Accelerated Crack Initiation Research Articles

Very-high-cycle fatigue behavior of AlSi10Mg manufactured by selective laser melting: Effect of build orientation and mean stress

This paper studies the influence of build orientation and mean stress on the very-high-cycle fatigue (VHCF) behavior of AlSi10Mg fabricated by selective laser melting (SLM). Horizontally built specimens demonstrate better mechanical performance than vertically built specimens. Four crack initiation modes are identified for the specimens in high-cycle fatigue (HCF) and VHCF regimes, namely initiation by lack-of-fusion, initiation by internal pore, initiation with no obvious defect and initiation by tearing. Moreover, it turns out that the mean stress accelerates crack initiation, and hence reduces fatigue life. A model is proposed to describe the effect of crack initiation location on fatigue strength.

A numerical study of the influence of interface morphology on the stress behavior in thermal barrier coatings near an inclined film-cooling hole

Early failure induced by the cooling-hole structure and the interface morphology has significant influence on the integrity of thermal barrier coatings (TBCs). Thus, understanding the influence of interface morphology in TBCs on the stress behavior near an inclined film-cooling hole is highly desirable for evaluating the reliability of TBCs. In this study, a two-dimensional (2D) submodel of TBCs around a cooling hole was established to numerically investigate the influence of interface morphology in TBCs on the stress behavior near an inclined film-cooling hole. The results showed that the rough interface aggravated the stress state at both the top coat/thermally grown oxide (TC/TGO) and bond coat/thermally grown oxide (BC/TGO) interfaces. The maximum tensile stress appeared at the cooling-hole edge in the TC, whereas the maximum tensile stress occurred at the peaks of the BC. The different starting locations of the interface morphology from the cooling-hole edge had a significant effect on the stress behavior near the cooling-hole edge in TC, but they had no influence on the stress behavior in the BC. With increasing amplitude, the maximum tensile stress in the TC initially increased and then decreased. The interface asperity may have accelerated crack initiation at the peaks of the BC, but it had no influence on the stress level of the zone near the cooling-hole edge.

Strain rate effect on the acoustic emission characteristics of concrete under uniaxial tension

Abstract Concrete is an important engineering material whose tensile property plays an important role in structural safety. Thus, the effect of strain rate on crack evolution in concrete during tension cracking cannot be neglected. Within a strain rate range of 10−6 to 10−4 s−1, an acoustic emission monitoring test for the whole process of concrete under uniaxial tension including the post-peak softening stage was conducted. Moreover, damage evolution, along with the cracking mechanism of concrete at various strain rates was discussed with respect to the effect of strain rate on acoustic emission. The results show that the acoustic emission activity of concrete is delayed due to an increase in strain rate. This indicates that the hysteresis of deformation and cracking can be observed in a uniaxial tension test of concrete. During the whole loading process, as the strain rate increased, the average level of the acoustic emission hit rate increased significantly, indicating that an increase in strain rate accelerates crack initiation and propagation in concrete. Average acoustic emission values, duration and energy also show an increasing trend, and the scatter distribution range between them and the acoustic emission amplitude changes significantly, a fact that can be used to identify the damage degree of concrete under different strain rates. A peak frequency and cd4 band wavelet energy spectrum coefficient tends to decrease, while the ca8 band wavelet energy spectrum coefficient increases. In other words, the proportion of low frequency acoustic emission signals increases, indicating that an increase in strain rate increases the proportion of macroscopic cracks in concrete. At various strain rates, the average level of acoustic emission characteristic parameters and the proportion of high frequency signals at the post-peak stage in concrete are higher than those at the pre-peak stage, indicating that the development of microcracks in concrete mainly concentrates during the post-peak softening stage.

