Timothy Briggs

Detection of spatially distributed damage in fiber-reinforced polymer composites:

This work describes a novel method of embedded damage detection within glass fiber–reinforced polymer composites. Damage detection is achieved by monitoring the spatially distributed electrical conductivity of a strain-sensitive multiwalled carbon nanotube thin film. First, thin films were spray-deposited directly upon glass fiber mats. Second, using electrical impedance tomography, the spatial conductivity distribution of the thin film was determined before and after damage-inducing events. The resolution of the sensor was determined by drilling progressively larger holes in the center of the composite specimens, and the corresponding electrical impedance tomography response was measured by recording the current–voltage data at the periphery of the monitored composite sample. In addition, the sensitivity to damage occurring at different locations in the composite was also investigated by comparing electrical impedance tomography spatial conductivity maps obtained for specimens with sets of holes drilled at different locations in the sensing area. Finally, the location and severity of damage from low-velocity impact events were detected using the electrical impedance tomography method. The work presented in this study indicates a paradigm shift in the available possibilities for structural health monitoring of fiber-reinforced polymer composites.

Spatial Sensing Using Electrical Impedance Tomography

The need for structural health monitoring has become critical due to aging infrastructures, legacy airplanes, and continuous development of new structural technologies. Based on an updated structural design, there is a need for new structural health monitoring paradigms that can sense the presence, location, and severity with a single measurement. This paper focuses on the first step of this paradigm, consisting of applying a sprayed conductive carbon nanotube-polymer film upon glass fiber-reinforced polymer composite substrates. Electrical impedance tomography is performed to measure changes in conductivity within the conductive films because of damage. Simulated damage is a method for validation of this approach. Finally, electrical impedance tomography measurements are taken while the conductive films are subjected to tensile and compressive strain states. This demonstrates the ability of electrical impedance tomography for not only damage detection, but active structural monitoring as well. This paper acts as a first step toward moving the structural health monitoring paradigm toward large-scale deployable spatial sensing.

Quasi-Static Indentation Analysis of Carbon-Fiber Laminates.

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 by changing the thickness 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 progressions. 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 drops not clear from NDE. Second, the simulations demonstrate insufficiencies in the code and can then direct future efforts for development.

A micro to macro approach to polymer matrix composites damage modeling : final LDRD report.

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.

Determination of Stress Free Temperature in Composite Laminates for Residual Stress Modeling

As the complexity of composite laminates rises, the use of hybrid structures and multi-directional laminates, large operating temperature ranges, the process induced residual stresses become a significant factor in the design. In order to properly model the initial stress state of a structure, a solid understanding of the stress free temperature, the temperature at which the initial crosslinks are formed, as well as the contribution of cure shrinkage, must be measured. Many in industry have moved towards using complex cure kinetics models with the assistance of commercial software packages such as COMPRO. However, in this study a simplified residual stress model using the coefficient of thermal expansion (CTE) mismatch and change in temperature from the stress free temperature are used. The limits of this simplified model can only be adequately tested using an accurate measure of the stress free temperature. Only once that is determined can the validity of the simplified model be determined. Various methods were used in this study to test for the stress free temperature and their results are used to validate each method. Two approaches were taken, both involving either cobonded carbon fiber reinforced polymer (CFRP) or glass fiber reinforced polymer (GFRP) to aluminum. The first method used a composite-aluminum plate which was allowed to warp due to the residual stress. The other involved producing a geometrical stable hybrid composite-aluminum cylinder which was then cut open to allow it to spring in. Both methods placed the specimens within an environmental chamber and tracked the residual stress induced deformation as the temperature was ramped beyond the stress free temperature. Both methods revealed a similar stress free temperature that could then be used in future cure modeling simulations.

Effect of Process Induced Stresses on Measurement of FRP Strain Energy Release Rates

Fiber reinforced polymer composites are frequently used in hybrid structures where they are co-cured or co-bonded to dissimilar materials. For autoclave cured composites, this interface typically forms at an elevated temperature that can be quite different from the part’s service temperature. As a result, matrix shrinkage and CTE mismatch can produce significant residual stresses at this bi-material interface. This study shows that the measured critical strain energy release rate, Gc, can be quite sensitive to the residual stress state of this interface. If designers do not properly account for the effect of these process induced stresses, there is danger of a nonconservative design. Tests including double cantilever beam (DCB) and end notched flexure (ENF) were conducted on a co-cured GFRP-CFRP composite panel across a wide range of temperatures. These results are compared to tests performed on monolithic GFRP and CFRP panels.

Verification and Validation of Residual Stresses in Simple Composite Structures

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 and the shrinkage upon cure exhibited by 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 could result in interlaminar delamination or fracture within the composite. 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 time and cost prohibitive. 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 SIERRA/SolidMechanics code Adagio. Concurrent with the model development, two simple, bi-material structures composed of a carbon fiber/epoxy composite and aluminum, a flat plate and a cylinder, are fabricated and the residual stresses are quantified through the measurement of deformation. Then, in the process of validating the developed modeling approach with the experimental residual stress data, manufacturing process simulations of the two simple structures are developed and undergo a formal verification and validation process, including a mesh convergence study, sensitivity analysis, and uncertainty quantification. The simulations’ final results show adequate agreement with the experimental measurements, indicating the validity of a simple modeling approach, as well as a necessity for the inclusion of material parameter uncertainty in the final residual stress predictions.

Criteria for initiation of delamination in quasi-static punch-shear tests of a carbon-fiber composite material.

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.