Uniaxial tests on composite laminates require the highest level of precision and stiffness - so that alignment can be established and maintained. Without a precision testing system (load frame), results can be compromised because composite materials are so sensitive to alignment.
Instron offers three precision platforms for testing composites and a wide range of load string accessories for tension/compression testing:
- Electromechanical: twin-screw systems for quasi-static mechanical testing from "0" to 600 kN (130,000 lbs)
- Servohydraulic: 2- or 4-column systems for static and dynamic testing from "0" to 5 MN (1,000,000 lbs)
- Electrodynamic: oil-free systems for static and dynamic testing in 1 kN, 3 kN, and 10 kN capacities (200, 600, and 2,000 lbs)
Accessory Items for Tensile Testing
A variety of grips are available for tensile testing flat composites coupons. The most popular are mechanical wedge grips and hydraulic wedge grips. For the most part, it is a matter of selecting a grip with the correct capacity, and making sure that the grip faces and associated dimensions are suitable for the length and thickness of the specimen tabs.
If it is necessary to meet the alignment requirements of NADCAP, for example, hydraulic grips are necessary. This is mainly due to the precision of the grip, and the highly repeatable nature of its operation. (Manual grips are subject to higher variability simply due to differences in operator techniques).
For non-ambient testing, extended hydraulic grips are available that keep the hydraulic "driver" portion of the grip outside the chamber, yet provide the convenience of hydraulic operation to the grip faces.
Adapters are available to attach fixtures directly to the hydraulic grips such that they can be left in place while performing other types of tests - such as compression, etc.
Instron is pleased to submit layout drawings to meet the needs of specific customer configurations.
Advanced composites are very sensitive to alignment, in that poor alignment will generate low strength values for tensile tests. There are 2 paths forward for alignment:
- Rely on the inherent alignment of the machine and load string (and align with shims if needed).
- Include hardware to enable alignment modifications
If a system is found to need alignment adjustment, an "AlignPro" fixture is recommended. The AlignPro fixture is placed between the load cell and the crosshead, and includes "jack screws" that allow small lateral (X-Y) and angular adjustments to a load string under load. This means that, with the AlignPro fixture installed, it is possible to adjust the alignment of the system in real time with an alignment cell installed, in order to minimize bending.
In order to guarantee that a system will meet the bending requirements of NADCAP, an AlignPro fixture is required for all electromechanical systems with high temperature extended hydraulic grips, and all servohydraulic systems.
Examples of Static Composites Testing Applications
- Tensile Testing Composite Laminates in a Chamber with Hydraulic Grips Tensile
- Testing Carbon Fiber Strips in an Environmental Chamber (ASTM D3039)
- Tensile Testing To NADCAP Alignment Standards
- Flatwise Tensile Strength Fixtures (ASTM C297)
- Tensile Properties of Isotropic and Orthotropic Fiber-Reinforced Plastic Composites (ISO 527-4)