Fatigue Crack Propagation Stages Research Articles

Microstructure and properties nonuniformity of Ti6Al4V component fabricated by high-power laser melting deposition

High-power laser melting deposition provides an efficient solution for the fabrication of large-sized titanium alloy components. In this study, Ti6Al4V blocks with well-formed structures were prepared using a 7 kW laser power, and their internal defect distribution, microstructure along the deposition direction, tensile properties, and fatigue performance were investigated. The results showed that the density of the as-deposited Ti6Al4V blocks could reach 99.94%, and the main internal defects were cavities, primarily spherical and dispersed, with a maximum diameter of 326.7 μm. Among them, 65.53% of cavities had a diameter smaller than 100 μm, and 92.76% had a diameter smaller than 200 μm. The microstructure at the top region of the specimen consisted of a needle-shaped α phase, the middle region had a mixture of needle-shaped and lamellar α-phase, and the bottom region showed a lamellar α-phase structure. With an increase in deposition height, the aspect ratio of α-phase increased, and the average width decreased from 3.22 μm to 0.88 μm. The internal structure was mainly composed of a basket-weave structure, with a small amount of Widmanstätten structure present at grain boundaries. Hardness and tensile properties exhibited significant non-uniformity along the deposition height. The tensile strength and yield strength at the top region were approximately 50 MPa higher than those in the middle and bottom regions, while the elongation was about 2% lower. The top region's tensile fracture surface displayed a sawtooth pattern, whereas the middle and bottom regions exhibited fractures along the 45° direction of the principal stress. The length of α-bundles and the interface density within the bundles were crucial factors affecting crack propagation. In the top region, α bundles were the longest, and the interface density was the highest. During crack propagation, when the crack extended perpendicular to the α bundles, it encountered high resistance, and when it extended non-perpendicular, it rapidly propagated along the bundle boundaries, exhibiting high strength and low plasticity. The fatigue performance of the Ti6Al4V specimens fabricated by high-power laser melting deposition showed significant dispersion, with cracks originating from subsurface lack of fusion defects. At different stages of fatigue crack propagation, the role of α phase laths varied: during the fatigue source region, cracks primarily propagated along the boundaries of α laths, displaying significant cleavage fracture characteristics. In the fatigue propagation region, crack propagation velocity was mainly influenced by the direction of the laths, and in the fatigue final rupture region, ductile fracture was predominant. Α laths parallel to the tensile direction exhibited the best plasticity, promoting the formation of large dimples and high tear ridges.

Experimental study on residual monotonic mechanical properties of high-performance Q690 steel with pre-fatigue damage

In this paper, we conduct an experimental study of the fatigue performance and monotonic tensile properties with pre-fatigue damage of seismic and corrosion resistant Q690 structural steel (abbreviation high-performance Q690 steel). Based on fatigue cumulative damage theory, S-N curves and fatigue cumulative damage curves of high performance Q690 steel were obtained. According to the static mechanical properties with different degree of fatigue damage, the related curves between fatigue damage degree and residual mechanical properties were proposed. Finally, based on the Rambeger-Osgood constitutive relationship, a constitutive model for high performance Q690 steel considering fatigue damage is proposed. The results indicated that the high performance Q690 steel showed good fatigue performance. Its limit fatigue strength is 630 MPa (0.86fy) approximately, which is higher than fatigue limit values specified in steel design standards. In addition, the fatigue crack propagation stage and transient fracture stage accounted for a relatively small proportion of fatigue life. The static mechanical properties with pre-fatigue damage of high performance Q690 steel is similar to the AISI 1045 steel. As the fatigue damage degree increases, the elasticity modulus of high performance Q690 steel decreases and yield strength increases. Whereas, pre-fatigue damage has little effect on ultimate strength and elongation. Finally, a new constitutive model considering fatigue damage was proposed, which was found better simulating tensile curves with different fatigue damage.

The interaction of crack and dislocations in cyclic loading in the body-centered cubic (BCC) crystal

In engineering, the mode of fatigue crack propagation in crystals is very important for studying the first stage of fatigue crack propagation. In this paper, a model of fatigue crack propagation in crystals is established by using the discrete dislocation method. The propagation rate of fatigue cracks in different crystals and the dislocation movement and distribution on the slip plans (SP) in front of cracks during loading and unloading are obtained by calculation the model. The effects of different dislocation motion behaviors on fatigue crack growth rate during loading and unloading are analyzed, including dislocation emission, rebound and annihilation. Compared with the case without considering dislocation shielding effect, the influence of dislocation shielding effect on fatigue crack propagation is given. At the same time, the effects of different grain sizes and different grain boundary (GB) angles on fatigue crack propagation are studied.

