Single Fatigue Crack Research Articles

The Low-Cycle Fatigue Behavior, Microstructure Evolution, and Life Prediction of SS304: Influence of Temperature.

To study the fatigue failure and microstructure evolution behavior of SS304, low-cycle fatigue tests are conducted at room temperature (RT), 300 °C, and 650 °C. The results indicate that, because of the influence of the dislocation walls, carbon-containing precipitates, and deformation twins, the cyclic hardening behavior is presented at RT. However, different from the cyclic hardening behavior at RT, the cyclic softening behavior of SS304 can be observed due to the dynamic recovery and recrystallization containing dislocation rearrangement and annihilation at 300 °C and 650 °C. In addition, two fatigue crack initiation modes are observed. At RT, the single fatigue crack initiation mode is observed. At high temperatures, multiple crack initiation modes are presented, resulting from the degradation of material properties. Furthermore, a new fatigue life prediction model considering the temperature is conducted as a reference for industrial applications.

A Robot-Assisted Microscopy System for Digital Image Correlation in Fatigue Crack Growth Testing

BackgroundDigital image correlation (DIC) with microscopes has become an important experimental tool in fracture mechanics to study local effects such as the plastic zone, crack closure, crack deflection or crack branching. High-resolution light microscopes provide 2D images but the field of view is limited to a small area and very sensitive to its alignment. A flexible positioning system is therefore needed to collect such DIC data during the entire fatigue crack growth process.ObjectiveWe present in our paper a new experimental setup for local high-resolution 2D DIC measurements at any location and at any time during fatigue crack growth experiments with a non-fixed DIC microscopy system.MethodsWe use a robot to move the 2D DIC microscope to any location on the surface of the specimen. Optical and tactile methods automatically adjust the system and ensure highest image quality as well as accurate alignment. In addition, an advanced repositioning method reduces out-of-plane motion effects.ResultsThe robot is able to achieve a repositioning accuracy of less than 0.06 mm in vector space, resulting in very low Von Mises strain scattering of 0.07 to 0.09% in the DIC evaluation. The system minimizes systematic errors caused by translation and rotational deviations. Effects such as crack deflection, crack branching or the plastic zone of a fatigue crack can be investigated with a field of view of 10.2 x 6.4 mm2.ConclusionsThe robot supported DIC system generates up to 8000 high-quality DIC images in an experiment that enables the application of digital evaluation algorithms. Redundant information create confidence in the results as all revealed effects are comprehensible. This increases the information content of a single fatigue crack growth test and accelerates knowledge generation.

Nonlinear Resonant Ultrasonic Spectroscopy (NRUS) for Monitoring Fatigue Crack Growth in Aluminum

ABSTRACT Nonlinear resonant ultrasonic spectroscopy (NRUS) is a resonance-based acoustic testing technique that yields the hysteretic elastic nonlinearity parameter by measuring the resonance frequency shift with increasing driving amplitude. NRUS offers great potential for nondestructive evaluation since it is relatively simple to implement and can detect incipient damage thanks to the high sensitivity of hysteretic nonlinearity parameter to micro-damage. Previous research has shown that NRUS can monitor distributed damage in a wide variety of materials, but the application of NRUS for detection of local defects in metals is less explored. In this study, the feasibility of using NRUS to assess local progressive damage in aluminum is investigated. We use three-point bending fatigue test to initiate a single fatigue crack in a large aluminum specimen. The cyclic loading is interrupted at several stages in order to image the crack using Scanning Electron Microscopy (SEM) and to perform NRUS and other tests. As the crack grows in length, NRUS records a gradual increase in the resonance frequency shift. However, the trend for the amplitude dependency of resonance frequency shift is different from what has been previously reported in materials with distributed damage; the resonance frequency changes are larger at low strain than at high strains. In addition, the utility of multi-modal NRUS for locating the fatigue crack is demonstrated. Finally, we compare the results of NRUS and those from an impact-based NRUS (INRUS) that uses an automated impact hammer as the excitation source in differentiating the fatigue-damaged from the intact specimen. Our findings suggest the potential of NRUS and INRUS in detecting local damage in metals.

