Bolted Joint Connection Research Articles

Structural Modeling and Failure Assessment of Spar-Type Substructure for 5 MW Floating Offshore Wind Turbine under Extreme Conditions in the East Sea

Not only the driving for offshore wind energy capacity of 12 GW by Korea’s Renewable Energy 2030 plan but also the need for the rejuvenation of existing world-class shipbuilders’ infrastructures is drawing much attention to offshore wind energy in Korea, especially to the diverse substructures. Considering the deep-sea environment in the East Sea, this paper presents detailed modeling and analysis of spar-type substructure for a 5 MW floating offshore wind turbine (FOWT). This process uses a fully coupled integrated load analysis, which was carried out using FAST, a widely used integrated load analysis software developed by NREL, coupled with an in-house hydrodynamic code (UOU code). The environmental design loads were calculated from data recorded over three years at the Ulsan Marine buoy point according to the ABS and DNVGL standards. The total 12 maximum cases from DLC 6.1 were selected to evaluate the structural integrity of the spar-type substructure under the three co-directional conditions (45°, 135°, and 315°) of wind and wave. A three-dimensional (3D) structural finite element (FE) model incorporating the wind turbine tower and floating structure bolted joint connection was constructed in FEGate (pre/post-structural analysis module based on MSC NASTRAN for ship and offshore structures). The FEM analysis applied the external loads such as the structural loads due to the inertial acceleration, buoyancy, and gravity, and the environmental loads due to the wind, wave, and current. The three-dimensional FE analysis results from the MSC Nastran software showed that the designed spar-type substructure had enough strength to endure the extreme limitation in the East Sea based on the von Mises criteria. The current process of this study would be applicable to the other substructures such as the submersible type.

60188 The coupled dynamic analysis of hydroelectric unit and powerhouse considering the bolt connection characteristics

As the increasing of the capacity and water head of the hydropower stations, the hydroelectric unit’s vibration and its induced powerhouse vibrations become a critical problem. The system of hydraulic-mechanical-electric-structural is coupled and with strong non-linear characteristics. The frame and head cover are all connected with the powerhouse pier via high-strength bolt. The paper is concentrated on the dynamic coupling mechanism and simulation model considering the bolt’s connection and load transmission characteristics. With the theory analysis and numerical simulation, the connection and transmission behaviour of the high-strength bolt would be studied and a more accurate and simple mechanical expression would be derived. The connection sliding law and dynamics modelling method of bolts are discussed. Based on the displacement-load curve of single lap and single bolt analysis it is concluded that the increasing preloading will improve the tangential bearing capacity of the bolted joint connection. Then the FEM coupling model of the lower bracket and powerhouse is established with the virtual medium method. Based on the free-vibration and dynamic response analysis the effects of the loosening of the bolt on the static stiffness of the coupling structure are discussed. The von-Mises stress’s increasing due to some of the bolt’s loosening would result to the material fatigue damage. Finally five preloading schemes of the bolts are discussed and the simulation results show that the preloading force has a greater lateral impact on the structure than on the longitudinal direction. The preloading force improves the coupling structure’s stiffness and reduces the vibration amplitude of the frame. As for the preloading force, when it is small the connection effect is not strong enough and the vibration level may be higher, so the actual preloading state should be exactly simulated. When the bolt preloading reaches 60-70% of the material yield strength, the rigid connection can be directly used in the modelling for the convenience of calculation.

Modeling and parameters identification of the connection interface of bolted joints based on an improved micro-slip model

This paper presented a non-linear modeling approach for bolted joint connection interfaces and identified the micro-slip model’s parameters through quasi-static experiments based on an improved micro-slip model. Firstly, considering the contact pressure’s spatial distribution characteristics on the connection interface, a new density function of critical sliding force was proposed. The contact pressure is distributed annularly on the interface. Then, the critical sliding force’s density function was applied to the Iwan model, and an improved micro-slip model was proposed. The residual shear stiffness on the connection interface during macro-slip and separation was also taken into account simultaneously. The piecewise linear model and the improved micro-slip model were adopted to reproduce the different tensile and compressive stiffness of the bolted joint and the friction on the interface, and the non-linear mechanical model of the bolted joint was obtained. Finally, this paper proposed a method for parameter identification of the micro-slip model on the connection interface. The hysteresis loops were obtained through quasi-static shear experiments, and corresponding parameters were identified.

