Fatigue Crack Tip Research Articles

Restoration of fatigue damage in steel tube by eddy current treatment

In this study, the ECT (eddy current treatment) was applied to 1045 steel tubular specimen, and the propagation of fatigue cracks within tubular specimens was delayed evidently at different fatigue stages. The delay could be ascribed to the restoration of the fatigue damage. The Finite Element simulation results indicate that the thermal effect in vicinity of fatigue crack tip could be responsible for the healing of fatigue crack. The thermal stress was mainly caused by the detour of eddy current in the vicinity of fatigue crack tips, which was influenced by the crack orientation and eddy current direction.

A model of energy dissipation at fatigue crack tip in metals

Experimental and numerical studies investigating fatigue crack growth were conducted on flat samples made of stainless steel AISE 304 and titanium alloy Grade 2. The heat flux from the crack tip caused by plastic deformation localization was measured using the contact heat flux sensor previously developed by the authors. This provides a possibility to find a correlation between energy dissipation and crack propagation rate under fatigue uniaxial loading with constant stress intensity factor and under biaxial loading with constant stress amplitude. The experiments with constant stress intensity factor have shown a decrease in energy dissipation at constant crack rate and R=-1 (R=0). A theoretical analysis of the stress field at the fatigue crack tip has been carried out to explain this phenomenon. Based on the obtained results, the heat flux from the crack tip is represented as the sum of two functions describing energy dissipation in monotonic and reversible plastic zones separately. It has been shown that dissipation in a reversible plastic zone is a function of the applied stress amplitude only. This causes energy dissipation to decrease at constant stress intensity factor. The proposed phenomenology was successfully verified by testing both materials under biaxial loading.

A novel algorithm for crack path identification based on infrared images

The present work aims at developing a new approach towards the analysis of the crack path through the access to the information from the infrared (IR) video imaging of the steady fatigue crack growth. We first test some commonly available infrared image filtering techniques, and then we introduce a new robust algorithm to identify the position of the dynamic crack. The proposed procedure performs the local analysis of the temperature distribution around the fatigue crack tip in the 316L stainless steel. The developed MATLAB framework simplifies the routine processing and permits analyzing multiple experiments reasonably easily. Two filtering procedures - Gaussian and wavelet shrinkage - were implemented and applied successively to denoise the IR images. Both filters produce comparable and good quality results. The new algorithm capable of automatic locating the crack tip position from consecutive filtered IR images is proposed. Remarkably successful results are demonstrated with only small errors caused by materials distortion.

Corrosion fatigue behavior and crack-tip characteristic of 316LN stainless steel in high-temperature pressurized water

The corrosion fatigue (CF) behavior and crack tip characteristic of 316LN stainless steel in 325 °C water at different strain rates were investigated under a strain amplitude of 0.6%. With decreasing the strain rate from 0.4% s−1 to 0.0004% s−1, the CF life decreased linearly in log-log plot. More obvious reduction of CF life at low strain rate was due to the environmental damage. The morphology of cracks and the dislocation density near the crack-tip front exhibit a significant different under different strain rate conditions. The hydrogen enhanced localized plasticity mechanism is discussed to explain the micro-damage process at different strain rates in high-temperature water.

The experimental study of heat dissipation during fatigue crack propagation under biaxial loading

The work is devoted to experimental study of heat flow evolution at the fatigue crack tip during biaxial loading. The plane samples of titanium alloy Graid-2 with thick of 1 mm were weakened by notch to initiate fatigue crack at the center of samples. Infrared thermography and the contact heat flux sensor based on the Seebeck effect are used to monitor the dissipated thermal energy. During tests the samples were subjected to cyclic loading of 10 Hz with constant stress amplitude and different biaxial coefficient. The experimental results confirm the previous conclusions of authors about two regime of energy dissipation at fatigue crack tip. The heat dissipation curve can be divided in two stages. Under the first stage the power of heat flux is proportional to the multiplication of the crack rate by its length. In the second stage is characterized by classical linear relationship between crack rate and heat flux rom the crack tip. The qualitative correspondence of the energy approach to the classical representation based on the stress intensity factor was shown.

