Masataka Nomura

Proposition of Helical Thread Modeling With Accurate Geometry and Finite Element Analysis

Distinctive mechanical behavior of bolted joints is caused by the helical shape of thread geometry. Recently, a number of papers have been published to elucidate the strength or loosening phenomena of bolted joints using three-dimensional finite element analysis. In most cases, mesh generations of the bolted joints are implemented with the help of commercial software. The mesh patterns so obtained are, therefore, not necessarily adequate for analyzing the stress concentration and contact pressure distributions, which are the primary concerns when designing bolted joints. In this paper, an effective mesh generation scheme is proposed, which can provide helical thread models with accurate geometry to analyze specific characteristics of stress concentrations and contact pressure distributions caused by the helical thread geometry. Using the finite element (FE) models with accurate thread geometry, it is shown how the thread root stress and contact pressure vary along the helix and at the nut loaded surface in the circumferential direction and why the second peak appears in the distribution of Mises stress at thread root. The maximum stress occurs at the bolt thread root located half a pitch from nut loaded surface, and the axial load along engaged threads shows a different distribution pattern from those obtained by axisymmetric FE analysis and elastic theory. It is found that the second peak of Mises stress around the top face of nut is due to the distinctive distribution pattern of σ z .

Evaluation of Thermal Contact Resistance at the Interface of Dissimilar Materials

When jointed portions of structures and machines are subjected to thermal loads, various problems and troubles occur due to the difference in thermal expansions between mating parts. In order to accurately analyze thermal and mechanical behaviors of the joints, the effect of thermal contact resistance must be taken into account. In this paper, thermal contact coefficient, which is the reciprocal of thermal contact resistance, at the interface of dissimilar materials is quantitatively measured by infrared thermography. The target materials are common engineering materials such as carbon steel, stainless steel and aluminum alloy. It has been shown in the previous papers that there exists a significant directional effect in thermal contact coefficients when the mating surface is composed of different materials. That is, thermal contact coefficient has a larger value when the heat flows from the material with lower thermal conductivity to the one with higher thermal conductivity. The effects of contact pressure and surface roughness on the coefficient are also evaluated in this work. Using the measured data, an empirical equation to estimate thermal contact coefficient is proposed, for the purpose of engineering applications, which correlates closely with the experimental data.

Analysis of Thermal and Mechanical Behavior of Pipe Flange Connections by Taking Account of Gasket Compression Characteristics at Elevated Temperature

Sealing of contained fluids is the primary performance required for pipe flange connections. It is well known that the leakage is likely to occur when contained fluids at high temperature are concerned. Due to the differential thermal expansion of each part, bolt preloads that tighten a pair of pipe flanges tend to decrease. Therefore, it is significantly important for the joint safety to estimate the amount of bolt preload reduction at the design stage. In this paper, a finite element approach is proposed to analyze thermal and mechanical behavior of pipe flange connections at elevated temperature, by incorporating the stress―strain curves of sheet gaskets measured in the temperature range which covers its usual service condition. Then, the reduction rate of bolt preloads at elevated temperature is systematically evaluated. The analytical objects are pipe flange connections tightened with aramid sheet gaskets. When a pipe flange connection is subjected to thermal load, the gasket stress usually decreases along unloading curves. The temperature dependency of gasket stiffness is considered by defining Youngs modulus in unloading E * , which can be introduced into FE formulation using ordinary solid elements. Numerical results show that bolt preloads are decreased by as much as 30% of the initial value when using aramid sheet gaskets of 3 mm thickness. The effectiveness of the proposed numerical method has been confirmed by comparing the numerical results of bolt preload reduction to experimental ones.

True Cross Sectional Area of Screw Threads With Helix and Root Radius Geometries Taken Into Consideration

When evaluating the strength of threaded fasteners under external loads, stress area is commonly used. However, in order to elucidate the mechanical behavior of bolted joints more rigorously and extensively, it is desired to derive an analytical expression of the true cross sectional area of screw threads, with the effects of the helix and root radius taken into account. In this paper, a series of closed-form algebraic equations, which can calculate true cross sectional areas of internal and external screw threads, is derived. The equations obtained can be applied to both metric and inch screw threads with a coarse or a fine pitch. Then, the equivalent diameter of the true sectional area is defined and compared with the pitch diameter and stress area diameter.

A New Experimental Approach for Measuring Friction Coefficients of Threaded Fasteners Focusing on the Repetition of Tightening Operation and Surface Roughness

Torque method is widely used when tightening threaded fasteners. Although it has a great advantage of easy operation, the scatter of bolt preloads inevitably occurs due to the scatter of friction coefficients. Friction coefficients on the contact surfaces are affected by various factors such as joint materials, surface roughness, tightening speed, etc. To evaluate the effects of those factors with high accuracy, experimental errors must be suppressed as low as possible.In this study, a new simple test equipment for measuring the friction coefficients is designed, in which the strain gages are attached to the equipment to eliminate the experimental errors caused by the gages being attached to each test specimen. Using the equipment, friction coefficients on the thread surface and nut loaded surface are measured separately. Experimental results show that the surface roughness has a smaller effect when using threaded fasteners made of stainless steel than the case of carbon steel fasteners. As for the repetition of tightening operations, it is found that the removal of metal power, which is generated by the galling between the mating surfaces, is effective for reducing the scatter of friction coefficients.Copyright

Development of Test Equipment for Measuring Compression Characteristics of Sheet Gaskets at Elevated Temperature