Failure and root cause analysis of vehicle drive shaft

This paper investigates the failure mode and root cause of drive shaft failure in a vehicle through examination of the macroscopic and microscopic morphologies of the fracture surface, the chemical composition, metallographic analysis, and mechanical properties of the material, and theoretical finite element calculations of the drive shaft. The results show that fatigue was the dominant mechanism of drive shaft failure due to obvious benchmarks on the fracture surfaces. Fatigue cracks initiated from the root fillet region of the spline gear. The absence of material defects in this region indicates that the main cause of fatigue cracks is attributable to a high stress concentration owing to the small geometrical dimensions of the root fillet. Meanwhile, the poor quality of the root fillet due to relatively high manufacturing tolerances also produces a higher stress concentration, and thus accelerates crack initiation and propagation. Furthermore, all the fatigue fractures occur on the right splines owing to higher stress amplitudes at this location. Improper stiffness matching between the left and right smooth shafts, which results in a larger torque transmitted to the right spline, is the root cause of the drive shaft failure. These results indicate that re-evaluating drive shaft design to ensure an equal stiffness between the two smooth shaft sections is recommended for a longer service life.

Contact Fatigue Failure Analysis of Helical Gears with Non-Entire Tooth Meshing Tests

This study presents an experimental approach based on partial tooth meshing contact fatigue with constant amplitude and different lengths of load cycles. To achieve this, 20CrH helical gears were considered, limiting the region of interest to one-third of the tooth to accelerate crack initiation and propagation. Different factors, such as number of fatigue cycles, tooth flank morphology, residual stress, hardness, and microstructure evolution under fatigue, were considered. The ultimate goal was to relate these variations with the observed failure modes. The results showed that with increasing cycle number, the residual stress at the same position in the meshing zone of the tooth flank increased first and then decreased. The residual stress values at the lower position of the pitch circle were larger than that of the addendum and dedendum. After cyclic loading, the content of the twin martensite within the microstructure below the pitch circle decreased and the corresponding hardness value of the tooth flank was reduced. After long-term cyclic loading, the precipitated phases aggregated and grew and the discontinuous distribution hindered the grain boundary slip. The dislocation density increased, and the pile-up of dislocation resulted in stress concentration at the grain boundary and the phase boundary, which induced the initiation of fatigue crack of the tooth flank.

Experimental and numerical prediction of effect of frequency on bending fatigue performance of polyamide 66/hectorite nanocomposite

ABSTRACTAn increased use of thermoplastics in components and structures that are subjected to cyclic loads necessitates a specific attention to variables that affect the hysteretic heating. Hysteretic heating effect in polyamide 66/hectorite nanocomposite has been investigated under bending strain control mode using a custom-built bending fatigue test setup in a laboratory environment. Dynamic mechanical analysis (DMA) results revealed a considerable rise in loss modulus with a decrease in frequency from 1 to 0.1 Hz irrespective of the temperature of the specimen. Alternatively, a reduction in fatigue test frequency from 2 to 0.5 Hz resulted in a significant decrease in cyclic softening. Fatigue behaviour predicted from DMA results using coupled structural/thermal finite element analysis is fairly in agreement with the experimental one. An accelerated crack initiation at decreased specimen temperature and high cyclic steady state stress reduced the fatigue life at 0.5 Hz compared with 2 Hz.

Stress corrosion cracking initiation and short crack growth behaviour in Alloy 182 weld metal under simulated boiling water reactor hydrogen water chemistry conditions

The effect of dissolved hydrogen on the stress corrosion cracking initiation and short crack growth behaviour of Alloy 182 weld metal was evaluated in 274°C hydrogenated high-purity water using accelerated crack initiation and growth tests with sharply notched fracture mechanics specimens and in-situ crack initiation and growth monitoring. A maximum in initiation susceptibility and crack growth rates was observed at the Ni/NiO phase transition line. Grain boundary misorientations and mismatch in Schmid factor were measured by electron backscattered diffraction along intergranular stress corrosion cracks and at crack initiation sites. Low-angle boundaries seem to be particularly resistant to cracking.

