•The intensity of permittivity peaks increase with increasing sintering temperatures.•The value of piezoelectric constant increases with increasing sintering temperatures.•Second crack and crack deflection are observed in the sample sintered at 1200 °C.•Domain switching mechanism contributes to the nonlinear ferroelastic deformation.For Nb/Ce co-doped Pb(Zr0.52Ti0.48)O3 ceramics {Pb(Zr0.52Ti0.48)0.95Nb0.05O3 + 0.2 wt.%CeO2} sintered between 1150 °C and 1250 °C, with increasing sintering temperatures, a gradual lattice distortion associated with an increased grain size are identified. Electrical test reveals that the ceramics exhibit a diffused phase transition, and the intensity of permittivity peaks increase with increase in sintering temperatures. The samples sintered at higher temperatures present a stronger piezoelectric property because of a larger grain size. While the mechanical test demonstrates that the samples sintered at lower temperatures exhibited a higher hardness because of a smaller grain size. Both the second crack and crack deflection are observed in the sample sintered at 1200 °C. The domain switching caused by a compression load contributes to the nonlinear ferroelastic deformation of ceramics. At last, the sample sintered at 1125 °C gains some good properties such as: Tc = 312 °C; d33 = 443 pC/N; KIC = 0.66 MPa m1/2 and σc = 430 MPa.Download high-res image (240KB)Download full-size image

•The significant degeneration of fatigue limit was detected in VHCF.•The fatigue failure process was congruously divided into four stages.•Fatigue crack propagation life occupied less than 3% of total life in VHCF.•The effect of stress ratio on crack propagation life was not distinct.Ultrasonic fatigue tests were carried out to investigate the effect of stress ratio on the very high cycle fatigue (VHCF) behaviors of titanium alloy (Ti-8Al-1Mo-1V). The fatigue strength at stress ratios of 0.1 and 0.5 were lower than the safely modified Goodman approximation, and a significant deviation was observed in the VHCF regime. Based on fractography and fracture mechanics, the fatigue failure process was congruously divided into four stages: (1) crack initiation induced by cleavage of primary α grains and its coalescence; (2) microstructure-sensitive slow crack propagation; (3) microstructure-insensitive fast crack propagation; and (4) final fatigue failure. Meanwhile, for surface crack initiation, fatigue crack propagation life only occupied less than 3% of total life in the VHCF regime, and over 95% of the fatigue crack propagation life was expended in the microstructure-sensitive crack propagation stage. The effect of stress ratio on crack propagation life was not distinct.

This paper presents an experimental and finite element analysis based mathematical modelling of cyclic behaviour of the component of hollow structural section (HSS) columns under bolt tensile loads. This component acts as an important constituent part to the blind bolted connections with HSS columns. Two typical failure modes and their related hysteretic curves are examined experimentally. It is shown that force versus displacement curves under monotonic loading can be regarded as the upper bound of the envelope of the counterpart cyclic response. Moreover, the hysteretic force–displacement curves of test columns with relatively thick tube walls demonstrate significant pinching manner thus less energy dissipative capacity. Based on the calibrated finite element model, the flexibility of tube wall and its influences on the connection response are compared for two failure modes. Parametric analyses are also performed to investigate the influence of geometric parameters on the elastic stiffness and yield strength. The mathematical hysteretic models allowing for the behaviour of the connections in two typical failure modes are proposed that distinguishes the developed model from those reported in the literature. It is demonstrated that the proposed model incorporating degradations of stiffness and/or energy dissipation can achieve a good representation of test hysteretic behaviours.

•In-plane shear compression of composite beams with laminated glass webs are experimentally evaluated.•Typical failure modes related to edge delamination and stress trajectories are analyzed.•Discussions are made for the strength improvement of composite beams due to adhesive joining.•Proposed analytical in strength prediction of studied structural mechanism is demonstrated to be satisfactory.Steel-glass composite beams with the combination of glass webs and steel flanges are known to be able to attain good ductility in contrast to pure glass panels. As a concentrated load is introduced away from the middle of the beam span, its resultant stress concentration can induce shear compression failure on the glass web which is distinguished itself from flexure failure as reported in the literature. This paper presents an experimental study on the in-plane shear compression behaviour of composite beams with laminated glass webs. Distinct failure mode related to load induced edge delamination traces on the glass web is reported. Based on the test results, the distribution of acting strains and stresses within the composite beams is plotted. The formation of inclined cracks on the glass web is justified as the stress trajectories are prone to bend to form a diagonal tension stress state. It is shown from the response of composite beams that the strength and elastic stiffness of composite beams are improved when the adhesives with better mechanical properties are used. Afterwards, it is shown that the strengths of test specimens are greater than these of counterpart glass stabilizing fins or stiffening fins due to the flexibility of adhesive joining. Finally, the test strengths have been evaluated based on the edge delamination traces and the equilibrium of the energy dissipation of proposed mechanism. It is demonstrated that the proposed mechanism in contrast to referred formulae is able to give a better prediction of strength.

