SEARCH · Search NASA

Results for “damage”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3

All sources NASA NTRS DOE OSTI

NASA NTRS2017

Damage Simulation in Composite Materials: Why It Matters and What Is Happening Currently at NASA in This Area

Use of lightweight composite materials in space and aircraft structure designs is often challenging due to high costs associated with structural certification. Of primary concern in the use of composite structures is durability and damage tolerance. This concern is due to the inherent susceptibility of composite materials to both fabrication and service induced flaws. Due to a lack of general industry accepted analysis tools applicable to composites damage simulation, a certification procedure relies almost entirely on testing. It is this reliance on testing, especially compared to structures comprised of legacy metallic materials where damage simulation tools are available, that can drive costs for using composite materials in aerospace structures. The observation that use of composites can be expensive due to testing requirements is not new and as such, research on analysis tools for simulating damage in composite structures has been occurring for several decades. A convenient approach many researchers/model-developers in this area have taken is to select a specific problem relevant to aerospace structural certification and develop a model that is accurate within that scope. Some examples are open hole tension tests, compression after impact tests, low-velocity impact, damage tolerance of an embedded flaw, and fatigue crack growth to name a few. Based on the premise that running analyses is cheaper than running tests, one motivation that many researchers in this area have is that if generally applicable and reliable damage simulation tools were available the dependence on certification testing could be lessened thereby reducing overall design cost. It is generally accepted that simulation tools if applied in this manner would still need to be thoroughly validated and that composite testing will never be completely replaced by analysis. Research and development is currently occurring at NASA to create numerical damage simulation tools applicable to damage in composites. The Advanced Composites Project (ACP) at NASA Langley has supported the development of composites damage simulation tools in a consortium of aerospace companies with a goal of reducing the certification time of a commercial aircraft by 30%. And while the scope of ACP does not include spacecraft, much of the methodology and simulation capabilities can apply to spacecraft certification in the Space Launch System and Orion programs as well. Some specific applications of composite damage simulation models in a certification program are (1) evaluation of damage during service when maintenance may be difficult or impossible, (2) a tool for early design iterations, (3) gaining insight into a particular damage process and applying this insight towards a test coupon or structural design, and (4) analysis of damage scenarios that are difficult or impossible to recreate in a test. As analysis capabilities improve, these applications and more will become realized resulting in a reduction in cost for use of composites in aerospace vehicles. NASA is engaged in this process from both research and application perspectives. In addition to the background information discussed previously, this presentation covers a look at recent research at NASA in this area and some current/potential applications in the Orion program.

McElroy, Mack↗

NASA NTRS1998

A New Approach to Overcoming Spatial Aliasing in Structural Damage Detection

Aircraft, reusable launch vehicles,unmanned aircraft, and other advanced structures are being built using lightweight composite materials/metals with design safety factors as low as 1.25. These advanced structures operate in uncertain and severe environments and are susceptible to damage such as delamination, fiber/matrix damage, hydrothermal strain in composite materials, and fatigue and cracking in metals. To ensure human safety and load-bearing integrity these structures must be inspected to detect and locate often invisible damage and faults before they become catastrophic. Conventional methods of non-destructive evaluation sometimes miss significant damage and are time consuming and expensive to perform. In contrast, vibrometry or vibration signature techniques are a global method of structural integrity monitoring that potentially can efficiently detect damage on large structures, including damage that is away from sensor locations, and in the interior of structures. However, a barrier problem in damage detection using vibration measurements is the need to measure the vibration response at a large number of points on the structure. Typically, model reduction or expansion procedures such as Guyan reduction or dynamic expansion are attempted to overcome the problem of insufficient measurements or spatial aliasing. These approaches depend on using information from the healthy model, and thus put error into the reduction used to represent the damaged structure. In this paper, a Frequency Response Function technique is used to detect damage to a fixed-free beam. The technique uses measured frequency response functions from the healthy structure as reference data, and then monitors vibration measurements during the life of the structure to detect damage. In an analytical simulation using a finite-element model of a beam, damage was located using only sparse measurements because the technique uses both rotation and translation measurements from the damaged structure.