On the Microstructure, Hardness and Impact Toughness of 356 and 413 Alloys

The present study was performed on 356 and 413 alloys containing various amounts of hydrogen (H2), titanium diboride (TiB2) and strontium (Sr). The results show that the eutectic silicon particles in non-modified alloys undergo coarsening during the solutionizing treatment. A new mechanism, based on the Ostwald theory, has been proposed to explain the observed phenomenon. Although the variation in melt treatment parameters has no significant influence on the alloy hardness, it does have a great effect on the alloy impact toughness. Best results are obtained when the alloy is degassed, Sr-modified and grain refined. Also, Sr-modified, degassed 413 alloys have been found to possess good impact toughness. Precipitation of Mg2Si phase particles in 356 alloy is more effective as a hardening agent than the Al2Cu phase precipitated in the 413 alloy. The increased Si content in non-modified 413 alloys accelerates crack initiation and propagation, leading to low toughness values. Modifying the 413 alloys with 200 ppm Sr results in a more even distribution of the spheroidized eutectic Si particles and hence improves the alloy toughness.

Effect of metallurgical parameters on the microstructure, hardness impact properties, and fractography of Al-(6.5–11.5) wt% Si based alloys

The present study was performed on 356 and 413 alloys containing various amounts of hydrogen (H2), titanium diboride (TiB2), and strontium (Sr). The results show that the eutectic silicon particles in non-modified alloys undergo coarsening during the solutionizing treatment. High energy levels are obtained when the alloy is degassed, Sr-modified, and grain refined. Precipitation of Mg2Si phase particles in 356 alloys is more effective as a hardening agent than the Al2Cu phase precipitated in the 413 alloy which contributes to the alloy strength/hardness. The increased Si content in non-modified 413 alloys accelerates crack initiation and propagation, leading to low toughness values. Modifying the 413 alloys with 200ppm Sr results in a more even distribution of the spheroidized eutectic Si particles and hence better the alloy toughness. Exceeding the required amount of Ti-containing grain refiner may result in the introduction of a large amount of brittle Al3Ti platelets that would reduce the alloy toughness.

Fatigue Performance of Pre-corroded 6xxx Aluminum Alloy Laser Beam Welds with Dissimilar Heat Treatment

The role of existing corrosion damage on the fatigue performance of 6156 AA laser beam welds in T3 state, with additional post weld heat treatment T8, has been assessed. The welds were subjected to salt fog environment for 720hours and then metallographically examined to characterize corrosion behaviour. The corrosion results demonstrate that T3/T8 treatment provides high corrosion susceptibility in the weld area, where localized damage in the form of large pits prevails. Fatigue performance of corroded samples is drastically degraded by the presence of corrosion pits which act as stress raisers and accelerate crack initiation in the corroded samples. Fatigue cracks initiate at the root of corrosion pits on the large specimen surfaces in the weld metal area and propagate in through thickness direction. The obtained results have been compared to results from 6156 welds with T4 treatment in as welded conditions.

S–N fatigue properties of a stable high-aluminum austenitic stainless steel for hydrogen applications

The fatigue properties of a novel high aluminum austenitic stainless steel with a high resistance against hydrogen embrittlement were investigated. S–N tests in 40 MPa H2 at −50 °C resulted in a reduction in fatigue life by a factor of about 2 compared to air. Striation analysis revealed no acceleration of crack growth rate but accelerated crack initiation or accelerated short crack growth in H2. No apparent difference in fatigue fracture characteristics and striation morphology between the air and H2 tested specimens could be identified.

Thermal Fatigue Behavior and Cracking Characteristics of High Si-Mo Nodular Cast Iron for Exhaust Manifolds

The thermal fatigue cracking behavior of high Si-Mo nodular cast iron (NCI) is investigated by means of optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectroscope (EDS), in order to find a new material used in exhaust manifolds in First Automotive Works (FAW). Nodular cast irons with silicon content about 4.7%, in combination with up to 1.1% molybdenum, were produced by Jilin University and FAW. The repeated heating/cooling test was performed under cyclic heating at various maximum heating temperatures (Tmax) ranging from 800 to 900 °C. Experimental results indicate that the thermal fatigue cracking resistance of high Si-Mo NCI decreases with increasing the maximum heating temperature. The periods for crack initiation are 24—36, 40—50 and 70—90 times associated with heating temperature of 900, 850 and 800 °C, respectively, when the holding time is about 10 min at Tmsx. When thermal fatigue cracking occurs, the cracking always initiates at the bigger surface of specimen. The major positions of cracks propagation are generally at the eutectic oxide boundary region and the region of the graphite disappearance. At the same time, the oxidation may accelerate crack initiation and propagation. On the other hand, micro-crack number varied from large to little because of shielding effect. As exhaust manifolds, the reasonable working temperature of high Si-Mo NCI is no more than 840 °C by test and analysis.

