Estimation of Tensile Strength Parallel to Grain of Wood Species

Abstract

According to the ABNT NBR 7190: 1997 standard, tensile strength of wood samples shall be defined as the ratio between the ultimate strength from a stress test and the specimen cross-sectional area (350 mm 2). However, due to the complex wood anatomy and experimental difficulties, the fractured surface in tested samples are not perpendicular to the loading direction; therefore, such fractured surface becomes larger than the sample cross section, overestimating tensile strength by this method. Using an alternative approach, we aimed to determine the approximate fracture surface of wood specimens subjected to tensile test. For this purpose, we used the least square method to estimate tensile strength of four hardwood species, which were equally divided into strength classes. Then, we determined the relationship between the cross-sectional areas of intact and fractured specimens. The results showed that the approximate area of a fractured surface was 2.14 higher than that of an intact sample. As a result, tensile strength estimates were 47% lower than those currently estimated, which is unfavorable for the safety of construction structures.

Degradation of Mechanical Properties of Pine Wood Under Symmetric Axial Cyclic Loading Parallel to Grain

“…However, due to the anisotropic characteristics of wood and the different strength of late wood and early wood layers, the fracturing mechanism does not occur in one direction [ 4 ]. Therefore, the rupture of the pine wood samples occurred at an angle of approximately 30 degrees to the direction of the fiber, as shown in Figure 10 d. Because the failure plane is not perpendicular to the applied load direction, the tensile strength consists of the total or effective stress, which has normal and shear stresses as components, as claimed by Christoforo et al [ 4 ] who studied the same phenomenon, but on tropical wood species. Analyzing the fracture of the pine wood samples subjected to tensile - compression uniaxial parallel to the fibers, it was found that the direction of normal stresses does not coincide with the main elastic directions, in addition to normal strains appear shearing strains, as shown by the generalized law of Hooke.…”

“…Their widths, proportion and density vary, depending on the specific environment and soil conditions from year to year [ 3 ]. Wood consists of a very complex system of fibers, cells, cementing substances, etc., whose physical and chemical cohesion poses particular problems in the calculations of strength and rigidity, compared to homogeneous and isotropic materials such as, for example, metals [ 4 , 5 , 6 ]. Nowadays, wood and wood-based materials have re-entered the attention of architects and builders, being a material with numerous physical, mechanical, acoustic, and workability qualities.…”

“…Taking into account the structural changes produced by cyclic stresses as well as the modification of the tensile breaking section of wooden samples, Christoforo et al [ 4 ] present a mathematical model for determining the shape coefficient that expresses the relationship between the actual breaking area and the theoretical area. The tension is distributed at the request of parallel traction with fiber of the different wood species from Brazil.…”

Research on the Compression Performance and Damage Mechanism of Larix Wood With Smooth Grain

“…4), and continues to extend to the plane perpendicular to the free surface of the expansion, the specimen will ultimately form a shear damage. Since the damage surface is not perpendicular to the direction of the applied load, the compressive strength consists of the total or effective stress, of which the normal and shear stresses are the components, Christoforo [6] et al conducted fracture analysis of pine specimens subjected to uniaxial compression of the tropical wood with a conforming grain and found that the direction of the positive stresses did not coincide with the main elastic direction, and that there were shear strains in addition to the positive ones, in accordance with the generalized Hooke's law. At compression parallel to the fibers, the structural change begins with an increase in the stress level, and the instability of the fibers is observed by the appearance of a number of macroscopic kinks, where the yielding of the material indicates a gradual transition of the unit cell from the initial state of an elastic material to an irreversible state.…”