•Cement dosages have limited effects on the strength recovery.•Crack distribution on the cross section was analyzed.•Longer exposure time leads to higher strength loss but better recovery.•H400 can be used to evaluate the synergetic effects of exposure time and temperature.Post-fire-curing is an effective method to recover fire-damaged concrete. In order to investigate the effects of cement dosage and cooling regimes on the recovery of fire-damaged concrete, three kinds of concrete specimens, with different cement dosages of 412, 392, 372 kg/m3, were prepared and tested. After cooling either in a furnace or ambient environment to the room temperature, these specimens were first soaked in water for 24 h and then subjected to 29 days’ post-fire-curing. Based on temperature history, residual compressive strength and cracks in the cross sections were tested and analyzed. And scanning electronic microscope (SEM) was used to help the morphology analysis. A new parameter named heat accumulation factor was proposed to estimate the strength loss and the recovery potency of the fire-damaged concrete. It was known from the experiments that the difference of relative residual compressive strength of specimens with three cement dosages is less than 3 percentage points. After post-fire-curing, the relative residual compressive strength of the specimens cooled in the air recovered from 77% to 83%, while that of the specimens cooled in the furnace recovered from 65% to 87%. The longer exposure under high-temperature due to the cooling in the furnace was responsible for the lower relative residual compressive strength and higher recovered strength. This is because the high temperature can promote dehydration processes, accompanied with more rehydration products with a denser microstructure. The crack lengths were observed much longer in the specimens cooled in the ambient environment due to the higher temperature gradient during the cooling process. The higher temperature and temperature gradient near the surface can result in more intensive cracks.

Temperature effects on the mobility of < c + a > dislocations on pyramidal planes are characterized by molecular dynamics simulations. On the first-order pyramidal planes, near-edge dislocations always decompose, while the critical resolved shear stress (CRSS) for near-screw dislocations increases with increasing temperatures. On the second-order pyramidal planes, the CRSS decreases to a minimum at ~ 400 K, then increases with temperatures. This indicates that first-order pyramidal slip is favorable at low temperatures, while at elevated temperatures both pyramidal slip planes are active, leading to profuse plasticity.Download high-res image (130KB)Download full-size image

The elastic-plastic deformation behaviors of nano-grained and coarse-grained yttrium aluminum garnet (YAG) transparent ceramics are investigated using nanoindentation. An inverse Hall-Petch relation is observed for the nano-grained YAG ceramic and a forward Hall-Petch relation is observed for the coarse-grained YAG ceramic. In addition, the plastic work ratio as a function of applied load for the nano-grained YAG ceramic shows a different trend than that for the coarse-grained YAG ceramic. These observations suggest that the plastic deformation of the nano-grained YAG ceramic cannot be attributed to the normal dislocation mechanism and is controlled by grain boundary sliding. A generalized self-consistent model for studying the mechanical behavior of the nano-grained YAG ceramic is developed and validated by experimental results. The stress-strain relationship predicted by this model is embedded in finite element simulations which confirmed that the plastic deformation of the nano-grained YAG ceramic indeed can be attributed to grain boundary sliding.Download high-res image (123KB)Download full-size image

The high-cycle fatigue behavior of Q345B structural steel was investigated experimentally. Highfrequency vibration fatigue testing machine and scanning electron microscopy were used to study the high-cycle fatigue S–N curve characteristics and crack initiation mechanism at ambient temperature. The surface temperature of the specimens was monitored. The relation between the fatigue limit and the amount of heat dissipation was also investigated. It was found that the fatigue life changed inversely with the stress amplitude in the high-cycle range. The fatigue limit in high cycle range was obtained from heat dissipation in the specimen and found to have good agreement with the S–N curve. The crack initiation was attributed to the surface defects and the persistent slip bands due to the cycle slip in fatigue loading.

•Prepared the nano-grained YAG transparent ceramic by high pressure technique.•Obtained the compressive yield with different temperature.•Obtained the compressive yield of nano-grained YAG transparent ceramic.Nano-grained ceramics have their unique mechanical characteristics that are not commonly found in their coarse-grained counterparts. In this study, nano-grained YAG transparent ceramics (NG-YAG) were prepared by low-temperature high-pressure technique (LTHP). The peak profile analysis of the X-ray diffraction was employed to investigate the compressive yield strength of NG-YAG. During the temperature at 450 °C, the residual micro-strain (RMS) increased with increasing loading pressure. However when the loading pressure was exceeded to 4.0 GPa the RMS exhibited a severe negative slop. The temperature effects on the compressive yield strength were also studied. It shows that the compressive yield strength of NG-YAG is 4.0 GPa and 5.0 GPa respectively at 450 °C and 350 °C. More importantly according to this investigation, a feasible technique to study the nano-grained ceramics is provided.Fig. 2 shows the significant slope changes of calculated residual micro-strain (RMS) associated with five selected pressure-temperature conditions. Another the grain size estimated from Scherrer's formula, especially when it changes with the pressure-temperature condition is also plotted in Fig. 2.

