| In recent years,housing construction has been moving towards multi-layered construction and the structural layout has been diversified.This puts forward higher requirements on the structural load-bearing model.Beams and columns have an important influence on the strength of the building.The beam-column node connects the beam and the column together as a whole.At present,studies have been conducted on the pseudo-static loading of beam-column joints.However,it may also be subjected to dynamic loads during service.Under dynamic loading,not only the influence of strength is considered,but also inertial forces and the influence of material properties must be taken into consideration.In actual,most structures are damaged,and the dynamic response of damaged structures deserves more attention.Therefore,it is necessary to study the dynamic fracture of beam-column specimens with prefabricated cracks.According to the different node forms,in the case of different positions,number of cracks and boundary conditions,theoretical analysis,numerical simulation and laboratory experiments were conducted to investigate the dynamic fracture characteristics of the joints.The results provide guidance for the actual project.?1?Using MTS multi-functional testing machine,combined with transmissive optical path and high-speed camera,the fracture mechanic property of T shaped and portal specimens with prefabrated cracks under pseudo-static loading was obtained.The T shaped beam-column specimens with prefabricated cracks at the column end have the minimum bearing capacity;for specimen with cracks located at beam and column end,crack at beam ends does not initiation,while crack at column end initiation.The bearing capacity of specimens is not much different with crack prefabricated only at beam end.The crack trajectory change is not obvious.The oscillating change of velocity is less than dynamic failure.For the specimens with cracks located at column end,when the crack begin to curve,the velocity gradually decreases to zero.For the portal type specimens,the crack at the bottom of the beam exerts a"pressure relief"effect,cracks at the bottom initiate first and the extended cracks do not penetrate the specimen.For specimens with prefabricated cracks at the ends of the beam and column,the crack at beam end or column end initiatiates after the beam bottom crack intiation.When the crack begin to curve,the crack will arrest soon.The load-displacement curve appears three times of loading-unloading process,and the slope of the loading process curve decreases,indicating that after the crack initiation,the bearing capacity of the structure is gradually reduced.The maximum bearing capacity of specimens with prefabracaied crack at the column end is 1.9kN,which is36%higher than that of specimens with crack at the beam end.The maximum bearing capacity of specimens without prefabricated crack at the bottom of the beam is significantly higher than specimens with prefabricated cracks at the bottom of the beam;the unloading process of specimens with oblique cracks at the beam ends shows?plasticity?characteristics,and the fracture surface is rough and the bearing capacity is maximum.Specimens with oblique cracks have two loading and unloading processes;Specimens with straight cracks at beam and column end exhibit"plastic"characteristics during the second unloading process.The crack propagation trajectory satisfies the quadratic polynomial fitting curve,the effect of shear force on specimens with straight cracks is smaller.?2?Using transmissive digital laser caustic system,impact fracture tests were performed on different types of flaws,such as prefabricated single cracks,single-bias cracks,and multiple cracks,and the interaction between stress waves and beam internal defects and the propagation principles of motion cracks were revealed.When the internal crack located at the axis of symmetry,the lower end of the crack first initiates then the upper end of the crack initiates.The unloaded stress wave is excited through the lower end of the beam crack,which increases the stress intensity factor at the upper end and accelerates the initiation of the upper crack.With the increase of the distance from the crack to the lower boundary of the specimen,the difference between the upper and lower end of the crack gradually decreases,and the maximum value of the stress intensity factor at the upper end gradually increases.For the same offset distance,the specimens with large distance from the crack to the lower boundary have larger crack initiation angles at the upper and lower ends of the crack;When the distance to the lower beam boundary is the same,the larger of the offset distance,the larger of crack initiation angle when the crack initiates.The experimental results are basically consistent with the theoretical calculations.For specimens with the same internal crack offset distance,the upper crack propagation speed of the specimen is large,and the lower crack propagation speed is small."Empty Hole"can significantly increase the secondary fracture toughness of the specimen,the increase of magnitude more than 100%,the crack propagation speed increases significantly after the secondary initiation;The specimen containing an asymmetrical