Thread Root Radius Research Articles

Investigation on Flexural Fracture Behaviour of Bolted Spherical Joints with Crack Propagation in Screw Threads.

Bolted spherical joints, due to their prominent merits in installation, have been widely used in modern spatial structures. Despite significant research, there is a lack of understanding of their flexural fracture behaviour, which is important for the catastrophe prevention of the whole structure. Given the recent development to fill this knowledge gap, it is the objective of this paper to experimentally investigate the flexural bending capacity of the overall fracture section featured by a heightened neutral axis and fracture behaviour related to variable crack depth in screw threads. Accordingly, two full-scale bolted spherical joints with different bolt diameters were evaluated under three-point bending. The fracture behaviour of bolted spherical joints is first revealed with respect to typical stress distribution and fracture mode. A new theoretical flexural bending capacity expression for the fracture section with a heightened neutral axis is proposed and validated. A numerical model is then developed to estimate the stress amplification and stress intensity factors related to the crack opening (mode-I) fracture for the screw threads of these joints. The model is validated against the theoretical solutions of the thread-tooth-root model. The maximum stress of the screw thread is shown to take place at the same location as the test bolted sphere, while its magnitude can be greatly reduced with an increased thread root radius and flank angle. Finally, different design variants related to threads that have influences on the SIFs are compared, and the moderate steepness of the flank thread has been found to be efficient in reducing the joint fracture. The research findings could thus be beneficial for further improving the fracture resistance of bolted spherical joints.

The effect of cortical thickness and thread profile dimensions on stress and strain in bone-anchored implants for amputation prostheses

Skeletal attachment of limb prostheses ensures load transfer between the prosthetic leg and the skeleton. For individuals with lower limb amputation, these loads may be of substantial magnitude. To optimize the design of such systems, knowledge about the structural interplay between implant design features, dimensional changes, and material properties of the implant and the surrounding bone is needed. Here, we present the results from a parametric finite element investigation on a generic bone-anchored implant system of screw design, exposed to external loads corresponding to average and high ambulatory loading. Of the investigated parameters, cortical thickness had the largest effect on the stress and strain in the bone-anchored implant and in the cortical bone. 36%–44% reductions in maximum longitudinal stress in the bone-anchored implant was observed as a result of increased cortical thickness from 2 mm to 5 mm. A change in thread depth from 1.5 mm to 0.75 mm resulted in 20%–22% and 10%–18% reductions in maximum longitudinal stress in the bone-anchored implant at 2 mm and 5 mm cortical thickness respectively. The effect of changes in the thread root radius was less prominent, with 8% reduction in the maximum longitudinal stress in the bone-anchored implant being the largest observed effect, resulting from an increased thread root radius from 0.1 mm to 0.5 mm at a thread depth of 1.5 mm. Autologous transplantation of bone tissue distal to the fixture resulted in reductions in the longitudinal stress in the percutaneous abutment. The observed stress reduction of 10%–31% was dependent on the stiffness of the transplanted bone graft and the cortical thickness of surrounding bone. Results from this investigation may guide structural design optimization for bone-anchored implant systems for attachment of limb prostheses.

Development of C/SiC Fasteners for High-Temperature Applications

Continuous carbon fiber reinforced silicon carbide (C/SiC) ceramic matrix composites are the candidate materials for high-temperature structural applications owing to their high specific strength, low coefficient of thermal expansion, and moderate thermal conductivity. C/SiC based fasteners provide reliability with oxidation resistance up to 1,650°C for joining such C/SiC structural components used in reusable launch vehicles for space applications. The present study describes the methodology to realize C/SiC based M8 fasteners by isothermal-isobaric chemical vapor infiltration process using 2D-stitched carbon fabric preforms. The realized fasteners exhibited a density of 2.10 g/cm3, a tensile strength of 191 ± 3 MPa, a shear strength of 230 ± 69 MPa in non-threaded and 150 ± 86 MPa in threaded regions of shank at room temperature, a high-temperature tensile strength of 170 ± 12 MPa, and a tensile modulus of 70 ± 8 GPa at 1,100°C in the atmospheric conditions. The tensile failure of the C/SiC bolts was observed to depend upon the relative dimensions of fillet radius and thread root radius, wherein a small fillet radius was observed to be detrimental, as it resulted in failure at the head-shank interface of the bolts. The scanning electron microscope images of tensile tested C/SiC bolts indicate extensive fiber pullout, characteristic of quasi-ductile failure of bolts. It is clearly evident from the studies that the C/SiC fasteners are a promising alternative over the currently used molybdenum alloy fasteners.

