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Haynes 282 is a γ ′ precipitation-strengthened nickel-based superalloy known for its exceptional high-temperature creep resistance and excellent fabricability. Recent advancements in powder-bed fusion-based additive manufacturing (PBF-AM) have enabled the fabrication of Haynes 282 with accurate, site-specific control of the grain orientation and morphology at the microscale. This ability opens up new avenues for microstructure design, to improve the material's fatigue crack resistance and service life. This paper investigates the fatigue crack growth behavior of hybrid microstructure Haynes 282 fabricated via PBF-AM. Previous experiments revealed a higher crack propagation rate in the coarse columnar-grained microstructural regions when compared against fine-grained areas. Here, a strain gradient crystal plasticity model was adapted to study the fracture-related mechanical fields at the crack tip in the two microstructures. The simulation results showed a consistent influence of grain structure and texture on crack propagation, as was seen in the experiment. The model analysis revealed higher crack propagation driving force along crack direction in the coarse-grained sharply textured microstructure and higher driving force for crack kinking in fine-grained more diffusely textured microstructure, which is ascribed to the combined effect of yield stress, hardening rate, texture and grain morphology. The presented modeling approach will facilitate the development of the AM-based accurate microstructure design by deepening the fundamental study in AM-specific microstructure-properties relations. [ABSTRACT FROM AUTHOR]

The tension-tension fatigue behavior and damage mechanism of basalt fiber-reinforced epoxy polymer (BFRP) composites at different stress ratios are studied in this paper. The fatigue experiments were performed under stress ratios, R = σmin/σmax of 0.1 and 0.5, while the lifetime and the stiffness degradation were monitored and analyzed to investigate the effect of stress ratios. The damage propagation during fatigue loading was periodically monitored by using an in situ scanning electron microscope (SEM). The results show that the fatigue life decreases and the fatigue life degradation rate increases with the decrease of stress ratio for examined BFRP composites. The stiffness degradation is also sensitive to different stress ratios, showing a greater stiffness loss before failure at lower stress ratio. From the SEM images, it is indicated that the micro-damage mode shifts from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios. [ABSTRACT FROM AUTHOR]

This paper investigates the fatigue properties and deformation behavior of the T6-treated Mg-2.99Nd-0.18Zn-0.38Zr (NZ30K) and an AZ91D-T6 magnesium alloys with the same grain size. Compared with the AZ91-T6 alloy, the NZ30K-T6 alloy shows significant increases in mechanical properties, achieving 17 MPa in YS, 51 MPa in UTS, 2.4 % in elongation, and 15 MPa in FS, respectively. The NZ30K-T6 alloy also shows higher cyclic stress amplitudes in comparison with the AZ91-T6 alloy. This is attributed to increased matrix strength from the higher precipitate strengthening in the NZ30K-T6 alloy. The cyclic stress amplitude of the AZ91-T6 alloy increases with increasing the number of cycles. For the NZ30K-T6 alloy, the cyclic stress amplitude first increases and then decreases during cyclic deformation. Hysteresis loops were symmetrical at all total strain amplitudes due to the lessened extents of texture in these cast magnesium alloys and the absence of twinning-detwinning during cyclic deformation. For both AZ91-T6 and NZ30K-T6 alloys, the cracked slip bands initiate the fatigue cracks and also assist the crack propagation. The Coffin-Manson law and Basquin equation can be used to evaluate the fatigue parameters and predict the LCF lives of both AZ91-T6 and NZ30K-T6 alloys. The multi-scale fatigue (MSF) life models and the same model parameters were also used to predict the HCF lives of these alloys. [ABSTRACT FROM AUTHOR]

This paper aims to statistically estimate the dynamic fatigue strength in brittle materials under a wide range of stress rates. First, two probabilistic models were derived on the basis of the slow crack growth (SCG) concept in conjunction with two-parameter Weibull distribution. The first model, Model I, is a conventional probabilistic delayed-fracture model based on a concept wherein the length of the critical crack growth due to SCG is enough larger than the initial crack length. For the second model, Model II, a new probabilistic model is derived on the basis of a concept wherein the critical cracks have widely ranging lengths. Next, a four-point bending test using a wide range of stress rates was performed for soda glass and alumina ceramics. We constructed fracture probability-strength-time diagrams (F-S-T diagrams) with the experimental results of both materials using both models. The F-S-T diagrams described using Model II were in good agreement with plots of the fracture strength and the fracture time of both materials more so than Model I. [ABSTRACT FROM AUTHOR]

