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Damage Mechanics for Static and Fatigue Applications
Published in Raul D.S.G. Campilho, Strength Prediction of Adhesively-Bonded Joints, 2017
Ian A. Ashcroft, Aamir Mubashar
The micromechanical approach of continuum damage modelling was applied to adhesive joints by Read et al. (2000), where a modified form of the GTN model for ductile materials was derived for rubber-toughened adhesives. As the yield surface of rubber-toughened adhesives shows hydrostatic stress dependence, parameters were included to account for the effect of pressure on the yield surface. The influence of void interactions on matrix shear banding was also included in the model and the model accounted for the cavitation of rubber particles. This phenomenon is usually visible as stress whitening in the failure of rubber toughened plastics. The modified criterion allows for the changing composition of the polymer matrix during void nucleation. The proposed modified yield function (Φ) is as follows Φ=σe2σm2-(q1f)2+2q1fcosh3σk2σm-1-μσkσm2=0 $$ \Phi = \frac{{\sigma _{e}^{2} }}{{\sigma _{m}^{2} }} - (q_{1} f)^{2} + 2q_{1} f{\text{~cosh~}}\left(\frac{{3\sigma _{k} }}{{2\sigma _{m} }}\right) - \left(1 - \frac{{\mu \sigma _{k} }}{{\sigma _{m} }}\right)^{2} = 0 $$
Deformation Mechanisms in Toughened PMMA
Published in Gabriel O. Shonaike, George P. Simon, Polymer Blends and Alloys, 2019
Philippe Béguelin, Christopher J. G. Plummer, H. H. Kausch
There has been considerable effort to develop models for toughening in glassy polymers toughened with elastomeric particles, much of which has centered on criteria for cavitation in isolated particles in an infinite matrix. Even given that cavitation of the second-phase particles is the overriding consideration, there remain questions relating to particle-particle interactions at high filler contents for which simple analytical approaches are inadequate, and for which numerical approaches are also limited (applicable in the elastic limit only and restricted to relatively small numbers of particles). Our aim here has not been to seek to validate such approaches, but to report on what occurs in the damaged material well beyond the damage threshold and the onset of linearity. Surprisingly, different morphologies and/or modifier contents result in similar low-rate toughness and a similar macroscopic appearance of the damaged (stress whitened) zone. This has been observed both in dog bone tensile specimens and in the damaged zone surrounding a stable crack generated during fracture tests. As demonstrated by measuring the light transmittance during the tensile tests, the abrupt increase in irreversible stress whitening corresponds to the overall yielding of the material. In-situ measurements of the volume change in specimens submitted to uniaxial tension have shown cavitational strain to be an important deformation mechanism, tending generally to be favored at high strain rates. Furthermore, at room temperature, the micromechanisms of deformation in the matrix appear similar for the materials modified by spherical particles (either 2L or 3L particles). Widespread crazing of the matrix, triggered by the cavitation of the rubbery phase of the particles, is believed to be the predominant damage mechanism. Results from tensile tests on thin films (55), and to a certain extent from the bulk samples, indicate that damage initiation in the form of crazes is sensitive to clustering of the modifier particles. Further, in-terparticle interactions clearly have a strong incidence on the way the damage zone and in particular crazes develop. Finally, based on the results for the IPNs, it appears that crazing itself is not a prerequisite for toughening, but that toughening does require the existence of an interconnected network of voided material and/or material with a low shear modulus (either crazes or elastomer) close to the crack tip in both. This may in turn be an indication of the important role of matrix shear in energy dissipation as the crack tip advances. Nevertheless, at low-strain-rate and/or high-temperature tensile tests, and in particular at high rubber contents, shearing of the matrix may occur at stress levels too low to initiate cavitation in the rubbery phase and crazing in the glassy matrix.
Compression behavior and deformation mechanism of 3D-printed Kagome lattice materials
Published in Mechanics of Advanced Materials and Structures, 2023
Zisu Li, Liang Gao, Xingchen Zhao, Minhui Xie, Botao Xie, Kai Li
Moreover, some stress whitening emerges near the connection-nodes due to the large bending and even torsional deformation of core-struts, especially in the lower core-layer. It is a visual phenomenon caused by the complete reflection of incident light. When the polymer material undergoes a large deformation, the local defects gradually initiate and converge in the high-stress region. Some crazing or micro-cracks form in the vertical direction of the stress, which would lead to a reduction of refractive index of the polymer material. As the increase of compression deformation, the stress whitening becomes more visible, but these crazing do not evolve into the obvious macroscopic damage in the tests. As shown in Figure 10(d), the characteristic of layer-printing can be clearly observed. Some micro-defects, such as void, resin faultage, and resin starved, are discovered. If these process defects just unfortunately locate in the high-stress region, it would arouse more serious crazing, even core-node fracture. On the fracture surface of connection-node between two core-layers, the distinct convergent texture is captured, which is considered as the important inducement of core-strut splitting.