Incipient Crack Research Articles

A FEM fretting map modeling: Effect of surface wear on crack nucleation

Much research demonstrated that the fretting sliding condition greatly influences fretting damage. Small displacement amplitudes, inducing partial slip, favor cracking, whereas large dissipative sliding gross slip amplitudes favor wear. Considering a Ti–6Al–4V/Ti–6Al–4V cylinder/plane contact, this typical evolution was quantified by plotting the evolution of maximum crack length versus displacement amplitude. Under partial slip, the crack nucleated above a critical tangential loading, related to a threshold δCN_PS displacement amplitude. Above the sliding transition (δt), although tangential loading remained high, crack length decreased to zero at the gross slip threshold δCN_GS, due to surface wear extension which reduced contact stress and removed incipient nucleated cracks. This fretting damage evolution was simulated using an FEM code, enabling synergic modeling of wear and crack phenomena. The crack nucleation risk was quantified using an SWT parameter combined with a linear cumulative damage law. Surface wear evolution was simulated by a local friction energy density wear approach. The three displacement values, δt, δCN_PS and δCN_GS, were shown to be accurately predicted if, respectively, the FEM simulation takes account of the tangential accommodation of the test system, the damage law is calibrated using reverse analysis of experimental partial slip crack nucleation results, and the energy wear rate is determined from the wear volume analysis in gross slip regions next to the sliding transition. This very good correlation enabled “Material Response Fretting Map” modeling and optimization of palliative coating strategy.

Experimental and numerical investigation of failure mode in geometrically imperfect DP590 steel

Various microstructure-level finite element models were generated according to the real microstructure of DP590 steel to capture the mechanical behavior and fracture mode. The failure mode of the dual-phase (DP) steels, mainly resulting from microstructure-level inhomogeneity and initial geometrical imperfection, was predicted using the plastic strain localization theory. In addition, dog-bone-type tensile test specimens with different edge qualities were prepared and the deformation processes were recorded using a digital image correlation system. When the steel exhibited no initial geometrical imperfection, void initiation was triggered by decohesion between martensite and ferrite which was predicted based on the severe strain concentration, or tensile stress in areas where stress triaxiality and strain values were high. Final failure was caused by shear localization in the vicinity. Moreover, the initial geometrical imperfections severely affected the overall ductility and failure mode of the DP590 steel. When initial geometrical imperfections were deeply ingrained, an incipient crack began at the site of initial geometrical imperfection, and then caused progressive damage throughout the microstructure, from the area of shear localization to the final fracture. Overall, the depth of the geometrical imperfection was the critical factor in determining whether internal decohesion or a local crack plays a dominant role.

Structure, mechanical characteristics, and deformation and fracture features of quenched structural steel under static and cyclic loading after combined strain-heat nanostructuring treatment

In the work, we studied the special features of deformation and fracture of quenched steel 50 (0.51%) under static and cyclic tension after combined strain-heat nanostructuring treatment, which includes fictional treatment with subsequent tempering at 350°C. It is shown that the combined nanostructuring treatment of quenched steel 50 changes the character of plastic flow, making it more uniform, in the loaded material. Under static tension, this shows up as disappearance of the yield plateau early in the process, and under cyclic loading, as suppression of the deformation relief formed by shear and rotational deformation modes. Despite incipient cracks, the hardened surface layer thus escapes complete fracture throughout the fatigue loading and preserves its resistance to mechanical contact action.

Impedance-based fault detection methodology for rotating machines

Visual examination, ultrasonic tests, and dye penetrant inspection are some examples of nondestructive techniques widely used for crack detection in rotors. These methods have proved to be costly, since satisfactory results rely on detailed and periodic inspections. Significant research effort has been directed in recent years to online monitoring techniques, that is, based on vibration signals measured during rotor operation. However, most of them are able to only detect deep cracks. The uniqueness of this article relies on the possibility of detection of incipient transverse cracks in rotating shafts using the so-called, electromechanical impedance method. This method has become a promising tool for structural health monitoring of systems due to its sensitivity to small local damage. Basically, the method monitors changes in the electric impedance of piezoelectric transducers, bonded to (or embedded into) the host structure, through specific mathematic functions, the so-called damage metrics, to detect damage. This is possible because the electrical impedance of the transducer is directly related to the mechanical impedance of the structure. In this context, successful experimental tests were performed in a horizontal rotor supported by roller bearings. Lead zirconate titanate (PZT) patches were bonded along the shaft of the rotor in which saw cuts approximating a breathing transverse crack were machined. The technique was validated under different rotating speeds and unbalance conditions.

