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NDT: Practical quantification of residual stress measurement

Residual stresses (RS) are crucial to the strength, fatigue life and corrosion resistance of structural components, and are often inherent to fabrication processes like welding, forging and heat treatments.  They primarily arise from the contraction of neighbouring material and differences in thermal expansion.

The magnitude of RS at expected failure locations is included in engineering critical assessments (ECAs), thus the more accurate the residual stress measurement, the less conservative the ECA can be.  This can lead to increased load limit state and fatigue life, enabling greater confidence and/or more severe loading conditions to be accounted for.  In addition, if the fatigue life could be extended, the time between inspection periods could be lengthened, leading to reduced maintenance costs as a result of minimising repairs.

Unless the RS field can be measured reliably, the magnitude of RS to be input to an ECA is assumed.

Background

In order to maintain lower operating costs and prevent catastrophic failures, it is crucial that RS are measured accurately, allowing decisions on component repair or replacement to be made efficiently.  Several RS measurement methods exist currently, classified as: destructive – contour mapping, block removal, splitting and layering (BRSL), slitting and boring; semi-destructive – hole-drilling; and non-destructive – neutron, synchrotron and X-ray diffraction.  These techniques are well validated with some providing high-resolution RS distributions.  However, they are often costly and time consuming to perform with most requiring a laboratory with dedicated facilities.  For on-site measurements, hole-drilling can be used, but it provides only a surface measurement and the site often requires repair after the measurement is performed.  To date, no on-site, through-thickness RS measurements can be routinely made.

Objectives

The project objectives are to:

  • develop and investigate the capability, accuracy and limitations of non-destructive ultrasonic techniques for quantification and monitoring of RS in metallic materials
  • develop a procedure for using the ultrasonic method to assess the RS of materials and welds on-site
  • generate tables with acoustoelastic constants and calibration blocks for the most common materials, to be used in combination with the above procedure
  • use the method to validate weld repair regions, in addition to providing further data for use in validating the stress relaxation clauses stipulated in current structural integrity standards
Figure 1. Monitoring changes in stress using electromagnetic transducers and phased array ultrasonic testing at different tensile loads.
Figure 1. Monitoring changes in stress using electromagnetic transducers and phased array ultrasonic testing at different tensile loads.

Work programme

This involves the following:

  • acquiring suitable material and then manufacturing specimens for acoustoelastic characterisation
  • defining and producing a procedure for determining the acoustoelastic constant for each material
  • establishing the ability and accuracy of each ultrasonic technique to measure known uniform and non-uniform stress fields in each material type
  • measuring the effect of cyclic loading on RS in parent material containing a known RS distribution and welded material
  • characterising the effect of initial RS magnitude, cyclic loading magnitude and stress-ratio on RS relaxation and fatigue life
  • disseminating the findings to industry and communicating the test method to TWI Members.
Avatar Francisco Torres Site Services Team Manager, Non-destructive Testing

Francisco joined TWI in 2017, initially as a Senior Project Leader before being promoted to his current role of Team Manager. He has a double MSc in Mechanical Engineer and Economics, and hands on experience in construction/inspection, pre-commissioning, and the commissioning of energy and petrochemical projects. Francisco originally entered the non-destructive testing (NDT) industry as an HSE Closed Bell Diver and progressed through PCN Level III certifications in Ultrasonic Testing, Phased Array Ultrasonic Testing and Time of Flight Diffraction. He has over 15 years’ onsite experience in Europe, the Middle East and Africa, and specialist expertise in the development of new techniques for phased array and electromagnetic transducers, such as the use of matrix probes and quantification of residual stresses.

For more information please email:


contactus@twi.co.uk