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At least 271 records · Page 15

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NASA NTRS2015

Correlation of Electrical Resistance to CMC Stress-Strain and Fracture Behavior Under High Heat-Flux Thermal and Stress Gradients

Because SiCSiC ceramic matrix composites (CMCs) are under consideration for use as turbine engine hot-section components in extreme environments, it becomes necessary to investigate their performance and damage morphologies under complex loading and environmental conditions. Monitoring of electrical resistance (ER) has been shown as an effective tool for detecting damage accumulation of woven melt-infiltrated SiCSiC CMCs. However, ER change under complicated thermo-mechanical loading is not well understood. In this study a systematic approach is taken to determine the capabilities of ER as a relevant non-destructive evaluation technique for high heat-flux testing, including thermal gradients and localized stress concentrations. Room temperature and high temperature, laser-based tensile tests were conducted in which stress-dependent damage locations were determined using modal acoustic emission (AE) monitoring and compared to full-field strain mapping using digital image correlation (DIC). This information is then compared with the results of in-situ ER monitoring, post-test ER inspection and fractography in order to correlate ER response to convoluted loading conditions and damage evolution.

electrical resistance↗

NASA NTRS1996

Progressive Fracture of Fiber Composite Builtup Structures

The damage progression and fracture of builtup composite structures was evaluated by using computational simulation to examine the behavior and response of a stiffened composite (0 +/- 45/90)(sub s6) laminate panel subjected to a bending load. The damage initiation, growth, accumulation, progression, and propagation to structural collapse were simulated. An integrated computer code (CODSTRAN) was augmented for the simulation of the progressive damage and fracture of builtup composite structures under mechanical loading. Results showed that damage initiation and progression have a significant effect on the structural response. Also investigated was the influence of different types of bending load on the damage initiation, propagation, and final fracture of the builtup composite panel.

Gotsis, Pascal K.↗

NASA NTRS2003

Buckling Of Long Compression-Loaded Anisotropic Plates Restrained Against Inplane Lateral and Shear Deformations

An approach for synthesizing buckling results and behavior for thin balanced and unbalanced symmetric laminates that are subjected to uniform axial compression loads and elastically restrained against inplane expansion, contraction, and shear deformation is presented. This approach uses a nondimensional analysis for infinitely long, flexurally anisotropic plates (coupling between bending and twisting) that are subjected to combined mechanical loads and is based on nondimensional parameters. In addition, nondimensional loading parameters are derived that account for the effects of the elastic inplane deformation restraints, membrane orthotropy, and membrane anisotropy on the induced prebuckling stress state. The loading parameters are used to determine buckling coefficients that include the effects of flexural orthotropy and flexural anisotropy. Many results are presented, for some selected laminates, that are intended to facilitate a structural designer's transition to the use of the generic buckling design curves that are presented and discussed in the paper. Several buckling response curves are presented that provide physical insight into the behavior for combined loads, in addition to providing useful design data. An example is presented that demonstrates the use of the generic design curves, which are applicable to a wide range of laminate constructions. The analysis approach and generic results indicate the effects and characteristics of laminate orthotropy and anisotropy in a very general and unifying manner.

Nemeth, Michael P.↗

NASA NTRS2026

Mechanical Properties of Carbon Fiber Reinforced Composites Exposed to Cryogenic Conditions and Space Radiation via Simulation and Testing

As NASA missions extend beyond low Earth orbit, increasing reliance is placed on carbon fiber reinforced polymer (CFRP) composites for spacecraft structures where mass efficiency, durability, and long-term reliability are critical. In service, these materials are subjected to a combination of ultraviolet radiation, vacuum, ionizing radiation, atomic oxygen, and extreme thermal excursions under sustained mechanical loading. Flight systems such as the Boeing Starliner and SpaceX Dragon employ external composite structures that will experience these environments for extended durations. Although prior spaceflight and ground studies have reported limited changes in bulk mechanical properties, the synergistic effects of these environments on composite microstructure, particularly at the fiber matrix interphase, remain insufficiently characterized and represent a potential qualification and reliability risk. This study investigates the effects of short-term cryogenic exposure on a radiation shielding carbon epoxy composite, SC2020, as a ground-based analog for space relevant thermal extremes. The SC2020 material system has previously flown on the International Space Station under the Materials International Space Station Experiment (MISSE) program. Composite specimens were exposed to liquid nitrogen for 6 and 24 hours and evaluated using a multiscale characterization framework that combined ASTM D3039 tensile testing, Atomic Force Microscopy (AFM) based interphase analysis, and helium gas permeability measurements. Tensile testing showed no statistically significant or permanent degradation in global strength or modulus following cryogenic exposure. In contrast, AFM measurements revealed reductions in interphase modulus, weakened adhesion, and increased nanoscale heterogeneity, indicating localized degradation mechanisms not captured by conventional bulk testing. Gas permeability measurements showed a progressive increase in helium diffusion with exposure duration, consistent with micro-void formation or partial interfacial debonding. The results indicate that cryogenic exposure initiates degradation at the fiber matrix interphase while leaving global mechanical properties largely unchanged over short durations. These findings underscore the importance of multiscale diagnostics for identifying early-stage damage mechanisms that may influence long term performance and qualification margins for spaceflight composite structures. The data presented establish a cryogenic baseline for comparison with forthcoming MISSE flight exposure results and support ongoing NASA Established Program to Stimulate Competitive Research (EPSCoR) efforts aimed at improving composite qualification methodologies, risk assessment, and reliability prediction for space environments.

