Fatigue Crack Propagation Process Research Articles

The smooth and notched three-point bending fatigue behavior of directionally solidified high-Nb TiAl alloy

To understand the fatigue behavior of directionally solidified (DS) TiAl alloys, the Ti46Al7Nb alloy fabricated by cold crucible DS method was investigated by microstructure characterization and three-point bending fatigue tests. Results show that the microstructure of the alloy is composed of α 2 /γ lamellae with β segregation, and the angle between lamellar arrangement and DS growth direction is 45°. Smooth and notched specimens were prepared to evaluate the fatigue performance of the TiAl alloy under pre-damage conditions. The finite element results indicate that the notched have more obvious stress concentration. The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. The crack propagation path indicates that there are cleavage microcracks with different shapes around the main crack in the notched specimen, while the main crack in the smooth one is surrounded by many corrugated cracks perpendicular to the lamellar direction. The whole process of fatigue crack initiation and propagation under three-point loading is established. Fracture morphology shows that notched specimens are mostly brittle cleavage fracture along the lamellar facets, while the smooth are trans-lamellar fracture with plastic characteristics. The degree of fatigue deformation of the smooth specimen is greater than that of the notched one, which mainly reflects on high density dislocation and dislocation intersection, as well as more sub-grain boundaries and twins. • The high-Nb TiAl alloy with well-aligned lamellae was fabricated by CCDS. • The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. • The corrugated secondary cracks formed around the main crack are conducive to the release of stress concentration. • The smooth specimen exhibits high-density dislocation intersection, as well as more sub-grain boundaries and twins.

Experiment-Based Fatigue Behaviors and Damage Detection Study of Headed Shear Studs in Steel–Concrete Composite Beams

Many in-service bridges with steel–concrete composite beams are currently aging and experiencing performance deterioration. Under long-term cyclic loads from traffic on bridges, headed shear studs in steel–concrete composite beams are vulnerable to fatigue damage. The comprehensive understanding of fatigue behaviors and the feasible detection of fatigue damage of headed shear studs is, thus, crucial for the accurate numerical simulation of the fatigue crack propagation process. The paper, thus, experimentally investigates the fatigue behaviors of headed shear studs through push-out tests of three specimens. The fatigue failure modes and cyclic strain evolution of specimens are analyzed. The fatigue lives of headed shear studs are compared with the S–N curves of the AASHTO, Eurocode 4 and BS5400 codes. The fatigue crack details of shear studs in push-out tests are then detected using the ultrasonic non-destructive testing. The results show that the root fracture is the main fatigue failure mode of shear studs under fatigue loading. The fatigue life estimations based on the three current codes (i.e., AASHTO, Eurocode 4 and BS5400) can be safely guaranteed only with different safety redundancies. The strain at the shear stud with fatigue damage shows a consistent increasing trend followed by decreasing behavior after reaching the peak value with the loading cycles. Moreover, the feasibility of the ultrasonic non-destructive testing with the combination of a strain measurement for fatigue crack details detection of headed shear studs in composite beams is proved.

Investigation of the fatigue damage mechanism of Inconel 617 at elevated temperatures obtained by in situ SEM fatigue tests

In the present study, fatigue damage evolution of Inconel 617 at 600 °C and 700 °C was investigated by using an in situ scanning electron microscope (SEM) fatigue test. The fatigue crack initiation and propagation processes were directly observed and recorded online. Fractography and microstructure analyses were performed to further reveal the mechanism of high-temperature failure. Experimental results showed that Inconel 617 was not sensitive to an artificial notch even if the curvature reached approximately a hundred microns, and it was more vulnerable to intergranular fracture. This alloy showed multiple crack initiation and propagation behavior at high temperatures. Multiple crack initiation sites did not occur in the same planes, and the number of crack initiation sites increased with increasing environmental temperature. Intergranular cracking behavior was observed in the near surface region, and transgranular crack initiation and propagation were mainly observed in the interior. Coarsening and discrete M23C6-type carbides formed at grain boundaries were the main factors responsible for intergranular cracking. The twin boundary (TB) was directly observed as the crack initiation site, and it retarded crack propagation along the TB and changed the crack propagation mode.

Ultra-low cycle fatigue properties and fracture mechanism of corroded structural steel

ABSTRACT Ultra-low cycle fatigue (ULCF) tests of corroded steel specimens were conducted to illustrate the effects of corrosion on the ULCF properties and fracture mechanism of structural steel. Fatigue crack initiation and propagation process and fracture morphology were observed respectively. The results indicate that the ULCF fracture life of corroded specimens decreases more with increasing surface roughness. The crack-initiation life of corroded specimens is less than half of the fracture life. With corrosion degree increases, the area of the ULCF crack propagation zone expands observably, where it gets harder to find dimples, indicating a transition from ductile to brittle fracture mode.

