Publications

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Physical aging of polymers is a thermodynamic phenomenon that occurs in the glassy regime. Upon cooling, the thermal contraction is restricted by a lack of adequate free volume within the polymer structure. This leaves the polymer in a state of thermodynamic non-equilibrium which relieves itself over long timescales. Time temperature superposition is typically used to accelerate this aging process to achieve validation of properties over the service life of the material. The shift factors determine the degree to which the material time is accelerated in an isothermal environment at elevated temperature. This is typically achieved with dynamic mechanical thermal analysis (DMTA). This method works well for neat polymers but fiber reinforced polymer composites (FRPC) have significantly higher stiffnesses and typical DMTA testing is limited to under 20 N of force. Due to the large unit cell for a fabric composite and geometrical limitations in the thickness of a ply, a higher force method would be more useful. In this study, an electrodynamic test frame was used to determine the shift factors for a glass fiber reinforced polymer (GFRP) composite which has a thermoset matrix. The goal is to determine whether the shift factors differ for different orientations of the composite. For an orthotropic material, directional dependent shift factors would increase material model complexity significantly.

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Delaminations are of great concern to any fiber reinforced polymer composite (FRPC) structure. In order to develop the most efficient structure, designers may incorporate hybrid composites to either mitigate the weaknesses in one material or take advantage #of the strengths of another. When these hybrid structures are used at service temperatures outside of the cure temperature, residual stresses can develop at the dissimilar interfaces. These residual stresses impact the initial stress state at the crack tip of any flaw in the structure and govern whether microcracks, or other defects, grow into large scale delaminations. Recent experiments have shown that for certain hybrid layups which are used to determine the strain energy release rate, G, there may be significant temperature dependence on the apparent toughness. While Nairn and Yokozeki believe that this effect may solely be attributed to the release of stored strain energy in the specimen as the crack grows, others point to a change in the inherent mode mixity of the test, like in the classic interface crack between two elastic layers solution given by Suo and Hutchinson. When a crack is formed at the interface of two dissimilar materials, while the external loading, in the case of a double cantilever beam (DCB), is pure mode I, the stress field at the crack tip produces a mixed-mode failure. Perhaps a change in apparent toughness with temperature can be the result of an increase in mode mixity. This study serves to investigate whether the residual stress formed at the bimaterial interface produces a noticeable shift in the strain energy release rate-mode mixing curve.

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.

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This work is to characterize the mechanical performances of the selected composites with four different overlap lengths of 0.25 in, 0.5 in, 0,75 in and 1.0 in. The composite materials in this study were one carbon composite (AS4C/UF3662) and one glass (E-glass/UF3662) composite. They both had the same resin of UF 3362, but with different fibers of carbon AS4C and E-glass. The mechanical loading in this study was limited to the quasi-static loading of 2 mm/min, which was equivalent to 5x10( -4 ) strain rate. Digital cameras were set up to record images during the mechanical testing. The full-field deformation data obtained from Digital Image Correlation (DIC) and the side view of the specimens were used to understand the different failure modes of the composites. The maximum load and the ultimate strength with consideration of the location of the failure for the different overlap lengths were compared and plotted together to understand the effect of the overlap lengths on the mechanical performance of the overlapped composites. 4 6

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Process-induced residual stresses occur in composite structures composed of dissimilar materials. As these residual stresses could result in fracture, their consideration when designing composite parts is necessary. However, the experimental determination of residual stresses in prototype parts can be time and cost prohibitive. Alternatively, it is possible for computational tools to predict potential residual stresses. Therefore, the objectives of the presented work are to demonstrate an efficient method for simulating residual stresses in composite parts, as well as the potential value of statistical methods during analyses for which material properties are unknown. Specifically, a simplified residual stress modeling approach is implemented within Sandia National Laboratories’ SIERRA/SolidMechanics code. Concurrent with the model development, bi-material composite structures are designed and manufactured to exhibit significant residual stresses. Then, the presented modeling approach is rigorously verified and validated through simulations of the bi-material composite structures’ manufacturing processes, 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.

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Experiments were performed to characterize the mechanical response of a 15 pcf flexible polyurethane foam to large deformation at different strain rates and temperatures. Results from these experiments indicated that at room temperature, flexible polyurethane foams exhibit significant nonlinear elastic deformation and nearly return to their original undeformed shape when unloaded. However, when these foams are cooled to temperatures below their glass transition temperature of approximately -35 o C, they behave like rigid polyurethane foams and exhibit significant permanent deformation when compressed. Thus, a new model which captures this dramatic change in behavior with temperature was developed and implemented into SIERRA with the name Flex_Foam to describe the mechanical response of both flexible and rigid foams to large deformation at a variety of temperatures and strain rates. This report includes a description of recent experiments. Next, development of the Flex Foam model for flexible polyurethane and other flexible foams is described. Selection of material parameters are discussed and finite element simulations with the new Flex Foam model are compared with experimental results to show behavior that can be captured with this new model.

