چكيده به لاتين
In this study, the effect of loading rates on the interlaminar fracture toughness of unidirectional laminated composites was investigated. An overview of different factors causes delamination and a categorization of the loading rates were presented. Then, the theoretical methods for calculating the strain energy release rate and a review of other researches about the effect of loading rate and behavior of double cantilever beam composite specimens were presented. The experimental results show that various factors such as test method, sample type, loading rate, interface angle, type of fiber and matrix affect the initiation and propagation fracture toughness. A number of researchers predicted increased fracture toughness with increasing loading rates, a number of decreases, and some predicted the ineffectiveness of increasing loading rates on fracture toughness. The use of the cohesive zone model (CZM) using the traction- separation law is studied. In order to determine the parameters of the CZM, 5 groups of loading rate was performed and the fracture toughness of each rate was extracted.
Determining the initiaton of crack growth, the length of bridging and the fracure energy at high rates requires appropriate recording equipment and images. To determine the initiaton of crack growth, the non-linear point of the load-displacement curve was considered. The images recorded by the software slowed down to 1000 frames per second to determine location of the crack accurately. To determine precisely the tipping point, a novel method was written using the image processing and GUI in MATLAB software, which has an acceptable accuracy for determining the length of the cracks. Four parameters, applied load, length of crack, crack opening and crack tip opening displacement to is used to determine CZM for every loading rate.
The results show that, with increasing loading rates up to 100 mm/min, the interlaminar fracture toughness is not significantly different from static rate, but at higher rates the fracture toughness begins to decrease which is due to the reduction of the bridging length and a brittle behavior of matrix. Using the experimental results and the CZM model, an exponential model has been used in ABAQUS. The simulation results show that ignoring the effect of bridging stress causes an error in the prediction of the fracture behavior, which can be achieved using the exponential model.
