Mechanical fracture parameters of concrete drill-core specimens supported by a slenderness ratio study

The detailed analytical and experimental investigation of the fracture behaviour of quasi-brittle materials is an endeavour which has been ongoing worldwide for many years. Such materials are usually characterized in terms of their mechanical fracture parameters, which are determined based on the evaluation of quasi-static fracture experiments. One of the most commonly used building materials with a quasi-brittle response is concrete, which is most often based on a cement matrix. It is sometimes also necessary to characterize concrete included in existing structures. In this case, test specimens are obtained by core drilling, and the investigation is conducted with the requirement to maximize the number of parameters obtained while minimizing the number of test specimens drilled from the structure. This paper focuses on the mechanical fracture parameters of core-drilled specimens taken from a selected concrete structure. Tests were performed on cylindrical specimens with a chevron-notched stress concentrator in the three-point bending configuration in order to determine modulus of elasticity, fracture toughness and fracture energy. Subsequently, theoretical compressive strength was estimated and tests for the determination of compressive strength values were performed focusing on dependence on the slenderness ratio, i.e. the relationship between the compressive strength and the length to diameter ratio of the cylindrical specimens. In relation to the obtained mechanical fracture parameters, selected specimens were analysed and three-dimensionally characterized via high-resolution X-ray computed tomography.

Effect of Loading Angles and Implant Lengths on the Static and Fatigue Fractures of Dental Implants

Mechanical properties play a key role in the failure of dental implants. Dental implants require fatigue life testing before clinical application, but this process takes a lot of time. This study investigated the effect of various loading angles and implant lengths on the static fracture and fatigue life of dental implants. Implants with lengths of 9 mm and 11 mm were prepared. Static fracture tests and dynamic fatigue life tests were performed under three loading angles (30°, 40°, and 50°), and the level arm and bending moment were measured. After that, the fracture morphology and fracture mode of the implant were observed. The results showed that 9 mm length implants have a higher static failure load and can withstand greater bending moments, while 11 mm length implants have a longer fatigue life. In addition, as the loading angle increases, the static strength and bending moment decrease linearly, and the fatigue life shows an exponential decrease at a rate of three times. Increasing the loading angle reduces the time of the implant fatigue test, which may be an effective method to improve the efficiency of the experiment.

Mixed Mode Crack Propagation in Polymers Using a Discrete Lattice Method

The fracture behavior of polymeric materials has been widely studied in recent years, both experimentally and numerically. Different numerical approaches have been considered in the study of crack propagation processes, from continuum-based numerical formulations to discrete models, many of the latter being limited in the selection of the Poisson’s coefficient of the considered material. In this work, we present a numerical and experimental analysis of the crack propagation process of polymethylmethacrylate beams with central and eccentric notches subjected to quasi-static three-point bending tests. The developed discrete numerical model consists of a regular triangular lattice model based on axial and normal interaction springs, accounting for nearest-neighbor interactions. The proposed model allows solving the above mentioned limitation in the selection of Poisson’s coefficient, incorporating a fracture criterion defined by a bilinear law with softening that includes the fracture energy in the formulation and allows considering a progressive damage. One of the main objectives of this work is to show the capacity of this lattice to simulate quasi-static fracture problems. The obtained results show that the proposed lattice model is capable of providing results close to the experimental ones in terms of crack pattern, peak load and initial stiffening.

On-chip environmentally assisted cracking in thin freestanding SiO2 films

An on-chip fracture mechanics method is extended to characterize subcritical crack growth in submicron freestanding films. The method relies on a self-actuated concept based on MEMS fabrication principles. The configuration consists of a notched specimen attached to actuator beams involving high internal stress. Upon release, a crack initiates at the notch, propagates, and arrests. Several improvements are worked out to limit the mode III component and to avoid crack kinking. The method is applied to subcritical crack growth in 140-nm-thick SiO2 films under different humidity conditions. The data reduction scheme relates crack growth rate to stress intensity factor. The static fracture toughness value is ~ 0.73 MPa $$\sqrt{\mathrm{m}}$$ m , with standard error of 0.01 MPa $$\sqrt{\mathrm{m}}$$ m and standard deviation of 0.17 MPa $$\sqrt{\mathrm{m}}.$$ m . Subcritical crack growth rates are much smaller than in bulk specimens. A major advantage is that many test samples can be simultaneously monitored while avoiding any external equipment. Graphic Abstract

Analysis of the stress-strain state in carburized gearwheels

This article studies the static strength, static fracture and stiffness of the teeth of a wheel made of structural alloy steel of 12KhN3А grade before and after the carburizing of the working surface. The results of the analysis show that the static strength and static destruction of the part before and after chemical heat treatment are approximately equal, however, the gear wheel, strengthened by this method, has a higher hardness. The study and simulation of the applied loads were carried out using a 3D-model in the SolidWorks 2018 Simulation software.

ISSI 2020: Mechanism and Method of Testing Fracture Toughness and Impact Absorbed Energy By Spherical Indentation Tests

Aimed at the problem that conventional approaches for mechanical property determination all need destructive sampling, which may be improper for in-service structures, the authors proposed a method to determine the quasi-static fracture toughness and impact absorbed energy from spherical indentation tests (SITs) in this study. The stress status and damage mechanism of SIT, Mode I fracture, Charpy impact tests, and related tests were first investigated through finite element (FE) calculations and scanning electron microscope (SEM) observations, respectively. It was found that the damage mechanism of SITs is different from Mode I fracture, while the Mode I fracture and Charpy impact test share the same damage mechanism. Taking the difference between SIT and Mode I fracture into consideration, the uniaxial tension and pure shear were introduced to correlate SIT with Mode I fracture. Based on which, the widely used critical indentation energy (CIE) model in fracture toughness determination from SITs was modified. The quasi-static fracture toughness determined from the modified CIE model was used in evaluating the impact absorbed energy by means of the dynamic fracture toughness and energy to crack initiation. Effectiveness of the new proposed method was verified through experiments on four kinds of steels, i.e. Q345R, SA508-3, 18MnMoNbR, and S30408.