Poor Fatigue Performance Research Articles

Mechanical properties of Hastelloy X produced by laser powder bed fusion and affected by spatter redeposition

Defects in materials manufactured via laser powder bed fusion challenge this manufacturing process' dependability and may prevent it from being comprehensively used for structural components, particularly those intended to operate under dynamic loading conditions. This study aims to investigate the effect of spatter-driven lack of fusion on the mechanical properties of Hastelloy X, with a particular focus on fatigue performance. Mechanical test specimens were manufactured in two builds with different build rates and monitored through in-situ optical tomography. The images acquired in situ were analyzed to detect spatter redeposits to predict the defect content in the gauge section. Selected specimens were measured ex-situ using X-ray computed tomography to map the defect populations. Afterward, the specimens were tensile and fatigue tested, and their performance was analyzed based on the measured and expected defect populations. It was confirmed that a higher build rate is associated with more extensive detection of spatter redeposits in optical tomography images and lack of fusion defects. The fatigue lives of specimens manufactured at higher build rates presented higher scatter but significantly higher average, despite the more critical defect population. Surprisingly, the confirmed presence of lack of fusion defects in the gauge section of test specimens did not necessarily result in a poorer fatigue performance. It was concluded that the grain refinement obtained through the increase in nominal layer thickness has a life-prolonging effect that overrides the effects of spatter-induced lack of fusion defects.

Effect of water cooling assistance on fatigue behavior of laser welded QP980 steel

Quenching and partitioning 980 (QP980) steel has been applied to new energy vehicles, but even laser welding joints show poor fatigue performance. In this study, water-cooling assistance is applied to improve fatigue resistance of welded joints. Hardness, tensile and fatigue tests are performed together with microstructure observation (scanning electron microscope, transmission electron microscope and electron back scattered diffraction). It is found that tempering martensite showing low hardness and small average grain boundary angles are responsible for poor fatigue performance. Results show that water-cooling assistance significantly improves welding softening and fatigue crack growth behavior of welded joints.

STUDY ON SURFACE INTEGRITY AND ITS INFLUENCE ON HIGH TEMPERATURE FATIGUE BEHAVIOR WHEN TURNING POWDER METALLURGY NI-BASED SUPERALLOY

As a member of the latest generation of powder metallurgy Ni-based superalloy, FGH96 has already been widely applied in the manufacturing of aero-engines because of its distinguished mechanical performances. The surface integrity plays an essential role in the final fatigue life of the machined parts. However, surface modification induced by the machining process is inevitable, which profoundly affects the high temperature fatigue behavior of the manufactured part during the service lifetime. To cover the gap, the present work was carried out to specially study the surface integrity and its influence on high temperature fatigue behavior of powder metallurgy Ni-based superalloy when subjected to the mechanical turning process. The surface integrity, including surface morphology, micro-hardness, and depth-dependent residual stress, was initially characterized under different input parameters. High temperature fatigue tests were then conducted on the processed specimens to correlate the surface integrity and its fatigue behavior. Results showed that input parameters greatly influence the surface condition and thus the high temperature fatigue behavior. Poor integrity on machined surface leads to poor fatigue performance with typical fatigue fracture morphology, including crack initiation, propagation, and final fracture. While fine integrity on machined surface contributes to better fatigue performance. More interestingly, the fracture morphology in this situation shows similarity to the static tensile fracture, which presents shear failure along the direction of maximum shear stress.

Hysteretic behaviours of a novel lead-viscoelastic coupling beam damper: Numerical analysis and Bouc-Wen based model

The replaceable coupling beam damper (RCBD) is widely recognized as an effective device to improve the seismic performances of shear wall structures. However, the extensively studied metallic-type RCBD has poor fatigue performance, while the viscoelastic-type RCBD is sensitive to the loading frequency. To overcome the drawbacks of the RCBDs mentioned above, the novel lead-viscoelastic coupling beam damper (LVCBD), which is mainly composed of lead rods, composite viscoelastic layers and steel plates, was proposed by the authors recently. Cyclic loading tests revealed that the lead rods of the LVCBD can overcome the drawback of frequency dependency of the conventional viscoelastic damper, making this innovative damper frequency-independent, fatigue resistant and stable in energy dissipation. In order to clarify the influence of the lead rod on the LVCBD, this paper performs numerical studies to investigate the effects of the lead rod number and lead rod diameter on the hysteretic behaviours of the LVCBD. Theoretical formulae are derived to calculate the shear-bearing force and stiffness of the damper and validated by the test and numerical results. Finally, by using the genetic algorithm (GA), a Bouc-Wen based hysteretic model is developed to depict the hysteretic behaviours of the LVCBD. Results show that the properly designed LVCBD with four lead rods exhibits a favorable shear-bearing capacity with uniform stress distribution. Increasing the lead rod diameter can effectively enhance the shear-bearing capacity of the damper. The developed formulae and hysteretic model yield good predictions of the hysteretic behaviours of the LVCBD, which can be used in the design and analysis of the LVCBD.

