Metallurgical Observations on Fatigue Failure of a Bent Copper Tube
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TECHNICAL ARTICLE—PEER-REVIEWED
Metallurgical Observations on Fatigue Failure of a Bent Copper Tube G. Pantazopoulos
Submitted: 13 October 2008 / in revised form: 9 February 2009 / Published online: 26 February 2009 Ó ASM International 2009
Abstract Deoxidized high phosphorus copper (C12200, DHP-Cu) is the principal construction material in hydraulic and HVAC installations due to its superior thermal/electrical conductivity, formability, corrosion resistance, and antibacterial properties. However, design and installation deficiencies or aggressive environmental conditions lead very often to unexpected failures due to abnormal operation or improper handling of material during construction. Copper water tubes experiencing leakage at the bend area, after short-time period in service, were disconnected from a hydraulic installation and brought for failure investigation. Optical and scanning electron microscopy, employed for microstructural and fractographic evaluation, were used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the leakage was caused by low-cycle fatigue failure (\104 cycles), initiated from the tube outer surface and followed by brittle intergranular fracture. Final fracture occurred via the brittle intergranular failure process, when the remaining tube wall thickness reaches a critical size. The crack then becomes unable to sustain the operating load. Review of the design and installation conditions along with the special attention to material handling to avoid surface flaws (pits, scratches, gouges, etc.), which might be potential fatigue crack initiation sites due to increased stress concentration, are suggested as preventive actions to minimize the potential for further fatigue damage.
G. Pantazopoulos (&) ELKEME Hellenic Research Centre for Metals S.A., 252 Pireaus Str., 17778 Athens, Greece e-mail: [email protected]
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Keywords DHP-Copper Fatigue failures Brittle intergranular cracking
Introduction and Background Information Deoxidized-high-phosphorus copper tubes are extensively used in water, sanitary, heating, and refrigerant application due to their high electrical conductivity, heat transfer characteristics, formability, corrosion resistance, and antimicrobial properties. Characteristic failures of copper tubes may be attributed to stress-corrosion cracking, intergranular corrosion, fatigue and microbiologically influenced corrosion (MIC), see also Refs. [1–5]. However, design and installation deficiencies or aggressive environmental conditions (such as improper water chemistry or chemical substances that cause chemical attack) are very serious contributors of tube unexpected failures. Fatigue life of tube and related components, affected by thermal and/or mechanical stress cycling occurring during operation, is very sensitive to the design of the installation. In bent tube areas, tensile residual stresses are superimposed on the alternating stresses and increase the mean stress level (rm), th
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