Nonlinear ultrasonic phased array with fixed-voltage fundamental wave amplitude difference for high-selectivity imaging of closed cracks.

The nondestructive evaluation of closed cracks is a challenging subject in ultrasonic testing. Recently, nonlinear ultrasonic phased array with fixed-voltage fundamental wave amplitude difference (fixed-voltage FAD) has been proposed as a practical approach. In this study, the maximum incident wave amplitude, which is one of the most critical parameters in closed-crack imaging, was investigated. First, a theoretical model was formulated to explicitly show the essence of the fundamental principle of FAD and the advantage of fixed-voltage FAD over different-voltage FAD. In experiments, the authors imaged a closed fatigue crack using a nonlinear ultrasonic phased array with fixed-voltage FAD while varying the incident wave amplitude. It was found that when the incident wave amplitude was sufficiently high, the nonlinear image visualized the closed crack tip, which could not be visualized in linear images. In addition, the incident-wave-amplitude dependence of the nonlinear responses was quantified. It was found that different parts within a single fatigue crack showed different nonlinear behaviors. This suggests that fixed-voltage FAD is useful not only for practical application of closed crack imaging but also for examining the nonlinear dynamics at various parts of closed cracks with a high spatial resolution.

Dynamic acousto-elastic response of single fatigue cracks with different microstructural features: An experimental investigation

Nonlinearity of damaged materials is often assumed to be correlated with the cumulative length of micro-cracks discounting the influence of crack width and roughness. To investigate the influence of microstructural features of realistic cracks on the nonlinear elastic properties, Dynamic Acousto-Elastic Testing (DAET) is performed on two aluminum alloy samples (30 × 40 × 170 mm3) with a single fatigue-crack. The cracks on the two samples are of similar length but have dissimilar microstructural features due to the different stress intensity factors used during the fatigue tests. DAET is carried out with a surface wave probe to measure the near-surface nonlinearity at a number of locations along each crack. The geometric features of the cracks, including crack width and roughness, are extracted from a series of scanning electron microscope images. DAET results demonstrate that nonlinearity varies along the crack; both samples exhibit greater nonlinearity near the notch, where the crack is wider than near the crack tip. However, the nonlinearity parameters obtained from the sample with a smoother crack are one order of magnitude larger than those of the sample with a relatively rough crack. In addition, the nonlinearity signatures of the two cracks exhibit very different tension/compression asymmetry. This study provides valuable data that not only directly correlates the nonlinear ultrasonic signatures and crack interface features but also illuminates the micro-mechanisms and origins of the measured nonlinearities.

Relating fracture mechanics and fatigue lifetime prediction

This article presents how to link together the results of fatigue crack growth tests, analytic fracture mechanics and experimental methods of fatigue lifetime predictions. The study at the beginning investigates the effect of mechanical load redistribution among failed and intact micro-structural bonds along fatigue crack growth to final crack at vulnerable locations in materials and structures under cyclic loads. The microstructural load redistribution model is analytically formulated as a mechanical interaction between fatigue crack growth and fatigue endurance on the macroscopic level. The article in continuation investigates how to link the parameters of fatigue crack growth in fracture mechanics to parameters of fatigue life directly from the work done on crack growth determined by testing. The article at the end illustrates the application of the analytic procedure for fatigue lifetime prediction that combines fracture mechanics and the load redistribution model for determination of S–N curve parameters important in structural analysis and design. In this research the fatigue life time parameters are derived from a single fatigue crack growth experiment instead from normally required sets of fatigue tests for different loading conditions.

Corrosion-Initiated Rotating Bending Fatigue Failure of a Fertilizer Conveyor Belt Head Roller

This paper presents the experimental process and findings of a forensic investigation of a failed fertilizer plant conveyor belt head roller shaft. A 25-year service life was expected but failure occurred following only 10 years. A gearbox which provided direct power to the roller had been replaced only several weeks earlier. The primary aim for the plant owner was to determine if the gearbox replacement had caused failure of the shaft, or, if the two incidents were unrelated. If incorrect installment of the gearbox had caused failure, liability would fall on the gearbox suppliers in terms of mitigation. However, it was revealed that the shaft had failed due to low-stress rotating bending fatigue, over an extended period of time. A premeditated change of material selection at the manufacturing stage, substituting carbon steel for 304L stainless steel, resulted in reduced corrosion resistance. Fertilizer ingredients tend to pose limited problems in their dry form in terms of corrosion; in the presence of moisture, however, can change the ingredients into aggressive corrosion species. Thus, corrosion allowed multiple fatigue cracks initiated from corrosion pitting under rotating bending load. The cracks then eventually joined together to form a single fatigue crack which propagated through the cross section.

