Low-velocity impact of 2D woven glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) composite laminates was studied experimentally and numerically. Hybrid laminates containing blocked layers of GFRP/CFRP/GFRP with all plies oriented at 0° were investigated. Relatively high impact energies were used to obtain full perforation of the laminate in a low-velocity impact setup. Numerical simulations were carried out using the in-house transient dynamics finite element code, Sierra/SM, developed at Sandia National Laboratories. A three-dimensional continuum damage model was used to describe the response of a woven composite ply. Two methods for handling delamination were considered and compared: (1) cohesive zone modeling and (2) continuum damage mechanics. The reduced model size achieved by omission of the cohesive zone elements produced acceptable results at reduced computational cost. The comparison between different modeling techniques can be used to inform modeling decisions relevant to low velocity impact scenarios. The modeling was validated by comparing with the experimental results and showed good agreement in terms of predicted damage mechanisms and impactor velocity and force histories.
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.
The concept of progressive failure modeling is an ongoing concern within the composite community. A common approach is to employ a building block approach where constitutive material properties lead to lamina level predictions which then lead to laminate predictions and then up to structural predictions. There are advantages to such an approach, developments can be made within each step and the whole workflow can be updated. However, advancements made at higher length scales can be hampered by insufficient modeling at lower length scales. This can make industry wide evaluations of methodologies more complicated. For instance, significant advances have been made in recent years to strain rate independent failure theories on the lamina level. However, since the Northwestern Theory is stress dependent, for adequate use in a progressive damage model, a similarly robust constitutive model must also be employed to calculate these lamina level stresses. An improper constitutive model could easily cause a valid failure model to produce incorrect results. Also, any global strain rate applied to a multi-directional laminate will produce a spectrum of local lamina level strain rates so it is important for the constitutive law to account for strain rate dependent deformation.
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.
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.
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.
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.
Multi-material composite structures develop residual stresses during the curing process due to dissimilar material properties, which eventually may lead to failure in the form of fracture, delamination, or disbonding. Experimentally determining residual stresses can be both time and cost prohibitive, whereas accurate simulated predictions of residual stresses can be cheaper and provide equivalent information during the design process. Residual stresses can be simulated through several different approaches of varying complexity. The method employed in this study assumes the majority of residual stresses are developed due the mismatch of coefficients of thermal expansion and polymer shrinkage, which is indirectly accounted for by calibrating the simulation with an experimentally determined stress free temperature. This method has shown success in predicting the residual stress states across different material combinations and structures in previous studies. Simply using single, or nominal, inputs to the simulation may provide a reasonable prediction, but will be unable to provide any confidence when failure could occur. Therefore, one must consider the natural stochastic behavior of the materials and geometry through an uncertainty quantification study. However, a limitation in performing uncertainty quantification studies for more complex models exists due to the large number of material and geometry parameters that need to be considered. The results from a previously conducted survey of sensitivity analysis methods were leveraged to reduce the number of parameters considered during an uncertainty quantification study, as well as decrease the computational cost in determining the sensitive parameters. This allowed the application of uncertainty quantification methods to validate more complex multi-material structures against experimental results. The structure that will be considered is a multi-material split ring comprised of three layers: Aluminum, glass fiber composite, and carbon fiber composite.
Hybrid composites allow designers to develop efficient structures, which strategically exploit a material's strengths while mitigating possible weaknesses. However, elevated temperature curing processes and exposure to thermally-extreme service environments lead to the development of residual stresses. These stresses form at the hybrid composite's bi-material interfaces, significantly impacting the stress state at the crack tip of any pre-existing flaw within the structure and affecting the probability that small defects will grow into large-scale delaminations. Therefore, in this study, a carbon fiber reinforced composite (CFRP) is co-cured with a glass fiber reinforced composite (GFRP), and the mixed-mode fracture toughness is measured across a wide temperature range (-54°C to +71°C). Upon completion of the testing, the measured results and observations are used to develop high-fidelity finite element models simulating both the formation of residual stresses throughout the composite manufacturing process, as well as the mixed-mode testing of the hybrid composite. The stress fields predicted through simulation assist in understanding the trends observed during the completed experiments. Furthermore, the modeled predictions indicate that failure to account for residual stress effects during the analysis of composite structures could lead to non-conservative structural designs and premature failure.
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, a process modeling methodology was developed and implemented into Sandia National Laboratories' SIERRA/Solid Mechanics code. This method requires the specification of many model parameters to form accurate predictions. These parameters, which are related to the mechanical and thermal behaviors of the modeled composite material, can be determined experimentally, but at a potentially prohibitive cost. Furthermore, depending upon a composite part's specific geometric and manufacturing process details, it is possible that certain model parameters may have an insignificant effect on the simulated prediction. Therefore, to streamline the material characterization process, formal parameter sensitivity studies can be applied to determine which of the required input parameters are truly relevant to the simulated prediction. Then, only those model parameters found to be critical will require rigorous experimental characterization. Numerous sensitivity analysis methods exist in the literature, each offering specific strengths and weaknesses. Therefore, the objective of this study is to compare the performance of several accepted sensitivity analysis methods during the simulation of a bi-material composite strip's manufacturing process. The examined sensitivity analysis methods include both simple techniques, such Monte Carlo and Latin Hypercube sampling, as well as more sophisticated approaches, such as the determination of Sobol indices via a polynomial chaos expansion or a Gaussian process. The relative computational cost and critical parameter list are assessed for each of the examined methods and conclusions are drawn regarding the ideal sensitivity analysis approach for future residual stress investigations.
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.
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.
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.
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.
