Abstract

The effect of nanoclay on the interlaminar fracture toughness of glass fiber composites was studied. Mode I and mode II tests were conducted to measure the interlaminar fracture toughness of nanoclay reinforced composites. A reduction in interlaminar fracture toughness (both mode I and mode II) was observed in the nanoclay reinforced fiber composites. Alignment of nanoclay along the fiber axis was found to be a possible reason for this reduction.  

Abstract

Inspired by the need for an efficient molecular simulation technique, we have developed engineering molecular mechanics (EMM) as an alternative molecular simulation technique to model high temperature (T > 0K) phenomena. EMM simulations are significantly more computationally efficient than conventional techniques such as molecular dynamics simulations. The advantage of EMM is achieved by converting the dynamic atomistic system at high temperature (T > 0K) into an equivalent static system. Fundamentals of the EMM methodology are derived using thermal expansion to modify the interatomic potential. Temperature dependent interatomic potentials are developed to account for the temperature effect. The efficiency of EMM simulations is demonstrated by simulating the temperature dependence of elastic constants of copper and nickel and the thermal stress developed in a confined copper system.  

Abstract

The equivalence of the virial stress and Cauchy stress is reviewed using both theoretical arguments and numerical simulations. Using thermoelasticity problems as examples, we numerically demonstrate that virial stress is equivalent to the continuum Cauchy stress. Neglecting the velocity terms in the definition of virial stress as many authors have recently suggested, can cause significant errors in interpreting MD simulation results at elevated temperatures (T > 0 K).   [full-text]

Abstract

Fracture behavior of vinyl ester resin and the methods that can be used to toughen vinyl ester resin were studied. Neat resin, 5% by weight nanoclay, 5% by weight core shell rubber (CSR) and hybrid system (3% nanoclay and 2% CSR by weight) were the material systems considered for comparing fracture toughness. Three types of cracks were used to determine the stress intensity factors at failure, viz., sharp crack, blunt crack and notch. The critical stress intensity factor in the case of sharp cracks improved significantly when compared to neat resin. In the case of notched and blunt cracked specimens, a reduction in stress intensity factors (at failure) was observed for reinforced systems. However, for notched and blunt cracked specimens, it was shown from the morphology of the fracture surface that the stress intensity factor calculated by assuming a notch or a blunt crack as an ideal crack was not the controlling parameter for fracture. A method for quantifying the crack tip sharpness using fracture surface roughness has been proposed.   [full-text]

Abstract

Morphology of nanoclay dispersed in resin and suspended in acetone was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show intercalation of resin in the gallery spaces of nanoclay andregions of exfoliated clay with random orientation. A vacuum assisted wet lay-up (VAWL) process was used for the inclusion of nanoclay in conventional fiber reinforced composites. The VAWL specimendisplayedimprovement ino?-axis compressive strength for nanoclay enhanced fiber composites. Addition of nanoclay produced a substantial increase in longitudinal compressive strengths (extracted from off-axis tests) of glass fiber reinforced composites. An elastic–plastic model was used to predict the compressive strength of fiber reinforced composites based on the matrix properties. The model predictions matched well with the experimental results.   [full-text]

Abstract

Solid-solid phase transformation is studied from a nanoscale mechanistic point of view. We performed molecular simulations on Ni-Al shape memory alloys to study the mechanisms that drive phase change. A new molecular simulation method called “engineering” molecular mechanics (EMM) is used in addition to MD simulations to study the phase change behavior. The approach is named "engineering" molecular mechanics (EMM) because it utilizes engineering properties like thermal expansion coefficient to represent temperature. The concept of equivalent atomistic thermal stress is introduced to model thermal activation. It is shown that EMM is accurate and much faster than MD simulations. The non-stoichiometric composition of NiAl leads to a non-homogenous distribution of nickel and aluminum atoms which makes the system anisotropic. This anisotropy makes it difficult to use continuum energy based theories to predict phase change. Molecular simulations show a clear phase change from Austenite to Martensite phase in a temperature range of 450-350 K. The reverse phase change from Martensite to Austenite was also modeled by heating the system from 100 K to 1000 K. On average, the system switched from Martensite to Austenite phase at a temperature of 400 to 500 K. The predicted results agree well with experimental data. Stress induced phase transformation and the associated pseudo-elasticity is also modeled. The atomistic mechanisms driving both thermally activated and stress induced phase transformations are explained.

