Multiple Crack Initiation Sites Research Articles

Influence of TiC particle size on microstructural evolution and fracture mechanism of TiC/Al-Cu composites

Using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and molecular dynamics simulations, this work investigated the microstructure, texture evolution, strengthening mechanisms, and fracture progression of TiC/Al-Cu composites with various particle sizes following hot extrusion and heat treatment. TiC particles significantly refine grain sizes, shifting the dominant coincidence site lattice (CSL) boundaries from Σ5 to Σ3. The addition of TiC particles alters the preferred grain orientation from (111)/ED to (001)/ED, and modifies the type and intensity of the texture. The primary strengthening mechanisms attributed to TiC include grain boundary strengthening, dislocation strengthening, Orowan strengthening, and load transfer strengthening. However, the strong tendency of smaller TiC particles to agglomerate substantially diminishes these strengthening effects; thus, under a TiC addition of 0.5 wt%, larger TiC particles exhibit better mechanical properties. Fractographic analysis and molecular dynamics simulations revealed that microcracks originate from dissociation at the interfaces between the TiC agglomerates and the Al matrix. The subsequent propagation, expansion, and interconnection of multiple crack initiation sites lead to the formation of macroscopic cracks that result in material failure.

Enhancing fatigue performance of AZ31 magnesium alloy components fabricated by cold metal transfer-based wire arc directed energy deposition through LPB

Cold Metal Transfer-Based Wire Arc Directed Energy Deposition (CMT-WA-DED) presents a promising avenue for the rapid fabrication of components crucial to automotive, shipbuilding, and aerospace industries. However, the susceptibility to fatigue of CMT-WA-DED-produced AZ31 Mg alloy components has impeded their widespread adoption for critical load-bearing applications. In this study, a comprehensive investigation into the fatigue behaviour of WA-DED-fabricated AZ31 Mg alloy has been carried out and compared to commercially available wrought AZ31 alloy. Our findings indicate that the as-deposited parts exhibit a lower fatigue life than wrought Mg alloy, primarily due to poor surface finish, tensile residual stress, porosity, and coarse grain microstructure inherent in the WA-DED process. Low Plasticity Burnishing (LPB) treatment is applied to mitigate these issues, which induce significant plastic deformation on the surface. This treatment resulted in a remarkable improvement of fatigue life by 42%, accompanied by a reduction in surface roughness, grain refinement and enhancement of compressive residual stress levels. Furthermore, during cyclic deformation, WA-DED specimens exhibited higher plasticity and dislocation density compared to both wrought and WA-DED + LPB specimens. A higher fraction of Low Angle Grain Boundaries (LAGBs) in WA-DED specimens contributed to multiple crack initiation sites and convoluted crack paths, ultimately leading to premature failure. In contrast, wrought and WA-DED + LPB specimens displayed a higher percentage of High Angle Grain Boundaries (HAGBs), which hindered dislocation movement and resulted in fewer crack initiation sites and less complex crack paths, thereby extending fatigue life. These findings underscore the effectiveness of LPB as a post-processing technique to enhance the fatigue performance of WA-DED-fabricated AZ31 Mg alloy components. Our study highlights the importance of LPB surface treatment on AZ31 Mg components produced by CMT-WA-DED to remove surface defects, enabling their widespread use in load-bearing applications.

Using X-ray imaging for the study of crack development in solder reliability testing

The possibility of monitoring the progressive damage and cracking in solder joints is investigated through a non-destructive method. As X-ray 3D Computed Tomography is known to require small sample dimensions in order to ensure a good resolution, Computed Laminography was used to get a high-resolution top view of various types of solder joints located on a large printed circuit board. In a first step, the assemblies were cross-sectioned after X-ray imaging in order to verify the findings and confirm the crack morphology. Then, by avoiding a destructive sample preparation step, it was possible to identify in a single sample the effects of solder fatigue during a standard reliability test. Firstly, multiple crack initiation sites could be highlighted in a single X-ray analysis. Moreover, the study gave an interesting insight of the time evolution of voids in solder joints. X-ray laminography produces 3D-like images that contain more information than a conventional cross section. By improving the recognition of horizontal cracks in this unusual point of view of solder joints, the refinement of this imaging and monitoring method can result in a real progress in failure analysis time and efficiency.

