Engenharia Agrícola ISSN: 1809-4430 (on-line) www.engenhariaagricola.org.br 1 Universidade Federal de São Carlos (UFSCar)/ São Carlos - SP, Brasil. 2 Fundação Universidade Federal de Rondônia (UNIR)/ Porto Velho - RO, Brasil. 3 Universidade Federal de São João del-Rei (UFSJ)/ São João del-Rei - MG, Brasil. 4 Escola de Engenharia de São Carlos (EESC) da Universidade de São Paulo (USP)/ São Carlos - SP, Brasil. Area Editor: Edilson Costa Received in: 7-21-2018 Accepted in: 12-10-2019 Engenharia Agrícola, Jaboticabal, v.40, n.2, p.238-242, mar./apr. 2020 Doi: http://dx.doi.org/10.1590/1809-4430-Eng.Agric.v40n2p238-242/2020 TECHNICAL PAPER INFLUENCE OF FATIGUE ON BENDING OF Pinus caribaea WOOD André L. Christoforo 1* , Thaina Q. Barbosa 1 , Diego H. de Almeida 2 , Tulio H. Panzera 3 , Francisco A. R. Lahr 4 1* Corresponding author. Universidade Federal de São Carlos (UFSCar)/ São Carlos - SP, Brasil. E-mail: christoforoal@yahoo.com.br | ORCID: https://orcid.org/0000-0002-4066-080X KEYWORDS fatigue, Pinus caribaea, static bending, analysis of variance, regression models. ABSTRACT This research aimed to evaluate the effect of the number of fatigue cycles (NC, 0 – reference, 450, 4500, 45000, and 90000) for frequencies (Fr) of oscillation equal to 0.5 and 1.0 Hz on the modulus of elasticity (E m ) and operating stress (σ m ) in the static bending of Pinus caribaea wood. E m and σ m were determined taking into account three specimens (SPEC) manufactured for reference condition and another three for each of the four NC levels and each Fr level, resulting in 27 SPEC. Sixty experimental determinations were obtained, being the approximate total time of sample exposure to fatigue of 350 hours. The same specimens used to determine E m and σ m for the reference condition were also used (via non-destructive tests) to obtain these properties for all fatigue cycles. The frequency and number of cycles significantly influenced both investigated properties. Reductions in E m and σ m values were observed after 45000 cycles. The progressive increase in the number of fatigue cycles caused more marked reductions in mechanical properties when compared to the increase in the frequency of oscillation. INTRODUCTION Wood is a material widely used in the manufacture of building components, such as partition panels, doors, frames, wainscoting, ceilings, and floors. Such employment has been growing despite some well-known prejudices inherent to wood, especially related to the insufficient dissemination of technological information already available about its properties under different operating conditions, and also due to the almost systematic lack of specific projects developed by qualified professionals (Nogueira et al., 2018). Eucalyptus and Pinus are the species most commonly used in construction because, in addition to their mechanical characteristics, there is an incentive to plant in reforestation areas (Almeida et al., 2018). The physical and mechanical properties of woods should be known to be used in construction. In Brazil, the project of wooden structures, as well as the methods and premises for obtaining physical and mechanical properties, are regulated by the Brazilian standard ABNT NBR 7190 (1997), named as “Project of Wooden Structures.” For dynamic and/or cyclical stresses, such as structures of bridges, silos, and others, in which the effect of mechanical fatigue is appreciable, the Brazilian standard does not present information regarding the variation of strength and stiffness properties of the wood due to fatigue cycles, which motivates the development of research on this subject. In Brazil, the only study found that deals with fatigue in solid wood, which is also the objective of this research, is that of Guimarães et al. (2012), who evaluated the effect of the cyclic loading on mechanical properties obtained from the static bending test (strength and stiffness) of 4 forest wood species: Dipteryx odorata, Pouteria guianensis, Cedrelinga catenaeformis, and Tectona grandis. The specimens were subjected to 40,000 and 100,000 fatigue cycles, an oscillation frequency of 0.4 Hz, and strength equal to 40% of the rupture strength in the static bending test.