Pappu Murthy - Academia.edu (original) (raw)

Papers by Pappu Murthy

Research paper thumbnail of Second generation integrated composite analyzer (ICAN) computer code

This manual updates the original 1986 NASA TP-2515. "Integrated Composite Analyzer (lCAN) Users a... more This manual updates the original 1986 NASA TP-2515. "Integrated Composite Analyzer (lCAN) Users and Programmers Manual." The various enhancements and newly added features are described to enable the user to prepare the appropriate input data to run this updated version of the ICAN code. For reference. the micromechanics equations are provided in an appendix and should be compared to those in the original manual for modifications. A complete output for a sample case is also provided in a separate appendix. The input to the code includes constituent material properties. factors reflecting the fabrication process. and laminate configuration. The code performs micromechanics. macromechanics. and laminate analyses. including the hygrothermal response of polymer-matrix-based fiber composites. The output includes the various ply and composite properties, the composite structural response. and the composite stress analysis results with details on failure. The code is written in FORTRAN 77 and can be used efficiently as a selfcontained package (or as a module) in complex structural analysis programs. The input-output format has changed considerably from the original version of ICAN and is described extensively through the use of a sample problem.

Research paper thumbnail of Designing of a Fleet-Leader Program for Carbon Composite Overwrapped Pressure Vessels

50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2009

Composite Overwrapped Pressure Vessels (COPVs) are often used for storing pressurant gases on boa... more Composite Overwrapped Pressure Vessels (COPVs) are often used for storing pressurant gases on board spacecraft when mass saving is a prime requirement. Substantial weight savings can be achieved compared to all metallic pressure vessels. For example, on the space shuttle, replacement of all metallic pressure vessels with Kevlar COPVs resulted in a weight savings of about 30 percent. Mass critical space applications such as the Ares and Orion vehicles are currently being planned to use as many COPVs as possible in place of all-metallic pressure vessels to minimize the overall mass of the vehicle. Due to the fact that overwraps are subjected to sustained loads during long periods of a mission, stress rupture failure is a major concern. It is, therefore, important to ascertain the reliability of these vessels by analysis, since it is practically impossible to show by experimental testing the reliability of flight quality vessels. Also, it is a common practice to set aside flight quality vessels as "fleet leaders" in a test program where these vessels are subjected to slightly accelerated operating conditions so that they lead the actual flight vessels both in time and load. The intention of fleet leaders is to provide advanced warning if there is a serious design flaw in the vessels so that a major disaster in the flight vessels can be averted with advance warning. On the other hand, the accelerating conditions must be not so severe as to be prone to false alarms. The primary focus of the present paper is to provide an analytical basis for designing a viable fleet leader program for carbon COPVs. The analysis is based on a stress rupture behavior model incorporating Weibull statistics and power-law sensitivity of life to fiber stress level.

Research paper thumbnail of Probabilistic simulation of long term behavior in polymer matrix composites

35th Structures, Structural Dynamics, and Materials Conference, 1994

A methodology to compute cumulative probability distribution functions (CDF) of fatigue life for ... more A methodology to compute cumulative probability distribution functions (CDF) of fatigue life for different ratios, r of applied stress to the laminate strength based on first ply failure criteria has been developed and demonstrated. Degradation effects due to long term environmental exposure and mechanical cyclic loads are considered in the simulation process. A unified timestress dependent multi-factor interaction equation model developed at NASA Lewis Research Center has been used to account for the degradation/aging of material properties due to cyclic on ply thickness and matrix strength in order to achieve high reliability of the structure.

Research paper thumbnail of Effect of cyclic thermo-mechanical load on fatigue reliability in polymer matrix composites

36th Structures, Structural Dynamics and Materials Conference, 1995

A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has be... more A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has been developed and demonstrated. Matrix degradation effects caused by long term environmental exposure and mechanicallthermal cyclic loads are accounted for in the simulation process. A unified time-temperature-stress dependent multifactor interaction relationship developed at NASA Lewis Research Center has been used to model the degradation/aging of material properties due to cyclic loads. The fast probability integration method is used to compute probabilistic distribution of response. Sensitivities of fatigue life reliability to uncertainties in the primitive random variables (e.g., constituent properties, fiber volume ratio, void volume ratio, ply thickness, etc.) computed and their significance in the reliability-based design for maximum life is discussed. The effect of variation in the thermal cyclic loads on the fatigue reliability for a (O/rt45/90), graphite/epoxy laminate with a ply thickness of 0.127 mm, with respect to impending failure modes has been studied. The results show that, at low mechanical cyclic loads and low thermal cyclic amplitudes, fatigue life for 0.999 reliability is most sensitive to matrix compressive strength, matrix modulus, thermal expansion coefficient, and ply thickness. Whereas at high mechanical cyclic loads and high thermal cyclic amplitudes, fatigue life at 0.999 reliability is more sensitive to the shear strength of matrix, longitudinal fiber modulus, matrix modulus, and ply thickness.