composite reliability↗

NASA NTRS2002

Buckling Behavior of Long Anisotropic Plates Subjected to Elastically Restrained Thermal Expansion

An approach for synthesizing buckling results for, and behavior of, thin balanced and unbalanced symmetric laminates that are subjected to uniform heating or cooling and elastically restrained against thermal expansion or contraction is presented. This approach uses a nondimensional analysis for infinitely long, flexurally anisotropic plates that are subjected to combined mechanical loads and is based on useful nondimensional parameters. In addition, stiffness-weighted laminate thermal-expansion parameters and compliance coefficients are derived that are used to determine critical temperatures in terms of physically intuitive mechanical-buckling coefficients. The effects of membrane orthotropy and membrane anisotropy are included in the general formulation. Many results are presented for some common laminates that are intended to facilitate a structural designer's transition to the use of generic buckling design curves. Several curves that illustrate the fundamental parameters used in the analysis are presented, for nine contemporary material systems, that provide physical insight into the buckling response in addition to providing useful design data. Examples are presented that demonstrate the use of generic design curves. The analysis approach and generic results indicate the effects and characteristics of elastically restrained laminate thermal expansion or contraction, membrane orthotropy and anisotropy, and flexural orthotropy and anisotropy in a very general and unifying manner.

Nemeth, Michael P.↗

NASA NTRS2002

Effect of Environment on the Stress- Rupture Behavior of a C/SiC Composite Studied

Advanced reusable launch vehicles will likely incorporate fiber-reinforced ceramic matrix composites (CMC's) in critical propulsion and airframe components. The use of CMC's is highly desirable to save weight, improve reuse capability, and increase performance. One of the candidate CMC materials is carbon-fiber-reinforced silicon carbide (C/SiC). In potential propulsion applications, such as turbopump rotors and nozzle exit ramps, C/SiC components will be subjected to a service cycle that includes mechanical loading under complex, high-pressure environments containing hydrogen, oxygen, and steam. Degradation of both the C fibers and the SiC matrix are possible in these environments. The objective of this effort was to evaluate the mechanical behavior of C/SiC in various environments relevant to reusable launch vehicle applications. Stress-rupture testing was conducted at the NASA Glenn Research Center on C/SiC specimens in air and steam-containing environments. Also, the oxidation kinetics of the carbon fibers that reinforce the composite were monitored by thermogravimetric analysis in the same environments and temperatures used for the stress-rupture tests of the C/SiC composite specimens. The stress-rupture lives obtained for C/SiC tested in air and in steam/argon mixtures are shown in the following bar chart. As is typical for most materials, lives obtained at the lower temperature (600 C) are longer than for the higher temperature (1200 C). The effect of environment was most pronounced at the lower temperature, where the average test duration in steam at 600 C was at least 30 times longer than the lives obtained in air. The 1200 C data revealed little difference between the lives of specimens tested in air and steam at atmospheric pressure.