Long-term prediction of fatigue crack growth in ballastless track of high-speed railway due to cyclic train load

This paper proposes a vehicle-ballastless track coupled model with consideration of concrete cracks in high-speed railways through finite element method. Cracked Euler beam elements are used to model the concrete slab and the supporting layer with through-transverse cracks. In addition, an iterative method is proposed to estimate the fatigue crack growth of concrete under long-term train dynamic load on the basis of the Forman model. The vehicle-ballastless track coupled model with concrete cracks is employed to obtain the crack-tip stress intensity factor (CTSIF) of concrete under the condition of initial crack depth. The CTSIF induced by a high-speed vehicle is adopted in the Forman model to calculate the crack growth rate. The crack depth is updated by the number of passing vehicles in each iteration step, which is determined by a crack growth increment threshold. The dynamic characteristics of the vehicle-ballastless track coupled system are re-calculated based on the updated crack depth. Then, the effects of the crack depth, the stiffness of CA mortar and subgrade on the CTSIF of the concrete slab and the supporting layer are investigated in detail. The fatigue crack propagation process and residual service life under different initial crack depths of the supporting layer are discussed. Some practical conclusions are drawn, which can provide some theoretical guidances for the routine maintenance of slab tracks.

In-situ observation and finite element analysis of fretting fatigue crack propagation behavior in 1045 steel

In this paper fretting fatigue crack behavior in 1045 steel is studied by in-situ observation and finite element analysis. in-situ fretting fatigue experiments are conducted to capture real-time fretting fatigue crack formation and propagation process. The fretting fatigue tests under different load conditions are carried out, then the lifetime and fracture surface are obtained. The crack propagation rates under different loading conditions are measured by in-situ observations. With in-situ observation, crack initiation location and direction are analyzed. Finite element model is used to calculate J-integral which then is applied to fitting with experimental crack growth rate, and establishing crack growth rate model. From fitted S-N curve, it turns out that smaller load ratio leads to higher lifetime. Crack initiates slightly below the point equivalent to line contact of the contact surface in different test conditions, and crack direction shows no obvious relationship with load parameters. The established crack growth rate model well agrees with the test results.

Maintenance optimisation for systems with multi-dimensional degradation and imperfect inspections

In this paper, we develop a maintenance model for systems subjected to multiple correlated degradation processes, where a multivariate stochastic process is used to model the degradation processes, and the covariance matrix is employed to describe the interactions among the processes. The system is considered failed when any of its degradation features hits the pre-specified threshold. Due to the dormancy of degradation-based failures, inspection is implemented to detect the hidden failures. The failed systems are replaced upon inspection. We assume an imperfect inspection, in such a way that a failure can only be detected with a specific probability. Based on the degradation processes, system reliability is evaluated to serve as the foundation, followed by a maintenance model to reduce the economic losses. We provide theoretical boundaries of the cost-optimal inspection intervals, which are then integrated into the optimisation algorithm to relieve the computational burden. Finally, a fatigue crack propagation process is employed as an example to illustrate the effectiveness and robustness of the developed maintenance policy. Numerical results imply that the inspection inaccuracy contributes significantly to the operating cost and it is suggested that more effort should be paid to improve the inspection accuracy.

Research on fatigue crack propagation process of fracturing pumphead

Fracturing pumpheads are a key and crucial component in fracturing technology. Fatigue crack failure greatly decreases the pumphead working life and operational efficiency. This paper describes an analytical model for a five-cylinder fracturing pumphead using the submodel method with the re-meshing technique to analyze its fatigue crack propagation process. Changes in the crack face, a-KI curve (crack length a and stress intensity factor KI), and a-N curve (number of fatigue loading cycles N) during crack propagation are considered. The results show that the crack failure of the fracturing pumphead results primarily from mode-I cracks. During crack propagation, the curvature of the crack front gradually decreases due to the higher stress intensity factor KI at the pressured inner surfaces of the pumphead. The stress distribution on the pumphead greatly influences the change law for the a-KI curve. When the crack tip localizes in the high stress zone, KI increases with the crack length a; when the crack tip localizes in the low stress zone, KI decreases with the crack length a. The step structure of the pumphead also influences changes to the a-KI curve. Around the step structure, a wave peak appears in the a-KI curve, but the influence is localised and its effect on the a-N curve is not significant.

