Experimental And Numerical Hybrid Method Of Measuring Traction-Separation Relation Of Cohesive Law In Composite Materials

Short Plant fiber reinforced composites have gained a considerable ground in the high performance applications,such as automotive interior board or aerospace structure parts,due to its environmental protection,light weight,and good mechanical properties.Besides tensile mechanical properties,fracture mechanical property is also the key of random short fiber reinforced composites structure design.The cohesive zone model is capable of characterizing the process of crack bridging,fiber pull out and branching of crack propagation of composites.The cohesive zone model is adopted in many applications,especially in theoretical research and numerical analysis of composites.At present,there is no recognized mature method of determining the material's cohesive zone law,therefore,experimental method of estimating cohesive zone law is an important subject of composite materials fracture mechanicals.On the basis of the three methods for estimating cohesive zone laws,an efficient hybrid procedure combining experimental measurements and an inverse algorithm is presented.First,an compact tension experiment is performed for spruce/PP composites,and digital image correlation(DIC)method is used for measuring the cohesive separations behind crack tip and the displacement field of specimens.Second,the displacement field is dispersed into many nodes according to finite element idea as measurement field,then corresponding auxiliary fields is established,and the stress and strain of both fields are gained by finite element method(FEM)calculation.At last,the conservation nature of the interaction J-integrals between these auxiliary fields and the measurement field are used to establishing equations for obtaining the cohesive tractions.In this thesis,Python is used to implement the above algorithms,including processing the experimental data measured by DIC,calculation of interaction J-integrals and so on.Two cohesive zone models are used for numerical experiments to verify the effectiveness of this method,and the influence of such factors like integral contour,mesh density and material property are discussed.At last,the method is used to extract the cohesive zone law of spruce/PP composites with 20% and 49% fiber volume content.The formation of cohesive zone is studied by estimating the cohesive zone laws of specimens under different load.The influence of errors in experimental measurement on determining cohesive zone laws is also discussed.