Modeling plastic deformation of post-irradiated copper micro-pillars

We present here an application of a fundamentally new theoretical framework for description of the simultaneous evolution of radiation damage and plasticity that can describe both in situ and ex situ deformation of structural materials [1]. The theory is based on the variational principle of maximum entropy production rate; with constraints on dislocation climb motion that are imposed by point defect fluxes as a result of irradiation. The developed theory is implemented in a new computational code that facilitates the simulation of irradiated and unirradiated materials alike in a consistent fashion [2]. Discrete Dislocation Dynamics (DDD) computer simulations are presented here for irradiated fcc metals that address the phenomenon of dislocation channel formation in post-irradiated copper. The focus of the simulations is on the role of micro-pillar boundaries and the statistics of dislocation pinning by stacking-fault tetrahedra (SFTs) on the onset of dislocation channel and incipient surface crack formation. The simulations show that the spatial heterogeneity in the distribution of SFTs naturally leads to localized plastic deformation and incipient surface fracture of micro-pillars.

OS2139 非比例混合モード負荷による共平面き裂成長速度に及ぼす材質の影響

Experiments have been performed for the coplanar fatigue crack growth under non-proportional mixed mode loading. In the experiments, sequential and overlapping mode I and mode II, or mode I and mode III cycles were applied to the crack in the specimen made of wheel or rail steel. The growth rates were obtained and compared in terms of equivalent stress intensity factor ranges. Fracture surfaces near the crack tip region of tested specimens were observed by SEM. For the investigation of incipient crack path direction, the elasto-plastic finite element analyses were performed with the combination of the maximum tangential stress criterion.

Foreign Object Damage Analysis of Leading Edge of Fan Rotor Blade

Foreign object damage (FOD) arises due to ingestion of small hard particles like small pebbles and sand particles in gas turbine engines. The damage caused due to impact of foreign object on leading edge of rotating blade results in premature crack initiation and thereafter failure of the blade due to the time-varying tensile loads in running engine. Residual stresses which arise due to impact of foreign object play an important role in controlling the rate of crack initiation and propagation. Acceleration and retardation of crack growth depends on the characteristics of stress whether it is tensile or compressive in nature ahead of the incipient crack. In the present study impact of spherical projectile against leading edge of a typical fan blade test specimen was simulated using commercially available finite element solver LS-DYNA. Deformed geometry of crater in test specimen is compared with the experimental result. Variation of residual stress in three directions along depth has been studied to predict location of crack initiation for various impact speeds. Finally, stress concentration factor has been obtained for the evaluation of fatigue life of the test specimen.

Incipient crack detection in a composite wind turbine rotor blade

This article presents a performance optimization approach to incipient crack detection in a wind turbine rotor blade that underwent fatigue loading to failure. The objective of this article is to determine an optimal demarcation date, which is required to properly normalize active-sensing data collected and processed using disparate methods for the purpose of damage detection performance comparison. We propose that maximizing average damage detection performance with respect to a demarcation date would provide both an estimate of the true incipient damage onset date and the proper normalization enabling comparison of detection performance among the otherwise disparate data sets. This work focuses on the use of ultrasonic guided waves to detect incipient damage prior to the surfacing of a visible, catastrophic crack. The blade was instrumented with piezoelectric transducers, which were used in a pitch-catch mode over a range of excitation frequencies. With respect to specific excitation frequencies and transmission paths, higher excitation frequencies provided consistent detection results for paths along the rotor blade’s carbon fiber spar cap, but performance fell off with increasing excitation frequency for paths not along the spar cap. Lower excitation frequencies provided consistent detection performance among all sensor paths.

Fracture Strains, Damage Mechanisms and Anisotropy in a Magnesium Alloy Across a Range of Stress Triaxialities

We report on the effect of stress-state triaxiality on damage accumulation leading to fracture at ambient temperature in magnesium alloy AZ31. We find that the strain to failure is weakly sensitive to triaxiality for the conditions investigated, at variance with the behavior of most alloy systems. Using plastic anisotropy measurements, post-mortem fractography and transverse cross-sectioning of specimens at incipient cracking, we discuss the contributions of plastic anisotropy, shear failure and coalescence-controlled cracking to limiting the net effect of stress triaxiality.

