Quasi-static Mode Ⅰ Fracture Toughness and Damage Behaviors of 3-D Angle-interlock Woven Composites

Author:Amna SiddiqueRu Dan

Supervisor:gu bo hong


Degree Year:2019





Three-dimensional angle-interlock woven composites(3-D AIWC)have been widely used for engineering applications due to their high inter-laminar shear strength and impact damage tolerance.Compared with other damage categories,delamination fracture is considered as a prevalent life-limiting failure mode for composite structures.Delamination may be introduced during manufacturing or caused by damage events during service,thus hampering structural integrity and durability.Keeping in view the adverse impact of delamination on composite structures,it is necessary to consider in structural design process as well as in verification testing to ensure safe application.This project is focused on fracture toughness characterization of 3-D AIWC to unveil fracture behaviors and toughening mechanisms.Quasi-static Mode I fracture toughness tests were conducted using double cantilever beam(DCB)specimen.A finite element analysis(FEA)was conducted at micro-structure level to simulate Mode I fracture test and to reveal failure morphologies.Moreover,effects of pre-crack length on Mode I toughness of 3-D AIWC have been considered.Fracture mechanisms obtained from FE model were confirmed by comparing with experimental tests and fractographic results.Mode I fracture tests were performed on plain woven composite(PWC)and 3-D AIWC to elaborate the effect of reinforcement architecture on fracture toughness.Fracture process was recorded to reveal the effects of two-dimensional(2-D)plain woven and three-dimensional(3-D)angle-interlock woven reinforcement architecture on damage evolution.It was observed that 3-D AIWC presents superior fracture toughness properties compared to 2-D PWC.The fracture toughness of 3-D AIWC was supplemented by energy absorbing mechanisms,such as resin breakage,yarns bridging,yarns pull-out and interface debonding.Secondary cracks formation as well as their propagation was also observed as an important energy absorbing mechanism.On the other hand,the fracture toughness resistance of 2-D PWC was due to interface delamination along with intra-layer fibers bridging.Loading rate effects were analyzed on 3-D AIWC by means of load-displacement curve,strain energy release rate and failure morphology.These responses depict significant dependence of Mode I fracture toughness on loading rate.Load-displacement data obtained at varying loading rates from 0.5mm/min to 100mm/min revealed decrease in critical level of load at which crack propagation occurred.Fractographic images also confirmed rate dependent fracture response of 3-D AIWC in terms of damage morphologies,which suggest to consider this factor during structural designs.Micro-structure FEA model has been established to study the fracture behaviors of 3-D AIWC and to reveal effect of pre-crack length variation(7mm,9mm and 11mm).We have found that pre-crack length has a significant effect on initial stiffness,critical load,strain energy release rate and a decreasing trend for all is observed with increase in pre-crack length.Fracture responses revealed the dependence of Mode I fracture toughness on pre-crack length and increase in precrack length results to decrease in fracture toughness.Interlocked network of reinforcement influenced the strain energy release rate by restricting crack growth.Stress evolution details were obtained from FEA.It is revealed that higher pre-crack length results in stress concentration at crack tip position;therefore,increased crack propagation occurs.Whereas at smaller pre-crack length,stress spread was observed.We hope the results of this research will help in selection and design of damage tolerant materials and structures to ensure safe application.