Mode ? Quasi-static And Fatigue Delamination Growth In Composite Laminates

It is important to have a further understanding of the damage mechanism in advanced composite materials,with an increase use of them from secondary to primary structures in aerospace engineering for the requirement on lightweight structures and fuel efficiency.Delamination,interlaminar failure occurs between adjacent laminas,has been demonstend to be one of the most critical failures in composite laminates.It will lead to the strength and stiffness degradation and finally cause catastrophic failure of the structures during their service lives.Fibre bridging,appearing at the wake of crack front,is another important phenomenon during delamination growth in composite materials.Bridging fibres,acting as a shielding mechanism,will tie both fracture surfaces during crack growth and result in a significant interface resistance increase.A series of mode I quasi-static and fatigue delamination tests were conducted on double cantilever beam(DCB)specimens.Theoretical analysis,fractography observatioin and numerical simulation approaches were subsequently applied to reveal the delamination damage mechanism in composite materials.The major work of this study can be categorized into two parts,i.e.quasi-static delamination growth study and fatigue delamination growth study in composite laminates.In chapter 1,the state of the art on delamination growth in composite laminates under both quasi-static and fatigue loading has been critically discussed and reviewed.In the second chapter,lay-up sequences for both unidirectional and multidirectional DCB specimens were designed first.Then,sereral groups of quasi-static delamination tests were performed on DCB specimens with typical lay-up sequences and different thicknesses,to investigate the effect of interface configuration and thickness on interlaminar resistance.According to the experimental data,there is sufficient evidence that the toughness will initially rise from a low level to a plateau with crack growth.And the toughness related to onset crack growth is interface geometry independent.However,the resistance associated with the crack propagation is interface configuration dependent significantly,due to the different amount of bridging fibres generated at the wake of crack front in different fiber orientations.The concept of resistance curve(R-curve)was applied to phenomenonly describe the toughness increase in a typical stacking sequence.To quantitative determine the effect of fibre bridging,the bridging stress distribution along the bridging region wascalculated for both interfaces.The bridging stress in multidirectional DCB specimen is higher than that in unidirectional specimen,since more bridging appeared in the former one.In addition,the thickness of the specimen also had influence in the bridging.With the increase of thickness,the plateau resistance decreases and bridging zone dimension increases.In the third chapter,a detail discussion and analysis on the quasi-static delamination growth in composite laminates was conducted by the photography observation of the crack path and dominant features on the fracture surface at micro-scale.The crack propagation path in the unidirectional DCB specimens is located in the middle plane.As a result,the crack path is a straight line.While,it is different in the multidirectional DCB specimens,in which the crack growth would offset from the middle plane at certain local area with the increase of crack length,leading to zig-zag crack propagation.This phenomenon could be minimized by increase of specimen thickness,i.e.higher bending stiffness could minimize the zig-zag crack growth.Scan electron microscope(SEM)observation indicates that fibre print and cusps are two typical features distributing on fracture surface.And the scale of cusps in the multidirectional fracture surface is larger that it in the unidirectional.In chapter 4,the delamination growth in composite laminates combined with large-scale fibre bridging was simulated by a numerical model.In this model,the delamination growth can be captured by superposition of two bilinear constitutive relations,which could be derived from the R-curve.Specifically,one bilinear law with high strength but short process zone represents crack generation in the matrix,and the other one with low strength but long process dimension represents fibre bridging.This approach was subquently implemented in the cohesive zone element by the user defined material subroutine(UMAT)in the commercial finite element software ABAQUS.According to the comparison analysis between the experimental data and the predicted results,it can make a conclusion that this method can be used to predict delamination growth in composite laminates with significant fibre bridging.In chapter 5,a thorough discussion on the similitude parameter in fatigue crack propagation analysis was first finished,according to the similitude principle.And a reasonable definition of the strain energy release rate(SERR)range was used in the following fatigue delamination growth study.Subsequently,a further investigation of fibre bridging effect on mode I fatigue delamination growth was conducted based on a series of fatigue tests.The results indicate that fibre bridging will decrease the fatigue crack growth significantly.Specifically,the obtainedParis resistance curves in the log-log graph shift from left to right and finally converge to be a single curve.The most left one represents fatigue crack growth without or with limited fibre bridging.The most right one represents fatigue crack growth with fully developed fibre bridging.And the curves locating in-between describe the fatigue delamination resistance with fibre bridging to a certain degree.It is therefore insufficient to apply only one fatigue resistance curve to determine the delamination growth behavior in composite laminates with large-scale fibre bridging.Furthermore,the bridging created in fatigue delamination is different from that in quasi-static delamination at the same crack length.As a result,it is incorrect to use the quasi-static R-curve to normzlize the fatige experimental data,unless the damage state between these two loading conditions for a certain interface configuration is the same or at least very similar.In the sixth chapter,a new approach was proposed to predict fatigue delamination growth with consideration of fibre bridging effect,based on a further analysis of the curve fitting parameters in the Paris relation.The validity of this new method was finally verified by fatigue experimental data.The observation on the fatigue fracture surface demonstrated that typical features on the fatigue fracture surface are similar to these on quasi-static fracture surface.However,fatigue fracture surface seems smoother than quasi-static fracture surface.This thesis provides a thorough discussion and study on mode I quasi-static and fatigue delamination growth in composite laminates,by a combination of experimental investigation,theoretical analysis,fractography observation and numerical simulation.And a further understanding of the damage mechanism of mode I delamination growth in composite laminates is obtained.The fibre bridging effect on both quasi-static and fatigue delamination growth is investigated carefully.Specifically,the quasi-static delamination growth with bridging was successfully simulated by a numerical model.In case of fatigue delamination,a new approach was proposed to predict delamination growth with consideration of fibre bridging.