Krishna Kannan

Polyamide 66/hectorite nanocomposites exhibit superior mechanical properties compared with pure polymers and are promising for structural applications. X-ray diffraction results revealed reduced degree of exfoliation with increase in organoclay content. Flexural fatigue characteristics of polyamide 66/hectorite nanocomposites containing different quantities of clay content were investigated, under deflection control mode, using a custom built flexural fatigue test rig. Addition of organoclay improved the moduli of the material. An enhanced resistance to cyclic softening was noticed at high temperatures with the incorporation of organoclay. Nanocomposite samples exhibited a significant improvement in fatigue life compared with pure polymers; however, the degree of enhancement is governed by the nanostructure of organoclay in polymer matrix. The fatigue life of nanocomposite samples is strongly affected by specimen temperature and induced stress. Macroscopic fracture surfaces changed from flat featureless structure to a highly perturbed structure with increase in organoclay content. © 2010 Society of Plastics Engineers.

Product design for safe performance and operation in harsh environments triggers the importance of understanding property degradation due to prolonged exposure of advanced composite materials to such environments. The effect of seawater immersion on monotonic tensile, stress relaxation, and cyclic bending fatigue response of polyamide 66 nanocomposites containing 0%, 1%, and 5% hectorite organoclay were investigated. Seawater equilibrated nanocomposites demonstrated less moisture absorption compared to their pure counterparts. The imbibed seawater significantly increased the ductility, and reduced the tensile modulus and strength of nanocomposites. A nine-parameter Prony series viscoelastic constitutive model was used to estimate the influence of seawater on the viscoelastic behavior based on stress relaxation data. The phenomenological model evaluated a drop of 82% in the instantaneous and the residual elastic moduli indicating a significant plasticizing effect of seawater. However, the amount of increase in the rubbery nature of polyamide is observed from the reduction in stress relaxation time of first Maxwell element by about 40%. Nanocomposites exhibited improved fatigue life with an increase in organoclay content even after equilibration in seawater. The fracture surface observation of seawater equilibrated bending fatigue specimen revealed rubbery behavior indicating the positive difference between cyclic steady-state temperature and material glass transition temperature.

Imbibed moisture affects the mechanical properties of polymers and influences the performance of products made out of them during service. Flexural fatigue tests were conducted under deflection control mode using a custom built, table-top flexural fatigue test rig at laboratory condition on PA66/hectorite nanocomposites (PA66CN). Dynamic mechanical analysis studies of PA66CN revealed significant plasticization effect of water on moduli and damping factor with increase in imbibed moisture content. A decrease in induced flexural stress amplitude and rise in temperature of specimen with increase in moisture content result in increased fatigue life at a constant cyclic end deflection. The microstructure of failed flexural fatigue specimens manifested a rubbery behavior. The extent of rubberiness is directly related to the difference between specimen temperature at cyclic steady state and glass transition temperature. © 2010 Elsevier B.V.

An increased use of thermoplastics in components and structures that are subjected to cyclic loads necessitates a specific attention to variables that affect the hysteretic heating. Hysteretic heating effect in polyamide 66/hectorite nanocomposite has been investigated under bending strain control mode using a custom-built bending fatigue test setup in a laboratory environment. Dynamic mechanical analysis (DMA) results revealed a considerable rise in loss modulus with a decrease in frequency from 1 to 0.1 Hz irrespective of the temperature of the specimen. Alternatively, a reduction in fatigue test frequency from 2 to 0.5 Hz resulted in a significant decrease in cyclic softening. Fatigue behaviour predicted from DMA results using coupled structural/thermal finite element analysis is fairly in agreement with the experimental one. An accelerated crack initiation at decreased specimen temperature and high cyclic steady state stress reduced the fatigue life at 0.5 Hz compared with 2 Hz.

There are many machine components made of polymeric materials, such as gears, which are subjected to cyclic loading conditions. To design such components, it is necessary to arrive at a suitable mathematical model that can describe the mechanical response of polymeric materials. In this paper, we derive a mathematical model for rate-type solids using thermodynamical framework developed by Rajagopal and Srinivasa (K.R. Rajagopal, A.R. Srinivasa, A thermodynamic frame work for rate type fluid model, Journal of Non-Newtonian Fluid Mechanics 88 (2000) 207-227) (also see Section 5 of Kannan and Rajagopal (K. Kannan, K.R. Rajagopal, A thermomechanical framework for the transition of a viscoelastic liquid to a viscoelastic solid, Mathematics and Mechanics of Solids 9 (2004) 37-59)), which was used by Rajagopal and Srinivasa to derive a mathematical model for isotropic, rate-type liquids. Uniaxial cyclic loading and stress relaxation experiments were conducted. The predictions of the model agreed well with the experimental data. © 2009 Elsevier Ltd. All rights reserved.

Design with polymeric materials is difficult because moisture absorption during service affects their mechanical properties. Mechanical behavior of polyamide 66/hectorite nanocomposites (PA66CN) with different amounts of imbibed moisture and organoclay was studied using tensile and instrumented micro-indentation tests. PA66CN specimens with imbibed moisture content were obtained by equilibrating in laboratory air and distilled water. The amount of moisture absorbed by the PA66CN samples is decreased with an increase in the amount of organoclay. The imbibed moisture greatly improved the ductility of PA66CN, which was reduced with increase in the amount of organoclay, however with a simultaneous reduction in the tensile modulus and strength. Fractography analysis of failed dry and equilibrated PA66CN specimens showed a clear transition from glassy to rubbery behavior due to absorption of moisture. © 2010 Elsevier Ltd. All rights reserved.

Polymer nanocomposites are promising for structural applications due to their high modulus and good corrosive resistance. The influence of environment on flexural fatigue behavior of polyamide 66/hectorite nanocomposites has been investigated in still air and circulated water mist environments. High hysteretic energy dissipation per cycle and high induced stress in mist lead to a significant drop in fatigue life in mist compared with that in air. The strain and hysteretic energy dissipation per cycle against fatigue life curves exhibited linear relationships on log-log scale in air. The failure analysis revealed a distinct deformation and fracture mechanisms in air and mist. © 2010 Elsevier Ltd. All rights reserved.