Experimental monitoring and modeling of fatigue damage for 3D-printed polymeric beams under irregular loading

Studying the fatigue of 3D-printed (3DP) structures usually requires time-consuming and ad hoc material characterizations. This paper experimentally demonstrates a non-intrusive structural health monitoring (SHM) framework capable of monitoring and modeling fatigue damage of 3DP structures. The experiments emulate realistic working conditions of machinery with polyethylene terephthalate beam parts as a subsystem. The pre-notched specimens with 0-, 45-, and 90-degree raster angles are manufactured and tested under highly irregular fatigue loads. Based on the phase space warping algorithm, the smooth orthogonal decomposition identifies the slow-time higher-order damage feature space, whose leading subspace is used as the estimated scalar-damage-time history. The critical inflection points (CIPs), identified in the higher-order damage feature subspace, delineate the three fatigue crack propagation stages with different rates of damage evolution. The lower-order projection of the CIPs in the damage-time histories can serve as early and late damage indicators, with a maximally 50% ahead of any observable signs of structural degradation from the response. The first CIP indicates the damage as early as at 10% of the total life span irrespective of the raster orientation. Based on a normalized temporal damage model, a hypothesis is posed to ascribe the raster angle-induced damage mechanisms to inter-laminar and cross-laminar fractures, validated by an optical-microscopy-based postmortem analysis. This paper provides an ad hoc alternative for studying fatigue damage with minimal assumptions and prior knowledge of the 3DP structure. Furthermore, the identified damage mechanisms suggest aligning the raster orientation with the applied stress to maximize the machinery’s fatigue life.

Study on Fatigue Life Prediction and Acoustic Emission Characteristics of Sandstone Based on Mesoscopic Crack Propagation Mechanism

Even when the maximum stress is less than the peak stress under conventional loading, fatigue failure of rock is likely to occur, thereby showing its unique characteristics. The present study summarized the factors affecting rock fatigue life from the perspective of phenomenology and studied the fatigue damage process of rock from the microscopic perspective. However, the meso-mechanical mechanism of fatigue–tension failure of rocks is still not very clear, and there are few studies on rock fatigue life that use meso-crack propagation models. In this paper, a mesoscopic model considering wing crack propagation is introduced to examine the fatigue failure of sandstone. A fatigue life prediction formula of sandstone was deduced via a combination with the Paris formula. This formula can quantitatively characterize the impact of upper limit stress and lower limit stress on the fatigue life of sandstone and explain the reason why upper limit stress has a greater influence on the fatigue process of sandstone. Such a prediction formula is applicable only under the condition of low confining pressures, which mainly cause tensile failure due to mesoscopic wing crack propagation. Acoustic emission signals during fatigue failure were monitored and then analyzed using a clustering method and a moment tensor inversion method. Therefore, the tensile or shear properties of mesoscopic failure could be distinguished according to acoustic emission characteristics in different stages of fatigue crack propagation. The results showed that crack sources causing sandstone fatigue failure are mainly tension-type when confining pressure is less than 10 MPa, which further verifies the proposed prediction model of sandstone fatigue life under low confining pressures.

Effects of Al content and α2 precipitation on the fatigue crack growth behaviors of binary Ti–Al alloys

The influence of Al content and α2 precipitation on the fatigue crack growth (FCG) behaviors of binary Ti - xAl (x = 2, 4, 6, 8 wt %) alloys was investigated. Ti–Al binary alloys with different Al contents were solution-treated and followed by aging treatment to obtain Widmanstatten microstructure and promote α2 precipitation at higher Al content. Results show that the increase of Al content dramatically reduces the FCG rate. The enhanced resistance to FCG propagation is related to the increase of the critical resolved shear stress (CRSS) for dislocation slipping that acts as crack initiation sites, basing on solution strengthening with the addition of Al. Furthermore, roughness-induced crack closure with increasing the Al content also contributes to the decrease of FCG rate. With the increment of the Al content, the early fatigue crack propagation stage, in which the crack propagation significantly affected by microstructural factors, will be enlarged due to the decrease of crack tip plastic zone, leading to crack deflection and the reduction of FCG rate. In Ti–8Al alloy, the presence of α2 precipitates and the increment of its size and volume fraction after aging treatment will result in significant increase of FCG rate. Moreover, the activation of large-scale slipping in relatively larger colonies promotes the crack to propagate along the large-scale slipping that induces smoother crack path and higher FCG rate.