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 structures metallic materials. It is important to consider potential residual stresses 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. A simplified method for simulating residual stresses was previously validated with two simple bi-material structures. Continuing on, the objective of this study is to further validate the simplified method for simulating residual stresses for bi-material split rings of different composites and layup variations. The validation process uses uncertainty quantification to develop a distribution of possible simulated residual stress states that are compared to experimentally measured residual stress states of fabricated structures similar to those simulated. 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 and a temperature independent material model is adequate within the composites glassy region.
Residual stresses can form within composite structures composed of asymmetric laminates during the elevated temperature curing processes common to composite materials. These residual stresses are primarily the result of unbalanced thermal strains that develop throughout the structure due to the composite's orthotropic coefficients of thermal expansion. Furthermore, structures composed of textile, or woven, composite fabrics lend themselves to the formation of these residual stresses, as extreme care must be taken during the lay-up of such parts to ensure that adjacent plies are placed front-to-front or back-to-back, as opposed to front-to-back, to eliminate the potential for any unbalanced thermal strains. Depending upon the specific geometric details of the composite structure of interest, it is possible that these residual stresses could result in fracture within the composite. Therefore, the consideration of potential residual stresses formed throughout the manufacturing process is important. 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 objectives of this study are two-fold. First, a simplistic method for simulating the residual stresses formed in polymer matrix composite structures is developed within the Sandia National Laboratories' SIERRA/SolidMechanics code Adagio. Subsequently, the required level of model fidelity necessary to provide realistic predictions of a textile composite's residual stress state is determined. Concurrent with the computational activities, asymmetric plates of a woven carbon fiber/epoxy composite are manufactured with varying thicknesses and the residual stresses exhibited by the plates are quantified through the measurement of deformation. The developed computational approach is used to simulate the manufacturing process of these asymmetric plates and final comparisons of the predicted and experimental results show reasonable agreement.
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. A simplified method for simulating residual stresses was previously validated with two simple bi-material structures composed of aluminum and a carbon fiber/epoxy resin composite. Therefore, the objective of this study is to further validate the simplified method for simulating residual stresses for different composites and more complex structures. The simplified method accounts for both the coefficient of thermal expansion mismatch and polymer shrinkage through the calibration to an experimentally-determined stress-free temperature. This was implemented in Sandia National Laboratories' solid mechanics code, SIERRA, to model split rings with temperature independent and dependent material models. The split rings are comprised of two materials: Aluminum with either a carbon fiber/epoxy resin composite or a glass fiber/epoxy resin composite. Concurrent with the computational efforts, structures similar to those modeled are fabricated and the residual stresses are quantified through the measurement of deformations. The simulations' results are compared to the experimentally observed behaviors for model validation. 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 and a temperature independent material model is adequate within the composite's glassy region. Copyright 2017. Used by CAMX - The Composites and Advanced Materials Expo.
Residual stresses can form within composite structures composed of asymmetric laminates during the elevated temperature curing processes common to composite materials. These residual stresses are primarily the result of unbalanced thermal strains that develop throughout the structure due to the composite's orthotropic coefficients of thermal expansion. Furthermore, structures composed of textile, or woven, composite fabrics lend themselves to the formation of these residual stresses, as extreme care must be taken during the lay-up of such parts to ensure that adjacent plies are placed front-to-front or back-to-back, as opposed to front-to-back, to eliminate the potential for any unbalanced thermal strains. Depending upon the specific geometric details of the composite structure of interest, it is possible that these residual stresses could result in fracture within the composite. Therefore, the consideration of potential residual stresses formed throughout the manufacturing process is important. 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 objectives of this study are two-fold. First, a simplistic method for simulating the residual stresses formed in polymer matrix composite structures is developed within the Sandia National Laboratories' SIERRA/SolidMechanics code Adagio. Subsequently, the required level of model fidelity necessary to provide realistic predictions of a textile composite's residual stress state is determined. Concurrent with the computational activities, asymmetric plates of a woven carbon fiber/epoxy composite are manufactured with varying thicknesses and the residual stresses exhibited by the plates are quantified through the measurement of deformation. The developed computational approach is used to simulate the manufacturing process of these asymmetric plates and final comparisons of the predicted and experimental results show reasonable agreement.
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. A simplified method for simulating residual stresses was previously validated with two simple bi-material structures composed of aluminum and a carbon fiber/epoxy resin composite. Therefore, the objective of this study is to further validate the simplified method for simulating residual stresses for different composites and more complex structures. The simplified method accounts for both the coefficient of thermal expansion mismatch and polymer shrinkage through the calibration to an experimentally-determined stress-free temperature. This was implemented in Sandia National Laboratories' solid mechanics code, SIERRA, to model split rings with temperature independent and dependent material models. The split rings are comprised of two materials: Aluminum with either a carbon fiber/epoxy resin composite or a glass fiber/epoxy resin composite. Concurrent with the computational efforts, structures similar to those modeled are fabricated and the residual stresses are quantified through the measurement of deformations. The simulations' results are compared to the experimentally observed behaviors for model validation. 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 and a temperature independent material model is adequate within the composite's glassy region. Copyright 2017. Used by CAMX - The Composites and Advanced Materials Expo.
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.
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.
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.
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.
Extension springs are used to apply a constant force at a set displacement in a wide variety of components. When subjected to an abnormal thermal event, such as in a fire, the load carrying capacity of these springs can degrade. In this study, relaxation tests were conducted on extension springs where the heating rate and dwell temperature were varied to investigate the reduction in force provided by the springs. Two commonly used spring material types were tested, 304 stainless steel and Elgiloy, a cobalt-chrome-nickel alloy. Challenges associated with obtaining accurate spring response to an abnormal thermal event are discussed. The resulting data can be used to help develop and test models for thermally activated creep in springs and to provide designers with recommendations to help ensure the reliability of the springs for the duration of the thermal event.