Multi-site Damage Research Articles

Statistical Performance Assessment of an NDE-Based SHM-DP Methodology for the Remaining Fatigue Life Prediction of Monitored Structural Components and Systems

Integrated structural health monitoring and damage prognosis (SHM-DP) methodologies, coupled with sensor-based nondestructive evaluation (NDE) techniques, are becoming increasingly important for the near-real-time condition assessment (i.e., SHM) and future performance predictions (i.e., DP) of aging mechanical systems, civil structures, and infrastructure networks, as well as automotive, naval, and aerospace vehicles. A successful SHM-DP strategy, capable of identifying all critical damage mechanisms while accounting for all relevant sources of uncertainty, can be used as an advanced tool to effectively and optimally manage the life-cycle of the monitored system, recursively forecast its remaining useful life (RUL), and ultimately reduce the overall ownership cost through dynamic reliability-based inspection and maintenance (RBIM) plans, system downtime minimization, catastrophic failure prevention, and potential RUL extension. In this perspective, fatigue damage propagation is one of the most critical and unpredictable deterioration processes for a large variety of structural and mechanical systems that are subjected repeatedly to cyclic and/or random operational loading during their service life. Within this limited scope, the authors developed a comprehensive NDE-based SHM-DP framework for recursively predicting the time-varying system reliability and the remaining fatigue life (RFL) of monitored systems subjected to deterioration by multi-site fatigue damage propagation. This paper provides a brief overview of the proposed framework and then uses a set of experimental fatigue test data to perform a thorough statistical performance assessment of the developed methodology at the local reliability component level (i.e., single damage mechanism and single damage location) including NDE detectability and measurement uncertainty as well as both load and model parameter uncertainty.

Multiple crack propagation by DBEM in a riveted butt-joint: a simplified bidimensional approach

A Multi-Site Damage (MSD) crack growth simulation is presented, carried out by means of Dual Boundary Element Method (DBEM), in a two-dimensional analysis of a cracked butt-joint made of aluminium 2024 T3. An equivalent crack length is proposed for an approximated 2D analysis of a 3D problem where the crack front assumes a part elliptical shape due to secondary bending effects. The assumptions made to perform such simplified bidimensional analyses are validated by comparing numerical results with experimental data, the latter obtained from a fatigue tested riveted butt-joint.

Numerical predictions of carburisation and crack evolution using a combined diffusion rate and remaining multi-axial creep ductility damage model

A novel numerical damage model has been developed to combine Fick’s second law of diffusivity with microstructure modelling of environmentally assisted creep damage. The environmental acceleration of material degradation has been modelled and compared with experimental observations. The combined multi-site damage and crack growth model for creep and environmentally assisted time-dependent material oxidation/carburisation based on a gas/solid interface diffusion and non-linear time-dependent creep mechanism is proposed. Numerical predictions are presented to develop a methodology for component lifing. The model allows for the development of a hardened layer due to surface oxidation and predicts damage and cracking during subsequent creep under an applied load. The simulated grain mesh structure used can replicate surface healing or diffuse intergranular cracking and material depletion emanating from the gas/solid surface interface by quantifying the strength ratios between grain and grain boundaries. In this article, oxidation/carburisation is estimated both analytically and numerically using Fick’s diffusion laws and carbon/steel diffusion flux properties available in the literature. It is also shown that carbon diffusion distribution can be related to grain hardening due to carburisation as well as grain/grain boundary strength ratios which could vary as much as a factor of 0.5. The model is validated by comparing with actual oxidation/carburisation data for the long-term oxidised 9-12 Cr steels operating at high temperatures. Finally, it is shown that the mode and rate of surface oxidation and hardening, depending on whether the material is homogenous or contains micro-cracks substantially affects the life time of a component under high temperature creep loading.

Bleeding sites in elderly trauma patients who required massive transfusion: a comparison with younger patients

IntroductionAmong elderly patients with severe trauma, the sites of massive hemorrhage and their clinical characteristics are not well understood. Therefore, we investigated the sites of massive hemorrhage in patients with severe trauma, and compared the results for younger and elderly patients. MethodsA cohort of severe trauma patients (Injury Severity Score ≥16) admitted from March 2007 to December 2014 was reviewed retrospectively. The inclusion criterion was massive bleeding, which was defined as bleeding that required the transfusion of ≥10 red cell concentrate units within 24 hours of admission, or as cases of early death that occurred despite continuous blood transfusion and before the patient could receive ≥10 red cell concentrate units within the first 24 hours after their admission. ResultsEighty-four patients met our inclusion criterion. The younger group (<65 years old) included 40 patients (48%), whereas the older group (≥65 years old) included 44 patients (52%). The percentage of nondiagnosable cases at the primary survey (massive bleeding due to multisite damage caused by a bone fracture or contusion, retroperitoneal hematoma without a pelvic ring fracture and with stable pelvic ring fracture) was 14% in the younger group and 40% in the older group (odds ratio, 3.92; 95% confidence interval, 1.37-11.27, P = .017). ConclusionsEven if no abnormalities are observed at the primary survey of elderly patients with severe trauma, physicians should consider the possibility of massive bleeding.

