Cyclic Impact Loading Research Articles

Analysis of the Dynamic Characteristics of the Top Flange Pile Driving Process of a Novel Monopile Foundation without a Transition Section

As the most widely used foundation type, the pile-driving capacity of large-diameter monopile foundations and the hammer force of pile top flanges is increasing, as are the stress and strain of traditional flange structures under the resistance to the cyclic impact load of pile hammers. This paper conducts an empirical study on the Rudong 150 MW offshore wind farm project in Longyuan, Jiangsu Province. The numerical simulation and calculation method of large-diameter pile fatigue damage is mastered and proposed, and a simulation analysis and comparison is conducted, as is an analysis of the sensitivity and feedback of the key software parameters. Based on the results of the above analysis, a pile simulation analysis of the offshore wind power projects under construction and those that are about to be started is performed. The pile hammer stroke energy and pile top hammer stroke process curve are extracted. According to the time range curve of limit and hammer stroke forces, the flange limit stress and distribution of the pile roof are simulated. Combined with the process characteristics of large-diameter pile sinking and analysis in the case of high hammer energy and high acceleration, the effect of the impact on the sensor is discussed. The results show that the instantaneous loading analysis could load at the flange of the pile, the maximum instantaneous stress was above 500 MPa, and the final maximum stress was approximately 307 MPa. Different loading methods vary greatly in the calculation results of the structure, so choosing the appropriate loading simulation method is key to pile sinking analysis.

Dynamic mechanical response and damage evolution of cemented tailings backfill with alkalized rice straw under SHPB cycle impact load

Dynamic disturbances such as blasting excavations cause damage to backfills. To ascertain and improve the dynamic mechanical response of backfill under frequent disturbance, single and cyclic impact load tests were conducted on cemented tailings backfill mixed (CTB)with different contents of alkalized rice straw (ARS) using a split Hopkinson pressure bar. The evolution laws of the dynamic mechanical properties and damage characteristics of CTB with the impact times and the ARS content were analyzed. The results showed that (1) The dynamic compressive strength (DCS) of CTB was improved by the ARS under single impact load with different strain rates. (2) Under the cyclic impact loads, the DCS, dynamic elastic modulus, and peak strain of CTB decreased with the increase in the cyclic impact times. Under the same impact times, the DCS of the backfill first increased and subsequently decreased with the increase in the ARS content, and 0.3% was the best content. (3) The unit volume absorbed energy of CTB was increased by the ARS and the longitudinal wave velocity (LWV) decay rate of CTB was decelerated under the cyclic impact loads. (4) The established damage evolution equation of CTB based on the LWV showed that its cumulative damage degree was increased by the cyclic impacts. Moreover, the incorporation of ARS was beneficial for decelerating and reducing the damage evolution rate and degree. The ARS was wrapped by hydration products in the CTB matrix, and its adhesion with the matrix promoted the ARS to play a bridging role, and inhibited and slowed down the development, propagation degree and propagation rate of cracks in CTB under cyclic impact. When the content of ARS was further increased, the probability of overlapping was significantly improved, which was not conducive to the adhesion between ARS and CTB matrix, resulting in the decline of the mechanical properties of CTB. The research results provide a reference for the understanding of the dynamic mechanical response of CTB with ARS under frequent blasting disturbances.

Tribo-Surface Variation Caused by Ti2AlC and Ti3AlC2 Particles-Containing Lubricant under Cyclic Impact Loading

Mn+1AXn (MAX) phase materials present an attractive potential for friction reduction and wear resistance applications due to the ternary layered structure. This work was done to investigate how the combination of Ti2AlC and Ti3AlC2 MAX phase particles with zinc dialkyl dithiophosp (ZDDP) additives in the lubricant affected the tribo-surface by means of a reciprocating test rig with cyclic impact loading. The results indicated that the friction and wear properties of Ti3AlC2-containing lubricant were better than those of Ti2AlC-containing lubricant. The distinctive microstructure of worn surface caused by the Ti3AlC2 particles was characterized by the uniform distribution of many fine scratches, while the other was distributed with more peeling pits by the Ti2AlC particles. The tribo-chemical reaction of ZDDP involving Ti3AlC2 particles promoted a larger regional distribution of the tribofilm and the generation of short-chain phosphates. The synergistic effect of Ti3AlC2 particles and ZDDP additives led to excellent tribological properties.