Experimental study on fracture and fatigue crack propagation processes in concrete based on DIC technology

A method to determine crack tip position and fracture process zone (FPZ) in concrete was proposed based on Bažant Crack Band Model (CBM) and digital image correlation (DIC) technique, which was described as: firstly, the critical crack opening displacement of concrete (wcr) was determined based on CBM and the basic mechanical properties; secondly, DIC method was used to obtain the displacement field on the surface of specimens; thirdly, the position where the crack opening displacement equaled to wcr was defined as the crack tip position. In the fatigue crack propagation tests, the whole fatigue crack was FPZ at the beginning of fatigue loading until the non-cohesive crack grew and the length of FPZ decreased. The length of FPZ tended to be stable when the number of loading cycles was close to half of the fatigue life. Fatigue crack propagation curves (a-N curves) were obtained and fitted by logistic function to reduce the influence of the data scattering. Fatigue crack propagation rate da/dN was separated into descending and ascending crack propagation functions based on the increment of crack Δa and stress intensity factor ΔK, respectively. The da/dN-Δa curves for the descending segment showed obvious scatter. Paris’ law was applied to describe fatigue crack propagation rate well for the ascending segment.

Experimental Study on Fatigue Crack Propagation of High-Strength Steel Wire with Initial Defects for Bridge Cables

In order to study the effect of initial defects on fatigue crack propagation law, a test method to identify fatigue crack propagation rate and path based on load waveform variation was presented, and a new test device was designed to apply fatigue pulsation loads to multiple wires for bridge cables simultaneously in this paper. To simplify the corrosion defect formation process, a machine-cut notch was used to describe the initial defect on the steel wire surface. Firstly, fatigue crack propagation tests were conducted on the surface notched steel wire specimens. By using crack front marking technique, the “beach-like patterns” visible to the naked eyes on the cross sections of the steel wires were formed, and the process of fatigue crack propagation can be tracked and reproduced. Then Autodesk Computer Aided Design (AutoCAD) software was used to describe the morphology of “beach-like patterns” and accurately measure the depth and width of cracks. Finally, the influence of initial defect morphology on fatigue crack propagation rate was investigated according to the relationship between fatigue cracks depth and cyclic loading numbers. The results show that the test device designed in this paper can effectively realize the synchronous fatigue crack propagation test of multiple wires, and significantly shorten the fatigue test period. By observing and analyzing the change of load waveform, the moment of fatigue crack propagation can be directly and accurately determined. The larger the depth, the smaller the width and the sharper the morphology of initial defect, the faster the crack propagation rate and the shorter the life of notched wire specimens under the combined action of fatigue loads and corrosive medium.

The numerical simulation of fatigue crack propagation in Inconel 718 alloy at different temperatures

Inconel 718 alloy is the most commonly used material of aero-engine turbine today. However, Inconel 718 alloy is known to suffer from the fatigue crack propagation during engine operation, which will lead to unstable fracture and seriously threatens the safety of the engine. In this paper, we research the fatigue crack propagation process of Inconel 718 alloy unilateral notched standard CT specimen at room temperature (298.15 K), 573.15 K, and 823.15 K under the type I cyclic fatigue load. The ABAQUS is used for numerical simulation. First, parametric models are established. Second, the extended finite-element method (XFEM) is used to in the calculation process for describing the crack state. Third, the stress intensity factor at the crack tip under different temperatures is calculated, and the extended finite-element crack length is solved to obtain the fatigue crack growth rate da/dN curve. The innovation is we use the XEFM method to predict the fatigue crack growth process of Inconel 718 alloy, and compare it with the test results to verify the reliability of the XEFM method under low stress intensity factor. We also provide some possible reasons for the error of XEFM results under high stress intensity factors, and the development of simulation methods should be emphasized.

In-situ investigation on fatigue behaviors of Ti-6Al-4V manufactured by selective laser melting

In-situ fatigue tests were conducted for exploring the fatigue crack propagation process for Ti-6Al-4V manufactured by selective laser melting (SLM). Fatigue specimens were prepared with two printing orientations by SLM and characterized by scanning electron microscopy (SEM). In-situ fatigue experiments were carried out at 25 °C, 200 °C, 400 °C and 600 °C, respectively. Results show that vertically printed specimens have more and larger defects than horizontally printed ones. The fatigue property of horizontally printed specimens is better than that of vertically printed specimens. The anisotropy caused by printing orientation is the reason for the difference in fatigue property. Crack propagation rate increases with temperature. A large number of secondary cracks occurred during the crack propagation at elevated temperature. The branching of secondary cracks releases the energy at the crack tip, and thus slows down the crack growth.