Schulz, Mark J.↗

NASA NTRS2007

The Effect of Delamination on Damage Path and Failure Load Prediction for Notched Composite Laminates

The influence of delamination on the progressing damage path and initial failure load in composite laminates is investigated. Results are presented from a numerical and an experimental study of center-notched tensile-loaded coupons. The numerical study includes two approaches. The first approach considers only intralaminar (fiber breakage and matrix cracking) damage modes in calculating the progression of the damage path. In the second approach, the model is extended to consider the effect of interlaminar (delamination) damage modes in addition to the intralaminar damage modes. The intralaminar damage is modeled using progressive damage analysis (PDA) methodology implemented with the VUMAT subroutine in the ABAQUS finite element code. The interlaminar damage mode has been simulated using cohesive elements in ABAQUS. In the experimental study, 2-3 specimens each of two different stacking sequences of center-notched laminates are tensile loaded. The numerical results from the two different modeling approaches are compared with each other and the experimentally observed results for both laminate types. The comparisons reveal that the second modeling approach, where the delamination damage mode is included together with the intralaminar damage modes, better simulates the experimentally observed damage modes and damage paths, which were characterized by splitting failures perpendicular to the notch tips in one or more layers. Additionally, the inclusion of the delamination mode resulted in a better prediction of the loads at which the failure took place, which were higher than those predicted by the first modeling approach which did not include delaminations.

Satyanarayana, Arunkumar↗

NASA NTRS2012

Use of Remote Sensing Data to Enhance NWS Storm Damage Toolkit

In the wake of a natural disaster such as a tornado, the National Weather Service (NWS) is required to provide a very detailed and timely storm damage assessment to local, state and federal homeland security officials. The Post ]Storm Data Acquisition (PSDA) procedure involves the acquisition and assembly of highly perishable data necessary for accurate post ]event analysis and potential integration into a geographic information system (GIS) available to its end users and associated decision makers. Information gained from the process also enables the NWS to increase its knowledge of extreme events, learn how to better use existing equipment, improve NWS warning programs, and provide accurate storm intensity and damage information to the news media and academia. To help collect and manage all of this information, forecasters in NWS Southern Region are currently developing a Storm Damage Assessment Toolkit (SDAT), which incorporates GIS ]capable phones and laptops into the PSDA process by tagging damage photography, location, and storm damage details with GPS coordinates for aggregation within the GIS database. However, this tool alone does not fully integrate radar and ground based storm damage reports nor does it help to identify undetected storm damage regions. In many cases, information on storm damage location (beginning and ending points, swath width, etc.) from ground surveys is incomplete or difficult to obtain. Geographic factors (terrain and limited roads in rural areas), manpower limitations, and other logistical constraints often prevent the gathering of a comprehensive picture of tornado or hail damage, and may allow damage regions to go undetected. Molthan et al. (2011) have shown that high resolution satellite data can provide additional valuable information on storm damage tracks to augment this database. This paper presents initial development to integrate satellitederived damage track information into the SDAT for near real ]time use by forecasters and decision makers.

Jedlove, Gary J.↗

NASA NTRS2013

Damage Detection Response Characteristics of Open Circuit Resonant (SansEC) Sensors

