Composites News

Issue 7

Strain Measurement: Techniques for Composites Coupon Testing

Characterization of the properties of anisotropic and inhomogeneous composite materials for use in demanding structural applications require a wide range of mechanical tests. Furthermore, tests need to be conducted over a range of temperatures on materials conditioned in a variety of different environmental conditions. The measurement of strain is a key requirement in tests to determine and monitor the tension, compression, and shear properties.

Currently, most approaches to strain measurement in composites coupon testing use contacting methods involving bonded strain gauges or clip-on extensometers. Recent developments in non-contacting strain measurement mean these systems now offer similar performance to traditional contacting systems, as well as providing significant other benefits such as the ability to provide full field strain maps.

General Aspects of Strain Measurement for Coupon Testing

Measurement of axial strain in tension tests can be achieved using a single strain measurement on one side of the specimen. However, more consistent and accurate results can be achieved by using the average of a pair of measurements on opposite sides of the test specimen, in order to compensate for the effects of bending due to misalignment. For compression testing, the use of an average strain value derived from measurements on opposite sides of the specimen is required by most standards.

Biaxial Clip-On Extensometer

In principle, since the strains in tension and compression test specimens are generally uniform, either local strain can be measured using a strain gauge or an average strain over an extended gauge length can be measured using an extensometer. In some cases, the small size of the specimen gauge section used in an unsupported compression tests may prevent the use of extensometers and require the use of strain gauges.

Tests to determine shear properties (e.g. In-Plane Shear, Rail Shear, and Vee-Notch Shear tests) require the measurement of shear strain. Shear strain can be determined from measurements of axial and transverse strain. In Vee-Notch Shear tests, the strain distributions are non-uniform with the strain being concentrated between the notches; accurate measurements of these local strains requires the use of strain gauges.

Composites on the Move: The Growing Need for Dynamic Testing

In a talk at the recent GOCarbonFibre 2015 Conference in Manchester, UK, Dr. Peter Bailey outlined the history of developments in the performance and uses of composite materials in transportation and gave his view of the current state of composite materials testing and analysis.

The talk compared the well-established body of research into effects such as fatigue, high straining rates, and fracture in metals with the emerging work on the dynamic performance of composite materials. He also looked at some specific challenges of testing composites in fatigue and at high-strain rates. The need for improved methods of monitoring damage and defining “failure” of a composite material was emphasized and examples of Digital Image Correlation (DIC) and Thermoelastic Stress Analysis (TSA) techniques were given.

Exhibition Update

Advanced Engineering 2015

We have just returned from exhibiting at the Advanced Engineering exhibition in Birmingham, UK, where we were happy to demonstrate our capabilities.Our testing experts stayed busy speaking with both existing and new potential customers on a variety of topics.

This year, we demonstrated machines from both our Electromechanical (Static) and Dynamic product lines. A model 5969 tensile tester equipped with our AutoX750 Automatic Extensometer and Bluehill® 3 Testing Software provided an example of a powerful high-throughput testing solution. An ElectroPuls™ Test Instrument, equipped with our latest dynamic version of the Advanced Video Extensometer (AVE 2) and WaveMatrix™ Software, demonstrated the capabilities of our dynamic testing systems. We took this opportunity to showcase the optional Digital Image Correlation (DIC) Software that can be used with the Advanced Video Extensometer. The DIC software allows users to determine the 2D strain distributions. The power of this system lies in the simplicity and the consistency of the fully integrated configuration.

Advanced Engineering 2015

CAMX Exhibition 2015

Instron recently attended the 2015 CAMX exhibition - a joint venture between SAMPE and ACMA - and demonstrated our latest developments in non-contacting strain. This year we saw increased interest in:

  • Non-contacting extensometers due to their robustness and ability to test composites without damaging the device, offering comparable accuracy to bonded strain gauges without expensive consumable
  • Full-field strain mapping, which allows user to received more information about the performance of the materials when used in complex geometries as well as to validate their FEA models
  • Composites fatigue testing with a renewed interest in high-cycle fatigue testing in tension and flex
Instron at CAMX 2015