Research paper thumbnail of Discontinuously Stiffened Composite Panel under Compressive Loading

Journal of Reinforced Plastics and Composites, 1995

The design of composite structures requires an evaluation of their safety and durability under se... more The design of composite structures requires an evaluation of their safety and durability under service loads and possible overload conditions. This paper presents a computational tool that has been developed to examine the response of stiffened composite panels via the simulation of damage initiation, growth, accumulation, progression, and propagation to structural fracture or collapse. The structural durability of a composite panel with a discontinuous stiffener is investigated under compressive loading induced by the gradual displacement of an end support. Results indicate damage initiation and progression to have significant effects on structural behavior under loading. Utilization of an integrated computer code for structural durability assessment is demonstrated.

Research paper thumbnail of Progression of damage and fracture in composites under dynamic loading

31st Structures, Structural Dynamics and Materials Conference, 1990

A new computational simulation method is presented to evaluate the dynamic aspects of composite s... more A new computational simulation method is presented to evaluate the dynamic aspects of composite structural response and durability that have not been simulated previously. Composite structural behavior under any loading condition, geometry, composite system, laminate configuration, and boundary conditions can now be simulated. Structural degradation, delamination, fracture, and damage propagation are included in the simulation. An angle-plied composite plate structure under normal impact loading is used as an example to demonstrate the versatility of the simulation method.

Research paper thumbnail of Micromechanics based fatigue life prediction of a polycrystalline metal applying crystal plasticity

Mechanics of Materials, 2015

Research paper thumbnail of Simplified Design Procedures for Fiber Composite Structural Components/Joints

Composite Structures, 1992

Slmplified step-by-step design procedures are summarized, which are suitable for the preliminary ... more Slmplified step-by-step design procedures are summarized, which are suitable for the preliminary design of composite structural components such as panels (laminates) and composite built-up structures (box beams). Similar procedures are also summarized for the pre]Imlnary design of composite bolted and adhesively bonded Joints. The summary is presented in terms of sample design cases complemented with typical results. Guidelines are provided which can be used in the design selection process of composite structural components/joints. Also, procedures to account for cyclic loads, hygrothermal effects and lamination residual stresses are included.

Research paper thumbnail of Reliability and Creep/Fatigue Analysis of a CMC Component

Volume 1: Turbo Expo 2007, 2007

Available electronically at http://gltrs.grc.nasa.gov Level of Review: This material has been tec... more Available electronically at http://gltrs.grc.nasa.gov Level of Review: This material has been technically reviewed by technical management.

Research paper thumbnail of Progressive fracture in composites subjected to hygrothermal environment

32nd Structures, Structural Dynamics, and Materials Conference, 1991

The influence of hygrothermal environmental conditions on the load carrying ability and response ... more The influence of hygrothermal environmental conditions on the load carrying ability and response of composite structures are investigated via computational simulation. An integrated computer code is utilized for the simulation of composite structural degradation under loading. Damage initiation, damage growth, fracture progression, and global structural fracture are included in the simulation. Results demonstrate the significance of hygro-thermal effects on composite structural response, toughness, and durability. flawless structural performance even when exposed to adverse loading and environmental "Associate Professor, Department of Civil and Environmental Engineering. tAerospace Engineer, Structures Division. tSenior Aerospace Scientist, Structures Division. research at NASA-Lewis Research Center on polymer matrix multi-layer angle-plied composites. ICAN is capable of determining the ability of the laminated composite to endure 17. SECURITY CLASSIFICATION

Research paper thumbnail of Probabilistic modeling of high-temperature material properties of a 5-harness 0/90 Sylramic fiber/CVI-SiC/MI-SiC woven composite

39th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference and Exhibit, 1998

An integrated probabilistic approach has been developed to assess composites for high temperature... more An integrated probabilistic approach has been developed to assess composites for high temperature applications. This approach was used to determine thermal and mechanical properties and their probabilistic distributions of a 5-harness 0/90 Sylramic fiber/CVI-SiC/MI-SiC woven Ceramic Matrix Composite (CMC) at high temperatures. The purpose of developing this approach was to generate quantitative probabilistic information on this CMC to help complete the evaluation for its potential application for HSCT combustor liner. This approach quantified the influences of uncertainties inherent in constituent properties called primitive variables on selected key response variables of the CMC at 2200°F. The quantitative information is presented in the form of Cumulative Density Functions (CDFs), Probability Density Functions (PDFs) and primitive variable sensitivities on response. Results indicate that the scatters in response variables were reduced by 30 to 50 percent when the uncertainties in the primitive variables, which showed the most influence, were reduced by 50 percent.