Crack growth at fastener holes containing intergranular cracking

This paper describes the results of a continuing study into a new class of multi-site damage problems, viz: the interaction between intergranular cracks and cracks that grow from small naturally occurring material discontinuities at a fastener hole, and extends the findings of the prior investigation by considering the effect of contact between opposing faces of the intergranular cracks. This study suggests that, for the particular problem analysed, the level of contact between the opposing surfaces of the intergranular cracks has little effect on crack growth and that intergranular cracks have little effect, i.e. less than approximately 11%, on crack growth.

An extended diffraction tomography method for quantifying structural damage using numerical Green’s functions

Existing damage imaging algorithms for detecting and quantifying structural defects, particularly those based on diffraction tomography, assume far-field conditions for the scattered field data. This paper presents a major extension of diffraction tomography that can overcome this limitation and utilises a near-field multi-static data matrix as the input data. This new algorithm, which employs numerical solutions of the dynamic Green’s functions, makes it possible to quantitatively image laminar damage even in complex structures for which the dynamic Green’s functions are not available analytically. To validate this new method, the numerical Green’s functions and the multi-static data matrix for laminar damage in flat and stiffened isotropic plates are first determined using finite element models. Next, these results are time-gated to remove boundary reflections, followed by discrete Fourier transform to obtain the amplitude and phase information for both the baseline (damage-free) and the scattered wave fields. Using these computationally generated results and experimental verification, it is shown that the new imaging algorithm is capable of accurately determining the damage geometry, size and severity for a variety of damage sizes and shapes, including multi-site damage. Some aspects of minimal sensors requirement pertinent to image quality and practical implementation are also briefly discussed.

Improved Probabilistic Remaining Useful Life Estimation in Structures: Modeling Multi-site Fatigue Cracking in Oil and Gas Service Structures

The purpose of this research is to develop a multi-site damage probabilistic life prediction model that could be used to assess the integrity of engineering structures susceptible to fatigue in presence of neighboring cracks. Both experiments and simulation were used to produce the data required for the model development. The experiments were performed to investigate the interaction of two adjacent semi-elliptical cracks under cyclic loading. A series of tests at different loads and for different crack aspect ratios were conducted under uniaxial constant amplitude fatigue loads on API-5L grade B steel samples. Crack growth rate of two initial semi-elliptical cracks was investigated both on the sample surface and in the depth direction. Moreover, Crack growth and interaction was investigated using a simulation technique that incorporates the stress intensity factor of a single crack with an existing cracks interaction correction factor models from the literature. Finally, a Bayesian inference modeling technique is adopted to estimate the life prediction model parameters, assess any model bias and uncertainty and validate it.

PROCEDURE RESEARCH MULTI-SITE DAMAGE ON THE SPECIMEN WITH HOLES

Aviation construction subjected to cyclic load, in consequence of which, near stress concentrators (rivet holes), emerge and develop fatigue cracks. The development of fatigue crack reduces the residual strength of the structure and further, its destruction. This work describes the selection of the sample to assess multi-site damage. Technique drilling rivet holes. Mounting the sample on the test machine. Processing of the results of the experiment. The data for crack growth in the aviation alloy D16AT. The coefficients C and n equations Paris Law that characterize properties of the material. The dependence of the crack growth rate of the stress intensity factor (SIF). The rate of crack growth and the number of cycles until a crack of specified length. According to experimental data obtained prediction of fatigue cracks. о� ����� �Ĺ �о экспериментальным данным получен прогноз развития усталостных трещин.

Boundary element simulation of fatigue crack growth in multi-site damage

This paper presents an efficient and automatic scheme for modelling the growth of multiple cracks through a two-dimensional domain under fatigue loading based on linear elastic fracture mechanics. The dual boundary element method is applied to perform an analysis of the cracked domain and the J -integral technique is used to compute stress intensity factors. Incremental crack propagation directions are evaluated using the maximum principal stress criterion and a combined predictor–corrector algorithm implemented for propagation angle and increment length. Criteria are presented to control the mesh used on the slower growing cracks in the domain, improving computational efficiency and accuracy by the use of virtual crack tips to avoid the need for severe mesh grading. Results are presented for several geometries with multi-site damage, and sensitivity to incremental crack length is investigated. The scheme demonstrates considerable advantages over the finite element method for this application due to simplicity of meshing, and over other boundary element formulations for modelling domains with large ranges of crack growth rates.

