Multiple Site Damage Cracks Research Articles

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.

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.

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.

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.

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.

Crack Growth Life Estimating for MSD Panel

The problem of multiple site damage (MSD) has got more attention in ageing structures. Cumulative effects of interacting cracks may significantly degrade the damage tolerance capacity of structures. Cracks caused by MSD are extremely difficult to detect and greatly reduce the residual strength, fatigue life and overall structural integrity of aircraft panels. This paper presents a simple numerical method, which use the principles of fracture mechanics and the computation results, to predict the fatigue crack growth life of MSD structure. Comparing with calculating crack growth life cycle by cycle, this method will save much time. To verify the validity of the proposed method, experiment was conducted and reported with simulation specimen of representative MSD structure with 5 details. The comparison between the calculated a-N curves and the crack growth lives and the test results shows that the prediction result with this fast method is acceptable. A discussion was carried out by numerical analysis; in with typical MSD structures with different initial crack length were adopted. Crack interaction effect was found obviously, but it occurred mainly in the last part of the crack growth lives. The relative size of MSD cracks depends significantly on the distribution of the initial cracks.

Stress Intensity Determination for MSD Riveted Stiffened Panels in the Presence of Corrosion

Accurate stress intensity solutions for multiple site damage (MSD) cracks in riveted stiffened panels are difficult to determine due to geometric complexity along with variations in rivet load transfer and corrosion, especially the interaction of MSD cracks. A methodology was proposed for efficiently depicting rivet in stiffened panel using finite elements. Rivet material properties were determined based on an empirical force-displacement relationship, for highly refined rivet model as well as for idealized spring element representations of rivets. Parametric studies of panels with a middle crack and a central stiffener indicate that rivets can provide comparable load transfer and relative displacement if the rivets closed to the crack are explicit modeled. Using idealized combination of explicitly and spring element representations of rivets, the stress intensity factors (SIFs) for uncorroded and corroded one-bay stiffened panels were predicted. The results show that the effect of MSD and thinning of the sheet is to increase substantially SIFs values compare to that of a single crack without corrosion. But the SIF is insensitive to corrosion of stiffener. The particular rivet material has relatively little effect on SIF values, while the rivet diameter has a significant effect.

Fatigue Testing of a Stiffened Lap Joint Curved Fuselage Structure

In April 1988, Aloha Airlines flight 243 experienced an explosive midair decompression that resulted in the separation of an 18-foot section of the fuselage crown of the Boeing 737 airplane. Investigations revealed that the linkup of small cracks emanating from multiple rivet holes in a debonded lap joint contributed to the catastrophic failure. This cracking scenario, known as multiple-site damage, is one of two sources of widespread fatigue damage; a type of structural degradation characterized by the simultaneous presence of fatigue cracks at multiple structural elements that are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement This study, sponsored by the National Aging Aircraft Research Program initiated by the Federal Aviation Administration in response to the Aloha accident, investigates multiple-site damage initiation and growth behavior in a pristine narrow-body fuselage panel. The test panel, a curved 6 x 10 ft stiffened structure containing six frames, seven stringers, and a longitudinal lap joint, was tested at the Federal Aviation Administration Full-Scale Aircraft Structural Test Evaluation and Research facility. The panel was subjected to a fatigue test with constant-amplitude cyclic loading, simulating the major modes of load associated with fuselage pressurization. Nondestructive inspections were conducted during the fatigue test to detect and monitor crack formation and growth. Multiple-site damage cracks were visually detected after about 80% of the fatigue life. Cracks developed and linked in the upper rivet row of the lap joint in the outer skin layer and formed a long fatigue crack before the termination of the fatigue test A residual strength test was then conducted by subjecting the panel to quasi-static loads until catastrophic failure. Fractographic examinations were conducted to reconstruct crack growth history. Preliminary results show multiple crack origins and significant subsurface crack growth.

A Numerical Study of MSD in Aircraft Lap Joints

Many military aircraft have reached or exceeded their original design life, and have been subject to significant increase in maintenance and repair cost due to multiple site damage (MSD). In order to assessing the effects of MSD on the structural integrity of aircraft lap joints, the wing lap joint of certain model military aircraft with MSD was analyzed using special code FRANC2D/L. The rivet holes along the top row of the outer skin of lap joint were considered as the independent structural unit for the simulated MSD cracks. The stress intensity factors (SIFs) at each crack tip with different distribution loads at the rivet holes were computed and show that the analysis results have good coherence with the available literature data. It also shows that the SIF at each crack tip s a function of crack length can be calculated by the crack growth simulation capability of FRANC2D/L. The SIF values are not sensitive to the rivet load distribution manner, which has seriously influence on MSD crack growth direction. Rivet loading can be best molded quadratic load distribution over one half of rivet hole relative to uniform load distribution and point load. As a result of this analysis, it is postulated that for MSD in aircraft lap joints, compliance measurements may provide a useful tool for assessing the structural integrity of the lap joints.

