Effect of substrate surface roughening and cold spray coating on the fatigue life of AA2024 specimens

The effects of cold spray coating and substrate surface preparation on crack initiation under cyclic loading have been studied on Al2024 alloy specimens. Commercially pure (CP) aluminum feedstock powder has been deposited on Al2024-T351 samples using a cold-spray coating technique known as high velocity particle consolidation. Substrate specimens were prepared by surface grit blasting or shot peening prior to coating. The fatigue behavior of both coated and uncoated specimens was then tested under rotating bend conditions at two stress levels, 180 MPa and 210 MPa. Scanning electron microscopy was used to analyze failure surfaces and identify failure mechanisms. The results indicate that the fatigue strength was significantly improved on average, up to 50% at 180 MPa and up to 38% at 210 MPa, by the deposition of the cold-sprayed CP-Al coatings. Coated specimens first prepared by glass bead grit blasting experienced the largest average increase in fatigue life over bare specimens. The results display a strong dependency of the fatigue strength on the surface preparation and cold spray parameters.     

Nitriding duration reduction without sacrificing mechanical characteristics and fatigue behavior: The beneficial effect of surface nano-crystallization by prior severe shot peening

Generally a clear beneficial effect of nitriding duration on resultant mechanical characteristics is reported in the literature. Considering the high energy cost in the competitive business environment, this work explores any opportunities to reduce nitriding duration while not sacrificing the resultant mechanical characteristics and fatigue behavior. To this end prior shot peening is applied with particularly severe parameters to generate ultra-fine grains and nano-structures in the surface layers. It was recently shown that the local fatigue strength improvement by combination of severe shot peening and 15 h nitriding could not eventually contribute in further increasing the fatigue limit of high strength low alloy steel smooth specimens as compared to only 15 h nitriding. In the present research combination of severe shot peening with nitriding at 7.5 h is assessed. It is affirmed that improvement by hybrid treatment can be actively exploited in the form of duration reduction. The characterization is carried out by optical and scanning electron microscopy observation, micro-hardness test, surface roughness measurement and X-ray diffraction measurement of residual stress. Fatigue limit of the treated specimens is experimentally determined. A critical comparison between the hybrid process with 50% nitriding duration reduction and the original nitriding process is presented. Based on the result of this study, nitriding duration can be successfully reduced without losing improvements in mechanical characteristics and fatigue behavior if a suitable prior severe shot peening, aimed to surface nano-crystallization, is performed.     

High- and very high-cycle plain fatigue resistance of shot peened high-strength aluminum alloys: The role of surface morphology

The present paper is aimed at investigating the effect of shot peening on the high and very-high cycle plain fatigue resistance of the Al-7075-T651 alloy. Pulsating bending fatigue tests (R = 0.05) were carried out on smooth samples exploring fatigue lives comprised between 10⁵ and 10⁸ cycles. Three peening treatments were considered to explore different initial residual stress profiles and surface microstructural conditions. An extensive analysis of the residual stress field was carried out by measuring with the X-ray diffraction (XRD) technique the residual stress profile before and at the end of the fatigue tests. Fatigue crack initiation sites were investigated through scanning electron microscopy (SEM) fractography. The surface morphology modifications induced by shot peening were evaluated using an optical profilometer. The influence of surface finishing on the fatigue resistance was quantified by eliminating the surface roughness in some peened specimens through a tribofinishing treatment. The capability of shot peening to hinder the initiation and to retard the subsequent propagation of surface cracks is discussed on the basis of a model combining a multiaxial fatigue criterion and a fracture mechanics approach.     