Study on Fatigue Crack Propagation and Fracture Characterization of 7050-T7451 Friction Stir Welded Joints

The fatigue crack propagation (FCP) behavior of FSW welded 7050-T7451 joints was studied using compact tensile specimens. Based on Paris law and three-stage theory of FCP, the FCP rate of joint was obtained by data fitting and calculation. The fracture morphologies at different stages of FCP were characterized by metallography and scanning electron microscopy (SEM). Results showed that the FCP rate in each region for FSW joint was as follows: heat affected zone (HAZ) > weld nugget zone (WNZ) > base metal (BM). In the first stage of FCP, the fracture morphologies of BM and HAZ were mainly tearing ridges and river pattern. There were a small number of dimples on the fracture of BM, but no dimples on the fracture of HAZ. In the second stage of FCP, the fatigue striations of the BM were denser and finer than those in the HAZ, and have dimple fracture characteristics. In the third stage of FCP, dimples still existed in the fracture surface of the BM, and the fatigue striations disappeared. The HAZ was dominated by lamellar tearing and cleavage fracture without dimples. During the three stages of FCP, the fracture morphology of the WNZ in the first and second stages was fine dimple fracture with significant equiaxed deep dimple morphology. In the third stage, the fracture dimple in the WNZ changed from equiaxed deep dimples to tearing dimples.

Experimental study and residual fatigue life assessment of corroded high-tensile steel wires using 3D scanning technology

Fatigue damage of bridge suspender is a serious problem under long-term environmental erosion and repeated vehicle loads. This paper quantifies the effects of corrosion on the residual fatigue life of steel wires in bridge suspender. A predictive model was proposed to estimate the fatigue life of corroded steel wires based on the equivalent initial flaw size method, in which both corrosion growth stage and fatigue crack propagation stage were considered. The pitting corrosion-induced stress concentration was incorporated into the stress intensity factor model. The theoretical residual fatigue life can be obtained by integrating the fatigue crack growth model from the equivalent initial crack size to a critical crack length. The effects of corrosion on the fatigue performance of steel wires with various corrosion degrees were investigated by fatigue loading tests. The fracture morphology of the steel wires after fatigue was observed using scanning electron microscopy. The relationships between corrosion degrees and fatigue lives of the steel wires were provided. The surface morphology of corroded steel wire was scanned by 3D scanning technology, and a three-dimension model was established. A finite element model of corroded steel wires was established after the scanning model was processed by the Geomagic software. Following that, the relationship between stress concentration factor and corrosion degree was analyzed. A fatigue life prediction method based on the equivalent initial crack size was verified by the experimental observation of various corroded steel wires and the data in open literature.

Fatigue performance of U-rib butt welds in orthotropic steel decks

This study focuses on the orthotropic steel deck (OSD) of a cable-stayed bridge with a plate-truss composite structure. Fatigue tests and numerical simulations were conducted to investigate fatigue features in an OSD with a broad U-rib and embedded sections, considering the assembly tolerance. For the purpose of this study, the pre-stressing force method was adopted to simulate the initial axial pressure in the deck. Fatigue tests were done on three full-scale segment specimens (a total of 6.5 million variable amplitude fatigue cycles). Initial long macro cracks are observed in the arc segment of butt welds. The length of these initial cracks is proportional to the stress amplitude. Fatigue crack propagation stages in the broad U-rib can be divided into four parts with clear demarcation points. The crack growth rate is proportional to the crack length; this increases as a result of the axial force. The initial axial pressure accelerated the crack propagation rate of fatigue details of butt welds. It is recommended that the butt welds of the broad U-rib embedded section can be designed using the fatigue strength of category FAT112 in Eurocode 3. In addition, empirical correction formulas for fatigue strength correction for assembly tolerance of the embedded section are proposed.

Metallurgical Investigation and Life Cycle Assessment of a Piston Rod of Thin Slab Caster

This work focusses on failure analysis of piston rod of a pendulum shear machine in a thin slab caster. Failure of the piston rod led to plant breakdown and huge production loss. In-depth analysis using macro- and micro-fractography, chemical analysis, microstructural analysis using optical and scanning electron microscope coupled with energy-dispersive x-ray spectroscopy and tensile testing was carried out. Macroscopic features like smooth fracture surface with multiple ratchet marks and microscopic features like striations confirmed fatigue mode of failure. Fatigue cracks initiated from the sharp step that acted as stress concentration site. The presence of deep machine or tool marks near the step further aggravated the situation. A number of manufacturing issues like inferior grade of material, undesired heat treatment and the presence of manganese sulfide inclusions were observed. All these factors led to much reduced service life of 5 months as against an expected service life of 5 years. A methodology for calculation of stress intensity factor at different stages of fatigue crack propagation and calculation of number of fatigue cycle for crack propagation has been presented based on quantitative fractography using scanning electron microscope. Based on the study, counter-measures to prevent similar failures in future have been recommended.

