Publications

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Fiber-reinforced composite materials offer light-weight solutions to many structural challenges. In the development of high-performance composite structures, a thorough understanding is required of the composite materials themselves as well as methods for the analysis and failure prediction of the relevant composite structures. However, the mechanical properties required for the complete constitutive definition of a composite material can be difficult to determine through experimentation. Therefore, efficient methods are necessary that can be used to determine which properties are relevant to the analysis of a specific structure and to establish a structure's response to a material parameter that can only be defined through estimation. The objectives of this study deal with demonstrating the potential value of sensitivity and uncertainty quantification techniques during the failure analysis of loaded composite structures; and the proposed methods are applied to the simulation of the four-point flexural characterization of a carbon fiber composite material. Utilizing a recently implemented, phenomenological orthotropic material model that is capable of predicting progressive composite damage and failure, a sensitivity analysis is completed to establish which material parameters are truly relevant to a simulation's outcome. Then, a parameter study is completed to determine the effect of the relevant material properties' expected variations on the simulated four-point flexural behavior as well as to determine the value of an unknown material property. This process demonstrates the ability to formulate accurate predictions in the absence of a rigorous material characterization effort. The presented results indicate that a sensitivity analysis and parameter study can be used to streamline the material definition process as the described flexural characterization was used for model validation.

Simulations of low velocity impact with a flat cylindrical indenter upon a carbon fiber fabric reinforced polymer laminate are rigorously validated. Comparison of the impact energy absorption between the model and experiment is used as the validation metric. Additionally, non-destructive evaluation, including ultrasonic scans and three-dimensional computed tomography, provide qualitative validation of the models. The simulations include delamination, matrix cracks and fiber breaks. An orthotropic damage and failure constitutive model, capable of predicting progressive damage and failure, is developed in conjunction and described. An ensemble of simulations incorporating model parameter uncertainties is used to predict a response distribution which is then compared to experimental output using appropriate statistical methods. Finally, the model form errors are exposed and corrected for use in an additional blind validation analysis. The result is a quantifiable confidence in material characterization and model physics when simulating low velocity impact in structures of interest.

The kinetics of thermoset resin cure are multifaceted, with flow and wet-out being dependent on viscosity, devolatilization being a function of partial pressures, and crosslinking being dependent on temperature. A unique cure recipe must be developed to address and control each factor simultaneously. In the case of thick-section composites, an uncontrolled exotherm could cause the panel to cure from the inside out, causing severe process-induced residual stresses. To identify and control the peak heat generation from the exothermic crosslinking reaction, differential scanning calorimetry (DSC) was conducted for different candidate cure schedules. Resin rheology data and dynamic mechanical analysis (DMA) results were used to confirm a viable resin viscosity profile for each cure schedule. These experiments showed which isothermal holds and ramp rates best served to decrease the exothermic peak as well as when to apply pressure and vent the applied vacuum. From these data, a cure cycle was developed and applied to the material system. During cure, embedded thermocouples were used to monitor heat generation and drive cure temperature ramps and dwells. Ultrasonic testing and visual inspection by microscopy revealed good compaction and < 1 % porosity for two different composite panels with the same resin system. DSC of post-cured samples of each panel indicated a high degree of cure throughout the thickness of the panels, further qualifying the proven-in process.

