Simultaneous tailoring of pulsed thermography experimental and processing parameters for enhanced defect detection and sizing in adhesive bonds in carbon fibre composites

Thermographic inspection provides opportunity to tailor non-destructive evaluation to specific applications. The paper discusses the opportunities this presents through consideration of adhesive bonds between composites, such as those joining structural members and outer skins, where access is restricted to a single side. To date, literature focusses on the development of either an experimental procedure or data processing approach. This research aims to demonstrate the importance of tailoring both of these aspects to an application to obtain improved defect detection and robust quantification. Firstly, the heating stimulus is optimised to maximise the thermal contrast created between defect and non-defect regions using a development panel. Traditional flash heating is compared to longer square pulse heating, using a developed shutter system, compromising between experimental duration and heat input. A pulse duration of 4 seconds using two 130 W halogen bulbs was found double the detection depth from 1 mm to 2 mm, revealing all defects in the development panel. Temporal processing was maintained for all data using thermal signal reconstruction. Spatial defect detection routines were then implemented to provide robust defect/feature detection. Spatial defect detection encompassed a combination of image enhancement and edge detection algorithms. A two-stage kernel filter/binary enhancement method followed by the use of Canny edge detection was found most robust, providing a sizing error of 1.8 % on the development panel data. This process was then implemented on adhesive bonds with simulated bond line defects. The simulated defects are based on target detection threshold of 10 mm diameter void found at 1- 2 mm depth. All simulated void defects were detected in the representative bonded joint down to the minimum diameter tested of 5 mm. By considering the tailoring of multiple aspects of the inspection routine independently, an overall optimised approach for the application of interest has been defined.

Modelling adhesively-bonded T-joints by a meshless method

Bonding of non-parallel substrates has many applications in the transport industry. Adhesively-bonded T-joints are employed for that purpose. However, geometrical dimensions, substrates’ shape and material choices are broad. The effect that varying upper substrate thickness has on the joint strength (P max ) was investigated in this work through numerical analyses. The numerical analysis was performed using a meshless method, the natural neighbour radial point interpolation method (NNRPIM), which has been proven accurate and robust in another adhesive joint configuration. Materials were considered elastic-plastic. A yield criterion developed for rubber-like materials, the Exponent Drucker-Prager, was used for the adhesive layer, while the metallic substrates were analysed with the von Mises yield criterion. P max was determined numerically using a strain-based continuum mechanics failure criterion. Normalised peel and shear stress distributions along the bond-line are presented. Effective strains, both elastic and plastic, were also obtained. The estimated P max was compared with experimental data; a good agreement was found. The stress distribution along the bond line becomes asymmetric in joints with unbalanced substrate thicknesses. At P max , from 10 to 25% of the bond-line has entered into plastic regime. The results indicate that the proposed methodology is suitable to analyse adhesively-bonded joints under different load solicitations.

A RADIATION SENSITIVE ADHESIVE SYSTEM FOR FUNCTIONALLY GRADED COMPOSITE JOINTS

Adhesively bonded composite joints can help reduce weight in structures and avoid material damage from fastener holes, but stress concentrations formed at the edges of the adhesive bond line are a main cause of failure. Stress concentrations within the adhesive can be reduced by lowering the stiffness at these edges and increasing the stiffness in the center of the joint. This may be achieved using a dual-cure adhesive system, where conventional curing is first used to bond a lap joint, after which high energy radiation is applied to the joint to induce additional crosslinking in specific regions. Anhydride-cured epoxy resins have been formulated to include a radiation sensitizer enabling the desired cure behavior. Tensile testing was performed on cured systems containing varying levels of radiation sensitizer in order to evaluate its effects on young’s modulus as a function of radiation dose.

OPTIMIZATION OF PROCESS PARAMETERS DURING CO-CURE OF HONEYCOMB SANDWICH STRUCTURES

Honeycomb sandwich structures are co-cured to bond partially cured thermoset prepreg facesheets with an adhesive layer to both sides of the honeycomb core under a pre-defined pressure and temperature cycle in an autoclave. High dependency of the co-cure process on the materials and process parameters makes it susceptible to defect such as poorly consolidated facesheet and highly porous bondline which can cause premature failure of the structure. The temperature and pressure in the autoclave and pressure in the vacuum bag are the parameters that describe the cure cycle of the process. In this work, an optimization of the process cycle for the co-cure process of sandwich structures that maximizes the fiber volume fraction within the prepreg and reduces the porosity is presented. The objective function is constructed to reflect the quality of both the facesheet consolidation and bond-line porosity. The Surrogate Optimization Algorithm is employed to find the cure cycle resulting in the highest facesheet consolidation level and the lowest porosity within the bond-line.

A Comparison of Adhesion Behavior of Urea-Formaldehyde Resins with Melamine-Urea-Formaldehyde Resins in Bonding Wood

This paper reports a comparison of adhesion behavior of urea-formaldehyde (UF) with those of melamine-urea-formaldehyde (MU) resins in bonding wood by analyzing the results published in literatures. For this purpose, the adhesion behavior of UF resins prepared by blending low-viscosity resin (LVR) with high-viscosity resin (HVR) at five different blending and two formaldehyde/urea (F/U) molar ratios (1.0 and 1.2) was compared with those of two MUF resins synthesized by either simultaneous reaction (MUF-A resins) or multi-step reaction (MUF-B resins) with three melamine contents (5, 10, and 20 wt%). As the blending (LVR:HVR) ratio increased from 100:0 to 0:100, the viscosity and molar mass (Mw and Mn) of the blended UF resins increased while the gelation time decreased. The interphase features such as maximum storage modulus (E′max), resin penetration depth, and bond-line thickness of the UF resins increased to a maximum and then decreased as the blending ratio increased. In addition, both MUF-A and MUF-B resins also showed an increase in the Mw and Mn as the melamine content increased from 5% to 20%. However, the E′max, resin penetration depth, and bond-line thickness of the MUF resins decreased as the molar mass or melamine content increased. These results indicated that the adhesion of UF resins heavily depends on the interphase features while that of the MUF resins highly depends on the cohesion of the resins.

Semiconductor Chip Electrical Interconnection and Bonding by Nano-Locking with Ultra-Fine Bond-Line Thickness

The potential of an innovation for establishing a simultaneous mechanical, thermal, and electrical connection between two metallic surfaces without requiring a prior time-consuming and expensive surface nanoscopic planarization and without requiring any intermediate conductive material has been explored. The method takes advantage of the intrinsic nanoscopic surface roughness on the interconnecting surfaces: the two surfaces are locked together for electrical interconnection and bonding with a conventional die bonder, and the connection is stabilized by a dielectric adhesive filled into nanoscale valleys on the interconnecting surfaces. This “nano-locking” (NL) method for chip interconnection and bonding is demonstrated by its application for the attachment of high-power GaN-based semiconductor dies to its device substrate. The bond-line thickness of the present NL method achieved is under 100 nm and several hundred times thinner than those achieved using mainstream bonding methods, resulting in a lower overall device thermal resistance and reduced electrical resistance, and thus an improved overall device performance and reliability. Different bond-line thickness strongly influences the overall contact area between the bonding surfaces, and in turn results in different contact resistance of the packaged devices enabled by the NL method and therefore changes the device performance and reliability. The present work opens a new direction for scalable, reliable, and simple nanoscale off-chip electrical interconnection and bonding for nano- and micro-electrical devices. Besides, the present method applies to the bonding of any surfaces with intrinsic or engineered surface nanoscopic structures as well.