Browsing by Subject "Tensile strength"
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Item Characterizing the ultimate properties of triblock styrene-diene thermoplastic elastomers(2005-05) Zeng, Qiumei; Leggoe, Jeremy W.; Idesman, Alexander V.; McKenna, Gregory B.; Dai, Lenore L.Styrene-diene triblock copolymers with polystyrene end blocks and polydiene midblock are an important group of thermoplastic elastomers (TPEs) and have been widely used in footwear, adhesives, automotive and the automotive industry. These materials exhibit a phase separated structure, with the polystyrene end blocks forming glassy domains distributed throughout the polydiene matrix. Styrenic TPEs have excellent mechanical properties comparable to those of vulcanized rubber, but can be processed like thermoplastics. The purpose of this research is to investigate the phenomena governing the failure process and ultimate properties in block-copolymer TPEs, and to propose a model to predict the theoretical strength of the materials. Styrene diene block copolymers were chosen as the model materials for this investigation since this class of TPE dominates commercial production of TPEs, and have simple, well characterized molecular structure, from which the properties of other block copolymer can be inferred. To support the development of the tensile strength model, uniaxial tensile test were performed for three commercial styrene-diene TPEs, polystyrene-polyisoprenepolystyrene (SIS) with 18 and 30 wt% polystyrene (PS) from Dexco Polymers and polystyrene-poly(ethylene-co-butylene)-polystyrene (SEBS) with 30 wt% PS from Kraton Polymers. Repeated loading tests confirmed that SIS with 18 wt% PS exhibited spherical morphology while the other two materials with 30 wt% PS exhibited continuous hard domains, as expected from the phase separation theory for block copolymers. The stress-extension curves for the second loading of SIS and SEBS with 30 wt% PS exhibit extensive stress softening, suggesting that the continuous domains break up progressively during deformation. The investigation on the influence of loading rate on tensile strength show that the tensile strength of SIS increase with increasing loading rate at low rates, eventually reaching a plateau or peak. The tensile strength of SEBS remained relatively constant over the entire loading rate range studied. The ultimate strain was found to be relatively independent of the loading rate. To investigate the fracture mechanism governing the ultimate properties of styrenediene TPEs, fractographic studies of the specimens broken during tensile testing were undertaken. SIS18 specimens were fond to contain gross processing flaws, as a result of the increased thickness of the specimens prepared for this material. The fracture surfaces of SIS30 specimens exhibited distinct rough and smooth regions, with the roughness of the surface decreasing with increasing strain rate. The fractures surfaces of the SEBS30 specimens indicated that failure in this material initiated at near surface flaws. The fracture surface in the initiation area was relatively smooth, becoming rough with the onset of rapid crack propagation (in a manner more akin to the behavior of glassy polymers than vulcanized elastomers). The appearance of the rapid crack propagation surface in this material was found to be unaffected by the loading rate. Based on the findings of the tensile test and fractographic investigations, it is apparent that flaws play a significant role in the fracture surface. In order to predict the ultimate strength of styrene-diene TPEs, it will be necessary to formulate models that account for the effects of flaws and the strength of the undamaged material adjacent to the flaws. In order to predict the effect of material parameters on the theoretical strength of undamaged material, a theoretical strength model has been proposed. The model takes into account the chain pullout and chain scission fracture mechanism expected to prevail at the molecular level. The theoretical strength is formulated as the maximum force the material can sustain per unit area based on the force supported by the glassy PS domains and the elastomeric mid-block chain sections intersecting a planar unit area. The strength contributed by the hard domains and elastomer matrix is related by the maximum force a chain can sustain without undergoing chain pullout or chain scission. The influence of degree of phase separation is also incorporated in the model. The model captures most features observed from experiment. Combined with the influence of flaws, the model is able to explain the different tensile strength behavior reported by different groups.Item Characterizing the ultimate properties of triblock styrene-diene thermoplastic elastomers(Texas Tech University, 2005-05) Zeng, Qiumei; Leggoe, Jeremy W.; Idesman, Alexander V.; McKenna, Gregory B.; Dai, Lenore L.Styrene-diene triblock copolymers with polystyrene end blocks and polydiene midblock are an important group of thermoplastic elastomers (TPEs) and have been widely used in footwear, adhesives, automotive and the automotive industry. These materials exhibit a phase separated structure, with the polystyrene end blocks forming glassy domains distributed throughout the polydiene matrix. Styrenic TPEs have excellent mechanical properties comparable to those of vulcanized rubber, but can be processed like thermoplastics. The purpose of this research is to investigate the phenomena governing the failure process and ultimate properties in block-copolymer TPEs, and to propose a model to predict the theoretical strength of the materials. Styrene-diene block