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At least 109 records · Page 6

Detection of Delamination in Composite Beams Using Broadband Acoustic Emission Signatures

Delamination in composite structure may be caused by imperfections introduced during the manufacturing process or by impact loads by foreign objects during the operational life. There are some nondestructive evaluation methods to detect delamination in composite structures such as x-radiography, ultrasonic testing, and thermal/infrared inspection. These methods are expensive and hard to use for on line detection. Acoustic emission testing can monitor the material under test even under the presence of noise generated under load. It has been used extensively in proof-testing of fiberglass pressure vessels and beams. In the present work, experimental studies are conducted to investigate the use of broadband acoustic emission signatures to detect delaminations in composite beams. Glass/epoxy beam specimens with full width, prescribed delamination sizes of 2 inches and 4 inches are investigated. The prescribed delamination is produced by inserting Teflon film between laminae during the fabrication of composite laminate. The objectives of this research is to develop a method for predicting delamination size and location in laminated composite beams by combining smart materials concept and broadband AE analysis techniques. More specifically, a piezoceramic (PZT) patch is bonded on the surface of composite beams and used as a pulser. The piezoceramic patch simulates the AE wave source as a 3 cycles, 50KHz, burst sine wave. One broadband AE sensor is fixed near the PZT patch to measure the AE wave near the AE source. A second broadband AE sensor, which is used as a receiver, is scanned along the composite beams at 0.25 inch step to measure propagation of AE wave along the composite beams. The acquired AE waveform is digitized and processed. Signal strength, signal energy, cross-correlation of AE waveforms, and tracking of specific cycle of AE waveforms are used to detect delamination size and location.

Okafor, A. C.↗

Use of an Ultrasonic/Sonic Driller/Corer to Obtain Sample Powder for CHEMIN, a Combined XRD/XRF Instrument

A miniature CHEMIN XRD/XRF (X-Ray Diffraction/X-Ray Fluourescence) instrument is currently being developed for definitive mineralogic analysis of soils and rocks on Mars. One of the technical issues that must be addressed in order to enable XRD analysis on an extraterrestrial body is how best to obtain a representative sample powder for analysis. For XRD powder diffraction analyses, it is beneficial to have a fine-grained sample to reduce preferred orientation effects and to provide a statistically significant number of crystallites to the X-ray beam. Although a 2-dimensional detector as used in the CHEMIN instrument will produce good results with poorly prepared powders, the quality of the data will improve if the sample is fine-grained and randomly oriented. An Ultrasonic/Sonic Driller/Corer (USDC) currently being developed at JPL is an effective mechanism of sampling rock to produce cores and powdered cuttings. It requires low axial load (< 5N) and thus offers significant advantages for operation from lightweight platforms and in low gravity environments. The USDC is lightweight (<0.5kg), and can be driven at low power (<5W) using duty cycling. It consists of an actuator with a piezoelectric stack, ultrasonic horn, free-mass, and drill bit. The stack is driven with a 20 kHz AC voltage at resonance. The strain generated by the piezoelectric is amplified by the horn by a factor of up to 10 times the displacement amplitude. The tip impacts the free-mass and drives it into the drill bit in a hammering action. The free-mass rebounds to interact with the horn tip leading to a cyclic rebound at frequencies in the range of 60-1000 Hz. It does not require lubricants, drilling fluid or bit sharpening and it has the potential to operate at high and low temperatures using a suitable choice of piezoelectric material. To assess whether the powder from an ultrasonic drill would be adequate for analyses by an XRD/XRF spectrometer such as CHEMIN, powders obtained from the JPL ultrasonic drill were analyzed and the results were compared to carefully prepared powders obtained using a laboratory bench scale Retsch mill.