The failure progression of a fiber-reinforced toughened-matrix composite (IM7/8552) was experimentally characterized at quasi-static (10−4 s−1) strain rate using crossply and quasi-isotropic laminate specimens. A progressive failure framework was proposed to benchmark the initiation and progression of damage within composite laminates based on the matrix-dominated failure modes. The Northwestern Failure Theory (NU Theory) was used to provide a set of physics-based failure criteria for predicting the matrix-dominated failure of embedded plies using the lamina-based transverse tension, transverse compression, and shear failure strengths. The NU Theory was used to predict the first-ply-failure (FPF) of embedded plies in [0/904]s and [02/452/−452/902]s laminates for the embedded 90° and 45° plies. The Northwestern Criteria were found to provide superior prediction of the matrix-dominated embedded ply failure for all evaluated cases compared to the classical approaches. The results indicate the potential to use the Northwestern Criteria to provide the predictive baseline for damage propagation in composite laminates based on experimentally identified damage response on a length scale-relevant basis.

The strain-rate-dependent matrix-dominated failure of multiple fiber-reinforced polymer matrix composite systems was evaluated over the range of quasi-static (10−4) to dynamic (103 s−1) strain rates using available experimental data from literature. The strain rate dependent parameter, m, was found to relate strain-rate dependent lamina behavior linearly to the logarithm of strain rate. The parameter was characterized for a class of laminates comprised of epoxy-based matrices and either carbon or glass fibers, and determined to be approximately 0.055 regardless of fiber type. The strain-rate-dependent Northwestern Failure Criteria were found to fit all data in superior agreement to classical approaches across all strain rates evaluated based on solely lamina-level properties. It was determined that using the determined m value with the Northwestern Failure Criteria provided an accurate prediction of material behavior regardless of fiber type for the identified material class, which significantly reduces the material characterization testing required for the typical building block approach used by industry for computational analysis validation.

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.

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Flexible open celled foams are commonly used for energy absorption in packaging. Over time polymers can suffer from aging by becoming stiffer and more brittle. This change in stiffness can affect the foam’s performance in a low velocity impact event. In this study, the compressive properties of new open-cell flexible polyurethane foam were compared to those obtained from aged open-cell polyurethane foam that had been in service for approximately 25 years. The foams tested had densities of 10 and 15 pcf. These low density foams provided a significant challenge to machine cylindrical compression specimens that were 1 “in height and 1” in diameter. Details of the machining process are discussed. The compressive properties obtained for both aged and new foams included testing at various strain rates (0.05. 0.10, 5 s-1) and temperatures (-54, RT, 74 °C). Results show that aging of flexible polyurethane foam does not have much of an effect on its compressive properties.

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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 cure shrinkage exhibited by polymer matrix materials. These residual stresses can have a profound effect on the measured performance of a loaded composite structure. A material property of particular interest when modeling the formation of damage in composite materials is the mode I fracture toughness. Currently, the standard method of measuring the mode I fracture toughness involves a double cantilever beam (DCB) experiment, where a pre-crack is introduced into a laminate and subsequently opened under tension. The resulting apparent fracture toughness from the DCB experiment may depend upon a coupled interaction between a material property, the mode I energy release rate, and the effect of residual stresses. Therefore, in this study, a series of DCB experiments are completed in conjunction with the solution of representative finite element models to quantify and understand the effect of process-induced residual stresses and temperature variations on the apparent fracture toughness. Specifically, double cantilever beam experiments are completed at three temperatures to characterize three types of specimens composed of carbon fiber/epoxy and glass fiber/epoxy materials: carbon bonded to carbon, glass bonded to glass, and carbon bonded to glass. The carbon-to-carbon and glass-to-glass specimens provide estimates of the composite's fracture toughness in the absence of significant residual stresses and the carbon-to-glass specimens indicate the effect of measurable process induced stresses. Upon completion of testing, the measured results and observations are used to develop high-fidelity finite element models simulating the residual stresses formed throughout the manufacturing process and the subsequent DCB testing of a laminate composed of the carbon/epoxy and glass/epoxy materials. The stress fields and delamination behavior predicted through simulation assist in understanding the trends observed during the DCB experiments and demonstrate the important relationship between experimental and computational efforts.

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Mode I fracture toughness is a key material property that is used in modeling the damage tolerance of a composite part. Current standard measurement practice involves using a double cantilever beam (DCB) test where a precrack is introduced into a laminate and the crack is opened in tension. Load, crack opening displacement, and crack length are measured as the crack extends down the length of the coupon. Despite careful effort, the crack length can be difficult to determine accurately and the resulting calculated fracture toughness values (G1c) can have significant scatter. In this study, standard fixed width DCB tests are compared to width-tapered DCB tests and in both cases the fracture toughness is calculated with the compliance method. The advantage of using a width-tapered DCB coupon is that stable crack growth occurs at a constant load so measurement of the crack length or crack tip opening displacement is unnecessary. In this study the equivalence of both the fixed width and width-tapered DCB tests is shown. Therefore, in situations where crack length or crack tip opening measurements can be difficult to obtain accurately (high rate, elevated temperature) the width-tapered DCB can be quite useful.