Verrilli, Michael J.↗

NASA NTRSDate not supplied

TBIRD, a rapid prototyping approach for design, assembly, and test of a LEO-to-ground lasercom CubeSat

The TeraByte InfraRed Delivery (TBIRD) system is a 3U payload on a 6U CubeSat launched in May 2022 which has now demonstrated space to ground links of >1 Terabyte (TB) per pass at a max data rate of 200Gbps. As a CubeSat mission, the development of the TBIRD payload was focused on low SWaP and a “rapid prototyping” approach which accepted higher risks to accelerate the schedule and reduce costs. The optomechanical design process followed standard in-house processes to develop a system that would be robust to LEO environmental loads, with a focus on the stability of the transmit (Tx) and receive (Rx) channel performance metrics. The driving requirement of maintaining 20μrad pointing error between the TX and Rx channels forced specific attention to thermal and mechanical load changes over operational conditions, which drove major design decisions. This paper describes some of engineering challenges overcome and approaches used to make TBIRD a successful program, as well as some of the tradeoffs of rapid prototyping precision optical payloads. TBIRD successfully met and exceeded the total downlink requirements listed above, with a bandwidth of 200Gbps and a total downlink of 4.8TB of information in a single pass.

TBIRD↗

NASA NTRS1986

Thermal barrier coating life prediction model development

A methodology is established to predict thermal barrier coating life in a environment similar to that experienced by gas turbine airfoils. Experiments were conducted to determine failure modes of the thermal barrier coating. Analytical studies were employed to derive a life prediction model. A review of experimental and flight service components as well as laboratory post evaluations indicates that the predominant mode of TBC failure involves thermomechanical spallation of the ceramic coating layer. This ceramic spallation involves the formation of a dominant crack in the ceramic coating parallel to and closely adjacent to the topologically complex metal ceramic interface. This mechanical failure mode clearly is influenced by thermal exposure effects as shown in experiments conducted to study thermal pre-exposure and thermal cycle-rate effects. The preliminary life prediction model developed focuses on the two major damage modes identified in the critical experiments tasks. The first of these involves a mechanical driving force, resulting from cyclic strains and stresses caused by thermally induced and externally imposed mechanical loads. The second is an environmental driving force based on experimental results, and is believed to be related to bond coat oxidation. It is also believed that the growth of this oxide scale influences the intensity of the mechanical driving force.

Demasi, J. T.↗

NASA NTRS1990

Definition of large components assembled on-orbit and robot compatible mechanical joints

One of four major areas of project Pathfinder is in-space assembly and construction. The task of in-space assembly and construction is to develop the requirements and the technology needed to build elements in space. A 120-ft diameter tetrahedral aerobrake truss is identified as the focus element. A heavily loaded mechanical joint is designed to robotically assemble the defined aerobrake element. Also, typical large components such as habitation modules, storage tanks, etc., are defined, and attachment concepts of these components to the tetrahedral truss are developed.

Williamsen, J.↗

NASA NTRS1989

Nonlinear shell analyses of the space shuttle solid rocket boosters

A variety of structural analyses have been performed on the Solid Rocket Boosters (SRB's) to provide information that would contribute to the understanding of the failure which destroyed the Space Shuttle Challenger. This paper describes nonlinear shell analyses that were performed to characterize the behavior of an overall SRB structure and a segment of the SRB in the vicinity of the External Tank Attachment (ETA) ring. Shell finite element models were used that would accurately reflect the global load transfer in an SRB in a manner such that nonlinear shell collapse and ovalization could be assessed. The purpose of these analyses was to calculate the overall deflection and stress distributions for these SRB models when subjected to mechanical loads corresponding to critical times during the launch sequence. Static analyses of these SRB models were performed using a snapshot picture of the loads. Analytical results obtained using these models show no evidence of nonlinear shell collapse for the pre-liftoff loading cases considered.

Knight, Norman F., Jr.↗

NASA NTRS1990

Nonlinear shell analysis of the Space Shuttle Solid Rocket Boosters

A variety of structural analyses have been performed on the Solid Rocket Boosters (SRB's) to provide information that would contribute to the understanding of the failure which destroyed the Space Shuttle Challenger. This paper describes nonlinear shell analyses that were performed to characterize the behavior of an overall SRB structure and a segment of the SRB in the vicinity of the External Tank Attachment (ETA) ring. Shell finite element models were used that would accurately reflect the global load transfer in an SRB in a manner such that nonlinear shell collapse and ovalization could be assessed. The purpose of these analyses was to calculate the overall deflection and stress distributions for these SRB models when subjected to mechanical loads corresponding to critical times during the launch sequence. Static analyses of these SRB models were performed using a 'snapshot picture' of the loads. Analytical results obtained using these models show no evidence of nonlinear shell collapse for the pre-liftoff loading cases considered.