Impact fatigue life improvement of bonded structures using the bi‐adhesive technique

AbstractOne of the most efficient methods of improving the strength of adhesive bonds is the bi‐adhesive technique. The main aim of the current study is to investigate the fatigue life of bi‐adhesive joints under cyclic impact loads based on the J‐N (impact energy vs. fatigue life) methodology. To achieve this, single lap joints were subjected to cyclic low energy impacts. The J‐N curves were constructed using the experimental data considering Basquin's equation. According to the results, the durability of the joints against impact fatigue loads can be improved significantly (up to 17 times higher for the tested joints) using the bi‐adhesive technique. The results also showed that the impact fatigue life is very sensitive to the length ratio of the ductile to the brittle adhesives in bi‐adhesive joints. It was found that the total absorbed energy of the joints increases significantly by the reduction of the impact energy.

Computational modelling of the effect of microstructure on the abrasive wear resistance of tungsten-carbide nickel composite coatings under sub-critical cyclic impact loading

Abrasive wear was simulated for tungsten carbide-nickel (WC–Ni) composite coatings with different volume fractions of reinforcing particles under abrasive cyclic loading. Using parallelized computing approaches, for the first time in the literature, the effects of reinforcing particle and normal load on material removal mechanisms and wear rates were numerically analyzed for multi-cycle loading. The results demonstrated how various material removal mechanisms compete with each other for varying load, cycles, and particle concentrations. This both confirmed previous experimental observations, as well as motivates future areas towards materials tailoring and optimization. Finally, a statistical predictive model was developed to define the relationship between the wear rate, reinforcing particle volume fraction, and load in order to inform future efforts in materials design.

Dynamic Characteristics and Damage Constitutive Model of Mudstone under Impact Loading.

The mechanical response characteristics of mudstone from the ingate roadway of the west ventilation shaft in Yuandian No. 2 coal mine, Huaibei City, Anhui Province, China to dynamic loads were quantified in single- and cyclic-impact compression tests, using the split-Hopkinson pressure bar test device. The dynamic stress–strain relationships and the failure characteristics of mudstone samples under different impact loads were analyzed systematically. Considering the “rate effect” of the mudstone dynamic strength, the dynamic strength criterion of mudstone was proposed, and the dynamic damage constitutive model of mudstone was established, based on the statistical damage theory. In response to single-impact loads, with increasing impact pressure, the mudstone peak stress and strain gradually increased, and the peak stress and average strain rate increased nonlinearly. In response to cyclic-impact loads, with an increasing number of impacts, the mudstone peak stress first increased and then decreased, and the peak strain increased gradually. With increasing impact pressure, the number of impacts to the samples’ failure decreased gradually. By parameter identification and comparative analysis of the test results, the proposed dynamic damage constitutive model of mudstone was validated. The model can be used for stability analysis of roadway-surrounding rock under dynamic loads.

Experimental Study on Dynamic Mechanical Characteristics and Energy Evolution of Sandstone under Cyclic Impact Loading

In order to study the dynamic mechanical properties of the surrounding rock of roadway in deep mines with rock burst, cyclic impact compression tests of sandstone under different impact velocities were carried out by using the split Hopkinson pressure bar (SHPB) test system, and the dynamic stress-strain curves of the samples under cyclic impact loads and the relationship between energy dissipation and impact numbers were analyzed. The results show that, as the impact loading times increase, the initial elastic modulus of sandstone samples decreases, the peak stress decreases, and the peak strain increases; the average strain rate increases. The cumulative damage of sandstone samples under impact load is significantly affected by impact velocity and times. During the cyclic impact test, the reflected energy of sandstone increases with the increase of impact numbers, while the transmitted energy decreases and the absorbed energy increases; the cumulative specific energy absorption variable is introduced, and the cumulative specific energy absorption value of the samples increases with the increase of impact numbers, which can better characterize the variation of energy accumulation and dissipation of sandstone samples under cyclic impact.

