Surface Strain Characteristics Of Composite Laminate With Impact Damage

The carbon fiber reinforced composite has been widely used in manyimportant engineering structure owing its higher specific strength andstiffness as well as the designability. Composite structures often suffersome low velocity impact in manufacture and service, but compositelaminate is highly sensitive to impact damage, it will have matrix cracking,delamination, fiber breakage etc. inner microscopic damage. Thesedamages will lead dramatic reduction of structural strength even they areonly barely visible on the surface and caused by low energy impact.Different damages produce different strength reduction at damage region:compression strength reduced by matrix damage and delamination;tension strength reduced by fiber damage. Thus, to a certain loadingsituation, there will be an obvious strain singularity in surface of thelaminate with damage compared with that without damage. It is a greatsignificance to composite structural health monitoring if it could find a lawand mechanism between strain singularity and damage, and then thedamage level and location may be determined with the help of full fieldstrain monitor technique.This paper studies the surface strain characteristic of damagecomposite laminate under different loading situation through finiteelement numerical simulation and full field strain measurement based ondigital image correlation (DIC) technique. This paper contains four mainstudies as below:1. A finite element (FE) model of carbon fiber composite laminate isdeveloped with ABAQUS，in which the cohesive element is employed tosimulate delaminations and the Hashin criterion to determine fiber andmatrix failure in the composite. In order to study the characteristic ofsurface strain,“Quasi-static” indentation numerical simulation isconducted at first, then the FE laminate model with damage is imported in the software though “data transmission” function. The surface strainresponses under tension，compression and shear load are obtained, whichshow that: the laminate with a certain damage may has a singular strainfield under limited load; singular strain field is caused by local bucklingunder compression load, and the area of singular region is determined bythat of delamination; the singular strain field is produced by the fiberfailure under tension and shear load, and the area of singular region isdetermined by that of fiber failure.2. Based on the composite beam mechanics theory, axis stiffness andbending stiffness formula of asymmetry laminate is deduced, which can beused to calculate load of delaminated sub-laminate and sub-laminate. It isconsidered that the critical buckling force is determined by the maximumbending stiffness of the ply in delaminated sub-laminate, differentdelaminated sub-laminate with same maximum bending stiffness plyshould have same critical buckling force. Knowing the critical bucklingforce of one delaminated laminate, the buckling strain of sub-laminatewith same maximum bending stiffness can be predicted. The finiteelement (FE) models of composite laminate with delamination of differentarea, depth and location are developed with ABAQUS, which is employedto verify the deduction above. The analytical and the numerical resultsindicate that: prediction results have a good agreement with results fromnumerical simulation, and can be employed to create the reference samplesfor delamination depth detection.3. Based on the digital image correlation (DIC) technique, a2-D fullfield strain monitoring software is developed with MATLAB. Thesoftware can rapidly calculate the correlation between a series of image,and then obtain the displacement and strain field of each image. Acalibration experiment is conducted. The software measurement is veryclose to strain gauge measurement and finite element simulation result,,and this software can be used to measure the displacement and the strainfield on structure surface. 4.A set of full field strain measurement experiment on compositelaminate with impact damage under tension and compression areconducted The surface strain singularity caused by impact damage isdetected. The singularity law is in accordance with finite elementsimulation, so it can be used in structural damage detection. The effects ofthe parameter in DIC on the detection are investigated.