Optimization of Selective Laser Melting Process Parameters Via Taguchi's Methods and Gray Relational Analysis for 3D Printing of 18Ni‐300 Maraging Steel

Taguchi methods and gray relational analysis are used to determine optimal processing conditions for the selective laser melting (SLM) of 18Ni‐300 maraging steel. The following SLM process parameters are selected for optimization: laser power (P), scanning speed (v), and hatch spacing (h). Statistical analysis indicates that SLM parameter configuration no. 5 (275 W, 700 mm s−1, 0.08 mm) as most suitable for the simultaneous improvement of tensile strength, elongation (or ductility), hardness, and impact toughness. The order of influence of SLM parameters is identified as follows: . Samples with improved mechanical properties can be produced using configurations with higher linear energy density (). This is because the increased laser energy input facilitates complete melting of metal powder, as well as better interlayer bonding between melt pools. Higher can be achieved by either increasing P or decreasing v. Digital image correlation analysis indicates that tensile test samples with higher porosity exhibit lower ductility due to its inability to accommodate plastic strain accumulation for prolonged durations. Fractography analysis indicates that during tensile loading, cracks will nucleate around the pore perimeter and propagate via microvoid coalescence. Fracture will occur at regions with higher porosity due to it having multiple crack initiation sites.

Fatigue life estimation of additively manufactured Ti-6Al-4V: Sensitivity, scatter and defect description in Damage-tolerant models

Damage-tolerant models for fatigue life estimation rely on fracture mechanics principles to model fatigue crack initiation and growth. In Laser-Based Powder Bed Fusion (LPBF) produced Ti-6Al-4V defect populations present non-uniform in size, shape, and location within components. Considering defect populations in a statistical nature offers one method to characterise defects to be included in fatigue life modelling for AM parts. In this study a novel numerical fatigue life model is built upon which the sensitivity to input parameters, short crack growth mechanics, defect morphology and location for LPBF-produced Ti-6Al-4V is established. Furthermore, we introduce the potential for multiple crack initiation sites and crack interaction. Fatigue strength is shown to be more sensitive to selected threshold parameters and short crack growth mechanics. Furthermore, the introduction of multiple crack initiations and interaction results in distributions of fatigue strength estimates sensitive to defect number, illustrating the importance of its inclusion as a source of fatigue scatter in LPBF produced Ti-6Al-4V parts. Overall, this study establishes areas of nuance and criticality required in the design and modelling of fatigue life unique to AM produced components.

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.

Transition from internal to surface crack initiation of a single-crystal superalloy in the very-high-cycle fatigue regime at 1100 °C

Transition from internal to surface crack initiation is controlled by oxidation assisted fatigue-crack process in the very-high-cycle fatigue regime. Between 760 and 1000 °C, single crack initiation site is associated with internal casting defect, followed by a crystallographic Stage I propagation. By contrast, multiple surface crack initiation sites appear at 1100 °C, as the consequence of internal oxide penetration. The fatal crack follows a Mode I propagation and oxygen can diffuse into the material along the crack path. γ′-phase depletion appears surrounding the oxidised and cracked regions, while localised rafting can occur close to the crack tip.

