Damage Evolution of Ballastless Track Concrete Exposed to Flexural Fatigue Loads:The Application of Ultrasonic Pulse Velocity,Impact-echo and Surface Electrical Resistance Method

In order to clarify the fatigue damage evolution of concrete exposed to flexural fatigue loads,ultrasonic pulse velocity(UPV),impact-echo technology and surface electrical resistance(SR) method were used.Damage variable based on the change of velocity of ultrasonic pulse(Du) and impact elastic wave(Di)were defined according to the classical damage theory.The influences of stress level,loading frequency and concrete strength on damage variable were measured.The experimental results show that Du and Di both present a three-stages trend for concrete exposed to fatigue loads.Since impact elastic wave is more sensitive to the microstructure damage in stage Ⅲ,the critical damage variable,i e,the damage variable before the final fracture of concrete of Di is slightly higher than that of Du.Meanwhile,the evolution of SR of concrete exposed to fatigue loads were analyzed and the relationship between SR and Du,SR and Di of concrete exposed to fatigue loads were established.It is found that the SR of concrete was decreased with the increasing fatigue cycles,indicating that surface electrical resistance method can also be applied to describe the damage of ballastless track concrete exposed to fatigue loads.

Effect of Curing Age on Tensile Properties of Fly Ash Based Engineered Geopolymer Composites(FA-EGC)by Uniaxial Tensile Test and Ultrasonic Pulse Velocity Method

Tensile properties of fly ash based engineered geopolymer composites(FA-EGC)at different curing ages were studied by uniaxial tensile test and ultrasonic pulse velocity(UPV)methods,which included uniaxial tensile properties,the correlation between ultrasonic pulse velocity and tensile properties,and characteristic parameters of microcracks.The experimental results show that obvious strain hardening behavior can be found in FA-EGC at different curing ages.With the increase of curing age,the tensile strength increases,the tensile strain decreases and the toughness becomes worse.The UPV of FA-EGC increases with curing age,and a strong correlation can be found between tensile strength and UPV.With the increase of curing age,the average crack width of FA-EGC decreases and the total number of cracks increases.This is because the strength of geopolymer increases fast at early age,thus the later strength development of FA-EGC tend to be stable.At the same time,the bond strength between fiber and matrix,and the friction of fiber/matrix interface continue to increase with curing age,thus the bridging effect of fiber is gradually strengthened.In conclusion,the increase of curing age is beneficial to the development of tensile properties of FA-EGC.

Statistical Modelling of Ultrasonic Pulse Velocity of Fly Ash Based Geopolymer Mortar using Response Surface Methodology

The fly ash based geopolymer has emerged as a capable and sustainable binder material in construction industry.Ultrasonic pulse velocity(UPV)method is a non-destructive technique for investigating the mechanical performance of concrete.Experimental investigation was performed for studying the effect of NaOH Molarity,Na2SiO3/NaOH and curing temperature on the ultrasonic pulse velocity of geopolymer mortar.Experiments were designed based on central composite design(CCD)technique of response surface methodology(RSM).Statistical model was developed and statistically validated and found significant as the difference between adjustable R-squared and predicted R-squared less than 0.2.Finally,the optimized mix proportion was assessed for maximized value of UPV.Experimental validation on the optimized mix reveals the close agreement between experimental and predicted values of UPV with significance level of more than 95%.The proposed technique improves the yield,the reliability of the product and the processes.

Creep properties and resistivity-ultrasonic-AE responses of cemented gangue backfill column under high-stress area

To investigate the creep and instability properties of a cemented gangue backfill column under a highstress area,the uniaxial compression creep tests were conducted by single-step and multi-step loading of prismatic samples made of cemented gangue backfill material(CGBM)under the high stressstrength ratio.The creep damage was monitored using an electrical resistivity device,ultrasonic testing device,and acoustic emission(AE)instrument.The results showed that the CGBM sample has a creep hardening property.The creep failure strength(CFS)is slightly larger than the uniaxial compressive strength(UCS),ranging in ratio from 108.9%to 116.5%.The instantaneous strain,creep strain,and creep rate increase with increasing stress-strength ratio in the single-step loading creep tests.The instantaneous strain and creep strain decrease first and then increase during the multi-step loading creep process.The axial creep strain of the CGBM column can be expressed by the viscoelastic-plastic creep model.Creep instability is caused by the accumulation of strain energy under multi-step loading and the continuous lateral expansion at the unconstrained middle position during the creep process.The creep stability of a CGBM column in a high-stress area can be monitored based on the variation of electrical resistivity,ultrasonic pulse velocity(UPV),and AE signals.