Process induced residual stresses commonly occur in composite structures composed of dissimilar materials. These residual stresses form due to differences in the composite materials' coefficients of thermal expansion as well as the shrinkage upon cure exhibited by most thermoset polymer matrix materials. Depending upon the specific geometric details of the composite structure and the materials' curing parameters, it is possible that these residual stresses can result in interlaminar delamination and fracture within the composite as well as plastic deformation in the structure's metallic materials. Therefore, the consideration of potential residual stresses is important when designing composite parts and their manufacturing processes. However, the experimental determination of residual stresses in prototype parts can be prohibitive, both in terms of financial and temporal costs. As an alternative to physical measurement, it is possible for computational tools to be used to quantify potential residual stresses in composite prototype parts. Therefore, the objective of this study is the development of a simplistic method for simulating the residual stresses formed in polymer matrix composite structures. Specifically, a simplified approach accounting for both coefficient of thermal expansion mismatch and polymer shrinkage is implemented within the Sandia National Laboratories' developed solid mechanics code, SIERRA. This approach is then used to model the manufacturing of two simple, bi-material structures composed of a carbon fiber/epoxy composite and aluminum: a flat rectangular plate and cylinders. Concurrent with the computational efforts, structures similar to those modeled are fabricated and the residual stresses are quantified through the measurement of deformation. The simulations' results are compared to the experimentally observed behaviors for model validation, as well as a more complex modeling approach. The results of the comparisons indicate that the proposed finite element modeling approach is capable of accurately simulating the formation of residual stresses in composite structures.

The kinetics of thermoset resin cure are multifaceted, with flow and wet-out being dependent on viscosity, devolatilization being a function of partial pressures, and crosslinking being dependent on temperature. A unique cure recipe must be developed to address and control each factor simultaneously. In the case of thick-section composites, an uncontrolled exotherm could cause the panel to cure from the inside out, causing severe process-induced residual stresses. To identify and control the peak heat generation from the exothermic crosslinking reaction, differential scanning calorimetry (DSC) was conducted for different candidate cure schedules. Resin rheology data and dynamic mechanical analysis (DMA) results were used to confirm a viable resin viscosity profile for each cure schedule. These experiments showed which isothermal holds and ramp rates best served to decrease the exothermic peak as well as when to apply pressure and vent the applied vacuum. From these data, a cure cycle was developed and applied to the material system. During cure, embedded thermocouples were used to monitor heat generation and drive cure temperature ramps and dwells. Ultrasonic testing and visual inspection by microscopy revealed good compaction and < 1 % porosity for two different composite panels with the same resin system. DSC of post-cured samples of each panel indicated a high degree of cure throughout the thickness of the panels, further qualifying the proven-in process.

A series of quasi - static indentation experiments are conducted on carbon fiber reinforced polymer laminates with a systematic variation of thicknesses and fixture boundary conditions. Different deformation mechanisms and their resulting damage mechanisms are activated b y changing the thickn ess and boundary conditions. The quasi - static indentation experiments have been shown to achieve damage mechanisms similar to impact and penetration, however without strain rate effects. The low rate allows for the detailed analysis on the load response. Moreover, interrupted tests allow for the incremental analysis of various damage mechanisms and pr ogressions. The experimentally tested specimens are non - destructively evaluated (NDE) with optical imaging, ultrasonics and computed tomography. The load displacement responses and the NDE are then utilized in numerical simulations for the purpose of model validation and vetting. The accompanying numerical simulation work serves two purposes. First, the results further reveal the time sequence of events and the meaning behind load dro ps not clear from NDE . Second, the simulations demonstrate insufficiencies in the code and can then direct future efforts for development.

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V arious phenomenological delamination initiation criteria are analyzed in quasi - static punch - shear tests conducted on six different geometries. These six geometries are modeled and analyzed using elastic, large - deformation finite element analysis. Analysis output is post - processed to assess different delamination initiation criteria, and their applicability to each of the geometries. These criteria are compared to test results to assess whether or not they are appropriate based on what occurred in testing. Further, examinations of CT scans and ultrasonic images o f test specimens are conducted in the appendix to determine the sequence of failure in each test geometry.

