Nam Phan

Energy harvesting wireless sensors and networked timing synchronization for aircraft structural health monitoring

Energy harvesting, combined with wireless sensors, could greatly improve our ability to monitor and maintain critical structures. This paper reports on the development of an integrated structural health monitoring and reporting (SHMR) system for use on Navy aircraft. Our goal was to develop and test a versatile, fully programmable SHMR system, designed to synchronize and record data from a range of wireless and hard wired sensor networks. Wireless sensors included strain gauges, accelerometers, and thermocouples. Hard-wired sensors included gyroscopes, accelerometers, and magnetometers. Data from an embedded Global Positioning System (GPS) provided position, velocity, and precise timing information.

Structural Health Management in the NAVY

There is a critical need for integrated system health management (ISHM) approaches to asset maintenance. Ideally, ISHM methodologies would track the system usage and the associated loads, monitor the system degradation and materials state, monitor relevant environmental parameters and their effects on system degradation, detect insipient system damage, diagnose failure mode, predict future system performance, and recommend maintenance actions. Even though there has been considerable progress in many subareas of ISHM over the past years, there is still ample room for future improvements in all technological aspects affecting ISHM. In fact, progress in ISHM has not been uniform. Some subsystems have experienced a far greater degree of development than others. For example, engine and machinery health monitoring and diagnostics, due to its criticality, has evolved at a faster pace than structural health monitoring. This article will review some of the aspects that need to be addressed in order to make structural health monitoring (SHM) of military systems a reality in the near future.

On the Fatigue Performance of Additively Manufactured Ti-6Al-4V to Enable Rapid Qualification for Aerospace Applications

To realize the potential benefits of additive manufacturing technology in airframe and ground vehicle applications, the fatigue performance of load bearing additively manufactured materials must be understood. Due to the novelty of this rapidly developing technology, a very limited, yet swiftly evolving literature exists on the topic. Motivated by these two points, we have attempted to catalogue and analyze the published fatigue performance data of an additively manufactured alloy of significant technological interest, Ti-6Al-4V. Focusing on uniaxial fatigue performance and crack growth, we compare to traditionally manufactured Ti-6Al-4V, discussing failure mechanisms, defects, microstructure, and processing parameters. We then attempt to identify key knowledge gaps that must be addressed before AM technology can safely and effectively be employed in critical load bearing applications.

Embeddable fiber optic strain sensor for structural monitoring

An extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor is presented for measurement of strain at high ambient temperatures. The sensor is fabricated using a femto-second (fs) laser. The EFPI sensor is fabricated by micromachining a cavity on the tip of a standard single-mode fiber and is then self-enclosed by fusion splicing another piece of singlemode fiber. The fs-laser based fabrication makes the sensor thermally stable to sustain temperatures as high as 800 °C. The sensor is relatively insensitive towards the temperature as compared to its response towards the applied strain. The sensor can be embedded in Carbon fiber/Bismaleimide (BMI) composite laminates for strain monitoring at high ambient temperatures.

Peridynamic Modeling of Cracking in Ceramic Matrix Composites

This study presents a peridynamic modeling approach to predict damage initiation and growth in fiber-reinforced Ceramic Matrix Composites (CMC). Damage prediction is based on the critical stretch which is directly related to the strain energy release rate failure criteria. The capability of this approach is verified against benchmark solutions and experimental observations available in the literature. A new nonuniform discretization capability in peridynamics is applied to investigate crack propagation in CMCs in the presence of a fairly dense matrix, and with fine discretization of the interphase (coating) regions. In the presence of a weak interphase between the fibers and matrix, it predicts the propagation of cracks through the dense matrix while deflecting around the fibers through the interphase region.

