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
Meeting the repeatability and bending requirements of NADCAP is challenging, and requires grips that are specifically designed for alignment-sensitive applications. Those include hydraulic wedge grips - originally designed for fatigue testing on hydraulic machines, and mechanical wedge grips that are intended for applications that require minimal bending.
Traditional manual wedge grips, which tend to be open-front/closed-back (not symmetrical), are not suitable for applications requiring NADCAP approval. For this reason, Instron has developed a new "Precision Manual Wedge Grip" specifically intended for high performance aerospace materials where superior alignment, minimum bending, and repeatability are paramount.