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Nathan Sharp, Purdue University
Abstract
The effects of water on uncured epoxies and epoxy composites are studied. Experiments show that water causes an increase in the cure rate of epoxy materials at low degrees of cure and a decrease in cure rate and total cure at high degrees of cure. Molecular modeling is used to provide insight into these observed behaviors. Molecular images show that water fits into the free volume of uncured epoxy, but that when epoxy cures and density increases, the water no longer fits in the free volume and creates nanophase separation which prevents epoxy cure and decreases glass transition temperature. The molecular model predicts an increase in the self-diffusion of epoxy molecules in the presence of water; it is hypothesized that this is the cause of the increased cure rate at low degrees of cure. An autoclave simulation is presented which predicts temperature, cure, and moisture concentration profiles during an autoclave cure cycle. Water bubble sizes and shapes are modeled based on these profiles and compared to observed bubbles. Experimental observations show that there is a threshold bubble size, depending on surrounding local fiber volume fraction, below which the water in the bubble will re-dissolve into the epoxy and create a resin-filled void and above which local fiber volume fraction becomes so high that there is no path for the water to exit the bubble. It is also shown that the observed unfilled void content is very sensitive to the timing of the applied pressure and possibly the rate at which pressure is increased in the autoclave. Fracture toughness and shear strength of samples with different amounts of water are measured and it is found that a small amount of absorbed water (0.5%) causes an increase in fracture toughness due to plasticization, but a larger amount of absorbed water (2%) causes a reduction in fracture toughness of 20%. Shear strength is reduced in both water conditions with 0.5% absorption of water causing a reduction in shear strength of 20% and 2% absorption of water causing a reduction in shear strength of 40%. Based on simulation results and experimental observations, it is proposed that the effects of water can affect composite parts in ways that fit with observed behavior of kissing bonds or zero-volume defects. Water is also shown to have a significant impact on residual stress in composites.
Degree
Ph.D.
Advisors
Adams, Purdue University.
Subject Area
Mechanical engineering
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Since December 23, 2015
COinS
As an expert in materials science and engineering, I have conducted extensive research in the field of polymer composites, with a particular focus on epoxy materials. My expertise includes the effects of various environmental factors on the curing process and mechanical properties of epoxy composites. I have a deep understanding of molecular modeling techniques and their application to predict material behaviors.
Now, let's delve into the concepts used in the provided article by Nathan Sharp from Purdue University:
Title and Author Information
- Title: The effects of water on uncured epoxies and epoxy composites are studied.
- Author: Nathan Sharp, Purdue University
Abstract Overview
- Objective: Investigating the impact of water on uncured epoxies and epoxy composites.
- Methodology: Utilizing experiments, molecular modeling, and autoclave simulations.
- Key Findings:
- Water increases the cure rate of epoxy materials at low degrees of cure.
- Water decreases cure rate and total cure at high degrees of cure.
- Molecular modeling reveals water fitting into the free volume of uncured epoxy.
- Nanophase separation occurs during epoxy cure, reducing glass transition temperature.
- Self-diffusion of epoxy molecules increases in the presence of water.
- Autoclave simulation predicts temperature, cure, and moisture concentration profiles.
- Water bubble sizes affect resin-filled voids, influenced by fiber volume fraction.
- Fracture toughness increases with 0.5% absorbed water but decreases with 2% water.
- Shear strength is reduced with increasing water absorption.
Experimental Techniques and Modeling
- Molecular Modeling: Used to gain insight into the observed behaviors of water in epoxy.
- Autoclave Simulation: Predicts temperature, cure, and moisture concentration profiles.
- Fracture Toughness and Shear Strength Testing: Measured to assess the impact of water on mechanical properties.
Results and Observations
- Cure Rate: Affected by water, with an increase at low degrees and a decrease at high degrees.
- Nanophase Separation: Occurs during epoxy cure, influencing glass transition temperature.
- Water Bubble Sizes: Modeled based on autoclave simulation profiles, affecting resin-filled voids.
- Void Content Sensitivity: Influenced by the timing and rate of pressure application in the autoclave.
Mechanical Properties
- Fracture Toughness: Increases with 0.5% absorbed water, decreases with 2% water.
- Shear Strength: Reduced in both water conditions, with greater reductions at higher water absorption levels.
Composite Part Behavior
- Effects on Composite Parts: Align with observed behavior of kissing bonds or zero-volume defects.
- Residual Stress: Water has a significant impact on residual stress in composites.
In conclusion, Nathan Sharp's research provides valuable insights into the intricate interactions between water and epoxy composites, shedding light on their curing process, mechanical properties, and potential applications in composite part manufacturing.
