Mixed-Mode Fracture Of Adhesively Bonded Joints

The study focuses on using fracture mechanics to evaluate mixed-mode fracture properties of adhesively bonded aerospace material systems. As a part of experimental efforts, mixed-mode fracture tests were performed using modified Arcan specimens consisting of several combinations of adhesive, composite and metallic adherends using a special loading device. By varying the loading angle, alpha from 0 degrees to 90 degrees, mode I, mixed-mode and mode II fracture data were obtained experimentally. Experimental and theoretical studies of mixed-mode fracture behaviour of adhesively bonded aluminium, steel and CF/PEI composite joints were also performed using an adhesive in the aerospace industry. Finite element analyses were carried out on specimens with different adherends. Based on those analyses, mixed-mode fracture criterions for the adhesively bonded systems under consideration have been determined

Modelling mixed-mode fracture in poly(methylmethacrylate) using peridynamics

AbstractPeridynamics (Silling (2000)) is a non-local continuum theory that is particularly suited to handle discontinuities in the displacement field, such as those arising during fracture. Peridynamics prescribes that each material point interacts with all its neighbors contained in a sphere of given radius; this assumption introduces a characteristic length scale in the continuum description. In a nutshell, the interactions between material points depend on their relative distance; in the peridynamics framework this distance is called the “bond length”. The equations of motion, holding at each material point, link the material point acceleration to the integral over the point neighborhood of a force density field, whose strength depend on bond-stretches, i.e. the ratio of the actual bond-length over the initial one. In these equations the displacement gradient does not appear, thus naturally allowing for discontinuities in the displacement field to occur. As to failure, the simplest possible damage description is provided by an interaction law prescribing the force to vanish when a critical bond-stretch threshold is crossed; this parameter can be related to the Mode I critical strain energy release rate. A single parameter is needed to describe failure, in principle under every possible loading condition.In this work the predictive abilities of peridynamics were checked against experimental results in the case of mixed-mode failure of brittle polymers. Pre-cracked poly(methylmethacrylate) (PMMA) samples were tested using different specimens, in order to obtain Mode I, Mixed-Mode and Mode II loading conditions. The material was assumed to behave according to a peridynamics brittle elastic material model; the parameters needed to calibrate the elastic behavior were determined from Mode I tests, as was the critical stretch.The peridynamics simulations of mixed-mode tests were able to catch the correct fracture initiation load and to provide a fair description of the crack path under different conditions. The peridynamics model was also able to qualitatively capture the typical “nail” shape assumed by the crack front during propagation

Investigation on the Effect of Different Pre-Cracking Methods on Fracture Toughness of RT-PMMA

Abstract In this study, eight techniques including coping saw, metal slitting saw, razor blade, cubic boron nitride (CBN) disc, scoring, die-cutting and guillotining, diamond disc, and laser cutting methods were used to produce pre-cracked fracture toughness (Kc) test specimens made of poly(methyl methacrylate)/graft-acrylonitrile butadiene styrene blends. The influences of notch shape (radial, rectangular, or angular-shaped and variety of thicknesses), pre-cracking method, chip removing or non-chip removing, and the contact or non-contact methods on the results obtained in fracture toughness tests were investigated. The results were analyzed by two methods, quantitatively and qualitatively, by comparing the obtained Kc results and studying the SEM and optical microscopy images, respectively. The results indicated that the different conditions of a produced pre-crack including; geometry of pre-crack due to geometry of tools, residual stress due to pre-crack creation, heat affected zone, damage of crack tip, and producing crazing around the crack tip could affect the fracture toughness. The maximum difference resulted from different pre-cracking methods was equal to 0.24 MPa.m0.5 and the lowest value of fracture toughness Kc, 2.53 MPa.m0.5, belonged to the scoring method but the highest value, 2.77 MPa.m0.5, belonged to the metal slitting saw method. Also, the results indicated that the effects of notching on toughness of RT-PMMA had a low notch sensitivity and the differences between minimum and maximum Kc values was found about 9%

3D Characterization of Mixed-Mode Fracture Toughness of Materials Using a New Loading Device

Abstract In this paper, a new loading device was employed to conduct a mixed-mode fracture test. Therefore, disadvantages detected in the previous mixed-mode fracture toughness test methods can be avoided. The new fixture has perfect symmetry which provides a uniform stress state, creates the pure plane strain conditions and eliminates the unwanted mode-III loading conditions. The values of non-dimensional stress intensity factors for pure mode-I, pure mode-II, and pure mode-III were obtained using 3D models of new loading device and modified Arcan method in order to compare the results and investigate the variation of fracture parameters. Furthermore, the values of correction factors for pure mode-III in various loading angles were studied for both fixtures and it was resulted that in modified Arcan device there was a larger contribution of unwanted third mode loading conditions and therefore the values of mode-I and mode-II non-dimensional stress intensity factors were affected. In the present study, in order to compare the results, the fracture toughness values of ABS (Acrylonitrile butadiene styrene) polymeric material were determined experimentally for both fixtures and a full range of mixed-mode loading conditions including pure mode-I and pure mode-II loading were created and tested. The differences in critical stress intensity factors of the fixtures were found about 7.00% and 42.65% in mode-I and mode-II loading conditions, respectively