Thomas Eason

Reengineering Aircraft Structural Life Prediction Using a Digital Twin

Reengineering of the aircraft structural life prediction process to fully exploit advances in very high performance digital computing is proposed. The proposed process utilizes an ultrahigh fidelity model of individual aircraft by tail number, a Digital Twin, to integrate computation of structural deflections and temperatures in response to flight conditions, with resulting local damage and material state evolution. A conceptual model of how the Digital Twin can be used for predicting the life of aircraft structure and assuring its structural integrity is presented. The technical challenges to developing and deploying a Digital Twin are discussed in detail.

High-Speed Digital Image Correlation Measurements of Random Nonlinear Dynamic Response

Future United States Air Force (USAF) high-speed vehicles will require innovative, non-contacting full-field measurement techniques to validate analysis and design practices. In this experimental investigation, the authors explore the feasibility of using high-speed 3D digital image correlation (DIC) to measure the geometrically nonlinear and stochastic response of a compliant panel representing thin-gauge aircraft-like structure. Existing measurement techniques typically employed for this application include laser vibrometry, accelerometers, and discrete strain gages. However, these approaches are limited to a few points or direct contact resulting in altered structural response. The possibility of full-field noncontact displacement and strain measurement is an attractive alternative for this type of dynamic response testing, particularly as one is not limited to predetermined sensor location. The technical challenges of using DIC for this application include extending the technique from quasi-static or extremely short duration transient dynamic measurement technique to steady-state, long-duration (seconds of data) random response. Multiple, long-time sample records are desired for ensemble averaging, and correspondingly high sample rates generate appreciable volumes of digital images never before attempted with this type of analysis. DIC displacement and strain results are compared to the more traditional measurement methods to establish accuracy. Results demonstrate the feasibility of using DIC for nonlinear dynamic displacement and strain response measurements. The ability to obtain full-field displacement data was beneficial towards identification and differentiation of the dynamic panel response from the inherent dynamic response of the experimental facility.

Characterizing Dynamic Transitions Associated With Snap-Through: A Discrete System

Geometrically nonlinear structures often possess multiple equilibrium configurations. Under extreme conditions of excitation it is possible for these structures to exhibit oscillations about and between these co-existing configurations. This behavior may have serious implications for fatigue in the context of aircraft surface panels. Snap-through is a name often given to sudden changes in dynamic behavior associated with mechanical instability (buckling). This is an often encountered problem in hypersonic vehicles in which severe thermal loading and acoustic excitation conspire to create an especially hostile environment for structural elements. In this paper, a simple link model is used, experimentally and numerically, to investigate the mechanisms of snap-through buckling from a phenomenological standpoint.

Reduced-Order Models for a Shallow Curved Beam Under Combined Loading

Future U.S. Air Force vehicles require structures that can withstand extreme combined environments. Examples include vehicles exposed to launch, sustained hypersonic velocities, reentry, and stealthy aircraft with buried engines and ducted exhaust. Two of the many conditions that a structure in these environments will experience are elevated temperatures and high acoustic loading. Computational methods are needed to rapidly explore the design space for extreme environment structures. There has been a significant amount of work toward developing reduced-order modeling to address the issue of sonic fatigue. These methods have been demonstrated to be useful for predicting the geometric nonlinear response of aircraft structures to stochastic loading. Recent work also demonstrates that these methods are able to predict the response of planar structures in these combined environments. The present study demonstrates that the implicit condensation reduced-order modeling method can also be extended to curved structures experiencing combined thermal-acoustic loading with changing thermal conditions. Successful displacement and strain comparisons for a curved beam structure are made between results from a commercial finite element code and reduced-order models, using a single random pressure load (162 dB) and varying temperature cases.

Decomposition-based calibration/validation metrics for use with full-field measurement situations:

Calibration and validation metrics that involve decomposition of simulation and test data have been developed for potential use in the quantification of margin and uncertainty. The uniqueness of these validation metrics allows for nearly full-field, simulation and test data over a wide range of spatial realizations (three-dimensional responses over multiple input conditions) and temporal (time or frequency) information, as needed. Currently, no other calibration/validation metrics have been developed that span multiple spatial realizations and temporal information simultaneously. A demonstration example utilizing two datasets explains how the calibration/validation metrics are formed and how they can be used to quantify the margin between the simulation and the test data as well as how it can quantify the uncertainty. The primary advantage of a proposed principal component analysis validation metric is that it preserves the engineering units of the original data so that the quantifications of margin and uncertainty can be made in engineering unit. A second advantage of the principal component analysis validation metric is that it can be used over a wide range of temporal information. The potential case of using sets of data with mismatched degree of freedom information is also explored. The general approach of using decomposition methods as the basis for calibration/validation metrics is extended to image decomposition methods. All decomposition methods successfully quantify margin and uncertainty in this general calibration/validation metric approach.

