The top twenty - TWI's score of top achievements in 1997

TWI Bulletin, May/June 1998

Structural integrity

• Conventional deterministic flaw assessment procedures have been conducted which incorporate partial safety factors, as recommended in BSI PD 6493: 1991. The target reliabilities implied by these procedures were compared with reliabilities derived from probabilistic fracture mechanics analyses. Significant differences were noted and a cautious approach in applying the partial safety factors is recommended.

• Extensive numerical stress analyses have been carried out by TWI on a medical device (stent) used to enlarge clogged-up arteries. The analysis results have been used to evaluate the fatigue life of the devices, which are subjected to fatigue loading caused by the heartbeat.

• A database has been developed to manipulate a variety of NDT data from about 500 weld flaws in butt welds, including actual flaw dimensions revealed by sectioning. The database has been employed to check and improve the suggested European Standards NDT defect detection acceptance criteria by evaluating the required number of ultrasonic probe angles to be used, and by assessing the appropriateness of ultrasonic signal amplitude thresholds.

• An extensive fatigue test programme and finite element analyses have been carried out by TWI on steel lighting columns. TWI's major input led to the first European Draft Fatigue Design Rules for Steel Lighting Columns (EN40).

• A procedure has been developed for assessing the structural integrity of corroded nozzles in pressure vessels. In many cases the procedure is less conservative than alternative procedures such as the draft AP1579 procedures. The code will allow lives of serving pressure vessels to be usefully extended by as much as several years.

Metals weldability

• Hardened weld areas in C-Mn and low alloy steels are susceptible to cracking in sour H ₂S media. Development of unique equipment capable of measuring pH in the test cell, at elevated pressure and during the test, has enabled reliable and direct determination of the effect of pH on hardness limits to avoid sulphide stress corrosion cracking in aqueous environments containing various partial pressures of CO ₂ and H ₂S.

• Intermetallic formation during a weld thermal cycle can reduce corrosion resistance of ferritic/austenitic stainless steels. Work has shown that there is no direct correlation between intermetallic volume fraction and loss of pitting resistance. Hence, in welding procedure qualification tests, reliance should be placed on direct measurement of corrosion properties rather than on quantification of intermetallic content by metallographic examination.

• Fabrication hydrogen cracking in welded ferritic steel assemblies is normally manifest only some time after welding is complete. Hence it is necessary to allow a delay period before inspection is carried out. Work has enabled certain combinations of weld joint details, procedure and materials to be defined where delay times can safely be reduced to as little as 12 hours. Equally important, the work has indicated situations where current guidelines are necessary, or even unconservative.

• Because of the attractive corrosion resistance and mechanical properties offered, industrial interest in the use of titanium alloy is increasing, noticeably in the offshore area. It has been demonstrated that titanium alloy pipes, including variants with Pd or Ru, can be welded using high joint completion rate procedures by a range of arc and non-arc processes, such as keyhole plasma, reduced pressure electron beam and radial friction methods.

Metals joining and surfacing

• A multimedia CD-ROM 'Staying in Shape' has been produced which is aimed at training welding engineers in understanding the cause of distortion and advising them of techniques which can be used to prevent distortion when welding and cutting. The program covers the different types of distortion, factors affecting distortion in welding, design steps and practical techniques to avoid distortion and actions to correct distortion after welding or cutting. The CD-ROM represents a significant advance in techniques for presenting visual information such as animations and video.

• A PC-based monitor has been produced for recording arc welding parameters. The PC monitor is capable of recording the current, voltage and wire feed waveforms for a range of arc welding processes (TIG, MIG, MMA and SAW). The suitability of the PC for monitoring arc welding processes has been demonstrated in recording the short term fluctuations in the dip and pulse waveforms. The development of the software enables welding engineers to use a standard PC for welding parameter monitoring which would otherwise require specialised instrumentation. 

• There is considerable interest in the application of laser welding to magnesium alloys to exploit the benefits of a low heat input process. Successful welds have been made with both CO ₂ and Nd:YAG lasers on a range of magnesium alloys, including trials on diecasting alloys which are generally considered difficult to weld.

• New tools have been developed for the friction stir welding of aluminium alloys which are capable of joining material over the thickness range 1.2-50mm in one pass. In addition, welding speeds have been increased by up to 90% in the case of 6mm thick 5083-0 aluminium alloy.

• A friction stir welding facility has been developed and constructed capable of horizontal and circumferential welding with a working envelope of 3.4m x 4m x 1.2m. An aluminium alloy cylindrical vessel, lm diameter x 2m length, has been successfully welded and pressure tested.

• The first ever friction stir welds have been made successfully in steel and titanium sheet. The welds were achieved using special tools and procedures.

• A new technique for combating wear has been developed which involves the fastening of inserts of a relatively hard material into the surface of components requiring protection. Specially designed self-flaring inserts - called Tribserts ^TM - are employed and, unlike self-piercing riveting, no die is required on the back face of the component.

• A 60kW, 200kV reduced pressure electron beam machine has been developed and commissioned for orbital welding of steel pipe. The system can handle a range of pipe diameters and thicknesses with either a horizontal or tilted electron gun. A novel external pipe seal and real time seam tracker were also commissioned successfully. 

Plastics and adhesives joining

1. A 400W diode laser has been used to manufacture through-transmission laser welds in a range of thermoplastic materials. The laser wavelength and the two materials to be welded are selected to ensure that the heat is generated preferentially at the interface between the two pieces. The outer surfaces of the component are largely unaffected by the process.
   
   
2. The plastics industry has been restricted to the use of pressure testing for the determination of the performance and long term degradation of plastics pipes and pipe joints. This method rarely tests the joints themselves. TWI has recently developed and manufactured a unique whole pipe tensile creep rupture test, to allow the true determination of joint performance and service life.
   
   
3. New procedures have been developed for the offshore industry to use adhesively bonded joints. The guidelines have been drawn up with the collaboration of DnV and Lloyds and are, therefore, a significant step towards the acceptance of adhesives in the offshore environment. Further to this, the quality assurance guidelines produced in the project will be submitted to ISO TC61 as a Committee Draft for discussion during 1998 and will then be circulated as a Draft International Standard, prior to taking up as an International Standard.