Corrosion fatigue and multiple site damage reliability analysis

This paper proposes a comprehensive seven-stage life prediction model for the reliability analysis of aging aircraft structures under multiple site damage (MSD) due to corrosion fatigue. The damage process is divided into an interaction stage and a non-interaction stage in order to consider the interactions of the MSD cracks. The structural initial state at the interaction stage is determined by bridging the pitting corrosion and MSD. Once interaction sets in, progressive failure analysis is implemented through an iterative finite element analysis procedure. The onset of crack coalescence is assumed to correspond to the onset of structural failure. The probability of crack coalescence under MSD is evaluated by using system reliability concepts. An adaptive multi-modal importance sampling technique is developed for efficiency and accuracy in probabilistic life estimation under MSD due to corrosion fatigue.

Residual Strength Prediction of Aircraft Fuselages Using Crack-Tip Opening Angle Criterion

Ductile tearing simulation and residual strength prediction of wide-body, lap-jointed fuselage panels are conducted. The damage configurations of the fuselage panels consist of a single lead crack and a lead crack with small multiple-site damage (MSD). The crack-tip opening angle fracture criterion and elastic-plastic finite element analysis are used to control stable crack advancement under conditions of general yielding. Thin-shell finite elements are used to model the fuselage structures except in the vicinity of cracks. In this region special plane-strain shell elements are used to capture three-dimensional constraint effects during stable crack growth. A global-local modeling procedure is employed in the numerical analyses. We show that accurate representation of the load transfer through the rivets is crucial for the stress distribution prediction. Modeling fatigue crack closure is essential to capture the fracture behavior at ductile tearing initiation. Breakage of a tear strap can have a major influence on residual strength prediction. Predicted crack growth and residual strength considering fatigue crack closure and tear strap failure agree reasonably well with test results. Observed and predicted results both indicate that the occurrence of small MSD cracks substantially reduces the residual strength.

An experimental and numerical analysis of residual strength of butt-joints panels with multiple site damage

The paper presents a combined experimental and computational study of the residual strength of a butt-joint panel with multiple site damage (MSD). The computational model uses a special strip-yield model adapted to MSD, which is implemented into a geometrically nonlinear finite element model. Crack initiation is modelled by a critical crack opening displacement and tearing by a constant crack opening angle criterion. Centre cracked tension tests were first performed to determine the basic fracture parameters. These fracture parameters were then used in the analysis of butt-joint panels with a large lead crack and smaller MSD cracks inserted along the upper row. The simultaneous propagation of multiple cracks in the tests was monitored continuously by aid of a specially developed image analysis technique. Loads for crack initiation, propagation and subsequent crack link-up and panel failure were computed and compared with test results. The geometrical nonlinear effects and the sensitivity to variations in fracture parameters were quantified. An excellent overall agreement was found for the most refined models.

Strength of Stiffened 2024-T3 Aluminum Panels with Multiple Site Damage

Two modie edlinkup modelsweredevelopedfordetermining thecritical stressbased on linkup (ligamentfailure ) of2024-T3aluminumpanelswithmultiplesitedamage.ThesemodelsweredevelopedforusewithstandardMilitary Handbook for Metallic Materials and Elements for Aerospace Vehicle Structures (MIL-HDBK-5G )yield strength values. For this investigation, ligament failure stresses predicted by these models are compared with test stresses determined from a variety of stiffened panels including single-bay panels with the lead crack centered between stiffeners and two-bay panels with the lead crack centered beneath a severed stiffener. The stresses predicted by the modie ed linkup models correlate well with the test data. The results of this investigation should add to the understanding of the extent to which nonlinear behavior can be modeled with simplie ed engineering models. Nomenclature a = lead crack half-length an = nominal lead crack half-length c = multiple site damage (MSD) crack length D = hole diameter Fcol = collapse stress ` = half-length for MSD crack and hole, cC D/2 L = ligament length t = panel thickness tS = stiffener thickness W = panel width WS = stiffener width

An energy approach to the link-up of multiple cracks in thin aluminum alloy sheets