A novel ultrafast-convert hybrid pulse square-wave VP-GTAW process for aluminum alloys

A novel ultrafast-convert hybrid pulse square-wave variable polarity arc welding power source was developed. The variable polarity current which crossed zero with no dead time possessed the ultrafast converting speed and the series of ultrasonic pulse current were superimposed in the positive polarity current duration. A high-efficiency hybrid pulse variable polarity gas tungsten arc welding （HPVP-GTAW）process for aluminum alloys was achieved. With X-ray inspection, microstructure analysis, tensile tests and scanning electron microscopy （SEM） for fracture surface, the square butt welding cbaracteristics of 5A06, 2A14 and 2219 aluminum alloys were tested, respectively. Experimental results show that microstructure and mechanical properties of these aluminum alloy welded joints are influenced significantly by the introduction of ultrasonic pulse current. The weld quality is improved predominantly by the novel HPVP-GTA W process.

The geomechanical properties of soils treated with nanosilica particles

This study examines the effect of nanosilica(NS)additive to improve the mechanical properties of clay,clayey sand,and sand.The engineering properties of the soils were investigated through Atterberg limits,compaction,unconfined compression,ultrasonic pulse velocity(UPV),freeze-thaw,and direct shear tests.The NS content varied from 0%to 0.7%and cement content was 5%and 10%by the dry weight of the soil.The curing period varied from 7 d to 150 d.The consistency,compaction,and strength properties of the soils were affected by the presence of NS and cement.The optimum NS contents in clay specimens with 5%and 10%cement were 0.5%and 0.7%,respectively.It was 0.7%in sand specimens with both cement ratios,as well as 0.3%and 0.7%in clayey sand specimens with 5%and 10%cement,respectively.In terms of freeze-thaw resistance,clayey sand specimens containing 0.5%NS and 10%cement had the minimum strength loss.Exponential relationships existed between the ultrasonic pulse velocity(UPV)and the unconfined compressive strength(UCS)of soil specimens having the same curing period.The shear strength parameters of the soils also improved with the addition of NS.Scanning electron microscope(SEM)images demonstrated that cement and NS contributed to the improvement of the soils by producing a denser and more uniform structure.It was concluded that the minor addition of NS could potentially improve the geomechanical properties of the soils.

Compressive Strength of Basic Magnesium Sulfate Cement Coral Aggregate Concrete(MCAC)on Non-Destructive Testing

Basic magnesium sulfate cement coral aggregate concrete(MCAC)is a new type of concrete consisting of basic magnesium sulfate cement,coarse coral aggregate,coral reef sand and seawater.The rebound hammer(RH),the ultrasonic pulse velocity(UPV)and the compressive strength(fcu)tests of 14 sets of cube specimens of the MCAC after 28 d of aging were conducted.The impact of the content and length of sisal fiber on the relationship between the fcu-RH and the fcu-UPV was determined.A mathematical model was established to predict the strength of the MCAC using the UPV,RH,and comprehensive UPV/RH methods and to obtain the curves of test strength.The applicability of the test strength curves of ordinary portland concrete(OPC),light-weight aggregate concrete(LAC),and coral aggregate concrete(CAC)to MCAC was assessed.The results showed that the test strength curves of OPC,LAC and CAC were inappropriate to determine the strength of MCAC using non-destructive method.The relative standard error of the curves of test strength of the RH method and the comprehensive method met the specifications,whereas that of the UPV method did not.

Artificial Neural Network(ANN)Approach for Predicting Concrete Compressive Strength by SonReb

The compressive strength of concrete is one of most important mechanical parameters in the performance assessment of existing reinforced concrete structures.According to various international codes,core samples are drilled and tested to obtain the concrete compressive strengths.Non-destructive testing is an important alternative when destructive testing is not feasible without damaging the structure.The commonly used non-destructive testing(NDT)methods to estimate the in-situ values include the Rebound hammer test and the Ultrasonic Pulse Velocity test.The poor reliability of these tests due to different aspects could be partially contrasted by using both methods together,as proposed.in the SonReb method.There are three techniques that are commonly used to predict the compressive strength of concrete based on the SonReb measurements:computational modeling,artificial intelligence,and parametric multi-variable regression models.In a previous study the accuracy of the correlation formulas deduced from the last technique has been investigated in comparison with the effective compressive strengths based on destructive test results on core drilled in adjacent locations.The aim of this study is to verify the accuracy of Artificial Neural Approach comparing the estimated compressive strengths based on NDT measured parameters with the same effective compressive strengths.The comparisons show the best performance of ANN approach.