Knight, N. F., Jr.↗

NASA NTRSDate not supplied

Fracture of Charring Avcoat With Meshfree Material Response-Coupled Fracture Approach

Fracture of thermal protection systems (TPS) is typically unfavorable, but often unavoidable. TPS can fracture during entry, manufacturing or from impact. Many NASA missions use ablative materials for TPS, including the fore-body for Dragonfly (PICA) and Orion for Artemis-I (Avcoat). The material response to thermal and mechanical loads during entry is integral to TPS sizing, design and analysis. The fracture of TPS also depends, and can be caused by, the change of material properties during entry. Simulations that resolve fracture of TPS due to thermo-mechanical forces can identify TPS failure mechanisms and be applied to conditions inaccessible to ground testing. We present simulation work using a mesh-free/Lagrangian approach to solving continuum mechanics, coupled to material response. Two different materials are simulated under different model entry-like boundary conditions. The dynamic crack structure is analyzed and compared across simulations. Crack analyses inform the role of design features, such as entry trajectories and manufacturing-influenced material properties, play in TPS fracture.

Andrew P Santos↗

NASA NTRS1994

Creep and stress relaxation modeling of polycrystalline ceramic fibers

A variety of high performance polycrystalline ceramic fibers are currently being considered as reinforcement for high temperature ceramic matrix composites. However, under mechanical loading about 800 C, these fibers display creep related instabilities which can result in detrimental changes in composite dimensions, strength, and internal stress distributions. As a first step toward understanding these effects, this study examines the validity of a mechanism-based empirical model which describes primary stage tensile creep and stress relaxation of polycrystalline ceramic fibers as independent functions of time, temperature, and applied stress or strain. To verify these functional dependencies, a simple bend test is used to measure stress relaxation for four types of commercial ceramic fibers for which direct tensile creep data are available. These fibers include both nonoxide (SCS-6, Nicalon) and oxide (PRD-166, FP) compositions. The results of the Bend Stress Relaxation (BSR) test not only confirm the stress, time, and temperature dependencies predicted by the model, but also allow measurement of model empirical parameters for the four fiber types. In addition, comparison of model tensile creep predictions based on the BSR test results with the literature data show good agreement, supporting both the predictive capability of the model and the use of the BSR text as a simple method for parameter determination for other fibers.

Dicarlo, James A.↗

NASA NTRS1984

Mechanical proof testing in cell processing

Fracture mechanics test methods are applied to evaluate the proof test characteristics of silicon Cz wafers. The results indicate that the strength distribution of silicon wafers is truncated by proof testing and no subcritical crack growth in silicon is observed during proof loading. Mechanical proof testing appears to be an effective method to eliminate weak samples before cell processing.

Chen, C. P.↗

NASA NTRS1997

Estrogen regulates the rate of bone turnover but bone balance in ovariectomized rats is modulated by prevailing mechanical strain

Estrogen deficiency induced bone loss is associated with increased bone turnover in rats and humans. The respective roles of increased bone turnover and altered balance between bone formation and bone resorption in mediating estrogen deficiency-induced cancellous bone loss was investigated in ovariectomized rats. Ovariectomy resulted in increased bone turnover in the distal femur. However, cancellous bone was preferentially lost in the metaphysis, a site that normally experiences low strain energy. No bone loss was observed in the epiphysis, a site experiencing higher strain energy. The role of mechanical strain in maintaining bone balance was investigated by altering the strain history. Mechanical strain was increased and decreased in long bones of ovariectomized rats by treadmill exercise and functional unloading, respectively. Functional unloading was achieved during orbital spaceflight and following unilateral sciatic neurotomy. Increasing mechanical loading reduced bone loss in the metaphysis. In contrast, decreasing loading accentuated bone loss in the metaphysis and resulted in bone loss in the epiphysis. Finally, administration of estrogen to ovariectomized rats reduced bone loss in the unloaded and prevented loss in the loaded limb following unilateral sciatic neurotomy in part by reducing indices of bone turnover. These results suggest that estrogen regulates the rate of bone turnover, but the overall balance between bone formation and bone resorption is influenced by prevailing levels of mechanical strain.

