Multiple Site Damage Research Articles

Experimental investigation on multiple-site fatigue failure of the hollow compressor impeller

This study conducts a systematic experimental investigation on multiple-site damage (MSD) in aero-engine hollow compressor impellers, focusing on MSD fatigue fracture growth and coalescence behaviors and mechanisms. Comparative analyses of literature research, standard specimen tests, and full-scale disk test experiences conclude that elevated cyclic radial stresses, especially in thin-walled disk webs, facilitate the occurrence of MSD fractures. Extending from previous numerical studies, a full-scale hollow impeller fatigue test on a high-speed spin tester revealed an over 153 mm circumferential through-wall MSD failure in the outlet web, accounting for 33 % of its circumference. Challenges in traditional nondestructive inspections for such complex structures emphasize the effectiveness of synergetic fractography analysis in investigating the MSD failure. Thousands of scanning electron microscope (SEM) images, examined at macro and micro levels with the assistance of metallography and stereomicroscopy, unfolded the MSD fracture features of crack initiation, propagation, coalescence, and interaction. Besides the well-known macroscopic rachet marks, this study is the first to identify and analyze various microscopic MSD merging features, such as terraces, plastic ridges, shear sliding surfaces, cliffs, plateaus, and lowlands. Differences in fracture merging and growth features enable us to distinguish MSD component cracks. Quantitative fractography analysis revealed that the crack initiation occurred 1126 test cycles earlier, and that the overall crack propagation life slightly surpassed 1646 cycles as new cracks merged and joined. Diverse crack growth durations and rates across 11 fracture paths indicate asynchronous initiations and confirm interactions among component cracks. Cracks initiate flatter near the central crack colony but appear bumpier and more pronounced plastic deformation at the periphery. The overall MSD fracture evolution process was backtracked through synergetic fracture assessments, detailing the merging process of varied component crack colonies, enhancing our understanding of the spatial–temporal evolvement of cracking scenarios in actual aero-engine impellers.

Study on multiple fatigue cracks growth in the web of a hollow compressor impeller

The fatigue crack growth characteristics of a hollow impeller whose thin-walled web has the propensity to nucleate multiple site damage (MSD) were investigated. To study its MSD crack growth, a simulation framework was proposed to accomplish noncoplanar crack coalescence and user-defined growth. It employed a modified linkup criterion and could account for the geometric constraints of cracked bodies, crack closure, and the kinking and twisting angle introduced by KII and KIII. Interaction features of the multisite crack growth were studied by simulating carefully designed cracking scenarios, and the crack cascade’s typical evolution over time and space was deduced. Results showed that larger adjacent cracks would introduce a stronger interaction effect. For crack colonies, the stress intensity factor would increase and remain higher between different merging positions. The crack growth was predominantly in tensile mode (greater than 95%) before coalescence, whereas the in-plane and out-of-plane shear modes increased noticeably to about 30% during and after coalescence. MSD will adversely affect the impeller fatigue evaluation due to faster crack growth, more complex crack profiles, and difficulty in determining the critical crack size due to the jump-like increase of crack colonies that makes applying damage tolerance difficult.

Multiple crack growth simulation for lap-joints based on three-dimensional finite element analysis

PurposeThe fuselage riveted lap-joints are susceptible to multiple site damage (MSD) and should be considered in damage tolerance analysis. This paper aims to investigate the stress intensity factor (SIF) and crack growth simulation for lap-joints based on three-dimensional (3D) finite element analysis.Design/methodology/approachThe 3D finite element model of lap-joints is established by detailed representation of rivets and considering the rivet clamping force and friction. Numerical study is conducted to investigate the SIF distribution along the thickness direction and the effect of clamping force. A predictive method for the cracks propagation of MSD is then developed, in which an integral mean is adopted to quantify the SIF at crack tips, and the crack closure effect is considered. For comparison, a fatigue test of a lap-joint with MSD cracks is conducted to determine the cracks growth live and measure the cracks growth.FindingsThe numerical study shows that the through-thickness crack at riveted hole in lap-joints can be treated as mode I crack. The distribution of SIF along the thickness direction is inconstant and nonmonotonic. Besides, the increase in clamping force will lead to more frictional load transfer at the faying surfaces. The multiple crack growth simulation results agreed well with the experimental data.Originality/valueThe novelty of this work is that the SIF distribution along the thickness direction and the MSD cracks growth simulation for lap-joints are investigated by 3D finite element analysis, which can reflect the secondary bending, rivet clamping, contact and friction in lap-joints.