copolymers were chosen as the model materials for this investigation since this class of TPE dominates commercial production of TPEs, and have simple, well characterized molecular structure, from which the properties of other block copolymer can be inferred. To support the development of the tensile strength model, uniaxial tensile test were performed for three commercial styrene-diene TPEs, polystyrene-polyisoprenepolystyrene (SIS) with 18 and 30 wt% polystyrene (PS) from Dexco Polymers and polystyrene-poly(ethylene-co-butylene)-polystyrene (SEBS) with 30 wt% PS from Kraton Polymers. Repeated loading tests confirmed that SIS with 18 wt% PS exhibited spherical morphology while the other two materials with 30 wt% PS exhibited continuous hard domains, as expected from the phase separation theory for block copolymers. The stress-extension curves for the second loading of SIS and SEBS with 30 wt% PS exhibit extensive stress softening, suggesting that the continuous domains break up progressively during deformation. The investigation on the influence of loading rate on tensile strength show that the tensile strength of SIS increase with increasing loading rate at low rates, eventually reaching a plateau or peak. The tensile strength of SEBS remained relatively constant over the entire loading rate range studied. The ultimate strain was found to be relatively independent of the loading rate. To investigate the fracture mechanism governing the ultimate properties of styrenediene TPEs, fractographic studies of the specimens broken during tensile testing were undertaken. SIS18 specimens were fond to contain gross processing flaws, as a result of the increased thickness of the specimens prepared for this material. The fracture surfaces of SIS30 specimens exhibited distinct rough and smooth regions, with the roughness of the surface decreasing with increasing strain rate. The fractures surfaces of the SEBS30 specimens indicated that failure in this material initiated at near surface flaws. The fracture surface in the initiation area was relatively smooth, becoming rough with the onset of rapid crack propagation (in a manner more akin to the behavior of glassy polymers than vulcanized elastomers). The appearance of the rapid crack propagation surface in this material was found to be unaffected by the loading rate. Based on the findings of the tensile test and fractographic investigations, it is apparent that flaws play a significant role in the fracture surface. In order to predict the ultimate strength of styrene-diene TPEs, it will be necessary to formulate models that account for the effects of flaws and the strength of the undamaged material adjacent to the flaws. In order to predict the effect of material parameters on the theoretical strength of undamaged material, a theoretical strength model has been proposed. The model takes into account the chain pullout and chain scission fracture mechanism expected to prevail at the molecular level. The theoretical strength is formulated as the maximum force the material can sustain per unit area based on the force supported by the glassy PS domains and the elastomeric mid-block chain sections intersecting a planar unit area. The strength contributed by the hard domains and elastomer matrix is related by the maximum force a chain can sustain without undergoing chain pullout or chain scission. The influence of degree of phase separation is also incorporated in the model. The model captures most features observed from experiment. Combined with the influence of flaws, the model is able to explain the different tensile strength behavior reported by different groups.Item Investigation on the tensile properties of individual cotton (Gossypium hirsutum L.) fibers(2012-08) Hosseinali, Farzad; Hequet, Eric F.; Abidi, Noureddine; Simonton, James L.This thesis consists of three chapters. The first chapter covers introductory definitions on the subject of tensile properties of textile fibers. Concepts such as individual fiber tensile strength, fiber bundle strength, and instruments such as apparatus for measuring tensile properties of fibers, creep, and stress are briefly reviewed in this chapter. The aim of the second chapter is to determine the relationships between individual cotton fiber tensile properties and their length, maturity, and fineness, within-sample. To this end, six samples were selected among 104 reference cotton samples and each one was sorted into seven length groups using the array method. Tensile properties of each length group were tested using the FAVIMAT, the individual fiber tensile tester. In order to measure their maturity-ratio and fineness, samples were examined using the Advanced Fiber Information System (AFIS) Pro-2. It is observed in all samples that, within-sample, short cotton fibers have on average a lower tensile force. Also, within a sample, AFIS results indicate that longer fibers are on average more mature than shorter fibers. It can be concluded that throughout mechanical processing, the least mature cotton fibers may be broken into smaller segments. In the third chapter, we compared two common methods of testing fiber tensile strength, namely individual fiber and fiber bundle tests. Our objective is to investigate the relationships between individual fiber tensile force and other properties of cotton fibers and to identify the best predictors of individual cotton fibers’ tensile force using regression techniques. According to the results, almost two-thirds of the variability observed for the average tensile force of individual cotton fibers can be explained by fiber maturity and elongation.Item Tensile strength of