A Comparative Study on Mechanical Behavior of Pipe-Socket Threaded Joints With Taper-Taper Threads and Taper-Parallel Threads Combinations by Finite Element Analysis and Experiments

There are two types of combination between external and internal threads used in threaded pipe connections for pressure piping specified in industrial standards like JIS as well as ISO. One is the combination that taper external thread of pipe is engaged with taper internal thread of a fitting. The other is that taper external thread of pipe is engaged with parallel internal thread of a fitting. Taper thread is always used for external thread outside the pipe wall. Both taper thread and parallel one are applicable to internal thread inside the fittings. This paper evaluates the mechanical behaviors of threaded pipe-socket joints (or pipe-coupling joints) and the difference due to the thread type combinations by means of axisymmetric finite element analysis for 3/4” and 3” joints. The analysis shows that the taper-taper threads combination establishes the full-length contact over the engaged threads but the taper-parallel has only a pair of threads in contact at the 1st engaged thread from the end of socket, and the difference results in the different behaviors of the joints. Stress and strain pattern also completely differ due to the difference in the engaged thread length. No significant effect of the size has been found in the present analysis for 3/4”and 3” joints. Experimental tightening tests and pressure leak tests have also been carried out for 3/4” and 3” joints with taper-taper threads combination. The measured experimental stress for 3/4” joints has shown an agreement with the simulated one fairly well. The pressure leak tests have demonstrated that the taper-taper threaded pipe-socket joints can hold internal pressure without leakage without using thread seal tape or jointing compound under low-pressure service condition. The 3/4” joints have started leaking at 1–4MPaG of internal pressure. The 3” joints haven’t shown leakage even at 6MPaG of internal pressure applied.

On the Design of Contact Member Surface Shape of Bolted Joints to Minimize Clamping Load Loss

Metal to metal contact between joint surfaces is widely used in bolted joints to obtain a rigid and a high performance connection. However, a significant amount of clamping load is lost when the joint is subjected to mechanical and thermal loading including creep and fatigue. In practice, to prevent bolt loosening, additional parts such as spring washers, double nut, spring lock washers, Nyloc nut and so on are used. Those methods are costly and influence the stability of the joint and affect its structural integrity. It is well established that a small compression displacement in clamping parts leads to a big clamping load loss in stiff joints. This paper discusses the relationship between connection stiffness and clamping load and presents a method that improves clamping load retention during operation by a careful design of the member contact surface shape. A single bolted joint with two clamping parts is modeled using finite element method (FEM). A method is proposed to obtain a specific stiffness by an optimized geometrical shape of the joint contact surfaces. The result shows that the contact surface shape based on a gradually varying gap can improve the retention of the initial clamping load. Furthermore, a formula of the connection stiffness based on the curve fitting technique is proposed to predict residual clamping load under different external load and loosening.

Experimental Assessment of the Dynamic Behavior of Polyolefin Thermoplastic Hot Melt Adhesive

In the last decades, the use of adhesives has rapidly increased in many industrial fields. Adhesive joints are often preferred to traditional fasteners due to the many advantages that they offer. For instance, adhesive joints show a better stress distribution compared to the traditional fasteners and high mechanical properties under different loading conditions. Furthermore, they are usually preferred for joining components made of different materials. A wide variety of adhesives is currently available: thermoset adhesives are generally employed for structural joints but recently there has been a significant increment in the use of thermoplastic adhesives, in particular of the hot-melt adhesives (HMAs). HMAs permit to bond a large number of materials, including metal and plastics (e.g., polypropylene, PP), which can be hardly bonded with traditional adhesives. Furthermore, HMAs are characterized by a short open time and, therefore, permit for a quick and easy assembly process since they can be easily spread on the adherend surfaces by means of a hot-melt gun and they offer the opportunity of an ease disassembling process for repair and recycle. For all these reasons, HMAs are employed in many industrial applications and are currently used also for bonding polypropylene and polyolefin piping systems. In the present paper, the dynamic response of single lap joints (SLJ) obtained by bonding together with a polyolefin HMA two polypropylene substrates was experimentally assessed. Quasi-static tests and dynamic tests were carried out to investigate the strain rate effect: dynamic tests were carried out with a modified instrumented impact pendulum. Relevant changes in the joint performance have been put in evidence. Failure modes were finally analysed and compared. A change in the failure mode is experimentally found: in quasi-static tests SLJ failed due to a cohesive failure of the adhesive, whereas in dynamic tests the SLJ failed due to an interfacial failure, with a low energy absorption.

Effect of Autoclave Cure Time and Surface Preparation on Film Adhesive Bond in Lightweight Material Joints

This study investigates the effect of cure time and surface roughness on mechanical performance of single lap joints (SLJ). Test joints are made of aluminum/aluminum or aluminum/magnesium adherends that are autoclave-bonded using a commercially available film adhesive. Joint mechanical performance is assessed in terms of the static load transfer capacity (LTC), fatigue life and failure mode. Except for the cure time, all the rates of the other autoclave-bonding parameters are kept constant; namely, the level of cure temperature and pressure, as well as the rates of autoclave heating, cooling, pressurization and depressurization. Test data, failure mode analysis, discussion, observations and conclusions are provided.

SCC Propagation Analyses in a Welded Joint Under Various Assumed Residual Stresses and Anisotropic SCC Propagation Law

In this paper, the results of evaluations of the stress intensity factor and of natural SCC (stress corrosion cracking) growth simulations under weld residual stresses are presented. The results reveal some characteristic SCC propagation behaviors. A surface crack is assumed at the welded joint of a mock-up of a pressurized water reactor (PWR) pressurizer nozzle. SCC propagation law for the weld metal is assumed to be anisotropic. SCC propagation in the surface direction is assumed to be 10 times slower than that in the thickness direction of the welded joint. The anisotropy is caused by different solidification rates of the weld metal in these directions. The material in terms of mechanical properties is set to be homogeneous as differences in the Young’s moduli and the Poisson’s ratios are not large between the weld and the mother metal. The analytical procedures based on the finite element method with an automatic mesh generation scheme for cracked structures, the results of analyses under a various residual stress distributions and some discussions are presented in this paper.