An energy approach has been used in the study of the coalescence or linkage of multiple cracks in aluminum alloy sheets. The study was motivated by concern for the structural integrity of aging aircraft. Forty reported tests for 2024-T3 aluminum panels with a major crack and several multiple-site damage (MSD) cracks have been analyzed via a simple computational model with a Dugdale–Barenblatt [D.S. Dugdale, J. Mech. Phys. Solids 8 (1960) 100–104; G.I. Barenblatt, in: H.L. Dryden, Th. VonKarman (Eds.), Advances in Applied Mechanics, vol. II, 1962, pp. 55–130] type of plastic or inelastic deformation. For simplicity, the computational model considers only the plastic interaction between the major crack and two symmetrically adjacent MSD cracks in an infinite sheet under remote tensile stress. By following the approach given in [B. Cotterell, J. K. Reddel, Int. J. Fract. 13 (1977) 267–277], the specific work to cause ligament failure is found to be a linear function of the normal extent of the confined plastic region for most tests considered. A few exceptions to this linear relation are attributed to the limitation of the employed computational model. A new criterion and an engineering method to predict crack link-up in an MSD sheet are proposed based on this specific work concept, and they have been demonstrated through application to stiffened panels.

Residual strength of unstiffened aluminum panels with multiple site damage

This study investigated the residual strength of unstiffened aluminum panels with widths of 381 mm and 229 mm containing multiple site damage (MSD). The MSD usually occurs at rivet holes, or other stress concentration locations within an aircraft structure. This study simulated rivet holes with MSD, by using holes of constant diameter with small cracks, evenly spaced across the midspan of specimens. The panels were prepared by either fatiguing MSD damage at rivet holes or simulating fatigue damage by saw cuts at each hole. Each specimen was subjected to a monotonically increasing tensile load until failure occurred across the midspan of the gauge section. Five different failure criteria which do not model the stable crack extension were evaluated to predict the residual strength (failure load) for each specimen geometry. These criteria provided a wide range of residual strength predictions for wide and narrow panels with MSD. A failure criterion which involved the plastic zone (yielding) of the lead and MSD cracks gave the most accurate prediction of failure load for panels with MSD damage. The width of the specimens did not affect, in general, the trends in the prediction of failure loads from the five failure criteria.

Fracture and fatigue analysis of curved or kinked cracks near fastener holes

The finite element alternating method is an attractive and efficient method for calculating the stress intensity factor of cracks emanating from fastener holes such as the problem of multiple site damage (MSD) at the fuselage skin of aging aircraft. While the alternating method has been used to solve various engineering problems in assessing the structural integrity of aging aircraft, this method does have some limitations on the modeling of MSD cracks. This alternating method can only model cracks with mode-I fracture behavior and requires the cracks to be straight and propagate in a self-similar fashion. As a result, there is some concern if the alternating method can be applied for the case when some of the cracks enamating from the fastener holes are slightly kinked. In the present research, cracks from fastener holes with the initial kink angles of +30° and −30° are studied. It is found that the fracture and fatigue life for slightly kinked cracks near the fastener holes can be very well approximated using the stress intensity factor for pure mode-I behavior obtained by replacing the slightly kinked cracks with simple straight cracks. Henceforth, the alternating method can be used to model slightly kinked cracks with straight cracks, thereby simplifying various fracture and fatigue analyses.

Residual strength of panels with multiple site damage

This article describes research aimed at predicting the residual strength (RS) of unstiffened aluminum panels that contain multiple site damage (MSD). The initial damage consists of through-the-t hickness cracks emanating from a row of holes in the center of a 23-cm- (9-in.-) wide panel. The row of holes is aligned perpendicular to a remote tensile load, and three failure criteria are employed to predict the fracture resistance of the cracked panel. RS tests of 2024-T3 aluminum panels with MSD were conducted to evaluate the analyses. The RS tests validate a failure criterion that includes plastic zone considerations for panels with a lead crack spanning the central holes and with small MSD cracks at other holes. The RS tests also demonstrate that significant loss of strength occurs when MSD cracking exists, even when the cracks are relatively small.

MULTIPLE‐SITE DAMAGE IN AIRCRAFT FUSELAGE STRUCTURES

Abstract— Since the Aloha accident the multiple site damage (MSD) problem of riveted lap joints in aircraft fuselages has drawn much attention. The failure scenarios for a lead crack and more small MSD cracks as discussed by Broek and Swift are summarized, including recent results of a relevant test series by Broek. It shows that small MSD cracks can significantly reduce the load for unstable crack extension. Prevention of catastrophic consequences requires crack arresting capability of the structure. Related aspects of the problem are discussed with reference to failure criteria for ligament failure, the MSD problem for existing and new aircraft, and different options for crack stopper bands.