Failure Analysis of Bolt of Rear Mounting Trunion of an Aero-Engine: A Case of Bending Induced Chevron Pattern as well as Fatigue Failure on the Same Fracture Surface

Present work describes the failure investigation of failed bolt of starboard rear mounting trunion of an aero-engine. Multiple fracture initiation points are noticed. This is a classic case of a single bending type of load initiating reversed bending fatigue as well as chevron pattern on the same fracture surface. Visually observed bending phenomenon supports the each type of failure mode. More interestingly, point of initiation of fast as well as the reversed bending fatigue failure is the same, although those two events have been found to be separate phenomena. It has been established that two different fatigue crack fronts, typical of reversed bending fatigue phenomenon propagated towards each other to make half of the cross-section fractured, while the another half failed by chevron patterned fast fracture. In this, the fast fracture of one half has preceded the reversed bending fatigue fracture of the other half, although the former is not responsible for happening of the later. Modes of fracture and factors influencing have been established in this article with emphasis on circumstantial evidence involving background information and visual examination, supported often by the open literature. Presence of cadmium (Cd) and its possible source, residence time and relative presence on differently fractured surfaces have offered important clues on establishing the sequence and relative inter-dependence of the said two fracture types. Presence of cadmium on the fracture surfaces, multiple crack initiation sites and numerous well-revealed secondary cracks on Branson cleaned fracture surface indicate that the cracks pre-existed on the material even before the cadmium plating and manifestation of chevron pattern is its extreme revelation. This pre-existing chevron pattern primarily aggravated the present failure through bending fatigue phenomenon in the later stages. Low alloy steel (ASTM grade 16) with presently used hardness (340 HV) level does not seem to suit the present application, as it is clear from its extreme brittleness as manifested by pre-existing cracks.

Influence of Plasma Electrolytic Oxidation on Fatigue Behaviour of ZK60A-T5 Magnesium Alloy

Magnesium alloys are used in the motorsport and aerospace fields because of their high specific strength. However, due to their low corrosion resistance, protective surface treatments, such as conversion coating or electroless plating, are necessary when they are used in humid or corrosive environments. The present study aimed at evaluating the effect of plasma electrolytic oxidation (PEO), followed by the deposition of a polymeric layer by powder coating, on the rotating bending fatigue behaviour of the wrought magnesium alloy ZK60A-T5. The specimens were extracted from forged wheels of racing motorbikes and were PEO treated and powder coated. Microstructural characterization was carried out by optical (OM) and scanning electron microscopy (SEM) to analyse both the bulk material and the multilayer, consisting of the anodic oxide interlayer with the powder coating top layer (about 40 µm total thickness). Rotating bending fatigue tests were carried out to obtain the S–N curve of PEO-treated specimens. The results of the rotating bending tests evidenced fatigue strength equal to 104 MPa at 106 cycles and 90 MPa at 107 cycles. The results of the investigation pointed out that PEO led to a reduction in fatigue strength between 14% and 17% in comparison to the untreated alloy. Fracture surface analyses of the fatigue specimens, carried out by SEM and by 3D digital microscopy, highlighted multiple crack initiation sites at the interface between the PEO layer and substrate, induced by the concurrent effects of coating defects, local tensile stresses in the substrate, and increased roughness at the substrate–coating interface.

Corrosion and Corrosion Fatigue Properties of Additively Manufactured Magnesium Alloy WE43 in Comparison to Titanium Alloy Ti-6Al-4V in Physiological Environment.

Laser powder bed fusion (L-PBF) of metals enables the manufacturing of highly complex geometries which opens new application fields in the medical sector, especially with regard to personalized implants. In comparison to conventional manufacturing techniques, L-PBF causes different microstructures, and thus, new challenges arise. The main objective of this work is to investigate the influence of different manufacturing parameters of the L-PBF process on the microstructure, process-induced porosity, as well as corrosion fatigue properties of the magnesium alloy WE43 and as a reference on the titanium alloy Ti-6Al-4V. In particular, the investigated magnesium alloy WE43 showed a strong process parameter dependence in terms of porosity (size and distribution), microstructure, corrosion rates, and corrosion fatigue properties. Cyclic tests with increased test duration caused an especially high decrease in fatigue strength for magnesium alloy WE43. It can be demonstrated that, due to high process-induced surface roughness, which supports locally intensified corrosion, multiple crack initiation sites are present, which is one of the main reasons for the drastic decrease in fatigue strength.