•Elastic and plastic deformation behaviors of Yb:YAG were studied by nanoindentation.•The deformation mechanism was investigated following the Sakai model.•The true hardness of Yb:YAG was determined based on the proportional specimen resistance model (PSR).The elastic and plastic deformation behaviors of ytterbium-doped yttrium aluminum garnet (Yb:YAG) transparent ceramics which affect its machinability were investigated by nanoindentation and in situ scanning probe microscopy (SPM) measuring method. This work aimed at studying the influence of applied load on the resistance to plasticity. It showed that the resistance to plasticity decreased with increasing the applied load, and the ductile machining became easier. But when the applied load exceeded 8 mN, microcracks were observed around the nanoindentation. Meanwhile, the indentation size effect was also analyzed for both measured nanoindentation hardness and Young’s modulus. The true hardness was determined based on the proportional specimen resistance (PSR). This study lends credence to the realization of manufacture for high surface quality for Yb:YAG transparent ceramic in abrasive machining.

•Full field strain evolution was characterized using DIC method in fatigue test.•The differences of fatigue failure mechanism between HAZ and FZ were discussed.•Porosity in FZ significantly influenced high cycle fatigue behaviours of the weld.The effects of mechanical heterogeneity on the tensile and high cycle fatigue (104–107 cycles) properties were investigated for laser-arc hybrid welded aluminium alloy joints. Tensile–tensile cyclic loading with a stress ratio of 0.1 was applied in a direction perpendicular to the weld direction for up to 107 cycles. The local mechanical properties in the tensile test and the accumulated plastic strain in the fatigue test throughout the weld’s different regions were characterized using a digital image correlation technique. The tensile results indicated heterogeneous tensile properties throughout the different regions of the aluminium welded joint, and the heat affected zone was the weakest region in which the strain localized. In the fatigue test, the accumulated plastic strain evolutions in different subzones of the weld were analyzed, and slip bands could be clearly observed in the heat affected zone. A transition of fatigue failure locations from the heat affected zone caused by accumulated plastic strain to the fusion zone induced by fatigue crack at pores could be observed under different cyclic stress levels. The welding porosity in the fusion zone significantly influences the high cycle fatigue behaviour.

It was frequently observed in experiments that stacking fault tetrahedron (SFT) can be completely absorbed by dislocation and generate defect-free channels in irradiated materials, but the mechanism is still open. In this paper, molecular dynamics (MD) was used to explore the dislocation mechanism of reaction between SFT and screw dislocation in copper. Our computational results reveal that, at high temperature, the SFT is completely absorbed by screw dislocation with the help of Lomer–Cottrell (LC) lock transforming into Lomer dislocation. This complete absorption mechanism is very helpful to understand the defect-free channels in irradiated materials.

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.

In this study, the effect of pre-existing corrosion pits on the fatigue behavior of 7075/T6 aluminium alloy in very long life range and in the near threshold regime was investigated by using piezoelectric accelerated fatigue test. The results indicate that the presence of pre-existing corrosion pits, produced by 1-day, 4-day, and 7-day immersion in salt water significantly reduces the fatigue life of the aluminum alloy by a factor of 10–100.

•The CFRP strengthening effect on the fatigue failure modes of open-hole plates is experimentally demonstrated.•The characteristic strain distribution and stiffness of CFRP strengthened open-hole plates are analyzed in details.•The fatigue life improvement effect of CFRP strengthening is discussed from a comparison of life curves related to details.•Proposed model allowing for stress ratios is shown to be effective in evaluating fatigue life enhancement after retrofit.•The influences of CFRP on the stress concentrations of open-hole details are clarified by finite element modelling.Open-hole details have gained widely application in bolted connections subjected to fatigue loading. Although the use of carbon fibre reinforced polymer (CFRP) can be efficient in strengthening critically stressed tension area, its benefit in improving the fatigue life of open-hole details has not been extensively evaluated. This paper presents the results of an experimental and finite element analytical study on the fatigue behaviour of CFRP strengthened open-hole steel plates. The merits resulted from CFRP strengthening are investigated and compared with some others from referred open-hole fabrication methods and these with artificial initial cracks or notches as reported in the literature. Typical failure modes, local strain evolution and stiffness degradation behaviour of the test open-hole details are reported. The fatigue life results are evaluated and compared against the design curves of the AASHTO. The results show that the use of single and triple layered CFRP strengthening results in fatigue life enhancement of nearly 20% and 60% respectively in contrast to bare steel open-hole plates. Incorporating the stress ratios allowing for fibrous rupture and steel-adhesive interface failure, the fatigue life of CFRP strengthened specimens can be properly evaluated. Finally, the geometric and configuration parameters of CFRP on the stress concentration of the open-hole details are studied based on finite element modelling. It is demonstrated that the studied stress concentration can be reduced with the increase of the modulus of carbon fibre and the number of CFRP layers. The contribution of the effective width of CFRP to the decrease of stress concentration is also discussed.

