Study On Crack Arrest And Fracture Parameters Of Fine Aggregate Concrete Under Impact Loads

Brittle materials such as rock and concrete are widely used in engineering structures such as hydropower dams,nuclear power plants,bridges and tunnels,etc.Due to the advantages of low price,wide distribution,easy to obtain materials,high compressive strength,easy to pour into a variety of shapes and so on,concrete has gradually become one of the most commonly used building materials for modern engineering structures.However,during the pouring process,defects such as pores,inclusions,microcracks or cracks will inevitably appear in the concrete.Under the action of dynamic loads such as impact or explosion,they will condense into nuclei and continue to develop macroscopic cracks.It even leads to structural damage,which significantly affects the safety or service life of industrial and civil buildings.Therefore,studying the dynamic fracture behavior of cracks and developing crack arrest technologies have important guiding significance for the design,evaluation and maintenance of engineering structures.The destruction process of concrete structure includes the whole process of internal microcracks gathering into nucleus,macroscopic cracks appearing and expanding,and finally leading to the whole structure destruction.In this paper,several new configuration specimens are proposed based on the characteristics of concrete internal crack propagation,in order to study the dynamic crack arrest and fracture characteristics,so as to develop new configuration specimens suitable for the study of dynamic crack arrest and fracture parameters and corresponding testing method.First,the numerical simulation of impact fracture of single cleavage trapezoidal opening(SCTO)specimen was carried out,and the crack arrest phenomenon was observed,but the crack arrest phenomenon was random,scattered and irregular.The crack arrest phenomenon was not detected in the impact fracture test of SCTO specimen,so the crack arrest phenomenon could not be detected by SCTO specimen.Then,in order to effectively monitor the crack arrest phenomenon,based on the idea of crack arrest by stress wave reflection,some large-size specimens were proposed after the configuration of the SCTO specimen was changed,such as a semi-circular edge crack with V-shaped bottom(SECVB)specimen configuration,and a trapezoidal opening crack with arc bottom(TOCAB)specimen configuration and a large semicircular edge crack with two arrest-holes(LSECTH)specimen configuration.Through the impact fracture test and numerical simulation of these three configuration specimens,the fracture characteristics and crack arrest mechanism of cracks are studied.In addition,a double-cracked concave-convex plate(DCCP)specimen was proposed to measure the fracture toughness of type I crack and type II crack at the same time.Through the summary and analysis of the research,The research of this paper can be divided into the following parts:(1)A large-size single-side cracked trapezoidal opening(SCTO)specimen configuration was proposed,and impact tests were conducted by using the SCTO specimen under the impact of a falling plate,and high-precision strain gauges were used to measure the impact loads.In order to verify the rationality of the designed configuration specimens,the explicit dynamic software AUTODYN was used to establish a corresponding numerical model,and the dynamic crack growth rate,crack initiation time and crack growth path were numerically studied.The dynamic crack initiation and propagation toughness were calculated by ABAQUS software.The research results indicate that:(1)because the SCTO specimen is simple and easy to pour,it is suitable for making concrete specimens,and the SCTO specimen is suitable for studying the effect of loading rate on the dynamic fracture characteristics of cracks.(2)The dynamic fracture toughness,energy release rate and crack speed all increase with the increase of the loading rate,but the crack initiation time decreases with the increase of the loading rate.(3)Numerical simulation shows that there is a crack arrest phenomenon during the dynamic crack propagation process,but it is beyond the monitoring range of the crack growth meter,and the crack arrest phenomenon cannot be monitored in the dynamic fracture test of the SCTO specimen;(4)At the moment of crack arrest,the particle velocity vector is opposite to the propagation direction of the crack,and the reflected compressive stress wave caused by the incident plate and the transmission plate plays a key role in crack arrest;(5)The initial fracture toughness is greater than the propagation fracture toughness,and due to the inertia effect,the energy release rate at the beginning is greater than that in the expansion process.(2)In order to study the dynamic crack propagation behavior of fine aggregate concrete under dynamic load,a large-size semicircular edge crack(SECVB)specimen with a V-shaped bottom edge was proposed.The V-shaped bottom of the SECVB specimen is designed as three angles of 180°,150° and 120°.The impact test was carried out on three SECVB specimens with a drop-weight impacts,and crack propagation meter was used to measure the relevant parameters of crack growth.The AUTODYN software was used to numerically study crack growth behavior,and the ABAQUS software was used to calculate the dynamic fracture toughness of the crack.The experimental and numerical results show that:(1)the horizontal component of the compressive stress wave generated by the interaction between the V-shaped bottom of the specimen and the transmission rod can inhibit the downwardly moving cracks,and the configuration of the SECVB specimen can prevent The crack propagation has a crack arrest function,and the crack arrest function of the 120°SECVB specimen is greater than that of the 150°SECVB specimen.