Eric A. Lindgren

Investigation of a Model‐Assisted Approach to Probability of Detection Evaluation

This paper presents a model‐assisted probability of detection (MAPOD) study for inspection of a two‐layer airframe structure. Eddy current measurements for varying crack length around fastener holes in a two‐layer aluminum structure are studied using both experimental and model‐generated data. New statistical algorithms are used to calculate the probability of detection. Good agreement was achieved between empirical and model‐assisted approaches.

MODEL‐ASSISTED PROBABILITY OF DETECTION EVALUATION FOR EDDY CURRENT INSPECTION OF FASTENER SITES

A model‐assisted approach for the design and execution of probability of detection (POD) studies is proposed. General agreement was achieved between experimental and full‐model assisted results for eddy current inspection of cracks at fastener sites located at both the first and second layers. The accuracy of the POD results was found to be dependent upon the NDE model and assumptions in the model‐assisted POD evaluation. Insight is presented for improving the quality of future studies.

Aging Aircraft NDE: Capabilities, Challenges, and Opportunities

US Air Force aircraft are managed by Damage Tolerance Assessment (DTA) and Nondestructive Evaluation (NDE) plays a critical role in insuring their flight safety. This paper provides an overview of current NDE capabilities for aircraft structures. The potential migration to Condition‐Based Maintenance (CBM) will require NDE to evolve from detection of defects to characterization of their location and size. This represents multiple significant challenges as aircraft have many material and geometry factors that complicate this analysis. This paper provides a strategy to resolve these factors.

Ultrasonic plate waves for fatigue crack detection in multi-layered metallic structures

A representative area of concern for fatigue crack growth in aircraft occurs in multi-layered metallic structures. Ultrasonic plate waves are currently being investigated by multiple initiatives to detect these types of flaws with a minimal number of sensors to enable Structural Health Monitoring (SHM). Previous work has focused on structures with one or two layers, coupled with modeling of the wave propagation within these representative samples. However, it is common for multi-layered structures to have more than two layers in many areas of interest. Therefore, this study investigates ultrasonic wave propagation and flaw detection in a multi-layered sample consisting of 2 to 4 total layers with fatigue cracks located in only one layer. The samples contain fastener holes configured as would be expected to find on typical aircraft structure. The flaws in this study are represented by electric discharge machined (EDM) notches. Preliminary measurements show that EDM notches can be detected by the guided ultrasonic waves, but that the sensitivity to EDM notch location is dependent on the boundary conditions of each layer. The boundary conditions are changed by applying various loads on the surface of each layer by tightening and loosening the fasteners that hold the sample together. This variation depicts representative conditions found of aircraft. The experimental results are supplemented by modeling of the guided wave propagation within the structure using the Finite Element Method. The primary parameter studied in the modeling effort is the effect of the changes in the boundary condition on the mode and amplitude of the guided wave. The results of this investigation establish some guidelines for the use of guided waves in multi-layered structures, plus challenges that exist for their use in SHM applications and strategies to address these challenges.

CASE STUDIES FOR MODEL‐ASSISTED PROBABILISTIC RELIABILITY ASSESSMENT FOR STRUCTURAL HEALTH MONITORING SYSTEMS

This paper describes progress on the development of a protocol and presents several case studies for model‐assisted probabilistic reliability assessment of structural health monitoring (SHM) technologies. The design and progress of an experimental demonstration study is presented highlighting the protocol and test plan. A simulation‐based study for an SHM system incorporating vibration methods is also introduced to demonstrate the protocol process. Probability of detection and probability of correct characterization curves were generated for different transducer locations providing key insight on sensor placement and expected detection and characterization performance.

MODEL‐ASSISTED PROBABILISTIC RELIABILITY ASSESSMENT FOR STRUCTURAL HEALTH MONITORING SYSTEMS

This paper describes a model‐assisted probabilistic methodology to ensure the reliability of SHM systems for damage detection, localization, and sizing. A hierarchical approach is presented that attempts to minimize the number of samples, the length of time, and degree of full‐scale testing required for statistically meaningful characterization results. The feasibility of applying this approach to typical sensing methods found in SHM systems is investigated, and additional challenges concerning model reliability and uncertainty propagation are addressed.

Probabilistic Risk Assessment: Impact of Human Factors on Nondestructive Evaluation and Sensor Degradation on Structural Health Monitoring

Managing human factors in nondestructive evaluation is critical for maintaining inspection reliability. Reliability of structural health monitoring systems is particularly sensitive to sensor degradation over time. To investigate the impact of these issues, probabilistic models for risk assessment and cost‐benefit analysis tools have been developed. Quantitative studies are presented evaluating the effects of variations in probability of detection associated with human factors, plus in‐situ sensor degradation effects on life cycle measures such as cost and probability of failure.

ANALYTICAL MODEL FOR THE SCATTERING OF AN OBLIQUELY INCIDENT PLANE SHEAR WAVE FROM A CYLINDRICAL CAVITY

A 3D analytical model was derived for the propagation and scattering of an obliquely incident plane shear wave from a cylindrical cavity. The generation and decay of the spiral creeping waves was found to be present for typical angled‐beam shear wave inspections of fastener sites although dependent on the angle of incidence and polarization. An extension of the model using an image method was derived to study a special case of plane wave scattering by a truncated cylinder with an infinite crack.

ANALYTICAL MODEL OF THE SCATTERING OF GUIDED WAVES FROM FASTENER SITES WITH A STIFFNESS INTERFACE

In guided wave crack detection for structural health monitoring, there is a critical need to understand the sensitivity to joint interface conditions. Analytical models are presented for the interaction of ultrasonic waves from a cylindrical hole with an elastic insert coupled by a stiffness interface. Parametric studies are presented investigating the effect of variation in the contact conditions on the reflection and generation of secondary waves from the fastener.

CHALLENGES AND OPPORTUNITIES IN NDE, ISHM AND MATERIAL STATE AWARENESS FOR AIRCRAFT STRUCTURES: US AIR FORCE PERSPECTIVE

As one of the primary data and information sources in the maintenance of USAF Aging Military Fleet, NDE plays a major role in the definition and operation of maintenance processes on these aircraft. To focus new NDE developmental efforts, the AFRL NDE R&D group has the charter to research, develop and transition new capabilities to the field and depot users. This multi‐faceted task is achieved through a balanced NDE and on‐board sensor development program with the ultimate goal to transition technology to the Air Force user Commands. Technology requirements for NDE and Material State Awareness emerge from Air Force Initiatives to realize Condition Based Maintenance and to develop the “Depot of the Future”. This evening session will present an overview of Air Force Initiatives, emerging R&D issues for Structural Health Monitoring and NDE methodologies as well as basic research initiatives within the Air Force Research Laboratory. It is intended that the session provide an open forum to pursue paths for new...