Development of design charts for stress intensity factors at tips of multi-site cracks in unstiffened curved panels

Multi-site damage need to be addressed and evaluated in order to assess the integrity of aging aircraft structures. One of the problems recognized in the recent times is the effect of interaction between two or more cracks in the close neighborhood in such structures. The present paper deals with such a problem and presents numerical estimates of stress intensity factors at a crack tip in an un-stiffened curved panel with a secondary crack in the vicinity of a primary crack. The results are presented in the form of design charts. These results should be useful in evaluation in the damage tolerance evaluation of aircraft structures with multi-site damage.

Application of supersonic particle deposition to enhance the structural integrity of aircraft structures

Aircraft metal components and structures are susceptible to environmental degradation throughout their original design life and in many cases their extended lives. This paper summarizes the results of an experimental program to evaluate the ability of Supersonic Particle Deposition (SPD), also known as cold spray, to extend the limit of validity (LOV) of aircraft structural components and to restore the structural integrity of corroded panels. In this study [LU1]the potential for the SPD to seal the mechanically fastened joints and for this seal to remain intact even in the presence of multi-site damage (MSD) has been evaluated. By sealing the joint the onset of corrosion damage in the joint can be significantly retarded, possibly even eliminated, thereby dramatically extending the LOV of mechanically fastened joints. The study also shows that SPD can dramatically increase the damage tolerance of badly corroded wing skins.

A study into the interaction of intergranular cracking and cracking at a fastener hole

This paper describes the results of a study into a new class of multi-site damage problems, viz: the interaction between multi-layer intergranular cracks and cracks that grow from small naturally occurring material discontinuities at a hole. The nature of the intergranular cracks, which are perhaps better thought of as delaminations rather than cracks, are chosen so as to approximate those found at fastener holes in RAAF AP3C (Orion) aircraft which arise due to ingress of the environment down the holes in the wing structure. The particular form of the multi-site damage studied involves two intergranular cracks (delaminations) that (in the vicinity of the hole) divide the plate thickness into thirds) interacting with cracks that emanate from the bore of the hole in a 7075-T6 aluminium plate.

Lamb-Wave Based Technique for Multi-Site Damage Detection

The propagation characteristic of Lamb waves activated by Piezoelectric actuators and collected by sensors in a stiffened panel has been investigated. A network of actuators is used to scan the structure before and after the presence of damage. A diagnostic imaging algorithm has been developed based on the probability of damage at each point of the structure measured by the signal reading of sensors in the baseline and damaged structure. A damage localization image is then reconstructed by superimposing the image obtained from each sensor-actuator path. Three-dimensional finite element model with a transducer network is modelled. Damage is introduced as a small softening area in the panel. Applying the imaging algorithm, the damage location was predicted with good accuracy. The validity of the algorithm was tested for multiple damages.

Analytical Determination of Residual Strength and Linkup Strength for Flat Panels, with Multiple Site Damage

The problem of multiple site damage in aged airplane fuselage is handled in this paper. The analytical and numerical procedures used for the estimation of the strength of a flat panel with such multi-site damage are presented. Further, numerical results are presented on the residual strength of the panel using fracture mechanics-based approach and the stress levels when the leading crack is likely to link up with multiple site damage cracks. The presence of multiple site damage cracks in the vicinity of leading crack significantly decreases the residual strength of the panel. The model is verified using experimental data from the open literature and the predictions are in good agreement with the measured residual strength.

Evaluation of the Effects of Corrosion on Fatigue Life of Clad Aluminum Alloy 2024-T3-Riveted Lap Joints with Acoustic Emission Monitoring

Corrosion affects the fatigue life of clad aluminum alloy-riveted lap joints, such as those found on an aircraft fuselage structure. Single-, double-, and triple-column-riveted lap joint specimens were fabricated and corroded in a Q-Fog accelerated corrosion chamber for five months using an ASTM G85-A5 prohesion test. Specimens were taken out of the chamber every 4 weeks, and the corrosion products which had been deposited on them were removed by immersion in concentrated nitric acid. For each corroded specimen, the mass loss with corresponding corrosion rate was determined. The specimens were fatigue loaded to failure on an MTS Universal Testing Machine with acoustic emission monitoring. Results indicate that exposure of lap joint specimens to this corrosive environment increased corrosion (mass loss), corrosion rate, and significantly reduced fatigue life. For a prolonged exposure in the corrosive environment, the fatigue life was reduced to zero, which has significant implication for aging aircraft. Acoustic emission monitoring successfully detected fatigue failure. Two failure modes, multisite crack damage and shear of the rivets, were observed.

