ABSTRACT Pretensioned hybrid joints that combine adhesive bonding and mechanical fastening are widely used in lightweight manufacturing, yet their performance depends strongly on manufacturing-defined boundary conditions. In this study, aluminium single-lap hybrid joints were tested under quasi-static tensile – shear loading and compared with purely bonded and purely bolted reference joints while specimen width, fastener configuration, spacer-based gap control, and tightening torque were varied within production-relevant ranges. Hybrid joints exhibited staged load transfer, combining high initial capacity with non-zero residual load-carrying capability after the adhesive-dominated peak. The distance spacer primarily acted as a process-control feature by constraining squeeze flow and preventing adhesive intrusion into threaded regions, whereas spacer omission introduced an uncontrolled assembly state that affected robustness and scatter and, for certain configurations, increased mean breaking load. Tightening torque and bolt diameter showed weak influence on peak breaking load within the tested window, while width-related trends were confounded with fastener count. The results emphasised that predictable performance requires robustness metrics in addition to mean capacity and provide practical guidance for selecting spacer strategies in production.

ABSTRACT Bolted joints are critical to engineering structures and their integrity is essential for safety and functionality. Traditional monitoring methods are often expensive and intrusive, necessitating the development of more efficient approaches. Preload loss in pretensioned bolts is inevitable in practice, making the reliable detection of loosening vital for structural reliability. This study presents a novel structural health monitoring (SHM) method based on vibration and percussive audio-emission signals generated during controlled percussion. A single bolted lap joint was subjected to percussion, and the resulting audio and vibration signals were recorded and analyzed in both the time and frequency domains to assess bolt tightness. As the bolt torque increased from 0 Nm to 25 Nm, significant variations were observed in signal characteristics. For the vibration signals, the Signal Energy, Peak-to-RMS ratio, and kurtosis changed by 48.84%, 61.02%, and 90.15%, respectively. For audio signals, the corresponding variations were 39.53%, 44.77% and 80.79%. Fast Fourier Transform (FFT) analysis showed a correlation between bolt tightening levels and frequency amplitudes, with slight frequency increases in both signal types with an increase in bolt torque. The results demonstrate that percussion-induced signals effectively reflect bolt tightness. Comparative analysis of vibration and audio responses highlights the potential of this multi-modal approach to enhance the reliability of bolted joint SHM applications.

The surrounding rock between two ultra-closely spaced roadways, under biaxial compression (BC) condition, exhibits high failure propensity and consequently poses serious threats to the coal mining operations. The counter-pulled rockbolt (hereinafter referred as “bolt”) is widely employed to improve the stability of this kind of surrounding rock. However, the strength, deformation, macro- and micro-failure behaviors of bolted surrounding rock under BC condition are insufficiently understood. Therefore, these performances of the bolted samples (BSs) are investigated by using biaxial compressive experiments and acoustic emission (AE) technology. The research results indicate that the bolt diameter has an obvious capacity in improving the peak and residual strength of BSs, the bolt pretension force can significantly enhance the peak strength of BSs, yet has a minimal impact on the residual strength. Three types of axial stress-axial strain curves were categorized: post-peak instantaneous stress drop type, post-peak multi-step stress drop type and post-peak stress delay drop type. A new reinforcement effect, termed as the all-directional reinforcement effect of the bolt was observed. Insights into the influence of bolts on the elastic modulus and lateral displacement of the samples under BC condition were obtained, and two mechanisms for the reinforcement effect of bolt on the lateral displacement of samples were revealed. When the lateral pressure σ2 is 8.89 MPa, the effect of the bolt on the failure pattern of BSs is not obvious. Nevertheless, the fracture angle increases with the increase of bolt diameter and pretension force. The micro-failure crack types were classified based on AE parameters of AF–RA and it was found that the micro-failure of the BSs was dominated by shear cracks. The micro-failure process of the BSs underwent three stages, with most micro-failures occurring in Stage II. A new approach capable of computing the peak strength or the equivalent cohesion (c*) and equivalent internal friction angle (φ*) of sample reinforced by pretension bolt is proposed and used to calculate c* and φ* of the BSs under BC condition.

In western mining areas, roadways excavated in weakly cemented thick coal seams often experience significant deformation and support failure due to poor roof conditions, low cementation strength, and stress redistribution. Taking the 11,703 transportation roadway of the Dananhu No. 7 Coal Mine in Xinjiang as a case study, this paper investigates the deformation mechanisms of the surrounding rock and proposes an optimized support technology. Using numerical simulations, theoretical analysis, and field monitoring, the study reveals that roof failure evolves through stress redistribution, plastic softening, interlayer separation, and fragmentation, ultimately forming a 3.5 m plastic failure zone. The existing support system fails to establish effective deep–shallow coordination, with shallow bolts yielding and deep cables underutilized. To address this, a Graded Anchor Composite Support Technology is proposed, consisting of surface concrete, shallow high pre-tensioned bolts, mid-depth short cables, and deep long cables. The optimized design enhances support performance and restricts plastic zone expansion. Field application and simulation results show a 73.3% reduction in roof subsidence and a 40% decrease in plastic zone depth. The study provides engineering guidance for roadway support in similar geological conditions.

