Novel Methods For Measuring The Mode ? And Mixed Modes ?/? Interlaminar Fracture Toughness Of Composites

Due to the high specific strength and modulus,composite material has been increasingly used in aerospace and automotive industries.However,composite materials have a major disadvantage,delamination.delamination failure is prone to occur when composite materials are subjected to impact load,thereby threatening the safety of composite aircraft/automobile structures.Therefore,simple and accurate methods for measuring the interlaminar fracture toughness are very important for the analysis and design of composite materials and structures.The measurement of the mode I and mixed modes I/II interlaminar fracture toughness of composites are studied in this paper.In fact,the ASTM test standards D5528 and D6671 has been widely used for determining the mode I and mixed modes I/II interlaminar fracture toughness,respectively.However,these two standards have some shortcomings:1)The D5528 standard recommends double cantilever beam?DCB?specimen for mode I delamination test and provides three data reduction methods,MBT,CC and MCC.These data reduction methods require the force,displacement and crack propagation length during the DCB test.The measurement of the crack length will not only cause a lot of trouble for the test but also increase the measurement uncertainty.In addition,these three methods are difficult to be applied in the high and low temperature DCB test in a environmental chamber.2)The D6671 standard recommends a Mixed Mode Bending?MMB?specimen for mixed modes I/II delamination test.Kinloch et al's energy release rate solution was used to calculate interlaminar fracture toughness.However,this method requires to conduct three different additional tests to obtain three material elastic constants,which makes the mixed mode interlaminar fracture toughness test become very time-consuming and expensive.To simplify these test methods,the following work has been conducted in this paper:?1?A double compliances method is proposed to determine the mode I interlaminar fracture toughness.The method is based on highly accurate compliance solution to composite DCB,which was derived in this paper.During the DCB test,the initial and final compliances can be obtained from the load-displacement curve of the DCB test.The initial and final crack lengths can be measured by A-scan.Using the compliance equation,the initial and final compliances and crack lengths.The material dependent parameters can be determined,which will be used to calculate the fracture toughness.The interlaminar fracture obtained by the double compliances method are consistent well with the results by using the ASTM standard method.In addition,the double compliances method is successfully used in DCB tests in low temperature environments.?2?The double compliances method is applied for studying the static and unstable crack propagation of woven composite materials.In this paper,it is found that unstable crack propagation usually happens in the woven composite DCB test.Therefore,the dynamic delamination problem of DCB is studied with the quasi-static woven composite DCB test.The double compliances method and high-speed camera are used to quantitatively analyze the crack propagation speed,the kinetic energy and material surface energy density during crack propagation.It is found that the surface energy density during the rapid crack propagation process is smaller than that obtained in a static process.?3?A combined strain and compliance method for determining the mixed mode interlaminar fracture toughness test of MMB specimens is proposed.To solve the problem of requiring additional tests for material properties in the D6671 standard,two kinds of strain and compliance methods are proposed in this paper.1)The MMB test is conducted with an additional DCB test with strain gauge attached;2)Strain gauges are attached to the MMB surface and the pure mode I crack opening displacement is recorded during the test by means of digital image correlation?DIC?techniques.Both of these strain and compliance methods are to obtain the elastic modulus E₁₁ and the material-dependent constant?for calculating the interlaminar fracture toughness.The interlaminar fracture toughness obtained by the two methods is within 5%of difference,compared to the reference solution obtained according to D6671.In summary,the main work of this paper is to overcome the shortcomings of ASTM test standards to determine the mode I and mixed modes interlaminar fracture toughness.Therefore,a double compliances method and a combined strain and compliance method are proposed for the DCB and MMB tests,respectively.These two methods not only overcome the shortcomings in the ASTM standards,but also ensure the accuracy of the test,which makes them more suitable for engineering applications.