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At least 199 records · Page 11

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NASA NTRS1994

Thermomechanical Fatigue Damage/Failure Mechanisms in SCS-6/Timetal 21S [0/90](Sub S) Composite

The thermomechanical fatigue (TMF) deformation, damage, and life behaviors of SCS6/Timetal 21S (0/90)s were investigated under zero-tension conditions. In-phase (IP) and out-of-phase (OP) loadings were investigated with a temperature cycle from 150 to 650 deg C. An advanced TMF test technique was used to quantify mechanically damage progression. The technique incorporated explicit measurements of the macroscopic (1) isothermal static moduli at the temperature extremes of the TMF cycle and (2) coefficient of thermal expansion (CTE) as functions of the TMF cycles. The importance of thermal property degradation and its relevance to accurate post-test data analysis and interpretation is briefly addressed. Extensive fractography and metallography were conducted on specimens from failed and interrupted tests to characterize the extent of damage at the microstructure level. Fatigue life results indicated trends analogous to those established for similar unidirectional(0) reinforced titanium matrix composite systems. High stress IP and mid to low stress OP loading conditions were life-limiting in comparison to maximum temperature isothermal conditions. Dominant damage mechanisms changed with cycle type. Damage resulting from IP TMF conditions produced measurable decreases in static moduli but only minimal changes in the CTE. Metallography on interrupted and failed specimens revealed extensive (0) fiber cracking with sparse matrix damage. No surface initiated matrix cracks were present. Comparable OP TMF conditions initiated environment enhanced surface cracking and matrix cracking initiated at (90) fiber/matrix (F/M) interfaces. Notable static moduli and CTE degradations were measured. Fractography and metallography revealed that the transverse cracks originating from the surface and (90) F/M interfaces tended to converge and coalesce at the (0) fibers.

Castelli, Michael G.↗

NASA NTRS1987

A loading system for creep testing under constant compressive stress

A critical analysis is made of layouts assuring constant stress under creep, pointing out the difficulties of using these layouts for compressive testing. A new mechanical system to maintain stress during creep testing is proposed. Constant stress is achieved by a definite motion of the load. The proposed system can be used in tensile or compressive testing. Practical measurements reveal that the stress constancy error is not greater than plus or minus 1.5 percent up to relative deformation of 30 percent.

Dobes, F.↗

NASA NTRS2003

Cryogenic Insulation Bondline Studies for Reusable Launch Vehicles

Cryogenic insulations bonded to metallic substrates were characterized under simulated mission conditions representative for a reusable launch vehicle. The combined thermal and mechanical test consisted of 50 to a 100 cycles. These combined thermal and mechanical cycles simulated flight missions with temperatures ranging from -423 F to 450 F and a maximum mechanical tension load ranging from 20,000 lbs. to 97,650 lbs. The combined thermal and mechanical (uniaxial tension) test apparatus (1 ft. by 2 ft. Test Apparatus) developed at the NASA Langley Research Center, was used to perform cyclic tests on cryogenic insulations bonded to tank wall substrates. No visual delamination or degradation was observed in the cryogenic insulation-to-metallic substrate bondline or butt joints between cryogenic insulation panels. In addition, after cyclic testing was performed, residual property results from tension-pull and closed-cell content tests of the cryogenic insulations indicated a decrease in the bondline strength and closed-cell content.

Johnson, T. F.↗

NASA NTRS2013

Mechanical Testing of Carbon Based Woven Thermal Protection Materials

Three Dimensional Woven thermal protection system (TPS) materials are one of the enabling technologies for mechanically deployable hypersonic decelerator systems. These materials have been shown capable of serving a dual purpose as TPS and as structural load bearing members during entry and descent operations. In order to ensure successful structural performance, it is important to characterize the mechanical properties of these materials prior to and post exposure to entry-like heating conditions. This research focuses on the changes in load bearing capacity of woven TPS materials after being subjected to arcjet simulations of entry heating. Preliminary testing of arcjet tested materials [1] has shown a mechanical degradation. However, their residual strength is significantly more than the requirements for a mission to Venus [2]. A systematic investigation at the macro and microstructural scales is reported here to explore the potential causes of this degradation. The effects of heating on the sizing (an epoxy resin coating used to reduce friction and wear during fiber handling) are discussed as one of the possible causes for the decrease in mechanical properties. This investigation also provides valuable guidelines for margin policies for future mechanically deployable entry systems.