The capability to assess the current or future state of the health of an aircraft to improve safety, availability, and reliability while reducing maintenance costs has been a continuous goal for decades. Many companies, commercial entities, and academic institutions have become interested in Integrated Vehicle Health Management (IVHM) and a growing effort of research into "smart" vehicle sensing systems has emerged. Methods to detect damage to aircraft materials and structures have historically relied on visual inspection during pre-flight or post-flight operations by flight and ground crews. More quantitative non-destructive investigations with various instruments and sensors have traditionally been performed when the aircraft is out of operational service during major scheduled maintenance. Through the use of reliable sensors coupled with data monitoring, data mining, and data analysis techniques, the health state of a vehicle can be detected in-situ. NASA Langley Research Center (LaRC) is developing a composite aircraft skin damage detection method and system based on open circuit SansEC (Sans Electric Connection) sensor technology. Composite materials are increasingly used in modern aircraft for reducing weight, improving fuel efficiency, and enhancing the overall design, performance, and manufacturability of airborne vehicles. Materials such as fiberglass reinforced composites (FRC) and carbon-fiber-reinforced polymers (CFRP) are being used to great advantage in airframes, wings, engine nacelles, turbine blades, fairings, fuselage structures, empennage structures, control surfaces and aircraft skins. SansEC sensor technology is a new technical framework for designing, powering, and interrogating sensors to detect various types of damage in composite materials. The source cause of the in-service damage (lightning strike, impact damage, material fatigue, etc.) to the aircraft composite is not relevant. The sensor will detect damage independent of the cause. Damage in composite material is generally associated with a localized change in material permittivity and/or conductivity. These changes are sensed using SansEC. The unique electrical signatures (amplitude, frequency, bandwidth, and phase) are used for damage detection and diagnosis. An operational system and method would incorporate a SansEC sensor array on select areas of the aircraft exterior surfaces to form a "Smart skin" sensing surface. In this paper a new method and system for aircraft in-situ damage detection and diagnosis is presented. Experimental test results on seeded fault damage coupons and computational modeling simulation results are presented. NASA LaRC has demonstrated with individual sensors that SansEC sensors can be effectively used for in-situ composite damage detection of delamination, voids, fractures, and rips. Keywords: Damage Detection, Composites, Integrated Vehicle Health Monitoring (IVHM), Aviation Safety, SansEC Sensors

Dudley, Kenneth L.↗

NASA NTRS2015

Development of a Near Real-Time Hail Damage Swath Identification Algorithm for Vegetation

Every year in the Midwest and Great Plains, widespread greenness forms in conjunction with the latter part of the spring-summer growing season. This prevalent greenness forms as a result of the high concentration of agricultural areas having their crops reach their maturity before the fall harvest. This time of year also coincides with an enhanced hail frequency for the Great Plains (Cintineo et al. 2012). These severe thunderstorms can bring damaging winds and large hail that can result in damage to the surface vegetation. The spatial extent of the damage can relatively small concentrated area or be a vast swath of damage that is visible from space. These large areas of damage have been well documented over the years. In the late 1960s aerial photography was used to evaluate crop damage caused by hail. As satellite remote sensing technology has evolved, the identification of these hail damage streaks has increased. Satellites have made it possible to view these streaks in additional spectrums. Parker et al. (2005) documented two streaks using the Moderate Resolution Imaging Spectroradiometer (MODIS) that occurred in South Dakota. He noted the potential impact that these streaks had on the surface temperature and associated surface fluxes that are impacted by a change in temperature. Gallo et al. (2012) examined at the correlation between radar signatures and ground observations from storms that produced a hail damage swath in Central Iowa also using MODIS. Finally, Molthan et al. (2013) identified hail damage streaks through MODIS, Landsat-7, and SPOT observations of different resolutions for the development of a potential near-real time applications. The manual analysis of hail damage streaks in satellite imagery is both tedious and time consuming, and may be inconsistent from event to event. This study focuses on development of an objective and automatic algorithm to detect these areas of damage in a more efficient and timely manner. This study utilizes the MODIS sensor aboard the NASA Aqua satellite. Aqua was chosen due to an afternoon orbit over the United States when land surface temperatures are relatively warm and improve the contrast between damaged and undamaged areas. This orbit is also similar to the orbit of the Suomi-National Polar-orbiting Partnership (NPP) satellite. The Suomi NPP satellite hosts the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument, which is the next generation of a MODIS-like sensor in polar orbit.

MODIS VIIRS↗

NASA NTRS2017

Structural Health Management of Damaged Aircraft Structures Using the Digital Twin Concept

