Crack Propagation Zone Research Articles

Mechanical and corrosion behavior of titanium alloys additively manufactured by selective laser melting – A comparison between nearly β titanium, α titanium and α + β titanium

Mechanical and corrosion behaviors of SLM-processed Ti-13Nb-13Zr alloy with primarily β phase and acicular martensitic α′ microstructure, CP Ti in martensitic α′ structure and Ti-6Al-4V with martensitic α′ and a small amount of β phase microstructural conditions were studied in this work. A factor f (tensile strength/elastic modulus) was used to define the mechanical performance and the Ti-13Nb-13Zr alloy exhibits the highest f (15.22 ± 0.19) comparing with CP Ti (9.90 ± 0.10) and Ti-6Al-4V (12.47 ± 0.14). Among the three materials, Ti-13Nb-13Zr alloy demonstrates lower stress-controlled fatigue resistance but higher strain-controlled fatigue performance than CP Ti and Ti-6Al-4V. Fatigue crack initiation occurred in the surface/subsurface pores and the river pattern was observed in the crack initiation zone and crack propagation zone. The results of the potentiodynamic polarization indicate that the Ti-13Nb-13Zr alloy has the highest corrosion potential comparing with that of CP Ti and Ti-6Al-4V alloy because that Nb2O5 is more chemically stable than V2O5 and solid layers of Nb2O and ZrO2 are existed in passive TiO2 layers during the corrosion process which could reduce the Cl− ingress into the oxide layer and increase the structural integrity of the oxide film.

Strain-controlled fatigue testing of TRIP800 high-strength quenching and partitioning steel

In this paper, TRIP800 steel plates of thickness of 1.2 mm were subjected to the quenching and partitioning (Q&P) process and underwent strain-controlled fatigue tests. The respective experimental fatigue curves were plotted, and the influencing factors of fatigue resistance were identified and analysed, while the specimen fractographies were examined. The test results show that the existence of residual austenite inhibition of crack extension and strain induced the martensite phase transformation in the residual austenite. The residual compressive stresses improved the fatigue strength of Q&P steel plates under study. The fatigue fracture nucleation occurred in surface inclusions, the crack propagation zone exhibited the ductile fracture mode, while the transient area possessed the brittle fracture features.

Corrosion fatigue behavior of S135 high‐strength drill pipe steel in a simulated marine environment

The fatigue and corrosion fatigue lives of S135 drill pipe steel were tested in air and in a simulated marine environment. The empirical equations for fatigue lives in different environments were obtained via regression analysis, and fatigue fracture mechanisms were elucidated. Results showed that an endurance limit existed in air, and fatigue cracks were initiated from or near the surface of specimens. The morphologies in air were cleavage fracture at the crack initiation zone and fatigue striations at the crack propagation zone. In the marine environment, fatigue striations were less at higher stress amplitudes, and fatigue lives were lower than that in air. Fatigue cracks were initiated from the corrosion pits on and near the surface of specimens. The morphologies were characteristic cleavage fracture at the crack initiation zone and cleavage fracture and secondary cracking at the crack propagation zone. Less fatigue striations occurred on the main fracture surface. The fracture mechanism in the marine environment can be summarized as follows: corrosion pit formation, crack initiation, crack propagation, piercing, and fracture.

On the failure mechanism for high pressure die casting A390 hypereutectic alloy in low cycle and high cycle fatique

The fatique failure behavior of A390 hypereutectic aluminum alloy plate was studied with laboratory CT and such technique provided a new method for analyses of the mechanical failure. Results were based on the reconstructed fractography and SEM observation and special attention was paid to crack propagation mechanism in the hypereutectic alloy. The fatique crack surface was separated into crack initiation site around pores, flat crack propagation zone and instantaneous crack zone. While the flat crack propagation zone only occurred in high cycle fatique and the height fluctuation of flat propagation was within 50 µm. The disappearance of flat propagation in low cycle fatique showed the different crack propagation mechanism between hypereutectic and hypoeutectic alloy. Different crack propagation stage changed with the stress intensity of crack tip. The flat propagation zones for high cycle fatique attributed to the relatively parallel extension of crack and debonding between PSPs and matrix in hypereutectic alloy. While under higher stress intensity like the case in LCF or later stage of HCF, the crack branched and extended instantaneously, tortuous crack propagation route formed and many PSPs with minor cracks or striations remained.