Fatigue Failure Initiation Modeling in AA7075-T651 Using Microstructure-Sensitive Continuum Damage Mechanics

A continuum damage mechanics (CDM) model for high-cycle fatigue (HCF) is presented to study crack initiation in AA7075-T651. This study is based on the experimental observation of dependence of crack initiation life on microstructure of alloys. We investigate the effect of microstructural features such as grain size and grain orientation on crack initiation life. A crystal plasticity finite element model (CPFEM) is implemented in conjunction with CDM model to simulate damage evolution at grain scale. Finite element program ABAQUS has been used and the CPFEM–CDM model is written using a user material subroutine. Simulations are performed for constant amplitude, completely reversed loading. In order to provide a prediction for fatigue scatter, we consider different realizations of the microstructure as well as uncertainty in fatigue parameters. Given probability density function of damage parameters, we can transport it into a lifetime probability density function using simulations results. Good agreement is observed between simulations results and available experimental data. Further investigation is needed to develop the CPFEM–CDM model for HCF under variable loading conditions.

Comparison of SGBEM-FEM Alternating Method and XFEM Method for Determining Stress Intensity Factor for 2D Crack Problems

The two most promising approaches to determine Stress Intensity Factor (SIF) developedover the past decade are the Symmetric Galerkin Boundary Element Method - Finite Element Method(SGBEM-FEM) based alternating method and the Extended Finite Element (XFEM) method. Thepurpose of this paper is to determine the SIFs for a number of 2-D crack problems by the two ap-proaches and measure their relative effectiveness in terms of accuracy, speed and computational re-sources.In the SGBEM-FEM alternating method, a finite element analysis is carried out on the un-crackedbody using the externally applied loading and next a boundary element analysis is performed byreversing the stresses found on the crack location from the finite element analysis, and the residualstresses on the boundary of the finite body are determined. The steps are repeated until convergenceis achieved where the residual stresses on the boundaries and traction on the crack surfaces are closeto zero.In the XFEM method, the mesh is created without considering the topology of the crack configura-tion and the discontinuities are handled by special discontinuity enrichment functions. The enrichmentfunctions increase the degrees of freedom and the regular stiffness matrix is augmented by additionalterms corresponding to the extra degrees of freedom but the increase in computational requirement isoffset by not having the burden of remeshing the finite elements.Both SGBEM-FEM alternating method and XFEM method are used to solve a number of crackproblems and the example cases clearly show the computational efficiency of the SGBEM-FEM al-ternating method over the XFEM method.

Damage Prediction in Composites due to Compression after Impact by Using Peridynamics

This study presents the peridynamics simulation of compression-after-impact (CAI) in two subsequent stages of impact and compressive type of loading. In the first stage, a transient analysis is performed that simulates the motion of a rigid impactor hitting a laminated specimen, leading to damage in the specimen. The simulation continues for additional time steps after the impactor rebounds from the specimen; subsequent impacts are not allowed. Once the transient effects in the impact problem dies out, the damaged specimen is subjected to compressive in-plane loading, resulting in further damage and eventual structural failure of the laminate.

A hybrid prognosis model for predicting fatigue crack propagation under biaxial in-phase and out-of-phase loading:

A hybrid prognosis model has been developed to predict the crack propagation in aluminum alloys subject to biaxial in-phase and out-of-phase fatigue loading conditions. The novel methodology combines physics-based modeling with machine learning techniques to predict crack growth in aluminum alloys. Understanding the failure mechanisms under these complex loading conditions is critical to developing reliable prognostic models. Therefore, extensive fatigue tests were conducted to study the failure modes of carefully designed cruciform specimens. Energy release rate was used as the physics-based parameter and Gaussian process was used to model the complex nonlinear relationships in the prognosis framework. The methodology was used to predict crack propagation in Al7075-T651 under a range of loading conditions. The predictions from the prognosis model were validated using the data obtained from the biaxial tests. The results indicate that the algorithm is able to accurately predict the crack propagation under proportional, non-proportional, in-phase, and out-of-phase loading conditions.

Applications of SPD to Enhance the Structural Integrity of Corroded Airframes

Abstract In the past decades, particularly since the Aloha Airlines (AA243) in 1989, corrosion has become one of the primary considerations in both military and commercial aircraft. Numerous authors have noted that corrosion can impact heavily on the economics, maintenance and safety of aircraft fleets. A number of different types of corrosion have been detected on aircraft structural aluminium alloys with exfoliation, pitting, intergranular corrosion and stress corrosion cracking being the prominent types of corrosion generally occurring in structural components exposed to corrosive environments. The additive metal technology of Supersonic Particle Deposition offers an effective methodolgy of repairing such damage.