The development of multidisciplinary integrated Structural Health Management (SHM) tools will enable accurate detection, and prognosis of damaged aircraft under normal and adverse conditions during flight. As part of the digital twin concept, methodologies are developed by using integrated multiphysics models, sensor information and input data from an in-service vehicle to mirror and predict the life of its corresponding physical twin. SHM tools are necessary for both damage diagnostics and prognostics for continued safe operation of damaged aircraft structures. The adverse conditions include loss of control caused by environmental factors, actuator and sensor faults or failures, and structural damage conditions. A major concern in these structures is the growth of undetected damage/cracks due to fatigue and low velocity foreign object impact that can reach a critical size during flight, resulting in loss of control of the aircraft. To avoid unstable, catastrophic propagation of damage during a flight, load levels must be maintained that are below a reduced load-carrying capacity for continued safe operation of an aircraft. Hence, a capability is needed for accurate real-time predictions of damage size and safe load carrying capacity for structures with complex damage configurations. In the present work, a procedure is developed that uses guided wave responses to interrogate damage. As the guided wave interacts with damage, the signal attenuates in some directions and reflects in others. This results in a difference in signal magnitude as well as phase shifts between signal responses for damaged and undamaged structures. Accurate estimation of damage size, location, and orientation is made by evaluating the cumulative signal responses at various pre-selected sensor locations using a genetic algorithm (GA) based optimization procedure. The damage size, location, and orientation is obtained by minimizing the difference between the reference responses and the responses obtained by wave propagation finite element analysis of different representative cracks, geometries, and sizes.

Seshadri, Banavara R.↗

DOE OSTI2024

Broadband 920-nm mirror thin film damage competition

The 2023 Laser Damage conference thin-film damage competition was devoted to a survey on the state-of-the-art broadband near-IR multilayer dielectric (MLD) mirrors designed for ultra-short pulsed laser applications. The requirements for the coatings were a minimum reflection of 99.5% at 45-deg incidence angle for S-polarization from 830 nm to 1010 nm and group delay dispersion (GDD) < ± 50fs 2 . The participants were allowed to select the coating materials, coating design, and coating deposition method. Samples were damage tested at a single testing facility to enable direct comparison among the participants using a 25 ± 5 fs optical parametric chirped-pulse amplification (OPCPA) laser system operating at 5 Hz. The testing results from this set of 37 samples showed that dense coatings by ion-beam sputtering (IBS), magnetron sputtering (MS), and electron-beam ion assisted deposition (e-beam IAD) exhibited highest damage initiation onset (laser-induced damage threshold or LIDT) while e-beam coatings were low performers. In addition, multilayer coatings using tantala and/or hafnia as high index materials were top performers. Furthermore, this competition included for the first time the measurement of the damage growth onset (laser-induced damage growth threshold or LDGT). This latter performance metric plays an important role in establishing the safe operational conditions for larger aperture ultrashort pulsed lasers. Information pertaining to the morphology of the damage sites and their evolution under subsequent exposure to different laser fluences leading to damage growth is presented. Finally, not all coating samples in the survey met the GDD requirements stated above and associated measurements are discussed in the context of the present and past thin-film damage competitions focused on similar broadband near-IR MLD coatings.

23 broadband high reflectors↗

NASA NTRS2023

Damage Tolerance Study of Carbon Fiber/Epoxy Sandwich Structure in Support of the SLS Payload Adapter Fitting (PAF)

This study presents experimental results of impact and subsequent compression after impact (CAI) strength testing of sandwich structure representative of most of the acreage of a Payload Adaptor Fitting (PAF) that has been built at NASA’s Marshall Space Flight Center (MSFC). An instrumented drop weight tower was used to inflict different levels of impact damage to specimens. Load versus deflection of impact curves, visual damage, dent depth, damage as ascertained by thermography and cross-sectional microscopy were evaluated as part of the damage resistance of the structure. Compression After Impact (CAI) was performed to assess damage tolerance. The results show that at the amount of visual damage was highly dependent on whether the peel ply was present on the surface of the structure. The peel ply had some effect on the CAI strength values since the specimens with peel ply showed a 14% higher average CAI strength value. For all specimens tested in this study, CAI versus damage and CAI versus impact energy curves were plotted and it was found that damage size was a good indicator of CAI strength. It is suggested to use a b-basis damage tolerance curve based on damage size as a conservative measure of the load carrying capability of the sandwich structure with impact damage.