Reduction of Stress Concentration Factor (SCF) on the Bolted Joint Connection for a Large Wind Turbine Rotor Blade through Various Design Modifications

The importance of a reliable blade root connection has grown due to the higher-gravity-induced edgewise loads on the blade root that resulted from the recent increased size and weight of a wind turbine rotor blade. To avoid the loosening of a bolt joint connection or even consecutive blade failures, the stress concentration factor (SCF) at the bolt thread root that is sensitive to fatigue should be understood comprehensively. In this work, two-dimensional and three-dimensional finite element (FE) analysis methods were used to determine the SCF at the bolt threads both between an insert and a M42 bolt used for a large offshore blade, and between a M42 bolt and a nut. The effect of various geometric parameters on the SCF were also investigated, which included shank diameter, nut height, nut type, and relief cone. Results showed that the decreased diameter of a M42 bolt shank diameter was the dominant design driver in reducing the stress concentration factor by 40%, from 3.94 to 2.32. The round nut type was also a recommended factor to be implemented to connect bolts and inner pitch bearing with an additional 10% SCF reduction. The relief cones applied to bolt threads and insert threads also contributed to the reduction of SCF to 2.01, a 49% reduction in total. This work not only provides guidelines by which to choose the proper geometry of the bolt and nut for a large blade, but also could be beneficial in designing bolted joint connections of segment or modular blades.

Sealing performance analysis of composite gaskets made of silicone rubber filled with ramie natural fibers

Silicon rubber gaskets are commonly used in many industries for low pressure sealing conditions only. The research studies about the sealing performance improvement of a novel composite gasket made of the silicone rubber filled with ramie natural fibers in a bolted joint connection. In this research the sample of gasket consists of 4 different types of gasket i.e. a pure silicone rubber gasket and the silicone rubber composite gasket with 1, 2 and 3 layers of ramie fiber woven. The gaskets are tested using a water pressure testing equipment with a variation of internal fluid pressure and difference torques at 8 N.m, 12 N.m, 16 N.m and 20 N.m. The sealing performance is measured based on the maximum fluid pressure that can be hold by the gaskets before the fluid leaking occurred. The results show that the silicone rubber composite gaskets have better sealing performance comparing with the pure silicone rubber gaskets where the silicon rubber gaskets capable to hold the internal fluid pressure until around 5 times rather than pure silicon rubber gaskets (0.17 MPa comparing with 0.90 MPa). On the other hand the sealing performance of the composite gasket with 3 layers is almost 2 times better than the composite gasket with 1 layer and around 1.5 times better than the composite gasket with 2 layer of fiber woven.

Investigations of friction-relevant parameters to ensure reliable bolted joints

During the process of torque-controlled tightening of bolted joints, the reliability of load transmission is limited by the achieved accurate preload. The preload is mainly influenced by head‐ and thread friction. The knowledge of reliable coefficients of friction and the dependence on assembly parameters are essential. Although a variety of friction coefficients were published but the overall ranges ask for more precision. So called windows of friction coefficients are available in known guidelines, to define permissible scattering. However, there are practical doubts about useful friction coefficients, especially if newly developed lubricants, corrosion‐resistant coatings and securing devices against bolt-loosening are used. In this study, different parameters of a bolted joint connection, such as lubrication, the surface protection of the screw connection or used security systems, have been investigated on their frictional influence. To make a reliable conclusion about the coefficients of friction, only a measured examination like DIN EN ISO 16047:2013-01 can help. By means of a designed test bench those values could be detected by measurement and supplemented with the collected data of vibration test results at the TH Koln. On this way practically useful coefficients of friction and their scattering have been made available.