The influence of salt fog exposure on the fatigue performance of Alclad 6xxx aluminum alloys laser beam welded joints

Laser welding is increasingly used for the fabrication of lightweight and cost-effective integral stiff- ened panels in modern civil aircraft. As these structures age in service, the issue of the effect of corrosion on their damage tolerance requires attention. In this work, labora- tory data on the influence of salt fog corrosion on the fatigue behavior of cladded 6156 T4 aluminum alloy laser welded specimens are presented. The experimental inves- tigation was performed on 6156 T4 laser butt welded sheets. Prior to fatigue testing the welded joints were exposed to laboratory salt fog corrosion exposure for 720 h. The results showed that the clad layer offers suffi- cient corrosion protection both on base metal and the weld. Fatigue testing was followed by standard metallographic analysis in order to identify fatigue crack initiation sites. Crack initiation is located in all welded samples near the weld reinforcement which induces a significant stress concentration. Localized corrosion attack of the clad layer, in the form of pitting corrosion, creates an additional stress concentration which accelerates crack initiation leading to shorter fatigue life relative to the uncorroded samples. The potency of small corrosion pits to act as stress concentra- tion sites has been assessed analytically. The above results indicate that despite the general corrosion protection offered by the clad layer, the localized attack described above leads to inferior fatigue performance, a fact that should be taken under consideration in the design and maintenance of these structures.

Fatigue of Electroformed Nickel

A 2–3 mm nickel mould, manufactured solely by electroforming, showed early cracking under thermal load cycles. Possible causes were attributed to material, design, or service deficiencies. The root cause of early fracture has been investigated thoroughly and is reported here. Monotonic and cyclic properties of electroformed nickel have been obtained at room and elevated temperature. Fatigue mechanisms were identified and compared to pure nickel. Finite element modeling of the mould under thermal cycles enabled the study of material, design, and in-service applications of the mould. It was found that over constraining the mould, while it was in service, caused excessive thermal stresses which accelerated crack initiation and propagation. Remedies to avoid failure are proposed.

Detecting the onset, propagation and location of non-artificial defects in a slow rotating thrust bearing with acoustic emission

Condition monitoring using high-frequency acoustic emission (AE) technology has remained a subject of intense study since the late 1960s. This paper demonstrates the use of AE measurements to detect, locate and monitor natural defect initiation and propagation in a conventional rolling element thrust bearing rotating at 72 r/min. To facilitate the investigation a special purpose test-rig was employed, which allowed for accelerated crack initiation on a bearing race. It is concluded that the onset of sub-surface cracks and the subsequent propagation and development to a surface spall was detectable with the AE technology. The paper also investigates the effects of attenuation and source characterisation on AE signals associated with the natural initiation and propagation of cracks on a thrust bearing race whilst in operation.

Gross slip fretting wear of CrCN, TiAlN, Ni, and CuNiIn coatings on Ti6Al4V interfaces

Fretting is a low amplitude oscillatory wear that occurs at component interfaces and can accelerate crack initiation as well as interfacial degradation. Prevalent in Ti-alloy contacts, fretting wear often occurs at the blade/disk interfaces of fan and compressor stages in turbine engines, causing premature component failure. In many cases, plasma sprayed CuNiIn (copper–nickel–indium) coatings and solid lubricants are applied to blade roots to mitigate the fretting problem. However, the CuNiIn coatings can cause severe damage to the uncoated Ti-alloy counter parts once the solid lubricants wear out. In this study, bench level gross slip fretting wear tests were conducted at room temperature on unlubricated Ti6Al4V (titanium, 6% aluminum, 4% vanadium) surfaces mated with CuNiIn and commercially pure Ni plasma sprayed surfaces. Upon analysis, it was determined that the uncoated Ti6Al4V surfaces initially gall. Then the resulting adherent wear particles break up causing a transition into a third body type wear mode over time. This leads to an accumulation of wear that is similar to that of uncoated Ti6Al4V mating surfaces. Additional tests were conducted after applying 2-μm thick PVD deposited TiAlN (titanium–aluminium–nitride) and CrCN (chrome–carbon–nitride) coatings to the surfaces of a second set of Ti6Al4V mating pairs. When worn against the unlubricated CuNiIn and Ni coatings, the thin TiAlN and CrCN coatings wore gradually without delaminating or cracking and were able to mitigate galling and extend test life. The application of the hard coatings lead to a reduction in wear in all of the tests except with the combination of TiAlN worn against CuNiIn.

