The Importance of Strain Measurement in Materials Development

Interview conducted by Stuart MilneJan 6 2016

Ian McEnteggart, Composites Market Manager at Instron, talks to AZoM about the importance of strain measurement in materials development.

What is strain and why is strain measurement important to materials development and testing?

Strain is a measure of the amount of stretch or compression along a material (Normal strains), or the amount of distortion associated with the sliding of layers within a material (Shear strains).

Strain measurement is a key element of materials testing. The physical properties of materials are usually represented by a stress-strain curve and knowledge of the stress-strain curve allows engineers to compare different materials, and predict the behavior of a part or structure made from a particular material  (e.g. stiffness and failure strength) during processing operations (e.g. pressing and forging) and during service. Strain measurement also plays a vital role in Low-Cycle Fatigue testing that is used to determine the durability of materials subject to alternating strains during service (e.g. engine parts). Devices designed to measure strain are referred to as extensometers.

What are the challenges of strain measurement and how do external influences impact the test?

Strain measurement is challenging for a number of reasons. Extensometers need to measure a very large range of deformations depending on the material and the test – from a few microns when determining the elastic modulus of a stiff material like a metal or a composite, to something approaching a metre when testing an elastomer. It is also important that the extensometer does not influence the behavior of the material being tested. In some cases, physical contact with the specimen can cause premature failure. In addition to making very precise measurements, extensometers have to be able to survive the shocks of repeated specimen breaks. Another challenge is the need to measure strain while testing in difficult environments such as low or high temperatures and fluid baths.

What types of strain measurement solutions does Instron provide?

Instron provides a large range of strain measurement solutions. In a few situations it is possible to use the extension of the testing system to determine strain, but in most cases, a dedicated strain measurement device is needed. Where required, strain gauges can be integrated into an Instron testing system, but usually strain measurement will be performed using an extensometer. Instron provides a variety of extensometers: manual clip-on and long travel, automatic contacting, and automatic non-contacting video.

How does the AVE Non-Contacting Video Extensometer provide fast and accurate non-contact strain measurement?

The Instron Non-Contacting Video Extensometer 2 utilizes a high-resolution digital camera and real-time image processing to track the movement of contrasting marks on a test coupon. Strain is determined from the change in the distance between the marks divided by the initial mark separation.

This is the second generation from Instron. What’s new about the latest version?

The latest generation of Instron’s video extensometers incorporates dedicated high-speed digital electronics to allow it to process images and generate strain data at speeds of up to 490 points per second. This means that the extensometer can faithfully track rapid changes in the material during a test. Also, the time taken (or lag) to produce a new strain reading is less than 8 mS, which means that the strain from the video extensometer can be used to control the testing machine.  

Why does Instron offer such a wide range of solutions?

Instron offers a wide range of strain measurement solutions in order to satisfy the wide range of materials (from biological materials to the latest high-temperature super alloys), test types (from simple tension tests to advanced multi-axis fatigue) and test environments (from -150 to over 1600 ⁰C).

How does the Digital Image Correlation (DIC) software work and how does it offer a clearer picture about the specimen being tested?

This technique works by applying a random pattern to the surface of a test specimen, capturing a series of images of the specimen during a test, and then analyzing the images with an algorithm that determines first the displacement field and then the strain field for each image. The first image is captured when there is no strain on the sample. The image is then split into small subsets and the patterns within each subset of subsequent images are compared to the reference image and displacements are calculated. From these displacements, a strain map is calculated. The strain maps of all the strain components (axial, transverse, shear strain), along with maximum and minimum normal strains, can be determined. Compared to traditional methods of local strain (e.g. strain gauges) or average strain over a large gauge length (e.g. extensometers) measurement, full-field strain measurement yields an enormous amount of additional information that can help engineers and scientists better understand the behavior of materials and structures.

What are the specific strain measurement challenges associated with plastics testing?

The term “plastics” covers a wide range of materials, from thin films and sheet through foams and cellular materials, to rigid plastics. Tests on thin plastic films and foams can be adversely affected by the weight or sharp contact points of a contacting extensometer. These problems can be overcome by the use of a non-contacting extensometer, but care must be taken to ensure that the method of marking does not influence the material behavior. When testing rigid plastics, the determination of the elastic modulus requires high levels of accuracy at small strains. Testing is often conducted over a range of temperatures and the extensometer used needs to perform under these conditions. Finally, for productivity along with consistency of results, automatic extensometer solutions reduce operator dependence and are the preferred solution.

How has the AutoX Automatic Extensometer improved the testing of sheet metals?

In the automotive industry, control of the properties of sheet metals is vital to ensure consistent results from forming operations. The Plastic strain ratio (r-value) is an important indicator of formability. The r-value is usually determined during a tensile test and requires the measurement of both axial and transverse strain. The accuracy of the r-value depends critically on the transverse strain measurement.

Use of an Instron AutoX Biaxial extensometer, incorporating both axial strain and transverse strain measurement, can improve throughput and ensure accurate and consistent r-values. Automatic contacting extensometers can be more than 30% faster per test, compared to two clip-on extensometers. The repeatability will also improve as the extensometer is always attached to the specimen in the same location.

How are strain measurements integrated with the other test data and why is this important for Dynamic strain and DIC?

Instron testing systems allow strain readings from different strain sources to be recorded synchronously with the other testing machine readings, such as force and extension, during the test. Materials properties can then be calculated and reported automatically. When using DIC, images are recorded concurrently with the other testing machine data, which allows the data to be matched to the full-field strain maps.

What are your views on the future of strain measurement?

Mechanical contacting methods and strain gauges will continue to be employed for standardized testing, but non-contact measurements will gain wider acceptance. Advances in digital electronics will mean that the performance of non-contacting video ext

ensometers will continue to improve. The wider use of Digital Image Correlation (DIC) to measure 2D strain distributions will lead to the development and standardization of new test methods.

About Ian McEnteggart

Ian McEnteggart has a physics degree from Birmingham University and works for Instron in the UK as Composites Market Manager.

He has been responsible for developing electronic controllers, software and transducers, including video extensometers, for material testing applications.

He is active in the development of international standards for materials testing and transducer calibration in both ISO and ASTM and produces articles on materials testing technology and techniques.

He is currently responsible for Instron’s range of composite testing fixtures, transducer products, Digital Image Correlation Software and the Bluehill^® Composite test method suite.