Mechanical and energy dissipation characteristics of granite under cyclic impact loading

Mechanical and energy dissipation characteristics of granite under cyclic impact loading

Методологічні підходи до прискорених випробувань подрібнювальних ножів ґрунтообробних котків. порівняльні випробування втрати різальної здатності

Abstract. The purpose of the research is to analyze the design features and parameters of grinding knives to compare the intensity of loss of cutting ability of each of the models of knives depending on the load cycles. Methods and materials. Conduct an analytical inspection and instrumental studies of the blades of grinding rollers type KR with different design features. Carry out bench accelerated tests based on the cyclic impact load of the knife fragment due to falling on the simulator of wear cycles and check the residual cutting capacity on the cutting simulator. Justify the coefficient of acceleration of tests as a derivative of hardness and density of the simulator, as well as the ratio of the energy of the fall of the knife fragment on the stand and its rotational motion in the field. Statistical analysis of experimental data was performed by analysis of variance and interpreted by standard computer programs Excel in the form of graphs. Results. Analyzing the proposed method for determining the intensity of loss of cutting ability of the knife when it falls on the simulator of plant debris allowed to rank different models and establish their relative resource. Unhardened models of both 45 and 30MnB5 Steel provide low life and their residual cutting ability is only 15-25% compared to hardened models. Analyzing the research results, the hardened model of a knife made of 45 Steel withstands 4 times more load cycles compared to unhardened models, but is inferior to the hardened model of Steel 30MnB5 (borista), which has the highest resource. Even after 150 load cycles (in terms of the work of a roller with a width of 12.5 m - operating time of more than 1000 ha), it retains more than 55% of the initial cutting ability, which is characterized by low intensity of its loss. Conclusions. According to the tests of the intensity of loss of cutting ability of knives, the most acceptable option for the equipment of roller-shredders is a hardened model made of 30MnB5 Steel (borista), which has the highest resource.

Investigation on the static fracture mechanical characteristics of marble subjected to impact damage based on the FDM-DEM coupled simulation

To investigate the mechanical characteristics of static fracture of marble subjected to dynamic damage after cyclic impacts, based on the modeling technology of the finite difference method (FDM) and the discrete element method (DEM) coupling, a three-dimensional numerical model of the split Hopkinson pressure bar (SHPB) was constructed, and the bar system and rock sample were modeled using FLAC3D and PFC3D programs, respectively. The numerical cyclic impact loading tests were carried out on notched semi-circular bend (NSCB) samples at a constant impact velocity, and then the static three-point bending fracture tests were simulated on these damaged samples. The coordinate data of the particles on fracture surfaces of the sample were extracted by compiling the Fish program, and then the fracture surface was reconstructed and the surface roughness was calculated quantitatively. The rationality and reliability of the numerical analysis were verified by comparison with the results of relevant laboratory tests. The results show that in the cyclic impact loading test, the stress-strain curve rebounds, resulting from the release of part of the stored strain energy during the unloading period. With the increase of the impact number n, the numbers of cracks and fragments generated increase. The connected force field becomes more and more chaotic and the number of broken force chains displays an increasing trend. The breakage of the force chains is the root cause of the deterioration of mechanical properties of the sample. The static fracture toughness of the sample after 5 times of impact is 53.35% lower than that of the natural sample, while the failure displacement increases. In the static loading process, more and more cracks and fragments generate as n increases. This is proof that the internal structure of the sample has been damaged in the cyclic impacts. The fracture surface roughness increases as the impact number increases. The research conclusions can provide certain guidance for the engineering practice.

Experimental research on cyclic impact load of concrete during curing period

In order to study the influence of cyclic impact load on the compressive strength of concrete during the curing period, we used a self-made drop-weight impact test device to conduct a daily impact test on C40 concrete specimens with a curing period of 28 days. At the same time, the longitudinal wave velocity test of the test block before and after the impact was carried out, and the 28-day uniaxial compressive strength of the concrete test block was finally measured. The results show that the cyclic impact load during the curing period will cause internal damage to the concrete, and the uniaxial compressive strength is significantly reduced; the acoustic wave test results show that the damage of the concrete under the impact energy of 11.76kJ, 5.88kJ, 7.84kJ, and 8.82kJ is 0.035, 0.022, 0.031, 0.031. The uniaxial compressive strength values after 28 days decreased by 11.93, 4.68, 7.6, 10.23 MPa. There is a positive correlation between impact load, damage and reduction in compressive strength.