(2)The critical stress intensity factor at the beginning of the crack is higher than the critical stress intensity factor at the time of crack propagation.In general,the critical DSIF decreases as the crack growth length increases,which indicates that the propagation toughness is not an independent material parameter,it is related to the crack growth speed.(3)The SECVB specimen is suitable for studying the crack growth behavior and crack arrest behavior of fine aggregate concrete under dynamic loads.(4)The crack initiation toughness increases as the loading rate increases,and finally it tends to a certain stable value.(3)In order to study the dynamic crack propagation and crack arrest laws of brittle materials,a trapezoidal opening crack with arc bottom(TOCAB)configuration specimen is proposed.Impact tests were carried out on TOCAB specimens with arcs of 0°,60°,90° and 120° under the drop weight impact equipment,and the crack initiation and growth time were monitored by a crack growth meter to obtain the crack growth rate.The finite difference software AUTODYN is used to numerically model the drop weight impact equipment and specimens,and the dynamic growth process of cracks and crack arrest laws are numerically studied.The dynamic fracture toughness of the crack is also calculated based on the finite element numerical model of the ABAQUS software.Both experimental and numerical results show that:(1)TOCAB specimens with three radians can achieve motion crack arrest,and this configuration can be used to study dynamic crack arrest.(2)The test results of the crack propagation path are basically consistent with the numerical results,which confirms the accuracy of the numerical model.(3)The critical dynamic stress intensity factor at the moment of crack initiation and arrest is greater than the critical dynamic stress intensity factor during the dynamic crack propagation process.(4)In the crack propagation path,the maximum horizontal compressive stress of the arc-bottomed specimen is greater than that of the flat-bottomed specimen,which has a significant inhibitory effect on dynamic crack propagation.(4)In order to study the interaction mechanism between the moving crack and the double arrest holes,a large semicircular edge crack with two arrest-holes(LSECTH)specimen is proposed.The impact test was carried out on LSECCH specimens with different distances between the double circular holes(35 mm,40 mm,45 mm,50 mm,55 mm,60 mm and 70 mm)using a drop weight impact device.CPG is used to monitor the crack propagation speed along the crack propagation path and the time it takes for the crack to propagate somewhere.The interaction characteristics of the crack and the two arrest holes are numerically studied by AUTODYN software.In the numerical model,the maximum tensile stress and shear stress failure criteria are used for brittle materials,and the crack propagation path,the circumferential stress field and the stress state between two crack arrest holes are numerically analyzed.The research results show that:(1)when two hole spacing is less than 55 mm,the crack propagation speed decreases greatly when the dynamic crack propagates to the area between the two holes.When two hole spacing is 60 mm and 70 mm,the crack propagation speed decreases less but still less than the crack velocity of the specimen without holes.(2)According to the different distances of the two arrest holes,the crack propagation path shows different characteristics.The main crack may branch into two sub-cracks,and may connect two arrest holes or be deflected and connected to one hole.The main crack may also be completely stopped.(3)The compressive stress wave under vertical loading causes the shape of the arrest hole to change from a circle to an ellipse,thereby generating a larger compressive stress in the area between the double holes,which slows down or inhibits the continuous growth of the vertical crack.(4)The larger the distance between the double holes,the longer the length of the main crack,the smaller the distance between the double holes,and the greater the maximum compressive stress at the midpoint between the double holes.Along the crack propagation path,the maximum compressive stress in the area between the two holes is the largest.(5)Under dynamic impact loads or explosive loads,tensile cracks and shear cracks are the main forms of crack propagation in brittle materials.In order to measure the initial fracture toughness of pure tensile and shear cracks in a specimen at the same time,a large-size double crack concavo-convex plate(DCCP)specimen configuration was proposed.The impact test is carried out on a drop impact device,and strain gauges are used to measure impact loads and crack initiation time.The dynamic difference program AUTODYN was used to simulate the stress state of the crack tip,and the initial fracture toughness of the crack was measured using experimental numerical methods and ABAQUS code.Through experiments and numerical studies,we have the conclusion:(1)The upper crack is dominated by shear stress at the beginning of the crack,and the tensile stress is dominated by the lower crack at the initiation of the crack.The DCCP specimen is suitable for simultaneous measurement of the initial fracture toughness of the pure tension and shear cracks;(2)Different loading drop weights have different loading rates,the crack initiation time of each specimen under five different loading rates is obtained by experiment.As the loading rate increases,the crack initiation time decreases,but the dynamic crack initiation toughness increases;(3)As the length of the lower crack increases,the dynamic crack initiation toughness of the lower crack gradually decreases,but the dynamic crack initiation toughness of the upper crack does not change much;(4)Under the same loading rate,the dynamic crack initiation toughness of the upper crack is about 0.5 times that of the lower crack.