CeO2 is a promising material for the utilization of solar light in photocatalytic reactions. However, the major obstacle for these studies is the lack of reliable methods to incorporate the desired elements such as nitrogen into the crystal lattice of CeO2. In the present study, a nitrogen-doped CeO2 thin film was synthesized by IBAD technique. With this technique, the nitrogen was heavily and uniformly doped into the CeO2 thin films. XPS analysis clearly demonstrates the greatest N concentration of 25 mol% can be doped in CeO2 thin films which is much higher than that obtained via traditional methods. The high resolution N 1s spectrum shows that nitrogen dopants are uniformly doped into CeO2 lattice by substituting O. The XRD results indicate ion bombardment on the growing film surface does not alter the crystal structure of the film by itself. Instead, the heavy nitrogen doping induces smaller grain size of CeO2. The SEM images show that with the increase of N doping, the surface become smoother with smaller particle size. The heavily nitrogen doping also induces a red shift of the visible light absorbance from 380 nm to 450 nm.

Fatigue in the very long life regime has assumed great significance in recent years particularly after it has been established that a fatigue limit doesn’t exist in most cases. The paper deals with investigations of gigacycle fatigue and near threshold fatigue crack growth behaviors of AA7075-T6, AA6061-T6 and AA2024-T3 Al alloys using piezoelectric accelerated fatigue at 20KHz. Experiments were conducted in air at room temperature on hourglass-shaped specimens under completely reversed loading conditions (R=−1). It was noticed that failure continues to take place right up to the gigacycle (109 cycles) range, at nominal cyclic stresses. The high cycle fatigue fractures are generally rough in appearance and seen to initiate predominantly from the surface. Furthermore, they contain a significant number of voids, exhibit crystallographic faceting, and discernible fatigue striations. Gigacycle fatigue fracture of AA2024-T3, however, shows features of ductile tearing instead of striations. The studies also show that fatigue crack growth rates of small cracks are greater than those of large cracks for almost the same stress intensity factor range ΔK, and that some small cracks may grow at ΔK values below the large crack threshold in the near-threshold crack growth regime.

•Mechanical properties and microstructures of basalt fibre reinforced RAC were studied.•Effects of the RCA replacement ratio on compressive behaviour of the RAC and BFRC columns.•The influence of the basalt fibre content on the mechanical properties and cyclic axial compression.•The basalt fibre could be used in RAC to enhance its structural behaviour.Concrete waste constitutes the major proportion of construction waste which counts on about 50% of the total wastes generated. The use of recycled aggregates serves to promote the recycling of concrete in the construction industry as well as to suit the reconstruction need in earthquake areas. This paper presents an investigation on mechanical properties and microstructures of basalt fibre (BF) reinforced recycled aggregate concrete (RAC). The main parameters considered in the study are the replacement ratio of recycled coarse aggregates (RCA) and the content of the basalt fibre. The research work is focused on the influences of the above parameters on the failure mode, compressive strength, tensile strength, elastic modulus, Poisson’s ratio and the ultimate strain of the BF reinforced RAC. The results show that the mechanical properties of RAC can be enhanced by using BF. The SEM observations of the concrete reveal that the BF accumulated in pores and on the surface of the attached mortar can not only strengthen the RAC, but also improve the microstructure of the interfacial transition zone (ITZ), which further enhances the strength and ductility of the RAC. Therefore, the basalt fibre reinforced RAC can be used in construction to reduce the environmental hazards from a large amount of earthquake waste from collapsed buildings.

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = −1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 107 cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.

The fatigue life of numerous aerospace, locomotive, automotive and biomedical structures may go beyond 108 cycles. Determination of long life fatigue behavior becomes extremely important for better understanding and design of the components and structures. Initially, before the invention of ultrasonic fatigue testing, most of the engineering materials were supposed to exhibit fatigue life up to 107 cycles or less. This paper reviews current understanding of some fundamental aspects on the development of accelerated fatigue testing method and its application in ultra-high cycle fatigue, crack initiation and growth mechanisms of internal fracture, S-N diagram, fatigue limit and life prediction, etc.