Effect of high temperature on the characterization of fretting wear regimes at Ti6Al4V interfaces

Fretting wear is a destructive phenomenon that can accelerate crack initiation in vibrating components, leading to premature catastrophic failures. Among the most important characteristics of fretting is the transition from mixed fretting wear to gross slip because of its detrimental effects on fatigue life. In this paper, a study was conducted that characterized the critical normal loads and displacements corresponding to this transition in Ti6Al4V interfaces at room temperature and 450 °C (842 °F). Experimental testing shows that there is a relationship between critical load and displacement that is temperature dependant. Tests conducted at or near the critical values of load and displacement exhibited extreme galling and evidence of cold welding. In the gross slip tests, the measured coefficient of friction also varied with applied load, which correlates with the Hertzian contact model.

Effect of Nonmetallic Inclusions on the Mechanical Properties of Hydrogenated Steels

We have studied intermediate-alloy Ni-Cu-Mo-V steel of open-hearth and converter (more pure as to nonmetallic inclusions) melting. In the case of middle hydrogen concentrations (up to 5 cm3 per 100 g), the strength characteristics, plasticity, and fracture energy of transverse and vertical specimens of converter steel decrease less than those for open-hearth steel. For higher hydrogen concentrations, the characteristics of steel become very low and almost identical for both materials. Under loading, cracks are initiated on nonmetallic inclusions; in hydrogenated steel, they propagate along the shortest path from one inclusion to another, without involving significant volumes of the specimen in plastic deformation. We have established that nonmetallic inclusions accelerate crack initiation under conditions of corrosion in pipe steels in the course of operation of pipelines.

Corrosion Fatigue in 7075-T6 Aluminum: Life Prediction Issues for Carrier Based Operations

Fatigue experiments performed on 7075-T6 Al alloy at various frequencies and stress levels and under alternating wet and dry environments indicate that simultaneous action of corrosion and fatigue substantially accelerates crack initiation and growth rates when compared to pure fatigue (dry air) conditions. In experiments performed under alternating wet and dry conditions, fatigue crack growth rate was observed to increase rapidly in the presence of mildly corrosive (salt solution) and decrease sharply when subjected to noncorrosive dry air. Crack arrest of various durations was observed at transition points between dry and wet cycles. Lowering the frequency of fatigue loading significantly reduced crack initiation and overall life time. S-N curves showed a continuous downward trend without reaching a plateau or threshold. These observations led to the conclusion that aircraft structural integrity can be seriously compromised even under mildly aggressive environments and at subcritical stress levels when cracks are present regardless of changing of environments and/or test frequency.

Characterization of mixed and gross slip fretting wear regimes in Ti6Al4V interfaces at room temperature

Fretting wear is a destructive phenomenon that can accelerate crack initiation in vibrating components, leading to premature catastrophic failures. Among the most important characteristics of fretting, is the transition from mixed fretting wear to gross slip. Research has shown that mixed fretting wear is the most detrimental to fatigue life. In this paper, a study was conducted that characterized the critical normal loads and displacements corresponding to the mixed fretting regime boundary in Ti6Al4V interfaces. Experimental testing shows that there is a linear relationship between critical load and displacement. Knowing the corresponding loads and displacements that induce mixed fretting conditions, will allow for the predetermination of wear regime in future tests. Tests conducted at or near the critical values of load and displacement exhibited extreme galling and evidence of cold welding. These tests also initially displayed mixed fretting wear; however, a cycle dependent transition to gross slip occurred in some tests.