An anti-noise real-time cross-correlation method for bolted joint monitoring using piezoceramic transducers

Bolted joint connection is the most commonly used connection element in structures and devices. The loosening due to external dynamic loads cannot be observed and measured easily and may cause catastrophic loss especially in an extreme requirement and/or environment. In this paper, an innovative Real-time Cross-Correlation Method (RCCM) for monitoring of the bolted joint loosening was proposed. We apply time reversal process on stress wave propagation to obtain correlation signal. The correlation signal's peak amplitude represents the cross-correlation between the loosening state and the baseline working state; therefore, it can detect the state of loosening. Since the bolt states are uncorrelated with noise, the peak amplitude will not be affected by noise and disturbance while it increases SNR level and increases the measured signals' reliability. The correlation process is carried out online through physical wave propagation without any other post offline complicated analyses and calculations. We implemented the proposed RCCM on a single bolt/nut joint experimental device to quantitatively detect the loosening states successfully. After that we implemented the proposed method on a real large structure (reaction wall) with multiple bolted joint connections. Loosening indexes were built for both experiments to indicate the loosening states. Finally, we demonstrated the proposed method's great anti-noise and/or disturbance ability. In the instrumentation, we simply mounted Lead Zirconium Titanate (PZT) patches on the device/structure surface without any modifications of the bolted connection. The low-cost PZTs used as actuators and sensors for active sensing are easily extended to a sensing network for large scale bolted joint network monitoring.

Experimental Vibration Analysis of a Cracked Laminated Glass Beam with Bolted Joint

In this study, in order to determine the very early load transfer behavior in the bolted joint connection, experimental dynamic analysis of different laminated glass beams including two surface cracks is considered. For this purpose, both three different plastic interlayers (i.e., three types of polyvinyl butyral—PVB) and three different glass-lamina thicknesses are taken into account. Effects of the plastic interlayer, thickness of the glass-lamina, number of surface cracks and their locations on the vibration characteristics/structural performances are examined experimentally. Vibration tests are performed to present free vibration characteristics of the laminated glass beams under clamped-free boundary conditions. Experimental dynamic analysis consists of six parts: (I) vibration analysis with no-crack and no-hole with a bolted joint; (II) vibration analysis with a surface crack and no-hole with a bolted joint; (III) vibration analysis with two surface cracks and no-hole with a bolted joint; (IV) vibration analysis with no-crack and a hole with a bolted joint; (V) vibration analysis with a surface crack and a hole with a bolted joint; (VI) vibration analysis with two surface cracks and a hole with a bolted joint. For these experimental steps, an impact hammer with a force transducer is used to excite the uncracked or cracked composite beams through the selected points. After the excitation, the responses are obtained by an accelerometer. The vibration measurements are completed using a microprocessor-based data acquisition system and nCode GlyphWorks software. Results are given in tabular and graphical forms.

Nonlinear system identification of frictional effects in a beam with a bolted joint connection

We perform nonlinear system identification (NSI) of the effects of frictional connections in the dynamics of a bolted beam assembly. The methodology utilized in this work combines experimental measurements with slow-flow dynamic analysis and empirical mode decomposition, and reconstructs the dynamics through reduced-order models. These are in the form of single-degree-of-freedom linear oscillators (termed intrinsic modal oscillators—IMOs) with forcing terms derived directly from the experimental measurements through slow-flow analysis. The derived reduced order models are capable of reproducing the measured dynamics, whereas the forcing terms provide important information about nonlinear damping effects. The NSI methodology is applied to model nonlinear friction effects in a bolted beam assembly. A ‘monolithic’ beam with identical geometric and material properties is also tested for comparison. Three different forcing (energy) levels were considered in the tests in order to study the energy-dependencies of the damping nonlinearities induced in the beam from the bolted joint. In all cases, the NSI methodology employed was successful in identifying the damping nonlinearities, their spatial distributions and their effects of the vibration modes of the structural component.