The paper focuses on modeling the strain fatigue lives of three commonly used cable insulation polymers, namely (1) polyvinyl chloride, (2) crosslinked polyethylene, and (3) polyphenylene ether under selected strain and temperature ranges. On the basis of results obtained from their fatigue tests, Coffin–Manson model, mean/maximum strain fatigue model, and a set of new semi-empirical equations were applied to establish the relationship between fatigue lives and strains. The unified strain model, herein we name it the Wei–Wong model, is developed to predict the fatigue lives of three polymers studied and their prediction capability was examined using our experimental data. It was found that the proposed Wei–Wong model can provide a better life prediction compared to the experimental data and other methods in the literature at selected temperatures, namely −40, 25, and 65 °C. [ABSTRACT FROM AUTHOR]

In this paper, yield and fatigue behavior of a polypropylene nanocomposite and a polyamide-6 nanocomposite has been studied. The Eyring equation was used to model the temperature and strain rate sensitivity of the yield strengths of these two nanocomposites. Both activation volume and activation energy of the polypropylene nanocomposite were higher than those of the polyamide-6 nanocomposite. The fatigue strength of the polyamide-6 nanocomposite was higher than that of the polypropylene nanocomposite. However, the ratio of maximum fatigue stress to yield strength of the polypropylene nanocomposite was higher than that of the polyamide nanocomposite. The fatigue failure in both composites was initiated at agglomerated nanoparticles. [ABSTRACT FROM AUTHOR]

This paper presents an overview of fatigue fracture modes in selected structural alloys employed in gas turbine engines. These include the mechanisms of fatigue crack growth in the near-threshold, Paris and high-δK regimes obtained from Ti-6A1-4V, Inconel 718 and PWA 1472 (a single crystal nickel-based superalloy of similar chemical composition to Inconel 718). Fatigue fracture modes in these materials are shown to be strong functions of the stress intensity factor range, AK, and the maximum stress intensity factor, Kmax. Fatigue mechanism maps are also presented to show the parametric ranges of ΔK and Kmax corresponding to the different fatigue fracture modes. [ABSTRACT FROM AUTHOR]

The Laue microdiffraction technique was used to investigate the strain field caused by the shot-peening operation and its redistribution after thermal hold or fatigue in a model nickel-based superalloy with an average grain size of 40μm. Micrometer and millimeter size mappings showed that the plastic deformation introduced by shot-peening in the whole sample partially relaxes after a thermal exposure at 450∘C and was fully redistributed by the fatigue of the material, except in the hardened layer close to the sample edge. Diffraction patterns permitted to measure separately the strains related to the average alloy (γ+γ′) and to the γ′ phase. No difference was observed between the two deviatoric strain fields. Even if there were small stresses in the inner part of the samples, the sensitivity of the Laue microdiffraction method was large enough to quantitatively characterize the crystal misorientations and the deviatoric strain redistributions. Useful data were provided not only at the grain scale but also at the mesoscopic scale, thus bridging the gap between the sin2ψ and Ortner’s methods used to determine residual stresses, respectively, in fine and single-grain microstructures. The obtained results are also of first interest for a quantitative comparison with HR-EBSD measurements in the scanning electron microscope. Energy coupled measurements with an energy-dispersive point detector were also performed to determine the full elastic strain tensors associated with the γ and γ′ phases. We demonstrated that, for Ni-based superalloys, the accuracy on strains and stresses was, respectively, of the order of 1×10-3 and 250 MPa for the diagonal components of tensors. The measurements suffered from the 150 eV resolution of the detector which made it difficult to the separate the energies of the γ and γ′ phases. Owing to large crystal misorientations, the microdiffraction technique was not able to determine elastic strains and hardening in the highly deformed layer, where a large amount of plastic strain and a number of defects were accumulated. Some improvements are proposed to overcome these difficulties. [ABSTRACT FROM AUTHOR]

