چكيده به لاتين
In the present research, initiation and propagation of delamination in laminated composites made of E-glass fibers and epoxy resin under mixed mode I/II loading were investigated. Asymmetric double cantilever beam (ADCB) specimens with mode mixity ratio of 14.735 % were manufactured by hand lay-up method. Mentioned specimens had the stacking sequence of [015 / θ // 0 / 07] and sixteenth ply direction was considered as 0˚, 30˚, 45˚, 60˚, and 90˚. The interface fiber orientation was changed in order to eliminate the effects of remote ply orientations. In unidirectional specimens containing nanoparticles, four interlayer zones around the delamination interface were reinforced by carbon nanofibers. By performing the delamination test, load - displacement curves of ADCB specimens were extracted. In order to full extraction of test results, values of two quantities of crack length and local displacement of initial crack tip were extracted using image processing method. The values of strain energy release rate for ADCB specimens were calculated by modified beam theory method and resistance curves of specimens were extracted.
Experimental test results showed that the use of carbon nanofibers at delamination interface, has not had a significant influence on load - displacement curve and resistance curve of unidirectional specimen. Furthermore, based on the test results, initiation and propagation values of fracture toughness for laminates with non-zero fiber orientation in underlay of crack are greater than ones of unidirectional laminates. This trend is reverse for bridging zone length parameter. This occurs due to overcoming fibers separation mechanism from the surface to bridging zone length mechanism in laminates with non-zero fiber orientation in underlay of crack. Experimental traction - separation law of ADCB specimens was extracted based on the J-integral method. According to the equations and ranges presented in literatures and results of numerical modelings, the scope of changes of quadlinear traction - separation law parameters was determined. Furthermore, the effect of parameters value changing in specified scope on load - displacement curve behavior of ADCB specimens was investigated qualitatively and quantitatively. Then, optimization process was implemented by genetic algorithm method and unknown parameters of quadlinear traction - separation law were extracted. Due to existing problems in genetic optimization process, a comprehensive proposed method, having suitable accuracy and simpler than genetic optimization algorithm was presented. Then, unknown parameters of trilinear and quadlinear laws were extracted using the proposed method.
Numerical modeling of ADCB specimens was implemented using user material (UMAT) coding of different traction - separation laws and load - displacement curves obtained of experimental, genetic algorithm and proposed methods were extracted. Then, results of experimental method were considered as the basis and traction - separation and load - displacement curves obtained of three mentioned methods were investigated. The results of study showed that compared with trilinear law of proposed method, quadlinear law of this method presents more accurate estimation of overall form of traction - separation and load - displacement curves. Furthermore, traction - separation and load - displacement curves obtained from quadlinear laws of proposed and genetic algorithm methods were close to each other. Therefore, due to numerous benefits of quadlinear law of proposed method such as ease of use and saving time, it can be concluded that mentioned method works more efficient than other methods in predicting the overall form of experimental traction - separation and load - displacement curves.