A physics-based defect tolerant design approach to predict the tensile performance of aluminium alloys with a defect

Inherent manufacturing defects are observed in additively manufactured and casted metallic alloys. During loading, the stress concentration due to defect results in poor tensile ductility and fatigue performance. A physics-based defect tolerant design approach is used in the present work to calculate the uniaxial tensile performance of cast ADC12 Aluminum alloys. The proposed model elucidates the correlation between tensile ductility and the dimensions of material defects. It derives a material constant referred to as the critical defect size, which is determined through a combination of uniaxial tensile testing on specimens with defects and employing finite element simulations. The proposed approach calculates well the tensile properties such as complete tensile stress–strain curve, ultimate tensile strength, and uniaxial tensile elongation.

On the low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing

The low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing (NIA) was examined at different strain amplitudes. We showed that NIA improved the low cycle fatigue life (more than 7000 cycles) by optimising the precipitate configuration within 5.5 h while maintaining comparable mechanical properties (570 MPa for tensile strength) and conductivity (nearly 39% IACS) to conventional isothermal ageing, simultaneously. Experimental observation combined with molecular dynamic simulation revealed that precipitation configuration manipulated by NIA had a crucial effect on fatigue resistance. A great number of repeatedly sheared and locally destructed GP zones enhanced co-planar slip and slip localisation in the under-aged alloy during the early stage of NIA, responsible for the dramatic displacement steps on the surface and resultant poor fatigue performance. As the NIA further proceeded, moderately coarsened precipitates with an average dimension of 6.0 nm and elevated number density effectively impeded the dislocation movement and weaken the slip localisation to a great extent, improving the fatigue performance within a few hours.

Experimental study on the cyclic behaviors of an innovative lead-viscoelastic coupling beam damper (LVCBD)

Previous researches showed that the replaceable coupling beam damper (RCBD) can effectively enhance the seismic performance of shear wall structures. However, the extensively studied metallic-type RCBD has poor fatigue performance, while the viscoelastic-type RCBD is sensitive to the loading frequency. To overcome the drawbacks of the RCBD mentioned above, this paper proposes a novel RCBD, i.e., the lead-viscoelastic coupling beam damper (LVCBD), which is not only independent of the loading frequency, but also has better fatigue performance with good energy dissipation capacity. The proposed LVCBD mainly consists of three components, i.e., the lead rods, composite viscoelastic layers and steel plates. Taking advantages of the hyper-elasticity property of rubber and the dynamic recrystallization capability of lead, the combined effects from the rubber and lead make the LVCBD reusable after a severe earthquake, and facilitate the post-earthquake recovery with less or without replacement of the RCBD. To examine the performance of the proposed novel damper, three specimens were manufactured and cyclic loading tests were carried out. The influences of the loading amplitude, loading frequency and fatigue loading on the mechanical properties of the specimen were systematically investigated and discussed. Experimental results indicated that the LVCBD showed a favorable deformation capability, frequency-independent performance, fatigue resistance and stable energy dissipation capacity under cyclic loading.

Stiffness Behavior and Micromechanical Modeling of Asphalt Mastic Composed of Different Fillers

Fillers are known as dust fractions in asphalt mixes having a predominant particle size less than 75 micrometer (μm). Typical filler concentration in an asphalt mix varies from 4% to 12% (by aggregate weight). The effects of filler intrinsic properties on mastic behavior can get magnified with the concentration of filler. Thus, in filler-rich mixes like stone matrix asphalt (SMA), the filler characteristics and concentration can be sensitive factors, playing significant roles in the performance of asphalt mastic and mix. The study considered six different types of fillers to capture the effect of filler characteristics on mastic. The mastics were prepared with a constant filler concentration by volume (F/Bvol=0.49), which was determined by a typical SMA mix design. The fillers were characterized based on their surface area, size distribution, shape, Rigden voids (RV), clay content, and chemical composition. The filler effects on mastic were evaluated based on the stiffness (complex modulus) and stiffness ratio. The weak correlation observed between the mastic stiffness and the fixed and free binder reveals that the RV and filler fixing factor (FFF) can mislead regarding the stiffening potential of filler material. The surface areas by Brunauer–Emmett–Teller (BET) and Blaine’s methods were insufficient to represent the stiffening potential while the stiffness showed a better correlation with the concentration of finer gradations in the filler evinced by a higher coefficient of P10 (percentage passing 10 μ sieve size) and fineness modulus (FM) parameters. Further, the micromechanics based understanding of mastic behavior explains the mastic stiffening phenomenon as the combined effects of adsorbed asphalt layer thickness and interparticle interaction. The linear relation observed between stiffness ratio and newly derived micromechanism stiffening factor (MMSF) indicates that in filler-rich mastics, the stiffness may be improved with a finer sized filler (lower FM) and higher volume concentration. However, the adsorbed layer thickness didn’t show correlation with any of the filler properties considered. The study recommends a limiting value for mastic stiffness in terms of stiffness ratio which is established based on MMSF factor. A limiting value for stiffness of mastic could be significant to avoid dry mixes and subsequent poor fatigue performance.