Research paper thumbnail of Stress Rupture Life Reliability Measures for Composite Overwrapped Pressure Vessels

48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007

Available electronically at http://gltrs.grc.nasa.gov Trade names and trademarks are used in this... more Available electronically at http://gltrs.grc.nasa.gov Trade names and trademarks are used in this report for identification only. Their usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Level of Review: This material has been technically reviewed by technical management.

Research paper thumbnail of Simulation of Fatigue Behavior of High Temperature Metal Matrix Composites

Life Prediction Methodology for Titanium Matrix Composites

A generalized relatively new approach is described for the computational simulation of fatigue be... more A generalized relatively new approach is described for the computational simulation of fatigue behavior of high temperature metal matrix composites (HT-MMCs). This theory is embedded in a specialty-purpose computer code. The effectiveness of the computer code to predict the fatigue behavior of HT-MMCs is demonstrated by applying it to a silicon-fiber/titanium-matrix HT-MMC. Comparative results are shown for mechanical fatigue, thermal fatigue, thermomechanical (in-phase and out-of-phase) fatigue, as well as the effects of oxidizing environments on fatigue life. These results show that the hey, approach reproduces available experimental data remarkably well.

Research paper thumbnail of Computational Simulation of Continuous Fiber-Reinforced Ceramic Matrix Composites Behavior

This report describes a methodology which predicts the behavior of ceramic matrix composites and ... more This report describes a methodology which predicts the behavior of ceramic matrix composites and has been incorporated in the computational tool CEMCAN (CEramic Matrix Composite ANalyzer). The approach combines micromechanics with a unique fiber substructuring concept. In this new concept, the conventional unit cell (the smallest representative volume element of the composite) of the micromechanics approach is modified by substructuring it into several slices and developing the micromechanics-based equations at the slice level. The methodology also takes into account nonlinear ceramic matrix composite (CMC) behavior due to temperature and the fracture initiation and progression. Important features of the approach and its effectiveness are described by using selected examples. Comparisons of predictions and limited experimental data are also provided.

Research paper thumbnail of Design for cyclic loading endurance of composites

The application of the computer code IPACS (Integrated Probabilistic Assessment of Composite Stru... more The application of the computer code IPACS (Integrated Probabilistic Assessment of Composite Structures) to aircraft wing type structures is described. The code performs a complete probabilistic analysis for composites taking into account the uncertainties in geometry, boundary conditions, material properties, laminate lay-ups, and loads. Results of the analysis are presented in terms of cumulative distribution functions (CDF) and probability density function (PDF) of the fatigue life of a wing type composite structure under different hygrothermal environments subjected to the random pressure. The sensitivity of the fatigue life to a number of critical structural/material variables is also computed from the analysis.

Research paper thumbnail of Microstructural Influence on Deformation and Fatigue Life of Composites Using the Generalized Method of Cells

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2015

A fully coupled deformation and damage approach to modeling the response of composite materials a... more A fully coupled deformation and damage approach to modeling the response of composite materials and composite laminates is presented. It is based on the semi-analytical generalized method of cells (GMC) micromechanics model as well as its higher fidelity counterpart, HFGMC, both of which provide closed-form constitutive equations for composite materials as well as the micro scale stress and strain fields in the composite phases. The provided constitutive equations allow GMC and HFGMC to function within a higher scale structural analysis (e.g., finite element analysis or lamination theory) to represent a composite material point, while the availability of the micro fields allow the incorporation of lower scale sub-models to represent local phenomena in the fiber and matrix. Further, GMC's formulation performs averaging when applying certain governing equations such that some degree of microscale field accuracy is surrendered in favor of extreme computational efficiency, rendering the method quite attractive as the centerpiece in a integrated computational material engineering (ICME) structural analysis; whereas HFGMC retains this microscale field accuracy, but at the price of significantly slower computational speed. Herein, the sensitivity of deformation and the fatigue life of graphite/epoxy PMC composites, with both ordered and disordered microstructures, has been investigated using this coupled deformation and damage micromechanics based approach. The local effects of fiber breakage and fatigue damage are included as sub-models that operate on the microscale for the individual composite phases. For analysis of laminates, classical lamination theory is employed as the global or structural scale model, while GMC/HFGMC is embedded to operate on the microscale to simulate the behavior of the composite material within each laminate layer. A key outcome of this study is the statistical influence of microstructure and micromechanics idealization (GMC or HFGMC) on the overall accuracy of unidirectional and laminated composite deformation and fatigue response.