STS-62 Shuttle Project↗

NASA NTRS2019

Structural and Functional Adaptation of the Vestibular Otolith to Altered Gravity from Microgravity to Hypergravity

Inertial acceleration and a change in head orientation with respect to gravity are sensed by mechanosensitive receptors in the inner ear otolith organs. These structures consist of calcium carbonate grains called otoconia that mechanically load the hair cell bundles and distribute the tangential shear force during movement, and changes in their density can alter hair cell sensitivity. A possible adaptive response to a chronic gravity change is a change in weight-lending otoconia. Another mechanism is a modification of the strength and number of synapses coupling the hair cells to nerve afferents that convey the signals into the brain. Here, we present the results obtained in 2 species exposed both to ÂµG (microgravity) and hyper-gravity (HG). Adult toadfish, Opsanus tau, were exposed to ÂµG (microgravity) in 2 shuttle missions and to 1.12-2.24G (force of gravity) [resultant] centrifugation for 1-32 days; readaptation was studied following 1-8 days after return to 1G. Results show a biphasic pattern in response to 2.24G: initial hypersensitivity, similar to that observed after ÂµG (microgravity) exposure, followed by transition to a significant decrease at 16-32 days. Recovery from HG exposure is approximately 4-8 days. Two major pieces of information are still needed: vertebrate hair cell response to altered gravity and impact of longer duration exposures on sensory plasticity. To address the latter we applied electron microscopic techniques to image otoconia mass obtained from 1) mice subjected to 91-days of ÂµG (microgravity) in the Mouse Drawer System (MDS) flown on International Space Station, 2) mice subjected to 91-days of 1.24G centrifugation on ground, and 3) mice flown on 2 shuttle missions. Images from MDS mice indicate a clear restructuring of individual otoconia, suggesting deposition to the outer shell. Images from their HG ground counterparts indicate the converse - an ablation of the otoconia mass. For 13-day exposures to ÂµG (microgravity) mice otoconia appear normal. Despite the permanence of gravity in evolution the animal senses exposure to a novel, non-1G, environment and adaptive mechanisms are initiated - in the short term compensation is likely confined to the peripheral sensory receptors, the brain or both. For longer exposures structural modifications of the otolith mass may also result.

Otoconia↗

NASA NTRS1998

Applications of Anabolic Vitamin D Analogs as Countermeasures to Bone Loss

The experiments in Round 2 were designed to extend the results of our efforts in Round 1 which led us to hypothesize that the seco-steroid, 1,25-dihydroxyvitamin D3[1,25(OH)2D3], acts in synergy with parathyroid hormone (PTH) to regulate bone calcium homeostasis. Our work centered on one particular target of 1,25(OH)2D3 action, the voltage-sensitive calcium channels (VSCC's), which are activated acutely by this steroid within milliseconds of exposure . A second area of research focused on the effects of mechanical strain on VSCC expression in bone. These experiments were performed in collaboration with Dr. Steven Goldstein (Univ. Michigan), who generously provided RNA extracted from dog bones that had been exposed to mechanical strain in vivo. Our results suggest that mechanical loading elevated VSCC expression in the long bones from 3 of the 6 animals tested. A second line of experimentation, carried out in collaboration with Dr. Randall Duncan, a NASA-funded investigator in Indianapolis, centered on RT-PCR analysis of effects of mechanical strain on Ca2(+) channel expression in cultured bone cells. Compared to unstrained controls, the expression of vitamin-D-sensidve Ca2(+) channels is elevated 3- to 5-fold over a 24 hr period.

Karin, Norman J.↗

NASA NTRS1998

Mechanical Characterization of Thermomechanical Matrix Residual Stresses Incurred During MMC Processing

In recent years, much effort has been spent examining the residual stress-strain states of advanced composites. Such examinations are motivated by a number of significant concerns that affect composite development, processing, and analysis. The room-temperature residual stress states incurred in many advanced composite systems are often quite large and can introduce damage even prior to the first external mechanical loading of the material. These stresses, which are induced during the cooldown following high-temperature consolidation, result from the coefficient of thermal expansion mismatch between the fiber and matrix. Experimental techniques commonly used to evaluate composite internal residual stress states are non-mechanical in nature and generally include forms of x-ray and neutron diffraction. Such approaches are usually complex, involving a number of assumptions and limitations associated with a wide range of issues, including the depth of penetration, the volume of material being assessed, and erroneous effects associated with oriented grains. Furthermore, and more important to the present research, these techniques can assess only "single time" stress in the composite. That is, little, if any, information is obtained that addresses the time-dependent point at which internal stresses begin to accumulate, the manner in which the accumulation occurs, and the presiding relationships between thermoelastic, thermoplastic, and thermoviscous behaviors. To address these critical issues, researchers at the NASA Lewis Research Center developed and implemented an innovative mechanical test technique to examine in real time, the time-dependent thermomechanical stress behavior of a matrix alloy as it went through a consolidation cycle.

Castelli, Michael G.↗