Experimental analysis of the fatigue life of threaded pipe connections under cyclic bending

When subjected to cyclic loads, fatigue cracks can cause failure of pipe systems joined by threaded pipe connections. In this study, the effect of contact conditions and coupling geometry is investigated experimentally. Fatigue tests under cyclic bending are carried out on three different threaded connection configurations. The resulting fatigue lives are compared and fracture surfaces are investigated. The fatigue cracks are found to initiate at the last engaged thread of the pin and fatigue life can be increased by applying PTFE tape or changing the coupling geometry

Fatigue damage identification in threaded connection of tubular structures through in-situ modal tests

The use of threaded connection is a valuable alternative to conventional welding in tubular constructions, e.g. pipelines, drill pipes and deep water risers. Those applications are normally exposed to environmental hazards – wave induced vibrations, temperature changes, etc. – as well as subjected to severe service conditions. A classical way to determine fatigue strength in many engineering situations is to calibrate material models by means of ad hoc designed experiments. Unfortunately, it is very difficult in this case even under laboratory conditions due to complicated stress and strain states in the contacting tapered helical thread surfaces of the connection. Therefore, a classical four-point bending fatigue test setup has been built with a real pipe specimen of 3.75 meter long, consisting of two standard API pipes connected by a threaded coupling, under unsymmetric (non-zero average) load control cycles. A complete vibration study has been carried out based on input-output modal tests for the entire period of the fatigue experiment. Input excitation is due to hammer impact and responses are recorded by accelerometers and by reusable dynamic strain gauges. The measured modal strains from the dynamic strain gauges allow for direct calculation of the modal curvatures, rather than deriving approximately from acceleration information. By comparing the measured modal parameters with those of a numerical model of the same structure in undamaged condition, damage detection, localization in the coupling and quantification are possible. This study leads to the following conclusion of practical use: the recent advancement in modal analysis, i.e. the reference based input-output combined deterministic-stochastic subspace identification, makes it possible to identify the structural modal properties from in-situ modal tests, which are performed while the fatigue test is ongoing. In this way the fatigue test is uninterrupted to avoid the problem of stress and strain disturbances happened in un-reversing load cycles test

Friction welding of ceramics to metals

This paper discusses the progress in a master thesis about friction welding of ceramics to metals. An existing friction welding machine has to be adapted for the experimental part of this research. The required capabilities of this machine are discussed in this paper. As an introduction, a general explanation about friction welding is given. The results of a literature survey on friction welding of ceramics to metals are discussed. The aim of this survey was to determine the process parameters required to obtain a good weld quality. It became clear that few literature exists on friction welding of these materials and that the cited values for the process parameters vary widely. Based hereon, a range of process parameter values was used to make certain design choices. Because a laboratory machine is aimed at, it has to be able to function at varying settings of the process parameters, including testing at higher rotational speeds. Finally, process windows illustrating sound combinations of friction pressure and specimen diameter are calculated. The design choices entail certain restrictions on the capabilities of the machine. These restrictions define the boundaries of the process windows

Weldability of micro-alloyed high-strength pipeline steels using a new friction welding variant

An innovative welding method for fully automatic joining of pipelines has been developed. The proposed welding procedure is a variant of the conventional friction welding process. A rotating intermediate ring is used to generate heat necessary to realise the weld. The working principles of the welding process are described. The weldability of the micro-alloyed high-strength pipeline steel API-5L X65 is experimentally investigated. It was found that the new welding process is suitable for joining this material. When welding with a sufficiently low heat input, a high weld quality is obtained. Under these circumstances the weld strength, ductility and impact toughness are high and fulfil the requirements of the commonly used standard EN 12732 for joining pipes

Extruded and injection moulded virgin PA 6/6 as abrasion resistant material

Polyamide (PA6/6) is often used as a tribological pair in abrasion prevalent applications such as hinges and sliders. PA6/6 is frequently processed by injection moulding and extrusion process. It is known that these processes influence the polymers mechanical behaviour, but their influence on the polymers wear response has not been studied. Hence the present research attempts to study the influence of different manufacturing processes on tribological behaviour for PA6/6. Wear tests were performed on a pin abrading tester (DIN 50322). Abrasion resistance of both extruded and injection moulded PA6/6 were tested at different loads (20 and 35 N). Single-pass (nonoverlapping mode) and multipass testing (overlapping mode) were used to understand the influence of clogging of wear debris. It is evidenced that with increasing load the specific wear rate decreases; moreover, fine abrasives tend to reduce the wear rate. In multipass testing a transfer layer clogged on the counterface that acted as a protective agent and lowers wear rate. Poor mechanical strength of injection moulded polymers is apparently compensated by microstructural response for having a similar wear behaviour between extruded and injection moulded PA 6/6. Hence a proper balance between microstructural and mechanical characteristics is an absolute must in PA 6/6 for better wear performance

Tribotesting of large-scale specimens: the necessity of proper design

Tribological tests have always been difficult and expensive to run, as the tests should as closely as possible mimic the application (Czichos, 1982). This has a number of implications: the tests cannot be sped up by applying higher contact pressure and/or velocity and the geometry and the environment of the test should match those of the application. Nonetheless most of the tribological tests are performed at small scale to minimize the cost of testing. This paper elaborates on the reasons why small scale testing, done on the cheap, often results in expensive mistakes

Experimental study of the friction and wear behaviour of a polymer disc / primer coating combination used in ball-joints by means of large-scale testing

C

Tribological behaviour of hardmetals

C