Effect of Rise and Fall Time on Dwell Fatigue Behavior of a High Strength Titanium Alloy

Frequency is an important factor influencing the fatigue behavior. Regarding to the dwell fatigue, it corresponds to the effect of rise and fall time, which is also an important issue especially for the safety evaluation of structure parts under dwell fatigue loading, such as the engines of aircrafts and the pressure hulls of deep-sea submersibles. In this paper, the effect of rise and fall time (2 s, 20 s, 110 s, and 200 s) on the dwell fatigue behavior is investigated for a high strength titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X with basket-weave microstructure. It is shown that the dwell fatigue life decreases with increasing the rise and fall time, which could be correlated by a linear relation in log–log scale for both the specimen with circular cross section and the specimen with square cross section. The rise and fall time has no influence on the crack initiation mechanism by the scanning electron microscope observation. The cracks initiate from the specimen surface and all the fracture surfaces present multiple crack initiation sites. Moreover, the facet characteristic is observed at some crack initiation sites for both the conventional fatigue and dwell fatigue tests. The paper also indicates that the dwell period of the peak stress reduces the fatigue life and the dwell fatigue life seems to be longer for the specimen with circular cross section than that of the specimen with square cross section.

Effect of stress ratio and mean stress on high cycle fatigue behavior of the superalloy IN718 at elevated temperatures

The present work shows the effect of mean stress on high cycle fatigue behavior of the superalloy IN718 at 400, 500 and 600 °C, at stress ratios (R) −1, 0.5, and 0.7. Fatigue life is decreased with increase in tensile mean stress. At high mean stress, decrease in fatigue life is associated with multiple crack initiation sites at the specimen surface. Fatigue limit is highly affected by the tensile mean stress and stress ratio, R = 0.7, since the maximum stress approaches near yield stress and causes cyclic ratcheting.

Effects of different surface modifications on the fatigue life of selective laser melted 15–5 PH stainless steel

Effects of machining, electro-polishing and laser surface re-melting (LRM) on the fatigue life of Selective Laser Melted (SLM) 15–5 precipitation hardening (PH) stainless steel are reported. Electro-polished surface showed ~97.4% reduction of surface roughness (Ra) and hence improved the fatigue life drastically (~100%) as compared to as-built specimens. Synergic effect of both compressive residual stress and reduction in surface roughness caused drastic improvement (~138%) in fatigue life of machined SLM specimens when compared to its as-built counterpart. Laser Surface Re-melting is found to be an effective technique to reduce the surface roughness (~91.2%) as well as sub-surface defects of geometrically complex as-built SLM specimen and thus improved its fatigue life by ~119%. Fractography analysis showed surface roughness, sub-surface defects and micro notches as the primary crack initiating factors for SLM specimens. High surface roughness in as-built part causes multiple crack initiation sites for specimens failed under both low and high stress amplitude. However, for machined, electro-polished and LRM SLM specimens distinct and multiple crack initiation sites could be observed when specimen failed under high cycle and low cycle regime respectively. However, for all the cases, fatigue life is found to be less compared to its wrought counterpart. Present study could be used as a guideline to select proper surface modification methods for SLM 15–5 PH stainless steel for a desired fatigue life.

Modelling of Fatigue Crack Initiation in Hydrogen Charged Polycrystalline Nickel

Hydrogen Embrittlement (HE) leads to deterioration of the fracto-mechanical properties of metals. In spite of vast literature, it is still not clearly understood and demands significant research on this topic. For better understanding of the hydrogen effect on fatigue behaviour of metals, present work focuses on developing a computational framework for fatigue crack initiation studies in metals in the presence of hydrogen. The developed framework consists of a nonlocal crystal plasticity model coupled with hydrogen transport model to study the fatigue behaviour of hydrogen charged metals. The nonlocal crystal plasticity model accounts for the statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) in polycrytalline metal. Hydrogen transport model, on the other hand, accounts for diffusion and trapping behavior of hydrogen due to concentration gradient, pressure gradient, plastic strain-rate with dislocations as the only trapping sites along the slip systems. A polycrystalline representative volume element (RVE) with periodic boundary conditions is used in this study. Fatigue crack initiation criterion is proposed for the simulated RVE with controlled microstructure by considering a critical value of the fatigue indicator parameter (FIP). FIP is formulated based on the experimental observations of several crack initiation sites along the grain boundaries, their normal direction with respect to loading direction and the accumulated plastic strain in nickel polycrystalline samples. Developed simulation framework correctly accounts cyclic stress-strain behavior and multiple fatigue crack initiation sites observed experimentally in the presence of hydrogen.

