4.2.2 Patch Size For patching, the parameter directly related to patch size is the overlap length (patch length). For double-sided laps, there is a critical lap length corresponding to the maximum available strength, typically 20 mm to 30 mm, and the use of a longer lap length does not increase the load carrying capacity. The requirement for optimal design is that the in-plane stiffness of the patch is consistent with that of the motherboard. Since the patch repair can be treated as half of a double-sided lap, in the case where the patch material and the mother board material have the same elastic modulus, the optimal patch thickness should be half the thickness of the mother board. The theoretical calculation results show that when the crack length is constant, increasing the width of the patch can improve the repair effect of the structure, but when the width of the patch increases to a certain value, the repair effect of the structure is improved little. On the other hand, there is an optimal value for the length of the patch, which minimizes the stress intensity factor at the crack tip. Increasing the thickness of the patch can improve the repair effect of the structure, but the maximum shear stress in the gel layer also increases. In order to avoid de-bonding, the thickness of the patch also has an upper limit value.
4.2.3 Patch Lamination In order to obtain the best adhesive repair effect, the fiber direction (main axis direction) of the composite patch should be consistent with the maximum force direction in the damage structure. Due to the limitations of the load direction and the many design constraints of laminates, there is usually no room for more choices for patch layups. The ratio of 0°, 45°, and 90° plies for fibers is generally around 30:55:15, and 0. The direction of the direction is the same as the direction of the main force.
5 Repair NDT and Test Verification
5.1 Repaired non-destructive testing The task of repair and inspection is to first select the appropriate non-destructive testing method, perform non-destructive testing on the in-service composite material structure and determine the damage location and damage size according to the specified procedures and cycles, and provide the composite materials repair technology for aerospace materials. Judgment basis; followed by non-destructive testing of the repaired quality of the damaged structure to assess the degree of damage repair and integrity. So far, domestic and foreign scholars have carried out some effective research work on nondestructive testing and interface bonding quality assessment. There are more than a dozen types of nondestructive testing methods proposed, but the methods that can be effectively used for composite material damage repair testing are mainly : Visual inspection, tapping, impedance method, resonance method, X-ray method, laser holography, infrared heat map method, ultrasonic method, acoustic emission method. According to reports in the literature, Chen Jinlong et al. introduced polarization phase shifting technology into dislocation speckles and established a test system with non-contact, high-precision, and full-field real-time observations to complete the bonding of the two-material interface. The study and quantitative analysis and processing were carried out to achieve quantitative non-destructive testing of bonding quality at the interface between two materials. Chi u et al. first put forward the concept of “smart structure†as a means to assess the in-service performance of the patch repaired on the mother structure. Y.L.Koh et al. applied smart material structure to non-destructive testing and used piezoelectric ceramics (PZT) as sensing/excitation elements. When PZT was stuck on the damage zone, real-time local detection was performed using impedance method to determine the damage. Appears; When remote sensing is required, the conversion function method is used to determine the extent of debonding in the repair. Combining the two methods, the damage in the repair and the injury extension in the mother can be detected
5.2 Verification of Repair Test Verification of the repair structure mainly includes performance verification, structural verification and component verification. Performance verification includes strength test, fatigue life test, durability test in damp heat environment and damage tolerance assessment test, etc. Whether the repaired structure meets the requirements of environmental adaptability and structural integrity.
6 patch patch repair research methods
The research methods for patch repairs mainly include: theoretical analysis and calculation and experimental research.
6.1 Theoretical Analysis and Calculations The purpose of the analysis and calculation is to give a basic assessment of the stiffness and strength reduction of the damage zone prior to the implementation of the repair, and to give the repaired stiffness and strength recovery conditions for the repair measures to be taken or determined. Estimates, thereby reducing the investment costs of test parts, shorten the repair cycle, and improve the repair design level. Theoretical analysis is particularly important for the problem of repairing damage in critical parts that cannot be verified experimentally. Theoretical analysis and calculation mainly use analytical methods and numerical methods.
6.1.1 Analysis method Erdogan et al. used the complex variable function method to analyze the stress distribution in the repaired structure and the stress intensity factor at the crack tip based on Musskhelishvilli's theory of plane elasticity. In this analytical model, the gel layer is considered as isotropic material and its elasto-plastic deformation is taken into account; both the crack plate and the patch are in the plane stress state, and the force between the two is achieved through the shear deformation of the glue layer. The shear forces of the gel layer on the cracked plate and the patch are evenly distributed in their respective thicknesses. The equations of the equation are established using the displacement coordination condition. The problem finally boils down to the solution of a Fredholm type system of the second type. Rose [18,19] et al. used the elastic inclusion theory of generalized plane stress to calculate the single dimension of the unilaterally repaired structure by analyzing the three dimensionless parameters of the stress concentration coefficient, the adhesive layer stress, and the crack tip stress component. The repair effect of the double-sided patching method was studied. The entire calculation process is divided into two stages: in the first stage of the analysis, it is assumed that the patch is glued to an undamaged metal plate and a rigid bonding assumption is assumed (no relative displacement between the metal plate and the composite patch) Calculate the stress value of the metal plate in the glued repair area according to the elastic inclusion theory; in the second stage analysis, introduce a crack in the metal plate, estimate the stress intensity factor value of the crack tip by an approximate method, and give The repair effect comparison, that is, double-sided patching is better than one-sided patching, because the out-of-plane bending in single-sided patching will lead to the decrease of patching efficiency and the increase of stress in patch and subbing layer. Thicker patches can effectively reduce the stress concentration of cracks under the patch. The analytical method has a short calculation time and low cost. It is very convenient when studying the influence of various parameter changes on the effect of adhesive repair. However, the parameters of patches are very limited; the calculation accuracy is low, and error analysis is difficult.