T1100/3960↗

NASA NTRS1991

Relevance of impacter shape to nonvisible damage and residual tensile strength of a thick graphite/epoxy laminate

A study was made to determine the relevance of impacter shape to nonvisible damage and tensile residual strength of a 36 mm thick graphite/epoxy motor case. The shapes of the impacters were as follows: 12.7 mm and 25.4 mm diameter hemispheres, a sharp corner, and a 6.3 mm diameter bolt-like rod. The investigation revealed that damage initiated when the contact pressure exceeded a critical level. However, the damage was not visible on the surface until an even higher pressure was exceeded. The impact energy to initiate damage or cause visible damage on the surface increased approximately with impacter diameter to the third power. The reduction in strength for nonvisible damage increased with increasing diameter, 9 and 30 percent for the 12.7 mm and 25.4 mm diameter hemispheres, respectively. The corner impacter made visible damage on the surface for even the smallest impact energy. The rod impacter acted like a punch and sliced through the composite. Even so, the critical level of pressure to initiate damage was the same for the rod and hemispherical impacters. Factors of safety for nonvisible damage increased with increasing kinetic energy of impact. The effects of impacter shape on impact force, damage size, damage visibility, and residual tensile strength were predicted quite well assuming Hertzian contact and using maximum stress criteria and a surface crack analysis.

Poe, Clarence C., Jr.↗

NASA NTRS1993

Impact damage tolerance of thin wall composite struts

An experimental investigation was made to study the impact damage tolerance of thin wall composite struts made of both brittle epoxy and toughened epoxy based composite materials. Damage parameters such as barely visible surface damage and internal damage represented by the ultrasonic C-scan, and residual compressive strengths were evaluated against impact energy for two impactor sizes. From both a damage resistance (internal damage vs. impact energy) and a damage tolerance (residual compressive strength vs. internal damage) point of view, the toughened IM7/977-2 struts exhibited better performance than the brittle epoxy based T50/934 struts. This is attributed to the toughening mechanism in 977-2 which impedes delamination initiation from impact, and delamination growth and subsequent buckling under a compression loading. At barely visible damage thresholds, regardless of the impactor sizes, a maximum strength reduction of 45-55 percent was observed for the T50/934 struts, and approximately 10 percent for IM7/977-2 struts. This is of great interest for developing a damage tolerance design approach and risk assessment methodology in which the design allowable would be defined by the residual strength at the threshold of barely visible damage.

Chen, G.-S.↗

NASA NTRS1994

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.↗

NASA NTRS1995

Impact damage resistance and residual property assessment of (0/+/-45/90)s SCS-6/Timetal 21S

The impact damage resistance and residual mechanical properties of (0/ +/- 45/90)s SCS-6/Timetal 21S composites were evaluated. Both quasi-static indentation and drop-weight impact tests were used to investigate the impact behavior at two nominal energy levels (5.5 and 8.4 J) and determine the onset of internal damage. Through x-ray inspection, the extent of internal damage was characterized non-destructively. The composite strength and constant amplitude fatigue response were evaluated to assess the effects of the sustained damage. Scanning electron microscopy was used to characterize internal damage from impact in comparison to damage that occurs during mechanical loading alone. The effect of stacking sequence was examined by using specimens with the long dimension of the specimen both parallel (longitudinal) and perpendicular (transverse) to the 0 deg fiber direction. Damage in the form of longitudinal and transverse cracking occurred in all longitudinal specimens tested at energies greater than 6.3 J. Similar results occurred in the transverse specimens tested above 5.4 J. Initial load drop, characteristic of the onset of damage, occurred on average at 6.3 J in longitudinal specimens and at 5.0 J in transverse specimens. X-ray analysis showed broken fibers in the impacted region in specimens tested at the higher impact energies. At low impact energies, visible matrix cracking may occur, but broken fibers may not. Matrix cracking was noted along fiber swims and it appeared to depend on the surface quality of composite. At low impact energies, little damage has been incurred by the composite and the residual strength and residual life is not greatly reduced as compared to an undamaged composite. At higher impact energies, more damage occurred and a greater effect of the impact damage was observed.