Chipera, S. J.↗

Assessment of impact damage of composite rocket motor cases

This contract reviewed the available literature on mechanisms of low velocity impact damage in filament wound rocket motor cases, MDE methods to quantify damage, critical coupon level test methods, manufacturing and material process variables and empirical and analytical modeling off impact damage. The critical design properties for rocket motor cases are biaxial hoop and axial tensile strength. Low velocity impact damage is insidious because it can create serious nonvisible damage at very low impact velocities. In thick rocket motor cases the prevalent low velocity impact damage is fiber fracture and matrix cracking adjacent to the front face. In contrast, low velocity loading of thin wall cylinders induces flexure, depending on span length and the flexure induces delamination and tensile cracking on the back face wall opposed to impact occurs due to flexural stresses imposed by impact loading. Important NDE methods for rocket motor cases are non-contacting methods that allow inspection from one side. Among these are vibrothermography, and pulse-echo methods based on acoustic-ultrasonic methods. High resolution techniques such as x-ray computed tomography appear to have merit for accurate geometrical characterization of local damage to support development of analytical models of micromechanics. The challenge of coupon level testing is to reproduce the biaxial stress state that the full scale article experiences, and to determine how to scale the composite structure to model full sized behavior. Biaxial tensile testing has been performed by uniaxially tensile loading internally pressurized cylinders. This is experimentally difficult due to gripping problems and pressure containment. Much prior work focused on uniaxial tensile testing of model filament wound cylinders. Interpretation of the results of some studies is complicated by the fact that the fabrication process did not duplicate full scale manufacturing. It is difficult to scale results from testing subscale cylinders since there are significant differences in out time of the resins relative to full scale cylinder fabrication, differences in hoop fiber tensioning and unsatisfactory coupon configurations. It appears that development of a new test method for subscale cylinders is merited. Damage tolerance may be improved by material optimization that uses fiber treatments and matrix modifications to control the fiber matrix interface bonding. It is difficult to develop process optimization in subscale cylinders without also modeling the longer out times resins experience in full scale testing. A major breakthrough in characterizing the effect of impact damage on residual strength, and understanding how to scale results of subscale evaluations, will be a sound micromechanical model that described progressive failure of the composite. Such models will utilize a three dimensional stress analysis due to the complex nature of low velocity impact stresses in thick composites. When these models are coupled with non-contact NDE methods that geometrically characterize the damage and acoustic methods that characterize the effective local elastic properties, accurate assessment of residual strength from impact damage may be possible. Directions for further development are suggested.

Paris, Henry G.↗

Free-Mass and Interface Configurations of Hammering Mechanisms

The design of the free-mass in an ultrasonic driller/corer (USDC) has been refined in order to improve the performance and operational reliability of the system. In one embodiment, the improvements in performance and operational reliability include decreasing the impact surface area of the free-mass to increase the transfer of impact energy from the piezoelectric transducer and reductions in the likelihood that the system will jam.

Bao, Xiaoqi↗

Characterization of Low-Velocity Impact Damage in Thermoplastic Laminated Composites

The impact damage response of two thermoplastic material systems were compared over a range of impact energies. Eight-inch-square impact specimens with a 24-ply quasi-isotropic layup were manufactured from TC1225 LMPAEK T700G from Toray Industries and APC AS4D/PEKK-FC from Solvay. The effect of the degree of crystallinity (DOC) on the damage response was also investigated using a third set of specimens from the Toray material manufactured with reduced crystallinity. Using a combination of data obtained from X-ray computed tomography (CT) scans and ultrasonic testing (UT), detailed damage maps were created for every interface and ply for selected specimens. Using these damage maps, the delamination size, shape, and area were evaluated at each interface. The presence of fractured fibers was also identified on each ply. The LMPAEK material was found to have fewer and smaller delaminations than the PEKK specimens for impacts at the same energy. However, the LMPAEK specimens contained a larger number of fiber breaks. A similar trend was observed when comparing the baseline LMPAEK specimens with the low-crystallinity LMPAEK specimens. The low-crystallinity specimens had very little delamination but had a large number of plies with fiber breaks. In addition, the LMPAEK specimens often contained lines of fiber fractures in near-surface plies emanating from the contact region that were not present in the PEKK specimens.

