials and methods Two commercial phenol-resorcinolformaldehyde (PRF) adhesives and two commercial polyurethane (PU) adhesives, all four of them approved in Germany for exterior grade structural application to glulam and fingerjointing, were tested dynamically by thermomechanical analysis (TMA) on a Mettler apparatus. Six samples of beech wood alone, and of two beech wood plys each 0.6 mm thick were bonded with each adhesive system at ambient temperature (25 C) and under pressure, and then conditioned at ambient temperature and to a constant equilibrium moisture content of 9%. Samples dimensions were of 21 · 6 · 1.2 mm. Triplicate samples were tested in non-isothermal mode between 25 C and 250 C at a heating rate of 10 C/min with a Mettler 40 TMA apparatus in three points bending on a span of 18 mm exercising a dynamic force cycle of 0.1/0.5 N on the specimens with each force cycle of 12 seconds (6 s/6 s). Triplicate samples were also tested in isothermal mode at 40 C, 45 C, 50 C, 100 C and 150 C under identical conditions as above. The classical mechanics relation between force and deflection E 1⁄4 [L/(4bh)][F/(f)] allows the calculation of the Young’s modulus E for each case tested, and this was done to follow the increase of modulus (MOE) as a function of temperature and time. The results are shown in Figs. 1 and 2.
Formation and Strength of Magnesia Whiskers
The formation of single-crystal magnesia whiskers during the compressive deformation of single crystals at room temperatures is described. These whiskers can have strengths of the order of millions of pounds per square inch. They are formed during fracture as a result of the discontinuous propagation of cracks which are mechanically relieving internal stresses resulting from plastic flow