Residual Stresses Of GFRP/Al-Li Laminates After Shot-peening Forming And During Alternating Thermal Field

Fiber metal laminates(FMLs)have been treated as the important structural materials for large aircraft skin owing to excellent fatigue properties and impact resistance.However,FMLs have complicated interfaces and suffer difficulty in forming due to the small failure strain of fiber layers,which limits their application.Aming at Chinese large aircraft project,our work mainly focused on residual stresses of the glass fiber/aluminum-lithium alloy laminates(GFRP/Al-Li laminates)under single shot and multi shots to establish the quantitative relationship between shot peening parameters and deformation curvature.Meanwhile,residual stresses characteristics and relaxation behaviors of shot-peened GFRP/Al-Li laminates under variable thermal field were explored to verify their adaptability to aircraft service environment.Our work provides theoretical basis for solving the shotpeening forming of fiber metal laminates(FMLs).Firstly,the residual stresses of GFRP/Al-Li laminates after single shot's indentation and impact were investigated by numerical simulation and experiments.The effect factors,including shot size,pressing depth and curing residual stress(residual stress of GFRP/Al-Li laminates after curing),on the residual stresses of single shot's indentation of GFRP/Al-Li laminates were revealed.The failure behavior and corresponding mechanism were analyzed and the accuracy of the numerical model prediction was also verified.Results indicated that the fiber layers significantly affected the local plastic deformation behavior under single shot indentation.The local plastic deformation of unidirectional GFRP/Al-Li laminates along the fiber direction was small and the craters were elliptical when the shot size exceeded 2 mm.Increased shot depth enlarged the residual stresses around the crater and excessive residual stress caused the failure of GFRP/Al-Li laminates.The depth of failure(the pressing depth of shot when failure occurred)increased first and then decreased with the shot size.Shot size dominated the failure behavior,including the possible fiber breakage or delamination,occurred at the depth of failure.Curing residual stress increased the residual stresses values of single shot's indentation on GFRP/Al-Li laminates and intensified their failures.Residual stresses after single shot's impact increased with the velocity of the shots and the velocity of failure(the velocity when failure occurred)decreased with the shot size.Residual stresses of single shot's impact were close to that of single shot's indentation when their pressing depths were similar,which verified the feasibility of equivalent replacement for them.Secondly,the uniform four-shots model of GFRP/Al-Li laminates was established to explore the residual stress field under multi-shots.The calculation method for the curvature prediction of shotpeened GFRP/Al-Li laminate was proposed through the model based on the idea of equivalent simulation.The equivalent bending moment(analytical calculation and numerical simulation)and equivalent stiffness of shot-peened GFRP/Al-Li laminates were respectively solved,which established the quantitative relationship between the shot peening parameters and the deformation curvature.It was found the error between the analytical and the test results was about 45% since the analytical calculation did not consider the curing residual stress of the GFRP/Al-Li laminates and weakened the interaction effects among the shots.However,the maximum error between the numerical simulation and the test results was-13.98%,while the minimum was only 5.21%.Furthermore,the equivalent simulation method was used to investigate the effect of curing residual stress on the residual stresses of shot-peened GFRP/Al-Li laminates.Curing residual stress increased the average residual stresses of shot-peened GFRP/Al-Li laminates and increased their deformation curvature.Finally,the high temperature test,thermal shock test and thermal fatigue test were applied to investigate the residual stress variation and relaxation behaviors of shot-peened GFRP/Al-Li laminates under the alternating thermal field.Results indicated that the curvature of the shot-peened GFRP/Al-Li laminates increased slightly with the temperature and residual stress field increased the degree of their deformation.The shot-peened GFRP/Al-Li laminates hardly deformed after 1100 thermal shock cycles(short high and low temperature residence time)from 0°C to 100°C,which revealed that residual stress field was stable.The initial residual stress relaxation(first 300 thermal fatigue cycles from-55°C to 100°C)of shot-peened GFRP/Al-Li laminates was not obvious.The residual stress relaxation was significant after 1100 thermal fatigue cycles(large temperature range,long high and low temperature residence time),while the overall degree of deformation satisfied the actual engineering requirements.Meanwhile,the temperature variation range,high and low temperature residence time and initial residual stress field of shot-peened GFRP/Al-Li laminates were positively related to their residual stress relaxation behaviors.