Analysis of the crack location in notched steel bars with a multiple DC potential drop measurement

The direct current potential drop (DCPD) technique is a frequently used method to measure crack propagation in metallic materials. With a single potential probe attached to the specimen, the size of an initiated crack can be estimated. Therefore, this method is used in crack propagation experiments to determine the crack propagation rate. However, with a single potential probe it is difficult to detect small cracks and impossible to determine the location of the initiated crack.In the present study, three potential probes were attached to cyclic loaded notched steel bars. For the case of an initiated single crack, this experimental set-up delivers different potentials depending on the distance of the probe to the initiated crack. A geometric model to determine the position of the initiated crack from the increase of the individual probe signals is presented.The experimental verification of the model has shown that in case of a single crack, the position can be determined clearly, even in the early stages of crack propagation. Moreover, the sensitivity of the potential drop measurement for the detection of an initiated crack is enhanced by this method. In case of multiple crack initiation sites the scatter of the calculated angle can be used as a criterion for crack detection.

A study of the parameters influencing mechanical properties and the fatigue performance of refill friction stir spot welded AlMgSc alloy

Friction spot welds of 1.6-mm-thick AlMgSc alloy were investigated in this work. A design of experiment method was used to evaluate the effect of process parameters on the shear static strength. The optimized condition of parameters was employed in the assessment of the fatigue behavior. The typical hook feature was minimized by restricting the tool penetration into only the upper sheet. As a consequence, shear strength was sensitive to the extension of the welded region rather than the hook morphology. The fatigue performance was affected by the multiple crack initiation sites that resulted from a complex stress state during the axial loading. Striations were observed in practically the entire crack propagation region, suggesting that unstable fatigue crack growth did not take place in this specific weld configuration.

Effects of Coatings on the High-Cycle Fatigue Life of Threaded Steel Samples

In this work, high-cycle fatigue is studied for threaded cylindrical high-strength steel samples coated using three different industrial processes: black oxidation, normal-temperature galvanization and high-temperature galvanization. The fatigue performance in air is compared with that of uncoated samples. Microstructural characterization revealed the abundant presence of small cracks in the zinc coating partially penetrating into the steel. This is consistent with the observation of multiple crack initiation sites along the thread in the galvanized samples, which led to crescent type fracture surfaces governed by circumferential growth. In contrast, the black oxidized and uncoated samples exhibited a semicircular segment type fracture surface governed by single-sided growth with a significantly longer fatigue life. Numerical fatigue life prediction based on an extended Paris-law formulation has been conducted on two different fracture cases: 2D axisymmetric multisided crack growth and 3D single-sided crack growth. The results of this upper-bound and lower-bound approach are in good agreement with experimental data and can potentially be used to predict the lifetime of bolted components.

Effect of Relative Humidity on Crack Growth Mechanism of Hot Dip Galvanized Steel in Atmospheric Environment