Multiscale Analysis of Concrete Damage and Crack Propagation Under High Cycle Loading

Concrete is a typical multiphase composite material in which the initiation and propagation of cracks under fatigue load are mainly determined by its mesoscopic structure. In this paper, a concrete multiphase mesoscopic model which considers thickness of interfacial transition zone (ITZ) varying with aggregate’s size is established by the integrated scripting method. The model comprehensively contains the stochastic characteristics of concrete mesoscopic structure. According to the fatigue crack propagation characteristics in different stages, a multiscale method is proposed by establishing the interactive mesoscopic and macroscopic models to targetedly analyze the whole process of fatigue crack initiation, propagation and failure of concrete specimen. Based on the technique of cycle block, concurrent simulation of concrete damage and crack propagation under high-cycle fatigue load is realized. The analysis results show that fatigue cracks in concrete mainly born near the ITZ and gradually enter into the cement mortar matrix to formulate the cracks with finite size. These cracks influence each other until the dominant crack appears and insatiably propagates to result in the failure of specimen.

In-situ SEM investigation on fatigue behaviors of additive manufactured Al-Si10-Mg alloy at elevated temperature

In-situ high temperature measurement in a scanning electron microscopy (SEM) environment is an important technique for exploring the microstructure evolution and crack propagation within refractory materials and other advanced alloys. In this paper, in-situ tensile fatigue experiments under SEM environment at room temperature and elevated temperature are performed for studying the fatigue performance and crack propagation process of selective laser melting (SLM) additive manufactured Al-Si10-Mg materials. Firstly, in-situ SLM additive manufactured Al-Si10-Mg fatigue tensile samples are prepared, and micro-CT are employed for the characterization of void defects within SLM additive manufactured Al-Si10-Mg with different laser scanning speeds and laser energy; Secondly, in-situ fatigue experiments are carried out under SEM environment at 25 °C, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C, respectively. The fatigue mechanical behaviors, microstructure evolution and crack propagations of as-fabricated Al-Si10-Mg samples are characterized. Finally, micro-CT tomography 3D imaging techniques are employed for exploring the link between SLM defects and fatigue performances.

A Study of Energy Balance in Deformed Metals for Assessment of Damage Accumulation Stage

The work is devoted to study of energy balance in metals under plastic deformation and failure. Based on our previous experimental results we develop a phenomenological thermodynamic model of heat dissipation in metals. It allows us to simulate the energy balance under plastic deformation, to study the failure process of metals based on the stored energy criterion and the process of fatigue crack propagation. New internal thermodynamic variable is introduced as defect induced deformation and calculated based on the solution of statistical problem of mesodefect evolution. The developed model is adapted for using in standard final-elements numerical code (Abaqus) to simulate energy dissipation under quasi-static and cyclic loading in 3D geometries. The theoretical results were illustrated by computing of energy dissipation both under tensile test of Armco iron specimens and crack propagation under cyclic loading.

Analysis of the Fatigue Crack Propagation Process of the Stress-Absorption Layer of Composite Pavement Based on Reliability

The stress-absorption layer in cement concrete pavement delays the development of reflection cracks and is good at fatigue resistance. Laboratory investigations of the anti-crack performance of the high viscous asphalt sand stress-absorption layer (HVASAL) and rubber asphalt stress-absorption layer (RASAL) were carried out by force-controlled fatigue crack propagation tests, for which three types of overlay structures with three types of pre-crack (i.e., the middle crack, the side crack, and the 45° inclined crack) were designed. A probability model was established to describe the propagation of the fatigue cracks. The fatigue crack propagation, the fatigue life, the crack propagation rate, and the crack propagation mechanism of the three types of overlay structure were compared and analyzed. The results show that the stress-absorption layers have good anti-crack fatigue performance, and that the RASAL is better than the HVASAL. The crack propagation patterns of the three types of overlay structure were found. In the double logarithmic coordinate, the curves of the three types of cracks are straight lines with different intercepts and slopes. The probability model quantifies the relationship between the crack propagation rate and ∆K. The influences of the three types of crack on the fatigue properties of the asphalt overlays are different.

