Tensile Testing Composite Laminates in a Chamber with Hydraulic Grips
Getting accurate, repeatable, and maximized results for aerospace composites requires a precision load strain and repeatable test conditions. In order to minimize variability of results and to get the best possible numbers for these rigid coupons, it is necessary to use a machine that has accurate guidance and is as stiff as possible - in all directions, and powered grips that clamp the specimen the same way for each and every test. This is especially true when performing tests in accordance with NADCAP, Airbus, GE, or other alignment specifications. Current standards to which this could apply include ASTM D3039, D5083, D5766, EN 2561, EN 2597, prEN 6035, ISO 527-4. ISO 527-5, and AITM 1-0007.
It is also quite common to have to perform these tensile tests at high and low temperature, which calls for an environmental chamber. The combined requirements for stiffness, alignment, repeatability, and non-ambient testing typically leads to a system configuration as shown in the photo: a high capacity, floor-mounted load frame (100 kN or 250 kN capacity), with extended hydraulic grips. In this fashion, the hydraulic "powered" part of the grip remains outside the chamber, whereas the gripping part that holds the specimen is extended into the chamber. This technique ensures that the entire length of the specimen is at test temperature - thus minimizing thermal gradients along the gauge length.
Precise alignment of the grips is ensured by the use of an AlignPro alignment device - this device alows for precise adjustment of the angularity and concentricity of the grips under both no load and loaded conditions. The alignment is verified using a strain gauged alignment cell (various cells are available conforming to different standards e.g. NACAP AC7101 and AC7122/1).
This configuration allows testing to be performed at temperatures ranging from -70° C to +350° C (-94° F to +662° F).
Strain measurement can be done via clip-on extensometry (over applicable temperature ranges), bonded strain gauges (logging strain via the test machine strain channels), or video extensometry, through the optical grade window on the front of the chamber.
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