Research paper thumbnail of Structural durability of stiffened composite shells

33rd Structures, Structural Dynamics and Materials Conference, 1992

The durability of a stiffened composite cylindrical shell panel is investigated under several loa... more The durability of a stiffened composite cylindrical shell panel is investigated under several loading conditions. An integrated computer code is utilized for the simulation of load induced structural degradation. Damage initiation, growth, and accumulation up to the stage of propagation to fracture are included in the computational simulation. Results indicate significant differences in the degradation paths for different loading cases. Effects of combined loading on structural durability and ultimate structural strength of a stiffened shell are assessed. Nomenclature O'gll-ply longitudinal stress 0"g22-ply transverse stress at33-ply normal stress at12-ply in-plane shear stress ae2a-ply out-of-plane shear stress *Associate Professor, Department of Civil and Environmental Engineering ?Graduate Student, Department of Civil and Environmental Engineering tAerospace Engineer, Structures Division. §Senior Aerospace Scientist, Structures Division. aa3-ply out-of-plane shear stress atilTply longitudinal tensile stress atilt-ply longitudinal compressive stress Ort22r-ply transverse tensile stress MDE-modified distortion energy failure criterion R-radius of cylindrical shell RR-delamination due to relative rotation by NASA-Lewis Research Center under grant NAG-3-1101.

Research paper thumbnail of Best Practices for Reliable and Robust Spacecraft Structures

48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007

A study was undertaken to capture the best practices for the development of reliable and robust s... more A study was undertaken to capture the best practices for the development of reliable and robust spacecraft structures for NASA's next generation cargo and crewed launch vehicles. In this study, the NASA heritage programs such as Mercury, Gemini, Apollo, and the Space Shuttle program were examined. A series of lessons learned during the NASA and DoD heritage programs are captured. The processes that "make the right structural system" are examined along with the processes to "make the structural system right". The impact of technology advancements in materials and analysis and testing methods on reliability and robustness of spacecraft structures is studied. The best practices and lessons learned are extracted from these studies. Since the first human space flight, the best practices for reliable and robust spacecraft structures appear to be well established, understood, and articulated by each generation of designers and engineers. However, these best practices apparently have not always been followed. When the best practices are ignored or short cuts are taken, risks accumulate, and reliability suffers. Thus program managers need to be vigilant of circumstances and situations that tend to violate best practices. Adherence to the best practices may help develop spacecraft systems with high reliability and robustness against certain anomalies and unforeseen events.

Research paper thumbnail of Probabilistic micromechanics and macromechanics for ceramic matrix composites

38th Structures, Structural Dynamics, and Materials Conference, 1997

The properties of ceramic matrix composites (CMC's) are known to display a considerable amount of... more The properties of ceramic matrix composites (CMC's) are known to display a considerable amount of scatter due to variations in fiber/matrix properties, interphase properties, interphase bonding, amount of matrix voids, and many geometry-or fabrication-related parameters, such as ply thickness and ply orientation. This paper summarizes preliminary studies in which formal probabilistic descriptions of the material-behavior-and fabrication-related parameters were incorporated into micromechanics and macromechanics for CMC's. In this process two existing methodologies, namely CMC micromechanics and macromechanics analysis and a fast probability integration (FPI) technique are synergistically coupled to obtain the probabilistic composite behavior or response. Preliminary results in the form of cumulative probability distributions and information on the probability sensitivities of the response to primitive variables for a unidirectional silicon carbide/reaction-bonded silicon nitride (SiC/RBSN) CMC are presented. The cumulative distribution functions are computed for composite moduli, thermal expansion coefficients, thermal conductivities, and longitudinal tensile strength at room temperature. The variations in the constituent properties that directly affect these composite properties are accounted for via assumed probabilistic distributions. Collectively, the results show that the present technique provides valuable information about the composite properties and sensitivity factors, which is useful to design or test engineers. Furthermore, the present methodology is computationally more efficient than a standard Monte-Carlo simulation technique; and the agreement between the two solutions is excellent, as shown via select examples.

Research paper thumbnail of Optimal fabrication processes for unidirectional metal-matrix composites: A computational simulation

A method is proposed for optimizing the fabrication process of unidirectional metal matrix compos... more A method is proposed for optimizing the fabrication process of unidirectional metal matrix composites. The temperature and pressure histories are optimized such that the residual microstresses of the composite at the end of the fabrication process are minimized and the material integrity throughout the process is ensured. The response of the composite during the fabrication is simulated based on a nonlinear micromechanics theory. The optimal fabrication problem is formulated and solved with non-linear programming. Application cases regarding the optimization of the fabrication cool-down phases of unidirectional ultra-high modulus graphite/copper and silicon carbide/titanium composites are presented.