Effect of stress peening on surface layer characteristics of (TiB + TiC)/Ti–6Al–4V composite

Residual stresses and microstructure on surface layer of (TiB + TiC)/Ti–6Al–4V are investigated after stress peening. The values of domain sizes and microstrain of surface deformation layers are calculated from the integral breadth of diffractive peaks via Voigt method. The results show that the compressive residual stresses and microhardness are improved significantly after stress peening, and the variations of residual stresses are affected by both the prestresses and the directions of measurement. Microstructure investigations reveal that, the deformation amount increase after stress peening, and smaller domain grain sizes and higher density dislocations are introduced. The changes of microstructures are mainly influenced by the values of prestresses. According to these investigations, it is can be found that the stress peening is superior to the conventional shot peening treatments and it is an effective method to improve the surface properties of (TiB + TiC)/Ti–6Al–4V composite.     

Effect of surface nanostructure on tensile behavior of superalloy IN718

Peak aged nickel based superalloy Inconel 718 was subjected to ultrasonic shot peening for different durations of 45, 60 and 90 min. The transverse and longitudinal sections of the ultrasonic shot peened (USSP) specimens were examined by scanning electron microscope (SEM) and transmission electron microscope (TEM) and the shot peened microstructures were characterized. A nanostructure of nearly 12 nm grain size was observed to develop in surface region of the USSP specimen. X-ray diffraction (XRD) of the USSP samples did not reveal any phase change. Microhardness of the shot peened region was increased by 20% in the as shot peened condition and it was reduced to 8% in the stress relieved condition. Yield and tensile strengths were found to increase only marginally by 3.5% and 2.15%, respectively and the ductility was reduced by 3.4% following ultrasonic shot peening. The results are discussed in terms of grain refinement in the shot peened region.     

Field studies of steam oxidation behavior of austenitic heat-resistant steel 10Cr18Ni9Cu3NbN

The steam oxidation of austenitic steel 10Cr18Ni9Cu3NbN at about 605 °C after in service for 12,000 h and 34,696 h in a 660 MW USC power plant was investigated. Results show that a double layer structure consisting of a magnetite outer layer and Cr-rich inner layer was observed. After 12,000 h, shot blasting treatment improved the steam oxidation resistance of 10Cr18Ni9Cu3NbN steel compared with previous report. Scale oxide which was beneficial from shot blasting treatment spalled off within a short time before 34,696 h. After initial scale exfoliation, the newly formed scale oxide was more likely to be the oxide in 9–12%Cr steels, which was attributed to the low chromium under the initial oxide.     

Laser shock peening effect on the dislocation transitions and grain refinement of Al–Mg–Si alloy

This paper systematically investigates the effect of laser shock peening without coating parameters on the microstructural evolution, and dislocation configurations induced by ultra-high plastic strains and strain rates. Based on an analysis of optical microscopy, polarized light microscopy, transmission electron microscopy observations and residual stress analysis, the significant influence of laser shock peening parameters due to the effect of plasma generation and shock wave propagation has been confirmed. Although the optical microscopy results revealed no significant microstructural changes after laser shock peening, i.e. no heat effect zone and differences in the distribution of second-phase particles, expressive influence of laser treatment parameters on the laser shock induced craters was confirmed. Moreover, polarized light microscopy results have confirmed the existence of well-defined longish grains up to 455 μm in length in the centre of the plate due to the rolling effect, and randomly oriented smaller grains (20 μm × 50 μm) in the surface due to the static recrystallization effect. Laser shock peening is reflected in an exceptional increase in dislocation density with various configurations, i.e. dislocation lines, dislocation cells, dislocation tangles, and the formation of dense dislocation walls. More importantly, the microstructure is considerably refined due to the effect of strain deformations induced by laser shock peening process. The results have confirmed that dense dislocation structures during ultra-high plastic deformation with the addition of shear bands producing ultra-fine (60–200 nm) and nano-grains (20–50 nm). Furthermore, dislocation density was increased by a factor of 2.5 compared to the untreated material (29 × 10¹³ m^− 2 vs. 12 × 10¹³ m^− 2).     

