Included in this Issue: New AVE2 feature AverEdge32
The AverEdge32 is a new tool for the Instron Advanced Video Extensometer (AVE2) which primarily reduces the scatter and variability of ‘r’ value results during sheet metals testing, potentially reducing the need for costly retests. If you already have an Instron Advanced Extensometer you can add the AverEdge32 hardware and software to your existing system. The AverEdge32 can be purchased with a new AVE2 or purchased for an existing AVE2. Read on for more details.
What Does a Video Extensometer Bring to Your Universal Testing Machine?
It is very rare to find one product or tool that can meet all needs. In the field of materials testing, many different specimen materials, geometries and sizes often means a range of accessories are required. Wedge grips, side acting grips, pneumatic grips, hydraulic grips, high capacity grips, low capacity grips, high and low capacity load cells, and extensometers with various gauge lengths and travels are just some of the accessories that need to be considered when setting up your test system.
Extensometers are a tool designed for measuring strain, and in order to do so they need to be able to measure changes to a specimen's gauge length as it undergoes testing. There are many different types of extensometers suitable for different methods and applications:
- Circumferential – Measures the expansion of cylindrical specimens (often under compression)
- Axial – Typical direction of measurement when performing tensile and compressive tests
- Transverse – Usually measures strain across the specimen width - perpendicular to axial testing
- Averaging – Avoids the influence of axial misalignment
- Biaxial – Can measure strain in two planes at once, often axial and transverse
- Long travel – Used for high-elongation materials such as elastomers
- Crack opening displacement – Used with compact tension specimens and some concrete bend tests
- LVDT – Often used in compression testing
- Deflectometer – Often used with bend fixtures
- Capacitive – Used at elevated temperatures
- Bonded strain gauges – Strain gauges that are adhered directly to specimen
- Automatic – Improves productivity and repeatability
Contacting or Non-Contacting?
Many extensometers are contacting extensometers, meaning that they touch and may influence the specimen during testing. Contacting methods may be intrusive:
- Knife edge contact can induce localized damage, and care needs to be taken with gripping pressure to avoid slip at one extreme and damage at the other.
- A transducer hanging from the specimen may affect results (unbalanced mass applies torque)
- Inertia of moving parts may be problematic during high speed tests (example: long travel extensometers)
- Contacting methods at non-ambient temperatures require transducer components to resist environment
- Knife edges wear and need replacement to avoid slippage or further localized specimen damage.
- Some materials' ‘explosive’ failure behavior may necessitate the removal of a clip-on extensometer prior to failure.
Because non-contacting extensometers avoid these problems, they are the preferred solution for testing delicate specimens (latex, foil, film, etc) and components where clip-on devices are difficult to attach. They are also the best option when testing materials with high energy failures that can damage contacting devices, and when testing inside chambers where the environment is too hostile for contacting devices.
Instron's Non-Contacting Video Extensometers
The AVE2 and SVE2 are Instron's solution for non-contact strain measurement. The AVE2 and SVE2 use a digital camera and clever Instron algorithms to measure and track the position of marks on a specimen. The algorithm determines the center point of the two marks and states the measured gauge length. The algorithm will then continue to determine the center of the two marks throughout the test, even when they change shape as they elongate. Another benefit of the A/SVE2 is the ability to use any gauge length and travel combination which will fit into the field of view being used. No need for a number of different clip-on extensometers with fixed gauge lengths. The AVE2 uses patented CDAT technology to keep a constant density of air between the AVE2 and the specimen. The controlled and directional airflow in this low density air corridor reduces the deflection of light and improves accuracy.
Patented CDAT Fans
Normal laboratory ventilation creates air flows which diffract light, introducing errors that can be as high as several microns. The AVE2 uses patented CDAT fans (US 7,610,815 B2 and EP 1,424,547, B1) which create a constant airflow between the specimen and the camera, producing consistent results even if your air conditioner turns on during a test.
Polarised LED light is used to remove external lighting effects and create contrast between specimens and marks.
Patented Cross-Polarized Lighting System
No two labs have the same lighting conditions, and lighting can change throughout the day, which can lead to different results. To ensure the AVE2 always sees the same image, Instron uses a patented cross-polarized lighting system (US 7,047,819 B2 and EP 1,424,547, B1) that is able to produce repeatable results regardless of your lab's lighting condition.
The A/SVE2 is designed primarily for use on Instron frames, but the videos do have 0-10 volt capability to allow them to be used on frames which can accept a 0-10 volt signal but not an Instron strain connection. When used with a tripod it is a useful tool to use in a multi-machine type test environment.
2D DIC is a feature that has been added to the Instron AVE2 and can be used in combination with Bluehill Universal software. It allows for post-test 2 dimensional strain analysis of a test piece surface.
The A/SVE2 can be mounted for use at ambient or with a temperature chamber for measuring strain at non-ambient temperatures.
The A/SVE2 can be used with a biobath for measurement of strain in a submerged environment.
IF we start from the premise that the A/SVE2 are non-contact extensometers and that is where their advantages lie, then the A/SVE2 are great for testing films, foils, delicate elastomers, and anything where a contacting extensometer may influence the results.
The AVE2 is not just great for those non-contact applications and materials, it can also meet the requirements of ISO 527-2 with its 1 micron accuracy for the low strain modulus calculation requirement. Other standards the AVE2 can meet include but are not limited to ASTM D3039, ASTM D638, ISO 6892-1, ASTM E8, ASTM D412, and ISO 37.
Introducing the AverEdge32
The AverEdge32 is a new advanced feature of the AVE2 that allows users to comply with ISO 10113, ASTM E517, and JIS 2253 by measuring transverse strain using data captured from 32 points along the specimen gauge length.
The plastic strain ration (r-value) calculation requires an accurate and repeatable transverse strain measurement. AverEdge32 is the ideal solution for materials that exhibit localized plastic deformation (Luder-banding), such as 5000 series Aluminum sheet metal.
AverEdge32 is an optional advanced feature of Instron’s AVE 2 non-contacting extensometer. Standard extensometers with a single local transverse strain measurement can exhibit stepped changes in transverse strain data, making it challenging to obtain an accurate value. AverEdge32 measures transverse strain at multiple locations along the specimen edges and averages them to produce a smooth and repeatable value, which is essential when calculating the r-value of sheet metal.
Averedge32 utilizes the onboard processing power of the AVE 2 to calculate the averaged strain value and send it as a digital signal to the machine controller in real time. This functionality does not require test operators to add transverse markings to specimens; all specimens can continue to be prepared with axial markings only.
Features and Benefits
- Markless edge detection of specimen
- Improved results repeatability while simplifying the testing process
- Real-time averaging of 32 transverse gauge widths
- Thickness Correction eliminates the need for costly telecentric lenses that limit field of view and total elongation.
- Dual Use Lighting System utilizes the existing polarized LED light source and a passive back screen to simply and safely illuminate the front and rear of the specimen.
- Contrast Compensation provides robust tracking of marks or specimen edges even if lighting conditions change during a test.
The videos do not just test in tension, they can also be used for compression or bend tests.
The AVE2 in particular is close to answering the initial question of "is there a tool in materials testing that can meet all my needs?" The AVE2’s versatility and capability means for extensometers we can say…nearly.
Six decades of Instron optical extensometry
If you have any of these strain measurement devices shown below, we are no longer able to support their electronics and highly recommend you speak to us for an upgrade. Instron has been supplying video extensometers since the 1960’s.