A Principal Component Analysis (PCA) Decomposition Based Validation Metric for Use with Full Field Measurement Situations

A validation metric that involves principal component analysis (PCA) decomposition of simulation and test data is developed for potential use in the quantification of margin and uncertainty (QMU) for an upcoming USAF design exercise (OEM Aero-Thermo-Structures Design Study—USAF Task Order 0015: “Predictive Capability for Hypersonic Structural Response and Life Prediction”). This validation metric allows for use of nearly full-field, simulation and test data over a wide range of spatial realizations (3-D responses over multiple input conditions) and temporal (time or frequency) information, as needed. A demonstration example utilizing two datasets explains how the validation metric is formed and how it can be used to quantify the margin between the simulation and the test data as well as how it can quantify the uncertainty. The primary advantage of the proposed PCA validation metric is that it preserves the engineering units (EU) of the original data so that the quantifications of margin and uncertainty can be made in EU. A second advantage of the PCA validation metric is that it can be used over a wide range of temporal information. While the proposed method shows considerable promise, future work is identified in terms of exploring other decomposition methods commonly used in fingerprint, iris and facial biometric pattern recognition.

Experimental Modal Analysis of an Aircraft Fuselage Panel

Hypersonic aircraft structures must operate in complex loading conditions and very high temperatures, making the design of a robust and reusable platform very challenging. An analytical and experimental test program was developed by the Air Force Research Laboratory (AFRL) and industry. The objective of the program is to review the design process of a thin skinned aircraft panel subjected to combined thermal-acoustic-mechanical loading, through a series of laboratory experiments at the AFRL’s Structural Dynamics Laboratory.

Experimental Modal Analysis of an Aircraft Fuselage Panel: Part II

Hypersonic aircraft structures must operate in complex loading conditions and very high temperatures, making the design of a robust and reusable platform very challenging. An analytical and experimental test program was developed by the Air Force Research Laboratory (AFRL) and industry. The objective of the program is to review the design process of a thin skinned aircraft panel subjected to combined thermal-acoustic-mechanical loading, through a series of laboratory experiments at the AFRL’s Structural Dynamics Laboratory.

Nonlinear Dynamic Response Prediction of a Thin Panel in a Multi-Discipline Environment: Part II—Numerical Predictions

Hypersonic aircraft structures must operate in a complex loading environment, where the coupling of the aircraft structural response with the aerodynamics will lead to conditions involving rich nonlinear dynamics. The modeling of these fluid-thermal-structural interactions is complex and prohibitively expensive when high fidelity models are used (i.e., CFD and FEA). This aspect, and the lack of relevant flight-test and experimental data, have resulted in knowledge gaps, which have led to the design of overly-conservative structures in the past. Work at the Structural Sciences Center (SSC) of the USAF Research Laboratory has focused on addressing these knowledge gaps from a structures perspective. As discussed in Part I of this paper, 3 years ago the SSC began a series of wind-tunnel experiments to provide full-field experimental data for a clamped nominally flat panel exposed to supersonic flow. The present work will focus on numerical predictions of the panel dynamic response using a reduced order model (ROM) for the structural response and full-field measurement data to represent the loads on the panel.

Nonlinear Response of a Thin Panel in a Multi-Discipline Environment: Part I—Experimental Results

High-speed aircraft structures are susceptible to the extreme and transient effects of the associated aerodynamic environment. These structures can experience a myriad of limit states—yield, fatigue, creep, buckling, and the response is very often path-dependent. Hypersonics, defined as flight speeds greater than Mach 5 (Heppenheimer, NASA Technical Report, NASA SP-2007-4232, September 2007) where aerodynamic heating drives the analysis and design, often causing appreciable structural concerns, is a flight regime with very little practical experience. While the NASA Space Shuttle Orbiter and other space-access vehicles routinely transit the Mach 5 barrier, long-duration air-breathing flights represent but a scant portion of past flight-test programs. As a result, the aerospace industry accounts for the associated uncertainties in the structural response through overly-conservative, and often program-deleterious, design assumptions. The USAF Research Laboratory, Structural Sciences Center (SSC), is investigating and developing analysis methods to predict the changing, nonlinear response of hypersonic hot-structures; however, there is a lack of relevant flight-test and experimental data useful for validating these developing structures-centric methods. The SSC recently began a series of thorough wind-tunnel experiments to provide quality, full-field experimental data for a simple, clamped nominally flat panel exposed to supersonic flow, shock boundary-layer interactions (SBLI) and heated flow. External heating sufficient to buckle the test article during supersonic wind tunnel experiments is being explored. Early results are presented in the present study. Additionally, wind tunnel conditions will be sought that lead to panel snap-through dynamics. The present study documents the evolution of the experiments, emphasizing the nonlinear response of the panel in preparation for upcoming wind-tunnel experiments. Also discussed are the characteristics of the experimental conditions leading to the nonlinear structural response, and the full-field displacement, pressure and thermal results necessary for model validation. Part II of this study will present the results of a numerical study of the same structure in the supersonic environment.