Evaluation of the sealing performance of pipe flange connection is significantly important for the safety of pipe line structures. The compression characteristics of sheet gaskets primarily affect the mechanical behavior of flanged connections. It is known that the stiffness of sheet gaskets decreases with an increase in temperature. Therefore, the compression test must be conducted at various levels of elevated temperatures. From the experimental point of view, however, a great difficulty is involved in measuring the compression characteristics of gaskets at elevated temperature. For this reason, a definite testing procedure has not yet been established. In this paper, a prototype of compression test equipment has been developed for measuring the stress-strain curves of sheet gaskets at elevated temperature. The test equipment is compact and the experiments can be conducted with a fairly easy operation. It can control the gasket stress from zero to 30MPa while keeping the temperature of test specimen at different levels from room temperature to 300° C and higher. Aramid sheet gaskets are selected as test specimens. Experimental results show that the gasket stiffness drops with an increase in temperature. The shapes of the compression curves at different temperatures are similar, and those curves move in the direction of lower stiffness as the temperature is increased. It is concluded that the test equipment proposed here has a high promise to measure the stress-strain curves of sheet gaskets and estimate the sealing performance of pipe flange connections at elevated temperature.Copyright

Experimental Verification of Finite Element Approach for Designing Robust Bolted Joints Using Titanium and Titanium Alloy Bolts at Elevated Temperature

When bolted joints are subjected to thermal load, variations of the bolt clamping force are of great concern from the view point of joint safety. It is considered that titanium and titanium alloy bolts have high possibility for clamping machines and structures subjected to thermal load. Its specific characteristic of low thermal expansion expectantly works well to mitigate the reduction of bolt clamping force caused by thermal expansion. Low weight, low Young’s modulus and high resistance to corrosion of titanium and titanium alloy are also highly attractive.In this paper, the effectiveness of the numerical method proposed in the previous study is validated by experiments using bolted joints composed of titanium bolts and carbon steel plates. Then, thermal and mechanical behaviors of titanium and titanium alloy bolts are analyzed by finite element analysis in order to examine the applicability of those bolts for the joints under elevated temperature. Numerical analyses are executed as in the manner introduced in the previous paper, i.e., by incorporating the thermal contact coefficient into the finite element formulation. Numerical results suggest that titanium and titanium alloy bolts are favorably applied to the joints made of carbon steel whose clamping forces are likely to decrease under elevated temperature.Copyright

Evaluation of Specific Mechanical Behavior of Fine Screw Threads by Finite Element Analysis and Experiments

Fine screw threads are widely used for the bolted joints under severe running conditions. It is well known that they are effective to prevent thread loosening due to fine pitch. As for other mechanical characteristics, it has been reported in the previous paper using complex stress functions that the stress concentration at bolt thread root is higher than coarse screw threads and the fatigue strength of threaded fasteners shows a minimum value for varying pitch. However, the latter is questionable since the calculations were conducted under lots of hypotheses. In this study, stress concentration and stress amplitude along thread root are evaluated by three-dimensional finite element analysis, in which numerical models of the bolted joints are constructed so as to accurately represent the effect of thread helical geometry. It is shown that the stress concentration at thread root of fine screw threads is higher than that of coarse screw threads, and the maximum stress amplitude is likely to be lower on the contrary. Meanwhile, it is sometimes recognized that clamping forces of fine screw threads are smaller comparing to those of coarse screw threads when tightened with same torque. To clarify this contradictory phenomenon, tightening experiments are conducted, and it is found that the difference of the energy needed for tightening screw threads is found to be the major cause.Copyright

Finite Element Analysis of the Cyclic Stress Amplitude of Threaded Fasteners Using Helical Thread Models

Fatigue failures of bolted joints frequently lead to serious accidents in machines and structures. It is well known that fatigue failure is likely to occur around the first thread root of the bolt adjacent to the nut loaded surface and the run-out of bolt thread. That is because high stress amplitudes are generated there due to alternating external forces. Accordingly, it is significantly important to evaluate the stress amplitudes along the thread root in order to rigorously examine the fatigue failure mechanism of bolted joints. In this study, stress amplitude distributions along the thread helix including the thread run-out are analyzed by threedimensional finite element analysis. The numerical models of the bolted joints are constructed so as to accurately represent the effect of thread helical geometry, using the modeling scheme proposed in the previous study which analyzed the stress concentrations at the thread root. The analytical objectives are bolted joints with axisymmetric geometry except for the helicalshaped threaded portions that are subjected to axisymmetric external forces. It has been substantiated, based on the stress amplitude distributions along the thread helix, which the fatigue failures are likely to originate from the first bolt thread, as in the case of the maximum stress, and the run-out of threads. It has also been shown that a bolt with reduced diameter is effective for the purpose of lowering the stress amplitude at the first thread root of bolt. [DOI: 10.1115/1.4004559]

Analysis of the Tightening Process and the Cyclic Stress Amplitude of Studs and Tap Bolts

When subjected to alternating external forces, a fatigue failure of bolted joint is most likely to occur around the first bolt thread for the case of bolt-nut connections. It has been substantiated in the previous paper by three-dimensional finite element analysis, in which the numerical models of bolted joints are constructed so as to accurately take account of the effect of thread helical geometry. In the cases of bolted joints clamped by studs and tap bolts, however, fatigue failures sometimes occur at other than the first bolt thread and frequently initiate around the far end female threads. In this paper, using the FE models with correct helical thread geometry, stress amplitude distributions along the thread root including the thread run-out are evaluated for both male and female threads. It is shown that the maximum stress amplitude in the male threads occurs at the thread root slight away from the first thread and the maximum value in the female threads is generated near the far end of the engaged threads. Also shown is the contact pressure distribution pattern at the interface between the plate and the block, which is inherent to the bolted joints clamped by studs and tap bolts and may be the major source of the specific stress amplitude distributions.© 2011 ASME