Composite Materials↗

Characterization of Low-Velocity Impact Damage in Thermoplastic Laminated Composites

The impact damage response of two thermoplastic material systems were compared over a range of impact energies. Eight-inch-square impact specimens with a 24-ply quasi-isotropic layup were manufactured from TC1225 LMPAEK T700G from Toray Industries and APC AS4D/PEKK-FC from Solvay. The effect of the degree of crystallinity (DOC) on the damage response was also investigated using a third set of specimens from the Toray material manufactured with reduced crystallinity. Using a combination of data obtained from X-ray computed tomography (CT) scans and ultrasonic testing (UT), detailed damage maps were created for every interface and ply for selected specimens. Using these damage maps, the delamination size, shape, and area were evaluated at each interface. The presence of fractured fibers was also identified on each ply. The LMPAEK material was found to have fewer and smaller delaminations than the PEKK specimens for impacts at the same energy. However, the LMPAEK specimens contained a larger number of fiber breaks. A similar trend was observed when comparing the baseline LMPAEK specimens with the low-crystallinity LMPAEK specimens. The low-crystallinity specimens had very little delamination but had a large number of plies with fiber breaks. In addition, the LMPAEK specimens often contained lines of fiber fractures in near-surface plies emanating from the contact region that were not present in the PEKK specimens.

Composite Materials↗

Manufacturing and Mechanical Testing of TC1225/LM-PAEK and TC1200/PEEK Thermoplastic Composite Panels

Development of thermoplastic composites (TPCs) for aerospace structures is experiencing renewed enthusiasm attributed to the availability of rapid manufacturing technology, ease of joining through fusion welding processes, and the successful utilization of the material in flight critical structures. Semicrystalline thermoplastics such as polyether ether ketone (PEEK), polyether ketone ketone (PEKK), and polyphenylene sulfide (PPS) have been successfully demonstrated as thermoplastic skins, keel beams, and angle brackets for commercial aircraft. Composite specimens were fabricated from the Toray TC1225 and TC1200 unidirectional prepreg systems and tested to gain confidence with NASA in-house TPC processing as part of the Thermoplastics Development for Exploration Application (TDEA) project. TC1225 is a relatively new material system that uses T700GC fibers and low-melt polyaryl ether ketone (LM-PAEK) resin. TC1200 is the latest version of a material system that has been available for several decades with AS4 fibers and PEEK resin. Results for ultrasonic inspection, optical microscopy, acid digestion, and differential scanning calorimetry establish the quality of the panels. Mechanical test data for unnotched and notched laminate tension and compression as well as compression after impact provide data for equivalency assessment and support material selection in TDEA. Satisfactory equivalence of the TC1225 results and the National Center for Advanced Materials Performance (NCAMP) database was found. The results indicate high-quality manufacturing was achieved, and the resulting mechanical test data are in agreement with that in the literature.

Thermoplastic Composites↗

Characterization of Low-Velocity Impact Damage in Toughened Non-Crimp Fabric Laminates

The damage response of resin-infused composites was investigated using quasi-static indentation (QSI) and instrumented drop-weight impact testing. The test specimens were composed of six layers of non-crimp fabrics where each layer contained +45°, 90°, −45°, and 0° plies. The goals of the testing were to identify damage responses that may differ from traditional thermoset materials and provide damage characterization data to evaluate current damage prediction analyses and aid improvements to current damage models. The damage was characterized using X-ray computed tomography (CT) and ultrasonic testing (UT). Incremental QSI loading was used on a single specimen to investigate damage growth in nine steps at progressively higher loads. Realtime acoustic emission data were used to determine the unloading points. Unlike typical thermoset composites, the force-displacement was smooth and showed no obvious indications of damage. The primary damage consisted of large lines of fiber fracture on the plies near the impact surface and delaminations through the thickness. Nine specimens were impacted at nominal impact energies of 25 ft-lbs, 30 ft-lbs, and 35 ft-lbs. The force histories at each energy followed nearly identical paths. Again, no obvious signs of damage were observed in any of the force histories. The general damage response in the dynamic tests was similar to the QSI-loaded specimens. However, repeat specimens impacted with a given impact energy showed variations in the damage. Some characteristics of the damage response were different than those typically observed in thermoset composites and will need to be evaluated using current analysis methods.