Miller, Jennifer L.↗

NASA NTRS2008

An Integrated Approach to Damage Accommodation in Flight Control

In this paper we present an integrated approach to in-flight damage accommodation in flight control. The approach is based on Multiple Models, Switching and Tuning (MMST), and consists of three steps: In the first step the main objective is to acquire a realistic aircraft damage model. Modeling of in-flight damage is a highly complex problem since there is a large number of issues that need to be addressed. One of the most important one is that there is strong coupling between structural dynamics, aerodynamics, and flight control. These effects cannot be studied separately due to this coupling. Once a realistic damage model is available, in the second step a large number of models corresponding to different damage cases are generated. One possibility is to generate many linear models and interpolate between them to cover a large portion of the flight envelope. Once these models have been generated, we will implement a recently developed-Model Set Reduction (MSR) technique. The technique is based on parameterizing damage in terms of uncertain parameters, and uses concepts from robust control theory to arrive at a small number of "centered" models such that the controllers corresponding to these models assure desired stability and robustness properties over a subset in the parametric space. By devising a suitable model placement strategy, the entire parametric set is covered with a relatively small number of models and controllers. The third step consists of designing a Multiple Models, Switching and Tuning (MMST) strategy for estimating the current operating regime (damage case) of the aircraft, and switching to the corresponding controller to achieve effective damage accommodation and the desired performance. In the paper present a comprehensive approach to damage accommodation using Model Set Design,MMST, and Variable Structure compensation for coupling nonlinearities. The approach was evaluated on a model of F/A-18 aircraft dynamics under control effector damage, augmented by nonlinear cross-coupling terms and a structural dynamics model. The proposed approach achieved excellent performance under severe damage effects.

Boskovic, Jovan D.↗

NASA NTRS2015

Development of a Near-Real Time Hail Damage Swath Identification Algorithm for Vegetation

The Midwest is home to one of the world's largest agricultural growing regions. Between the time period of late May through early September, and with irrigation and seasonal rainfall these crops are able to reach their full maturity. Using moderate to high resolution remote sensors, the monitoring of the vegetation can be achieved using the red and near-infrared wavelengths. These wavelengths allow for the calculation of vegetation indices, such as Normalized Difference Vegetation Index (NDVI). The vegetation growth and greenness, in this region, grows and evolves uniformly as the growing season progresses. However one of the biggest threats to Midwest vegetation during the time period is thunderstorms that bring large hail and damaging winds. Hail and wind damage to crops can be very expensive to crop growers and, damage can be spread over long swaths associated with the tracks of the damaging storms. Damage to the vegetation can be apparent in remotely sensed imagery and is visible from space after storms slightly damage the crops, allowing for changes to occur slowly over time as the crops wilt or more readily apparent if the storms strip material from the crops or destroy them completely. Previous work on identifying these hail damage swaths used manual interpretation by the way of moderate and higher resolution satellite imagery. With the development of an automated and near-real time hail swath damage identification algorithm, detection can be improved, and more damage indicators be created in a faster and more efficient way. The automated detection of hail damage swaths will examine short-term, large changes in the vegetation by differencing near-real time eight day NDVI composites and comparing them to post storm imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Terra and Aqua and Visible Infrared Imaging Radiometer Suite (VIIRS) aboard Suomi NPP. In addition land surface temperatures from these instruments will be examined as for hail damage swath identification. Initial validation of the automated algorithm is based upon Storm Prediction Center storm reports but also the National Severe Storm Laboratory (NSSL) Maximum Estimated Size Hail (MESH) product. Opportunities for future work are also shown, with focus on expansion of this algorithm with pixel-based image classification techniques for tracking surface changes as a result of severe weather.

Bell, Jordan R.↗

NASA NTRS2017

A Model-Based, Bayesian Solution for Characterization of Complex Damage Scenarios in Aerospace Composite Structures

Ultrasonic damage detection and characterization is commonly used in nondestructive evaluation (NDE) of aerospace composite components. In recent years there has been an increased development of guided wave based methods. In real materials and structures, these dispersive waves result in complicated behavior in the presence of complex damage scenarios. Model-based characterization methods utilize accurate three dimensional finite element models (FEMs) of guided wave interaction with realistic damage scenarios to aid in defect identification and classification. This work describes an inverse solution for realistic composite damage characterization by comparing the wavenumber-frequency spectra of experimental and simulated ultrasonic inspections. The composite laminate material properties are first verified through a Bayesian solution (Markov chain Monte Carlo), enabling uncertainty quantification surrounding the characterization. A study is undertaken to assess the efficacy of the proposed damage model and comparative metrics between the experimental and simulated output. The FEM is then parameterized with a damage model capable of describing the typical complex damage created by impact events in composites. The damage is characterized through a transdimensional Markov chain Monte Carlo solution, enabling a flexible damage model capable of adapting to the complex damage geometry investigated here. The posterior probability distributions of the individual delamination petals as well as the overall envelope of the damage site are determined.