Experimental study on surface topography and fracture strength of worn saw wire in multi-wire sawing

In multi-wire sawing, reducing the wire wear loss and increasing wire consumption are the common means to avoid the risk of saw wire breakage because the worn saw wire experiences a complex process with strength degradation and surface damage. So, it is urgent and necessary to give a reliable wire wear loss in theory. This paper focused on the surface topography and fracture strength of worn wire with different wire wear loss. Furthermore, the saw wire maximum safety wear loss model was presented based on the experimental results. By observing the surface topography of the worn saw wire, the dented traces were distributed on the whole surface and a few grooves with large depth were shown in the local zone. The cross-sections of the worn saw wires with different wear loss were circular. In addition, its fracture strength of the wore saw wires was not reduced with the increasing of the saw wire wear loss. On the contrary, the fracture strength of the wire with large wear loss was improved by hardening in comparison with fresh saw wire. This phenomenon suggested that surface damages did not affect the fracture strength of saw wire under the experimental condition. Furthermore, fracture photograph of the fracture test samples was divided into the crack propagation zone and final fracture zone. By analyzing these fracture topography, almost all the crack propagation zones were circular and its diameter was about 48.27 μm. It is not correlated with the amount of wear loss. However, the shape became non-circular in some samples with large depth damage on the surface. By summarizing these results, the model and calculation method of the maximum safety wear loss of saw wire were established, which could provide the theoretical basis for the determining the maximum safety wear loss.

Effect of ultrasonic vibration on fatigue performance of AA 2024-T3 friction stir weld joints

Fatigue test of conventional friction stir welding (FSW) joints, ultrasonic vibration enhanced friction stir welding (UVeFSW) joints and base metal in 2024-T3 aluminum alloys are performed to investigate the influence of ultrasonic vibration on the fatigue strength and life of joints. It is found that for 50% survival probability the fatigue strength of UVeFSW joint is greatly improved, up to 96.13% of the base metal. On the advancing side, there exists a weak bonding region at upper part of transverse cross section of welds due to material flow. The stress concentration reaches maximum in the weak bonding region, which usually becomes the fatigue crack initiation site of FSW weld. The application of ultrasonic vibration narrows the weak bonding region and reduces the stress concentration there. In the crack propagation zone, the fatigue striations of UVeFSW joints fracture are parallel with each other and spaced apart, while the regular fatigue striations are not obvious but with larger spacing and poorer parallelism for FSW joint fracture. In the final fracture region, the application of ultrasonic vibration makes the secondary-phase particles smaller in size, more in quantity, and more uniform in distribution. All these contribute the improvement of fatigue performance in UVeFSW.

Effect of hydrogen on the fatigue behavior of the near-β Ti-5Al-5Mo-5V-3Cr alloy

The influence of hydrogen at low limits on the fatigue behavior of Ti-5553 alloy has been studied. The formation of hydrides was investigated using a high-resolution facility coupled to a transmission electron microscope that provided phase and orientation mappings at the nanometric scale. Besides the fatigue limit reduction for the highest hydrogen content, the predominance of subsurface crack initiation sites with cleavage planes and ductile features in contrast to quasi-cleavage facets in the unstable crack propagation zone suggested the influence of hydrogen. The formation of hydrides, in this case, was attributed to be the reason for the different fatigue behaviors.