This work is focused on evaluating the Stability of Dolerite Dyke at 740 Decline of Mupane Gold Mine. The wire meshes used to support the Dolerite Dyke, proved to insufficient and inadequate to sustain the pressure coming from the supported Dolerite Dyke and they are always at is a risk of unexpected rock falls. The work undertook the investigation of the strength of the Dolerite Dyke, and estimation of the required mean fall out weight evaluating the and extend of failure mechanism. Discontinuity data (mainly joints) obtained through the Scanline Mapping were used to analyze the rock using the RS2 software. The rock mass classification method includes RQD, which is 29.9, RMR is 58, and Q tunneling is 0.2666. These classification methods agree that the Dolerite Dyke is of inferior quality and support is recommended as per given support guidelines. It is found that the Dolerite rock has several instability problems and needs to be addressed. To reconfirm the failure limit, depth was increased to test the stability analyses of the Dolerite Dyke. This showed that as depth was increased, the in situ stress increases, creating deformations that create instability issues. Dips software analyzed the joints sets which were discovered to weaken the rock strength which made the Dolerite Dyke susceptible to failure. From unwedge analysis we the maximum volume of key books is 25cm2. Which individually cannot activate failure. From RS2 the dyke- section experiences deformation even after support installation. Installation of 2.3 m long full column pre-tensioned resin bolts into the hanging wall and sidewalls at an angle of 90 degrees, and application of 25mm thick shotcrete to the hanging wall and sidewalls after the installation are recommended to compensate for blocky ground conditions

Underground infrastructures are the essential assets to the transport, energy storage, and utilities. However, with the deepening development of urbanisation and modernisation, the safety of underground infrastructure is threatened by various natural or human-induced hazards. In the construction and operation phases, hazards can be single, cascade or combined in their origin and effects, the combined or potential interrelated effects of them may exacerbate consequences, leading to higher loss of functionality and cost of restoration. Determining the post-hazard capacity and recovery ability of these infrastructures in a quantitative approach remains ongoing research priority. This paper proposes a unified approach of implementing a quantified resilience assessment of underground infrastructures subjected to multiple hazards. Embedding potential abrupt hazards into long-term deterioration of a circular tunnel, coupled with modelling of restoration, the lifecycle performance is presented. The results show that the disturbance caused by excavation construction has an influence on the performance of subsequent operation phase. The deterioration hazards including ageing and corrosion of structure and creep deformation of rock mass during the whole service life continuously decreases the functionality of the tunnel. While abrupt hazards, such as earthquake and blasting, lead to a sharp decline in functionality in a short period of time. Continuous evolution over time of deformation and damage caused by various hazards, whether deteriorated or abrupt, will be inherited and accumulated. Timely repair, such as pretensioned bolts and grouting can effectively restore the bearing capacity of the infrastructure to a specific extent. Through this numerical approach, the restoration time can be estimated by the functionality recovered and unit consumed time. The dimensionless resilience index is then calculated for the post-hazard functionality based on the specific hazard scenarios and restoration solutions.

While most academic studies focus on the properties of cured joints, this research addresses the manufacturing process of hybrid joints in their uncured state. Hybrid joints that combine adhesive bonding with pre-tensioned bolts exhibit superior mechanical performance compared to exclusively bonded or bolted joints. However, the adhesive flow during manufacturing in hybrid joints often results in a nonuniform adhesive thickness, where obtaining an exact thickness is crucial for accurate load capacity predictions. This paper presents experiments involving three different adhesives, providing precise measurements of the adhesive layer thickness distribution, which served as a reference when evaluating and validating the subsequent numerical predictions. The numerical predictions were performed using computational fluid dynamics (CFD) to model the flow behavior of the adhesives during the bonding process and their interactions with the metal substrates. The CFD predictions of the adhesive layer thickness showed good agreement with the experimental data, with the relative differences between the average experimental and numerical thickness values ranging from 4.07% to 27.1%. The results were most accurate for the adhesive with sand particles, whose particles remained intact, ensuring that the adhesive's rheology remained unchanged. The results highlight the importance of the rheological behavior of the adhesive in the final distribution of the adhesive layer thickness, thereby expanding the understanding of these joints.