In this article, the fatigue dislocation sub-structures of Al–Mg–Si alloys were studied under the same equivalent stress amplitude of 93 MPa with multi-axial non-proportional loading paths, which were circle, ellipse, rectangle, and square loading paths. After fatigue test, the fatigue dislocation sub-structures of the failure specimens were also observed under different equivalent stress amplitudes, which were 47, 70, 90, 140, and 163 MPa with the transmission electron microscopy method. The results indicate that fatigue lives of the alloy are different under different loading paths with the same equivalent stress amplitude, and the fatigue life decreases with the increase in equivalent stress amplitude from 47 to 163 MPa. Cyclic additional hardening is found above medium effective stress amplitude of 93 MPa. The dislocation sub-structures strongly depend on equivalent stress amplitudes and loading paths. The dislocation sub-structures of parallel slip band, slight small dislocation cell (or labyrinth), and remarkable labyrinth feature are found under the equivalent stress amplitudes of 47, 93, and 140 MPa, respectively. The movability of the dislocation and the stress concentration degree are closely related to fatigue life and cyclic hardening of the alloy. [ABSTRACT FROM AUTHOR]

The effects of ageing on the mechanical properties and thermal stresses distribution of injection moulded, short glass fibre/clay/polypropylene composites were studied. Two different clays were studied—talc and sepiolite. The results obtained indicate that the incorporation of short glass fibre into clay/polypropylene composites improves the mechanical properties, independently of ageing treatment. Larger elastic modulus values were obtained for talc-filled samples, whereas higher strength values were obtained with the sepiolite-filled ones. The impact strength increased as a result of the incorporation of glass fibre into the sepiolite-filled composite, while a small decrease was detected for the talc-filled polypropylene sample. Sepiolite-filled compounds show higher mould shrinkage in the bar-axis direction than equivalent talc-filled grades. In contrast, the shrinkage obtained on annealing at various temperatures between 100 and 160 °C was generally greater for talc-filled compounds than for the sepiolite-filled compounds. The shrinkage behaviour in the transverse direction was more complex. The residual stress levels of clay-filled polypropylene compounds were generally lower than those reported in the literature concerning short glass fibre polypropylene compounds under similar conditions. Hybrid composites showed much higher stress levels than the corresponding clay-filled samples independently of ageing conditions. [ABSTRACT FROM AUTHOR]

It is pointed out that proposed mechanisms of fatigue fracture should be consistent with Coffin’s Law for constant strain-amplitude cyclic deformation. Most proposed mechanisms are not. The fracture of dislocation cores along their glide planes is consistent with the fact that the law describes the behavior during the first ¼ cycle as well as that after a very large number of cycles. Experimental evidences of the relative weakness of edge dislocation cores is described. Analysis of this weakness by Bullough, and in this article, discussed. It is concluded that this effect plays an important role in fatigue fracture. [ABSTRACT FROM AUTHOR]

An investigation has been carried out on the adiabatic shear band (ASB) in a Ti-3Al-5Mo-4.5V (TC16) alloy deformed at high strain rate by a split Hopkinson pressure bar (SHPB). ASB in TC16 alloy is a “white” band with a width of about 13 μm. Microhardness of the ASB is larger than that of the matrix. The elongated cell structures of width about 0.2–0.5 μm with thick dislocation exist in the boundary of the shear band. Results suggest that the fine equiaxed grains with α-phase and α″-phase coexist in the shear band. The “white” band is a transformation band. Calculation of the adiabatic temperature rise indicates that the maximum temperature within ASB is about 1,069 K that is above the phase transformation temperature. Finally, formation of an ASB in the TC16 alloy and its microstructure evolution are described. [ABSTRACT FROM AUTHOR]

A recording microindentation instrument equipped with optical observation and acoustic emission detection was built and used to examine, in situ, the crack initiation behavior of a commercial soda-lime-silica based float glass from contact with a Vickers diamond indenter. Indenter motion was provided by a piezoelectric actuator, while a strain gage load cell and capacitance displacement gages simultaneously monitored the indenter load and displacement, respectively, on the glass. It was found that both the maximum load and total indenter contact time influence the initiation behavior. Crack initiation occurred more readily when the total contact time and maximum load were increased. Increasing the maximum load from 0.2 kgf to 4 kgf resulted in the initiation of median-radial cracks on unloading at 6% Fmax up to 50% Fmax, respectively. In contrast, the initiation of lateral cracks was not significantly affected by the total contact time or the maximum load. Examination of the indentation cross-sections beneath 1 and 4 kgf indentation sites revealed well-developed median-radial and lateral crack systems. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a 72Se/72 As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β+ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed. [ABSTRACT FROM AUTHOR]