Research on Tunnel Surrounding Rock Failure and Energy Dissipation Based on Cyclic Impact and Shear Loading

Aiming at the cyclic impact deformation and failure of tunnel surrounding rock under shear stress, a self-developed rotation-impact simulation test platform was used to determine the number of failures, stress-strain curves, and energy in the process of cyclic impact failure. The failure process of rock under different impact velocities and shear stresses has been systematically studied. Results show that, under the same impact speed, the shear stress will increase with the increase in the rotation speed, but an upper limit will exist. When the rotation speed reaches this upper limit, the shear stress will no longer increase. The presence of shear stress will reduce the number of impacts required for rock failure. When the impact speed is 7.2 m/s, the number of impacts at the maximum rotation speed is 60% of the static state. When the impact velocity is 16.8 m/s, this value is only 33.3%. At the same impact velocity, the stress-strain curves under different rotation speeds do not change significantly, but with the increase in the rotation speed, the slope of the elastic stage of the stress-strain curve gradually decreases, and the corresponding stress of the rock sample decreases when the maximum strain is reached. With the increase in shear stress, the crushing specific energy required for rock failure gradually decreases. The greater the impact velocity, the more obvious the impact of shear stress on energy dissipation. In the tunnel process, when the surrounding rock is subjected to impact loads from different directions, only the axial strain analysis will have certain safety hazards, and timely support and reinforcement work are required.

Mechanical response and energy dissipation characteristics of granite under low velocity cyclic impact

In the process of metal ore blasting mining, the surrounding rock in the middle and far area will still be affected by the stress wave generated by the explosion, resulting in the damage and strength reduction of surrounding rock. The fracture process of rock under cyclic impact load is a process of energy absorption and dissipation. The research on the mechanical properties and energy dissipation law of surrounding rock under low velocity cyclic impact has a good reference for the research on the damage law of rock under weak impact environment. In this paper, the granite samples collected from Sanshandao Gold Mine in Laizhou City, Shandong Province were subjected to cyclic impact tests by using a 50 mm diameter split Hopkinson pressure bar (SHPB) test device. The stress-strain evolution law and energy dissipation law of the specimen in the process of damage and fracture are mainly studied. The results show that the dynamic process of a large number of void defects (such as voids, dislocations, microcracks, etc.) in granite under cyclic impact load is strengthened, and the damaged core is formed and propagated, leading to tensile fracture; With the accumulation of strain energy, the damage situation intensifies, and then affects the stress propagation, so that the ratio of incident energy to reflected energy increases linearly with the accumulation of strain energy, and the ratio of transmitted energy also decreases linearly.

A Modelling of Cyclic Thermal and Impact Loads on Excavator Bucket

Open brown coal mining is usually complicated with high temperatures of coal massif, caused by spontaneous combustion, which significantly accelerates wear rate and resource consumption of excavator bucket. During excavation of hot coals, subjected to spontaneous combustion, excavators work in extreme conditions, which lead to a great reduction of their service life. In order to prolong excavators’ service life, repair and maintenance techniques are employed in mines. However, making a proper schedule of those techniques in such dangerous mines is a difficult task. This is also aggravated by a lack of techniques of in-situ resource analysis. In this study, an attempt to assess resource consumption rate of a hydraulic excavator bucket, working at excavation of self-ignited coal, has been made. In order to calculate the resource of a bucket, a computer model of the bucket fatigue life assessment has been made, using finite element analysis. Proposed model allows calculating stresses, induced during one digging cycle, and assessing residual resource of the bucket, which is related to its resource. It has been determined that thermal loads significantly accelerate bucket resource consumption, especially the bottom part of the bucket and the tooth caps. According to the research, tooth caps will fail after 253 hours of digging, and bottom part after 102 days. It has been concluded that, in order to increase service life of excavators operating in mines subjected to spontaneous combustion, it is necessary to revise the existing maintenance and repair system.