Monitoring and instantaneous evaluation of fatigue crack using integrated passive and active laser thermography

This study proposes an integrated passive and active laser thermography (IPALT) system comprising a continuous wave (CW) laser and an infrared (IR) camera for fully noncontact monitoring and instantaneous evaluation of a fatigue crack in a metallic structure. The IPALT system operates in two phases: (1) passive thermography (PT) mode, and (2) active thermography (AT) mode. The PT mode monitors the fatigue crack initiation by measuring the thermoelastic effects on the fatigue crack tip in real time using the IR camera. Once the crack tip is identified, the AT mode is automatically triggered, and the corresponding fatigue crack is precisely quantified using the CW laser and the IR camera. Through this effective crack monitoring system under operating conditions, time dependent crack propagating behavior analysis, as well as early crack detection, can be achieved. The feasibility of the proposed IPALT system is experimentally validated by monitoring a metallic structure under a 10 Hz cyclic loading. The validation tests indicate that fatigue crack initiation is detected at 10,000 cycles by the PT mode, and the fatigue crack length is quantified as 10.45 mm with an accuracy of 99.43% compared to the microscope observed ground truth of 10.51 mm.

In-situ microscopy testing of plasticity variation ahead of fatigue crack tip in AL2024-T3

In this paper, an in-situ experiment is performed under the optical microscopy to investigate the continuous variation of the strain field in the vicinity of the fatigue crack tip in Al2024-T3. The specimen is a small thin plate with an edge-crack, whose surface is speckled with tiny and dense spots with random-shapes. The applied load cycle is divided into a certain number of steps at which high resolution images around the crack tip are taken and recorded by a microscope camera. Once collecting the series of images, the strain distributions at each load step can be evaluated through the digital image correlation technique. Consequently, the continuous variation of the strain field ahead of the crack tip within the applied load cycle can be estimated approximately. Then combining the experimental measurements with the material constitutive relationship, the plastic zone sizes at different load levels can be calculated. In the current study, the continuous variations of the retensile and reversed plastic zones with different stress ratios are measured. Furthermore, the investigation is extended to the simple variable loading case, in which the plastic zone variation is traced before, during and after the single overload. It is shown that the plastic zone experiences an immediate reduction after the overload and gradually restores to the previous level. The crack closure effect and plasticity-induced loading interaction are analyzed. Finally, several conclusions are drawn based on the current investigation.

Experimental and theoretical analysis of heat flux at fatigue crack tip under mixed mode loading

The work is devoted to experimental and theoretical study of heat dissipation at the fatigue crack tip during mixed mode loading. The plane samples of stainless steel (AISI 304) were weakened by notch to initiate fatigue crack. There were two types of samples for uniaxial loading and biaxial loading. Infrared thermography and the contact heat flux sensor based on the Seebeck effect were used to measure the dissipated thermal energy. The samples were subject to cyclic loading with constant stress amplitude and different biaxial coefficients. The experimental results confirmed the hypothesis about two stages of heat dissipation at fatigue crack tip under Paris regime. At the first stage, the power of heat flux is proportional to the product of the crack rate by the crack length. The second stage is characterized by a traditional linear relationship between the crack rate and the heat flux. The theoretical approach to calculate dissipation energy at fatigue crack tip was obtained. This technique based on link between the elastic-plastic and elastic strain fields at crack tip using the coupling between Young’s modulus and the secant plasticity modulus.

An dissipated energy based analysis of fatigue crack propagation law

The experimental study of heat flux evolution at the fatigue crack tip during biaxial loading was carry out in this work. The plane samples of stainless steel AISI 304 with thick of 3 mm were weakened by notch to initiate fatigue crack at the centre of samples. A contact heat flux sensor based on the Seebeck effect was used to monitor the dissipated thermal energy. During tests the samples were subjected to cyclic loading of 5 Hz with constant stress amplitude and different biaxial parameter. The experimental results confirm the previous conclusions of authors about two regime of energy dissipation at fatigue crack tip. The curve of the dissipated energy can be divided in two stages. In the second stage is characterized by classical linear relation between crack rate and energy dissipation. In the first stage the crack rate is proportional to the multiplication of the power of heat flux by crack length. The energy dissipation does not depend on the biaxial parameter during cyclic loading.

Estimation of the plastic zone in fatigue through the thickness based on synchrotron diffraction data

In this work we present a novel methodology to analyse the extent of crack tip plasticity inside engineering materials. The plastic zone developed ahead of a fatigue crack in a bainitic steel compact tension specimen is studied. The bulk of the steel is probed with synchrotron X-ray diffraction technique to measure the elastic strain field deep inside the material. The procedure involves combining the experimental synchrotron data with numerical data to obtain the equivalent strain map. This map is then converted into a stress field where the boundary of the plastic zone can be directly extracted. The procedure is tested on a real fatigue crack and the results are compared to plane stress and plane strain models. The promising results will enable key progress on understanding the mechanisms that generate the damage at the tip of fatigue cracks in engineering components.