Applying fracture mechanics to fatigue strength determination – Some basic considerations

A discussion is provided on demands that must be met in order to apply fracture mechanics to the determination of overall fatigue lifetime and strength, i.e., S-N curves and fatigue limits. These comprise the determination of the cyclic crack driving force for all stages of fatigue crack propagation, in particular for the short crack stage where the crack driving force has to be determined for elastic-plastic deformation and the gradual build-up of the crack closure phenomenon. Special emphasis is put on a fatigue damage relevant specification of the initial crack size. Different approaches in the literature are discussed. Another important aspect is the adequate treatment of multiple crack propagation. Finally, the discussion is illustrated by an example of a butt weld made of a medium strength steel.

Characterization of the Long Crack Propagation Behaviour in a Hardenable Aluminium Alloy in Very High Cycle Fatigue Regime

In the regime of Very High Cycle Fatigue (VHCF), it appears questionable whether all the well-known stages of fatigue crack propagation occur. Deviations must be expected through the very small dimension of the cyclic plastic zone ahead of the crack tip and, hence, a strong interaction with microstructural features seems very likely. Investigations have shown that “natural” crack initiation often takes place underneath the material surface leading to crack propagation without contact to atmospheric components. In order to reproduce the crack growth behaviour, an ultrasonic fatigue testing system (USFT) equipped with a small vacuum chamber was used for fatigue experiments in vacuum. The tests were carried out on the aluminium alloy EN-AW 6082 in the peak-aged (pa) condition. Micro-notches were prepared in the USFT specimens by means of the Focused-Ion-Beam technique. Systematic measurements in laboratory air in the region of the threshold value of long fatigue crack growth revealed that primary precipitates significantly influence the crack growth behaviour. Depending on the spatial distribution of the primary precipitates, strong crack retardation and localized crack arrest take place even far above the threshold value. In vacuum only shear-stress-controlled VHCF long crack propagation were detected in EN-AW 6082 (pa) due to very pronounced single dislocation slip associated with the secondary precipitates of the aluminium alloy.

Cyclic J-integral: Numerical and analytical investigations for surface cracks in weldments

The cyclic J-integral (ΔJ-integral) is a crack tip parameter of elastic-plastic fracture mechanics which can be used as governing parameter for the description of fatigue crack growth (FCG) in metallic structures. In this contribution, it is applied for modelling FCG in weldments. The ΔJ-integral is determined by means of analytical approximation formulas as well as numerical methods. An analytical solution, which takes into account effects of the local ligament plasticity, was derived. This solution is based on well established methods such as R6, BS7910 and SINTAP which were modified for cyclic loading. It incorporates methods for the description of short crack closure behaviour as well as the well known analytical (long) crack closure function of Newman. A specific code was written to evaluate the ΔJ-integral numerically in the course of finite element based crack growth simulations. The code was first validated for an infinite plate with centre crack by applying elastic and elastic-plastic material behaviour. Next, the ΔJ-integral was calculated for cracks in various butt and cruciform welded joints. The results were compared with the results of the derived analytical approximation formula. A good accordance was achieved between the results. Note that the work was part of the German research cluster IBESS the aim of which was the development of a method for fracture mechanics based determination of the fatigue strength of weldments. Since the question behind the present paper was restricted to the cyclic elastic plastic crack driving force needed for the short fatigue crack propagation stage, only the geometrical aspects of weldments (i.e. the weld toe notch) are addressed here whilst other characteristics such as material inhomogeneity (HAZ) or residual stresses are discussed by other papers of this special issue.

Fatigue behaviors of C/Mo double-coated SiC fiber-reinforced Ti6Al4V composites with varied interfacial microstructure

Fatigue crack propagation behaviors of the as-prepared and thermally exposed (700 °C/196 h, 800 °C/196 h) C/Mo double-coated SiC fiber-reinforced Ti6Al4V composites were investigated. The results show that interfacial microstructure evolution has significant effect on fatigue crack propagation behaviors. As for the as-prepared and 700 °C/196-h thermally exposed composites, fiber-bridging accompanied with interfacial debonding can decrease the crack growth rate significantly, while the latter one has lower crack growth rate at the early fatigue crack propagation stage, as the diffusion of Mo atoms makes the matrix close to the interface have more ductile β-Ti. However, fiber-bridging phenomena disappeared with the altered interface in the composite after 800 °C/196-h thermal exposure. Fatigue fracture analysis further reveals that multistep deflections of crack in interface contribute to the improvement of interface toughness. The formation of ductile β-Ti layer resulted from the diffusion of Mo atoms is beneficial to blunt the crack tip.