Research paper thumbnail of Second generation integrated composite analyzer (ICAN) computer code

This manual updates the original 1986 NASA TP-2515. "Integrated Composite Analyzer (lCAN) Users a... more This manual updates the original 1986 NASA TP-2515. "Integrated Composite Analyzer (lCAN) Users and Programmers Manual." The various enhancements and newly added features are described to enable the user to prepare the appropriate input data to run this updated version of the ICAN code. For reference. the micromechanics equations are provided in an appendix and should be compared to those in the original manual for modifications. A complete output for a sample case is also provided in a separate appendix. The input to the code includes constituent material properties. factors reflecting the fabrication process. and laminate configuration. The code performs micromechanics. macromechanics. and laminate analyses. including the hygrothermal response of polymer-matrix-based fiber composites. The output includes the various ply and composite properties, the composite structural response. and the composite stress analysis results with details on failure. The code is written in FORTRAN 77 and can be used efficiently as a selfcontained package (or as a module) in complex structural analysis programs. The input-output format has changed considerably from the original version of ICAN and is described extensively through the use of a sample problem.

Research paper thumbnail of Designing of a Fleet-Leader Program for Carbon Composite Overwrapped Pressure Vessels

50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2009

Composite Overwrapped Pressure Vessels (COPVs) are often used for storing pressurant gases on boa... more Composite Overwrapped Pressure Vessels (COPVs) are often used for storing pressurant gases on board spacecraft when mass saving is a prime requirement. Substantial weight savings can be achieved compared to all metallic pressure vessels. For example, on the space shuttle, replacement of all metallic pressure vessels with Kevlar COPVs resulted in a weight savings of about 30 percent. Mass critical space applications such as the Ares and Orion vehicles are currently being planned to use as many COPVs as possible in place of all-metallic pressure vessels to minimize the overall mass of the vehicle. Due to the fact that overwraps are subjected to sustained loads during long periods of a mission, stress rupture failure is a major concern. It is, therefore, important to ascertain the reliability of these vessels by analysis, since it is practically impossible to show by experimental testing the reliability of flight quality vessels. Also, it is a common practice to set aside flight quality vessels as "fleet leaders" in a test program where these vessels are subjected to slightly accelerated operating conditions so that they lead the actual flight vessels both in time and load. The intention of fleet leaders is to provide advanced warning if there is a serious design flaw in the vessels so that a major disaster in the flight vessels can be averted with advance warning. On the other hand, the accelerating conditions must be not so severe as to be prone to false alarms. The primary focus of the present paper is to provide an analytical basis for designing a viable fleet leader program for carbon COPVs. The analysis is based on a stress rupture behavior model incorporating Weibull statistics and power-law sensitivity of life to fiber stress level.

Research paper thumbnail of Probabilistic simulation of long term behavior in polymer matrix composites

35th Structures, Structural Dynamics, and Materials Conference, 1994

A methodology to compute cumulative probability distribution functions (CDF) of fatigue life for ... more A methodology to compute cumulative probability distribution functions (CDF) of fatigue life for different ratios, r of applied stress to the laminate strength based on first ply failure criteria has been developed and demonstrated. Degradation effects due to long term environmental exposure and mechanical cyclic loads are considered in the simulation process. A unified timestress dependent multi-factor interaction equation model developed at NASA Lewis Research Center has been used to account for the degradation/aging of material properties due to cyclic on ply thickness and matrix strength in order to achieve high reliability of the structure.

Research paper thumbnail of Effect of cyclic thermo-mechanical load on fatigue reliability in polymer matrix composites

36th Structures, Structural Dynamics and Materials Conference, 1995

A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has be... more A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has been developed and demonstrated. Matrix degradation effects caused by long term environmental exposure and mechanicallthermal cyclic loads are accounted for in the simulation process. A unified time-temperature-stress dependent multifactor interaction relationship developed at NASA Lewis Research Center has been used to model the degradation/aging of material properties due to cyclic loads. The fast probability integration method is used to compute probabilistic distribution of response. Sensitivities of fatigue life reliability to uncertainties in the primitive random variables (e.g., constituent properties, fiber volume ratio, void volume ratio, ply thickness, etc.) computed and their significance in the reliability-based design for maximum life is discussed. The effect of variation in the thermal cyclic loads on the fatigue reliability for a (O/rt45/90), graphite/epoxy laminate with a ply thickness of 0.127 mm, with respect to impending failure modes has been studied. The results show that, at low mechanical cyclic loads and low thermal cyclic amplitudes, fatigue life for 0.999 reliability is most sensitive to matrix compressive strength, matrix modulus, thermal expansion coefficient, and ply thickness. Whereas at high mechanical cyclic loads and high thermal cyclic amplitudes, fatigue life at 0.999 reliability is more sensitive to the shear strength of matrix, longitudinal fiber modulus, matrix modulus, and ply thickness.

