Carbon fiber testing

The texture, fiber angle and gap distribution, of carbon fiber fabrics, preforms and composites is a major testing task. For process engineers it is important to know that the fiber orientation of each ply in a stack remains unchanged throughout the entire manfacturing process. The EddyCus systems can help to investigate the inner structure of differently oriented multi-axial fabrics. Utilizing an advance eddy current scan (ECscan) the user can obtain information about

• the fiber orientation and stacking and
• the fiber distribution or spacing.

The EddyCus CF series provides advanced fiber structure analysis in high resolution across multiple layers. Defects can then be measured and easily classified by the user. The following errors are typically detected: 

• Distortions & misalignments,
• Wrinkles & overlaps,
• Fuzzy balls,
• Gaps & undulations,

The non-destructive testing method is particularly suitable for carbon fiber fabrics as well as for composites. Both, small and large parts can be scanned by the available EddyCus CF systems. 

The eddy current technology utilizes the electrical conductivity of the carbon fibers to generate the EC-scan. This feature differentiates the method strongly from other NDT methods. Therefore, the company has been awarded twice for its innovation strength. 

3D shaped parts particularly preforms can be investiated at critical locations, so called regions of interest (ROI), for potential wirnkles, gaps or distortion after forming and moulding of the part. Thefore a small sensor is moved by a robot across the surface to detect subsurface defects in the inner structure. The data can be analysed in the tool EddyEVA. Process engineers can compare this actual data on fiber flow or movement with their results from FEM or process simulation.

Main contributor of this solution is the Fraunhofer IKTS, which has the capability to use stereo camera to re-capture the shape of the part, then to create a path planning, and check for collisions before scanning the ROI with the eddy current sensor.

Automated evaluation is the key to control manufacturing process of carbon fiber production. The software EddyEva is a standalone tool to avoid errors when manually assessing parts. Once thresholds are defined all eddy current scans are evaluted the regarding fiber orientation, gaps or other flaws. Alterantively a reference can be used to compare and decide on good/ bad parts.

The novel eddy current-based technology allows to determine the fiber areal weight of carbon fiber fabrics without contact. The underlying assumption is: the quantity of conductivity corresponds with the amount of carbon fiber present in a certain area.

The EddyCus CF system is able to measure the grammage and carbon fiber volume content of carbon fiber non-wovens or short fibers at a high local areal resolution. Additionally, the system can be used for industrial inline process control.

• Non-contact
• Real-time
• Inline fiber areal weight monitoring or high resolution scan by individual fiber areal weight testing systems
• Monitoring via network and instant process adjustment upon material variation
• Time stamp or trigger controlled

The sensors can be stacked to monitor the entire textile width. This measurement information can be fed back to the processing machine, in order to adjust the manufacturing process for example of carbon non-wovens to keep it within tolerances. Additionally, each roll can be equipped with a datasheet showing the local fiber areal weight at any position of the roll.

The isotropy describes whether a material has the same properties in all directions e.g. a typical metal. Anisotropic materials have prevalent orientation and uneven distribution of mechanical load like UD-carbon fiber tapes. In particular for short fiber products such as carbon fiber SMC, it is therefore relevant to monitor the main angles in which the material is capable of distributing external stress.

The EddyCus technology allows to determine the state of isotropy by measuring the prevalent bulk fiber orientation. Similar to tensile tests in various direction the captured data provide an information the distribution of orientations, in the identical location. There is no cut-out and destructive test required.

Matrix defects, other composites and geometry/contour testing

The classic Eddy Current testing heavily relies on the presence of electrically conductive material. SURAGUS however advances the eddy current-based approach towards highly accurate and super-fast testing of low-conductive to non-conductive materials, such as glass reinforced plastics or pure polymers

For specific measurement tasks SURAGUS combines eddy current-based sensors with complementary ultrasonic or laser technology. These hybrid systems ensure an optimal solution for any challenging measurement task.

Matrix defects

Matrix system are typically non-conducting polymers with the task to protect and to stabilize the reinforcing fiber. From an electric perspective, CFRP  matrix material acts as dielectric separator between the conductive fibers. Dielectric properties can be captured using specific SURAGUS probes working at high frequencies, deeply analyzing the impact of chemical cross-linking, curing or overheating.

Other composites (e.g. GFRP)

SURAGUS high frequency eddy current technology is sensitive to a dielectric material property, called permittivity. The scanning system accurately measures the effective permittivity , displaying local variations in the analyzed sample. A SURAGUS sensor scan reveals the layup of a glass fiber reinforced plastic and any local inhomogeneities in high local resolution without any need for coupling media, X-ray radiation or even sample destruction.

Monitoring the permittivity change over time

• Cure monitoring
• Monitoring the degradation/aging

Mapping the homogenety of permittivity

• Identifying local curing defects
• Finding local damages / inclusions
• Analyzing the texture of GFRP

Quantitative permittivity control (via calibration)

• Measuring the permittivity
• Determining bulk material composition e.g. resin content of composite
• Dielectric layer characterization

Complementary sensors for contour and delamination testing

In addition to the eddy current-based sensors, the system can be equipped with laser profile sensors or ultrasonic transducers allowing to combine the advantages of each technology.

The laser profile sensor is ideal to measure surface roughness (topography) and contour. The line profile covers a large area of up to 100 mm length by scanning at vertical high resolution. This approach reveals information on loose stacks, fuzzy fringes, etc. in addition to the information on the inner texture of the sample captured by the EddyCus sensor.

SURAGUS hybrid systems with combined eddy-current and ultrasonic sensors, require a coupling media, but they complete the EddyCus solution by providing exact insights on delamination, porosity, cracks and inclusions. The eddy current sensors are fully ingress-protected and can be used in a water bath. It is also possible to integrate them to existing gantries for ultrasonic testing systems and to analyze the scan with the powerful SURAGUS EddyEVA software.