Crack growth arrester using “gourd-shaped-insert-plate”

In accordance with aging infrastructure systems such as steel bridges, fatigue of metals becomes serious risks. Fatigue crack may lead the structure to catastrophic failure. Countermeasures against fatigue failure are required. “Crack growth arrest and retardation system” using “gourd-shaped-insert-plate” has been developed as a high performance and easy construction method against fatigue crack, by generating fatigue crack closure force and reduce crack tip opening displacement and stress concentration at the fatigue crack tip. Arrester was applied to CT (Compact Tension) specimen and also mockup of steel bridge deck, in order to validate performance of the system. Fatigue crack growth rates of the specimens with arrester were 1/10~1/2 of those without arrester. And apparent threshold stress intensity factor range was increased by about 10~20 MPa√m through using only few gourd-shaped arresters. Also the arrester was applied to the fatigue crack in a mockup of steel bridge deck. Without arrester, fatigue crack growth occurred in accordance with the increase of loading cycles, on the other hand, fatigue crack growth was arrested after installing the arrester in the mockup of steel bridge deck. And to enable quantitative fatigue life assessment in fatigue design, fracture mechanics based-crack closure model with spring elements for the arrester has been verified with good accuracy for fatigue crack growth rate in the tests using CT specimens.

Low-cycle Fatigue and Fracture Behaviour of 9%Ni Steel Flux Cored Arc Welding joint at Cryogenic Temperature

In the present work, low-cycle fatigue (LCF) test and crack tip opening displacement (CTOD) test were performed for 9% Ni steel flux cored arc welding (FCAW) joint at room temperature (296 K) and cryogenic temperature (80 K). At cryogenic temperature, the strain amplitude had a far greater impact on fatigue life of 9%Ni steel welded joint and it decreased dramatically lead to a significant increase in fatigue life. It was found that most fracture initiation of joints located in fusion area at room temperature, while it occurred in weld seam at low temperature. The fracture toughness of weld seam was higher than that of fusion zone no matter the testing temperature. The effect of precipitated phase was the true reason. The fatigue cracks propagated in transgranular mode at room temperature, ultimately, and intergranular mode at low temperature in both LCF specimens and CTOD specimens.

Alloy design strategy for fatigue-resistant steels and associated microstructure observations at a small fatigue crack tip

Alloy design strategy for fatigue-resistant steels and associated microstructure observations at a small fatigue crack tip

Hydrogen Effect on the Fatigue Crack Growth in Austenitic Stainless Steel Investigated by a New Method Based on Nanohardness Distribution

A new method of nanohardness distribution for investigating the hydrogen effect on the fatigue crack in austenitic stainless steels was developed. The nanohardness distribution around the fatigue crack tip is dependent on the plastic zone and different microstructures in materials. Nanoindentation could provide a possibility to quantitatively estimate the size of the plastic zone and to expressly identify the strain-induced α′ martensitic transformation around the fatigue crack tip. The results of measured nanohardness distribution reveal that hydrogen reduces the size of the estimated plastic zone around the fatigue crack tip, especially in the specimen tested in hydrogen gas environment, which is attributed to hydrogen-enhanced localized plasticity. Both hydrogen and α′ martensite greatly influence the nanohardness and the gradient of hydrogen concentration near the crack tip, which will have a significant effect on the fatigue crack growth.

Effect of Electropulsing Treatment on the Fatigue Crack Growth Behavior of Copper.

Crack propagation was quantitatively evaluated to investigate the effect of electropulsing treatment (EPT) on fatigue crack growth of copper specimens. Varying fatigue cycles were obtained under six different load levels. The crack lengths were measured under two load levels to examine the effect of cyclic stress. The microhardness was measured around the vicinity of the crack tip. Furthermore, the fracture surface was observed by scanning electron microscopy. Results show that EPT with electric current density of 150 A/mm2 enhances the high-cycle fatigue life, and the effect tends to increase with the decrease in cyclic stress. Vickers microhardness (HV) near the crack tip decreases to normal levels after treatment, and the approaching cracks on two sides can be observed. Local annealing and recrystallization occur around the fatigue crack tip. Accordingly, crack propagation can be delayed, and fatigue life can be prolonged by EPT.