Research paper thumbnail of Discontinuously Stiffened Composite Panel under Compressive Loading

Journal of Reinforced Plastics and Composites, 1995

The design of composite structures requires an evaluation of their safety and durability under se... more The design of composite structures requires an evaluation of their safety and durability under service loads and possible overload conditions. This paper presents a computational tool that has been developed to examine the response of stiffened composite panels via the simulation of damage initiation, growth, accumulation, progression, and propagation to structural fracture or collapse. The structural durability of a composite panel with a discontinuous stiffener is investigated under compressive loading induced by the gradual displacement of an end support. Results indicate damage initiation and progression to have significant effects on structural behavior under loading. Utilization of an integrated computer code for structural durability assessment is demonstrated.

Research paper thumbnail of Progression of damage and fracture in composites under dynamic loading

31st Structures, Structural Dynamics and Materials Conference, 1990

A new computational simulation method is presented to evaluate the dynamic aspects of composite s... more A new computational simulation method is presented to evaluate the dynamic aspects of composite structural response and durability that have not been simulated previously. Composite structural behavior under any loading condition, geometry, composite system, laminate configuration, and boundary conditions can now be simulated. Structural degradation, delamination, fracture, and damage propagation are included in the simulation. An angle-plied composite plate structure under normal impact loading is used as an example to demonstrate the versatility of the simulation method.

Research paper thumbnail of Micromechanics based fatigue life prediction of a polycrystalline metal applying crystal plasticity

Mechanics of Materials, 2015

Research paper thumbnail of Simplified Design Procedures for Fiber Composite Structural Components/Joints

Composite Structures, 1992

Slmplified step-by-step design procedures are summarized, which are suitable for the preliminary ... more Slmplified step-by-step design procedures are summarized, which are suitable for the preliminary design of composite structural components such as panels (laminates) and composite built-up structures (box beams). Similar procedures are also summarized for the pre]Imlnary design of composite bolted and adhesively bonded Joints. The summary is presented in terms of sample design cases complemented with typical results. Guidelines are provided which can be used in the design selection process of composite structural components/joints. Also, procedures to account for cyclic loads, hygrothermal effects and lamination residual stresses are included.

Research paper thumbnail of Reliability and Creep/Fatigue Analysis of a CMC Component

Volume 1: Turbo Expo 2007, 2007

Available electronically at http://gltrs.grc.nasa.gov Level of Review: This material has been tec... more Available electronically at http://gltrs.grc.nasa.gov Level of Review: This material has been technically reviewed by technical management.

Research paper thumbnail of Progressive fracture in composites subjected to hygrothermal environment

32nd Structures, Structural Dynamics, and Materials Conference, 1991

The influence of hygrothermal environmental conditions on the load carrying ability and response ... more The influence of hygrothermal environmental conditions on the load carrying ability and response of composite structures are investigated via computational simulation. An integrated computer code is utilized for the simulation of composite structural degradation under loading. Damage initiation, damage growth, fracture progression, and global structural fracture are included in the simulation. Results demonstrate the significance of hygro-thermal effects on composite structural response, toughness, and durability. flawless structural performance even when exposed to adverse loading and environmental "Associate Professor, Department of Civil and Environmental Engineering. tAerospace Engineer, Structures Division. tSenior Aerospace Scientist, Structures Division. research at NASA-Lewis Research Center on polymer matrix multi-layer angle-plied composites. ICAN is capable of determining the ability of the laminated composite to endure 17. SECURITY CLASSIFICATION

Research paper thumbnail of Probabilistic modeling of high-temperature material properties of a 5-harness 0/90 Sylramic fiber/CVI-SiC/MI-SiC woven composite

39th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference and Exhibit, 1998

An integrated probabilistic approach has been developed to assess composites for high temperature... more An integrated probabilistic approach has been developed to assess composites for high temperature applications. This approach was used to determine thermal and mechanical properties and their probabilistic distributions of a 5-harness 0/90 Sylramic fiber/CVI-SiC/MI-SiC woven Ceramic Matrix Composite (CMC) at high temperatures. The purpose of developing this approach was to generate quantitative probabilistic information on this CMC to help complete the evaluation for its potential application for HSCT combustor liner. This approach quantified the influences of uncertainties inherent in constituent properties called primitive variables on selected key response variables of the CMC at 2200°F. The quantitative information is presented in the form of Cumulative Density Functions (CDFs), Probability Density Functions (PDFs) and primitive variable sensitivities on response. Results indicate that the scatters in response variables were reduced by 30 to 50 percent when the uncertainties in the primitive variables, which showed the most influence, were reduced by 50 percent.

