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Creep strain measurement

TWI patent© Haverkamp Patentanwälte

Component failure due to creep involves long-term deformation and exposure to high levels of stress below that of the yield strength of the material. Monitoring of creep deformation, which increases as a function of temperature, therefore, can be essential in preventing failure of components – particularly those used in high-temperature environments such as power plants and oil refineries.

Techniques for monitoring creep include replica metallography, strain gauges, X-ray diffraction, ultrasonic and magnetic methods which are often unreliable, and their application in high-temperature environments can be challenging.

Digital image correlation (DIC) is an optical technique for mapping the two-dimensional strain field over an area of the surface of a test specimen. A random speckle pattern is applied to this area using paint or via a surface coating. Strain calculations are based on correlation algorithms that compare images of the test specimen’s surface in different states, captured using digital cameras.

Drawbacks in high-temperature creep measurements using DIC include problems in ensuring adequate adhesion of the coating to the surface of the underlying parent material which may otherwise affect strain measurement. In techniques using speckle pattern interferometry, measurements can be limited to the coating rather than the parent material itself. In addition, a coefficient of thermal expansion mismatch between the coating and the underlying parent material may lead to the coating failing to withstand hightemperature cycling. Other drawbacks include the requirement to have a specific furnace to house the specimen under test and the need for a continuous inert gas supply for the duration of the test to prevent oxidation.

In October 2018, The Welding Institute was granted UK patent GB2528771, titled Creep Strain Measurement. The patent disclosed a method of using a device to monitor creep strain in an inspection surface of a component. The creep strain device, pictured, is mounted against the component.

It is positioned over the inspection area (2) and clamped to the surface (3). The device comprises a chamber with an aperture for receiving the inspection surface and is further arranged to provide a gas-tight seal so the internal environment within the chamber is sealed from that outside. This provides a localised sealing of the inspection surface.

An inert gas is used to protect the underlying parent material in the inspection surface from oxidation at high temperature for an extended period of time. The seal minimises the loss of this inert gas from the internal cavity (4) and the gas, e.g. argon, is introduced via a gas input component (5). The chamber has a window (6) through which the inspection surface may be imaged and DIC apparatus may be offered up to the window and used to acquire images of the inspection surface.

The method of using the creep strain device first involves providing two or more markings forming micro-indentations on the inspection surface of the component. The device is mounted on the component to place it within the internal environment of the chamber following indentation of the inspection surface. With stress applied to the inspection surface at an elevated temperature of 25 0C-1 ,OOO OC for a set time period, the change in geometry of the two or more markings is monitored in accordance with said time period using imaging apparatus.

The patented technology allows for in-situ in line assessment of creep deformation in the underlying parent material, and for high-temperature DIC to be incorporated without a continuous inert gas or specialist furnaces, and the use of a coating is non-essential. The localised sealing of an inspection surface enables numerous cyclic temperature changes from ambient temperature to over 600 0C so that long-term creep strain during operation of e.g. steam pipes and other high-temperature components in power plants, or other engineering facilities can be measured over many months, years or even decades.

Read the full patent here

This article first appeared in the Oct 2019 issue of Materials World, the member magazine of the Institute of Materials, Minerals and Mining.