Surface integrity of hybrid CM247LC/Inconel 718 components produced by laser-directed energy deposition

High-temperature alloys pose significant challenges in additive manufacturing. These materials have unique properties, such as high resistance to mechanical and chemical degradation when exposed to high temperatures. Furthermore, when these alloys are used to produce hybrid components with other similar alloys, investigating their surface integrity is critical because any residual stress can lead to early stage cracks and poor fatigue performance. In this research, a hybrid manufacturing approach is employed to produce components from difficult to weld alloys, i.e. CM247LC deposited on IN718 through a laser based direct energy deposition (L-DED) process. The surface integrity, mechanical properties and microstructure of such hybrid components is investigated, especially their welding/joint areas. Crack-free processing regimes were established to deposit CM247LC while mitigating the negative effects onto the microstructure of the Inconel substrate. Especially, the thermal gradients were managed to deliver crack free sections of CM247LC with good interface bonding, strength and fine microstructure. It is important to note that this is achieved without any significant preheating that contrasts with what is reported in other investigations so far. Furthermore, end-use hybrid blisks with deposited CM247LC blades onto Inconel 718 disks (HUB) were manufactured and then machined within a single processing set-up. The results show that the substrate thickness, the machining between the deposited layers and the final machining and heat-treatment play a role in reducing residual stresses. Ultimately, such hybrid manufacturing approach can be considered a new solution for producing such components and also for their subsequent repair.

Unnotched fatigue of Inconel 718 produced by laser beam-powder bed fusion at 25 and 600°C

Porosity—and hence poor fatigue performance—is a major concern for additively manufactured (AM) alloys using powders as the raw materials. Since the microstructures in these alloys are also often unique, a detailed understanding of the interactions between the fatigue cracks emanating from the pores and the microstructural features is necessary for the successful deployment of the AM alloys in industry. Keeping this in view, the microstructures and mechanical properties of the as-built Inconel 718 alloy, produced using laser beam powder bed fusion (LB-PBF) process, were investigated at room temperature (RT) and 600 °C. Emphasis was on the high cycle fatigue behavior evaluated using the rotating bend fatigue tests, and the role of the lack of fusion pores (LOFs) on the fatigue resistance. The experimental results show that the unnotched fatigue strength (σf) at 600 °C is 23% lower than that at RT. This was due to the lower work hardening rate at 600 °C, which facilitates easy crack initiation at LOFs that both favorably located and oriented. For stress amplitudes (σa) higher than σf, however, dynamic recrystallization at the crack tip regions of the specimens fatigue tested at 600 °C retards the short fatigue cracks (SFCs) and leads to a substantially higher fatigue life as compared to that at RT. Postmortem analyses were for understanding the initiation and growth mechanisms of SFCs and the roles of plasticity/oxidation induced crack closure on them. It shows that the LB-PBF induced microstructural characteristics such as solidification cells with high dislocation density are effective in resisting the growth of SFCs at 600 °C, as compared to RT. These results further highlight the roles of the unique microstructural aspects of additively manufactured alloys on the fatigue resistance, especially at high temperatures.

Fatigue life prediction of the additively manufactured specimen

Additively manufactured specimens generally exhibit comparable or improved tensile properties, such as yield stress, ultimate tensile strength, and uniform elongation, compared to conventionally manufactured specimens. However, the defects that are typically present in additively manufactured microstructures result in inferior fatigue performance. A representative volume element-based modeling technique incorporating these defects has been used to predict the reduction in endurance limit of an additively manufactured stainless steel compared to the conventionally manufactured material. This physics-based model can clearly demonstrate the poor fatigue performance of additively manufactured specimens based on the micro-plasticity generated by the defects in the microstructure under cyclic loading. A Neuber analytical model has also been applied to predict the fatigue life of additively manufactured materials for a given stress amplitude. Both the prediction from the finite element model and the analytical Neuber model are very close to the experimental endurance limit.

