On The Performance Of In-service Composite Structures Repaired Through In-situ Scarf Method

Composite materials have been increasingly used in aerospace and other fields due to their excellent properties.However,the high manufacturing cost,low impact resistance and nondegradation of composites limit their further applications.In addition,the repair cost of aircraft occupies a large proportion of their lifetime costs.Repair of composite structures which can impove the service life of components and reduce the cost becomes very important.The scarf bonded repair is a common and efficiency method.However,most of the present works are focusing on the in-plane load capacity and failure modes of scarf repaired structures.Rare works are focusing on the effect of environment condition and fatigue behavior.And the limitations of repair conditions and structure configurations have not been considered.In terms of in-service components,it is difficult to be disassembled to smaller parts and repaired in an autoclave.Thus the repair process has to be undertaken in-situ on aircraft structures using vacuum bag cure.However,for autoclave cured composite materials,vacuum bagging is difficult to achieve low-void laminates of good quality,which harms the mechanical properties of composites.So it is of significance to carry out research on the in-situ scarf repaired technology of composite structures and evaluate their performances comprehensively.In order to solve the above problems,a new approach was tried in this paper to perform in-situ scarf repair of autoclave cured composite material(5228A/CCF300)by using an alternative material(TC350-1/IM7-12K)which has better properties in vacuum bagging for the patch.Considering the service safety of aircraft,the performances and damage mechanism of in-situ scarf repaired composite structures were evaluated from various aspects,namely the in-plane load capacity,reliability,effect of hygrothermal condition,impact responses and fatigue behavior.The main works are described as follows:(1)Firstly,the feasibility of selecting TC350-1/IM7-12 K for patch was analyzed.Then,four groups of scarf repaired composites with various laminate thicknesses were fabricated and tested to elucidate the repair efficiency and failure modes.Experimental results indicate that the failure strengths of different groups are similar and the dominated failure mode is cohesive failure of adhesive,accompanied by partial 45° and 90° matrix cracks of composite patch.And a finite element model(FEM)based on continuum damage model(CDM)and cohesive zone model(CZM)was established to predict the failure strength and explain the damage mechanism.The numerical results show good agreement with test results and indicate that matrix cracks of composites initiate before the adhesive failure.Finally,based on the validated model,the effects of overlap patch and 3D defects on the ultimate strength were discussed.(2)A characteristic feature of adhesive-repaired structures is that large variations in joint strength occur due to surface treatment and curing conditions.So a reliability analysis process of in-situ scarf repaired composite structures was established by combined the Kriging surrogate model with FEM using MATLAB software programming.The reliability index of static strength for scarf repaired compoistes was calculated.And the global sensitivity analysis method based on variance was adopted to determine the influences of different adhesive parameters on the load capacity of repaired structures.The importance order of the above parameters was given to help structure design.(3)The moisture uptake process and residual strength of scarf repaired composites and bulk adhesive were experimentally studied.It is shown that moisture uptake process of bulk adhesive specimen and scarf repaired specimen can be described by Fickian model.And the properites of bulk adhesive decrease more seriously after the same aging time.In addition,there is a linear relation between the strength reduction and moisture contents for repaired composites.Due to the plasticization of adhesive caused by water vapor,the failure strength of aged specimens decreases.By introducing a degradation factor which is related to moisture absorption,a degraded FEM considering the influence of hygrothermal aging was established.The predicted strength and failure modes achieve good agreement with experimental results.(4)The impact responses and tensile behavior after impact of in-situ scarf repaired composite structures were studied under different impact energies.Experimental results indicate that the impact energy can be devived in two stages by 10 J.The impact load-time curve was smooth and the tensile strength after impact did not decrease before 10 J.When the impact energies are larger than 10 J,a sudden drop of impact load was observed and the tensile strength after impact decreased linearly with the increasd impact energy.At the same time,a impacttensile numerical model of scarf repaired composite structure was used to analyze the evolution process of impact and tensile mechanism after impact.Numercial results show that the impact damge sequence is that delamination occurs firstly,followed by matrix cracks,and adhesive failure happens in the end.In addition,adhesive failure is the main reason for the decrease of tensile strength after impact.(5)Experimental works were conducted to investigate the fatigue limit and fatigue mechanism of in-situ scarf repaired composites.It is found that the fatigue resistance of repaired composites is significantly lower than that of intact composites.The crack initiation stage occupies a large proportion of fatigue life,about 93±3 percent of total life based on the stiffness change and crack observation.Then,according to the fatigue mechanism,fatigue theory based on continuum damage mechanics was utilized to predict the fatigue life of repaired structures.At last,the safety fatigue life was evaluated using single side allowance factor in statistical method.