Influência das condições de contorno e geometria dos espécimes em resposta à compressão axial

Abstract

Axial compressive strength is a fundamental mechanical property in choosing the most appropriate material for a structure, so it is necessary that the compression test be performed correctly, giving accurate results without interference from the test environment. However, the frictional intensity between the loading platens and specimen contact faces can directly influence the mechanical behavior of the material, and factors such as specimen shape and height can even intensify this effect. Due to the friction, shear forces on the contact faces promote a triaxial state of stresses, this causes an increase in maximum strength, directly interferes in the post-peak behavior and even in the rupture mode of the material, giving it parameters that do not represent its real properties. For this work, an experimental program was developed with 121 specimens, divided into cylinders, height (h) = 20cm and base (b) = 10cm, prisms with h = 20cm and b = 10x10cm and cubes with 10cm sides, used two concretes with characteristic strength of 50MPa and 30 MPa. The compression tests were carried out in five boundary conditions, three are anti-friction methods: grease, teflon and brush plates, one is a reference: standardized traditional test and finally a maximum friction configuration: 3 mm thick metal sheets bonded on the contact faces of the specimens. Among the results, it can be highlighted that the lower the height/base ratio of the specimen, the greater the strength to axial compression and the toughness of the material. The strength variation between the h/b = 2.0 and h/b = 1.0 ratios found was 15% and 35% for 50MPa and 30MPa concretes, respectively. When using the anti-friction methods, the strength variation for the different adopted geometries was eliminated, as well as the post-peak softening of the tests was significantly reduced, which is the result of contact friction between the specimen and the steel plates of the test machine. As for the rupture mode, it was observed that the reference and maximum friction conditions presented a biconical hourglass fracture. For the anti-friction conditions, there was a columnar rupture. Based on these results, the variation in the material's behavior due to friction can harm not only in the academic field, but in engineering practice, since axial compression tests are usually performed to verify whether the material used on site has properties compatible with the estimated in the project.