Pham, John↗

NASA NTRS2004

Lamb Wave Stiffness Characterization of Composites Undergoing Thermal-Mechanical Aging

The introduction of new, advanced composite materials into aviation systems requires a thorough understanding of the long term effects of combined thermal and mechanical loading upon those materials. Analytical methods investigating the effects of intense thermal heating combined with mechanical loading have been investigated. The damage mechanisms and fatigue lives were dependent on test parameters as well as stress levels. Castelli, et al. identified matrix dominated failure modes for out-of-phase cycling and fiber dominated damage modes for in-phase cycling. In recent years, ultrasonic methods have been developed that can measure the mechanical stiffness of composites. To help evaluate the effect of aging, a suitably designed Lamb wave measurement system is being used to obtain bending and out-of-plane stiffness coefficients of composite laminates undergoing thermal-mechanical loading. The system works by exciting an antisymmetric Lamb wave and calculating the velocity at each frequency from the known transducer separation and the measured time-of-flight. The same peak in the waveforms received at various distances is used to measure the time difference between the signals. The velocity measurements are accurate and repeatable to within 1% resulting in reconstructed stiffness values repeatable to within 4%. Given the material density and plate thickness, the bending and out-of-plane shear stiffnesses are calculated from a reconstruction of the dispersion curve. A mechanical scanner is used to move the sensors over the surface to map the time-of-flight, velocity, or stiffnesses of the entire specimen. Access to only one side of the material is required and no immersion or couplants are required because the sensors are dry coupled to the surface of the plate. In this study, the elastic stiffnesses D(sub 11), D(sub 22), A(sub 44), and A(sub 55) as well as time-of-flight measurements for composite samples that have undergone combined thermal and mechanical aging for a duration of 10,000 hours are reported.

Seale, Michael D.↗

NASA NTRS1979

Space Shuttle Solid Rocket Booster decelerator subsystem - Air drop test vehicle/B-52 design

The air drop development test program for the Space Shuttle Solid Rocket Booster Recovery System required the design of a large drop test vehicle that would meet all the stringent requirements placed on it by structural loads, safety considerations, flight recovery system interfaces, and sequence. The drop test vehicle had to have the capability to test the drogue and the three main parachutes both separately and in the total flight deployment sequence and still be low-cost to fit in a low-budget development program. The design to test large ribbon parachutes to loads of 300,000 pounds required the detailed investigation and integration of several parameters such as carrier aircraft mechanical interface, drop test vehicle ground transportability, impact point ground penetration, salvageability, drop test vehicle intelligence, flight design hardware interfaces, and packaging fidelity.

Runkle, R. E.↗

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 NTRS2014

ADEPT for Secondary Payloads

Deployable entry vehicles provide an enabling capability for large payloads at destinations across the solar system. The primary advantage of such systems derives from the fundamental limitation of available launch vehicle shroads preventing tried-and-true rigid aeroshell technology from being applied. However, little mission-pull exists for deployable entry vehicle technology for missions where the entry vehicle is the primary payload and can fit within available shrouds. The propensity toward applying rigid aeroshell technology is logical given that deployable concepts are higher risk and provide no marginal scientific benefit for missions where a rigid aeroshell is an option. A movement toward flying smaller spacecraft has emerged in recent years and is revealing novel applications for deployable entry vehicles as secondary payloads. Secondary payloads must minimize interference with the primary payload in order to ease accomodation. Advancements in small spacecraft technology have caught the attention of government entities who are now recognizing the high value proposition of small spacecraft for frequent, incremental technology development and science return rather than the traditional high cost and infrequent big-bang approaches. Worldwide, well over 200 small satellites (mass < 180 kg) have launched in recent years for such purposes as eduation, technology development, science, commerce, and defense. The Adaptable Deployable Entry and Placement Technology (ADEPT), a mechanically-deployable entry vehicle technology, has been under development at NASA since 2011. An Ames Center Innovation Fund (CIF) proposal was recently awarded to study ADEPT for EDL of small payloads. The unique capability of ADEPT for small science payloads comes from its ability to stow within a slender volume and deploy passively to achieve a mass-efficient drag surface with a high heat rate capability. The low ballistic coefficient results in entry heating and mechanical loads that can be met by a revolutionary three-dimensionally woven carbon fabric supported with rib structures. This carbon fabric has test-proven capability as both primary structure and payload thermal protection. This presentation will inform the EDL community of the capability of ADEPT for delivery of small secondary payloads as well as provide status of on-going technology development activities. Results of a mission applications study for secondary payloads to Mars and Venus will be presented. A primary goal of these mission design activities is to integrate high-value verification approaches wherever possible. For example, for some mission applications it is possible to test in NASA arc jet facilities at full-scale. This enables endto- end thermostructural requirement verification with a single test. Another important challenge that needs to be solved is the reliability of the decelerator to achieve the desired shape using simple passive mechanical actuators (such as springs) that do not require power from the primary payload for deployment. A configuration of ADEPT is being prototyped and used to evaluate the functionality and reliability of the deployment mechanism through bench-top testing. The prototype will then be tested in a low speed wind tunnel in late 2014 to evaluate aerodynamic load-carrying capability and deflected shape. Results of the deployment testing and status of wind tunnel test plans will be discussed.