Effects of laser shock peening on the ultra-high cycle fatigue performance of additively manufactured Ti6Al4V alloy

Although additive manufacturing (AM) allows to fabricate metallic parts with a high static strength comparable to their forged counterparts, the fatigue strength of an AM-ed part is generally inferior, restricting AM from many critical applications. In this study, laser shock peening (LSP) was applied to modify the surface properties of selective laser melting fabricated (SLM-ed) Ti6Al4V titanium alloy. This study performs systematic tests and analyses on the fabricated alloy specimens to characterize their microstructures and mechanical properties including residual stress, tensile strength, ultra-high cycle fatigue (UHCF) strength. The results reveal that LSP can refine microstructure, suppress residual stresses, and delay crack propagation in the affected area. However, the inherent defects in an SLM-ed part, such as unmelted powders, lack of fusion and clusters of α phase, dominate the fatigue failure of the specimens especially in the UHCF regime, resulting in their poor fatigue performance. Meanwhile, The LSP processed specimens showed a lower S-N curve than that of specimens without LSP processing especially in the UHCF regime, which not only results from the inherent defects, but also the increased surface roughness and non-uniform residual stresses.

A machine-learning fatigue life prediction approach of additively manufactured metals

The defects retained during laser powder bed fusion determine the poor fatigue performance and pronounced lifetime scatter of the fabricated metallic components. In this work, a machine learning method was adopted to explore the influence of defect location, size, and morphology on the fatigue life of a selective laser melted Ti-6Al-4 V alloy. Both the high cycle fatigue post-mortem examination and synchrotron X-ray tomography were combined to acquire the geometric features of the critical defects, which were trained using a support vector machine (SVM). To accelerate the optimization process, the grid search approach with cross validation was selected for fitting the model parameters. It is found that the coefficient of determination between the predicted and experimental fatigue lives can reach up to 0.99, indicating that the SVM model shows strong training ability.

Effect of rolling process on fatigue performance of stir zone of AA6061‐T6 doube‐side friction stir welding joint

AbstractStir zone (SZ) of AA6061‐T6 double‐side friction stir welding (DS‐FSW) joint with a thickness of 6 mm had a poor fatigue performance compared with that of base metal (BM). The rolling process was employed to improve the fatigue performance of SZ. The results showed that rolling process increased the dislocation density in SZ and remarkably enhanced its fatigue performance. The crack initiation played a key role in the fatigue performance of SZ. It relied on the debonding of lamellar structure in SZ. The debonding of lamellar structure depended on the cracking of fine grain layer in lamellar structure.

Effect of nanoclay inclusion on mechanical and fatigue behavior of ABS/PP/HDPE thermoplastics

Thermoplastic Polymer materials are replacing conventional metals in the usage of commodities owing to their high specific properties. Due to their low mechanical properties and poor fatigue performance their usage is restricted in engineering applications. In the present study three variety of thermoplastic polymers Polypropylene (PP), High-density polyethylene (HDPE) and Acrylonitrile Butadiene Styrene (ABS) were considered and reinforced with nanoclay and their mechanical and fatigue behavior were tested. Results dictates that reinforcement of nanoclay improves their mechanical properties and fatigue performance considerably.

Experimental Method of Fatigue Performance of Mastic Asphalt for Bridge Deck Pavement

Mastic asphalt is a type of pavement material that has good fluidity and is self-levelling at construction temperature for the bridge deck. There are highly accurate methods and indexes for evaluating fluidity and high-temperature deformation resistance for mastic asphalt-design and construction-control systems. The fatigue cracking is one of the main failure forms of bridge deck pavement. Therefore, the method used to evaluate the fatigue properties of pavement materials is also essential. The anti-deformation capability of the mastic asphalt must be increased, that results in poor fatigue performance and consequent failure of the bridge deck pavement to avoid the rutting of bridge deck pavement. In this study, a simple method is put forward for evaluating mastic asphalt fatigue performance. Impact toughness is defined as the area under the load-displacement curve of a three-point bending beam specimen under impact load to evaluate the fatigue performance of mastic asphalt. The four-point bending beam fatigue test is used to verify the rationality of the impact toughness test method. The results showed that there is a good correlation between the impacts toughness index of mastic asphalt produced under different mixing conditions and the accumulative dissipative energy and fatigue life demonstrated by the four-point bending beam test. Therefore, to evaluate the fatigue performance of mastic asphalt by impact toughness test. Fatigue performance and rut resistance are two ways to evaluate road performance of asphalt mixtures, but they are mutually restrictive. The results show that impact toughness and dynamic stability are inversely correlated. As the impact toughness increases, dynamic stability decreases. Therefore, balancing the fatigue performance and high-temperature rutting resistance of mastic asphalt in the design and quality control is very important.

