SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
In this paper we use a photothermal imaging technique to characterize the damage caused to an imperfectly coated gold-coated Kapton sample exposed to successively increased fluences of atomic oxygen in a laboratory atomic source.
Detection of damage due to foreign object impact is an important factor in the development of new aerospace vehicles. Acoustic waves generated on impact can be detected using a set of piezoelectric transducers, and the location of impact can be determined by triangulation based on the differences in the arrival time of the waves at each of the sensors. These sensors generate electrical signals in response to mechanical motion resulting from the impact as well as from natural vibrations. Due to electrical noise and mechanical vibration, accurately determining these time differentials can be challenging, and even small measurement inaccuracies can lead to significant errors in the computed damage location. Wavelet transforms are used to analyze the signals at multiple levels of detail, allowing the signals resulting from the impact to be isolated from ambient electromechanical noise. Data extracted from these transformed signals are input to an artificial neural network to aid in identifying the moment of impact from the transformed signals. By distinguishing which of the signal components are resultant from the impact and which are characteristic of noise and normal aerodynamic loads, the time differentials as well as the location of damage can be accurately assessed. The combination of wavelet transformations and neural network processing results in an efficient and accurate approach for passive in-flight detection of foreign object damage.
Displacement damage effects from protons and neutrons are compared. Differences in the annealing properties were observed which are due to the different microscopic structure of the damaged regions from the two ty pes of particles.
Damage diagnosis in critical components is essential for ensuring the safety and reliability of operations across industries, spanning manufacturing, aerospace, and energy. Traditional acoustic nondestructive testing methods primarily focus on detecting defects through the direct scattering of single-mode incident waves from the damage, which limit their applicability to simple structures and small inspection areas. Our earlier research demonstrated that machine learning algorithms combined with sparse sensor networks can identify critical defect signatures even from multiply scattered, multi-mode acoustic signals, indicating the potential for improved defect inspection in complex, real-world structures. In this work, we demonstrate the successful implementation of this approach in a fixed sensor configuration to rapidly and accurately detect simulated defects in a geometrically complex, real-world structure, a brake rotor hub. Three different types of defects were physically simulated on the surface of the hub, and the collected data were used to train an autoencoder-based deep learning model. Two models were tested, one using single measurements and the other using multiple measurements taking advantage of the spatial distribution of the sensor network. After training, the multi-measurement model achieved 100 % accuracy in identifying, classifying, and locating unseen, unique damages. This work illustrates the potential of the proposed method for a wide range of industrial applications.
The mechanical response of a component is affected by defects, such as porosity, arising from the laser powder bed fusion (LPBF) fabrication process. Thus, it is important to develop accurate and efficient inspection methods for identifying porosity. In this work, porosity identified in an X-ray computed tomography (XCT) volume of a Ti-5553 coupon was compared to pores identified in a serial sectioned volume that represented the ground truth. The porosity of the XCT scan was identified using contrast-based, ISO-based, and machine learning (ML) methods for segmentation. Large inherent porosity was easy to identify, but the ISO thresholding still struggled due to the intensity gradient resulting from both the beam hardening in XCT and the uneven lighting of the serial sectioning panels. Further, the results show that ML-based methods were better suited for identifying small pores and reducing the amount of false positives. Additionally, high strain-rate impact testing was done on some of the XCT samples as well as post-mortem XCT inspection, and the same suite of segmentation and quantification tools were used to identify the large spallation cavities. The comparison of porosity pre- and post-mortem provides insight on the influence of the LPBF porosity on the formation of spall cavities.
X ray attenuation measurement systems that are capable of characterizing density variations in monolithic ceramics and damage due to processing and/or mechanical testing in ceramic and intermetallic matrix composites are developed and applied. Noninvasive monitoring of damage accumulation and failure sequences in ceramic matrix composites is used during room-temperature tensile testing. This work resulted in the development of a point-scan digital radiography system and an in situ x ray material testing system. The former is used to characterize silicon carbide and silicon nitride specimens, and the latter is used to image the failure behavior of silicon-carbide-fiber-reinforced, reaction-bonded silicon nitride matrix composites. State-of-the-art x ray computed tomography is investigated to determine its capabilities and limitations in characterizing density variations of subscale engine components (e.g., a silicon carbide rotor, a silicon nitride blade, and a silicon-carbide-fiber-reinforced beta titanium matrix rod, rotor, and ring). Microfocus radiography, conventional radiography, scanning acoustic microscopy, and metallography are used to substantiate the x ray computed tomography findings. Point-scan digital radiography is a viable technique for characterizing density variations in monolithic ceramic specimens. But it is very limited and time consuming in characterizing ceramic matrix composites. Precise x ray attenuation measurements, reflecting minute density variations, are achieved by photon counting and by using microcollimators at the source and the detector. X ray computed tomography is found to be a unique x ray attenuation measurement technique capable of providing cross-sectional spatial density information in monolithic ceramics and metal matrix composites. X ray computed tomography is proven to accelerate generic composite component development. Radiographic evaluation before, during, and after loading shows the effect of preexisting volume flaws on the fracture behavior of composites. Results from one-, three-, five-, and eight-ply ceramic composite specimens show that x ray film radiography can monitor damage accumulation during tensile loading. Matrix cracking, fiber-matrix debonding, fiber bridging, and fiber pullout are imaged throughout the tensile loading of the specimens. In situ film radiography is found to be a practical technique for estimating interfacial shear strength between the silicon carbide fibers and the reaction-bonded silicon nitride matrix. It is concluded that pretest, in situ, and post-test x ray imaging can provide greater understanding of ceramic matrix composite mechanical behavior.