Smith, B.↗

NASA NTRS1976

Axial fatigue of /0, +30, and -30 deg/6s graphite/epoxy

Results are presented for an investigation carried out on a /0, +30, and -30 deg/6s graphite/epoxy angle-ply laminate in order to characterize the overall fatigue behavior, to evaluate strength/moduli degradation due to cyclic loading, and to identify the failure modes and mechanisms. The specimen life determined from fatigue tests is measured as a function of cyclic stress amplitude for various stress ratios. Fractured specimens are sectioned and examined by conventional metallography and SEM to reveal failure modes and mechanisms. It is shown that the laminate studied is susceptible to axial fatigue failures under cyclic loading which involves both tensile and compressive mean stresses. Fracture surface observations indicate the occurrence of complex failure modes. A variety of local failure mechanisms are identified.

Ramani, S. V.↗

NASA NTRS2007

Zero-Gravity Locomotion Simulators: New Ground-Based Analogs for Microgravity Exercise Simulation

Maintaining health and fitness in crewmembers during space missions is essential for preserving performance for mission-critical tasks. NASA's Exercise Countermeasures Project (ECP) provides space exploration exercise hardware and monitoring requirements that lead to devices that are reliable, meet medical, vehicle, and habitat constraints, and use minimal vehicle and crew resources. ECP will also develop and validate efficient exercise prescriptions that minimize daily time needed for completion of exercise yet maximize performance for mission activities. In meeting these mission goals, NASA Glenn Research Center (Cleveland, OH, USA), in collaboration with the Cleveland Clinic (Cleveland, Ohio, USA), has developed a suite of zero-gravity locomotion simulators and associated technologies to address the need for ground-based test analog capability for simulating in-flight (microgravity) and surface (partial-gravity) exercise to advance the health and safety of astronaut crews and the next generation of space explorers. Various research areas can be explored. These include improving crew comfort during exercise, and understanding joint kinematics and muscle activation pattern differences relative to external loading mechanisms. In addition, exercise protocol and hardware optimization can be investigated, along with characterizing system dynamic response and the physiological demand associated with advanced exercise device concepts and performance of critical mission tasks for Exploration class missions. Three zero-gravity locomotion simulators are currently in use and the research focus for each will be presented. All of the devices are based on a supine subject suspension system, which simulates a reduced gravity environment by completely or partially offloading the weight of the exercising test subject s body. A platform for mounting treadmill is positioned perpendicularly to the test subject. The Cleveland Clinic Zero-g Locomotion Simulator (ZLS) utilizes a pneumatic subject load device to apply a near constant gravity-replacement load to the test subject during exercise, and is currently used in conjunction with the General Clinical Research Center for evaluating exercise protocols using a bedrest analog. The enhanced ZLS (eZLS) at NASA Glenn Research Center features an offloaded treadmill that floats on a thin film of air and interfaces to a force reaction frame via variably-compliant isolators, or vibration isolation system. The isolators can be configured to simulate compliant interfaces to the vehicle, which affects mechanical loading to crewmembers during exercise, and has been used to validate system dynamic models for new countermeasures equipment designs, such as the second International Space Station treadmill slated for use in 2010. In the eZLS, the test subject and exercise device can be pitched at the appropriate angle for partial gravity simulations, such as lunar gravity (1/6th earth gravity). On both the eZLS and the NASA-Johnson Space Center standalone ZLS installed at the University of Texas Medical Branch in Galveston, Texas, USA, the subject's body weight relative to the treadmill is controlled via a linear motor subject load device (LM-SLD). The LM-SLD employs a force-feedback closed-loop control system to provide a relatively constant force to the test subject during locomotion, and is set and verified for subject safety prior to each session. Locomotion data were collected during parabolic flight and on the eZLS. The purpose was to determine the similarities and differences between locomotion in actual and simulated microgravity. Subjects attained greater amounts of hip flexion during walking and running during parabolic flight. During running, subjects had greater hip range of motion. Trunk motion was significantly less on the eZLS than during parabolic flight. Peak impact forces, loading rate, and impulse were greater on the eZLS than during parabolic while walking with a low external load (EL) and rning with a high EL. Activation timing differences existed between locations in all muscles except for the rectus femoris. The tibialis anterior and gluteus maximus were active for longer durations on the eZLS than in parabolic flight during walking. Ground reaction forces were greater with the LM-SLD than with bungees during eZLS locomotion. While the eZLS serves as a ground-based analog, researchers should be aware that subtle, but measurable, differences in kinematics and leg musculature activities exist between the environments. Aside from space applications, zero-gravity locomotion simulators may help medical researchers in the future with development of rehabilitative or therapeutic protocols for injured or ill patients. Zero-gravity locomotion simulators may be used as a ground-based test bed to support future missions for space exploration, and eventually may be used to simulate planetary locomotion in partial gravity environments, including the Moon and Mars. Figure: Zero-gravity Locomotion Simulator at the Cleveland Clinic, Cleveland, Ohio, USA