6.1.2 Numerical Methods Jones et al. used the planar finite element model to study the problem of composite patch repairs with cracked structures. Due to the limitation of the plane analysis model, the influence of bending deformation and structural asymmetry caused by single-sided glue repair on the results is ignored in the analysis model. Sun et al., based on the Mindlin plane theory, assumes that the displacement is linearly distributed along the thickness of the plate, so that the influence of the bending deformation in the single-sided cemented repair structure on the calculation result can be considered. Randolph A.Odi and Clifford M.Friend compare the three finite element models commonly used in the repair of composite materials, namely SIENER's 2D plane strain model, BAIR's quasi-3D composite shell element model and 3D block element model. The calculations give the corresponding analysis conclusions: Compared with the traditional two-dimensional model, the calculated results of the in-plane stresses of the mother plate, patch, and adhesive layer are in good agreement with the traditional two-dimensional model, but the reliability of the resulting shear stress of the adhesive layer is poor. SIENER's 2D plane strain model cannot give accurate layup stress because of its own cell quality; 3D model calculation is more accurate, but calculation time is longer. The research work in this area mainly focuses on two aspects: firstly, improve the analytical model of the adhesive layer to improve the calculation accuracy; secondly, calculate the residual thermal stress on the stress distribution, crack tip stress intensity factor and fatigue crack in the adhesive repaired structure. The effect of the expansion rate. The finite element analysis method has a wide range of applications. There is no limitation on the shape of the structure and the layering of the patch, and the calculation accuracy is high. It is commonly used in the analysis of the actual glued joint repair of aircraft structures. Some scholars have also conducted some research work on certain specific issues. For example, M.J.Davis and D.A. Bond have studied the failure modes and failure mechanisms of several common aircraft adhesive structures and adhesive repairs. Through anatomical stripping of many specimens, various real failure modes and failure mechanism descriptions are given, and it is also pointed out that these failure modes and destruction mechanisms are correct for the correct selection of adhesives, repair materials, repair parameters, repair processes, and test methods. The importance of J.W. Choi et al. performed strength studies on graphite/epoxy composites using three repair methods (single-sided pre-cured patch method, double-sided pre-cured patch method, and solid-state curing method) and obtained strength recovery rates. 60% to 80% of the conclusions of the non-notched pieces. The fatigue life of the repaired specimen was predicted by adopting the Hwang and Han equation (MFLPE1 modified fatigue life equation) based on the fatigue stiffness reduction model and the reference stiffness, and compared with the traditional S-N curve fatigue life equation.
6.2 Experimental studies Ratwani et al. studied in detail the effects of patch material, dimensions and layup design, and environmental conditions (temperature and relative humidity) on the fatigue life of cemented repair structures. Sandow and Cannon examined the thickness of the 2024-T3 aluminum alloy plate, the type of adhesive (K138 cured at 40°C and AF163 cured at 120°C), type of fatigue loading (equal spectrum and random spectra), and patch layout design The effect of structural fatigue life. Denney studied the effects of debonding location, degumming size and initial crack length, and maximum stress and stress ratio on the fatigue crack growth rate and structural fatigue life. Alawi and Saleh studied the shape and size of the patch, the adhesive repair method (single-sided or double-sided) and the surface quality of the test piece on the fatigue crack propagation rate, and considered that the fatigue crack growth rate in the adhesive repair structure changed. The root cause is the change of the material structure constant during the process of Paris crack propagation. Therefore, through the statistical analysis of the fatigue test data, the material constants in the Paris crack propagation formula for various typical repair conditions are fitted.
7 Conclusion <br>Composite patch repairs are important for the aerospace and composites industries. Although in the past 30 years, many scholars have done a lot of detailed and useful work on the patch repair of composite materials, there are still some difficulties and unsatisfactory areas in the patch repair of composite patches. It needs to be further studied. . (1) Since the thermal expansion coefficients of the composite material patch and the metal material differ greatly, the residual thermal stress and residual thermal strain exist in the repaired structure when the structure is cooled to room temperature after curing at a high temperature. In the actual adhesive repair, what kind of composite material is used is economical and can match the thermal expansion coefficient of the metal matrix material. (2) When a carbon/epoxy composite patch repairs an aluminum alloy structure, the electrochemical reaction between the two is prone to occur. What measures can be taken to effectively prevent it.