Numeral modeling of fracture failure of recycled aggregate concrete beams under high loading rates

Fracture tests of recycled aggregate concrete (RAC) beams of different sizes were conducted under high loading rates. In order to characterize the effect of high loading rate on the behavior of RAC beams, two new material models were used together with the commercial finite element software \(\hbox {ABAQUS}^{\mathrm{R}}\). One model is a viscoelastic model that can predict the increase of stiffness (modulus of elasticity) of RAC with increasing loading rate, and the other model is a multiphase composite model that can determine the effective stiffness of RAC taking into account the special internal structure of recycled aggregate. Two different cases were considered in the numerical simulation. Case 1 is for fixed beam size under different loading rates, and Case 2 is for fixed loading rate with different beam sizes. For Case 1, the simulation results of the maximum loads under three different strain rates agreed with test data quite well. The Force-CMOD curves of the numerical simulation and test data showed similar trends. The higher the strain rates, the wider the high stresses spread in the crack propagation zone. The good agreements with the test data indicated that the two new material models can characterize the effect of high loading rate on RAC beams very well. For Case 2, three beam sizes and one loading rate was studied. The post-peak Force versus CMOD curves from the simulation follow the same trend of the test data. The stress distributions in the beams of different sizes are similar. On the other hand, the maximum loads predicted by the numerical model did not agree very well with test data. This is due to the fact that the maximum forces of RAC notched beams exhibited size effect, which was not considered in the fracture criteria adopted in \(\hbox {ABAQUS}^{\mathrm{R}}\) and not in the two new material models. This will be a topic for future research.

Mechanical and Fatigue Properties of Heavy Section Solution Strengthened Ferritic Ductile Iron Castings

Recently, standardized solution strengthened ferritic ductile irons have met the interest of the industrial world since their improved mechanical properties and workability compared to standard ferritic–pearlitic ductile irons. For this reason, mechanical and fatigue properties of heavy section solution strengthened ferritic ductile irons castings are examined. Mechanical tests are carried out on specimens taken from the center of castings with long solidification times. Metallographic analyses are performed by using optical microscopy. Fracture surfaces of fatigue specimens are investigated in order to evaluate crack initiation and propagation zones. Finally, fatigue curves of the analyzed alloy are compared with those obtained from pearlitic ductile iron with the same casting geometry.

Microstructural investigation of subcritical crack propagation and Fracture Process Zone (FPZ) by the reduction of rock fracture toughness under cyclic loading

This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to develop much-needed understanding of the micro-mechanical and micro-structural dynamics of sub-critical crack propagation. The strength variation in rocks under mechanical loading without corrosive chemical environment was investigated by investigating the mode-I (tensile) fracture toughness (KIC) response to static and cyclic loading. In some cases, cracks can grow at a lower load level compared to the static case. This phenomenon is called subcritical crack propagation and depends on the behaviour of the Fracture Process Zone (FPZ). The KIC response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damage mechanisms in front of the chevron crack. A maximum reduction of the static KIC of 43% was obtained for the highest amplitude increasing cyclic loading test. Detailed Scanning Electron Microscope (SEM) examinations were performed on the surfaces of the tips of the chevron notch cracks, revealing that both loading methods cause fatigue in the disc rock specimens. When compared with static rupture, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle failure under cyclic loading, while smooth and bright cracks along cleavage planes were formed under static loading. It is believed in this study that the rock texture characteristics such as interlocked and cemented grains, cement volume, cement mineralogy play very important role on damage behaviour of rocks and development of FPZ and subcritical cracking.

Impact Modification and Fracture Mechanisms of Core–Shell Particle Reinforced Thermoplastic Protein

Mechanical properties and fracture mechanisms of Novatein thermoplastic protein and blends with core–shell particles (CSPs) have been examined. Novatein is brittle with low impact strength and energy‐to‐break. Epoxy‐modified CSPs increase notched and unnotched impact strength, tensile strain‐at‐break, and energy‐to‐break, while tensile strength and modulus decrease as CSP content increases. Tg increases slightly with increasing CSP content attributed to physical crosslinking. Changes to mechanical properties are related to the critical matrix ligament thickness and rate of loading. Novatein control samples display brittle fracture characterized by large‐scale crazing. At high CSP content a large plastic zone and a slow crack propagation zone in unnotched and tensile samples are observed suggesting increased energy absorption. Notched impact samples reach critical craze stresses easily regardless of CSP content reducing impact strength. It is concluded that the impact strength of thermoplastic protein can be modified in a similar manner to traditional thermoplastics. image

