Direction Of Principal Tensile Stress Research Articles

Fibre alignment in fresh concrete with coarse aggregates and its visualization in transparent model concrete

Steel fibres distributed in concrete can greatly improve the mechanical performance of concrete, especially flexural and tensile behavior, if their orientations align to the direction of principal tensile stress. The study attempts to find diversified characters of fibre alignment in fresh concrete with consideration of adding coarse aggregates. A transparent concrete model, consisting of Carbopol gel, glass beads and crystals, is designed for the observation of fibre orientation. It shows good transparency and visibility, and can consider the effect of coarse aggregates since the volume fraction and particle size distribution of crystals are the same as coarse aggregates in real concrete mixture. The rheological properties of the model can be adjusted by the dosage of Carbopol and glass beads. Using this concrete model, the distribution and orientation of fibres can be directly observed and analyzed, considering the effect of aggregate sizes, concrete rheology and vibration time. The results show that coarse aggregates could limit the distribution space of fibres and lead to the decrease of fibre orientation coefficient. Fibre dispersion coefficient also decreases with the increase of aggregate sizes.

Experiment study on seismic behavior of squat UHPC shear walls subjected to tension-shear combined cyclic load

Five squat UHPC shear walls with high axial tension ratio ranging from 0.5 to 0.6 were tested under tension-shear combined cyclic load to replicate seismic performance of bottom shear walls in high-rise buildings. All specimens exhibited shear-sliding failure mode characterized by multiple shear-related cracks distributed in a dense array and full-length localization of horizontal crack along the end of anchorage rebars. The seismic behaviors were comprehensively evaluated by hysteretic responses, energy dissipation capacity, strength and stiffness degradation. Additionally, detailed analysis on the strain development of vertical reinforcement and web horizontal rebars were carried out to study their shear resistance contribution. It was found that, with the assistance of steel fibers providing an efficient shear resistance consistent with the varying direction of principal tensile stress, the shear capacity of UHPC shear wall was slightly improved even when axial tension ratio increased to 0.6. The ultimate drift capacity of UHPC specimens reached 3.3% drift with an excellent strength retention ability of more than 500kN in this test. Besides, the accumulated energy consumption of UHPC specimen after crack localization improved by 38% compared with the normal concrete shear wall. Results of strain analysis demonstrated that vertical reinforcement provided an effective clamping force in the shear-sliding resistance. A modified design formula considering efficient tension resistance offered by UHPC was proposed with adequate safety concerns and accuracy.

Effect of extrusion drawing and twist-orientation on mechanical properties of self-reinforced poly(lactic acid) screws

In order to improve torsional strength of a bioabsorbable poly(lactic acid) (PLA) screw, twist-orientation method was developed in this study. PLA screws were prepared through a series of routes including compression molding, extrusion method, twist-orientation, and forging. Shear and torsional strength were measured as mechanical properties of screw. As a result, twist-orientation improves torsional strength of PLA screw without the decrease in shear strength. The maximum torsional strength was obtained at helix angle of 45° and extrusion ratio of 8. This is because fibrous crystals were oriented close to the direction of principal tensile stress which occurred during torsion test. Since this principal stress occurs in the 45° direction on the surface of specimen, the maximum torsional strength is achieved by twist-orientation at this helix angle.

Analysis of the equal principal angles assumption in the shear design of reinforced concrete members

Several Compression Field Theories have been developed for the design of structural concrete members to resist shear. These theories adopt the simplifying assumption that the direction of principal tensile stress and the direction of principal tensile strain coincide. This assumption is known as the Equal Principal Angles assumption (EPAs), or Wagner’s hypothesis. Experimental evidence indicates that this is a reasonable simplification. This paper investigates potential differences in the directions (or angles) of principal stress and principal strain and evaluates the relative influence of design parameters on the divergence of these angles. Within practical ranges of these design parameters, the transverse reinforcement content is found to have the greatest influence on the applicability of the EPA assumption.