Effect of shot peening on fretting fatigue of Ti811 alloy at elevated temperature

The effects of compressive residual stress, surface roughness, microstructure hardening induced by shot peening (SP) on the fretting fatigue (FF) resistance of Ti811 titanium alloy at elevated temperature were evaluated. The results show that SP improves the FF resistance of Ti811 alloy at 350 °C, but decreases the resistance at 500 °C. Compressive residual stress is the predominant factor in improving the FF resistance of the alloy at 350 °C. Compressive residual stress induced by SP arrests crack growth. Microstructure hardening due to SP has a minor effect on FF resistance. Surface roughening induced by SP is detrimental to the FF resistance of the alloy at both 350 and 500 °C, as microcracks initiate easily in the roughened surface under these conditions.     

Effect of nitriding and shot-peening on the fatigue behavior of 42CrMo4 steel: Experimental analysis and predictive approach

In this work, we are interested in the bending fatigue resistance of the nitrided 42CrMo4 steel improvement by shot-peening. The micro-structure, the micro-hardness, the residual stresses distribution and the crack resistance of the hardened steel are determined. The gains, expressed in term of endurance limit, brought by these treatments are established by three-points bending fatigue tests and discussed in relation to the residual stresses evolution under the cyclic loading conditions. The fatigue fracture resistance after a combined process of surface hardening including shot peening followed by nitriding is analyzed by methods of fracture mechanisms. This reveals that the gain provided by the nitriding is about 8% against 35% for the nitriding with shot-peening. This is primarily allotted to a high level of compressive residual stresses for nitrided + shot-peened state compared to the nitrided state. The fast relaxation of these stresses in the low cycle fatigue domain is at the origin of the fatigue fracture at surface, which leads to a lower fatigue fracture resistance compared to the untreated state. Based on multiaxial HCF criterion of Sines and taking into account the different surface properties, a local predictive approach was developed.     

Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening

SAE 9260 spring steel specimens after enduring 50% of their mean fatigue life were subjected to laser shock peening using an in-house developed 2.5 J/7 ns pulsed Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser for studying their fatigue life enhancement. In the investigated range of process parameters, laser shock peening resulted in the extension of fatigue life of these partly fatigue damaged specimens by more than 15 times. Contributing factors for the enhanced fatigue life of laser peened specimens are: about 400 μm thick compressed surface layer with magnitude of surface stress in the range of −600 to −700 MPa, about 20% increase in surface hardness and unaltered surface finish. For laser peening of ground steel surface, an adhesive-backed black polyvinyl chloride (PVC) tape has been found to be a superior sacrificial coating than conventionally used black paint. The effect of repeated laser peening treatment was studied to repair locally surface melted regions and the treatment has been found to be effective in re-establishing desired compressive stress pattern on the erstwhile tensile-stressed surface.     

Mechanical behaviour of dissimilar friction stir welded tailor welded blanks in Al–Mg alloys for Marine applications

Tailor welded blanks (TWB) in Al alloys are an attractive structural solution for application in the shipbuilding sector, mainly due to reductions in weight and lower production costs. In the present study, the global and local mechanical properties of dissimilar friction stir welded TWB were assessed. The joints were manufactured with dissimilar Al–Mg alloys and thicknesses (6 and 8 mm) of particular interest to the shipbuilding sector (AA5083 and AA5059). A digital image correlation system (DIC) linked to a tensile test system was used to characterise the local strain fields, and true stress–strain curves were generated for several TWB sub-zones. Microhardness and DIC analyses showed that the stir zone of the TWB presented overmatching in relation to the weakest base material, and that the joints displayed excellent overall mechanical performance that was comparable to the AA5059 base material in terms of strength and ductility. The fatigue strength was evaluated by means of tension–tension fatigue tests, and the TWB joints reached the fatigue keen with a stress range of 70 MPa.     