An investigation into the performance of macro-fiber composites for sensing and structural vibration applications

This paper presents the use of macro-fiber composites (MFC) for vibration suppression and structural health monitoring. The major advantages of the piezoelectric fiber composite actuators are their high performance, flexibility, and durability when compared with the traditional piezoceramic (PZT) actuators. The recently developed MFC actuator provides these advantages and can be used in structural vibration applications. In addition, the ability of MFC devices to couple the electrical and mechanical fields is larger than in monolithic PZT. In this study, we showed that an MFC could be used as a sensor and actuator to find modal parameters of an inflatable structure. This sensor and actuator combination has also been used to reduce vibration in an inflated object. Once the sensing capability was identified, we developed a self-sensing circuit for an MFC. Our experimental results clearly indicate that this strategy can suppress structural vibration, while reducing the number of system components. Finally, the MFC has been implemented as impedance sensor for structural health monitoring (e.g. a of bolted joint connection). The experimental results presented in this paper show the potential of MFC for use in the dynamics and control of flexible structures.

The transformation method for the simulation and analysis of systems with uncertain parameters

In this paper, the transformation method is introduced as a powerful approach for both the simulation and the analysis of systems with uncertain model parameters. Based on the concept of α-cuts, the method represents a special implementation of fuzzy arithmetic that avoids the well-known effect of overestimation which usually arises when fuzzy arithmetic is reduced to interval computation. Systems with uncertain model parameters can thus be simulated without any artificial widening of the simulation results. As a by-product of the implementation scheme, the transformation method also provides a measure of influence to quantitatively analyze the uncertain system with respect to the effect of each uncertain model parameter on the overall uncertainty of the model output. By this, a special kind of sensitivity analysis can be defined on the basis of fuzzy arithmetic. Finally, to show the efficiency of the transformation method, the method is applied to the simulation and analysis of a model for the friction interface between the sliding surfaces of a bolted joint connection.

Analysis of Bolted Joints with Nonlinear Gasket Behavior

Design methods for full face gaskets in bolted pressure vessel joints have received little attention in the literature. Such gasketed joints play a prominent role in attaching rectangular plan from water boxes to rectangular tubesheets in condenser water boxes. With higher cooling water pressures becoming evident due to cooling tower circuits, the water box-tubesheet structure, and its bolted joint connection requires rigorous analysis for both structural integrity and leak tightness. Although it is well known that gasket material has a highly nonlinear stress strain behavior, very few analyses are available to calculate and evaluate the effect of the nonlinear gasket behavior in a bolted joint connection. In this paper, an approximate method for simultaneously analyzing structural integrity and leak tightness of typical bolted flange connections with nonlinear gasket material is developed. The flange is modeled as an elastic element, the bolt is simulated by a linear spring with bending and extensional resistance, and the gasket is modeled by a series of nonlinear compression springs. A simple nonlinear stress-strain relation for initial loading and unloading of the gasket is developed based on experimental data. The analysis technique employs an incremental procedure which follows the configuration through preloading and pressurization and checks structural integrity and gasket leakage. To illustrate the method, a typical full face gasket and flange construction is studied, and the effect of gasket properties on the final state is investigated. A series of simulation results are obtained which illustrate clearly the effect of gasket prestrain, undersizing of bolts, and wall rotational resistance. Of particular importance is a simulation comparing results obtained using actual nonlinear gasket stress-strain data with results obtained using linear models for the gasket. It is demonstrated that for full face gasket configurations, simulation of the nonlinear behavior is required to achieve accurate results. The procedure developed in this work is ideal for optimization of flange gasket configurations because of its cost effectiveness while simultaneously evaluating the interaction between structural integrity and joint leak tightness.