The Application of Nonlinear Reverberation Spectroscopy for the Detection of Localized Fatigue Damage

Nonlinear reverberation spectroscopy (NRS) is a non destructive evaluation method exploiting the amplitude dependent change of the resonance frequency in samples made of a nonlinear elastic material. After a sample has been excited near resonance, the immediate vibration frequency of the decaying reverberation signal decays with the decreasing amplitude. The frequency-amplitude dependence can be used to quantify the nonlinearity of the material, typically caused by the damage level. This paper handles the possibilities and difficulties of using NRS to detect a single fatigue crack in specimens made of an linear material like a composite or steel. In this case, the nonlinearity is concentrated in a small zone which is not necessarily affected by the low frequency vibrations used for NRS. First, a proof of concept is given by testing composite beams with increasing levels of fatigue damage. Tests prove that the nonlinear frequency change is more efficient for quantifying early damage than its linear counterparts such as the damping coefficient, at least when the crack is located in a vibration antinode. The method is subsequently used to test steel industrial samples with a complex geometry, showing that efficient damage detection indeed depends on the used vibration mode.

Giga-Cycle Property of a New Age-Hardened Aluminium Alloy Containing Excess Solute Magnesium

The giga-cycle property of a newly developed Al alloy, which contains 0.5wt.% excess Mg solute compared to a standard age-hardened 6061 alloy (6061-T6), was investigated by using smooth specimens subjected to ultrasonic fatigue. The fatigue strength of the new alloy was higher than that of a normal 6061 alloy particularly at relatively low stress amplitude level. Several analyses (surface crack observation, fractography, FIB cross-sectioning, etc.) were also conducted to reveal the micro-mechanism of the observed strength properties. The following results were obtained: i) No fatigue limit was confirmed for both 6061 and new alloy. ii) Total life (Nf) of 6061 and new alloys was determined by a single fatigue crack initiated from a surface PSB crack. iii) Crack initiation resistance defined by N25 (number of cycles to reach ρ = 25 mm-2, where ρ is the PSB crack number density) for new alloy was higher than that of 6061. iv) The higher fatigue strength of new alloy was explained by the effect of excess Mg solute which increased the resistance against the formation of PSB cracks.

Structural damage assessment with antiresonances versus mode shapes using parallel genetic algorithms

SUMMARY Antiresonances have become an attractive alternative in structural damage assessment. They can be identified easier and more accurately than mode shapes, and still providing the same information. Antiresonances are derived from point frequency response functions (FRFs) or from transfer FRFs. However, antiresonances from transfer FRFs are very sensitive to small structural changes, and the matching between numerical and experimental antiresonances is affected. This problem is solved if antiresonances from point FRFs are used. However, it implies an experimental procedure that differs from a common modal testing, which may become not practical or too expensive. This paper proposes a damage detection method able to deal with transfer antiresonances. The inverse problem is handled by a Parallel Genetic Algorithm. In this case, a perfect match between the antiresonances is not required because the optimization is not gradient based. Moreover, the matching can change at each step and the optimization is not affected. The algorithm is verified with two experimental cases: an exhaust system of a car with a single fatigue crack and a tridimensional space frame structure with single and multiple damage scenarios. Results are compared with the ones obtained using mode shapes. Damage detected is consistent with the experimental damage in both cases. Copyrightr 2010 John Wiley & Sons, Ltd. Received 23 January 2010; Revised 15 April 2010; Accepted 28 April 2010

