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A generalized sequence-dependent constitutive model is formulated and coupled with genetic algorithm (GA) to minimize the effect of bolt-on-bolt elastic interaction (EI) of a large size wind turbine blade joint during the process of joint preloading prior to turbine operation. The composite blade flange is bolted to a steel hub flange on the turbine tower side. A Python-based tool is developed for big data sets of elastic interaction among the 54-bolt wind turbine blade-to-hub connection. Trial tightening sequence and patterns include (a) clockwise sequential tightening of six bolts at each apex of three rotated triangles, (b) clockwise sequential tightening of nine bolts at each apex of two rotated triangles, (c) separately tightening one pair of bolts that are located 180° diagonally across each other and repeat 26 more times in a clockwise direction. A genetic algorithm is used to identify feasible tightening sequences that minimize the scatter of bolt preload and achieve the desired level of a uniform joint preload. 

This paper presents a relevant and timely study on the application of thermal loaded digital shearography for crack classification in glass fiber reinforced plastic (GFRP) structures, particularly air-cooled condenser (ACC) fan blades. A thermal loaded digital shearography system was applied to measure strain concentration caused by the cracks at different fatigue cycles. A thermomechanical model was introduced to estimate the heating temperature and the time to ensure heat can reach to the desired depth and that both shallow and deep cracks can be detected. In order to correlate the information of strain concentration in the shearograms to the different stages of cracks, fatigue testing with dynamic three-point bending was conducted. The fatigue tests demonstrated how the strain concentration evolved in the shearograms, while the crack developed from the early (no noticeable strain concentration), to the middle (strain concentration is forming), to the late stage (significant strain concentration is found). The relationships between the degrees of strain concentration in the shearograms and the different stages of cracks can be obtained from testing of the artificial cracks. Using the rules and experimental results obtained from artificial samples, digital shearography was applied to classify the crack stages in parts of ACC fan blades from industry. The combination of artificial crack testing, fatigue loading experiments, and validation with CT scans demonstrates a comprehensive approach and provides potential guidance for industry to determine criticality and maintenance criteria. 

Abstract The demand for effective de-bondable adhesive technology enabling substrate separation under small loads has grown in recent years. Thermally Expandable Particles (TEP) can be embedded in structural adhesives to promote mechanical separation of the adherends. However, the activation of TEP additives in joints with non-metallic adherends is challenging and can result in substrate thermal damage and poor de-bonding performance, due to the low thermal conductivity and dielectric loss factor typical of plastics and polymer-matrix composites. In this study, the effect of bondline stainless steel inserts on fully composite (Carbon Fiber Reinforced Polymer, or CFRP) bonded Single Lap Joints (SLJ) mechanical and de-bonding performance is evaluated. A centrifugal mixer is used to disperse the TEP in the adhesive. TEP additives are activated using induction heating of the bondline insert, which also helps control crack initiation and propagation. SLJ de-bonding tests are run under a constant 20 lb (89 N) load, and substrate temperature is recorded with thermocouples and an infrared thermometer. Joint strength is evaluated with quasi-static lap shear tests on a servo-hydraulic tensile test apparatus. Preliminary de-bonding testing is performed on a broad initial set of 316 stainless steel insert designs. Out of those, the four best-performing insert geometries are chosen for the complete study. Two TEP enrichment levels (10% and 20% wt.) are investigated. The mechanical and de-bonding performance of SLJs with steel inserts is compared to TEP-only baseline fully-composite and multi-material (AA 6061 Aluminum Alloy + CFRP) joints. The results show that bondline inserts enable fast de-bonding of fully-composite SLJs. Insert geometry and thickness affect joint de-bonding time and reliability, and can be optimized to allow for a partial recovery of lap shear strength. 100% de-bonding reliability is achieved with “block”-type inserts, with de-bonding performance similar to TEP-enriched metallic joints. Visual inspection of the fracture surfaces shows the relationship between TEP activation and crack propagation path. Discussion and conclusions are provided. 

Abstract This study investigates the effect of autoclave curing variables on the glass transition temperature of and the degree of cure and strength of epoxy film adhesive single lap joints (SLJs) under static tensile shear loading. Studied autoclave variables include the cure temperature, cure pressure, temperature, and pressure ramp rates on the glass transition temperature as well as the cure time duration. Test joints are made of Aluminum substrates that are autoclave-bonded using epoxy film adhesive (AF163-2k). For each variable combination of the autoclave process, the corresponding glass transition temperature of cured Epoxy film adhesive is obtained using Dynamic Mechanical Analysis (DMA-Q800). Test data are generated for both baseline joints [uncycled] as well as for joints that have been heat-cycled in an environmental chamber after initial autoclave bonding. Results show a strong correlation between the autoclave process variable combinations and the corresponding glass transition temperature bond strength, and the failure mode of test joints. 

Abstract Carbon and glass fiber-reinforced plastic (CFRP and GFRP) composites have gained popularity in various industries and settings owing to their exceptional strength-to-weight ratio, corrosion resistance, fatigue resistance, and design flexibility. Adhesives are commonly utilized to bond carbon fiber reinforced plastics to other materials or to themselves. Bonded joints have been found to enhance the mechanical characteristics of materials, while also reducing additional costs for practical applications. The bonding region’s strength can be influenced by different material combinations and substrate thicknesses. The present investigation employed a double-sided multi-digital image correlation (DS-Multi-DIC) system to quantify the deformation and fracture mechanisms of adherends during tensile testing. The primary objective was to assess the static impact on the strength of the bonded zone under tensile loading. The system comprises of two stereo vision DIC measurement systems, wherein each system comprises of two GigE cameras. These cameras are positioned at the front and back of the sample, respectively the process of system calibration involves the utilization of the double-sided calibration technique to integrate the coordinate systems of the two DIC measurement subsystems with the global coordinate system. This enables the direct measurement of deformation and strain in three dimensions. This paper examines the impact of adhesive thickness and type on the strength of the bonded area and fracture mechanism by analyzing the alteration in strain distribution and maximum strain during static stretching. The study yielded a conclusion that the magnitude of the bonded region’s strength is positively correlated with the thickness of the bonding material. The cohesive force of the bonded region is positively correlated with the malleability of the adhesive bond. Furthermore, an examination was conducted on the impact of adhesive thickness and type on peeling strain. As the bond’s bending stiffness diminishes, there is a corresponding increase in the peel strain it undergoes. Furthermore, a succinct description is provided regarding the unequal allocation of auditory alterations that occur during stationary stretching.