Joining by forming of piezoceramic macro-fiber arrays within micro-structured surfaces of aluminum sheets

Functional integration of smart materials in sheet metal enables lightweight composite parts which are enhanced by new functionalities. Locally integrated piezoceramic/metal composites consist of a prefabricated array of ten parallel piezoceramic macro-fibers with dimensions of 0.277 mm by 0.232 mm by 10 mm which are joined in micro-formed cavities within the surface of an aluminum sheet metal. By the use of joining by forming, the interference–fit, preload and form–fit of macro-fiber arrays are achieved in a single process step. The paper describes investigations of the joining by forming process in formal planned experiments using the design of experiments method. The influence of the dimensions and preparation of the joining partners, the maximum forming force and the velocity of the forming stamp are varied. The interference–fit and preload depend on the maximum forming force. In contrast, the quality of the form–fit is primarily related to the geometric dimensions and the forming force. Fiber fractures and incipient cracks are the major failure mechanisms during joining by forming of the macro-fibers. The number of cracks is significantly reduced by the use of lower die velocities, lower maximum joining forces and the introduction of additional geometric elements in the microstructure of the metal surface. Concluding, constraints with regard to the design of parts and the process are derived from the experiments.

Low-cycle fatigue of a multi-layered aluminum sheet alloy

Low-cycle fatigue (LCF) in engineering structures is caused by a relatively low-frequency strain cycling or thermal cycling. Regardless of the fact that it is much more dangerous than high-cycle fatigue, it has not been studied enough. Coated aluminum alloys are widely used in aerospace and transportation industries, mostly because of their high toughness and strength-weight ratio and improved surface properties. The effect of one-, two- and three-layer coatings, including an inner electroless nickel layer and, additionally, electrodeposited nickel, gold and silver, on the LCF behavior of 1.6-mm-thick 6061-T6 Al alloy was studied in a strain-controlled purely bending mode at maximum plastic strain varied from 0.003 to 0.010. The lifetime of the coated alloy drastically decreases as compared to the substrate. Incipient cracks were revealed, first of all, in the electroless nickel layer and in the substrate close to its surface.

Crack location identification of rotating rotor systems using operating deflection shape data

Crack location identification, as one key destination of structural health monitoring, is still a challenge for operating rotor systems. The operating deflection shape (ODS), which represents a visual description of the structural vibration patterns under operating conditions, has been gaining importance for structure damage detection in recent years. The ODS carries damage information of a structure, however, it is also difficult to detect weak cracks of rotor directly. The approximate waveform capacity dimension (AWCD) method was successfully applied to damage detection of plates and beam-like structures. In this paper, a strategic approach that combines ODS and weighted AWCD is proposed for crack location identification of the rotating rotor. To eliminate the false peaks of AWCD and obtain desirable results, a weight factor and ODS curvature data are introduced to the expression of the weighted AWCD. The effectiveness of the proposed method is validated by numerical simulation and experimental investigation in a cracked rotor system. The results indicate that the proposed approach not only provides good identifying performance for incipient rotor cracks, but also effectively eliminates the fault peaks introduced by the inflexion locations of ODSs. Moreover, the proposed approach proves promising in detecting crack locations of rotating rotor systems.

Improvement effects of vibration on cutting force in rotary ultrasonic machining of BK7 glass

Rotary ultrasonic machining (RUM) exhibits a high potential for a significant reduction in the cutting force, which directly associates with tool wear, machining accuracy, machining temperature, and surface integrity. However, the improvement mechanisms of the ultrasonic vibration on the cutting force are still not fully recognized, restricting the currently optimization methods for further reducing the cutting force occurred during the RUM process. In this research, by incorporating the kinematics principles of the abrasive, the evolution features of the material strain rate in the loading phase were first discussed with respect to the indentation mechanics theory. Taking these features into account, the RUM scratching tests were carried out on the polished specimen surfaces under various process parameters to capture the integrated damage patterns evoked in the high strain rate stage. Following, the comparative indentation tests were respectively conducted on the RUM scratches and the gentle polished surfaces. The indentation-induced damage structures and the load–displacement curves were characterized and assessed to investigate the improvement mechanisms of the superimposed ultrasonic on the cutting force in formal RUM process. It was found that superimposing an ultrasonic vibration led to the incipient cracks nucleated in the abrasive loading phase, and their propagations would increase the material removal rate (MMR) obtained in formal RUM process. Furthermore, the incipient cracks provided a shielding effect to the indentation force, which was a dominant factor in diminishing the cutting force of the diamond tool. The nucleation of the incipient cracks resulted in more energy dissipation after the abrasives penetrating into the hard substrate of the material, which would lead to a higher residual stress on final RUM surface. In addition, a failure pattern (plastic deformation or brittle fracture) evolution model involved in abrasive loading phase was developed with respect to the strain rate effects of the material.