Application of configurational mechanics in characterizing contact fatigue life and its crack propagation: a numerical lattice-based approach

A numerical lattice-based approach equipped with a brittle erosion algorithm and material forces in the context of LEFM and configurational mechanics is introduced to analyze contact fatigue life and crack propagation of brittle solids under low amplitude cyclic loading using the classical Paris’s law. Material force vectors at lattice nodes were employed in identifying potential crack path in high-cycle fatigue applications. A 2D plane strain pad-substrate system with contact formulations under constant compression and cyclic sinusoidal shear with constant amplitude was considered to investigate fatigue crack’s growth rate and its shape in the substrate, made of hydrided brittle Zircaloy-4 used in nuclear industry. The capability of the lattice in predicting the crack extension path using the material force vectors at the crack tip is verified by comparing with analytical solutions for a centered crack domain under pure shear and direct tension loading, and also for an initial small surface crack for the contact fatigue pad-substrate system. Obtaining the total fatigue life by assuming a failure value for the crack length and confirming the Paris’ law crack growth, the lattice results demonstrate that the fatigue crack path in the substrate is a curved trajectory which gradually reduces as the fatigue crack tip grows deeper into the substrate. Having simple constitutive formulation and straightforward erosion algorithm, this approach together with configurational mechanics implications can be employed in analyzing fretting fatigue problems.

ECCI Characterization of Dislocation Structures at a Non-propagating Fatigue Crack Tip: Toward Understanding the Effects of Mn-C and Cr-N Couples on Crack Growth Resistance

We attempted to clarify the underlying mechanisms of the enhanced small-fatigue-crack resistance of Fe-Mn-C twinning-induced plasticity (TWIP) steel and high-nitrogen austenitic steel. To this end, we performed electron channeling contrast imaging near the tips of non-propagating fatigue cracks in Fe-18Cr-14Ni steel without a significant amount of interstitials, Fe-23Mn-0.5C TWIP steel, and Fe-25Cr-1N austenitic steel. The fatigue crack non-propagation limits of the TWIP steel and high-nitrogen steel were higher than that of the steel without a significant amount of interstitials; the higher limits of the TWIP steel and high-nitrogen steel are attributed to the Mn-C and Cr-N interactions, respectively. The enhanced small-fatigue-crack resistance of the Fe-23Mn-0.5C steel is attributed to local hardening at the crack tip caused by an increase in the dislocation density via dynamic strain aging. The enhanced dislocation planarity of the Fe-25Cr-1N steel, which is a result of the Cr-N interaction, is a significant factor that influences (i.e., increases) the crack resistance. The enhanced dislocation planarity results in dislocation pile-up stress at the crack tip, thereby preventing dislocation emission from the crack tip.

Deformation in fatigue crack tip plastic zone and its role in crack propagation of titanium alloy with tri-modal microstructure

A combination of scanning electron microscopy and electron backscatter diffraction is used to investigate the deformation in the fatigue crack tip plastic zone and its role in the crack propagation of Ti-6Al-2Zr-1Mo-1V alloy with a tri-modal microstructure. The results show that heterogenous slip and secondary micro-cracks are the main features of the fatigue crack tip plastic zone. These features play important roles in fatigue crack propagation. In particular, the unique lamellar α in the tri-modal microstructure can deflect and delay the crack propagation effectively, thus, improving the fatigue life.

Resistance of High-Strength Steels to Stress Corrosion Cracking Depending on External Environment pH Factor

The paper deals with the study of stress corrosion cracking of high-strength steels in an aqueous environment with a varying pH factor ranging from 5.5 to 12.0. Steels were studied after quenching and tempering, one of the steels was prone to temper embrittlement.
 Single-edge notched pre-cracked specimens were used for the experiments. Changes in the pH factor at the crack tip were measured using an antimony electrode. The pH factor values at the crack tip dropped to 2.0. Steel prone to temper embrittlement showed significantly shorter incubation period and more accelerated development of corrosion process compared to the optimized heat treatment of the second steel. Proneness to intergranular fracture was observed close to the fatigue crack tip. The obtained results expand the existing knowledge about localized corrosion processes leading to the refinement of the stress corrosion cracking model when changing the pH factor on the crack tip.

The roles of internal and external hydrogen in the deformation and fracture processes at the fatigue crack tip zone of metastable austenitic stainless steels

Fatigue crack growth (FCG) tests were performed with two types of metastable austenitic stainless steels having different austenite phase stabilities under hydrogen-precharged conditions (internal hydrogen) and in gaseous hydrogen environments (external hydrogen). The materials showed a peculiarly slower FCG rate with internal hydrogen than with external hydrogen even though the hydrogen concentration was much higher under the internal hydrogen conditions. The results are interpreted in terms of hydrogen-modified plastic deformation character comprising inhibited cross-slipping or enhanced deformation twinning in combination with the sequence of hydrogen penetration and strain-induced α′ martensite formation in the local region surrounding the fatigue crack tip.