Numerical investigation of the behavior of combination connections with pretensioned high-strength bolts and longitudinal fillet welds

Abstract The joints of bars composed of flanges and high-strength bolts are currently used in steel structures as simple and economical construction solutions. This paper presents some theoretical aspects regarding the behavior and calculation of joints with flanges and partially pre-tensioned bolts. The overloading of the pretensioned high-strength bolts produced by the tensile effort resulting from the external actions is usually neglected in the technical materials and in the design norms for constructions, as long this tensioning effort does not exceed the pretensioning force of the high-strength bolts. The overloading effort of the bolts can be evaluated theoretically by estimating the relative stiffness of the flanges and bolts and confirmed through experimental tests and computer simulations. This paper also theoretically and computationally analyzes a joint for a tensioned bar made up of pre-stressed high-strength bolts and end flanges.

Abstract High-strength HV-fastener sets of dimensions M48 and M64 with property class 10.9 were employed in offshore wind turbine frameworks. The M64 were used in coupling flanges within monopiles. The M48 were employed in another offshore wind farm and encountered natural weathering. In both installations, time-delayed fractures of the nuts were observed. Owing to the presence of macroscopically visible corrosion products, hydrogen-induced stress corrosion cracking (Hi-SCC) was established as the probable cause of failure. However, a nut fracture in a properly pre-tensioned bolt assembly is atypical since the stresses in the bolt threads are higher than those in the nuts. Based on the Hi-SCC theory, the fracture should occur at the most stressed component, which is the bolt. During the root cause analysis, extensive examinations were conducted to determine the cause of the nut fractures. The focus was on investigating whether the nut material was more prone to Hi-SCC than the bolt material. The examination program included scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS) analysis of the fracture surfaces, optical microscopy of microspecimens, mechanical tests, and stress rupture tests of hydrogen-charged specimens. While the results suggest that the tested nuts comply with the requirements of the applicable standards regarding material properties, they also reveal that the nut material is, despite its lower tensile strength, significantly more susceptible to Hi-SCC than the bolt material. Therefore, a direct relationship between material susceptibility to Hi-SCC and the tensile strength, as standards and guidelines imply, is not given.

Hybrid joints consisting of pre-tensioned bolts and a bonded connection—The influence of adhesives on the load-bearing capacity

Bolted joints have been widely used in rotor assemblies of aircraft engines. A typical bolt joint is composed of two metal parts connected mechanically with pre-tensioned bolts. During operations, the bolting pretensions can vary with the changing contacting relationship between the parts, which makes the connection stiffness of the joint nonlinear on the macroscopic scale. The fundamental problems of the dynamical analysis for bolted joints are (1) to identify the transition of the change of the connection stiffness and properly model the overall connection stiffness and (2) to investigate the influence of such variable stiffness on the rotor vibration. In this paper, an analytical model is proposed considering the effect of bolted joints. The bolt of the joint is simplified as a two-node spring-like connection element considering different axial tension and compression stiffness, non-uniform preload of bolts and the contact interface friction described based on Coulomb friction. The dynamic equations of the rotor system are established based on the lumped mass modeling and the proposed connection element, and numerically solved through the Runge–Kutta method. The results indicate that the rotor system exhibits a variety of states, such as period-1 motion, multi-period motion, quasi-periodic motion and chaotic motion. The effects of friction coefficient and preload of the bolts on the nonlinear dynamic behaviors of the bolted rotor system are demonstrated in detail.

ABSTRACT The resurgence of interest in hybrid joints, combining adhesive bonding with mechanical fastening in engineering applications, is driven by the recognition of unique advantages and limitations inherent in each joining method. Unlike conventional bonded joints with precise adhesive thickness control, hybrid joints involving pre-tensioned bolts depend on a process termed squeeze-flow, influenced by joint geometry, adhesive viscosity, and adhesive filler presence. A method for the precise measurements of the adhesive layer thickness, conducted at 5 mm intervals, proved its accuracy and repeatability. The results revealed the significant impact of adhesive viscosity and filler characteristics on adhesive layer thickness. Viscosity, while playing a central role, is not acting in isolation, as filler properties collectively influence adhesive layer thickness distribution and bonding performance. The complexity of this relationship underscores the interplay between adhesive layer thickness, viscosity, filler content, and load-bearing capacity. Smaller-sized fillers result in thinner layers and reduced load capacities, while adhesives with larger particles exhibit higher load capacities. This complexity emphasizes the need for a nuanced approach when selecting adhesives for specific applications, considering not only viscosity but also filler attributes to optimize bonding performance in hybrid joints.