asphalt binder and influence of chemical composition on binder rheology and strength(2014-08) Sultana, Sharmin; Bhasin, Amit; Liechti, Kenneth M.; Prozzi, Jorge A.; Zhang, Zhanmin; Fowler , David W.Asphalt mixtures or asphalt concrete are used to pave about 93% of about 2.6 million miles paved roads and highways in the US. Asphalt concrete is a composite of aggregates and asphalt binder; asphalt binder works as a glue to bind the aggregate particles. The mechanical response of the asphalt binder is dependent on the time/rate of loading, temperature and age. An asphalt concrete mixture inherits most of these characteristics from the asphalt binder. Also the asphalt binder plays a critical role in providing the asphalt concrete the ability to resist tensile stresses and relaxing thermally induced stresses that can lead to fatigue and low temperature cracking, respectively. Hence, it is very important (but not sufficient) to ensure that asphalt binders used in the production of asphalt concrete are inherently resistant to cracking, rutting and other distresses that a pavement may undergo. Current binder specification (AASHTO M-320) to evaluate its fatigue cracking is based on the stiffness of the binder and not on its tensile strength. Also, measurements following current specifications are made on test specimens subjected to a uniaxial mode of loading that does not produce the same stress state in the binder as in the case of asphalt concrete. Another challenge in being able to produce binders with inherently superior performing characteristics is the fact that the asphalt binders produced in a refinery do not have a consistent chemical composition. The chemical composition of asphalt binder depends on the source and refining process of crude oil. There is a need to better quantify the tensile strength of asphalt binder and understand the relationship between the chemical composition of asphalt binders and its mechanical properties. The knowledge from this study can be used to engineer asphalt binders that have superior performance characteristics. The objective of this research was to quantify the tensile strength of asphalt binder, develop a metric for the tensile strength and identify the relationship between chemical composition and mechanical properties of asphalt binder. Laboratory tests were performed on binders of different grades using a poker chip geometry to simulate confined state by varying the film thickness, rate of loading and modes of loading. The chemical properties of asphalt binder were studied based on SARA fractionation. The findings from this research showed that the modified correspondence principles can unify and explain the rate and mode dependency of asphalt binder. This study also quantified the relationship between chemical composition, and rheological and mechanical properties of asphalt binder. Finally, a composite model was developed based on the individual properties of chemical fractions which could predict the dynamic modulus of the asphaltenes doped and resins doped binder.Item Use of ionic liquid for producing regenerated cellulose fibers(2011-05) Jiang, Wei, master of science in textile and apparel technology; Chen, Jonathan Yan; Xu, BugaoThe objectives of the research are to establish the process of obtaining regenerated fibers and films from wood pulp and bagasse pulp with the ionic liquid 1-Butyl-3-methylimidazolium Chloride (BMIMCl) as a solvent; to study the impacts on tensile strength of different spinning parameters; to find the optimal spinning condition, and to obtain regenerated cellulose products with flame retardant properties. Solutions were obtained by dissolving cellulose (wood/bagasse) pulp into the BMIMCl. The solutions were extruded in a dry-jet and wet-spinning method using water as a coagulation bath. The obtained fibers were tested to evaluate the properties such as tensile strength, thermal property, thermal mechanical property, crystal order, and ionic liquid residue in obtained fiber. The orthogonal experiments were designed to find out the strongest affective variable and the optimal condition of the spinning process. The regenerated cellulose films with melamine resin or zinc oxide were obtained. Their flame retardant properties were tested. Cellulose fiber with melamine resin was also obtained. Thermo-gravimetric analyzer (TGA) was used to measure the thermal properties of obtained products, and to calculate their activation energies. Dynamic mechanical analysis (DMA) was used to determine the thermal mechanical properties of obtained fibers. Wide angle X-ray diffraction (WAXD) was used to measure the degree of crystallinity and degree of crystal orientation. The tensile strength was tested by a tensile machine. To evaluate the quantity of ionic liquid residue in the regenerated fibers, the instrumental methods of FT-IR and Mass Spectrometry were applied. Research results indicated increases in the degree of crystallinity and storage modulus under a higher fiber drawing speed. Both regenerated bagasse fibers and regenerated wood fibers had similar thermal properties. However, the regenerated bagasse fibers showed a higher degree of crystallinity and a higher tenacity than the regenerated wood fibers obtained under the same condition. The study also revealed water treatment would be helpful for eliminating the ionic residue in regenerated fibers. It was also found the concentration of cellulose in the BMIMCl solution affected the tensile strength of regenerated fiber mostly. Certain amount of melamine or zinc oxide nanoparticles contained in the cellulose matrix could improve the flame retardant property effectively.