Noncontact Fatigue Crack Evaluation Using Thermoelastic Images

This paper proposes a noncontact thermography technique for fatigue crack evaluation under a cyclic tensile loading. The proposed technique identifies and localizes an invisible fatigue crack without scanning, thus making it possible to instantaneously evaluate an incipient fatigue crack. Based on a thermoelastic theory, a new fatigue crack evaluation algorithm is proposed for the fatigue crack-tip localization. The performance of the proposed algorithm is experimentally validated. To achieve this, the cyclic tensile loading is applied to a dog-bone shape aluminum specimen using a universal testing machine, and the corresponding thermal responses induced by thermoelastic effects are captured by an infrared camera. The test results confirm that the fatigue crack is well identified and localized by comparing with its microscopic images.

Bladed disc crack diagnostics using blade passage signals

One of the major potential faults in a turbo fan engine is the crack initiation and propagation in bladed discs under cyclic loads that could result in the breakdown of the engines if not detected at an early stage. Reliable fault detection techniques are therefore in demand to reduce maintenance cost and prevent catastrophic failures. Although a number of approaches have been reported in the literature, it remains very challenging to develop a reliable technique to accurately estimate the health condition of a rotating bladed disc. Correspondingly, this paper presents a novel technique for bladed disc crack detection through two sequential signal processing stages: (1) signal preprocessing that aims to eliminate the noises in the blade passage signals; (2) signal postprocessing that intends to identify the crack location. In the first stage, physics-based modeling and interpretation are established to help characterize the noises. The crack initiation can be determined based on the calculated health monitoring index derived from the sinusoidal effects. In the second stage, the crack is located through advanced detrended fluctuation analysis of the preprocessed data. The proposed technique is validated using a set of spin rig test data (i.e. tip clearance and time of arrival) that was acquired during a test conducted on a bladed military engine fan disc. The test results have demonstrated that the developed technique is an effective approach for identifying and locating the incipient crack that occurs at the root of a bladed disc.

Modified Kitagawa Diagram and Transition from Crack Nucleation to Crack Propagation

Kitagawa-Takahashi diagram combines the endurance limit of a smooth specimen and the crack propagation threshold in a fracture mechanics specimen into single diagram thus providing the connection between the stress or strain-life and damage tolerance approaches. The diagram is modified by considering that (a) fatigue requires two independent load-parameters for unambiguous description, (b) long crack growth behavior defines the material resistance under constant stress amplitudes along with the associated R-ratio effects, (c) remote applied stresses and localized plasticity-effects can be combined to provide the total mechanical force opposing the material resistance leading to crack initiation, growth and failure describable in the diagram, (d) localized plasticity contributes to internal stresses that either augment or retard the remote stresses, and finally (e) the magnitude and gradient of these internal stresses determine the condition for propagation and/or non-propagation of the incipient cracks that form either at pre-existing stress concentrations or in situ formed stress concentrations due to localized plasticity. Localized plasticity forms the basis for the additional crack tip driving forces in either accelerating or decelerating crack growth kinetics thereby providing conditions for either crack arrest with resulting non-propagating cracks or for continuous uninterrupted crack growth. Internal stresses are generated during fatigue damage in the form of dislocation pile-ups, intrusions and extrusions. The analysis shows that critical magnitude and gradient of the internal stresses are required for an incipient crack to grow continuously, failing which crack arrest can occur. The methodology is based on separating the mechanically introduced crack tip driving forces vs the material resistance; the later can be extracted from long-crack growth data under constant amplitudes. Analysis of incipient short cracks growing under the elastic-plastic notch tip stress fields are analyzed systematically for various elastic stress concentrations, K t, and notch-tip radii, ρ. A general formulation is developed based on the calculations that can be incorporated into the unified life predication model that is being developed.