Quantitative analysis of intrinsic mode III fatigue thresholds in bcc metals

Intrinsic (effective) threshold is a material characteristic important for both engineering and science connected to fatigue crack propagation processes. This paper presents the first attempt to give a quantitative interpretation of the mode III effective threshold. The approach of a local growth mode previously applied to the remote mode II cracks, which led to a prediction of values of the mode II effective threshold in metallic materials, is applied to remote mode III cracks in materials with coplanar shear-mode crack growth (bcc metals). Local mode II stress intensity factors of a zig-zag shaped crack front loaded in the remote mode III were determined by numerical modelling for various angles of in-plane crack front asperities. The results were compared with experimentally measured effective thresholds and geometry of real crack fronts. It was found that for relatively small angles of the in-plane precrack asperities (23° for the ARMCO iron and 25° for niobium) the remote mode III crack propagation can be realized purely by the local mode II growth mechanism. Experimentally measured average angles of asperities on real precrack fronts (25° for the ARMCO iron and 26° for niobium) were very close to those obtained theoretically. This represents a quantitative confirmation of a dominance of the local mode II mechanism previously deduced only qualitatively by observation of fractographical patterns.

Fatigue tests of welded connections between longitudinal stringer and deck plate in railway bridge orthotropic steel decks

This research presented the fatigue tests of longitudinal stringer-to-deck (SD) welded connections, which have been identified as the locations most sensitive to fatigue damage in the orthotropic steel decks (OSDs) of railway bridges. Four full-scale SD connections were fabricated, and two loading patches were considered. Static loading was first carried out to obtain the structural hot spot stresses at weld toes as well as stress concentration factors (SCFs), by which the hot spots providing the highest stresses were identified. Cyclic loading was then implemented next to the static loading, and the behaviors including fatigue crack initiation and propagation process, fatigue failure mode, characteristic fatigue life, as well as degradation of vertical rigidity, were all obtained from the test. The crack growing process can be totally divided into four stages, and the fatigue lives after the crack arrived at the deck edge were very short. Variations of crack dimensions were also obtained, and the simplified formulae of crack growth rate were numerically fitted so that the crack propagation lives can be predicted by using the crack dimensions. Comparisons also show that the FAT 100 curve in IIW fatigue design recommendation could overestimate the fatigue resistance of such connections where double-sided fillet welds were used to connect the stringer web and the deck plate, and therefore double-sided groove welds with partial or full penetrations are recommended for the stringer-to-deck connections in railway bridge decks.

Experimental study on fatigue failure of rib-to-deck welded connections in orthotropic steel bridge decks

The rib-to-deck (RD) welded connections are the most sensitive locations to encounter the fatigue failure in orthotropic steel decks (OSDs), and numbers of fatigue cracks arising from these areas have been found in existing OSD bridges. This research focus on the fatigue cracking process, fatigue characteristics as well as failure mechanics of RD connections under cyclic loading. Six full-scale RD welded joints were fabricated, and two load cases of centric and eccentric loading were considered. Static loading was first conducted with aims of measuring elastic strain distributions at potential hot spots. Structural hot spot stresses at weld toes as well as stress concentration factors (SCFs) were linearly extrapolated by using the recorded strains, based on which critical locations were identified. High-cycle repeated loading was subsequently implemented, from which the fatigue crack initiation and propagation process, fatigue failure mode, characteristic fatigue life, as well as degradation of vertical rigidity, were obtained. Four stages of crack propagation were mainly observed, and the remaining fatigue lives after the crack reached the deck edge were short to be neglected. Variations of crack dimensions including the longitudinal length and the depth in the plate thickness were also revealed. Comparison between experimental numbers of cycles and standard S-N formulae indicates that the FAT 100 curve provided in the IIW fatigue recommendation could be conservatively used to estimate the fatigue resistance of such rib-to-deck welded connections composed of 16mm thick deck plates and 80% PJP welds.

Influence of H2S Corrosion on Rotating Bending Fatigue Properties of S135 Drill Pipe Steel

Fatigue properties of S135 drill pipe steel 27CrMo under different conditions were investigated by rotating bending fatigue test. The results show that the fatigue limit at 106 cycles of the tested steel is 540 MPa. However, the fatigue cycles of the specimens after H2S corrosion shows a sharp decline from 106 to 104, when the stress is also 540 MPa. SEM observation of fatigue fracture surfaces reveal that the fatigue fracture has the characteristics of ductile fracture before H2S corrosion. After H2S corrosion, its shows the characteristics of brittle fracture. The main effect of H2S corrosion is that the hydrogen atoms gathered at grain boundaries, inclusions and defects, which promoted the process of fatigue crack initiation and propagation, reducing the fatigue properties.