On the cyclic fatigue of adhesively bonded aluminium: Experiments and molecular dynamics simulation

Direct bonding of metals with resin plays a critical role in the jointing of dissimilar materials. The adhesion strength is known to be dependent on strain (loading) rate due to the strain rate sensitivity of polymeric resin. This study evaluated the adhesion durability of an epoxy resin adhesive on an aluminum alloy under cyclic impact loading. First, we used a repetitive laser shock adhesion test (LaSAT), which enabled us to evaluate the impact strength of interfacial fractures (i.e., adhesion and its durability). In this method, a pulsed YAG laser was used to generate strong elastic waves, which resulted in the interfacial fracture of the adhesive/aluminum alloy. We prepared two types of specimens with different curing temperatures (20 °C and 100 °C) and found that the specimen with the higher curing temperature showed higher adhesion strength and durability. We revealed that the adhesion strength showed cyclic fatigue characteristics, and that higher curing temperature improved fatigue strength. To elucidate this mechanism on the molecular level, we conducted a molecular dynamics (MD) simulation for the epoxy resin adhesive on aluminum alloy. The model featured an all-atom model of the Al2O3/epoxy resin interface, and we performed cyclic tensile deformation until delamination. We found that the number of loading cycles until delamination increased when the applied tensile stress was lower. We also found that the 400 K curing model had greater adhesion strength than the 300 K curing model. This tendency was confirmed by the results of the LaSAT experiments. Our comprehensive study incorporating LaSAT experiments and MD simulations evaluated adhesion strength and revealed its fracture mechanism.

A Study of the Dynamic Characteristics of Red Sandstone Residual Soils Based on SHPB Tests

To explore the response characteristics of soil under dynamic loads, cyclic impact tests with varying axial compressions and impact velocities were conducted using SHPB tests. The test results were analyzed to study the mechanical properties and energy dissipation characteristics due to impact loads. In terms of mechanical properties, for the same impact times, the peak stress of the specimen with axial compression was found to be greater than that without axial compression, but the final strain was lower than that without axial compression, indicating that axial compression affects the peak stress and final strain of the specimen. The dynamic stress strengthening of the specimen under cyclic impact loads has also been analyzed. In terms of energy dissipation, the effects of axial compression, impact velocity, and impact times on the ratios of dissipated energy, reflected energy, and transmitted energy to incident energy were compared, and it was found that the ratio of the dissipated energy did not exceed 24% under any condition. In terms of damage evolution, the damage variable gradually decreases with an increase in impact times. A critical value of the damage variable was determined, above which the transmittance continued to decrease, the reflectivity gradually increased, and the dissipation rate remained stable.

Experimental Study on Impact Failure Characteristics of Large-size Boulder Specimen by SHPB

The issue of rock breaking is inevitable when shield excavating the strata rich in boulders with large size. Based on the one-dimensional elastic stress wave theory, the dynamic stress-strain relationship of rock specimens under single impact and cyclic impact load is obtained by using split Hopkinson pressure bar test system, and the variation law of absorbed energy with the changes of impact pressure or impact frequency is analyzed. The experiment results indicate that under single impact, when the impact pressure is added, the peak value of strain and stress increase; the dynamic elastic modulus of the specimen increases in the elastic stage, while increases at first and then decreases in the non-linear elastic loading stage. Also, the optimal impact energy exists corresponding to the maximum absorbed energy. Under the cyclic impact, the rock failure needs to meet the requirement of lowest impact energy. With the increase of impact pressure, the peak strain and single absorbed energy decrease at first and then increase, and the cumulative absorbed energy increases gradually. When the impact energy exceeds the energy required for rock failure, the rock is subject to primary impact failure. The research results can provide reference for the design of rock breaking cutters and the control of shield tunnelling parameters.

Effects of Confining Pressure on Mechanical Properties and Damage Evolution of Granite under Cyclic Impact Loading