H. Reed↗

NASA NTRS2020

Evaluation of Fatigue Damage Accumulation Functions for Delamination Initiation and Propagation

The present report follows on the cohesive fatigue damage model methodology proposed in NASA-TP-2018-219838. In that report, an empirical function describing the incremental damage due to cyclic loading was used to calculate fatigue damage within a cohesive formulation. The form of the function was developed such that, when integrated at a constant stress amplitude from no damage to failure, it produces a life versus load response that is consistent with an S-N diagram. Therefore, the parameters of the damage model could be obtained by fitting the model predictions to an S-N diagram. The finite element analyses performed demonstrate that the cohesive fatigue accumulation function provides a link between the S-N diagram that describes crack initiation, and the Paris law that characterizes the rate of crack propagation. However, when the model was proposed, it was not known whether the form of the damage accumulation function associated with a desired S-N diagram is unique and, if not, if the link between S-N and the Paris law is unique and independent of the fatigue function selected. In the effort described herein, several alternative forms of the damage function that reproduce the desired features of S-N diagrams were found and evaluated. The effects of each of these functions on the predicted parameters of the Paris law and the propagation threshold are discussed. The results indicate that the predicted exponent m of the Paris law is indeed independent of the damage accumulation function. However, different functions predict different values for the pre-factor C of the Paris law. Therefore, the proper damage accumulation function must be selected by comparison with experiments. One of the new damage accumulation functions proposed herein was found to be particularly useful for analysis because of the ease with which the model parameters can be determined with a minimal amount of experimental information. The effectiveness of the proposed methodology and damage function was demonstrated by conducting analyses of a double cantilever beam test, a mixed-mode bending test, and a three-point bending test of a skin/doubler specimen. The results indicate that the same set of model parameters can provide accurate predictions of the rate of fatigue crack propagation for a variety of material interfaces, mode mixities, load levels, and stress ratios.

Cohesive elements↗

NASA NTRS2021

Modular Composite Damage Detection System (MCDDS)

The 2020 CIF funded Modular Composite Damage Detection System (MCDDS) aims to advance the technology of damage detection in composites in the areas of thin, flexible damage detection panel construction, encapsulation techniques in composite matrixes, innovative integration and interfacing techniques with flexible embedded sensory panels, repair methods of composite materials, and improved packaging of the MCDDS. Custom thin, flexible polyimide printed circuit boards (PCBs) were designed and procured for fabrication of test specimens. A custom interface PCB was designed and fabricated to monitor the newly developed damage detection test specimens. Test specimens were fabricated using the novel flexible circuitry that were either fully or partially encapsulated. The fully encapsulated specimens were used to demonstrate embedded damage detection concepts and the partially encapsulated specimens were used to demonstrate surface-mounted (laminate) damage detection concepts. The new composite damage detection test specimens were tested and evaluated to determine if the novel damage detection sensory panels could withstand low-cycle threshold fatigue loading per a modified version of American Society for Testing and Materials (ASTM) D 7264 for up to 50 cycles without void formation. The data for multiple sensory panels indicates that the newly fabricated sensory panels are robust and do indeed withstand three-point bend flexural loading for more than 50 cycles. Additionally, testing was performed to determine if damage could be detected in damage detection test specimens subjected to failure using a 3-point bend test per a modified version of ASTM D 7264. Again, multiple sensory panels (both fully and partially encapsulated) showed the ability to detect damage to specimens subjected to failure per a modified version of ASTM D 7264. Additionally, a novel concept for tracking and monitoring of cracking (and potentially delamination) in composites was designed and developed that utilizes painted-on conductors.

Christopher Forney↗