Abstract

Simulation of atomic/molecular systems has been successfully employed to study material behavior at the nanometer size scale. The two most widely used numerical simulation techniques to study mechanical behavior of solids at the nanometer level are Molecular Statics (MS) and Molecular Dynamics (MD). Molecular dynamics simulations solve for the dynamics of the system governed by Newton's equations of motion at the pico second level. This fine thermal vibration detail of the motion of atoms is irrelevant for engineering purposes. Most often engineering problems need averaged solutions. Hence the time integration required to solve the dynamic problem at the pico second level has hundreds of thousands of steps of wasted computational time which leads to prohibitively long computation times and astronomical amount of data sets. On the other hand, Molecular Statics (MS) simulations based on minimizing the potential energy of the system do not require time integration of the governing equations, and hence reduce the computation time and the amount of data generated. But, the price we pay for using MS simulations is that it is valid at a temperature of 0 K and hence only gives a qualitative (and idealized) view of the problem. We propose a modified approach to molecular statics, wherein the initial lattice constant (or the characteristic length of the system like bond length for polymers) will be modified to account for temperature (higher than 0K) before using molecular statics to minimize the potential energy of the system. In principle, this approach will work because there is absolutely no change in the inter-atomic potential used in MD due to change in temperature. When temperature increases above 0K, the global changes observed in the system are thermal expansion and the increase in ground state (total) energy. Modifying the lattice constant (or characteristic length) of the system to account for thermal expansion automatically includes the change in the ground state energy. Also, thermal expansion data is readily available for most materials of interest and hence can be used to modify the lattice constant of the system. Thus, we can account for the temperature effects without the expensive computation involved in MD simulations. In order to demonstrate that the modified MS (which we will henceforth refer to as "Engineering" Molecular Mechanics (EMM)) works, Several thermo-mechanical problems were studied using MD and EMM. The first problem we considered was the variation of elastic constants with temperature. Copper and Nickel systems with 500 atoms were chosen to study the dependence of elastic constants on temperature. The elastic constants were evaluated by applying a uniaxial strain incrementally. Comparisons of the MD and EMM results for Copper and Nickel over a range of temperatures show that EMM predicts the thermal dependence of elastic constants very well even at very high temperatures (75 % of melting temperature). Temperature induced martensitic phase transformation in shape memory alloy system containing Nickel and Aluminum will also be analyzed using EMM. The critical transformation start and end temperatures will be investigated using EMM and the results will be compared with MD simulations and experiments. The effect of composition of the alloy on phase transformation during cooling and heating will also be investigated.

Abstract

Fracture behavior of vinyl ester resin with and without fiber was studied. Fracture toughness of vinyl ester resin was enhanced using nanoclay and Core Shell Rubber (CSR) particles. Hybrid specimens with both nanoclay and CSR particles were tailored to optimize the improvements produced by both nanoclay and CSR particles. Nanoclay was also used in fiber reinforced composites to enhance the mechanical properties. However, a reduction in mode I and mode II interlaminar fracture toughness of the composite was observed due to the addition of nanoclay. Possible reasons for the reduction in fracture toughness of fiber reinforced composites were studied.   [full-text]

Abstract

An elastic-plastic model was developed to predict the compressive strength of off-axis fiber reinforced composites based on the matrix properties. The model predictions matched well with the experimental results, hence proving that the compressive strength of off-axis fiber reinforced laminates can indeed be predicted based on matrix elastic-plastic properties alone. Using the model, it was also verified that addition of nanoclay produces a substantial increase in pure compressive strengths (extracted from off-axis tests) of glass fiber reinforced laminates.   [full-text]

Abstract

Morphology of nanoclay dispersed in vinyl ester resin was studied using Transmission Electron Microscopy (TEM). Addition of 3 %, 5 % and 8 % by weight of nanoclay increased the elastic modulus of resin by 16 %, 19 % and 20 % respectively. However, the best elastic and elastic-plastic behavior was observed for 5 % by weight nanoclay loading. Manufacturing process for inclusion of nanoclay in fiber reinforced laminates was developed. Off-axis compression tests showed substantial improvement in compressive strength of glass fiber reinforced laminates due to the addition of nanoclay. Pure compressive strength extracted from off-axis tests also show significant increase in compressive strength by addition of 3 % and 5 % by weight of nanoclay.   [full-text]

Abstract

Effect of nanoclay on the compressive strength of glass fiber composites was studied. It was observed that the addition of 5% (by weight) nanoclay increased the elastic modulus of the resin by 20%. Glass fiber composites were made using VARTM and Vacuum Assisted Wet Lay up procedures. A number of curing procedures were studied. There was a reduction in compressive strength due to the addition of nanoclay in specimen made using VARTM. However, for specimen made using the wet lay up procedure, an increase of 20-25% in compressive strength was observed due to the addition of nanoclay. The decrease in strength in the VARTM specimen might be caused due to the filtering of nanoclay by the resin distribution media and the fiber packing. It was found that curing with elevated temperatures initially may result in better compressive strengths in the nanoclay enhanced composite.   [full-text]