Specimen Gripping Effects in Composites Fatigue Testing - Concerns from Initial Investigation
With an increasing market for fatigue testing of composites, where new customers are surprised and concerned by almost all specimens failing at the tab, we decided to take a look at whether there are some easy solutions in test setup.
The paper, Specimen Gripping Effects in Composites Fatigue Testing – Concerns from Initial Investigation, developed in partnership between Peter Bailey, Ph.D (Instron Senior Applications Specialist) and Dr. Austin Lafferty (University of Sheffield), presents the findings of an initial investigation into effects of end tabs and gripping force on specimens for fatigue testing of structural composites. Live measurement of specimen compliance comparing extensometer measurements with grip displacement showed the impact of gripping on the dynamic behavior of the specimen. Use of thermal imaging enabled assessment of how load is introduced and carried along the specimen.
“Is There an Easy Solution?”
Sadly, the answer is “probably not”, but the measurements do offer some insight into how easily results can be affected. This short study shows that the gripping pressure at the ends of the specimen can affect results more significantly than may be anticipated, and solutions which are well established for quasi-static testing may cause problems in cyclic tests.Download: Specimen Gripping Effects in Composites Fatigue Testing Paper
Understanding the behavior of inhomogeneous, anisotropic composite materials subject to complex loading patterns, such as Open Hole Tension/Compression, Compression after Impact (CAI), Bearing Load, and Vee Notch Shear, requires the measurement of 2D strain fields. Digital Image Correlation (DIC) is a non-contact, optical technique that compares images of a tested specimen’s surface to generate, full field, displacement and strain maps.
on Composite Specimen (Axial Strain)
on Composite Specimen (Shear Strain)
DIC works by capturing a series of images throughout a test and analyzing them afterwards. A typical system includes a camera system, lighting, and a software package to control image capture and to conduct post-test analysis. Recently, we introduced an integrated DIC System option to our very successful range of Video Extensometers.View: An Introduction to DIC
In today's world, conformance to standards is serious business. The cost of non-compliance is now enormous, especially where litigation can be involved. This is why every testing system, no matter how old or new, should be routinely verified for conformance to applicable national and international standards. It is not just good measurement practice, it is essential business sense.
Within the composites industry, manufacturers have to demonstrate that their testing systems meet the alignment requirements specified in many ASTM standards that reference tolerances for either bending stresses or alignment. It would be wrong to think that just because a material testing system is regularly calibrated for force, strain, and displacement that you are assured correct and reliable materials testing data. Load frame alignment can change for a number of reasons including changing grips or fitting new or replacement fixtures. This is something that Nadcap, the global cooperative accreditation program for aerospace engineering, defense, and related industries, is particularly interested in.
We are closely involved in the multi-disciplinary requirements needed to succeed with Nadcap approval. Working directly with Nadcap and ASTM to understand the criteria for alignment, we can provide the professional services that help a business meet those needs and remove the complexity. Our services include alignment test of the load string in conformance with Nadcap requirements for AC7122 for testing composite materials and also covers a recent requirement from Nadcap regarding alignment transducers.
Alignment transducers are precision-manufactured specimens designed to the geometry dictated by the testing standard. Resistance strain gauges are then attached to the surface of the specimen and the deflection is read with the use of a multi-channel data acquisition device. The quality of the manufacture of every item in this measurement system is critical for the accurate measure of strain across the specimen and calculation of bending in the machine.
The Nadcap alignment procedures require the use of essentially flawless alignment transducers, as they do not permit corrections for imperfections in the transducers. In response to these and other stringent requirements, we have developed a new and improved method for verifying the strain measurement capability of its alignment transducers.
This methodology has been accredited by NVLAP under lab code 200301-0 and enables us to provide superior alignment services to ASTM E1012 and AC7122. We can also provide stand-alone, accredited calibrations of the conditioning electronics and data acquisition devices and/or the alignment transducers strain measurement capability.
In addition to alignment, the Instron Nadcap service package includes system calibration and a regular preventative maintenance schedule that will ensure measurements and results are accurate and repeatable. Training to reinforce health and safety policies and software support, which enables immediate online interventions, ensures compliance and productivity.
Join us in Copenhagen, Denmark (19 - 24 July) where we will be showcasing our non-contacting strain measurement using Digital Image Correlation (DIC), as well as advanced techniques for dynamic and fatigue testing.
Ian McEnteggart (Instron Composites Market Manager) and Dr. Peter Bailey (Instron Senior Applications Specialist) will be attending the seminar to stay abreast of developing research needs and will be on hand to discuss the latest testing solutions with you. During the conference, Peter will be presenting a paper examining the application of electrical monitoring for fatigue tests on CFRP.
Downloadable White Papers
- Composites Gripping Guide
- Characterizing Complex Materials with Impact Testing
- A Review of Contacting Strain Measurement Techniques for Composites Laminate Testing
- The Effect of Frame Alignment on Tensile Test Data
- A Review of Current In-Plane Composites Compression Testing Methods & Standards
About Strain Measurement
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As Featured In
- AM&P Magazine: Using Digital Image Correlation to Measure Full Field Strain
- Aerospace Manufacturing: Taking the Strain