The single most critical factor in getting good, consistent results from mechanical testing of composites is alignment.
The critical factors involved in achieving and maintaining excellent alignment include:
The Load Frame
Precision alignment starts with a precision frame. Meeting the bending requirements of NADCAP means that crosshead displacements need to be repeatable to within microns. This points to stiff, well-guided, zero-backlash electromechanical (5900 Series), servohydraulic (8800 Series) or electrodynamic (ElectroPuls) load frames.
Alignment Fixtures, Software, and Services
Even the most excellent load frames may require fixtures to align the load string - extended hydraulic grips for example. For this, Instron offers the AlignPro fixture - which fits between the load cell and the crosshead and with associated AlignPro Software allows the user to make adjustments under load in real time, to minimize bending.
With the AlignPro fixture installed, the user has the option to:
- Align the system using strain-gaged alignment samples supplied by Instron or manufactured in-house
- Have Instron provide alignment services
Instron is committed to providing all the technology and services necessary to meet the most demanding aerospace requirements.
Grips
Hydraulic grips are the most consistent, operator-independent, repeatable grips for alignment-sensitive applications. Whereas mechanical grips are also suitable for testing composite coupons, meeting the repeatability and bending requirements of NADCAP is best accmplished with hydraulic grips.