Based On Strain Research Of Evaluating Repaired Structural Performance Of Composite Material

Carbon fiber composite laminates have many advantages,such as high strength,high rigidity,light weight and corrosion resistance,and are widely used in aerospace,wind power,high speed train,ship and automobile industries.However,no matter how superior the material properties is,it will inevitably produce a variety of damage during the period of manufacture,transportation and service process,such as cracks due to fatigue,stratification or penetrating damage caused by birds or hail impact,resulting in laminated plates comprehensive performance decline.In order to avoid the high cost of direct replacement of materials,repairing the damaged composite laminates has important engineering application value.The detection and evaluation of the recovery state of the repaired laminated plates have been the focus of the research.However,the most studied methods are the destructive static tensile tests,compression tests and bending tests to obtain the ultimate strength of the restored structure.This destructive assessment method can not meet the requirements of on-site repair or evaluation of the actual application components.Based on the application background of the aerospace engineering,this paper mainly studies the bonding repair technology of composite laminates,and builds a dynamic piezoelectric non-destructive testing system based on strain modal and non-destructive testing based on static strain.The performance of the composite laminate was evaluated systematically under damage and repair conditions.The main contents of this paper are as follows:(1)The macroscopic mechanical properties of composite laminates are studied,and the constitutive equations of stress and strain of the laminated plates are deduced according to the stress-strain relationship of the single layer of laminated plates,which can provide the theoretical basis for bending test.The advantages and disadvantages of both the natural frequency and the strain modal are also theoretically analyzed.Both the natural frequency method and the strain modal method can detect the state of damage or repair.the former can not locate damage and repaired position.And the latter can not only locate,but also can conduct quantitative analysis of damage and repair,to provide theoretical guidance for piezoelectric testing experiments.(2)The experimental study of bonding repair technology of composite laminates.The effects of the repairing parameters such as the surface treatment of the damaged holes,resin adhesive proportion,the thickness of the adhesive layer,the diameter and the thickness of the patch,and the temperature curing curve on the repairing effect were studied.The repairing experiment will be finished according to the reasonable repair parameters,and the repaired laminates are prepared for subsequent testing.(3)A dynamic piezoelectric detection system of carbon fiber laminates with strain modal parameters was established.The dynamic piezoelectric strain signals of the laminated plates were collected by PVDF piezoelectric plates.After the original signals are decoupled,filtered,and wavelet transform processed,the amplitude and frequency signals of the laminates will be found.The amplitude of each order frequency could be found and processed by cubic spline interpolation to get the first-order,third-order and fourth-order strain modal curves of the damaged and repaired structure.The changes of local strain rate and strain modal of repaired laminated plates under different factors of patch diameters,single and double-sided repair methods and damage pore size were analyzed.Then the performance of repaired laminate could be evaluated.(4)The non-destructive cantilever bending test method is used to measure the static strain of laminated plates under different bending moments,we can evaluate the performance of repaired laminated plates according to the change of strain.Relative damage rate is taken as the performance evaluation parameters.In the case of a small damage aperture,single-sided repaired structure can meet the performance requirements.Even if the double-sided repair of laminated panels can not meet the performance requirements in the case of a big damage aperture,so it is no repair value.Increasing the diameter of the patch can reduce the average relative damage rate and improve the recovery effect of the repaired structure,but as the diameter of the patch increases,the extent average relative damage rate decreases and the efficiency of performance recovery deteriorates.