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Presented is a model verification and validation effort using low - velocity impact (LVI) of carbon fiber reinforced polymer laminate experiments. A flat cylindrical indenter impacts the laminate with enough energy to produce delamination, matrix cracks and fiber breaks. Included in the experimental efforts are ultrasonic scans of the damage for qualitative validation of the models. However, the primary quantitative metrics of validation are the force time history measured through the instrumented indenter and initial and final velocities. The simulations, whi ch are run on Sandia's Sierra finite element codes , consist of all physics and material parameters of importance as determined by a sensitivity analysis conducted on the LVI simulation. A novel orthotropic damage and failure constitutive model that is cap able of predicting progressive composite damage and failure is described in detail and material properties are measured, estimated from micromechanics or optimized through calibration. A thorough verification and calibration to the accompanying experiment s are presented. Specia l emphasis is given to the four - point bend experiment. For all simulations of interest, the mesh and material behavior is verified through extensive convergence studies. An ensemble of simulations incorporating model parameter unc ertainties is used to predict a response distribution which is then compared to experimental output. The result is a quantifiable confidence in material characterization and model physics when simulating this phenomenon in structures of interest.

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Fiber-reinforced composite materials offer light-weight solutions to many structural challenges. In the development of high-performance composite structures, a thorough understanding is required of the composite materials themselves as well as methods for the analysis and failure prediction of the relevant composite structures. However, the mechanical properties required for the complete constitutive definition of a composite material can be difficult to determine through experimentation. Therefore, efficient methods are necessary that can be used to determine which properties are relevant to the analysis of a specific structure and to establish a structure's response to a material parameter that can only be defined through estimation. The objectives of this study deal with the computational examination of the four-point flexural characterization of a carbon fiber composite material. Utilizing a novel, orthotropic material model that is capable of predicting progressive composite damage and failure, a sensitivity analysis is completed to establish which material parameters are truly relevant to a simulation's outcome. Then, a parameter study is completed to determine the effect of the relevant material properties' expected variations on the simulated four-point flexural behavior. Lastly, the results of the parameter study are combined with the orthotropic material model to estimate any relevant material properties that could not be determined through experimentation (e.g., in-plane compressive strength). Results indicate that a sensitivity analysis and parameter study can be used to optimize the material definition process. Furthermore, the discussed techniques are validated with experimental data provided for the flexural characterization of the described carbon fiber composite material.

Capabilities are developed, verified and validated to generate constitutive responses using material and geometric measurements with representative volume elements (RVE). The geometrically accurate RVEs are used for determining elastic properties and damage initiation and propagation analysis. Finite element modeling of the meso-structure over the distribution of characterizing measurements is automated and various boundary conditions are applied. Plain and harness weave composites are investigated. Continuum yarn damage, softening behavior and an elastic-plastic matrix are combined with known materials and geometries in order to estimate the macroscopic response as characterized by a set of orthotropic material parameters. Damage mechanics and coupling effects are investigated and macroscopic material models are demonstrated and discussed. Prediction of the elastic, damage, and failure behavior of woven composites will aid in macroscopic constitutive characterization for modeling and optimizing advanced composite systems.

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Damage evaluation for fiber-reinforced polymer composites has been a topic of interest for more than 30 years, and for good reason. With damage modes significantly different than monolithic alloys, engineers have had to design composite structures to tolerate delamination, fiberbreakage, matrix cracking, and fiber-matrix debonding. Accomplishment of this goal has required understanding how and why these damage modes manifest themselves and grow to critical levels, even when the damage is barely visible from the surface. To this end, many nondestructive evaluation techniques have been developed, each with their advantages and disadvantages to characterize these damage forms. In this study, a series of non-destructive evaluation techniques are performed and evaluated on a set of damaged carbon fiber reinforced plastic (CFRP) specimens that have been subjected to varying levels of incident kinetic energy from low velocity impact (LVI). Specifically, 3-D x-ray computed tomography (CT), active thermography, electrical impedance tomography (EIT), and vibrothermography have been systematically utilized for evaluation of the specimens. The advantages and disadvantages are thoroughly explored and reported for each method in order to gain insight into the limitation of each of the damage detection methods and the damage morphology resulting from LVI.