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Threaded fasteners are widely applied in mechanical systems, providing the functions of connection, fastening, and sealing. However, loosening is vulnerable to occurring in harsh environment. The importance of loosening detection cannot be emphasized. Percussion-based loosening detection method has attracted much attention due to the convenience and low cost. However, the simultaneous loosening detection of multiple-threaded fasteners based on percussion method is still a challenging issue that needs to be addressed. This study proposes a novel multi-bolt loosening detection method combining percussion method, and deep learning. The method consists of three integrated modules, that is, signal preprocessing, loosening information enhancement, and loosening detection modules. In the first module, variational mode decomposition is used to decompose the original signal into a series of intrinsic mode function to eliminate the interference of noise. In the second module, compressive sampling matching pursuit is applied to represent the denoised signal sparsely, and the sparse signal is fused with the denoised signal to enhance loosening information in the signal. Last, DenseNet-CBAM network structure combining attention mechanism is proposed for multiple classification task. Experimental results showed that the proposed method achieved the detection accuracy of more than 97% in three different types of mechanical structures with multiple-threaded fasteners, indicating its great potentials in engineering applications. [ABSTRACT FROM AUTHOR]

Aerospace vehicle connection constructions are in urgent need of joint structures with excellent aerodynamic profiles and environmental adaptability. To address issues such as poor aerodynamic profile, material thermal expansion coefficient mismatch, and limited joint structure evaluation indexes, a multi-bolt, countersunk C/SiC composite joint structure is presented in this study. The development of a 3D Hashin progressive damage model and its dedicated solver code is presented. The validity of the model is confirmed by comparing simulation results with experimental data. Three evaluation indexes are proposed, peak load, weight increment efficiency, and bolt load distribution, to thoroughly evaluate the mechanical performance of multi-bolt, countersunk C/SiC composite joint structures. Using the proposed model and evaluation indices, we evaluate sixteen different designs of multi-bolt, countersunk C/SiC composite joint structures and analyze how design parameters affect their mechanical properties and damage patterns. The results show that the best mechanical properties of the joint structure are achieved when the ratio of bolt pitch to through hole diameter is 3, the ratio of bolt spacing between columns to through hole diameter is 4, the ratio of the distance between the free edge of the substrate to through hole diameter is 1.5, the ratio of through hole diameter to specimen thickness is 1.7, and the ratio of the distance between the edge of the substrate to through hole diameter is 1.5. [ABSTRACT FROM AUTHOR]

The capacity of connection in pultruded glass fiber reinforcement polymers (PGFRP) is usually considered a significant factor in the design of the composite structure. Pasting glass fiber sheets (GFS) can be used to strengthen new structures or enhance existing PGFRP connections' serviceability. This study presented experimental results when a bolted connection in PGFRP was strengthened by glass fiber sheets (GFS). There were 72 specimens of multi-bolted in the connection strength experiment. The test parameter was divided into three series of GFS types that were made from woven roving laminate or chop strand mat. The specimens were also constructed from other parameters: end distance and the number of bolts. The strengthening PGFRP connection by GFS resulted in up to 177% increase in the capacity in compare with non-strengthening connection. On the other hand, the equations used to estimate the joint strength after being strengthened by GFS were proposed. The method's effectiveness and suggested calculating formula can provide to designers trying to enhance PGFRP multi-bolted connections. [ABSTRACT FROM AUTHOR]

Aerospace vehicle connection constructions are in urgent need of joint structures with excellent aerodynamic profiles and environmental adaptability. To address issues such as poor aerodynamic profile, material thermal expansion coefficient mismatch, and limited joint structure evaluation indexes, a multi-bolt, countersunk C/SiC composite joint structure is presented in this study. The development of a 3D Hashin progressive damage model and its dedicated solver code is presented. The validity of the model is confirmed by comparing simulation results with experimental data. Three evaluation indexes are proposed, peak load, weight increment efficiency, and bolt load distribution, to thoroughly evaluate the mechanical performance of multi-bolt, countersunk C/SiC composite joint structures. Using the proposed model and evaluation indices, we evaluate sixteen different designs of multi-bolt, countersunk C/SiC composite joint structures and analyze how design parameters affect their mechanical properties and damage patterns. The results show that the best mechanical properties of the joint structure are achieved when the ratio of bolt pitch to through hole diameter is 3, the ratio of bolt spacing between columns to through hole diameter is 4, the ratio of the distance between the free edge of the substrate to through hole diameter is 1.5, the ratio of through hole diameter to specimen thickness is 1.7, and the ratio of the distance between the edge of the substrate to through hole diameter is 1.5.

Multi-bolt joint distributed around the connecting members are generally adopted to meet the high-performance assembly requirements in aerospace, energy and power industries. However, the initial preload could be low due to non-optimized preload sequence and bolt stress relaxation, especially at elevated temperature. Thus, it is necessary to take elastic interaction and bolt stress relaxation into account before jointing. In this article, a general multi-bolt elastic interaction with bolt stress relaxation is modelled analytically. First, the multi-bolt joint is characterized by ‘spring-node’ model and elastic interaction stiffness. Second, the bolt residual preload can be estimated according to linear superposition of elastic interaction and bolt stress relaxation under the condition of node displacement equilibrium. Further, the influence of preloading sequence and bolt stress relaxation on residual preload distribution was numerically analyzed using a typical circular ring with 8-bolt joint. Two bolts’ preloading sequences were planned, star sequence and counterclockwise sequence, respectively. The bolt creep simulation time was set as 10 h using the power-model at intermediate temperature. From comparison, the predicted results using the developed model were consistent with the FE simulations both with and without bolt stress relaxation. [ABSTRACT FROM AUTHOR]

Abstract: Accurately accessing the three-dimensional stress distribution through the thickness surrounding the bolt hole is necessary to identify the most effective multi-bolt single-lap joint design in large-scale thick laminate structures. To decrease computing time and resources, a two-dimensional to three-dimensional global–local finite element numerical method is presented, in which both the local zone selection strategy and boundary constraints applying method are discussed in detail. And both the contact stress and the stress components around the fastener hole are analyzed by using the proposed global–local method. Through analysis, it is found that the contact stress is concentrated in the layers that are close to the shear plane; the is much higher than other stress components; most of the stress components conform cosine distribution around the fastener hole; the out-of-plane stress and interlaminate shear stress are quite high in the layers close to the shear plane; the stress distributions of different layers are depended on the ply angles of the specified layer; the effectiveness of the global–local model procedure in reducing the computation effort is obvious and this methodology is applicable to other detailed stress/strain analysis problems in large-scale structures. [Copyright &y& Elsevier]

The stress distribution surrounding the fastener hole in thick laminate mechanical joints is complex. It is time-consuming to analyze the distribution using finite element method. To accurately and efficiently obtain the stress state around the fastener hole in multi-bolt thick laminate joints, a global-local approach is introduced. In the method, the most seriously damaged zone is 3D modeled by taking the displacement field got from the 2D global model as boundary conditions. Through comparison and analysis there are the following findings: the global-local finite element method is a reliable and efficient way to solve the stress distribution problem; the stress distribution around the fastener hole is quite uneven in through-the-thickness direction, and the stresses of the elements close to the shearing plane are much higher than the stresses of the elements far away from the shearing plane; the out-of-plane stresses introduced by the single-lap joint cannot be ignored due to the delamination failure; the stress state is a useful criterion for further more complex studies involving failure analysis. [ABSTRACT FROM AUTHOR]