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The mode I interlaminar fracture toughness of dissimilar-material joints with thermal residual stresses is experimentally evaluated. The double cantilever beam (DCB) test is commonly used to assess the interlaminar fracture toughness of composites but is inadequate when dissimilar materials are joined owing to the mixed mode, which arises from differences in the bending stiffness and the thermal residual stresses formed during manufacturing because of differences in the linear expansion coefficient of the adherends. To address these difficulties, a new test, named the constant-load DCB test, was developed to evaluate the mode I interlaminar fracture toughness. In this study, a testing machine, based on the theoretical method previously established to derive the mode I interlaminar fracture toughness of dissimilar-material joints, is manufactured, and the fracture toughness is experimentally determined. As a result, good agreement between the theoretical value and experimental results is obtained, and the mode mixity ratio is significantly decreased compared with that of the general DCB test.

In recent years, for the aim of weight reduction of transportation equipment, carbon fiber reinforced thermoplastics (CFRTPs), which have high recyclability and formability, are becoming suitable for mass production.Additionally, with the development of multi-material structures, excellent technologies are required for joining metals and CFRTPs. Presently, adhesive bonding and mechanical joining methods are employed for joining dissimilar materials,however, these methods still have some problems. Therefore, an alternative bonding method that does not use adhesives or employ mechanical joining is required for joining CFRTPs and metals. This study focuses on direct bonding between the CFRTP laminate and an aluminum alloy by fabricating a nanostructure on the aluminum alloy surface. The nanostructure penetrates the CFRTP matrix, causing an anchoring effect that improves the bonding strength significantly. The influence of the nanostructure on the energy release rateof the directly bonded CFRTP and aluminum was evaluated bystatic double cantilever beam testing. Because of the difference in thermal expansion coefficients of the CFRTP laminate and the aluminum alloy, significant residual stresses are generated. The effect of the thermal residual stresses on the energy release ratealong with the resulting mode mixity (mode I and II) was determined. Results reveal that the critical energy release rate is improved by the nanostructure and mode I contribution of the energy release rate is increased for the nanostructure case.

The current work studies the fatigue damage initiation and progression in a quasi-unidirectional non-crimp fabric based fibre composite used for wind turbine blades. This is done by combining in situ transilluminated white light imagining (TWLI) with ex-situ X-ray computed tomography (CT) experiments along with tension clamp X-ray CT experiments. TWLI is used to monitor the off-axis cracks in the thin supporting backing fibre bundles present in quasi-UD composites, and a crack counting algorithm is applied to automatically count the cracks in images obtained in situ during fatigue testing. It is found that off-axis cracks not only initiate at the specimen edges but also at isolated locations inside the specimen, which could be related to the microstructural features. In addition, a clear effect of strain level on the measured off-axis crack density is observed. From the X-ray CT experiments, it is found that the UD fibre fractures initiate and progress from regions where the off-axis backing fibre bundles are ‘in contact’ with a UD fibre bundle. Damage is seen to first initiate at a cross-over region of the backing fibre bundles, and later at a region with only one backing fibre bundle. In addition, applying tension to the specimen during X-ray CT scanning is found to reveal additional UD fibre fractures that are not visible in scans performed the unloaded state. With load applied, a significant number of UD fibre fractures were observed earlier in the fatigue life than expected. Based on the observations of the study a damage progression scheme is presented for quasi-UD fibre composites.

The data published with this paper is obtained during fatigue testing of a unidirectional non-crimp fabric based glass fibre composite by means of ex-situ X-ray CT and in-situ transilluminated white light imaging experiments. The data experimentally shows the damage initiation and progression under fatigue loading both in terms of off-axis cracks in the thin supporting backing fibre bundles and fibre fractures in the load carrying fibre bundles. X-ray CT data comparing the loaded and unloaded state of damage regions by means of a tension clamp solution are also published with this paper.

±The current study presents a workflow to import a fibre bundle structure of a non-crimp fabric based fibre composite obtained by X-ray CT to a solvable 3D model in the finite element software ABAQUS. The considered fibre composite is similar to that used for the load carrying parts of wind turbine blades, and each layer of the non-crimp fabric contains fibre bundles oriented in the 0º, 90º, and 45º directions. The 3D fibre bundle geometry is first segmented in the software AVIZO and then imported to Geomagic Wrap where the geometry is smoothened and converted into a nurbs surface that can be imported into ABAQUS. The resulting stress distribution is qualitatively compared to previous experimental observations and discussed.