Prediction Of Delamination Threshold Load For Composite Laminate Subjected To Low Velocity Impact Based On Test Samples

There are two types of models of delamination threshold load (DTL):one is based on model II energy release rate of composite material, theother is based on interlaminar shear strength (ILSS) of composite material.The first model indicates DTL shows linear relationship with3/2power ofthickness of plate. The second model indicates the square of DTL showslinear relationship with the cube of thickness and the radius of impactor.Direct use of these two models when we predict the DTL in impactexperiment of thin plates is not accurate, because the above calculationdoesn’t consider the deflection of plates. If plates are thin, greaterdeflection will induce the “wrapping” effect on the tup, resulting insignificantly increasing contact area and decreasing the shear stress, so theaccuracy of prediction will be greatly influenced. It’s better to modify thecontact radius and improve the prediction model based on ILSS. Throughdifferent geometry equations between plate and impactor, two modifiedmethods which consider the deflection of thin plates are deduced toimprove the accuracy of prediction of DTL.Contrasting the numerical simulation models with theoreticalcalculation models of two modified methods, the increase of contact radiuswhich considers the deflection of plate and the shear stress in differentcontact radius from composite half-space and thin plate are verified. Theresults show they are in great agreement and the modified methods can beused to improve the accuracy.In order to study the factors affecting the increase of contact radius,there are six factors studied by response surface methodology (RSM) and least square method (LSM): impact force P, equivalent elasticity modulusE, bending stiffness of plate D, radius of plate b，radius of impactor Rand ply number. The increase of contact radius increases linearlyproportional to increase of radius of impactor and impact force andnonlinearly proportional to other factors. Increase of contact radius hasnegative correlation with bending stiffness and positive correlation withother factors. Impact force and plate bending stiffness have the largestinfluence on increase of contact radius. Then Taylor approximatecomputation method is used to fit the explicit function of increase of radiusbetween bending stiffness and impact force.Finally an impact experiment on one carbon-fiber composite wingbox structures conducted to verify the modified method model. The resultsshow high prediction accuracy can be got by using these modified models.Multi-zone prediction method is better than unmodified methods, but notbetter than modified methods. Additionally the accuracy of multi-zonemethod is unstable and only useful in the intermediate regions. To thediversified problem of experimental data, RSM is re-used to calculate themargin of error which is in line with the experimental data. The mostinfluenced factors of dispersity of DTL are flexural stiffness and platethickness, and the angle of inclination takes the second place.