Paleoenvironmental framework of dinosaur tracksites and other ichnofossils in Early Cretaceous mixed siliciclastic-carbonate deposits in the Neuquén Basin, northern Patagonia (Argentina)

The study of the uppermost section of the Early Cretaceous Agrio Formation in northern Patagonia (Neuquén Basin) where dinosaur tracks assigned to cf. Therangospodus pandemicus are exposed (tracksites I and II) evidence mixed marginal marine siliciclastic-carbonate deposits. The succession was divided in two intervals. A lower one containing theropod tracks, recorded on top of subtidal oolithic limestones with tiny wave ripples suggesting shoreline fluctuations and subaerial exposure. Tidal influence is recognised by fining upward and prograding cycles starting with subtidal carbonates and ending with fine-grained siliciclastic deposits at the top, or rarely laminites. Dolomitization affects subtidal deposits generated in an alkaline media stressful for tracemakers. Intertidal facies include abundant heterolithic deposits, coquinas composed of gastropods encrusted by multilayered bryozoans and muddy levels with incipient mud cracking. Invertebrate ichnofossils recognized from tidally dominated deposits include Arenicolites, Kouphichnium, and Rhizocorallium. The upper interval is a transgressive–regressive cycle that starts with dark shales, deficiently oxygenated, and covered by prograding sandstones and finally sand flat deposits. This interval contains Gyrochorte, Hillichnus, and Ophiomorpha documented in wave-influenced sandstones. Dinosaur tracks as well as Hillichnus, attributed to tellinoid bivalves, and Kouphichnium assigned to xiphosurans, imply the activity of producers rarely recorded previously as body fossils in marginal marine deposits of southern South America. Previous paleogeographic schemes are questioned by our analysis, which shows evidence of extremely shallow and tide-controlled sedimentation, sometimes with subaerial exposure, with high cyclicity related to a marginal marine depositional setting and lack of significant erosion by the overlying unit, as traditionally was suggested.

Failure Prediction in Sheet Metal Forming Depending in Pre-Straining and Bending Superposition

The automotive industry nowadays, uses numerical simulation systems to determine process safety of car body parts. Forming simulations are usually used to predict local necking and cracks during the deep-drawing operation or to calculate the spring-back behaviour. Furthermore, FEA is also used for optimizing the hemming process. In this contribution, further development and the use of an enhanced failure criterion for the evaluation of flanging and hemming processes are shown. This criterion describes material failure caused by incipient surface cracks on the bending edge keeping the predominant bending load conditions in consideration. The investigations of the bending conditions in this criterion include loads from previous forming operations and geometrical aspects, such as bending radii. The approach presented in this contribution can deliver a more reliable prediction regarding the expected material failure.

Comparison of Minimum Detectable Crack Size in a Geared System From Three Different Vibration Transducer Types

Acoustic and vibration signals contain distinctive patterns useful for detecting defects in machinery. A number of signal processing strategies have been proposed for detecting and quantifying incipient defects in gears. Nonintrusive measurement techniques like those based on laser Doppler vibrometry (LDV) and microphones need to be studied in order to help users in selecting the appropriate transducer according to the application. This article presents an experimental design to determine, as a novel contribution, the minimum detectable crack size in a geared system comparing three different transducers: a microphone, an accelerometer, and an LDV. The comparison intends to discriminate the three transducers with respect to its early detection capability. Experiments were conducted in a closed loop torque test rig for several torque levels and speeds. Experimental factorial design was used to determine the main effects and their interactions in the detection process. The well-known Hilbert and wavelet transforms have been used as signal processing technique. Their advantages in the detection of incipient defects are highlighted in this article. The results indicate that the acoustic signal stands out as the method that first detects an incipient progressive crack in gears (it detected 1.3-mm-long cracks), although, as a drawback, the results obtained using the microphone signal are more sensitive to speed and torque. The second place was for LDV with 1.8-mm crack detection, and the third place for the accelerometer with 2.3-mm crack detection.

205102 レール鋼における非比例混合モードI+II負荷における疲労き裂成長方向(OS04 材料・機械・構造物における強度と寿命2,オーガナイズド・セッション)

Two criteria for the prediction of incipient crack path direction of non-proportional mixed mode I+II loading in rail steel are investigated. The analysis is based on 2D plane strain FE-simulations in which actual elasto-plastic stress distribution is used for the maximum tangential stress (MTS) criterion and maximum tangential stress range (MTSR) criterion. The purpose of the analysis is to compare the numerical results with experimental results. It is shown that both the MTS and MTSR criteria including plasticity and sophisticated work hardening law are well suited to predict incipient crack growth directions for the rail steel under non-proportional loading.