Investigation on mechanical behavior of pre-tensioned bolt in fractured rock mass using Continuum Discontinuum Element Method(CDEM)

This article discusses the issue of force behavior in a pre-tensioned bolt that is part of a symmetrical flange connection, loaded with an operating load parallel to the bolt axis. The first part describes the issue being dealt with in detail. The next part offers a brief description of the generally known theoretical approach to the solution, and also outlines the limitations of this approach. This part is followed by a theoretical analysis of working load behavior in the bolt of the analyzed device, using Finite Element Method (FEM). In this article, a comparison of several different approaches to the solution can be also found, followed by a description of the experiment preparation and its results. The obtained values also serve as a subject for reverse iterative analysis carried out using ANSYS software. The aim is to find a proper modification to the symmetric calculation model that would be consistent with the results of measurements from technical practice. All results achieved are described in the final part of the article.

To evaluate the stability of a rock slope a common practice is to calculate the factor of safety by dividing the driving forces on the resisting. Hoek and Bray [1] proposed a formula for the calculation of the factor of safety FS for rock slopes reinforced by pre-tensioned bolts and anchors, so-called active support devices. The formulas have been used worldwide for active support of rock slopes in the past 40 years and are also incorporated in different rock slope stability software. The reinforcement force of the active support is in the formula considered a negative driving force. On the other hand, for passive support, referring to reinforcement by non-tensioned bolts and anchors, the reinforcement force is counted as an additional resisting force in the calculations. In this paper, the two definitions to the factor of safety are examined with the help of an example of simple block equilibrium, which demonstrates issues regarding the formula for active support. It is concluded that the reinforcement force should always be counted as a resisting force in the calculation of the factor of safety regardless it is active or passive support. Concepts of factors of safety for shear failure and collapse are proposed and verified with a calculation example.

Numerical modelling and field monitoring study on large-span tunnelling using pretensioned bolt–cable combined support system

This paper presents the results of an experimental study aiming to investigate the behavior of steel connections that combine pretensioned high-strength bolts and longitudinal fillet welds on a common faying surface. A total of 75 double-shear tension splices were tested under direct tension loading to quantify the effect of various connection variables on the load-deformation behavior of the connection. These variables include the (1) bolt pattern (2×2 and 2×3), (2) bolt size (3/4 in. and 1 in.), (3) bolt grade (ASTM F3125 Grade A325, A490, and F1852), (4) bolt pretensioning method (turn-of-nut and tension control bolts), (5) faying surface class (Class A and B), and (6) weld/bolt strength ratio. The variation in the connection characteristics covered a wide range of weld/bolt strength ratios from 0.50 to 2.00. The bolts were installed in oversized holes, and the specimens were assembled in a negative bearing condition to allow for a maximum slip distance. The load-deformation behavior of the combination connections was recorded and compared to that of the bolted- and welded-only control specimens. In all tests, the addition of welds increased the capacity of the connection. The investigation shows that the capacity of the combination connection with pretensioned high-strength bolts and longitudinal fillet welds can be computed by adding the capacities of the individual connecting elements while considering the strain compatibility.

In general, bolted connections are exposed to vibrations or repeated over-loads that could lead to self-loosening due to loss of preload. Pre-tensioned bolts in ring flanges are critical parts in Offshore Floating Wind Power Systems, and normally a certain percentage of the installed bolt connections are checked and re-tightened every year. This re-tightening is often done at a high cost and a short weather window due to strong winds and high waves. In this paper, three bolt dimensions (M20, M30 and M42) of the anti-loosening bolt system have been tested. The M30 and M42 bolt systems were preloaded and exposed to transverse oscillating loading, and the loss of preload as a function of load amplitude and number of cycles were measured and compared to standard bolts of HV type, exposed identically. The tested novel bolt system has shown superior capacities to withstand self-loosening, compared to standard bolts.

ABSTRACT Strength prediction of bonded joints remains a challenging task made even further complicated for hybrid joints in which adhesive is supplemented by mechanical fasteners. The present paper complements a series of experimental investigations on hybrid joints combining adhesive bonding with pre-tensioned bolts on galvanised and coated steel substrates, and considering two adhesives. The paper starts with modelling the strength of the bonded connection under the simultaneous action of tensile and compressive normal stresses, and shear. It then presents how these data are moulded into appropriate failure criteria and used the latter as input data for a subsequent full probabilistic finite element analysis. Following the paths of Weibull and subsequent researchers, joint capacity is predicted with reasonable accuracy for a dozen different combinations of substrates, adhesive, and pre-tension level – for which, on average, an accuracy slightly below 10% has been achieved. The results deliver meaningful insights into the subtle relationship between adhesive bonding, mechanical fastening, failure criteria, and resulting predictions methods.