Flange Gasket Behavior Characterization for Service in Arctic Environment

With Oil and Gas exploration and production in low temperature arctic environment, new concerns arise concerning the application of the existing design rules for these conditions. Most codes and standards for bolted flange calculation do not include specific rules concerning application at low temperature. Especially, gasket performances under low temperature are neither tabulated nor easily available from other public sources. In this context, in collaboration with TOTAL, SCHLUMBERGER and SAIPEM sponsors, the CETIM has performed a program called ARCTICSEAL to characterize the flange gasket behavior in arctic environment. The goal is to check that gasket references, commonly used by these companies, are still usable under low temperature arctic conditions without main performance loss. The test program involves mechanical and sealing tests performed on 4 gasket types (sheet fiber based, sheet PTFE based, sheet graphite based and graphite spiral wound) with both NPS 8 and NPS 16 gasket sizes. Gasket performances are determined at −60°C, after loading at −25°C. Additional test involving hot/cold thermal cycling ageing (simulation of intermittent hot process fluid in the bolted flange) have also been performed. The result shows the ability of the tested gasket references to maintain good sealing behavior in most cases. Moreover, the hot/cold thermal cycling ageing improves the sealing performances for some of the tested gasket references. Moreover, a dedicated test stand has been developed to study the behavior of NPS 8 flange connection exposed to quick heat-up after exposure to low temperature environment.

The Influence of Elevated Temperature in Creep Relaxation of Various PTFE Gaskets Production Methods

Polytetrafluoroethylene (PTFE) products have been widely used for industrial sealing applications due to their outstanding chemical resistance as well as their electrical, anti-stick, impact resistance and low friction properties. Diverse PTFE flat gaskets production methods are available in the industry, which include the combination of a selected Resin Type and a determined manufacturing process, being able to provide a wide range of several types of PTFE gaskets, with different mechanical characteristics. The production method has significant impact on the mechanical properties of the PTFE gaskets, especially the creep relaxation property at elevated temperatures. Therefore, the gasket behavior at high temperatures and, consequently, the bolted flanged joint (BFJ) performance, directly depends on both selected Resin Type and gasket manufacturing process. One of the most used PTFE gasket types is produced from multiaxially expanded PTFE sheets. As it is known, different types of stretching, among other factors, confer to the gasket particular mechanical characteristics. This type of gasket exhibits a very low gasket seating stress, and it is the preferred type for most low bolt load flanges. This paper presents a study of the creep relaxation property at room and elevated temperatures of expanded PTFE gaskets manufactured by different Resin Types and processes of expanding PTFE materials. Experimental results are presented comparing EN 13555 [1] Qsmax, PQR and ΔeGc parameters of the produced PTFE gaskets.

Application of Miniaturized Experiments for Constitutive Modeling of Creep Relaxation of a Novel Textured Gasket Product

The material attributes that are fundamental for developing a candidate textured, ceramic-filled PTFE gasket, such as texture style/dimensions, filler material, thickness and so on, create a set of potential combinations that are not practical to experimentally characterize at the component-level one-by-one. Optimizing gasket performance, however, is essential to the operation of bolted connections associated with pressurized vessels that transfer media from one location to another. Gaskets are essential for these systems since they confer high levels of leak mitigation across a range of operating environments. A balance of both compressibility and sealability must be displayed in an optimal candidate gasket to be subjected to aggressive operating conditions. A novel textured PTFE material (termed textured) characterized using a miniaturized test platform. This new-to-market viscoelastic material features a dual-face, raised honeycomb pattern. Experiments on both flat (termed Flat) and textured are used to identify viscoelastic constitutive model constants associated with Burger theories. Considering that the test platform contains an elastic bolt that is tightened to a prescribe torque level, the gasket is subjected to creep relaxation. Test results on the small samples contribute to constitutive modeling. The influence of parameters such as filler material selection, torque level, dwell period, etc. are presented.

Effect of Autoclave Process Variables on Film Adhesive Bond With Polycarbonate Adherends

This study investigates the effect of autoclave curing process variables on the bond strength between film adhesive and polycarbonate adherends. Bond strength is measured in terms of the tensile shear load transfer capacity (LTC). Studied variables include cure temperature, cure pressure and their respective rates as well as the duration of cure time. Test coupons are made of two layers’ polycarbonate lexan adherends that are autoclave-bonded using aliphatic polyether film adhesive (Huntsman PE399). The relative significance of variables and variable combinations are investigated for their effect on the bond strength. Experimental test data shows interaction between autoclave variable cure temperature in combination with cure time, temp ramp rate and pressure ramp rate have significant effect on bond strength and failure mode.

Ratchet Boundary for Superimposed Biaxial Membrane Stress States

Thermal membrane and bending stress fields exist where the thermal expansion of pressure vessel components is constrained. When such stress fields interact with pressure stresses in a shell, ratcheting can occur below the ratchet boundary indicated by the Bree diagram that is implemented in the design Codes. The interaction of primary and thermal membrane stress fields with arbitrary biaxiality is not implemented presently in the thermal stress ratchet rules of the ASME Code, and is examined in this paper. An analytical solution for the ratchet boundary is derived based on a non-cyclic method that uses a generalized static shakedown theorem. The solutions for specific applications in pressure vessels are discussed, and a comparison with the interaction diagrams for specific cases that can be found in the literature is performed.