Long-term mechanical performance and durability of fiber reinforced polymer (FRP) composite structures are strongly influenced by the evolution of their microstructure. A multi-scale analysis framework of composite structures that recognize microstructural deterioration due to coupled time-stress-temperature-moisture effects are needed in conjunction with experimental works for microstructural characterization to understand and predict material/structural behaviors. The proposed CAREER program aims to develop a method for linking the evolution of composite constituent (fiber-matrix-interphase) to the overall viscoelastic and damage behaviors of FRP structures and to effectively model and characterize the microstructural properties. The research plan consists of (1) An analytical/computational formulation phase to develop a multi-scale material and structural modeling framework. The framework is based on a synthesis of three major components: numerical algorithms of time-dependent constitutive models with hygrothermal, stress, and damage effects at the constituent (matrix-interphase) levels, hierarchical micromechanical constitutive models for several composite reinforcements, and layered structural elements that incorporate through-thickness material variability and transverse shear deformations. The framework is implemented in a general three-dimensional (3D) nonlinear finite element (FE) code. (2) Experimental phase to characterize in-situ microstructural properties and verify the proposed multi-scale framework. Macro scale testing includes axial and shear creep tests on FRP specimens with Digital image correlation (DIC) to record full field surface displacements. Micro scale testing includes creep indentation and scratch tests on fiber, matrix and interphase regions. While major progress has been made in multi-scale material constitutive modeling, the proposed research plan is significant and unique because it provides an overall microstructural-global material and structural framework that couples time, mechanical, hygrothermal, and damage behaviors from the micro-level (micron scale) to the nonlinear response at the structural level (meters scale). The outcome of this research will enhance understanding of microstructural behaviors of heterogeneous materials that strongly influence the global responses of composite structures. This will benefit both civil and aerospace infrastructures, such as aircraft structural components, bridges, tunnels, fluid conveying pipes, and many others. The research and education plans are integrated so that the education initiatives draw upon the research results. The interdisciplinary nature of the CAREER Plan will overreach and impact students in the mechanical, civil, aerospace, and petroleum engineering departments. The educational plan focuses on strengthening solid mechanics curriculum at Texas A&M by developing graduate course in Computational Inelasticity and preparing undergraduate and graduate students who can contribute to nonlinear mechanics of heterogeneous systems both as engineers and researchers. To enhance student learning in material and structural behaviors, visualization and animations that are correlated with real testing using DIC technique will be developed. A summer research program that targets high-school teachers will be established to disseminate research in the evolution of microstructural materials to public. In addition, collaborations with industry will be initiated to apply the research finding in the analyses of oil drilling and aircraft structures.
Research Outcome for the General Public Principal Investigator: Anastasia Muliana Organization: Texas Engineering Experiment Station (TEES) Title: CAREER Time-dependent Multi-scale Frameworks for Mechano-thermo-hygro-visco and Damage Behaviors of Composite Materials and Structures Award ID: 0546528 Intellectual Merit: This NSF CAREER project introduces a model to predict the overall time-dependent performance of several structures and/or structural components made of fiber reinforced polymer layered composites subject to coupled mechanical and hygrothermal external stimuli. The use of layered fiber reinforced polymer composites makes it possible to build lighter bridge-decks, aircraft fuselages, transmission towers, large wind-turbine blades, ship decks, and many others. The composites used in these applications often experience complex mechanical loading and hostile environmental conditions, which cause degradation in the structures. The response of polymeric based composite changes over time due to the existence of soft polymers and continuous changes in the environmental conditions, i.e. high temperatures during summer and low temperatures in winter in addition to changes in the humidity, intensify the deterioration of the microstructures which cause material damages and structural collapses. Thus, it is necessary to bridge the microstructural response of the composites to the overall performance of the composites structures. This can be achieved through the development of a multi-scale model of composite structures that include time-stress-temperature-moisture effects and damage behaviors (Fig. 1). In order to support the development of the multi-scale framework, experimental data on the performance of composites at different environmental conditions are needed. This project has successfully developed the predictive model for the performance of composite structures subject to mechanical loading and hostile environmental conditions at multiple time and length scales. Figure 2 illustrates time-dependent deformations (short- and long-term responses) from the experiment and multi-scale prediction of layered composites, commonly used in bridge decks, at several isothermal temperatures. Another example is on predicting time-dependent response of aircraft fuselage made of fiber reinforced polymer and aluminum layered composites, termed as fiber metal laminates. The time-dependent deformations from the multi-scale have been verified by comparing their results with available experimental data on fiber metal laminates reported in the literature. Figure 3 shows the overall field deformation of an aircraft fuselage, made of fiber metal laminate having glass fiber epoxy composite and aluminum layers, which was generated from the multi-scale model. The multi-scale model has also been used to analyze response of other composite systems such as polymeric based functionally graded materials and foam core sandwich composites at different isothermal temperatures and moisture contents. Furthermore, predictions of time-dependent delamination or separation between layers in the layered composites, such as separation between composite skins and foam core of sandwich composites undergoing diffusion of a fluid, have been made. Broader Impact: The research on the development of a multi-scale model for predicting time-stress-temperature-moisture dependent response of polymeric based composites have been disseminated through several publications: 26 peer reviewed journals, 9 conference proceedings, 3 PhD dissertations, and 6 master theses. The PI has involved 3 PhD, 6 Master, and 2 Undergraduate students in this NSF CAREER project. The students have been trained in conducting experiments on composites, analyzing overall time-dependent responses of composite structures subject to combined mechanical, thermal, and moisture stimuli, developing models for predicting deformations of polymers and composites, formulating multi-scale models for various composite systems, and characterizing the thermal, physical, and mechanical properties of the composites. Both graduate and undergraduate students have been trained in writing technical publications and giving technical presentations and poster presentation. In addition, the PI advised 2 high-school teachers from Crystal City and Brownville, Texas, which have high populations of Hispanic students, during the summer of 2007. The teachers were involved in testing of layered composites at several temperatures and they came up with final projects in which they constructed a testing prototype to simulate creep testing. The final project was shared with other teachers during final project presentations and presented to high-school students. A graduate course in Design and Modeling of Viscoelastic Structures was developed and became part of the curriculum in the mechanical engineering department at Texas A&M University since Fall 2009. The PI has used the multi-scale framework to create visualization and animation of mechanical responses in materials and structures subject to various loading histories. This visualization helps the students learn about changes in stress, strain, and displacement fields with loadings and time and identifying critical locations based on these field variables. Finally, the time-dependent models for polymers and composites developed in this NSF CAREER project have been modified and extended to predict short-term and long-term responses of hot-mixed asphalt and asphalt binders, and deformation in biological tissue and biodegradable polymers undergoing changes in their properties with diffusion of fluids. Figure 4 illustrates a time-dependent degradation in a polymeric stent of a mesh tubular structure. The regions with a high degradation level (red color) are critical that could lead to stent collapses.