The investigation of crack growth in specimens with rectangular cross-sections under out-of-phase bending and torsional loading

The paper presents the fatigue test results of rectangular cross-section specimens made of 10HNAP (S355J2G1W) steel. The specimen height to width ratio was 1.5. The tests under bending with torsion were performed for the following ratios of bending to torsional moments M_aB/M_aT = 0.47, 0.94, 1.87 and the loading frequency 26.5 Hz. Nominal stresses were chosen for the equivalent stress according to the Huber-Mises hypothesis equal to 360 MPa. The tests were performed in the high cycle fatigue regime for the stress ratio R = −1 and phase shift between bending and torsion loading equal to ϕ = 0 and 90°. Crack initiation and propagation phases were observed on the specimen surface using the optical microscope (magnification 20×) with an integrated digital camera. The test results for the fatigue crack growth rate versus the stress intensity factor range for mode I and mode III have been described with the Paris equation.     

The effect of twinning and detwinning on the mechanical property of AZ31 extruded magnesium alloy during strain-path changes

In order to investigate the effect of twinning–detwinning on the mechanical properties of AZ31 extruded magnesium alloy pre-compression and pre-stretch deformation were conducted along extrusion direction (ED) at 1%, 3%, 5% strain levels. After pre-strain, the strain-path was inverted by performing tensile or compressive tests at room temperature. Results showed that the detwinning behavior occurred during the inverse tension after the pre-compression. Although due to the aforementioned effect the tensile yield strength decreased, by increasing the pre-compressive levels both fracture elongation and peak strength improved. In the inverse compressive tests after pre-stretch the {1 0 −1 2} twinning was restrained and the volume fraction of twins decreased, leading to the improvement of yield strength by increasing in pre-stretching levels.     

Mechanical,fatigue and microstructural properties of friction stir welded 5083-H111 and 6082-T651 aluminum alloys

In this study, 5083-H111 and 6082-T651 aluminum alloy plates in 6 mm thickness that are used particularly for shipbuilding industry were welded using Friction Stir Welding (FSW) method as similar and dissimilar joints with one side pass at PA position with the parameters of 1250 rpm tool rotation, 64 mm/min welding speed and 2° tool tilt angle. Tensile tests results showed sufficient joint efficiencies and surprisingly high yield stress values. Bending fatigue test results of all joint types showed fatigue strength close to each other. Fatigue strength order of the joints were respectively FSWed 5083-5083, and 6082-6082 similar joints and 5083-6082 dissimilar joint. Cross sections of the weld zones have been analyzed with light optical microscopy (LOM) and fracture surface of fatigue test specimens were examined by scanning electron microscopy (SEM). Although there were no voids in radiographic and microscopic analyzes, 5083-6082 joint showed rarely encountered voiding effect under fatigue load. Microhardness measurements revealed rare result for FSWed AW5083 and novel result for FSWed 6082 aluminum alloy.     

Experimental study and numerical simulation of the damage mode of a square reinforced concrete slab under close-in explosion

Terrorist attacks using improvised explosive devices on reinforced concrete buildings generate a rapid release of energy in the form of shock waves. Therefore, analyzing the damage mode and damage mechanism of structures for different blast loadings is important. The current study investigates the behavior of one-way square reinforced concrete (RC) slabs subjected to a blast load through experiments and numerical simulations. The experiments are conducted using four 1000 mm × 1000 mm × 40 mm slabs under close-in blast loading. The blast loads are generated by the detonations of 0.2–0.55 kg trinitrotoluene explosive located at a 0.4 m standoff above the slabs. Different damage levels and modes are observed. Numerical simulation studies of the concrete damage under various blast loadings are also conducted. A three-dimensional solid model, including explosive, air, and RC slab with separated concrete and reinforcing bars, is created to simulate the experiments. The sophisticated concrete and reinforcing bar material models, considering the strain rate effects and the appropriate coupling at the air–solid interface, are applied to simulate the dynamic response of RC slab. The erosion technique is adopted to simulate the damage process. Comparison of the numerical results with experimental data shows a favorable agreement. Based on the experimental and numerical results, the damage criteria are established for different levels of damage. With the increase of the explosive charge, the failure mode of RC slab is shown to gradually change from overall flexure to localized punching failure.     