Experimental and Numerical Investigations of Fretting Fatigue Behavior for Steel Q235 Single-Lap Bolted Joints

This work aims to investigate the fretting fatigue life and failure mode of steel Q235B plates in single-lap bolted joints. Ten specimens were prepared and tested to fit theS-Ncurve. SEM (scanning electron microscope) was then employed to observe fatigue crack surfaces and identify crack initiation, crack propagation, and transient fracture zones. Moreover, a FEM model was established to simulate the stress and displacement fields. The normal contact stress, tangential contact stress, and relative slipping displacement at the critical fretting zone were used to calculate FFD values and assess fretting fatigue crack initiation sites, which were in good agreement with SEM observations. Experimental results confirmed the fretting fatigue failure mode for these specimens. It was found that the crack initiation resulted from wear regions at the contact surfaces between plates, and fretting fatigue cracks occurred at a certain distance away from hole edges. The proposed FFD-Nrelationship is an alternative approach to evaluate fretting fatigue life of steel plates in bolted joints.

Low cycle fatigue behavior of casting A319 alloy under two different aging conditions

To determine the influence of peak- and over-aging on low-cycle fatigue (LCF) behavior of casting A319 alloy, the strain controlled low-cycle fatigue tests was studied at room temperature under different total strain amplitudes. At lower total strain amplitudes (0.2–0.3%), the A319 alloy presented initial cyclic hardening and then, steady stage and cyclic softening; while at higher total strain amplitudes (0.35–0.4%), it presented continuous hardening in the peak-aged specimens and constant softening in the over-aged specimens. At a given total strain amplitude, the hysteresis loops in the peak-aged specimens were relatively narrower than those in the over-aged specimens. The fatigue life of the peak-aged samples was higher than that of the over-aged samples at any given total strain amplitude. Fractographic morphology indicated that the size of the fatigue crack propagation zone at peak-aged condition was larger than that at over-aged treatment. The crack characteristic in the propagation region and the fast fracture zone also showed some differences between two aging conditions.

Experimental investigation on low cycle fatigue of DZ125 with film cooling holes in different processes of laser drilling

Due to the different low cycle fatigue (LCF) properties and fatigue fracture behavior around film cooling holes on DZ125, the LCF tests are carried out using tension cycling under stress control conditions (stress ratio R=0.1) at 900°C. The specimens were designed as thin-wall plate with single hole and multi holes under picosecond and nanosecond laser drilling processes. Comparative analyses of the differences between fatigue life and microscopic fracture morphology are conducted. It is shown that under the same stress condition, the relationship between fatigue life is as follows: picosecond laser single-holed specimen>nanosecond laser single-holed specimens>picosecond laser multi-holed specimens>nanosecond laser multi-holed specimens. Scanning electron microscope (SEM) analyses of the fracture revealed that the crack initiates from the film cooling holes where fatigue source zone, fatigue crack propagation zone and fatigue fracture zone can be found. However, the different processes lead to slightly different fracture morphology: radial-type ridge centering on the fatigue source zone is more apparent and uniform in picosecond laser drilling specimens than in the nanosecond laser drilling ones. On the other hand, the radial-type ridge is biased toward large-aperture side with nanosecond laser drilling.

Low cyclic fatigue behavior of electron-beam-welded Ti–6Al–4V titanium joint

The low cycle fatigue (LCF) tests were carried out using symmetrical cyclic loading under total strain amplitude control conditions. The present paper is devoted to investigating the cyclic deformation response of Ti–6Al–4V titanium and the electron-beam-welded (EBW) joint in the following aspects, i.e., cyclic deformation behavior, fatigue life and fatigue fracture behavior. The results show that the softening of the joint is significant at larger strain ranges, while not obvious at smaller strain ranges. The joint shows shorter fatigue life at larger strain ranges and equivalent fatigue life at smaller strain ranges compared with Ti–6Al–4V base metal. A fatigue crack of the joint not only originates at the surface or subsurface, but also at defects in the fusion zone (FZ). The crack propagation zone of Ti–6Al–4V base metal shows ductile fracture mechanism, while the joint shows brittle fracture mechanism. In all the fatigue fracture zones many dimples appear, showing the typical ductile fracture.