longitudinal crack tends to start initiating,the end closer to the impact loading head initiates and has a lower fracture toughness;The specimens with symmetric cracks initiate at both ends,and the secondary fracture toughness is slightly less than the first.When there are two cracks in the specimen,the cracks at the axis of symmetry begin to initiate;both cracks are biased,the cracks are all initiates,and the lower end of the crack runs through the specimen to promote the expansion of the upper crack.The extended trajectory of the lower end of the main crack will be biased towards the lower end of the secondary crack.The crack on the upper end of the secondary crack will be biased towards the new surface formed by the upper end of the main crack.?3?Using the transmissive digital laser caustic system,impact test was performed on a prefabricated crack with different positions and inclination angles at the beam end and cantilever beam-column specimen with different pretilt cracks at the end of the column,the dynamic fracture mechanics parameters such as the prefabracated crack propagation trajectory,crack propagation velocity,and crack tip stress intensity factor were obtained.For cantilever beam specimen with a single vertical edge crack,the tensile stress plays a major role in the crack propagation process,and the fracture mode is type I;the fracture mode with an oblique crack specimen is I-II composite type.The beam end oblique cracks increase the oscillation of the velocity and acceleration during crack propagation.The straight crack energy accumulation process is short,and the fracture toughness is reached before the cantilever beam component with a single inclined edge crack and the expansion path is relatively straight.The distance between the prefabricated cracks and the core area of the node increases,the crack initiation time shortens,and the fracture mode gradually changes from tensile fracture to gradient tensile fracture.The horizontal distance between the crack breakthrough point and the prefabricated crack gradually increases,and the degree of cracking increases.The average velocity decreases,the time needed from initiation to penetration increases.The KId value decreases gradually as the specimens initiaties,and the KdII value gradually increases.The average expansion toughness of the specimen gradually decreases.When the distance from the crack to the core zone is 32mm,the composite fracture feature is the most obvious.For structures containing micro-crack near the core area of the node,reinforcement measures should be taken on the extended path to suppress the propagation of dynamic cracks.The fracture modes of specimens with prefabricated crack at different ends of the column are all I-II composite.With the increase of the crack angle,the slower the accumulation of energy at the crack tip,the longer the crack initiation time and the greater the degree of crack propagation.Slant cracks have a greater influence on the speed.The increase of the prefabricated crack angle results in an increase in the maximum and average expansion speed.The inclined crack causes the KdIC to decrease,the KdIIC increases,and the shear stress proportion increases when the specimen starts to initiate.?4?Using the dynamic caustic method,the effects of different types of T-beam-column support conditions,prefabricated crack position and number change on the crack initiation toughness,propagation velocity and crack tip stress intensity factor were studied.The stress field distribution characteristics of T-beam-column specimens obtained by using the hyperdynamic strain gauge and CEEMD waveform denoising method.Under the effect of stress wave,cracks on the beam end near the loading head are prone to initiate.Under the effect of the reflected tensile wave,stress concentration occurs at the intersection of the beam and the column,and the crack propagates.When the one free end of beam is restrained,cracks at the end of the column are hard to initiate,and cracks stop propagating immediately after the cracks initiate.The two cracks start cracking apart from the beam end on the side of the loading head and the column end.The initiation of cracks in the beam end promotes the propagation of column cracks,and the speed of the two changes alternately.The crack propagation path of the beam is biased toward the fracture surface formed by the column crack.Constraint is imposed on one side of the beam,and specimens with different numbers of cracks exhibit different fracture characteristics.For the specimens with prefabricated crack at the end of the column and two beam ends,the cracks of the beam close to the loading point initiate;only after the specimen containing the crack at the beam end breaks,the oscillation deformation of the specimen due to the impact causes the formation of the beam-column intersection.The cracks are expanded and appear in a"C"shape in the core area.In the T-type test piece with internal cracks,the end where the stress concentration is large starts to initiate and after the expansion penetrates,the other end of the crack starts to crack.Under the effect of unloading the stress wave,the stress intensity factor at the crack initiation point of the post-cracking stage is