Fatigue Life Scatter Modeling and Failure Probability Prediction of Multisite Damage Structural Part under Variable Amplitude Load History

An “index stress” (a constant amplitude cyclic stress) is introduced to characterize the scatter of the fatigue life under a variable amplitude load history. It is defined as a fatigue damage weighted average of the amplitudes of the individual stress cycles in a variable amplitude load history. The fatigue test results show that the scatter of fatigue life under variable amplitude load histories can be well characterized by such defined index stress. For probabilistic fatigue life prediction and failure probability estimation, a multiple-site damage structural part is taken as a series system, and the total probability theorem is applied to reflect the effect of load history uncertainty on failure probability.

Математическая модель многоочагового усталостного повреждения заклепочных соединений

Multiple Site Damage (MSD) is one of the significant damaging factors that limit the airworthiness of aging fleet aircrafts. In case of MSD multiple fatigue cracks initiates and propagates at the rivet holes. Those cracks are relatively short in length, but with a sufficiently large number of them and an unfavorable arrangement along the rivet joint, they can join together and form a crack of a dangerous length. To prevent this type of damage it is necessary to have adequate methods for predicting the boundary state of riveted joints during MSD. A useful approach is a numerical experiment based on Monte-Carlo simulation of the MSD main random factors – the formation of initial cracks and their growth. This paper presents a probabilistic model for predicting the initial stage of MSD – destruction of at least one bridge between the adjacent holes. A level I model is considered, which describes the process of fatigue failure of specimens without rivets but with multiple holes, which are typical for riveted joints. The initiation of fatigue cracks and their growth are modeled taking into account the laws of damage development obtained experimentally on specimens with multiple cracks. So, to simulate the random initiation of cracks in time the Weibull distribution is used. The parameters of this distribution depend on the applied stress. The growth of cracks is described by the Paris' equation, taking into account the experimentally confirmed correlation between the coefficients of this equation. The model assumes that each initiated crack propagates according to a random value of the Paris' equation exponent. The distribution of such a random value corresponds to a logarithmically normal law with experimentally obtained parameters. The criterion for the possible join of opposite cracks growing from adjacent holes is the uniting of plastic deformation zones at the tips of such cracks. The results of modeling are presented in the form of multiple site damage realization field of points in the coordinates of the number of cycles before the initiation of cracks vs. the number of cycles before the destruction of the bridge between holes.

Fatigue performance prediction of Al-alloy 2024 plates in riveted joint structure

The purpose of this paper is to estimate the fatigue life of 2024 aluminum plates in riveted joints under the influence of cyclic loading. First, the factors affecting the plastic equivalent strain resulting from the riveting process have been identified using the finite element method. For this purpose, a three-dimensional elastoplastic model is used for simulation. The relevant geometrical parameters such as the rivets shank length, the rivets shank diameter, and dimensional tolerances as well as pattern of where the rivets are and the material of the rivet joints are determined and subsequently the fatigue life of the aluminum joint is obtained by experimental tests. Then the fatigue life of the riveted joints is calculated using multiple site damage (MSD). To prove the validity of the analytical results, numerical modeling and experimental tests are performed on 2024 aluminum plates in riveted joints. Numerical analysis has been performed using linear fracture mechanics and boundary element method (BEM) for the fatigue crack growth of 2024 aluminum plates in riveted joints. Finally, the results of the analytical method for fatigue life are compared with experimental tests and numerical method that results in acceptable achievements.

A Novel Assisted Artificial Neural Network Modeling Approach for Improved Accuracy Using Small Datasets: Application in Residual Strength Evaluation of Panels with Multiple Site Damage Cracks

An artificial neural network (ANN) extracts knowledge from a training dataset and uses this acquired knowledge to forecast outputs for any new set of inputs. When the input/output relations are complex and highly non-linear, the ANN needs a relatively large training dataset (hundreds of data points) to capture these relations adequately. This paper introduces a novel assisted-ANN modeling approach that enables the development of ANNs using small datasets, while maintaining high prediction accuracy. This approach uses parameters that are obtained using the known input/output relations (partial or full relations). These so called assistance parameters are included as ANN inputs in addition to the traditional direct independent inputs. The proposed assisted approach is applied for predicting the residual strength of panels with multiple site damage (MSD) cracks. Different assistance levels (four levels) and different training dataset sizes (from 75 down to 22 data points) are investigated, and the results are compared to the traditional approach. The results show that the assisted approach helps in achieving high predictions’ accuracy (<3% average error). The relative accuracy improvement is higher (up to 46%) for ANN learning algorithms that give lower prediction accuracy. Also, the relative accuracy improvement becomes more significant (up to 38%) for smaller dataset sizes.

