1,721,074 research outputs found
Scratch hardness as a quasi-intrinsic parameter to measure the scratch resistance of polymers
Full text linkIn this work four different polymers (acrylonitrile-butadiene-styrene, high-impact polystyrene, rubber-toughened polybutylene terephthalate, linear low-density polyethylene) were characterized in terms of their bulk (modulus and yield stress) and surface (scratch hardness) mechanical properties. The intrinsic time-dependence of the materials was addressed by performing DMA and compression tests at varying testing speed/frequency, exploiting time-temperature superposition and Eyring’s model to obtain data at strain rates compatible with scratch experiments. The latter were performed by applying different loading histories (constant depth or load) and indenters. Scratch hardness was determined using Pelletier’s model; it was demonstrated that such a parameter provides a reliable and almost intrinsic (i.e. loading history independent) evaluation of scratch resistance, seen as the resistance the material opposes to indenter penetration. The material compressive yield stress (evaluated at the strain rate relevant to scratch phenomena) was found to be the key controlling factor in determining scratch hardness. It can therefore be taken as a measure of the mechanical scratch resistance when evaluating the possible effects of variables such as material composition, crystallinity, physical ageing... Its relation with other aspects of the scratch phenomenon (in particular deformation recovery) was also explored, accounting for the specific deformation regime imposed by the indenter (transitioning from elastic to predominantly plastic)
Modelling mixed-mode fracture in poly(methylmethacrylate) using peridynamics
Full text linkAbstractPeridynamics (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
An experimental investigation of the scratch behaviour of polymers – 2: Influence of hard or soft fillers
Full text linkVariation in the scratch behaviour of polymers with incorporation of different types and amounts of fillers was investigated both in terms of scratch visibility and scratch hardness. Scratch tests were carried out by a microscratch tester. The effect of fillers on the scratch deformation mode and hence on scratch visibility was evaluated by examining recorded penetration depth and acoustic emission traces together with optical microscope images of the groove left on the surface. Scratch hardness was determined by using a recently proposed scratch model. Adequacy of the model to interpret the local phenomenon induced by addition of fillers was questioned.
It was demonstrated that incorporation of hard fillers can enhance the scratch hardness of polymers as well as their basic mechanical properties but leads to an increase in scratch visibility due to a change in scratch deformation mode from ductile ploughing to a brittle failure with crack formation. On the contrary, addition of soft fillers causes a reduction in scratch hardness but it also reduces scratch visibility since it improves the recovery characteristics of the polymer
An experimental investigation of the scratch behaviour of polymers: 1. Influence of rate-dependent bulk mechanical properties
No full textThe scratch behaviour displayed by a series of polymers in constant penetration depth scratch tests conducted by a microscratch tester was examined in relation to their intrinsic rate-dependent mechanical properties. Scratch hardness, which is considered as a measure of the scratch performance, was assessed by using a recently proposed scratch model. The adequacy of the model to explain deformation recovery and pile-up characteristics of the material in the scratch test was questioned by comparing the predictions of the model with residual profile measurements.
The study demonstrated that the scratch phenomenon is dominated by the yield stress of the material. Besides, scratch hardness was found to be independent of the loading conditions. Indentation hardness and scratch hardness are comparable if the testing conditions are similar
Fracture of rubber-toughened poly(methylmethacrylate): measurement and study of cohesive zone parameters
No full text
Determination of a cohesive law for polymers from three-point bending experiments
No full textIl comportamento a frattura dei materiali polimerici può essere
efficacemente descritto mediante l’uso di modelli coesivi, basati
su una legge che correla sforzo e apertura della cricca. In questo
lavoro si applica un metodo per l’individuazione della legge
coesiva di due polimeri amorfi a partire da prove sperimentali di
flessione a tre punti
Scratch hardness of polymeric materials
No full textSeveral polymeric materials possessing widely different characteristics were investigated. Their scratch hardness was evaluated by a microscratch tester using Pelletier’s model, which allows calculation of the real contact area during the scratch process. The results were compared with
relevant bulk mechanical properties and indentation hardness: scratch hardness correlated quite well with the latter and it was found to be strongly dependent on the compressive yield stress
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