A detailed experimental investigation was performed at a single maximum cyclic stress (sigma max) level to physically characterize the progression of thermomechanical fatigue (lW) damage in continuously reinforced (0 deg) SCS-6/Timetal 21S, a titanium matrix composite. In-phase (IP) and out of-phase (OP) loadings were investigated at sigma max = 1000 MPa with a temperature cycle from 150 to 6500 C. Damage progression, in terms of macroscopic property degradation, was experimentally quantified through an advanced TMF test methodology which incorporates explicit measurements of the isothermal static moduli at the TMF temperature extremes and the coefficient of thermal expansion (CTE) as functions of the TMF cycles. Detailed characterization of the physical damage progression at the microstructural level was performed by interrupting multiple TMF tests at various stages of mechanical property degradation and analyzing the microstructure through extensive destructive metallography. Further, the extent of damage was also quantified through residual static strength measurements. Results indicated that damage initiation occurred very early in cyclic life (N less than 0.1Nf) for both the IP and OP TMF loadings. IP TMF damage was found to be dominated by fiber breakage with a physical damage progression in the microstructure which was difficult to quantify. OP TMF loadings produced matrix cracking exclusively associated with surface initiations. Here, damage progression was easily distinguished in terms of both the number of cracks and their relative inward progressions toward the outer fiber rows with increased cycling. The point at which the leading cracks reached the outer fiber rows (when localized fiber/matrix de-bonding and matrix crack bridging occurred) appeared to be reflected in the macroscopic property degradation curves.
The fatigue damage was characterized in specimens which consisted of a tapered composite flange bonded onto a composite skin. Quasi-static tension tests were performed first to determine the failure load. Subsequently, tension fatigue tests were performed at 40%, 50%, 60% and 70% of the failure load to evaluate the debonding mechanisms. For four specimens, the cycling loading was stopped at intervals. Photographs of the polished specimen edges were taken under a light microscope to document the damage. At two diagonally opposite corners of the flange, a delamination appeared to initiate at the flange tip from a matrix crack in the top 45deg skin ply and propagated at the top 45deg/-45deg skin ply interface. At the other two diagonally opposite corners, a delamination running in the bondline initiated from a matrix crack in the adhesive pocket. In addition, two specimens were cut longitudinally into several sections. Micrographs revealed a more complex pattern inside the specimen where the two delamination patterns observed at the edges are present simultaneously across most of the width of the specimen. The observations suggest that a more sophisticated nondestructive evaluation technique is required to capture the complex damage pattern of matrix cracking and multi-level delaminations.
The project had two objectives: 1) The primary objective was to characterize damage tolerance of composite materials. To accomplish this, polymer matrix composites were to be subjected to static indentation as well as low-velocity impacts and the results analyzed. 2) A second objective was to investigate the effects of laser shock peening on the damage tolerance of aerospace materials, such as aluminum alloys, in terms of crack nucleation and crack propagation. The impact testing was proposed to be performed using a Dynatup drop tower. The specimens were to be placed over a square opening in a steel platen and impacted with a hemispherical tup. The damage was to be characterized in the laminate specimens. The damage tolerance of aerospace alloys was to be studied by conducting fatigue tests on aluminum alloy specimens with prior shock peening treatment. The crack length was to be monitored by a microscope and the crack propagation rate, da/dN, determined.
This research is part of a program aimed to evaluate and demonstrate the ability of candidate CMC materials for a variety of applications in reusable launch vehicles. The life and durability of these materials in rocket and engine applications are of major concern and there is a need to develop and validate life prediction methodology. In this study, material characterization and mechanical testing was performed in order to identify the failure modes, degradation mechanisms, and progression of damage in a C/SiC composite at elevated temperatures. The motivation for this work is to provide the relevant damage information that will form the basis for the development of a physically based life prediction methodology.
Explore the source record for details and available documents.
Composite materials remain extremely vulnerable to out-of-plane impact loads, which may lead to severe losses in strength and stiffness. Impact induced damage is often a complex mixture of transverse cracks, delaminations and fiber failures. An experimental investigation was undertaken to quantify damage tolerance and resistance in composite materials impacted using the drop-weight method. Tests were conducted on laminates of several different carbon-fiber composite systems such as epoxies, modified epoxies, and amorphous and semicrystalline thermoplastics. In this paper, impacted composite specimens have been examined using destructive and nondestructive techniques to establish the characteristic damage states. Specifically, optical microscopy, ultrasonic and scanning electron microscopy techniques have been used to identify impact induced damage mechanisms. Damage propagation during post impact compression was also studied.