Low cycle fatigue tests for a hot extruded Nickel base alloy tube material have been performed at room temperature and at 204°C. The alloy shows a normal hardening and softening cyclic stress-strain response at room temperature. At 204°C, however, the cyclic stress-strain response shows a strain hardening first followed by a relatively stable stress and finally a secondary cyclic strain hardening. This stable stress disappears with increasing strain amplitude. The mechanisms of the secondary cyclic strain hardening have also been investigated by transmission electron microscopy (TEM). Besides dislocation multiplication, interactions between stacking faults and moving dislocations and between interstitial atoms and moving dislocations could also contributed to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204°C and its influence on the cyclic stress-strain response were also discussed. [ABSTRACT FROM AUTHOR]

The bipolar electric fatigue behaviour of antiferroelectric ceramics with the composition of Pb0.88Ba0.10La0.02 (Zr0.55Sn0.35Ti0.10)O3 was investigated under various cycling fields. The material exhibits a degradation in the maximum field induced strain, a diffuse AFE-FE phase transition and an enhancement in the diffusion character of the FE-AFE phase transition due to electric cycling. Those variations increase with cycle number, indicating a logarithmic fatigue up to 108 cycles. There is no indication for the variations to be recovered, and the symmetry of the negative and positive parts of the strain hysteresis loops still remains. Higher cycling field results in a stronger deterioration of the maximum field induced strain and a larger extent of diffusion in AFE-FE and FE-AFE phase transitions. The normalized maximum strain shows nearly the same scale of degradation when the materials are cycled at various electric fields. After a heat treatment at 500°C for 1 h, the variations in the AFE-FE and FE-AFE phase transition due to cycling disappear, whereas the maximum strain resumes almost to its original value. Electrochemical variations are considered to contribute to the main fatigue mechanism for the material under investigation. [ABSTRACT FROM AUTHOR]

Induced damage in brittle materials due to two interacting Vickers indentations at various orientations was investigated using a three-dimensional finite element model. The model considers `tensile cracking and compressive yielding' behavior of ceramics. Damage evolution due to both the simultaneous and sequential double indentations was studied. The simulation results indicated that the induced damage zone patterns are strongly a function of the relative orientation of the two indenters. The existence of another nearby indentation reduces the crack size on the side closer to the first indentation but increases the overall median damage zone size. These results were validated by sequential Vickers indentation experiments on borosilicate glass. The evolved damage patterns were further rationalized based on Bousinesq and blister field stress fields. Finally, the implication of these results on material removal mechanisms due to simultaneous interaction of several grits in a ceramic grinding process is discussed. [ABSTRACT FROM AUTHOR]

The continuous low cycle fatigue behaviors of a Fe-base superalloy, Nb-modified A286 alloy, have been evaluated at the test temperatures of 650°C and 350°C under various total strain ranges. It was found that the change of the slope in the Coffin–Manson plot was closely related to the fatigue cracking with the test temperature. In the high temperature low cycle fatigue (HTLCF) of Nb-A286 alloy, the fatigue cracking exhibited the intergranular mode at 650°C and the transgranular mode at 350°C. The intergranular fatigue cracking at 650°C was due to the precipitation of the η phase at the grain boundary assisted by the applied stress during low cycle fatigue. It is investigated whether the η precipitate at the grain boundary provides the site for the grain boundary cavitation, which induces the intergranular cracking in low cycle fatigue. This is confirmed by the results of low cycle fatigue at 25°C after heat treatment which forms the η phase at the grain boundary. [ABSTRACT FROM AUTHOR]

Mechanical behavior, such as tensile and fatigue strength, of the optical fiber sensor embedded within the composite laminate was investigated. Tensile and fatigue tests were performed to evaluate the static and fatigue characteristics of optical fibers embedded within three types of laminated composite specimens, [06/OF/06]T, [02/904/OF/904/02]T and [03/903/OF/903/03]T. The initiation of damage and fracture of the optical fiber were detected by observation of the intensity drop-off of laser signal transmitted through the optical fiber during test. Experimental results showed that the fatigue strength of optical fiber embedded within the cross-ply laminate is much lower than the fatigue strength of optical fiber within the unidirectional ply laminate. It was also found that the optical fiber embedded within unidirectional ply laminate fractured due to the fatigue damage accumulation of internal defects of optical fiber itself. However the optical fiber embedded within the cross-ply laminate fractured due to the growth of transverse matrix crack of host composite laminate. [ABSTRACT FROM AUTHOR]