greatly increased,and the maximum increase rate is 166%.For specimens with two internal cracks,cracks that started after cracking have a large influence on the speed and the extension trajectory of the previously expanded crack.The change of the boundary condition of the lower column has a great influence on the pre-existing crack propagation trajectory of the inverted T-shaped test piece.When the upper end is fixed and the lower end is hinged,the cracks in the upper column begin to crack and expand,cracks occur in the core area of the beam-column,and the expansion speed is small;when the upper and lower ends are fixed,the column cracks do not start initiating,and stresses concentration form at the intersections of the beams and columns,specimen fracture.The smaller the speed oscillation changes when the crack propagates.The change of the upper boundary condition affects the fracture of the specimen with cracks in the lower column.When the upper end is not constrained,the specimen containing the column crack starts to crack at the column crack and the intersection of the beam and the column.The number of cracks in inverted T-shaped specimens varies and the fracture morphology of specimens varies.The beam ends contain cracks in the specimen,cracks start at the beam end;the beam ends without cracks,and cracks extend at the intersection of the beam and the column.With the increase of the number of cracks,the structural instability increases and the speed oscillation changes during the structural failure.For specimens containing multiple cracks,the stress concentration is formed at each crack tip.As the beam ends or the intersection of the beam and the column initiate,the stress concentration at the other crack tip disappears.The dynamic stress field distribution of specimens containing prefabricated cracks at the end of the column has the following rules:In the horizontal direction,the point about the neutral axis of the beam is symmetrical and the tension-compression strain amplitudes are equal;the upper side of the beam is pulled,and the middle of the beam strain>In-node strain;Under the beam,as the distance to the core decreases,the strain changes from pull to pressure.In the vertical direction,the strain gauge under the loading head has obvious change characteristics,which can reflect the variation of the stress wave.The distribution of the stress field of the specimen with two pre-existing cracks is as follows:in the horizontal direction,the two sides of the neutral axis are symmetrically pulled and the other side is under pressure.As the distance from the core area of the beam-column decreases,the difference between the two strain gauges gradually decreases.In the vertical direction,the strain gauge is pulled under the loading head and the pressure characteristics are obvious.The stress wave is the most significant effect.The direction of force in the middle of the beam is opposite.The strain amplitudes are basically the same.The tension in one side of the core area of the node is affected,and the other side is affected by the expanding crack and is in the tension-pressure change stress field.?5?The impact fracture mechanics characteristics of the symmetrical beam-column specimen were obtained by the experimental method of dynamic caustics.The super dynamic strain gauge was used to measure the dynamic strain of the portal specimen and H-type specimen with pre-crack at the bottom of the beam.The distribution of stress field of fracture was obtained.Under the action of the impact stress wave,the cracks at the bottom of the beam start to crack at the portal specimens;then cracks at the ends of the beam and column initiate.Both the beam end and the column end contain cracked specimens,the cracks at the end of the column form stress concentration,but does not initiate.With the increase of the number of prefabricated cracks,the average velocity of crack propagation at the beam end and at the end of the column gradually decreases,and the velocity oscillation increases.For gate spencimen which does not contain prefabricated crack at beam bottom,the crack initiation time increases significantly.Only specimens with pre-formed cracks at the beam end or at the end of the column begin to crack after the point of intersection of the beam-column initiates.After full expansion,the beam end or column end crack propagates.Beam end and the column end contains cracks,the column end crack first initiates,when the crack extension to the middle of the column,the crack arrested.Then the intersection of beam and column crack initiates,after the expansion,the cracks in the column continue to expand.And the beam end cracks also begin to expand.The beam crack propagation toughness is greater than the column cracks.In the case of gate-type specimens with internal cracks,when the internal crack is located at the beam end,the beam-column first cracks at the intersection,and when it extends to the upper beam boundary,the upper crack of the beam cracks.After 30?s cracks penetrate the test piece,the excited?unloading?stress wave increases the stress intensity factor at the lower end of the crack,and the crack initiation toughness at the lower end of the crack is 1.85MN/m3/2.For the column contains cracks,the