Composite Materials↗

Characterization of Low-Velocity Impact Damage in Toughened Non-Crimp Fabric Laminates

The damage response of resin-infused composites was investigated using quasi-static indentation (QSI) and instrumented drop-weight impact testing. The test specimens were composed of six layers of non-crimp fabrics where each layer contained +45°, 90°, −45°, and 0° plies. The goals of the testing were to identify damage responses that may differ from traditional thermoset materials and provide damage characterization data to evaluate current damage prediction analyses and aid improvements to current damage models. The damage was characterized using X-ray computed tomography (CT) and ultrasonic testing (UT). Incremental QSI loading was used on a single specimen to investigate damage growth in nine steps at progressively higher loads. Realtime acoustic emission data were used to determine the unloading points. Unlike typical thermoset composites, the force-displacement was smooth and showed no obvious indications of damage. The primary damage consisted of large lines of fiber fracture on the plies near the impact surface and delaminations through the thickness. Nine specimens were impacted at nominal impact energies of 25 ft-lbs, 30 ft-lbs, and 35 ft-lbs. The force histories at each energy followed nearly identical paths. Again, no obvious signs of damage were observed in any of the force histories. The general damage response in the dynamic tests was similar to the QSI-loaded specimens. However, repeat specimens impacted with a given impact energy showed variations in the damage. Some characteristics of the damage response were different than those typically observed in thermoset composites and will need to be evaluated using current analysis methods.

Composite Materials↗

Effects of specimen resonances on acoustic-ultrasonic testing

The effects of specimen resonances on acoustic ultrasonic (AU) nondestructive testing were investigated. Selected resonant frequencies and the corresponding normal mode nodal patterns of the aluminum block are measured up to 75.64 kHz. Prominent peaks in the pencil lead fracture and sphere impact spectra from the two transducer locations corresponded exactly to resonant frequencies of the block. It is established that the resonant frequencies of the block dominated the spectral content of the output signal. The spectral content of the output signals is further influenced by the transducer location relative to the resonant frequency nodal lines. Implications of the results are discussed in relation to AU parameters and measurements.

Williams, J. H., Jr.↗

Fatigue Life and Damage Tolerance of Postbuckled Composite Stiffened Structures with Indentation Damage

The fatigue life and damage tolerance of composite stiffened panels with indentation damage are investigated experimentally using single-stringer compression specimens. The indentation damage was induced to one of the two flanges of the stringer of every panel. The advantages of indentation compared to impact are the simplicity of application, less dependence on boundary conditions, better controllability, and repeatability of the imparted damage. The tests were conducted using advanced instrumentation, including digital image correlation, passive thermography, and in situ ultrasonic scanning. Specimens with initial indentation damage ranging between 32 and 56 mm in length were tested quasi-statically and in fatigue, and the effects of cyclic load amplitude and damage size were studied. A means of comparison of the damage propagation rates and collapse loads based on a stress intensity measure and the Paris law is proposed. The stress intensity measure provides the means to compare the collapse loads of specimens with different damage types and damage sizes, while the Paris law is used to compare the damage propagation rates in specimens subjected to different cyclic loads. This approach enables a comparison of different tests and the potential identification of the effects that influence the fatigue lives and damage tolerance of postbuckled structures with defects.

Davila, Carlos G.↗

Damage assessment and residual compression strength of thick composite plates with through-the-thickness reinforcements

Damage in composite materials was studied with through-the-thickness reinforcements. As a first step it was necessary to develop new ultrasonic imaging technology to better assess internal damage of the composite. A useful ultrasonic imaging technique was successfully developed to assess the internal damage of composite panels. The ultrasonic technique accurately determines the size of the internal damage. It was found that the ultrasonic imaging technique was better able to assess the damage in composite panel with through-the-thickness reinforcements than by destructively sectioning the specimen and visual inspection under a microscope. Five composite compression-after-impact panels were tested. The compression-after-impact strength of the panels with the through-the-thickness reinforcements was almost twice that of the comparable panel without through-the-thickness reinforcement.

Smith, Barry T.↗

Damage assessment and residual compression strength of thick composite plates with through-the-thickness reinforcements

Damage in composite materials was studied with through-the-thickness reinforcements. As a first step it was necessary to develop new ultrasonic imaging technology to better assess internal damage of the composite. A useful ultrasonic imaging technique was successfully developed to assess the internal damage of composite panels. The ultrasonic technique accurately determines the size of the internal damage. It was found that the ultrasonic imaging technique was better able to assess the damage in a composite panel with through-the-thickness reinforcements than by destructively sectioning the specimen and visual inspection under a microscope. Five composite compression-after-impact panels were tested. The compression-after-impact strength of the panels with the through-the-thickness reinforcements was almost twice that of the comparable panel without through-the-thickness reinforcement.