Microstructure and mechanical property of Ti and Ti6Al4V prepared by an in-situ shot peening assisted cold spraying

In the current study, for the first time, an in-situ shot peening (SP) is introduced into cold spray by mixing large sized stainless steel particles with spraying powders to prepare dense Ti6Al4V (TC4) and commercially pure Ti (CP Ti) deposits. It is attempted that via the in-situ hammering by these large sized SP particles, plastic deformation of the previously deposited layers could be greatly enhanced and thereby porosities can be declined. Results show that, as the SP particle proportion increases from 0 to 70 vol.%, porosities of the CP Ti and TC4 coatings decrease from 13.7% and 15.3% to 0.3% and 0.7%, respectively. SEM observations reveal that no SP particle is incorporated into TC4 coatings. A few SP particles (≤ 2.3 vol.%) are observed in CP Ti coatings due to the relatively low hardness of CP Ti. Only a slight decline trend in deposition efficiency of the CP Ti and TC4 powders is detected as increasing SP content. The in-situ SP results in remarkable work hardening. As the SP particle content increases from 0 to 70 vol.%, Vickers microhardness of the CP Ti and TC4 coatings increase from ~ 143 and 240 HV_0.3 to ~ 203 and 427 HV_0.3, respectively.     

"南侠9"沉船水下解体爆破控制技术

为了确保航行安全,对"南侠9"沉船进行爆破解体打捞.采用条形聚能药包接触爆破切割和集中药包撕裂相结合的方式,详细介绍了药包的布置、起爆药包的制作、爆破网路、安全距离的计算和环境的保护.沉船一次起爆后,船体按设计相互分离分解成小块并无过分飞散,船体残骸被全部打捞,爆破工程获得圆满成功.     

Improving low-cycle fatigue properties of rolled AZ31 magnesium alloy by pre-compression deformation

The effect of pre-compression deformation on the low-cycle fatigue properties and cyclic deformation behavior of as-rolled AZ31 alloy was investigated by performing the stress-controlled low-cycle fatigue tests at room temperature. Fatigue properties and cyclic damage process should be closely related to the twins. The present work aimed to investigate the deformation mechanism and fatigue life caused by the introduced {1 0−1 2} twinning–detwinning from the viewpoint of stress amplitude. The results reveal that the twins contribute to the fatigue properties and cyclic damage process of AZ31 alloy. There were noticeable changes in hysteresis loops, microstructures and fatigue lives when the stress amplitude increased from 120 to 150 MPa. The fatigue life of pre-compressed samples was more superior to that of the as-rolled sample under different stress amplitudes, especially under the stress amplitude close to the tensile yield strength of the as-rolled sample.     

Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels

Austenitic stainless steels have been used for over 100 years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1 weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C + N: 0.65–1.2) and ratios (C/N: 0.13–∞) of C and N. Manson–Coffin analyses revealed distinct differences in the fatigue behaviour to CrNi-alloyed C + N steels investigated earlier. This contribution presents these differences and discusses them in relation to microstructural characteristics as well their alterations under cyclic loading.     

The substructures of martensite in a TiNiHf10 high temperature shape memory alloy

The martensite of a TiNiHf₁₀ alloy has been determined to possess a monoclinic crystal lattice structure with lattice parameters of a = 0.289 nm, b = 0.408 nm, c = 0.468 nm and β = 98.9°. The (001) compound twins are found to be the substructures of the TiNiHf₁₀ alloy and the [011] type II twin is found to be one kind of accommodation twin of the (001) twins. The (001) compound twin plane is parallel to the K₁ plane of the [011] type II twins when they co-exist. Four spear-like martensite variants form a self-accommodation group by (011) type I twinning relationship.