The purpose was to characterize damage initiation and growth in notched titanium matrix composites at room temperature. Double edge notched or center open hole SCS-6/Ti-15-3 specimens containing 0 deg plies or containing both 0 and 90 deg plies were fatigued. The specimens were tested in the as-fabricated (ASF) and in heat-treated conditions. A local strain criterion using unnotched specimen fatigue data was successful in predicting fatigue damage initiation. The initiation stress level was accurately predicted for both a double edge notched unidirectional specimen and a cross-plied center hole specimen. The fatigue produced long multiple cracks growing from the notches. These fatigue cracks were only in the matrix material and did not break the fibers in their path. The combination of matrix cracking and fiber/matrix debonding appears to greatly reduce the stress concentration around the notches. The laminates that were heat treated showed a different crack growth pattern. In the ASF specimens, matrix cracks had a more tortuous path and showed considerable more crack branching. For the same specimen geometry and cyclic stress, the (0/90/0) laminate with a hole had far superior fatigue resistance than the matrix only specimen with a hole.
The purpose was to characterize damage initiation and growth in notched titanium matrix composites at room temperature. Double edge notched or center open hole SCS-6/Ti-15-3 specimens containing 0 deg plies or containing both 0 and 90 deg plies were fatigued. The specimens were tested in the as-fabricated (ASF) and in heat-treated conditions. A local strain criterion using unnotched specimen fatigue data was successful in predicting fatigue damage initiation. The initiation stress level was accurately predicted for both a double edge notched unidirectional specimen and a cross-plied center hole specimen. The fatigue produced long multiple cracks growing from the notches. These fatigue cracks were only in the matrix material and did not break the fibers in their path. The combination of matrix cracking and fiber/matrix debonding appears to greatly reduce the stress concentration around the notches. The laminates that were heat treated showed a different crack growth pattern. In the ASF specimens, matrix cracks had a more tortuous path and showed considerable more crack branching. For the same specimen geometry and cyclic stress, the (0/90/0) laminate with a hole had far superior fatigue resistance than the matrix only specimen with a hole.
An identification procedure proposed by Shen and Taylor to determine the crack characteristics (location and size of the crack) from dynamic measurements is tested. This procedure was based on minimization of either the 'mean-square' measure of difference between measurement data (natural frequencies and mode shapes) and the corresponding predictions obtained from the computational model. The procedure is tested for simulated damage in the form of symmetric cracks in a simply-supported Bernoulli-Euler beam. The sensitivity of the solution of damage identification problems to the values of parameters that characterize damage is discussed. A sensitivity formula is derived.
Contributions provided by the NASA Research Centers to key Space Shuttle return-to-flight milestones, with an emphasis on debris and Thermal Protection System (TPS) damage characterization, are described herein. Several CAIB recommendations and Space Shuttle Program directives deal with the mitigation of external tank foam insulation as a debris source, including material characterization as well as potential design changes, and an understanding of Orbiter TPS material characteristics, damage scenarios, and repair options. Ames, Glenn, and Langley Research Centers have performed analytic studies, conducted experimental testing, and developed new technologies, analysis tools, and hardware to contribute to each of these recommendations. For the External Tank (ET), these include studies of spray-on foam insulation (SOFI), investigations of potential design changes, and applications of advanced non-destructive evaluation (NDE) technologies to understand ET TPS shedding during liftoff and ascent. The end-to-end debris assessment included transport analysis to determine the probabilities of impact for various debris sources. For the Orbiter, methods were developed, and validated through experimental testing, to determine thresholds for potential damage of Orbiter TPS components. Analysis tools were developed and validated for on-orbit TPS damage assessments, especially in the area of aerothermal environments. Advanced NDE technologies were also applied to the Orbiter TPS components, including sensor technologies to detect wing leading edge impacts during liftoff and ascent. Work is continuing to develop certified TPS repair options and to develop improved methodologies for reinforced carbon-carbon (RCC) damage progression to assist in on-orbit repair decision philosophy.
A stochastic damage model for predicting the rupture of a brittle multiphase material is developed, based on the microcrack-macrocrack interaction. The model, which incorporates uncertainties in locations, orientations, and numbers of microcracks, characterizes damage by microcracking and fracture by macrocracking. A parametric study is carried out to investigate the change of the stress intensity at the macrocrack tip by the configuration of microcracks. The inherent statistical distribution of the fracture toughness arising from the intrinsic random nature of microcracks is explored using a statistical approach. For this purpose, a computer simulation model is introduced, which incorporates a statistical characterization of geometrical parameters of a random microcrack array.
Both the macro studies of crack layer propagation are presented. The crack extension resistance parameter R sub 1 based on the morphological study of microdefects is introduced. Experimental study of the history dependent nature of G sub c supports the representation of G sub c as a product of specific enthalpy of damage (material constant) and R sub 1. The latter accounts for the history dependence. The observation of nonmonotonic crack growth under monotonic changes of J as well as statistical features of the critical energy release rate (variance of G sub c) indicate the validity of the proposed damage characterization.