Residual Strength Prediction of Aluminum Panels with Multiple Site Damage Using Artificial Neural Networks.

Multiple site damage (MSD) cracks are small fatigue cracks that may accumulate at the sides of highly loaded holes in aging aircraft structures. The presence of MSD cracks can drastically reduce the residual strength of fuselage panels. In this paper, artificial neural networks (ANN) modeling is used for predicting the residual strength of aluminum panels with MSD cracks. Experimental data that include 147 unique configurations of aluminum panels with MSD cracks are used. The experimental dataset includes three different aluminum alloys (2024-T3, 2524-T3, and 7075-T6), four different test panel configurations (unstiffened, stiffened, stiffened with a broken middle stiffener, and bolted lap-joints), many different panel widths and thicknesses, and the sizes of the lead and MSD cracks. The results presented in this paper demonstrate that a single ANN model can predict the residual strength for all materials and configurations with high accuracy. Specifically, the overall mean absolute error for the ANN model predictions is 3.82%. Furthermore, the ANN model residual strength predictions are compared to those obtained using the most accurate semi-analytical and computational approaches from the literature. The ANN model predictions are found to be at the same accuracy level of these approaches, and they even outperform the other approaches for many configurations.

Calculation of Stress Intensity Factor on Typical MSD Structure of Aircraft Wing

The research object of this paper is Multiple site damage (MSD) structures which are common in the service of aging aircraft. A complex function modified method is proposed to calculate the stress intensity factor of MSD cracks. On the basis of Muskhelishvili complex function method and its approximate superposition method, the concept of equivalent crack is introduced to solve the accurate stress function of MSD cracks. The stress intensity factor of MSD hole edge crack of infinite plate under far end uniform load is calculated and compared with the finite element method. The method is applied to the typical MSD structure of aircraft wing, and the stress intensity factors of cracks in three modes are compared and analyzed. This method can be used to discuss the damage tolerance of MSD structure, it has certain application value in the reliability evaluation of aircraft.

A Compounding Method for Calculating Stress Intensity Factor of Pipelines with MSD Structure

Multiple site damage (MSD) often occurs during the service of the nuclear pipeline. The typical structure is multiple cracks on circumferential internal surface of the pipeline. In order to evaluate the reliability of the nuclear pipeline and discuss the damage tolerance of the pipeline with multiple cracks, a compounding method for calculating stress intensity factor (SIF) of pipelines with MSD structure is proposed. Compared with the finite element results, the compounding method is accurate and reliable. The SIF of double cracks and triple cracks are studied by using crack spacing as single factor variable. The result shows that the SIF of MSD cracks can be influenced by the crack spacing of the adjacent crack.

A modified residual strength prediction method for aluminum alloy plates with MSD

A novel modified ligament yield criterion to calculate the residual strength for aluminum alloy panels with Multiple Site Damage (MSD) is proposed. Different from the conventional ligament yield criterion the novel method is based on modifying the plastic zone of the crack tip. The microdefects can be represented by the coefficient of plastic zone determined after the material processing. Then modified plastic zone size is much closer to that of the fracture. The residual strength tests are performed on plate specimens in 7075-T651 aluminum alloy sheet with MSD. Furthermore, we also verify the availability and capability of the modified method with public literature. The predicted residual strengths are within 3% averagely of the corresponding test results, much more accurate in comparison with other methods.

Stress intensity factors numerical calculations for two penny shaped cracks in the elastic solid

In the face of enormous costs of developing new aircraft, high fuel prices, increasingly demanding environmental standards, harsh economic race and the fight against competing manufacturers, today's aviation is adapting not by producing brand new, more advanced aircraft, but through modifying existing ones by incorporating new more energy efficient engines, in accordance with today's leading criterion that requires minimal fuel consumption, enabling at the same time the longest possible range. The structure of these “new” aircraft is metallic and almost identical as it has been for decades, but now with the new requirement that its service life has to be extended, as long as it is economically justified. In such environment, it is clear that one of the leading tasks is to have the best possible assessment of the negative impact of the mechanism of Multiple Site Damage (MSD), which represents an introduction to the phenomenon of Widespread Fatigue Damage (WFD), the most insidious degradation mechanism of a metallic structure, whose significance was supposed to be diminished in recent decades with accelerated growth in the application of the composite structures. In line with the aforementioned trends, this paper aims to evaluate to what extent today's numerical methods are capable to contribute to the assessment of behavior of such phenomena and which of the proposed approaches could be used for investigating of the fracture problems, both accurately and efficiently. This assignment was conducted through numerical computations of stress intensity factors (SIFs) for two penny shaped cracks in elastic solid estimated by the methods that the authors of this paper consider to be the most relevant in the last few years.