Degradation of sensing properties of fiber Bragg grating strain sensors in fatigue process of bonding layers

Bonding layers, serving as the strain transmission mediums, may bring undesirable effects to the sensing properties of fiber Bragg grating (FBG) strain sensors during their fatigue process. To analyze the strain sensitivity and the reflected spectrum of FBG strain sensors in different fatigue stages of bonding layers, their strain sensitivities were derived according to strain transfer models. Resorting to T-matrix formalism, the affected reflected spectra were stimulated. Theoretical analysis results show that there is a gradual decline in the strain sensitivity during the stage of fatigue crack initiation, and that significant distortions emerge in the reflected spectrum during the stage of fatigue crack propagation. In addition, a cyclic loading fatigue test was conducted and the phenomenon observed in the test showed a good agreement with the theoretical prediction.

Fatigue small crack growth threshold determination of a high-Nb TiAl alloy at different temperatures by in-situ observation

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy (SEM) in-situ observation. The results indicated that the fatigue crack growth threshold ΔKth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750°C was determined as 12.89 MPa·m1/2 and 8.69 MPa·m1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.

Cohesive zone modelling of low cycle fatigue cracks in cracked and notched specimens

ABSTRACTA cyclic cohesive zone model (CZM) combined with the extended finite element method for the fatigue crack initiation and propagation in ductile materials was investigated under different loading conditions in C(T) specimens. Detailed experimental verification confirmed the known CZM could not predict effects of the loading ratio in fatigue crack properly. Including the loading ratio into the damage evolution equation described the loading ratio correctly. The CZM was used to predict both fatigue crack nucleation and propagation in a deep notched C(T) specimen. The simulations displayed reasonable agreement with experimental data in the stable stage of fatigue crack propagation and showed the potential for a unified description of fatigue crack nucleation and growth.

Analysis on Fatigue Crack Propagation and Fractography of A7075 Aluminum Alloy

The fatigue fractography and surface of A7075 aluminum alloy was investigated by means of optical microscope, scanning electron microscope and technology of metallurgic replica. The results show that the fatigue crack generally initiated in the surface or near the surface of the sample area, such as hole and inclusion. The crack propagation regions under high stress amplitude was characterized by furrow and hollow, and obviously presented fine ripple vein. While lots of fatigue striations and sidesteps with the holes and secondary cracks generated in the crack propagation regions of low stress amplitude. Fatigue transient breaking area is a mixed fracture of brittle and ductile fracture. In the fatigue crack propagation stage, with the increasing of cycle times, secondary cracks gradually merged into primary crack on the path of crack propagation. The dislocation of crack tip,grain boundaries and different grain orientations have great influence on the path of crack growth, which can make cracks bending.

A R-p-da/dN-ΔK Surface Formula and the Statistical Analysis of Effects of Stress Ratio on Crack Growth for GC4 Steel

surface formula based on stress ratio was proposed in consideration of the effects of stress ratio on fatigue crack growth rate and the scatter of fatigue crack growth. The parameters of surface formula were estimated by using the linear principle of no-linear function. The surface formula was applied on GC4 steel to analyze statistically the effects of stress ratio on fatigue crack growth rate. The results showed that the crack growth rate will increases as the stress ratio increases in the stable stage of fatigue crack propagation for GC4 steel. The increase trend is more obvious when higher reliability is demanded.

Fatigue behavior of plain C–Mn steel plates with fine grained ferrite in surface layers

Abstract The effect of fine-grained surface layers on the fatigue behavior of plain C–Mn steel plates is investigated. The plain C–Mn steel plates have been manufactured by a special thermo-mechanical controlled process (TMCP). For plates rolled by the special TMCP (designated special plates), the ferrite grain size approaches 5.5 μm in the surface layers and reaches 6.5 μm on average in the whole thickness of the plates, while for usually rolled plates (designated usual plates), the grain size is 15 μm on average in the whole thickness of the plates, without obvious difference between surface and central layers. Significant improvements of fatigue properties have been achieved by the ferrite grain refinement. Under the similar stress condition, the fatigue lifetime of the special plate is more than 10 times as long as that of the usual plate, and the first stage of fatigue crack propagation can be prolonged. With a similar lifetime of the usual plate under a load ratio R (σmin/σmax) approaching zero, the special plate can sustain a load 40 MPa higher than that of the usual plate. Furthermore, fatigue fractographs have been observed and analyzed by a scanning electron microscope (SEM).