Analytical And Experimental Determinations Of The Mode ? Interlaminar Fracture Toughness Of Composite Material Using Four Points Bend

Composite materials have good specific strength and specific stiffness,but they are prone to delamination failure after being impacted,which is the most important form of failure that threatens aircraft safety.Therefore,it is of great value to study the theory and method of testing the interlaminar fracture toughness which measures the composite material's resistance to delamination.This thesis focuses on the test method for Mode-? interlaminar fracture toughness of composite materials.ASTM D7905 issued by the American Materials Testing Commission using the three-point end-notched flexure(ENF)test to obtain the composite Mode-? fracture toughness.However,the ENF experiment has two defects:1)In the ENF experiment,the crack growth of the specimen is unstable,and the fracture toughness resistance curve of the material cannot be obtained.2)Using the ENF experiment,when the crack propagates to the loading point,due to the stress concentration at the loading point,the bending fracture sometimes precedes to the crack propagation.Thus,the crack propagation data cannot be obtained in the experiment.To overcome these two shortcomings,DAVIDSON and MARTIN proposed the four-point end-notched flexure(4ENF)test.Analysis based on simple beam theory showed that: In4 ENF test,cracks can achieve steady propagation,only the force of the testing machine is needed to obtain Mode-? fracture toughness,without the need to measure the crack length.In addition,in the 4ENF experiment,once crack propagation occurs,bending fracture does not generally occur.Because of these advantages,the 4ENF test has been considered as a potential method for testing the fracture toughness of Mode-? fracture toughness.However,4ENF experimental research shows that there is a relative difference of more than 10% in the fracture toughness of the Mode-? delamination of the same composite measured by 4ENF and ENF.For this reason,this paper mainly studies the causes of this difference,and proposes a symmetrical bilateral gap sample to overcome the defects of 4ENF.The main research contents of this article are:(1)Based on two-dimensional elasticity theory and finite element analysis,a highprecision 4ENF energy release rate solution is given.This solution is different from the energy release rate obtained by the simple beam formula.It is a function of crack length and material anisotropy.When the equivalent crack length(a/h)exceeds 40,the relative difference between the formula and FEA simulation is less than 1%.(2)The effects of friction and geometrical non-linearity on 4ENF are systematically studied.The results show that the friction between the fixture and the sample is about 5%? 10%.The geometric non-linear effect of the sample is about 3% ? 7%.The influence of friction between the cracked surfaces of the samples is 4% ? 6%.The geometric nonlinear effect of the sample thickness is about 1%.The geometric non-linear effect of the relative position of the loading point and the fulcrum is about 1%.(3)In order to overcome the shortcomings of the 4ENF experiment,a symmetric bilateral notch four-point bending specimen is proposed in this paper.An analytical solution of the energy release rate of the specimen with high accuracy is given by theoretical analysis.A compliance method for testing the Mode-? interlaminar fracture toughness of composite materials using a symmetrical bilateral notch four-point bending specimen is proposed.The Mode-? interlaminar fracture toughness obtained by the test is accurate and reliable.Compared with the ENF test proposed by ASTM D7905,the bilateral notched four-point bending specimen proposed in this paper can obtain the R resistance curve of Mode-? interlaminar.In summary,this paper presents a high-precision energy release rate solution in4 ENF,and quantitatively gives the degree of influence of friction and geometric nonlinearity on the 4ENF energy release rate.A symmetric bilateral notch four-point bending specimen is proposed,and a high-accuracy energy release rate solution and a compliance test method for the R resistance curve are given.The experimental results show that the Mode-? fracture toughness obtained by this method is reliable.The research in this paper provides a new compliance method for testing the interlaminar fracture toughness of composite materials,and has certain theoretical and engineering significance.