Study of multiple-site structural damage detection application using vibration response

The identification effects of different modal parameters (displacement mode, rotation mode, and curvature mode) of the beam structure on structural damage during crack initiation are studied in this paper. Firstly, the displacement mode, the rotation mode and the curvature mode of the two-span simply supported beam are calculated using the vibration response. Secondly, combining the multi-resolution and singularity analysis characteristics of continuous wavelet transform, the wavelet transform coefficients of three modal parameters are calculated to identify the singularity of structural modal parameters. Thirdly, the recognition effects of each method on structural damage are compared by adding different degrees of random noise to the simulation data. The results show that the damage identification methods based on three different modal parameters can locate small-scale structural damage, but the anti-noise performance of each method is different, the anti-noise performance of the rotation mode is the best, the displacement mode is second and the curvature mode is poor. Finally, the experimental data is used to verify the recognition effect of each damage identification method based on different modal parameters on the actual structural damage. The experimental results show that the damage recognition effect of the rotation mode parameters is better, while the displacement mode and the curvature mode parameters are similar, but the curvature mode method is more likely to be interfered at the smaller damage. These analysis results provide a reference for the selection of different damage identification parameters according to the environment and response characteristics in engineering practice.

A modified model of residual strength prediction for metal plates with through-thickness cracks

A new model, accurate and easy-to-apply, was proposed to predict the residual strength for the metal plates intact or with different damage degrees. In this model, we introduced the damage degree factor (DDF) to quantify the initial damage condition of a plate. The middle crack tension tests and multiple site damage (MSD) tension tests were performed on plate specimens in aluminum alloy LY12-CZ and 2524-T3 respectively. For various damage degrees, the predicted results of this new model show an improved correlation with test results compared to net section yield criterion, K-apparent criterion and Duong’s method (Duong et al., 2001).

Weight functions and stress intensity factors for two unequal-length collinear cracks in an infinite sheet

The configuration of two unequal-length collinear cracks is common in aircraft structures containing multiple site damage. In this paper, the weight functions for this crack configuration are derived from the exact crack opening displacements under remote tension, and verified by reducing to the existing exact weight functions of two equal-length collinear cracks and by good agreement with finite element analysis. These weight functions are used to calculate the stress intensity factors and crack opening displacements for different load cases. The results agree very well with those from literature and finite element analysis. The weight functions for two unequal-length cracks provide a highly efficient and accurate tool for damage tolerance analysis of structure with multiple site damage.

A multi-fidelity modelling approach to the statistical inference of the equivalent initial flaw size distribution for multiple-site damage

A new methodology for the statistical inference of the Equivalent Initial Flaw Size Distribution (EIFSD) using the Dual Boundary Element Method (DBEM) is proposed. As part of the inference, Bayesian updating is used to calibrate the EIFS based on data obtained from simulated routine inspections of a structural component from a fleet of aircraft. An incremental crack growth procedure making use of the DBEM is employed for the modelling of the simultaneous growth of cracks in the structure due to fatigue. Multi-fidelity modelling, in the form of Co-Kriging, is used to create surrogate models that act in place of the DBEM model for the expensive Monte Carlo sampling procedure required for the statistical inference of the EIFSD. The proposed methodology is applied to a numerical example featuring a long fuselage lap joint splice in the presence of Multiple Site Damage (MSD). Results show that the EIFSD can be accurately estimated within 10% error with data from just 50 inspections. The employed Co-Kriging models proved to be effective substitutes for the DBEM model, providing significant reductions in the computational cost associated with the implementation of the proposed statistical inference methodology.