生体用Ｔｉ‐２９Ｎｂ‐１３Ｔａ‐４．６Ｚｒ合金の切欠き疲労特性

The effects of microstructures on notch fatigue propertes of a Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) subjected to various thermo-mechanical treatments were investigated in this study. The notch fatigue strengths of TNTZ subjected to aging at 673 K and 723 K after solution treatment or cold rolling at stress concentration factors of 1 (without the notch), 2 and 6 in both low and high cycle fatigue life regions are greater than those of as-cold rolled and as-solutionized TNTZ, and TNTZ subjected to aging at 598 K after solution treatment or cold rolling. A single fatigue crack initiates on a specimen surface and at the tip of the notch at stress concentration factors of 1 and 2. While, a few cracks initiate the tip of the notch having a stress concentration factor of 6. Notch sensitivity factor of TNTZ is almost equal to or smaller than those of other materials such as Ti-6Al-4V, CP-Ti, S35C, S45C and SUS304. It is considered that the notch sensitivity factor decreases with decreasing the width of lath like α phase and its volume fraction.

외팔보에 대한 가진력수준제어를 통한 피로균열규명기법의 실험적 검증

In this study, a new damage identification method for beam-like structures with a fatigue crack is proposed, which does not require comparative measurement on an intact structure but require several measurements at different level of excitation forces on the cracked structure. The idea comes from the fact that dynamic behavior of a structure with a fatigue crack changes with the level of the excitation force. The 2nd spatial derivatives of frequency response functions along the longitudinal direction of a beam are used as the sensitive indicator of crack existence. Then, weighting function is employed in the averaging process in frequency domain to account for the modal participation of the differences between the dynamic behavior of a beam with a fatigue crack at the low excitation and one at the high excitation. Subsequently, a damage index is defined such that the location and level of the crack may be identified. It is shown from the analysis of vibration measurements in this study that comparison of frequency response characteristics of a beam with a single fatigue crack at different level of excitation forces enables an effective detection of the crack.

Cyclic cracking behavior of low-alloy pressure vessel steel in simulated BWR water

Fatigue crack initiation and growth behavior of low-alloy pressure vessel steel A533B has been investigated in simulated BWR water. Two distinguished crack morphologies were observed, closely depending on the cyclic strain rate. At a high strain rate, fatigue cracks grew in a tortuous manner. Final failure pre-dominantly resulted from the continuous growth of a single fatigue crack. A rough fracture surface was obtained on which hydrogen-induced cracking features were dominant. At a low strain rate, however, fatigue cracks developed in an entirely straight manner. Final failure was usually caused by the joint development of multi-site fatigue cracks. A flat fracture surface was obtained on which slight crack arrest rather than typical hydrogen-induced cracking feature was found. In addition, apparent acceleration in fatigue crack growth rate was observed in the temperature and strain-rate regions of dynamic strain aging (DSA). Related environmentally assisted cracking (EAC) mechanisms are also discussed.

Fractographic aspects of fatigue of sintered Ni-steels

This study examines the fatigue fracture morphology of two Ni-sintered steels with different contents of pores and MnS. These steels are produced to contain either MnS or pores or a combination of both. A total of 12 alloys are examined. Fatigue fracture of smooth rectangular and circular specimens were obtained by testing at constant amplitude and zero mean stress at 30-Hz frequency. Fractographic examinations were carried out to characterize the location and character of the fatigue origins as well as the propagation mechanisms of the fatigue crack initiated from these origins. For materials where pores are the most populous, multiple fatigue crack origins were observed at surface locations. In cases where MnS were the most populous, single fatigue crack origin was observed either at surface inhomogeneity or at subsurface inclusion. In all samples, fatigue crack growth occurred by four propagation modes: transparticle striated and serrated mechanisms, interparticle neck separation from microvoid coalescence and transparticle cleavage fracture. The combination of these fracture mechanisms depends mainly on the pore level and the applied stress amplitude.

Fatigue crack growth in unidirectional graphite-epoxy composites under cyclic compression

It is shown that in addition to the well-known failure process involving kink band formation, the application of cyclic compressive loads to notched plates of graphitr-epoxy composites can lead to the nucleation and growth of a single fatigue crack normal to the compression axis and to the fibre direction. This stable mode I compression fatigue fracture is a consequence of the generation of a zone of residual tensile stresses within the notch-tip region where deformation occurs in the form of matrix cracking, fibre breakage, fibre buckling nad/or interfacial debonding