Mechanical Behavior Of Carbon Fiber Composite Laminates With Engineered Structural Defects

Defects,such as voids and fiber wrinkles,are known to have detrimental effects on the structural integrity of fiber reinforced composite materials.Such defects may originate from sources such as raw materials,manufacturing processes and in-service conditions and very often are impossible to be completely eliminated.With the understanding of their formation mechanisms and their effects on composite mechanical performance,on the one hand,appropriate combinations of processing parameters could be derived to drastically reduce their occurrence or quantity;on the other hand,certain 'defects' could be deliberately designed to yield desirable material properties or phenomenon in specified directions for particular engineering applications,the latter is sometime being referred as‘Defect Engineering'Following a thorough technical survey on the defects pertaining to carbon fiber reinforced plastics(CFRP),the main objective of the project is set out to demonstrate the elimination,control and exploitation of defects by(1)establishing the effect of processing parameters on the generation of defects;(2)the effect of engineered defects on the mechanical behavior of composite laminates;and(3)how residual stresses could be actively exploited for the manufacturing of bi-stable laminates.The materials of interest are domestically produced carbon fiber prepregs for autoclave manufacturing process.The presence of signature defects(gaps and overlaps)in the ever popular automated fiber placement(AFP)technique acts as a motivation for the design of defects into composites.Firstly,the curing pressure during the autoclave process was used as the processing variable to manufacture laminates with varying levels of porosity to evaluate the manufacturability of the supplied carbon fiber prepregs and to understand the effect of curing pressure on matrix dominated properties in composite laminates.Experimentally,modes ?,? and the short beam shear test methods were employed to investigate the dependence of interlaminar property response on pressure.The work has shown that under mode ? and short beam shear testing conditions,there exist a threshold pressure in quasi isotropic laminates where a decline in property is recorded while all unidirectional laminates show no sensitivity to curing pressure.By this approach,it is possible to correlate pressure,voids and the interlaminar properties of composite laminates thereby providing sustainable routes to optimizing interlaminar properties in composite materials from the manufacturing stageIn the context of controlling defects,under the practical guidance of industrial collaborators,the primary type of defects-wrinkles were engineered into laminates by coupling gaps and overlaps This is set out to understand the influence of wrinkles on mechanical properties without the inclusion of third party interfaces within the laminate.It has been demonstrated that composites with varying levels of wrinkle severity can be designed and producedThe failure strength and failure mode dependence of the laminates on wrinkles is presented through experimental studies of the tensile and compression behaviors.The findings of the present work show that by coupling gaps and overlaps in a particular architecture,wrinkles of different geometries can be obtained.Also the work shows that in the presence of wrinkles,failure occurs around the defect region where under tensile loading stress concentration around the defect zone leads to strength knockdown while interlaminar shear stresses lead to the strength knockdown under compression.A new and simple analytical model has been proposed that is capable of predicting the wrinkle angle and also establishing the relation between wrinkle angle and the parameters used to induce the wrinkle.The proposed model provides useful insight into the effect of wrinkle angle on the mechanical performance which enables the control of wrinkles in practical manufacturing processes such as the AFP.Furthermore,numerical simulations have been carried out to inform the deformation behavior and failure mechanisms of the wrinkle-containing composite laminates.Through this,further understanding of wrinkles are established that will aid in the control of defects from the manufacturing standpointFinally,the commonly existing residual stress in composite laminates was explored for beneficial applications in bi-stable structures.Emphasis is placed on manufacturing practices and how they affect bi-stability in laminated composites.The results show that increased consolidation as a result of increasing curing pressure leads to a reduced bi-stable curvature while engineered defects show a detrimental effect on bi-stability.