Research paper thumbnail of Stress Rupture Life Reliability Measures for Composite Overwrapped Pressure Vessels

48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007

Available electronically at http://gltrs.grc.nasa.gov Trade names and trademarks are used in this... more Available electronically at http://gltrs.grc.nasa.gov Trade names and trademarks are used in this report for identification only. Their usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Level of Review: This material has been technically reviewed by technical management.

Research paper thumbnail of Simulation of Fatigue Behavior of High Temperature Metal Matrix Composites

Life Prediction Methodology for Titanium Matrix Composites

A generalized relatively new approach is described for the computational simulation of fatigue be... more A generalized relatively new approach is described for the computational simulation of fatigue behavior of high temperature metal matrix composites (HT-MMCs). This theory is embedded in a specialty-purpose computer code. The effectiveness of the computer code to predict the fatigue behavior of HT-MMCs is demonstrated by applying it to a silicon-fiber/titanium-matrix HT-MMC. Comparative results are shown for mechanical fatigue, thermal fatigue, thermomechanical (in-phase and out-of-phase) fatigue, as well as the effects of oxidizing environments on fatigue life. These results show that the hey, approach reproduces available experimental data remarkably well.

Research paper thumbnail of Computational Simulation of Continuous Fiber-Reinforced Ceramic Matrix Composites Behavior

This report describes a methodology which predicts the behavior of ceramic matrix composites and ... more This report describes a methodology which predicts the behavior of ceramic matrix composites and has been incorporated in the computational tool CEMCAN (CEramic Matrix Composite ANalyzer). The approach combines micromechanics with a unique fiber substructuring concept. In this new concept, the conventional unit cell (the smallest representative volume element of the composite) of the micromechanics approach is modified by substructuring it into several slices and developing the micromechanics-based equations at the slice level. The methodology also takes into account nonlinear ceramic matrix composite (CMC) behavior due to temperature and the fracture initiation and progression. Important features of the approach and its effectiveness are described by using selected examples. Comparisons of predictions and limited experimental data are also provided.

Research paper thumbnail of Design for cyclic loading endurance of composites

The application of the computer code IPACS (Integrated Probabilistic Assessment of Composite Stru... more The application of the computer code IPACS (Integrated Probabilistic Assessment of Composite Structures) to aircraft wing type structures is described. The code performs a complete probabilistic analysis for composites taking into account the uncertainties in geometry, boundary conditions, material properties, laminate lay-ups, and loads. Results of the analysis are presented in terms of cumulative distribution functions (CDF) and probability density function (PDF) of the fatigue life of a wing type composite structure under different hygrothermal environments subjected to the random pressure. The sensitivity of the fatigue life to a number of critical structural/material variables is also computed from the analysis.

Research paper thumbnail of Microstructural Influence on Deformation and Fatigue Life of Composites Using the Generalized Method of Cells

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2015

A fully coupled deformation and damage approach to modeling the response of composite materials a... more A fully coupled deformation and damage approach to modeling the response of composite materials and composite laminates is presented. It is based on the semi-analytical generalized method of cells (GMC) micromechanics model as well as its higher fidelity counterpart, HFGMC, both of which provide closed-form constitutive equations for composite materials as well as the micro scale stress and strain fields in the composite phases. The provided constitutive equations allow GMC and HFGMC to function within a higher scale structural analysis (e.g., finite element analysis or lamination theory) to represent a composite material point, while the availability of the micro fields allow the incorporation of lower scale sub-models to represent local phenomena in the fiber and matrix. Further, GMC's formulation performs averaging when applying certain governing equations such that some degree of microscale field accuracy is surrendered in favor of extreme computational efficiency, rendering the method quite attractive as the centerpiece in a integrated computational material engineering (ICME) structural analysis; whereas HFGMC retains this microscale field accuracy, but at the price of significantly slower computational speed. Herein, the sensitivity of deformation and the fatigue life of graphite/epoxy PMC composites, with both ordered and disordered microstructures, has been investigated using this coupled deformation and damage micromechanics based approach. The local effects of fiber breakage and fatigue damage are included as sub-models that operate on the microscale for the individual composite phases. For analysis of laminates, classical lamination theory is employed as the global or structural scale model, while GMC/HFGMC is embedded to operate on the microscale to simulate the behavior of the composite material within each laminate layer. A key outcome of this study is the statistical influence of microstructure and micromechanics idealization (GMC or HFGMC) on the overall accuracy of unidirectional and laminated composite deformation and fatigue response.