Perusek, Gail P.↗

NASA NTRS1994

Correlation of finite element free vibration predictions using random vibration test data

Finite element analysis is regularly used during the engineering cycle of mechanical systems to predict the response to static, thermal, and dynamic loads. The finite element model (FEM) used to represent the system is often correlated with physical test results to determine the validity of analytical results provided. Results from dynamic testing provide one means for performing this correlation. One of the most common methods of measuring accuracy is by classical modal testing, whereby vibratory mode shapes are compared to mode shapes provided by finite element analysis. The degree of correlation between the test and analytical mode shapes can be shown mathematically using the cross orthogonality check. A great deal of time and effort can be exhausted in generating the set of test acquired mode shapes needed for the cross orthogonality check. In most situations response data from vibration tests are digitally processed to generate the mode shapes from a combination of modal parameters, forcing functions, and recorded response data. An alternate method is proposed in which the same correlation of analytical and test acquired mode shapes can be achieved without conducting the modal survey. Instead a procedure is detailed in which a minimum of test information, specifically the acceleration response data from a random vibration test, is used to generate a set of equivalent local accelerations to be applied to the reduced analytical model at discrete points corresponding to the test measurement locations. The static solution of the analytical model then produces a set of deformations that once normalized can be used to represent the test acquired mode shapes in the cross orthogonality relation. The method proposed has been shown to provide accurate results for both a simple analytical model as well as a complex space flight structure.

Chambers, Jeffrey A.↗

NASA NTRS1982

Unsteady flow phenomena in industrial centrifugal compressor stage

The results of an experimental investigation on a typical centrifugal compressor stage running on an atmospheric pressure test rig are shown. Unsteady flow was invariably observed at low flow well before surge. In order to determine the influence of the statoric components, the same impeller was repeatedly tested with the same vaneless diffuser, but varying return channel geometry. Experimental results show the strong effect exerted by the return channel, both on onset and on the behavior of unsteady flow. Observed phenomena have been found to confirm well the observed dynamic behavior of full load tested machines when gas density is high enough to cause appreciable mechanical vibrations. Therefore, testing of single stages at atmospheric pressure may provide a fairly accurate prediction of this kind of aerodynamic excitation.

Bonciani, L.↗

NASA NTRS1995

Characterization of Two-Dimensionally Braided Composites Subject to Static and Fatigue Loading

This paper presents a characterization of damage processes in four different two-dimensionally braided architectures consisting of graphite fibers in an epoxy matrix, subjected to both static and fatigue loading. Both notched and unnotched specimens were tested from each architecture. Damage mechanisms in static compression testing were found to range from matrix cracking in an architecture controlled by shearing characteristics of the matrix to axial fiber bundle kinking and splitting in architectures controlled by unidirectional fiber bundles. In both static tension and tension-tension fatigue testing, it was found that all architectures sustained the same basic damage types and progression sequences. The first types of damage to occur were splits in the braider bundles and cracks in the resin rich areas. Once a sufficient density of this type of damage had occurred, the axial bundles began to disbond from the surrounding constituents, and continued to do so until ultimate failure. One architecture sustained an additional mode of failure, which involved splitting of the axial bundles along their length. Varied degrees of notch sensitivity were found among the different architectures. In one architecture, this was attributed to differences found in the stacking of the individual plys of the braided architecture. As the density of axial bundles adjacent to the notch increased, the rate of stiffness loss increased and the life decreased.