A Comparison Study of the Thermal Fatigue Properties of Three Ni‐Based Cast Superalloys Cycled from 20 to 1100 °C

The thermal fatigue properties of cycling from 20 to 1100 °C, a new kind of Ni‐based cast superalloy, named N3 are studied and the results are compared with those of other alloys, namely, K4002 and K403, corresponding to simulation analysis. The results show that the cracking models of the three alloys are basically the same; that is, when the temperature is alternately changed, cracks are easily formed at the interface of the matrix and carbide due to differences in the thermal expansion coefficients of the matrix and carbide. The N3 alloy contains a block‐shaped (γ + γ′) eutectic with a volume fraction of 15.5%, which effectively hinders crack propagation, thereby resulting in the best thermal fatigue performance among the investigated alloys. The K4002 alloy also has a sunflower‐like eutectic with a volume fraction of 12.5%, which also shows good structural stability and excellent thermal fatigue properties. However, the K403 alloy exhibits poor thermal fatigue performance for the transformation of the carbides during thermal fatigue; in this case, the carbides form chain‐like M23C6 carbides at the grain boundaries or interdendritic area because of unstable intrinsic character of the alloy. The results also demonstrate that a low eutectic content with a small volume fraction (2.2%) of the K403 alloy does not hinder crack propagation.

Laboratory evaluation on performance of porous polyurethane mixtures and OGFC

The application of open grade friction course (OGFC) is limited by problems such as mixture disintegration and relatively poor fatigue performance. In this study, polyurethane (PU), a polymer binder, and PU-bonded mixtures were studied for overcoming the limitations of traditional asphalt-bond OGFC mixtures. The viscosity-time and mechanical properties of two PU binders were characterized using Rheometer, tensile test and dynamic mechanical analysis (DMA). Then a laboratory evaluation of the PU-bonded porous mixtures (PPM) and asphalt-bonded porous mixtures (APM) were undertaken using the Marshall stability test, the Cantabro Loss test, and the fatigue test. The test results show the PPM obtained three times higher stability and over one order of magnitude greater fatigue life than APM. When compared with the low glass-transition temperature (Tg) PU binder, the high Tg PU binder significantly improved resistance to moisture damage and lowered Cantabro loss of PPM in the 60 °C immersion conditions. Results of this study will provide an important reference for utilizing PU porous mixtures as OGFC surface layer, and discover ways to improve OGFC durability.

Integration of Heat Treatment with Shot Peening of 17-4 Stainless Steel Fabricated by Direct Metal Laser Sintering

Direct metal laser sintering (DMLS) is a promising powder-based additive manufacturing process for fabrication of near-net-shape parts. However, the typically poor fatigue performance of DMLS parts must be addressed for use in demanding industrial applications. Post-treatment can be applied to enhance the performance of such components. Earlier attempts at inducing grain refinement through severe plastic deformation of part surfaces using shot peening improved the physical and mechanical properties of metals without chemical alteration. However, heat treatment can modify the surface-hardening effects attained by shot peening. Hence, we examined the feasibility of applying shot peening combined with heat treatment to improve the performance of DMLS-fabricated 17-4 stainless steel parts through microstructural evolution studies and hardness measurements. Compared to a specimen treated only by shot peening, the sample exposed to additional heat treatment showed increased hardness due to aging of the dominant phase.

A novel hydroforming approach in manufacturing thin-walled elbow parts with small bending radius

Bending of a thin-walled tube with small relative bending radius is a hot topic which has aroused extensive attention in recent years. The main disadvantage of tube parts with small bending radius fabricated by available bending approaches is non-uniform wall thickness, large springback, poor fatigue performance, and wrinkle that is difficult to control for thin-walled tube bending. In order to solve this problem, a Bend-Bulge Forming (BBF) process combining rotary draw bending and hydroforming that realizes the fabricating of thin-walled elbows with small bending radius has been put forward in this study, and both FEM simulations and experiments were carried out. Meanwhile, in order to reveal the deformation behavior and the failure during BBF process, theoretical and FEM analysis were conducted by free expansion with bent tubes. The results show that the axial feeds play a prominent role in the hydroforming process. It can promote the tube wall outside of the bend contact with hydroforming die and avoid the excessive thinning. At the same time, the tube wall inside of the bend which is thickened during bending operation can be deformed sufficiently. Finally, the final parts fabricated by the BBF process have approximately constant wall thickness and high precision.