Smith, Barry T.↗

A Module Experimental Process System Development Unit (MEPSDU)

Subsequent to the design review, a series of tests was conducted on simulated modules to demonstrate that all environmental specifications (wind loading, hailstone impact, thermal cycling, and humidity cycling) are satisfied by the design. All tests, except hailstone impact, were successfully completed. The assembly sequence was simplified by virtue of eliminating the frame components and assembly steps. Performance was improved by reducing the module edge border required to accommodate the frame of the preliminary design module. An ultrasonic rolling spot bonding technique was selected for use in the machine to perform the aluminum interconnect to cell metallization electrical joints required in the MEPSDU module configuration. This selection was based on extensive experimental tests and economic analyses.

Source record↗

Research Developments in Nondestructive Evaluation and Structural Health Monitoring for the Sustainment of Composite Aerospace Structures at NASA

The use of composite materials continues to increase in the aerospace community due to the potential benefits of reduced weight, increased strength, and manufacturability. Ongoing work at NASA involves the use of the large-scale composite structures for spacecraft (payload shrouds, cryotanks, crew modules, etc). NASA is also working to enable both the use and sustainment of composites in commercial aircraft structures. One key to the sustainment of these large composite structures is the rapid, in-situ characterization of a wide range of potential defects that may occur during the vehicle's life. Additionally, in many applications it is necessary to monitor changes in these materials over their lifetime. Quantitative characterization through Nondestructive Evaluation (NDE) of defects such as reduced bond strength, microcracking, and delamination damage due to impact, are of particular interest. This paper will present an overview of NASA's applications of NDE technologies being developed for the characterization and sustainment of advanced aerospace composites. The approaches presented include investigation of conventional, guided wave, and phase sensitive ultrasonic methods and infrared thermography techniques for NDE. Finally, the use of simulation tools for optimizing and validating these techniques will also be discussed.

Cramer, K. Elliott↗

Recent advances in nondestructive evaluation made possible by novel uses of video systems

Complex materials are being developed for use in future advanced aerospace systems. High temperature materials have been targeted as a major area of materials development. The development of composites consisting of ceramic matrix and ceramic fibers or whiskers is currently being aggressively pursued internationally. These new advanced materials are difficult and costly to produce; however, their low density and high operating temperature range are needed for the next generation of advanced aerospace systems. These materials represent a challenge to the nondestructive evaluation community. Video imaging techniques not only enhance the nondestructive evaluation, but they are also required for proper evaluation of these advanced materials. Specific research examples are given, highlighting the impact that video systems have had on the nondestructive evaluation of ceramics. An image processing technique for computerized determination of grain and pore size distribution functions from microstructural images is discussed. The uses of video and computer systems for displaying, evaluating, and interpreting ultrasonic image data are presented.

Generazio, Edward R.↗

Free-Mass and Interface Configurations of Hammering Mechanisms

A series of free-mass designs for the ultrasonic/ sonic driller/corer (USDC) has been developed to maximize the transfer of energy from the piezoelectric transducer through the horn to the bit, as well as to minimize potential jamming. A systematic development was made producing novel designs of free-mass configurations where the impact force is spread across a minimal area maximizing the impact on the bit. The designed free masses were made to operate at high temperatures (500 C) as on Venus, and they can be made to operate at extremely low temperature, too. In normal operation, the free mass bounces between the horn and the bit, impacting both repeatedly. The impact stress profile, maximum stress, contact time duration, and the required yielding stress for the materials of the free mass, bit, and horn are all affected by the contact area. A larger contact area results in lower stress in the contact region, and avoids yielding of the materials. However, before the excitation voltage is applied to the transducer, the horn, free mass, and the bit are pressed together. Larger contact area results in a stronger coupling of the bit to the horn transducer, which greatly changes the vibration characteristics of the transducer, and makes the USDC difficult to start. To obtain optimum performance, a catalog of free-mass designs is required, allowing maximum flexibility during trade-off for these conflicting contact area requirements. For this purpose, seven different designs were conceived: point contacts, circular contacts, point/circular contacts, line contacts, ring contacts, line/ring contacts, and dashed line contacts. Besides point/circular and line/ring contacts, the free mass can be designed as any of the above shapes. Depending on the ratio of the diameter to the height, and the free-mass retention method used (the cup or rod), the free mass can be configured with one or more sliding surfaces on the outside or inside diameter surface or both. Matching horn tips and free mass may also offer some utility in maximizing the stress pulse.