A hot dip zinc galvanizing is currently used as an effective corrosion protection technique for steels. There are a lot of reports concerning to the corrosion of coated metals. However, due to its difficulties for evaluation, the corrosion fatigue strength of galvanized steel has not been investigated. For this reason, the effect of galvanizing on the fatigue strength has not been understood yet. In the previous work, we studied the fatigue of zinc galvanized S45C steel at room humidity. In order to investigate the atmospheric corrosion fatigue strength of galvanized steel, we conducted the fatigue test focusing on the fracture mechanism of hot-dip galvanized steel in the corrosive atmospheric environment. The substrate of the test material was S45C steel rod. Prior to the galvanizing process, S45C steel rod was pickled to remove the flux. The specimens were immersed in the galvanizing bath at 450℃ for 3 min. The corrosive environment was prepared in a glass sealed container controlled by the humidity control system. Load controlled fatigue tests were carried out at a load ratio of 0.01 and a frequency of 10 Hz (sine wave). SEM observation was performed to study the morphology of fracture surface and to determine the crack initiation sites. In the cross sectional observation of galvanized layer, the Zn-Fe alloy layer was constructed in delta-one (δ 1 ) phase and zeta (ζ) phase, and the top of surface was eta (η) phase which was comprised of the pure zinc layer. When the maximum stress was 90% of yield stress, the fatigue strength at RH=80% was mostly equal to that at room humidity (approximately RH=30%). However, the great difference in the fatigue strength was observed between the specimen at room humidity and that at RH=80% when the maximum stress was less than 80% of yield stress. The number of cycles to failure at RH=80% were increased approximately two times compared with that at room humidity. Stage II was defined as the main crack stable propagation region and the shape of fracture surface at stage II at RH=80% was different from that at room humidity. Multiple crack initiation sites at room humidity were observed on the η phase or a Zn-Fe alloy layer/η phase interface. Those crack initiation sites can be distinguished individually. They were existed some fracture surfaces of stage II, and these shapes were all crescent. On the other hand, there were multiple crack initiation sites at RH=80%, and every crack initiation site was located on the top of the surface. The location of crack initiation sites existed close to each other. Although some crack initiation sites were observed, those were existed in a fracture surface of stage II. The shape was elliptical and was the same with that of S45C steel. This suggests that those cracks coalesced with each other in the process of multiple cracks growing, and finally a fracture surface of stage II was formed. The oxygen contents of oxide formed on the fracture surface when the maximum stress was less than 80% of yield stress showed a larger value at RH=80% than that at room humidity. We consider that the galvanized layer in high-humidity corrosive atmosphere had been hardened by the oxide layer and resulted in the increase of fatigue strength.

Experimental and calculation challenges of crack propagation in notches – From initiation to end of lifetime

Notches reduce significantly the lifetime of cyclically loaded components due to their stress concentration and early crack initiation. In this work the initiation and crack growth behaviour of cracks in notches has been studied with the direct current potential drop technique. Due to the notch geometry and semi-elliptical shape of the starting crack, significant differences between the crack length determination by the Johnson equation and the real crack length have been observed. By means of numerical simulations for single and multiple crack initiation sites, a calibration function for determining the crack length from the potential drop has been developed. The calibration function results in a good agreement between the estimated and measured changes of the potential drop and permits a quite accurate estimation of the crack initiation life time.

Chemical milling effect on the low cycle fatigue properties of cast Ti–6Al–2Sn–4Zr–2Mo alloy

The current research work presents the chemical milling effect on the low cycle fatigue properties of cast Ti–6Al–2Sn–4Zr–2Mo alloy. Chemical milling treatment is one of the final steps in manufacturing titanium alloy components that removes the brittle alpha-case layer formed during various thermal processes. The treatment includes immersion of the components in solutions containing hydrofluoric (HF) and nitric (HNO3) acids in relevant molar ratios. Although this treatment demonstrates advantages in handling components with complex net geometries, it may have detrimental effects on the surface, by introducing pitting and/or intergranular corrosion and thereby adversely affecting in particular the fatigue strength. The first series of specimens were tested in as-machined condition. Two more series were, prior to fatigue testing, subjected to 5 and 60min chemical milling treatment. It was found that the fatigue lives were substantially decreased for the chemically treated specimens. The fractographic investigation of all mechanically tested samples revealed multiple fatigue crack initiation sites in the chemically milled samples. These cracks were located either at the prior beta grain boundary or the prior beta grain boundary triple joints. The prior beta grain boundaries were found to have deep ditch-like appearance which depth increased with increasing milling time. These ditch-like grain boundaries acts as stress raisers and thereby promote early fatigue crack initiation and thus lower fatigue life.