Browsing by Subject "scratch"
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Item Evaluation of Packaging Film Mechanical Integrity Using a Standardized Scratch Test(2012-10-19) Hare, BrianPolymeric packaging films see widespread use in the food packaging industry, and their mechanical integrity is paramount to maintaining product appearance, freshness, and overall food safety. Current testing methods, such as tensile or puncture tests, do not necessarily correlate well with field damages that are observed to be scratch-like. The standardized linearly increasing load scratch test is investigated as a new means of evaluating the mechanical integrity of packaging films. Mechanical clamp and vacuum fixtures were considered for securing the films to a set of backing materials and tested under various testing rates and film orientation conditions. Film performance was evaluated according to their puncture load. Based on the above study, the vacuum fixture offers the most consistent and meaningful results by providing a more intimate contact between film and backing and minimizing uncontrolled buckling of the film during testing. Additional testing was also carried out on a commercial film to confirm similarity between damage observed in the scratched films and that from the field. The scratch test gives good correlation between field performance and scratch test results on a set of commercial films. The usefulness of the scratch test methodology for packaging film mechanical integrity evaluation is discussed. Scratch-induced damages on multi-layer commercial packaging films are investigated using cross- and longitudinal-sectioning. Scratch test results show clear distinction between the two tested systems on both the inside and outside surfaces. Microscopy was performed to investigate the feasibility of utilizing this methodology as a tool for packaging film structure evaluation by determining the effect each layer has on the resistance of scratch damages. It is shown that the film showing superior scratch test results also shows significantly better stress distribution through its layers during the scratch test, as well as better layer adhesion during severe deformation. The scratch test shows good ability to provide more in-depth film mechanical integrity testing by allowing for layer-by-layer analysis of damages and layer adhesion after testing.Item Scratch Behavior of Multiphase Styrenic Copolymers and Effects of Environmental Conditioning(2014-09-05) Moghbelli, EhsanScratch-induced surface deformation is a tribological research area that falls under the abrasive wear category. A variety of factors including high strain rate, large-scale deformation, non-linear material response, heat dissipation, and the evolution of a complex stress field, renders scratch a complex mechanical process. The dependence of polymers on testing rates, temperature, and pressure, along with the surface characteristics of the two materials in contact bring the rate, time, temperature, and pressure dependent behaviors of polymers, and the surface condition of the interacting surfaces also add to the complications of scratch analysis. In order to gain an in-depth understanding of polymer scratch behavior, this dissertation focuses on the scratch response of multiphase systems made up of a plastic matrix and a dispersed rubber phase. The introduction of a rubber phase and the effect it has on scratch behavior is explored through a number of factors including rubber size and type, environmental conditioning through heat processing, moisture exposure, and water immersion. A standardized progressive load scratch test (ASTM D7027/ISO 19252) is used to examine the mechanical response to scratch deformation in ASA and ABS systems with varying rubber particle size. Previous simulation results from finite element methods are used to assess the scratch response of the multiphase systems and comparisons are made to results based on single phase plastics and their respective scratch behavior. The key scratch damage transitions identified and studied are: (1) the onset of scratch groove formation, (2) the onset of periodic cracking, (3) the onset of material removal (plowing), and (4) the onset of scratch visibility. The onset of groove formation is generally related to the secant modulus at the point of compressive yielding. The onsets of crack formation and plowing are more complex to quantitatively evaluate, and are strongly influenced by the material tensile and/or shear strength. For ASA copolymers, enhanced scratch performance is observed in systems with rubber particles the size of 1 micron relative to 100 nm sized rubber particle systems, while ABS copolymers containing 100 nm sized rubber particles are more scratch resistant than ASA copolymer systems with similar rubber particle size and distribution. The fact that these three model systems exhibit similar mechanical properties in uniaxial tension and compression bulk testing does not explain their differences in scratch resistance based on our previous FEM modeling and experimental results for single phase systems. The local stress state generated by the rubber particles and the scratch process at the surface, along with changes in surface coefficient of friction, are used to explain these findings. In order to minimize orientation and residual stress effects from the injection molding process, heat treatments at temperatures above and below Tg were carried out on ASAs with varying rubber content, rubber size, and rubber type. Low temperature annealing (LTA) was seen to reduce rubber orientation while having no impact on bulk mechanical properties, surface characteristics, or scratch resistance. On the other hand, high temperature annealing (HTA) minimized orientation and residual stress and showed no impact on bulk mechanical properties or surface characteristics, while also leading to a significant improvement in scratch resistance for high rubber content systems.Item Scratch behavior of polymers(Texas A&M University, 2005-11-01) Lim, Goy TeckThis dissertation work is focused on the analytical and numerical examination of the mechanical response of polypropylene (PP) under scratch deformation by a semispherical indenter. The finite element (FE) method is employed as the analysis technique and ABAQUS??, a commercial FE package is adopted to perform the analysis. Important physical and computational considerations on the implementation of FE analyses for the scratch problem are reviewed. It is shown through the discussion of the generated results that a good understanding can be gained on how different scratch conditions can affect scratch behavior of PP. A phenomenological deduction of the scratch damage process and mechanisms is also established. Considering the two main damage modes of polymers, shear yielding and crazing, it is shown that the two damage modes not only exist in the scratch deformation, and moreover, that they may compete against each other for dominance. A parametric study is also performed to assess the influence of material and surface properties on scratch response of material. A secondary research effort is also made to investigate the material constitutive modeling of polymers. Focusing on elastomeric or rubbery materials, a new mixed network model between the Gaussian and eight-chain non-Gaussian models is proposed. This mixed model inherently preserves the good predictive power of these two models and yields better predictions over a wider range of deformation than that of the rubber model adopted by ABAQUS??.Item The development of scratch test methodology and characterization of surface damage of polypropylene(Texas A&M University, 2004-11-15) Wong, Min HaoA new scratch test methodology is proposed. The new test methodology is developed based on the principles of materials science and solid mechanics, which include the consideration of material parameters, use of microscopy for image analysis and the finite element method (FEM). The consistency and reproducibility of test results are shown using a new scratch test device on two sets of neat and talcfilled polypropylene (PP) systems. Three different test conditions, i.e., linear load increase under constant rate, constant load under constant rate, and linear rate increase under constant load, have been conducted to determine the most effective, informative test conditions for evaluation of scratch resistance of polymers. Experimental observations and FEM results show a good qualitative correlation. The unique advantages of the new scratch test method for evaluating scratch resistance of polymers are discussed. A systematic study of surface damage effected by a progressive scratching load is performed on model polypropylene (PP) systems. Mar-scratch and stress -whitening transitions can be readily observed, and the corresponding critical loads determined. Distinctive scratch hardnesses and surface damage features are found for different material systems. Visibility of scratched surface is quantified using gray level analysis via a flatbed scanner and a commercial image analysis tool. It is found that the onset of scratch visibility can be determined accurately and reproducibly using the custom -built scratcher under progressive loading condition. Talc particles are found to be responsible for the increased light scattering, leading to greatly increased visibility. The observed scratch visibility is also found to be related to the measured frictional force profiles. Approaches for producing scratch resistant PP are discussed.