Classification of multi-site damage using support vector machines

Pattern recognition is now well-known to be a powerful approach to addressing the higher levels of damage identification e.g. location and severity assessment of damage. However, a major problem in implementation for real structures is the need for training data associated with all possible damage states. Even if appropriate data were available for individual damage states, the combinatorial explosion in states which occurs when multiple simultaneous damages are present would usually prohibit a pattern recognition approach. One approach to the solution of this problem is to construct classifiers on the basis of single damage data which will generalise to multiple damage states; the current paper is a very preliminary step in this direction. In the first part, a comprehensive multiple damage feature database is established as the result of an experimental campaign on a full-sized aircraft wing structure; in the second part, a classifier based on the support vector machine paradigm is investigated. The paper also considers how data visualisation can shed light on which features are likely to generalise best from the single damage problem to the multiple damage case.

A PROBABILISTIC FATIGUE LIFE ESTIMATION APPROACH FOR COMPLEX STRUCTURE AND COMPONENT

Fatigue life corresponding to specific failure probability is much more significant than mean life, since fatigue fracture is a highly uncertain phenomenon. For a complex structural component with several damage sites, the contributions of the every damage site are stochastic in nature. Moreover, the damages or fatigue lives of the individual vulnerable sites are neither completely correlated nor perfectly independent of each other. The present paper develops a probabilistic fatigue life evaluation approach to predict the probabilistic fatigue life of complex mechanical or structural components containing multiple damage sites. By correctly dealing with the failure dependence of the every damage sites and characterizing the fatigue life random variable of a multi-site-damage (MSD) structure or component using the extreme statistic of the fatigue lives of the individual damage sites, probabilistic fatigue life of MSD structural component is reasonably predicted.

A simple and accurate method for computing stress intensity factors of collinear interacting cracks

The multi-site damage problem of aging aircraft has raised the issue of obtaining accurate stress intensity factors of interacting collinear cracks under external load. The simple approximate technique proposed by Kachanov (1987) [8] is specialized in this work to arbitrary length collinear cracks with arbitrary spacing under far field tension. This approach yields simple formulas that can be used to quickly and accurately assess the mode I stress intensity factors of any number of collinear cracks in an infinite body under far field tension considering interactions between all cracks. The advantage of this approach over others is that it is simple, has good accuracy, and is very efficient. Kachanov's method is further enhanced here such that a repeating subpanel of cracks of arbitrary number, size, and spacing can be modeled implicitly with resulting efficiency gains. The effect of interaction cracks can be used to build up stress intensity factor solutions for more complex geometries. Several numerical examples and timing studies are used to demonstrate the efficacy of this approach.

Multi-site damage localization in anisotropic plate-like structures using an active guidedwave structural health monitoring system

A new approach for structural health monitoring using guided waves in plate-like structureshas been developed. In contrast to previous approaches, which mainly focused on isotropicor quasi-isotropic plates, the proposed algorithm does not assume any simplificationsregarding anisotropic wave propagation. Thus, it can be used to improve the probability ofdetection.In this paper the mathematical background for damage localization in anisotropic plateswill be introduced. This is an extension of the widely known ellipse method. The formalismis based on a distributed sensor network, where each piezoelectric sensor acts in turn as anactuator. The automatic extraction of the onset time of the first waveform in thedifferential signal in combination with a statistical post-processing via a two-dimensionalprobability density function and the application of the expectation-maximization algorithmallows a completely automatic localization procedure. Thus, multiple damages can beidentified at the same time.The present study uses ultrasonic signals provided by the spectral element method. Thissimulation approach shows good agreement with experimental measurements. A local linearneural network is used to model the nonlinear dispersion curves. The benefit of using aneural network approach is to increase the angular resolution that results from the sparsesensor network. Furthermore, it can be used to shorten the computational time for thedamage localization procedure.

Advanced Feature Selection for Simplified Pattern Recognition within the Damage Identification Framework

The paper is concerned with adopting a data-driven approach to damage detection and location on an aerospace structure without recourse to an artificial neural network. Five advanced features are selected, each detecting the removal of only one of five inspection panels on the structure. The features give perfect classification for damage location for single-site damage and 98.1% correct classification for multi-site damage scenarios, using a statistically calculated threshold. However, if the threshold values for two of the five features are altered slightly, 100% correct classification would be possible for single- and multi-site damage.