Evaluation of the structural integrity and fatigue life of a hydraulic accumulator used for marine diesel engine according to the thread root radius

Evaluation of the structural integrity and fatigue life of a hydraulic accumulator used for marine diesel engine according to the thread root radius

Failure Analysis of a Ti6Al4V Screw Used in a RASL Procedure

A failure analysis investigation was performed on a Ti6Al4V medical implant screw that failed after 1 month of surgical implantation. Light microscopy, scanning electron microscopy, and finite element analysis (FEA) techniques were utilized to characterize the mode(s) of failure and fracture surfaces. Complete fracture of the screw was observed near the transition from the tapered thread at the back of the screw, to the flute portion towards the front of the screw. Fractographic analysis revealed significant fretting and flattening of the threads, indicating lack of fixation to the bone, and micromotion of the screw during implant life. Off-axis, reverse bending macroscopic beach marks, as well as microscopic fatigue striations were found on opposite sides of the annular fracture surface. These features suggest intermittent cyclic bending led to crack initiation, and propagation of crack fronts originating along circumference of the tapered thread transition region. Microvoid coalescence (overload features) were also found on the fracture surface, as well as secondary cracks along the screw circumference within the thread root. FEA was used to show the development of localized stress concentrations within the thread root radius at the taper transition region, consistent with observed crack initiation areas. The fracture morphology suggests that premature failure occurred because of interacting factors including: a lack of fixation between bone and screw, micromotion of the screw threads against the bone, and reverse bending fatigue within the tapered to flute transition region. These conditions may have been exacerbated by the use of an improper screw diameter or inadequate installation torque.

Finite Element Analysis of Worm Auger in Meat Grinder

The worm auger contributes to the pressures generated at the grinder plate face during grinding of meat. The axial pressure distribution along the worm auger during grinding was discussed. The pressure almost increased linearly along axial direction from feeding end to discharge end. The 3D model of the worm auger created in Pro/E software were imported into ANSYS 9.0 software and a linear static finite element analysis was performed. The visual deflection patterns the part experiences under "real world" loading conditions were obtained so as to complete the security check of strength. The results showed that the yield strength was largely improved with thread root radius increasing from 2 mm to 5 mm. Hence, this method can be applied to rapidly and correctly find those dangerous positions and existing imperfections of its structure, thus providing valuable data to judge its structural rationality before practical manufacture, so as to make further improvements and optimizations for its structural design.

Case study of Ti6Al4V pedicle screw failures due to geometric and microstructural aspects

Pedicle screws are widely used alternatives to improve the spine fusion in the treatment of several spine diseases. However, the screw fracture rates are still high nowadays. Efforts to provide design and material selection should be encouraged. This paper deals with the analysis of design issues related to failure event of 10 Ti6Al4V pedicle screws explanted from five patients who had rupture before 5years in use. The objective of this study was to correlate the mechanical failure with the geometric and microstructural concerns. Bending fatigue was the main mechanism responsible for the failures. Fractures occurred near the thread end or at intermediate thread level. In some cases environmental assisted cracking (EAC) was associated with failure initiation. The formation of lamellar or platelike microstructure, the small thread root radius and surface roughness were factors related to failures.

Fatigue assessment of piston rod threaded end

The present paper deals with the research on the influence of different thread type on the piston rod fatigue life. Research was done on hydraulic cylinder piston rod, which becomes subjected to fatigue failures after change of its design. Previously used piston rods used M74 × 3 threads, but redesigned rods are compromising M76 × 2. Due to changes in thread type the supplier was convinced that changes will affect lowering stresses in thread core which is true, but they have overlooked the fact that lowering the pitch will also lead to lowering the thread root radius which leads to the significant rise of stress concentration and therefore lower the fatigue life.