FEATURES OF FRACTURE OF METAL ITEMS (HARDWARE) MADE OF STEEL 30KhGSA

The reasons for destruction of fasteners made of steel 30KhGSA are analyzed. It is shown that in case of destruction of bolts in the package connecting frame elements, damage of bolts results from fretting corrosion, attributed to mutual friction between bolts and elements of skin, belt and tape. Fretting corrosion develops in the sites where cadmium coating of bolts is damaged upon friction. The occurrence of fracture in the belt, skin and tape is promoted by break-in scuffing of the openings and corrosion damages (fretting corrosion) triggering the intergranular fracture, most severe in the skin and tape. The reason for damage arises from structural and technological factors: increased stresses present in the package, micro-displacements of the mating parts, violation of the cadmium coating, insufficient corrosion protection and environmental impact. Destruction of self-locking nuts made of 30KhGSA steel with anti-corrosion cadmium coating used in the assembly of the auxiliary gas turbine engine is attributed to their significant overheating, i.e., the impact of molten coating on the surface of loaded nuts and the penetration of the liquid metal into the base material along the grain boundaries. The use of cadmium coatings in operation of steel parts at a temperature exceeding (even briefly) the melting temperature of Cd ( T melt = 320.9°C), under conditions of contact with stressed metal due to the Rebinder effect, leads to a sharp loss of strength and premature failure of the part. Study of the destruction of galvanized bolts made of 30KhGGSA steel during the repeated-static testing revealed that several factors contribute to the onset of fatigue multiple site damage: presence of the rough risks; sandblasting, performed before galvanizing using large sand fractions, which contribute to the premature origin of cracks; areas with deposits formed upon coating with a loose adhesion of the layer. The presence of a large number of pits along with the fatigue grooves indicates to high load of the bolt material upon operation. The study was carried out using optical and scanning electron microscopy and X-ray microanalysis.

Theoretical analysis of fatigue failure in mechanically fastened Fibre Metal Laminate joints containing multiple cracks

Mechanically fastened joints are susceptible to the presence of multiple-site damage (MSD) cracks in the critical fastener row. Different from the MSD growth in joints consisting of metallic substrates, the two coupled metal crack growth and interfacial delamination propagation failure mechanisms in Fibre Metal Laminates (FMLs) make the prediction of fatigue behaviour in FML joints with MSD scenario burdensome and impractical when considering all factors influencing the fatigue performance. This paper presents a theoretical study on the MSD crack growth behaviour in mechanically fastened FML joints with a focus of modelling the effects of bearing and bypass loads. The proposed model in this paper is built upon analytical models dealing with MSD growth in flat FML panels and single crack growth in FML panels subjected to a combined tension-pin loading case. This model would be particularly useful for symmetric FML joints where no secondary bending effects present. A deliberately designed symmetric FML joint was tested to validate the proposed model. The model captures the rapid crack growth in the vicinity of fastener holes due to bearing stresses and crack acceleration due to the interaction of cracks. It is identified that the load redistribution between intact fastener rows and the cracked fastener row accelerates crack growth with crack length. The effects of secondary bending stresses in FML joints on the crack growth behaviour is extensively discussed. The performance of the proposed model for single lap FML joints is also examined using test data from open literature. It is found that the proposed model provides a conservative prediction for the tested single shear lap FML joint from open literature.

Approximate determination of stress intensity factor for multiple surface cracks

In this paper a versatile and easy to use approximate procedure was used for the estimation of mode I stress intensity factors in case of multiple surface cracks in a three dimensional elastic body, subjected to remote uniaxial tensile loading. The mentioned method uses known solutions for either 2D or 3D configurations containing only one crack, and takes into consideration the interaction effect between cracks. This procedure, which is based on the principle of superposition, was applied on a configuration with three coplanar semi-elliptical cracks embedded in three dimensional elastic body, subjected to remote uniaxial tensile loading. All cracks are located in the same plane at the same distances, in the middle of the body and the applied stress is perpendicular to the cracks plane. For the verification purposes, the stress intensity factors solutions were obtained by using finite element method based computer program. The conducted analysis showed that approximate method is, above all, fast and efficient tool for stress intensity factors assessment even in the case of 3D configurations with multiple site damage. The comparison between results also showed the significance of accurate calculation of stress intensity factors, in order to provide a better understanding and prediction of 3D multiple cracks propagation.

Method to analyse multiple site damage fatigue before and after crack coalescence

The Multiple Site Damage is a phenomenon that appears in e.g. aircraft fuselages and weld geometries. This article introduces a method for solving the Multiple Site Damage fatigue in a 2D specimen. The cases studied are for Linear Elastic Fracture Mechanics situations, isotropic materials and crack growth governed by the Paris Law regime. Already known fracture mechanics concepts are merged together in an innovative algorithm that increases computational efficiency by optimizing the number of Finite Element Analyses necessary for fatigue calculations. A new approach to crack coalescence is presented by using the application limit of Linear Elastic Fracture Mechanics. Comparisons with analytical, experimental and other software results have shown the reliability of this method for several cases. A final explanatory example of a plate with multiple cracks and a hole shows the capabilities of the method proposed.