Experimental Study On Composite Fracture Performance Of C40 Steel Fiber Reinforced Conrete

Concrete is a widely used material in today’s structures,but its high compressive strength,low tensile strength and poor toughness limit its use.Steel fiber reinforced concrete is a new type of multiphase composite material in which steel fibers with smaller diameters and shorter lengths are added.Steel fiber can greatly improve the toughness and tensile strength of concrete,and has a very broad application value and market prospect.However,the research on fracture mechanics of steel fiber reinforced concrete is not yet mature,and composite fractures are particularly rare.Therefore,it is very necessary to conduct theoretical research on composite fracture properties of steel fiber reinforced concrete beams.In order to study theⅠ/Ⅱcomposite fracture properties of steel fiber reinforced concrete,a total of 21 groups of beam specimens with a size of 400mm×100mm×100mm were fabricated and subjected to three-point bending fracture tests.The work performance test and mechanical performance test are carried out to explore the influence of steel fiber on conrete performance.The test control variables in this paper are the specimen seam height ratio:0.2,0.3,0.4;the fiber content volume rate:0,0.5%,1.0%,1.5%;the bearing offset distance:0,40mm,60mm,80mm.The resistance strain gauge method was used to measure the crack initiation load P_ini and the fracture load P_max of the specimen.Draw a load-strain curve to analyze the difference between the fracture process and ordinary concrete.The digital image correlation method（DICM）is used to analyze the crack propagation path,The fracture opening displacement（CMOD）,the fracture tip slip displacement（CTSD）,and the length of the fracture process zone（FPZ）are calculated by this method.By drawing the influence curve,observe the influence rule of the control variable on the above parameters.By using ABAQUS finite element modeling software,The shape factors Y_I,Y_Ⅱ,and T stress of the specimen are calculated,and the load values are substituted.The stress intensity factors K_Ⅰ and K_Ⅱ of the specimen are calculated,and the fracture angle of the specimen is measured,and the variation trend of the calculation parameters with the three control variables is studied.The generalized strain energy density factor criterion（GSED）was used to predict the fracture behavior of the test piece,and the experimental results were compared with the theoretical prediction results to analyze the relative error with each criterion without considering the T stress.The specific conclusions are as follows:（1）After the work performance test,the slump of ordinary concrete is 60mm,and the slump of 1.5%steel fiber concrete is 40mm,which reduces the slump by20mm,which meets the requirements of work performance.It was found that when1.5%steel fiber was added,the compressive strength of concrete increased by 15.7%,the splitting tensile strength increased by 30.9%,and the elastic modulus increased by2.2%.（2）Measured by the resistance strain gauge method,when 1.5%steel fiber was added,the crack initiation load P_ini and the fracture load P_max increased by 73.5%and50.2%respectively.Moreover,the fracture ductility of steel fiber reinforced concrete is larger than that of ordinary concrete,and the falling section of the fracture curve is longer.Experiments show that the crack propagation of steel fiber reinforced concrete has multiple cracks,and the cracks are interlaced,the failure mode is that the steel fibers slip out of the matrix.When the specimen with 0.5%fiber content was selected,when the seam height ratio increased from 0.2 to 0.4,and the P_ini and P_max highest decreased by 38.9%and 33.9%respectively.When offset by 80mm,and the P_ini and P_max maximum increase of 38.2%,39.2%.（3）The CMOD,CTSD and FPZ values of the specimen were calculated.With the increase of the slit height ratio,the values of CMOD and CTSD increased,and the value of FPZ decreased.With the increase of the eccentric distance of the support,the values of CTSD and FPZ will increase,and the CMOD will decrease.With the increase of fiber content,CMOD increased by 61.1%,CTSD increased by 309%,and FPZ value increased by 124%.Therefore,the maximum value of CMOD occurred on the specimen TPB-1.5-0.4-60,which was 0.083mm;FPZ and CTSD reached the maximum value of 48.06mm and 0.045mm at TPB-1.5-0.2-60 and TPB-1.5-0.4-60,respectively.（4）Calculated by ABAQUS software,the shape factors Y_I,Y_Ⅱ and T^*stress all increase with the increase of the fracture height ratio.When the seam height ratio is0.2-0.3,the Y_I growth is slower,and when the ratio is 0.3-0.4,the growth is faster.Y_Ⅱ,the T^*stress increases more uniformly.With the increase of the offset distance of the support,the decrease of Y_I is slower at 0-40mm,and the decrease is faster at 40-80mm.The growth of is Y_Ⅱ relatively uniform,and the specimen with a seam height ratio of 0.2 has the fastest growth.Also the T^*stress increases,but the T^*stress change is small,and the Y_I,Y_Ⅱ change is large.（5）The joint height ratio of 0.5%fiber content calculated by finite element is 0.4,and the type I fracture toughness of the specimen is 1.043MPa m^√2.The calculated values Bαof the specimens are all less than 0.375.The traditional MTS criterion calculated K_IC is consistent with the finite element combined theoretical calculation.When 1.5%steel fiber was added and the bearing was offset by 60mm,the K_Ⅰ at fracture was 33.76%higher than that of ordinary concrete,and the K_Ⅱ was 31.25%higher.It is calculated that the stress intensity factor of the specimen increases with the increase of the seam height ratio,and the maximum increase of K_Ⅰ is 30.98%,and the maximum increase of K_Ⅱ is 70.58%.（6）When Me is 0.7,the relative errors between the fracture angles predicted by GSED,SED,and MTS criteria and the experimental values are 3.5%,25%,and 46%;when Me is 0.9,the relative errors are 2.1%,28%,and 51%,respectively.When the experimental value is(K_Ⅱ K_Ⅰ=0.407),according to the theoretical curve,the relative errors predicted by the GSED,SED,and MTS criteria are 2.8%,16.1%,and26.7%;when(K_Ⅱ K_Ⅰ=0.26),the relative errors are 2.3%,19.8%,and 44.8%,respectively.To sum up,the criterion considering T stress is much more precise than not considering it,and T stress has an important influence on the fracture of steel fiber reinforced concrete composites.