Microstructure Evolutions and Properties of Al-Cu Alloy Joint in the Pulsed Power Ultrasonic-Assisted GMAW

The pulsed power ultrasonic-assisted gas metal arc welding(PU-GMAW)is a new development hybrid welding method.The influences of the pulsed power ultrasonic on the microstructure evolution of the welded joint of Al-Cu alloy were investigated by using scanning electron microscopy,transmission electron microscope and electron back scattering diffraction.The results showed that the efficient heat input in the PU-GMAW was increased by above 100%compared with the traditional GMAW.The grain and eutectic of the PU-GMAW weld seam were refined compared with that of the GMAW.Cavitation and acoustic streaming induced the dendrite fragmentation(α-Al)and heterogeneous nucleation,which were the main reasons for the grain refinement.The microhardness of the PU-GMAW welded joint was improved compared with that of the GMAW owing to the change of the eutectic and grain.

Mechanism and microstructure of nickel-ceria composite coatings prepared by pulse current deposition under the ultrasonic field

In recent research, a novel method combined with pulse current （PC） deposition and the ultrasonic （U） field was used to fabricate pure nickel and nickel-ceria composite coatings, respectively. Morphology, crack propagation, and crystal texture were observed and analysed by using environment scanning electron microscopy （E-SEM） and transmission electron microscopy （TEM）. Orthogonal experiment [L16 （45）] was designed to optimize the parameters of pulsed power and the appropriate amount of RE addition based on microhardness. Effect of RE addition and pulsed current on the mechanism of co-electrodeposition was also investigated and compared. Experimental results indicated that it produced the alloying coatings, exhibiting compact grain and amorphous state. Nano-sized RE would preferentially occupy and pad at the edge of cracked gaps and micropore to limit the growing location and space for coarse Ni grain. Furthermore, during annealing at 480 ℃ for 2 h, a solid-solution precipitated phase named NiCexO1-x （0

Ultrasonic displacement field generated by laser pulse line source

Applying the Fourier transform to the wave equations of elastic medium at its surface a laser pulse line source is acted, the integral representations of solutions are obtained. Displacement waveforms are calculated numerically by using double FFT. The calculated results include the displacements of the elastic half space of Aluminum medium, and epicenter and off-epicenter of an Aluminum plate. The two exciting sources of thermoelastic and ablating generation are considered respectively. The experiment was made on the Aluminum medium with a Nd:YAG laser and the normal displacement signals are detected by a laser interferometer. The numerical results are quite in agreement with experiments.

Effect of diffraction on the ultrasonic velocity measuredby the pulse interference method in VHF range

The effect of diffraction on the ultrasonic velocity measured by the pulse interference method has been investigated in VHF range theoretically and experimentally. Two silicate glasses are taken as the specimens, their frequency dependences of longitudinal velocities are measured in the frequency range of 50-350 MHz, and the phase advances of ultrasonic signals caused by diffraction effect are calculated using A. O. Williams' theoretical expression. For the velocity error due to diffraction effect, the experimental results are in good agreement with the theoretical prediction. It has been shown that the velocity error due to diffraction effect is directly proportional to dθ21 (f)/df, whereθ21 (f) is the phase advances difference between the two partial reflection signals used in velocity measurement and f is the ultrasonic frequency.

Applications of Non-destructive Tests for Diagnosis of Heritage Buildings:Case Studies from Singapore and Malaysia

Non-destructive tests have been increasingly used in historic buildings not only for pre-restoration investigation but also as useful tools to evaluate the effectiveness of eventual interventions.These tests can complement and supplement destructive tests like laboratory analysis of extracted samples as long as a sound sampling plan has been put in place to commensurate with the pattern and extent of dilapidation and materials distribution.Common methods employed in projects in Singapore and Malaysia include the infrared thermography,ground penetrating radar,microwave moisture tomography and ultrasonic pulse echo tomography.The paper discusses about a selection of case studies,demonstrating the application of various techniques for the identification of hidden details,defects and deterioration,and moisture detection.

Effect of grain refinement on cutting force of difficult-to-cut metals in ultra-precision machining

The nickel-based superalloy Inconel 718 is treated with Coupled Ultrasonic and Electric Pulse Treatment(CUEPT),and the surface grain is refined from the average size of 9550.0 nm to287.9,216.3,150.5,126.3,25.8 nm by different effective treatment currents,respectively.The ultraprecision turning experiments are carried out on the processed workpiece after CUEPT.The experimental results show that the average cutting force increases with the decrease of surface grain size.Moreover,a mathematical model that can describe the relationship between grain size and cutting force is established,and the calculated results match the experimental results well.The calculated results also indicate that the variation of cutting force caused by the same variation of grain size decreases as the degree of grain refinement increases.Finally,the influence mechanism of grain refinement on cutting force is analyzed.The improvement of stability of grain boundaries and the increase of number of grain boundaries cause the increase of cutting force after grain refinement.