Xianjie Yang

A Mathematical Model for Vibration-Induced Loosening of Preloaded Threaded Fasteners

A mathematical model is proposed for studying the vibration induced loosening of threaded fasteners that are subjected to harmonic transverse excitation, which often causes slippage between the contact surfaces between engaged threads and under the bolt head. Integral equations are derived for the cyclic shear forces as well as the bearing and thread friction torque components. They depend on the ratio of the relative rotational to translational velocities. The relationship between the dynamic thread shear force and bending moment is developed. When the external transverse excitation is large enough, it causes the threaded fasteners to loosen. Numerical results show that the dynamic transverse shear forces on the underhead contact surface, and between the engaged threads, decrease the bearing, and thread friction torque components. The effect of bolt preload, bearing and thread friction coefficients, the amplitude of the harmonic transverse excitation, and the bolt underhead bending on the bolt loosening are investigated. Experimental verification of the analytical model results of the bolt twisting torque is provided.

Novel Formulation of the Tightening and Breakaway Torque Components in Threaded Fasteners

New formulas are developed for the torque-tension relationship, various torque components, and breakaway torque values in threaded fastener applications. The three-dimensional aspects of the lead helix and thread profile angles and the kinetic and static friction coefficients are all taken into account. Two scenarios of the contact pressure between threads and under the turning fastener head are considered, namely, uniformly distributed and linearly distributed contact pressure scenarios. The effect of thread pitch, lead helix and thread profile angles, friction coefficients, and fastener geometry is discussed. Results from the new formulas are compared with the approximate torque-tension relationship provided in the literature. A percentage difference analysis indicates that the new formulas provide a significant improvement that would enhance the reliability and safety of bolted connections, especially in critical applications.

Achieving Uniform Clamp Load in Gasketed Bolted Joints Using a Nonlinear Finite Element Model

A three-dimensional nonlinear finite element model is developed for achieving a uniform clamp load in gasketed bolted joints. The model is used for both multiple and single pass tightening patterns. Geometric nonlinearity of the gasket is taken into account and plastic model parameters are experimentally determined. The effect of the tightening pattern, gasket loading and unloading history, and the preload level is investigated. The validity of the FEA methodology is experimentally verified. This study helps improve the reliability of gasketed bolted joints by minimizing the bolt-to-bolt clamp load variation caused by elastic interaction among the various bolts in the joint during initial joint-bolt-up.

Effect of Adhesives on the Mechanical Behavior of Thick Composite Joints

In this paper, experimental and numerical methods are used to study the deformation and interfacial failure behavior of an adhesively-bonded thick joint made of multi-layer S2 glass/SC-15 epoxy resin composite material. The adhesive material is 3M Scotch-Weld Epoxy Adhesive DP405 Black. Continuum damage mechanics models are used to describe the damage initiation at or near the interface and final failure process. The effect of adhesive overlap length, thickness and plasticity on the interfacial shear stress and normal stress are studied. Experimental and analytical data are used to validate the proposed damage models.Copyright

Novel Formulation of Bolt Elastic Interaction in Gasketed Joints

Novel formulation was proposed for studying bolt elastic interaction during the tightening of a group of fasteners in flat faced gasketed joints. The model was used for developing tightening strategies that would achieve a more uniform clamp load in the flange at initial assembly. Clamp load distribution is investigated for various tightening sequences and values for the gasket modulus of elasticity, gasket thickness, and grip length. An experimental setup and test procedure were developed to verify the numerical results produced by the elastic interaction model. Analytical and experimental results were presented and discussed.

Pullout Performance of Self-Tapping Medical Screws

This study investigates the effect of the pilot hole size, implant depth, synthetic bone density, and screw size on the pullout strength of the self-tapping screw using analytical, finite element, and experimental methodologies. Stress distribution and failure propagation mode around the implant thread zone are also investigated. Based on the finite element analysis (FEA) results, an analytical model for the pullout strength of the self-tapping screw is constructed in terms of the (synthetic) bone mechanical properties, screw size, and the implant depth. The pullout performance of self-tapping screws is discussed. Results from the analytical and finite element models are experimentally validated.

Analytical and Experimental Investigation of Self-Loosening of Preloaded Cap Screw Fasteners

In an effort to establish a theoretical outline of a criterion for preventing the vibration-induced loosening of preloaded threaded fasteners, this paper provides an experimental and analytical insight into the effect of the initial bolt preload and the excitation amplitude on the self-loosening performance of a cap screw fastener. A nonlinear model is used for predicting the clamp load loss caused by the vibration-induced loosening of cap screw fasteners under cyclic transverse loading. Experimental verification was conducted on the twisting torque variation and the effect of the preload level and transverse displacement amplitude. Comparison of the experimental and analytical results on the clamp load loss with the number of cycles verifies that the proposed model accurately predicts self-loosening performance.

Effect of Thread and Bearing Friction Coefficients on the Self-Loosening of Preloaded Countersunk-Head Bolts Under Periodic Transverse Excitation

A nonlinear mathematical model is developed for studying the self-loosening behavior of preloaded countersunk threaded fasteners that are subjected to cyclic transverse loads. Torque components acting on the bolt are divided into pitch and resistance torque components; the net torque determines whether or not the bolt will rotate loose under the external excitation. The accumulation of the differential amount of loosening rotation increments is converted into the gradual loss of the bolt tension/clamp load. Although the loosening model incorporates several system variables, this study is focused on investigating the effect of thread and bearing friction coefficients on the loosening of fasteners with coarse and fine threads. Model prediction of the self-loosening behavior is experimentally validated.

An improved model for adhesively bonded DCB joints

An improved analytical model is proposed for characterizing the fracture behavior of an adhesively bonded double cantilever beam joint under Mode I loading. Novel interfacial normal stress distribution function is used with a key parameter c that is determined using continuum mixture theory. In addition to the mechanical and sectional properties of the adherends, crack length, and overlap area, the model also incorporates the adhesive thickness and material properties as well as the crack tip rotation. Model prediction of the fracture toughness of the joint is entered into finite element analysis to simulate crack propagation under peel loading. The effect of various parameters on the joint fracture properties is discussed. Results show that the proposed model provides better correlation with published experimental data.

Cylindrical Bending of Bonded Layered Thick Composites

In this paper, a mathematical model is developed to describe the mechanical behavior of adhesively bonded multi-layer thick composites. Based on the proposed novel deformation field, the stress field is derived for the layered composite joint. Unlike conventional shell theory, the proposed model developed takes into account the transverse shear deformation in the composite and adhesive layers; the adhesive layers are normally much more vulnerable to cracking. The validity and accuracy of the proposed model is investigated by comparing the results with finite element prediction for various loading scenarios.