Abstract To study the effects of the confining pressure (Cp) on the mechanical properties and damage evolution of deep rock masses under multiple impact loading, the improved split-Hopkinson pressure bar was used to perform cyclic impact on granite samples with Cp = 0, 4, 8, 12, and 16 MPa. The experimental results indicated that the total number of impacts increased with the increase in Cp. The sample with Cp = 0 MPa was destroyed with only one impact and exhibited a typical type-I stress–strain curve. The samples with Cp = 4, 8, 12, and 16 MPa exhibited typical type-II stress–strain curves. Under the same cyclic impact loading, a higher Cp yielded a lower rate of increase of the average strain rate and maximum strain, as well as a lower rate of decrease of the peak stress and elastic modulus. The peak stress and maximum strain exhibited a good negative linear correlation and positive linear correlation with the average strain rate, respectively. At Cp = 4 and 8 MPa, the cumulative damage of the sample gradually increased with the increase in the number of impacts. At Cp = 12 and 16 MPa, the compaction effect caused the cumulative damage to be negative in the first few impacts. Subsequently, the cumulative damage gradually increased owing to the impact damage-softening effect. Additionally, the sample with Cp = 0 MPa was pulverized, and the fragmentation was severe. At Cp = 4, 8, 12, and 16 MPa, the Cps inhibited the free expansion of internal cracks in the sample, and the sample exhibited a significant shear failure mode with less fragmentation.

Influence of Cyclic Impact Loading and Axial Stress on Dynamic Mechanical Properties of Burst‐Prone Coal

High in‐situ stress and frequent dynamic disturbances caused by the mining process in deep coal mines can easily induce dynamic disasters such as coal burst. We conducted laboratory experiments to assess the effects of the axial stress loading and dynamic cyclic impact loading on the dynamic mechanical properties of burst‐prone coals by using a modified split Hopkinson pressure bar (SHPB). Comparisons were made using two types of burst‐prone and burst‐resistant coal samples. The mineral components, organic macerals, and dynamic mechanical features of both burst‐prone and burst‐resistant coal samples were comparatively analyzed based on the obtained X‐ray diffraction (XRD), optical microscope observations, and dynamic compressive stress‐strain curves, respectively. The results of the microstructure analysis indicated a larger difference between the minimum and maximum reflectances of vitrinite for burst‐prone coal. Compared to the burst‐resistant coal samples, the burst‐prone coals contained less corpocollinite and fusinite. While applying a high axial static load combined with cyclic impact load, the coal samples showed the characteristics of fatigue damage. The results also demonstrated that preaxial stress affected the burst resistance of coal samples. The greater the preaxial stress was, the less the coal samples could withstand the dynamic cyclic impact load. In comparison to the burst‐resistant coal sample, the burst‐prone coal sample showed a larger dynamic compressive strength and a lower deformation. They were also more positively capable of the propagation and activation of the coal burst. We believe that the results of the study are conducive to further understanding of the distribution of microcomponents of burst‐prone coals. The results are also beneficial for realizing the dynamic mechanical characteristics of burst‐prone coals under the impact of cyclic dynamic load.

Effect of hardened surface layer obtained by frictional treatment on the contact endurance of the AISI 321 stainless steel under contact gigacycle fatigue tests

Abstract Previous research has shown that the contact endurance (resistance to the fatigue spalling) of the corrosion resistant AISI 321 steel is insufficient under cyclic impact loading. The application of surface hardening is effective for increasing the contact endurance of metal materials if the gradient hardened layer is formed. The aim of this work is to study the possibility of increasing the contact endurance of the AISI 321 steel by surface hardening. A hardened layer has been formed on the surface of the steel by means of the frictional treatment with a sliding indenter at the normal load of 294 N. Mechanical testing for gigacycle contact fatigue of the surface-hardened AISI 321 steel has been conducted in accordance with the pulsing impact “plane-to-plane” contact scheme with an ultrasonic loading frequency. Optical and scanning electron microscopy, energy-dispersive microanalysis, optical profilometry, X-ray diffraction analysis, microhardness measurement and instrumented microindentation are used as methods of investigation. It has been discovered that the surface frictional treatment forms a gradient hardened layer up to 500 μm deep, which has a fairly homogeneous dispersed structure, an excellent quality surface with a quite low roughness value (Ra = 0.23 μm), high microhardness (780 НV0.025), robust residual stresses (σα = −540 MPa), and higher resistance to elastic-plastic deformation according to the microindentation data. The hardened layer considerably increases the contact endurance of the steel under conditions of impact contact-fatigue loading. This manifests itself in a considerable decrease (from 20 to 1–2 μm) in the depth of contact damages on the steel surface and the absence of damage localization, that is, a more uniform failure. However, localization of the contact damages can be facilitated by defects in the hardened layer (particularly pores), therefore, their number must be minimal.