Probing the Elastic-Plastic, Time-Dependent Stress Response of Test Fasteners Using Finite Element Analysis

Abstract The evolution of global and local stress/strain conditions in test fasteners under test conditions is investigated using elastic-plastic, time-dependent finite element analyses (FEA). For elastic-plastic response, tensile data from multiple specimens, material heats and test temperatures are integrated into a single, normalized flow curve from which temperature dependency is extracted. A primary creep model is calibrated with specimen- and fastener-based thermal relaxation data generated under a range of times, temperatures, stress levels, and environments. These material inputs are used in analytical simulations of experimental test conditions for several types of fasteners. These fastener models are constructed with automated routines and contact conditions prescribed at all potentially mating surfaces. Thermal or mechanical room temperature preloading, as appropriate for a given fastener, is followed by a temperature ramp and a dwell time at constant temperature. While the amount of thermal stress relaxation is limited for the conditions modeled, local stress states are highly dependent upon geometry (thread root radius, for example), preloading history and thermal expansion differences between the test fastener and test fixture. Benefits of this FE approach over an elastic methodology for stress calculation will be illustrated with correlations of stress corrosion cracking (SCC) initiation time and crack orientations in stress concentrations.

Elasto-Plastic Fatigue Life Improvement of Bolted Joints and Introducing FBI Method #

The failures of bolted joints, which are the basic and popular fasteners in industry, are mostly due to fatigue. This paper presents an effective method to improve the elasto-plastic fatigue life of a bolt. A complete bolted joint is considered and modeled with the finite element approach to calculate stress and strain. The code based on crack initiation theory is developed to calculate the fatigue life of the bolted joint. Local plasticity and mean stress correction are both applied in this analysis. Among the many factors that influence failure and fatigue behavior in bolt-nut fasteners, geometry has the most important effect on fatigue life. The geometry of a bolt is classified to four decisive parameters: Thread Flank Type, Thread Root Radius, Thread Run-out, and Head Fillet Radius. However, sensitivity studies are performed to investigate the effect of each of the four geometrical parameters on fatigue life. Finally, a new bolt design configuration is proposed and the results are compared with the conventional bolt. This new design is named FBI which stands for “Fatigue-Bolt-Improvement” method. This modification in design and fabrication of bolts will greatly reduce the stress concentration at the critical area, and will increase the fatigue strength of the bolt as compared to the classic standard bolt.

Evaluating the Effects of Overtorque in Bolts

The correct tension or pre-load applied to a bolt is critical to the reliability of a bolted joint assembly. If the pre-load is too low and the joint is subject to cyclic loads, the fatigue reliability may be compromised. Alternatively, if the tension is too high, the reliability may again be questionable due to either yielding of the bolt or an increasing risk of environmental induced cracking. Simple laboratory tests have been performed to show that microhardness is sensitive to strain hardening in a bolt that has yielded. The thread root radius can also be measured to determine whether a threaded fastener has been stretched past the yield point. This paper summarizes the laboratory tests to evaluate yielding and presents a fracture mechanics-based approach to determine if a bolt is at risk from environmental effects because of too high a clamp load or tensile stress.

Effect of Shapes on the Delayed Fracture Strength of High Strength Friction Grip Bolts

As one of trial improvement of delayed fracture strength of high strength friction grip bolt, the mitigation of stress concentration of each part of bolt was carried out and there obtained the following results.(1) By cutting off incomplete thread up to effective diameter, the delayed fracture life can be extended.(2) The fine thread with bigger area of core section is more excellent than the coarse thread, but the effect is small in large diameter.(3) The delayed fracture strength is better if the thread root radius is larger.(4) Larger radius of head-to-shank fillet is desirable.(5) The usage of self-aligning nut is effective for the mitigation of the bending stress which grows in tightening and the said usage extends the delayed fracture life.

Low-Cycle Fatigue of Large-Diameter Bolts

New constant-deflection amplitude fatigue data on bolting materials are presented. The relationship of cyclic yield strength to static yield strength is examined. Several series of full-size stud fatigue tests are analyzed and compared to the basic failure curve to obtain experimental fatigue strength reduction factors for bolts and studs. The variation of fatigue strength reduction factors with thread root radius, stud size, thread taper, and other variables is noted. A new design fatigue curve for bolts is proposed which reflects the additional information available at the present time.