Burr, Scott T.↗

NASA NTRS2018

Development of the Multi Purpose Transportation System for the Space Launch System (SLS) Core Stage (CS) Flight Article

NASA’s Multi Purpose Transportation System (MPTS) is designed to transport the Space Launch System vehicle segments by waterway and roadway. It is tasked with transporting the vehicle from where it is manufactured to its intermediate test location and final launch destination. Its design incorporates mechanisms that release degrees of freedom to prevent excessive loading during transit and ensure a successful delivery of the vehicle to its intended destination. In addition to the CS (Core Stage) flight article, the system will also move three structural test articles and a simulated CS Pathfinder (weight, center of gravity, outer mold line dimensions, and overall length) over road terrain on four NASA centers and on the Pegasus barge. The MPTS independently supports the article at both ends while communicating as a combined unit through automated monitoring of its released degree of freedom and corrective responses. This allows the system to constrain its payload in a statically determinate manner while traversing across highly variable terrain. Multi-body simulation of the transportation route is useful to predict free-body motion within the specified travel ranges. The MPTS has completed its design and analysis developmental cycles. This unpublished paper will describe the design challenges encountered in developing this system of large scale structure which incorporates complex mechanisms. The unique techniques and methodologies developed for analytical assessment of the hardware will be also be discussed. Key results from analytical evaluations as well as ongoing performance testing will be presented.

Sandridge, Sarah↗

NASA NTRS2018

Development of the Multi-Purpose Transportation System for the Space Launch System (SLS) Core Stage (CS) Flight Article

NASA's Multi-Purpose Transportation System (MPTS) is designed to transport the Space Launch System (SLS) vehicle segments by waterway and roadway. It is tasked with transporting the vehicle from where it is manufactured to its intermediate test location and final launch destination. Its design incorporates mechanisms that release degrees of freedom to prevent excessive loading during transit and ensure a successful delivery of the vehicle to its intended destination. In addition to the Core Stage (CS) flight article, the system will also move three Structural Test Articles (STAs), the Dynamic Demonstration Unit (DDU), and a simulated CS Pathfinder (weight, center of gravity, outer mold line dimensions, and overall length) over road terrain at four NASA centers and on the Pegasus barge. The MPTS independently supports the article at both ends while operating as a combined unit through automated monitoring of its released degree of freedom and corrective responses. This allows the system to constrain its payload in a statically determinate manner while traversing highly variable terrain. Multi-body simulation of the transportation route is useful to predict free-body motion within its range of mobility. The MPTS has completed its design and analysis developmental cycles. This paper describes the design challenges encountered in developing this system of large-scale structure, which incorporates complex mechanisms. The unique techniques and methodologies developed for analytical assessment of the hardware are also discussed.

Sandridge, Sarah↗

NASA NTRS1973

Nonlinear response of boron/aluminum angleplied laminates under cyclic tensile loading: Contributing mechanisms and their effects

The nonlinear response of boron/aluminum angleplied laminates subjected to cyclic loads was investigated. A procedure is outlined and criteria are proposed which can be used to assess the nonlinear response. The procedure consists of testing strategically selected laminate configurations and analyzing the results using composite mechanics. Results from the investigation show the contributions to nonlinear behavior are from: premature random fiber breaks where the ply orientation angle is small relative to the load direction, ply relative rotation at intermediate values of the ply orientation angle, and nonlinear aluminum matrix behavior at large values of the orientation angle. Premature fiber breaks result in progressively more compliant material; large ply relative rotations result in progressively stiffer material; and pronounced matrix nonlinear behavior results in no significant change in the stiffness of the initial load portion.

Chamis, C. C.↗

NASA NTRS1973

Nonlinear response of boron/aluminum angleplied laminates under cyclic tensile loading - Contributing mechanisms and their effects

Chamis, C. C.↗

NASA NTRSDate not supplied

Initiation and Propagation of Skin/Stiffener Separation in Postbuckled Structures Subjected to Cyclic Loads

A methodology is proposed to evaluate experimentally and numerically the initiation and propagation of skin/stiffener separation in postbuckled panels subjected to cyclic loads. The response mechanisms that characterize skin/stiffener separation were investigated using a three point bend (3PB) test specimen, which is a simple specimen consisting of a laminate representing the skin and a doubler representing the stiffener flange. The 3PB tests were performed under quasi-static and cyclic loads. A fatigue cohesive model was used to analyze the initiation and propagation of skin/stiffener separation in the 3PB. The results indicate that the fatigue cohesive model with model parameters obtained from quasi-static fracture tests can provide accurate predictions of initiation from a pristine state, and the rate of fatigue crack propagation for a variety of mode mixities, load levels, and stress ratios.

Carlos G Davila↗