Bao, Xiaoqi↗

Engineering the Interface: Advanced Surface Technologies for Lunar Dust Management and Equipment Longevity

Through the Artemis program, NASA intends to develop a sustainable human foothold on the Moon, ultimately paving the way for crewed exploration of Mars. The Moon's hostile environment poses numerous obstacles, including exposure to radiation, temperature extremes, micrometeoroid threats, and particularly the persistent problem of lunar dust. Lunar dust impacts nearly every aspect of surface operations through adhesion and abrasion mechanisms, with contamination from anthropogenic activities (landing, rovers) far outweighing natural phenomena. Multiple adhesion pathways contribute to surface contamination in the lunar environment, including van der Waals forces, electrostatic forces, chemical reaction, and magnetic forces from elemental iron deposits. Sharp asperities from micrometeoroid bombardment and atmospheric absence increase interaction potential and enable mechanical interlocking. Low cohesion between dust particles exacerbates these challenges, as minimal interaction potential between dust and nearby surfaces overcomes particle cohesion, causing contamination. Lunar dust adhesion mitigation technologies can be categorized as either active, requiring external energy, or passive, relying on intrinsic material properties. Ultrasonic and electrodynamic technologies have been developed to the highest technology readiness level for active approaches. Passive strategies primarily focus on surface chemistry and topography modifications. At NASA Langley Research Center, approaches include surface migration agents to reduce surface energy, topographical modification using laser ablation patterning, and tailored surface conductivity to reduce intrinsic adhesion force. Performance has been evaluated using custom-built ultrasonic and centrifuge instruments. Plume-surface interactions from lunar landers can propel micrometer-sized particles at velocities up to 1000 m s-1.8 These particles pose risks to landers, habitats and infrastructure, leading to erosion, degradation, and reduced component lifespan. A panel recovered from Surveyor III was determined to have been severely abraded because of lunar dust displaced from the Apollo 12 lunar module that landed 160 m away. The performance of metallic surfaces has been evaluated via high velocity single particle impact using the laser-induced project impact test (LIPIT) facility at the University of Utah. Peridynamics modeling, a form of continuum mechanics that uses a nonlocal approach enabling greater simulation capabilities of crack initiation and fracture, has also been utilized to gain greater insight into material response during impact events. Lunar dust contamination challenges extend to power generation systems and moving equipment. Cables, rotation stages, and other mechanisms may experience limited range of motion and reduced lifetime due to dust infiltration. NASA Langley Research Center has evaluated traditional aerospace alloys, softgoods, wear resistant ceramics, and several polymer and polymer composite materials. Test methods have included traditional techniques like Taber abrasion testing, as well as designed test configurations developed in the DUSTE (dust, ultraviolet radiation, and space thermal environmental) chamber that reproduce mechanism functions in operational environment. Beyond laboratory experiments, several flight experiments have been conducted. Materials were exposed to the low Earth orbit environment on the Materials International Space Station Experiment (MISSE) and to the lunar surface environment through the Aegis Aerospace Regolith Adherence Characterization (RAC) payload and the Honeybee Robotics PlanetVac payload. Determining lunar dust's impact on surface exploration and habitation requires comprehensive experimental and computational capabilities combined with lessons learned from initial lunar activities. Identifying the greatest environmental challenges and developing mitigation technologies provides the clearest path toward successfully, expeditiously, and efficaciously completing NASA's mission. This presentation will discuss ongoing efforts at NASA Langley Research Center and collaborator contributions to these critical objectives.

Surface Engineering↗