Technical Insight: Full Scale Four Point Rotating Bend Testing

TWI has been providing our Industrial Members with support with full-scale four-point rotating bend testing for years. Also known as a flexural test, bending tests measure a material’s resistance to bending forces to provide information on fatigue, flexural strength, modulus and stress-strain behaviour.

Often used as a quality control procedure for butt-welded joints, these tests can be guided or unguided. With guided tests, the test piece is bent around a ‘former’ with a specified diameter, often in accordance with welding procedure and welder qualification specifications such as ASME IX, ISO 9606 and ISO 15614 Part 1.

Our projects include core research programme (CRP) work, where we conduct research and development work for the wider benefit of our Industrial Members, joint industry projects that allow interested parties to act as sponsors in return for exclusive access to the project results and the opportunity to guide the direction of the work, and public-funded projects where we typically join a consortium of other organisations to solve challenges and achieve goals for the wider benefit of industry and society. In addition, we undertake exclusive, impartial, independent, and often confidential projects on behalf of specific Industrial Members.

Some past examples of TWI projects that incorporated full scale four-point rotating bend testing, include…

Core Research Programme (CRP) Projects

Our core research programme (CRP) projects are created to meet the needs of industry, typically solving challenges for the benefit of our wider Industrial Membership base. For example…

- Modelling Damage in Composite Materials

Composite materials are used in a wide range of industries to realise benefits including reduced weight, better corrosion resistance, lower whole life cycle costs, improved thermal, acoustic and vibration properties and no requirement for hot work when retrofitting. However, assessing damage in composite materials via testing can increase costs and lead times. To address this, TWI has investigated the feasibility of using modelling tools to study damage initiation and propagation in composite materials. This was based upon our understanding of material properties obtained from standardised experimental tests and an evaluation of existing techniques for damage modelling. From this we were able to verify and validate a selected modelling approach by comparison with benchmark samples. This project studied two aspects of modelling damage in composites; damage initiation and damage evolution - both in aerospace grade carbon fibre reinforced polymer, T700-M21. This included the study of damage initiation and evolution via continuum damage mechanics (CDM) and experimental results generated from static four-point bending tests. The study showed the promising value of using virtual testing to support the analysis of composite parts, either during the design phase or during the integrity assessment of in-service structures. In addition to conventional approaches using readily available commercial finite element software capabilities, TWI’s phenomenological approach to the study of damage in composites, involving a framework of experimental and modelling activities, achieved a closer representation of the behaviour of real materials for industry (Figures 1-4).

Joint Industry Projects

Our joint industry projects (JIPs) are created to address specific challenges that are typically of interest to particular industry sectors. Industrial Member companies are invited to sponsor the projects in return for access to the results as well as being able to provide input into the direction of the research to optimise the results for their desired outcomes.

- Fatigue Behaviour of Lined Pipes

Not only are carbon steel pipes that are mechanically lined with a corrosion resistant alloy (CRA) cheaper than clad pipes, but they are also quicker and easier to manufacture. The liner is secured in place with weld overlay cladding at the ends, allowing girth welds to be produced between sections of lined pipe, just as with clad pipe. However, when mechanically-lined pipe (MLP) is subjected to fatigue loading, there is a risk that the point where the weld overlay interfaces with the liner (the ‘weld overlay/liner triple point’) may be a critical point for potential fatigue failure. TWI created this JIP to measure the fatigue performance of MLP via full scale testing before developing an ultrasonic inspection technique for the triple point and developing approaches for engineering critical assessment (ECA) and finite element analysis to model the wrinkling behaviour of the liner. This allowed us to provide recommendations to provide operators with confidence in the safety of installed lined pipe, leading to increased safety and reliability of riser systems and reduced costs and lead times for sour fields.

Public-Funded Projects

Full scale four point rotating bend testing has also been used in a number of public-funded projects, often as one of several areas of expertise and capabilities called upon at TWI as we partnered with other industrial and academic organisations.

- FLECTION: Fatigue Life Prediction for Greener Air Travel

Funded by the European Union's Horizon 2020 research and innovation programme, the FLECTION project saw TWI work alongside experts from the University of Cambridge and RTM Breda of Italy as part of the ‘Clean Sky’ European research initiative that aimed to reduce CO₂ emissions as well as lowering the levels of noise produced by aircraft. The FLECTION project focused on the development of the contra-rotating open rotor (CROR) engine concept, which has been shown to offer massive reductions in fuel burn and CO₂ emissions but creates more engine noise for both passengers and people on the ground. TWI assisted with fatigue predictions for the CROR engine blade, taking account of residual stresses from nicks, dents and scratches. An extensive experimental campaign was performed to characterise the material (Inconel 718), measure the properties of the damaged material, and provide data for the computational model validation. This included characterisation of the elasto-plastic, fracture and fatigue properties of the undamaged material (at room temperature and 550°C), and four-point bend fatigue testing of the damaged material. The computational model was calibrated and validated with experimental data provided by the partners, providing a useful tool to support the design of turbine blades (Figure 5).

Dedicated Industrial Member Support and Other Projects

Much of our work is conducted for the benefit of individual Industrial Member companies, who approach us with specific challenges in order to take advantage of our knowledge, expertise and experience, as well as being able to utilise our extensive facilities as an extension of their own.

- Fitness for Purpose of Duplex Stainless-Steel Risers

TWI provided reassurance to Single Buoy Moorings (SBM) Monaco, who had themselves been commissioned to design and build an offshore floating production, storage and offloading (FPSO) vessel to be used in the Wanaea and Cossack fields off NW Australia (Figure 6). During pipework welding, SBM discovered unexpectedly high hardness values outside the design requirements in the weld zones where duplex stainless steel had been used for the rigid arm and riser piping. Example welds were sent to TWI where we were able to confirm the high hardness values, but otherwise considered the weld to be perfectly normal. To confirm this, TWI conducted a range of simulated environment tests, including slow strain rate (SSR) tensile testing and four-point bent beam tests at elevated temperature and pressure in a hot, sour brine solution. The H ₂S and CO ₂ contents were used to simulate field conditions. The tests established suitability of the welds for service, showing that the high hardness values were related to the strain induced by welding, which was not likely to be a problem under service conditions.

- Detection of Fatigue Crack Initiation on Welded Joints

Four-point bend testing was also part of a research project investigating the detection of fatigue crack initiation on welded joints under bending loading using acoustic emission (AE). TWI aimed to demonstrate the advantages of using AE monitoring in standard fatigue bending tests of welded joints, with the tests being based on standard fatigue test requirements in which specimens are subjected to four-point bending load conditions to determine S N curves (Figures 7 and 8). This project’s objectives were to detect crack initiation and propagation in welded steel structures (specimens) during fatigue bending testing using AE, and then quantify the accumulated damage of the structure, using the recorded data to inform future AE pattern recognition for damage of welded structures. Fatigue tests were performed according to a specific standard bending test for welded joints and monitored with a Vallen AE acquisition system, using four-channel VS150 sensors to verify the repeatability and accuracy of the accumulated damage identification and crack initiation. This project showed that the AE monitoring technique represented an accurate method of identifying crack initiation and crack propagation, and a potential solution for structural monitoring.

TWI’s experts conduct a wide range of projects for the benefit of our Industrial Members and wider industry each year, using a range of techniques and capabilities, including full scale four-point rotating bend testing. You can find out more about the process at TWI, here:

https://www.twi-global.com/what-we-do/services-and-support/full-scale-four-point-rotating-bend-testing