Numerical model of crack growth in hydraulic re-fracturing

Crack propagation simulations with FEM employ remeshing to provide more accurate results. This raises a question about the direction and criterion of mesh modification. In the case of general-purpose CAE-packages, we have to deal with a stationary mesh, and the crack trajectory is usually represented as a chain of elements with degraded properties. The accuracy of the solution is heavily dependent on the choice of mesh topology, the degree of mesh refinement in the unpredictable crack propagation zone, and correct crack surface loading in this case is impossible. The algorithm proposed in this paper is based on the ANSYS Mechanical APDL language for stepwise geometry reconstruction and mesh modification in accordance with the current configuration of a growing crack and assures a more accurate description of its shape. The crack propagation process is divided into stages. Each subsequent stage differs from the previous one by the crack shape modified due to the crack length increment in the calculated direction, and the linear stationary boundary value problem of elasticity is solved under the assumption of small deformations. To check the adequacy of the model, an experiment on crack propagation in glass samples with an initial cutoff under uniaxial compression has been performed. Samples of size 200×100 mm with a 2.5×40 mm central cutout at an angle of 30 ° to the horizontal axis are made from 4 mm thick window glass. Vertical loading is increased until the crack passes through the sample. The relative difference between the calculated and experimental crack paths does not exceed 5%. The numerical model developed is used to solve the problem of secondary crack growth for different values of stress field anisotropy in the oil ground layer. The factors of crack propagation re-fracturing along the normal to the crack primary gap are defined: 1) stress anisotropy ratio> 0.8; 2) growth of discharge pressure; 3) increase of primary crack disclosure.

Effect of Different Welded Structures on Mechanical Properties of TA2 Tube-to-Tubesheet Joints

Four types of TA2 welded tube-to-tubesheet joints prepared by manual tungsten arc argon-shielded welding technique are studied in this paper. The pull-out tests and low cycle fatigue tests were performed to optimize welded structures of tube and tubesheet. The results show that fractures of welded TA2 tube and tubesheet samples occur at weld zone of TA2 steel tube for the pull-out tests and low cycle fatigue tests. The extension-tubesheet welded joints have the maximum pull-out forces and the best fatigue resistance, and the internal-bore welded joint with 45 deg bevel occupies second place. Fractures are both initiated from weld toe of the outside of tube for the pull-out tests and low cycle fatigue tests. Crack propagates along the direction of 45 deg for the pull-out test. However, crack propagates perpendicularly to the direction of the applied load for low cycle fatigue test, and then fractures immediately parallel to the direction of the applied load. Fatigue striations with a spacing of about 10 μm can be observed on the fatigue crack propagation zone. However, hemispheroidal dimples exist on instant rupture zone.

Surface Integrity and Fatigue Behavior for High-Speed Milling Ti–10V–2Fe–3Al Titanium Alloy

Influence of cutting parameters on surface integrity when milling Ti–10V–2Fe–3Al is investigated based on high-speed cutting experiments. Surface integrity measurements, fatigue fractography analysis, and fatigue life tests are conducted to reveal the effect of surface integrity on crack initiation and fatigue life. The results show that under given experiment conditions, surface roughness decreases linearly when increasing cutting speed or decreasing feed per tooth. The latter has a greater impact on surface roughness than the former. Compressive stress can be detected on all machined surfaces. With the increase of feed per tooth or cutting speed, respectively, residual stress presents a linear increase. Cutting parameters have no significant impact on micro-hardness. When the surface roughness ranges from 0.5 to 1.0 μm, the effect of surface residual stress on fatigue life is more than that of surface roughness. When the surface residual compressive stress increases, the fatigue life improves significantly. Compared with 60 m/min, when cutting speed is 100 or 140 m/min, the surface roughness decreases, the surface residual compressive stress increases, and the fatigue life improves by 124 and 59%, respectively. Under a tensile load, fatigue crack on machined surface of Ti–10V–2Fe–3Al titanium alloy originates at the cross-edge of the specimen surface. With the increase of surface roughness, the area ratio of fatigue crack propagation zone, and fatigue fracture zone decreases.