cracks in the column did not start cracking.Two symmetrical cracks were formed at the intersection of the beam and the column.For the H-beam-column specimens with prefabricated cracks at the bottom of the beam,the crack at the beam bottom begin to crack first,then the crack at beam end begin to initiate;for specimens with cracks at the end of the column,cracks at the intersections of beams and columns prior to initiate;for specimens with cracks at beam end and column end,after the cracks at the bottom of the beam begin to crack,the cracks on the beam end initiate and expand.The cracks at the end of the column finally start to initiate.The beam-end expansion cracks are subjected to compressive stress at the intersection of the beam and the column,then cracks stop propagating.During this time,the stress intensity factor at the end of the column was significantly increased by33%.The crack initiation time is closely related to the number of cracks,and the structure of many cracks is easily destroyed and the stability is weak.The number and position of cracks have a great influence on the dynamic fracture of H-type specimens with cracks in the upper column.For the specimens with cracks at bottom of the beam and the upper column,only crack at the bottom of the beam initiates.For the specimen with cracks at lower column,due to the expansion of the cracks in the lower column,the stability of the structure is reduced,and cracks in the upper column expand in the core area of the beam-column.For the specimens with cracks at the beam end,the upper column and the lower column,the cracks at the beam end and the lower column initiates,and the average crack propagation speed is small.The speed of the cracks extending to the joints of the beam and the column gradually decreases,and cracks stop propagating.After the secondary crack initiation,the propagation speed is smaller than the first.The dynamic fracture characteristics of H-type specimens with pre-crack only at the nodes were studied.During the expansion of specimens with beam end cracks,the cracks gradually diverge,the stress intensity factor at the main crack tip weakens,the main crack propagation speed decreases,and the secondary cracks disappear after a period of time.For specimens with column end cracks,the cracks at the end of the column are not initiated,and the cracks at the intersection point of the beam and column initiate cracking.They expand in the core zone and penetrate through the lower intersection point of the beam and column.The bifurcation cracking results in both the primary and secondary crack propagation speeds decreasing.The failure mechanism of the specimen containing internal cracks is different from that of specimens with edge cracks.Under the action of stress waves,the specimens with edge crack only have one crack tip.The cracks are located in the interior of the specimen with two crack tips,the stress wave interacts with both crack tips simultaneously,resulting in stress concentration.This dispersion of energy causes a longer time for crack initiation of specimens containing internal cracks.The two crack tips are easily cracked near the boundary,and the stress concentration is relatively large.The first crack tip can promote the later crack tip expansion.The super dynamic strain test was performed on a portal specimen with prefabricated crack at the bottom of the beam.The specimen strain at the end of the column containing prefabricated cracks has the following rules:the strain at the bottom of the beam reaches a maximum of 1320??at the initiation of cracking;the strain at the upper and lower sides of the beam decreases gradually as the distance to the loading point increases,and the stress distribution in the core region is symmetrical.The dynamic stress field in the horizontal direction of specimens with prefabricated cracks at the beam end has the following distribution law:the crack at the bottom of the beam is under tensile stress,and it is under compressive stress near the loading head.Due to the influence of the reflected wave,it gradually pulls;the beam near the end of the crack is subject to pressure after pulling.The stress field distribution of H-beam specimens with prefabricated cracks at the bottom of the beam has the following rules:In the horizontal direction,the tensile stress near the bottom crack of the beam is strong,the compressive stress at the lower point of the beam is larger,and the stress in the core region of the node is symmetrical.As the cracks at the bottom of the beam expand,the compressive strain near the load head gradually changes to tensile strain,and the crack at the beam end gradually changes from tensile to compressive.In the vertical direction,there is a strong compressive stress near the loading point,and the initial crack loading at the bottom of the beam is compressed,and then gradually pulled.When the beam ends contain prefabricated cracks,the strain changes in the inner and outer sides of the upper column are consistent,and the lateral strain value is slightly larger than the inner side;the inner and outer forces of the lower column are opposite,and the pressure strain amplitude inside the column is large. |
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