International Journal of Civil and Structural Engineering Research ISSN 2348-7607 (Online) Vol. 8, Issue 1, pp: (117-121), Month: April 2020 - September 2020, Available at: www.researchpublish.com
FATIGUE STRENGTH PROPERTIES OF LAMINATED VENEER LUMBER AND STRUCTURAL PLYWOOD Kiran M.C, Anand Nandanwar, Prakash V, Narasimhamurthy Indian Plywood Industries Research and Training Institute IPIRTI, PB No 2273, Tumkur road, Bangalore 560022, India
Abstract: Fatigue resistance of Laminated Veneer Lumber, Structural Plywood were investigated in this study for determining allowable design stresses for both materials as a percentage of Ultimate strength (Modulus of Rupture). Test were carried out at Selected stress levels to corresponded specific percentages of materials ultimate strength. Constant amplitude cyclic test of 1 Hz (60 cycles/minute) frequency were performed with six applied stress levels 80, 70, 60, 50, 40, 30 expressed as percentages of the materials ultimate strength. Results indicated that Laminated Veneer Lumber, Structural Plywood, were able to survive fatigue life of over 6 lakh cycles at stress level equal to 30 and 27% of average Modulus of Rupture (MOR) respectively. It was also observed that increase in the Rate of loading decreased the percentage of allowable design stresses for all the material and vice versa. The study shows that the allowable design stress for the design applications must be based on percentage of static MOR and the frequency of cyclic loading in order to satisfy safety and service requirements. Keywords: Allowable design stress, Laminated Veneer Lumber, Structural Plywood, fatigue, Modulus of Rupture (MOR).
1. INTRODUCTION In various applications, materials are subjected to repeated stresses. The behavior of materials under such load conditions differs from the behavior under a static load. When the material is subjected to repeated load cycles (fatigue) in actual use, designers are facing problems with predicting fatigue life, which is defined as the total number of cycles to failure under specified loading conditions. Fatigue testing gives more relevant data to predict the in-service life of materials and no sufficient data has been generated for Laminated veneer lumber & structural plywood which is used for furniture, housing and other structural applications viz; I Beams, ply-webbed box beams, floor joists and bearers, wall girts, lintel beams and roof framing beams, strutting beams, hanging beams, valley or hip rafters and ridge beams, header beams, rim board etc. The information related to fatigue strength properties of the wood composite becomes more essential. The strength properties available for the wood composites have primarily been determined by static tests only, as the strength properties determined by static loading do not provide any information of the material to predict its life in its actual use. Fatigue failure of a material results from the sustained or cyclic application of stress less than is required to cause in elastic behavior or fracture under monotonic loading conditions. Damage initiates as micro cracks that subsequently aggregate, leading eventually to macro cracking and failure. The definition of fatigue given above is similar to that employed by the American society for testing and materials (ASTM, 1993), but incorporates a time argument because of woods rheological nature. Any material exhibiting flow behavior must exhibit fatigue that depends on the number of stress cycles and the rate of stressing and/or time under stress. Unfortunately, this has not been widely recognized in the past.
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