Determination of fretting parameters of hoisting rope in coalmine and fretting-fatigue behavior of steel wires

Purpose – The objective of this paper is to determine fretting parameters of hoisting rope according to the hoisting parameters in coalmine and to explore the effect of contact load on fretting-fatigue behavior of steel wires. Design/methodology/approach – Based on the mechanical model of hoisting rope in coalmine, the dynamic tension simulation of hoisting rope was performed. Static equations of hoisting rope under tension and torsion and theories of contact mechanics were applied to obtain fretting parameters. Fretting-fatigue tests of steel wires at different contact loads were conducted using a fretting-fatigue test rig. The fretting regime, normalized tangential force and fretting-fatigue life were studied. The morphologies of fretting contact scars and fracture surfaces were observed by scanning electron microscopy and optical microscopy to examine wear and failure mechanisms. Findings – Dynamic tension changes from 0 to 30,900 N. In outer strand layer, contact loads between steel wires in certain wire layers are 60.5 and 38.3 N compared with 378 and 102.7 N between wire layers; relative displacements between wires are 62.5 and 113.2 μm, respectively. Mixed fretting regimes develop in all cases. Increasing contact load decreases the stabilized relative slip and normalized tangential force, reduces the fretting fatigue life, induces accelerated adhesive wear and fatigue wear and results in rougher fracture surface topographies. In all cases, fretting zone induces crack initiation; crack propagation and rupture zones present brittle cleavage and longitudinal splitting, respectively. Practical implications – This paper presents the systemic study on determination of fretting parameters of hoisting rope according to the hoisting parameters in coalmine and the fretting-fatigue behavior of its internal steel wires. The results of fretting-fatigue tests show that the increase of contact load decreases the stabilized relative slip in mixed fretting regime and normalized tangential force, reduces the fretting fatigue life, induces accelerated adhesive wear and fatigue wear and results in rougher fracture surface topographies. Originality/value – The authors warrant that the paper is original submission and is not being submitted to any other journal. And the research does not involve confidentiality, copyright infringement, leaks and other issues, all the responsibilities that the authors will take.

Tensile and fatigue properties of gravity casting aluminum alloys for engine cylinder heads

The mechanical properties were evaluated on specimens of AlSi9Cu3–T6 (333-T6) gravity casting (GC) alloy at room temperature. The GC 333-T6 alloy showed higher yield strength (YS), ultimate tensile strength (UTS) and quality Index but lower hardening capacity than GC 333 aluminum alloy without heat treatment. In addition, the GC 333-T6 aluminum alloy offered a five-fold higher hardening capacity in the cyclic deformation than in the monotonic deformation. Cyclic deformation characteristics of GC 333-T6 aluminum alloy were obtained from the LCF test. The alloy exhibited cyclic stabilization at low strain amplitudes (0.2%) and cyclic hardening at higher strain amplitudes (0.25–0.35%). The extent of cyclic hardening increased with increasing strain amplitude. The Basquin' s equation and Coffin–Manson relationships could be used to describe the fatigue lifetime of this alloy. Additionally, micro-cracks initiated at pores would preferentially pass through the elongated Si particles at lower strain amplitudes. The fatigue crack propagation was mainly characterized by the formation of dimples or fatigue striations at different strain amplitudes. Meanwhile, the larger fatigue crack propagation zone and smaller spacing of fatigue striations at the lower total strain amplitude (0.2%) gave rise to a longer fatigue life. Furthermore, final fast fracture tended to preferentially occur from the larger defects in the fast-fracture region, such as large voids, small pits and inclusions.