Earlier online detection of micro-cracks

Non-destructive testing (NDT) to determine the integrity of materials and components is of great interest to many manufacturing industries. OCAS detects micro-crack initiation earlier during fatigue testing, thanks to the use of acoustic emission.

Earlier detection of micro-cracks

OCAS has a wide range of test facilities to evaluate in-use properties. The faster micro-cracks can be detected, the more precise these measurements become. This is important for durability, life time estimation and prediction of structural performance.

Although most common NDT(*) techniques have great potential, some disadvantages cause limitations to their use. Not all NDT techniques can be applied to a wide thickness range, or have safety issues. Some NDT techniques have a too low sensitivity for micro-cracks, or only detect cracks at the surface. Other NDT techniques suffer from issues due to reflections at welds or are not fit to use as online detection during testing. Visual detection techniques cannot be used on corroded surfaces, etc.

In the case of fatigue resistance testing on welded components, OCAS tested several online micro-crack detection NDT techniques, including acoustic emission monitoring.

Acoustic emission is the phenomenon of radiation of acoustic waves in solids that occurs when a material undergoes irreversible changes in its internal structure, for example as a result of crack formation or plastic deformation due to ageing, temperature gradients or external mechanical forces. In particular, acoustic emission is occurring during the processes of mechanical loading of materials and structures accompanied by structural changes that generate local sources of elastic waves. This results in small surface displacements of a material produced by elastic or stress waves generated when the accumulated elastic energy in a material or on its surface is released rapidly.

The images below show results of crack detection during fatigue resistance testing on large-scale welded components at OCAS. Thanks to the distinctive acoustic signal in the time and frequency domain and the use of a noise filter, the (micro)cracks can be easily distinguished from machine and production noise. Acoustic events are detected to the microseconds range and time and frequency data is online available during the test.


Click images to enlarge

Acoustic emission monitoring was found to be highly useful in detecting crack initiation, propagation and final break through in case of fatigue testing of welded components. This NDT technique enabled OCAS to detect micro-crack initiation faster than by using other methods.

OCAS uses acoustic emission NDT for micro-crack detection for a range of in-use property performance tests such as e.g. bending tests:

Online raw data during bending test: micro-cracks + machine noise
Online filtered data during bending test: micro-cracks
Micro-cracks in bending test sample

(*)Non-destructive testing

Destructive tests are often used to determine the physical properties of materials such as ductility, strength, impact resistance, fracture toughness and fatigue strength. However, discontinuities and differences in material characteristics are often more effectively found by non-destructive testing (NDT).

According to the standards, non-destructive testing (NDT) is a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. NDT methods rely on the use of electromagnetic radiation, sound and other signal conversions to examine a wide variety of articles for integrity, composition, or condition with no alteration of the article undergoing examination.

Today, modern non-destructive tests are used during manufacturing and for in-service inspections to ensure product integrity and reliability. For constructions, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary for a safe and reliable fit-to-purpose use.

“In the world of online crack detection acoustic emission is a powerful tool, thanks to the direct online availability of frequency information up to 1 MHz.”

Maarten Van Poucke, Research Engineer Applications & Solutions department, OCAS