Research On Detecting The Effective Prestress Of Prestressed Strand By Using Blind-hole Method

The blind hole method is widely used in the r esidual stress measurement of metal components due to its advantages such as simple operation,high measurement accuracy,and small damage to the measured component.At present,the application and research of this method at home and abroad have been quite mature.Based on this,this paper intends to use blind hole method to measure the effect ive prestress of steel strands.It mainly contains the following work:First ly,this paper presents the stress release formula for an infinite slab wit h a through hole under ideal condit ions.Based on this,for the case of non-unifor m distribut ion of residual stress,the method of layer-by-layer drilling using the integral method is proposed.And then it summarizes the correct ion methods under the two kinds of errors: dr illing eccentricit y and plastic deformat ion at the edge;On the other hand,the finite element software ABAQUS was used to establish the monolithic prestressed hollow slab beam model,and the effect of excavating concrete on the existing prestress of the p restressed concrete hollo w slab was explored.Secondly,this paper establishes a linear-parabolic elasto-plastic constitutive model of steel strand material,and uses the finite element software ABAQUS to establish two models of single steel wire and steel strand.And then it explores the reasonable length of the model.On this basis,in the single steel wire model,the investigat ion of borehole release stress under elast ic and elastoplast ic constitut ion is carried out.It is found that under elastoplast ic const itution,the stress relief value of borehole will be larger than that under elastic constitutive structure.In addition,based on the aforesaid theoretical aspects,the parameters affect ing the borehole stress release—drilling depth,borehole diameter,tensile load and elast ic modulus—were analyzed in this paper.After analysis,it can be concluded that increasing the drilling depth after the depth ratio of the borehole reaches a certain value is no longer effect ive for stress relief of the drill hole.This value is 1.33 in the single wire model and 1.67 in the strand model.Under elast ic const itution,the stress relief value of borehole has a linear relat ionship with tensile load,and has nothing to do with the elast ic modulus.Finally,nine groups of bare steel strand drilling tests were designed and carried out to explore t he hole stress release values at different borehole depths,different borehole diameters,different tensile loads,and the influence of drilling to non-drilled boreholes around the strand.The experimental results obtained the same rules as the theoretical analysis,and verified the relat ionship found in the theoretical analysis part between the stress relief value of the steel strand edge and the parameters such as drilling radius a nd drilling depth.In order to solve the problem that the hole margins cannot be fixed when the strain gauges are patched,a cont iguous strain gauge with a fixed hole margin is designed.Based on the experimentally derived rule,six different sets of tensi le load steel strand drilling tests are performed.Based on these six sets of experimental data,the relatio n between the tensio n load and the release strain convergence value was fitted to provide a calculat ion method for the effective prestress testing o f steel strands based on the blind hole method.