Indentation fatigue testing is carried out for four kinds of zirconia ceramics; 6Y-FSZ, 3Y-TZP, Mg-PSZ and Ce-TZP; which have different strengthening-toughening mechanisms one another. Each material shows different indentation fatigue behavior. Large sized surface chipping is produced easily in early cycles for non-toughened 6Y-FSZ, while 3Y-TZP toughened by anelasticity has high resistance against chipping. There is a possibility that anelastic characteristic works as resistance against indentation fatigue damage. Stress induced phase transformation also improves fatigue damage, but this effect depends on the extent of transformation and/or occurrence of microcracking. [ABSTRACT FROM AUTHOR]

The low-cycle fatigue (LCF) behaviour of a Ni–Fe base superalloy Inconel 718 was studied at room temperature. Deformation mechanisms operating at different strain amplitudes were established by detailed transmission electron microscopy at various fractions of fatigue life. It was observed that there was an increase in the number of microtwins present in the microstructure with increase in number of cycles, N, at low strains, where very little slip activity was identified. The undeformed specimens contain a number of annealing twins in the microstructure which increased in number with increase in cyclic deformation. Electron microscopy also revealed the existence of grain-boundary ledges in the undeformed specimens. A typical region of the grain-boundary area of the deformed samples showed a front of microtwins in their “nascent” state, advancing from one grain into the other. It was concluded that in the low-strain regime, cyclic deformation was accommodated by forming new microtwin interfaces emerging from grain-boundary ledges present in the undeformed specimens. [ABSTRACT FROM AUTHOR]

The effect of strain rate on the behaviour of Fe3Al has been investigated experimentally in the range 90–1300 s-1 under tensile impact. The experimental results indicate that Fe3Al is strain-rate sensitive and its critical strain in maximum stress increases with increasing strain rate. According to the test results for Fe3Al in air and water, it is confirmed that environmental embrittlement induced the fracture of Fe3Al under tensile impact. With testing at faster strain rate, further hydrogen embrittlement is suppressed. This is consistent with the micrography of the fracture surface in Fe3Al specimens, which indicates that these iron aluminides are intrinsically quite ductile. [ABSTRACT FROM AUTHOR]

The energy for complete fracture in double edge-notched tension test specimens has been measured for a wide range of polymer films. Results indicated that the variation of the total specific work of fracture, wT, with ligament length, L, can be described by two straight lines, both of the form wT = we + β wpL, thus giving upper and lower intercept values at zero ligament length (i.e. we) for each film. The first term, we, is the energy absorbed per unit area of fracture, whereas the second term, wp, is the energy absorbed per unit volume of plastic deformation remote from the fracture surface. The lower we value was obtained from the extrapolation of the data within the mixed mode region (plane-stress/plane-strain) where the maximum net-section stress exceeded 1.15 times that of the tensile yield stress, σy, of the material, and the upper value was ascertained by extrapolating the data within the plane stress region where the net-section stress was 1.15 σy. It appears that the transition from plane stress to plane strain mode of fracture in thin films occurs at a ligament length much greater than 5B, where B is the specimen thickness. Moreover, it was found that the linearity of the data within the plane-stress region was not affected when ligament length values exceeded the plastic zone size. Moreover, variation of the extension to break with ligament length, for both pure plane stress and the mixed mode regions, was also linear; and the extrapolation values at zero ligament length were identified as crack opening displacements. Essential work estimated from the crack opening displacement agreed reasonably well with the extrapolated values. [ABSTRACT FROM AUTHOR]

Presents an interpretation of failure under cyclic contact loading. Claim that the fatigue effect is influenced by periodical friction between the grain-interlock bridges existing in such materials; Suggestion that delayed failure in contact fatigue tests is caused by a reduction of the friction term due to cyclic loading and by a related increase of the effective stress intensity factor until fracture toughness is reached.