Array Ultrasonic Measuring of Fatigue Crack Growth Rates

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Published on 20 November 2014

As part of a number of exploratory research projects, TWI has been trialling the use of array technology for the real-time monitoring of fatigue crack growth. Establishing the growth rate of cracks during failure is a key part of assessing the integrity of a structure, as it provides improved understanding of the behaviour of materials and components under different loading conditions.

Improving the fidelity of structural assessments

In standard test procedures, the crack growth is approximated using NDT methods which require the test to be stopped periodically in order for measurements to be taken. This project's objective was to generate a system that essentially collects a real-time ultrasonic video of a crack as it grows to failure. This then allows the measurement of crack growth rate, ie da/dt, enabling a more sophisticated assessment of a structure's integrity.

Measuring the microscopic

Fatigue cracks are difficult to detect and track for a number of reasons: they often start at unknown susceptible points or from existing small flaws; they can be very small, ie measured in micrometres; and their propagation is subject to frequent changes in trajectory. Our key challenge therefore was to optimise the ultrasonic instrumentation and software to ensure that it was capable of detecting the crack tip and of tracking and measuring its growth in real-time.

Ascertaining the method's effectiveness

To verify the techniques and methods, a standard fatigue test specimen with a starter notch was fabricated. The ultrasonic probe was positioned to monitor the notch, and then the test was run in the fatigue testing machine. The video of the test showing the growth and failure due to a fatigue crack can be viewed online: http://youtu.be/XADf8MWPNLU

Following the verification trials, full-scale resonance tests were undertaken on pipes with known susceptible points. Figure 1 shows the pipe setup and the modified system designed to track the crack in a 2D plane. The ultrasonic techniques being developed include active phased array methods and real-time full matrix capture methods.

Fig 1: A full-scale resonance test with a modified two-probe system shown below

Summary

Growth of fatigue cracks can be monitored and tracked in real-time using array ultrasonic techniques
The data is a plot of the crack size at specific times during the fatigue test and the gradient of this plot gives the growth rate of the crack at any given time, as given in Figure 2. This result provides insight into the nature of the material and the variable rate of the crack growth, presenting valuable input to failure models
The video frame rate and the repetition frequency of the instrument can be adapted to measure growth rate even at the fast fracture period at the end

Fig 2: Graph showing the growth of the fatigue crack measured using the position of the crack tip from the phased array ultrasonic image

A technology with diverse applications

The monitoring technology is being further developed using a variety of other related methods. The final system being envisaged is a tool for TWI's resonance testing facility with a two-step function: (1) detect the onset of a fatigue crack in all possible susceptible areas and (2) monitor and track its growth to failure.

This system has the potential to be configured for utilisation as a permanently mounted monitoring system for a range of critical structures and components in industry, including wind turbine blades, risers used in offshore oil and gas recovery, mooring chains and aircraft.

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