Research paper thumbnail of Structural durability of stiffened composite shells

33rd Structures, Structural Dynamics and Materials Conference, 1992

The durability of a stiffened composite cylindrical shell panel is investigated under several loa... more The durability of a stiffened composite cylindrical shell panel is investigated under several loading conditions. An integrated computer code is utilized for the simulation of load induced structural degradation. Damage initiation, growth, and accumulation up to the stage of propagation to fracture are included in the computational simulation. Results indicate significant differences in the degradation paths for different loading cases. Effects of combined loading on structural durability and ultimate structural strength of a stiffened shell are assessed. Nomenclature O'gll-ply longitudinal stress 0"g22-ply transverse stress at33-ply normal stress at12-ply in-plane shear stress ae2a-ply out-of-plane shear stress *Associate Professor, Department of Civil and Environmental Engineering ?Graduate Student, Department of Civil and Environmental Engineering tAerospace Engineer, Structures Division. §Senior Aerospace Scientist, Structures Division. aa3-ply out-of-plane shear stress atilTply longitudinal tensile stress atilt-ply longitudinal compressive stress Ort22r-ply transverse tensile stress MDE-modified distortion energy failure criterion R-radius of cylindrical shell RR-delamination due to relative rotation by NASA-Lewis Research Center under grant NAG-3-1101.

Research paper thumbnail of Best Practices for Reliable and Robust Spacecraft Structures

48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007

A study was undertaken to capture the best practices for the development of reliable and robust s... more A study was undertaken to capture the best practices for the development of reliable and robust spacecraft structures for NASA's next generation cargo and crewed launch vehicles. In this study, the NASA heritage programs such as Mercury, Gemini, Apollo, and the Space Shuttle program were examined. A series of lessons learned during the NASA and DoD heritage programs are captured. The processes that "make the right structural system" are examined along with the processes to "make the structural system right". The impact of technology advancements in materials and analysis and testing methods on reliability and robustness of spacecraft structures is studied. The best practices and lessons learned are extracted from these studies. Since the first human space flight, the best practices for reliable and robust spacecraft structures appear to be well established, understood, and articulated by each generation of designers and engineers. However, these best practices apparently have not always been followed. When the best practices are ignored or short cuts are taken, risks accumulate, and reliability suffers. Thus program managers need to be vigilant of circumstances and situations that tend to violate best practices. Adherence to the best practices may help develop spacecraft systems with high reliability and robustness against certain anomalies and unforeseen events.

Research paper thumbnail of Probabilistic micromechanics and macromechanics for ceramic matrix composites

38th Structures, Structural Dynamics, and Materials Conference, 1997

The properties of ceramic matrix composites (CMC's) are known to display a considerable amount of... more The properties of ceramic matrix composites (CMC's) are known to display a considerable amount of scatter due to variations in fiber/matrix properties, interphase properties, interphase bonding, amount of matrix voids, and many geometry-or fabrication-related parameters, such as ply thickness and ply orientation. This paper summarizes preliminary studies in which formal probabilistic descriptions of the material-behavior-and fabrication-related parameters were incorporated into micromechanics and macromechanics for CMC's. In this process two existing methodologies, namely CMC micromechanics and macromechanics analysis and a fast probability integration (FPI) technique are synergistically coupled to obtain the probabilistic composite behavior or response. Preliminary results in the form of cumulative probability distributions and information on the probability sensitivities of the response to primitive variables for a unidirectional silicon carbide/reaction-bonded silicon nitride (SiC/RBSN) CMC are presented. The cumulative distribution functions are computed for composite moduli, thermal expansion coefficients, thermal conductivities, and longitudinal tensile strength at room temperature. The variations in the constituent properties that directly affect these composite properties are accounted for via assumed probabilistic distributions. Collectively, the results show that the present technique provides valuable information about the composite properties and sensitivity factors, which is useful to design or test engineers. Furthermore, the present methodology is computationally more efficient than a standard Monte-Carlo simulation technique; and the agreement between the two solutions is excellent, as shown via select examples.

Research paper thumbnail of Optimal fabrication processes for unidirectional metal-matrix composites: A computational simulation

A method is proposed for optimizing the fabrication process of unidirectional metal matrix compos... more A method is proposed for optimizing the fabrication process of unidirectional metal matrix composites. The temperature and pressure histories are optimized such that the residual microstresses of the composite at the end of the fabrication process are minimized and the material integrity throughout the process is ensured. The response of the composite during the fabrication is simulated based on a nonlinear micromechanics theory. The optimal fabrication problem is formulated and solved with non-linear programming. Application cases regarding the optimization of the fabrication cool-down phases of unidirectional ultra-high modulus graphite/copper and silicon carbide/titanium composites are presented.