A number of examination methods can be used to identify weaknesses caused by incorrect fiber orientation. In particular, the fiber volume content and fiber distribution permit conclusions to be drawn regarding the stiffness and tensile strength behavior. A new ultrasound method offers the flexibility to capture these parameters on the finished component.

Ultrasound coupling: a shear wave probe contacts a glass fiber-reinforced plastic test specimen via a viscous couplant

Methods for Determining the Fiber Orientation in Reinforced Plastics

Ultraschall-Sensoren

Messtechnik

Sensoren / Messwertaufnehmer

Verbundwerkstoffe / Werkstoffverbunde

Kunststoff Werkstoffe

Computer-Tomografie-Systeme

Werkstoffprüfung

Optische Mess- und Prüftechnik

Ultraschallprüfgeräte

Faserverbundwerkstoffe

Andere Werkstoffe

Kunststofffasern

Verstärkungsstoffe

Röntgenprüfgeräte

Software für Optische Messtechnik

Groundbreaking Ultrasound  Shear Waves

To illuminate the inner workings of its products, one manufacturer of electrical appliances uses metrotomography, a non-destructive test method, for quality assurance. The method, which links computer tomography to the coordinate measuring, reduces the testing costs and extends testing capabilities, from product development to assembly control. 

Koordinatenmessgeräte

Zerstörungsfreie Werkstoffprüfung

Qualitätsnachweise

Metrotomography: Looking for Clues with X-Rays 

Measuring and testing include all the methods and equipment used for empirical investigation of standardized test samples or finished parts subject to mechanical, thermal or chemical loading. Increasing part quality and efficiency requirements make ongoing developments essential. 

Quality Control: Increased Pressure for Testing Provides a Spur 

The meantime, computer tomography has become a standard method in the industry for nondestructive part testing. The use and further processing of these data still present a significant challenge. GOM mbH, Braunschweig, Germany, has developed the GOM Inspect software, which is available free of charge and is intended as an evaluation software for 3D measurement data, including 3D inspections and STL mesh processing and permits independent evaluation of measurement results. 

GOM 

Auswerte- und Analysesoftware

Messtechniksoftware

Evaluation Software Free of Charge with CT Data Import  

During production, real time available data about the dimensional accuracy of products are invaluable. They do not only save material but also ensure that the production runs within its tolerances. Modern measurement methods can lead to devices, which are surprisingly easy to operate. 

Technical Progress Enables New Applications for Quality Control 

Bildverarbeitung, Machine Vision

Bildverarbeitungssysteme

Optoelektronische Bilderkennung, Bildverarbeitungssysteme

Kameras

Dickenmessgeräte

Radiografie-Geräte

High-Tech Inspires Real Time Measurement 

The requirements of measurement technology vary depending on the measurement task. In addition to the need for high accuracy or short measuring time, the computed tomography sensor also requires the measurability of objects that are difficult to penetrate with X-rays like multi-material workpieces. Different measurement methods are available for such applications.

The OnTheFly-CT (left) enables inline measurement and evaluation in the production cycle by saving the dead times for positioning the sample occurring in the conventional start/stop process (right)

16:9

Special Measuring Methods Extend the Range of Application for Computed Tomography

Bildverarbeitungs-Software

Fertigungsautomation

Bildverarbeitung

Geometrische Mess- und Prüfverfahren

Qualitätssicherung

Qualitätssicherung Kunststoff

Measuring at the Limit

New materials, complex designs and tight tolerances push conventional metrology to its limits in the quality assurance of lighting systems. That is why two Zeiss Metrotom computer tomographs are now in use at ZKW Lighting Systems. They quickly provide comprehensive findings and thus ensure stable production processes and shorter development times.

A modern headlamp consists of up to 300 individual parts that must be assembled and positioned with high precision.

Quality Assurance for Headlights

Inspection down to the Micrometer

The latest generation of coordinate measuring machines for computed tomography (CT) applications costs less, is smaller and easier to use. That is why these machines are increasingly replacing conventional 3D measuring machines. Werth Messtechnik has introduced two compact machines with new software capable of measuring injection molded or extruded plastic parts both atline and inline in real-time.

The TomoScope XS Plus compact machine with microfocus transmission tube offers Raster Tomography of several workpiece groups on various levels of the fixture.

New Compact Machines Accelerate the Spread of Computed Tomography

A Further Development  of Tube Technology

Wherever materials properties can contribute to weight savings, it is especially important to optimize the parts. A recent study by the Audi AG and Ottronic Regeltechnik GmbH shows which role non-destructive testing using active thermography can play. 

Active Thermography for Testing Mass-Produced Composite Parts 

Thermographie

A Fleeting Glance inside  Lightweight Materials 

Overmolded thermoplastic composites can give affordable weight savings for automotive applications. However, robust predictability procedures for these new materials are not yet state of the art. For this reason, Sabic has developed a test component that can be produced in a representative production process, and is able to validate predictions for all of the many different composite failure modes. To improve the simulations even further, an integrated simulation approach – one which takes into account the effects of processing on the material performance – is used. 

A New Test Beam Design Is Capable of Testing Different Composite Failure Modes 

Spritzgießsimulation

Faserverarbeitung

Spritzgießen

Werkstoffprüfung, mechanisch

Biegeprüfmaschinen

Zug-und Druckfestigkeitsprüfgeräte

Confidence in Composite Simulations 

A novel process enables the efficient consolidation of high-quality thermoplastic fiber-reinforced semifinished products. The material is heated with infrared radiation and consolidated quickly and efficiently under vacuum in the closed mold. 

Producing CFRP Efficiently in a Vacuum Using Infrared Radiation 

Kohlenstofffasern / -matten

Wärmebehandlung

Werkstofftechnik Wärmebehandlung

Gewebe, technisch

The Efficient Route to Tailored  Organo Sheets 

In-situ polymerization of Caprolactam to create fiber composite carrier structures with subsequent functionalization of the component through injection molding is very closely approaching the ideal of economic series production. One decisive equipment component here is the unit for preparing and injecting the reactive components. At K 2016, Engel Austria is defining the next milestone en route to series production, with a newly-developed reaction unit for which a patent has been registered. 

In-Situ Polymerization for Fully-Automated Production of Lightweight Structures 

Polyamide

Mehrkomponenten-Spritzgießen

Schäumen / Reaktive Verarbeitung

Controlled Reactivity 

Carbon-fiber-reinforced polymers continue to grow at double-digit rates. Polyurethane is the multitalented newcomer among these materials. Economic mass production makes innovative pultrusion methods very impressive. 

Fiber-Reinforced Polymers Are as Important as ever and Providing the Impetus for Global Growth 

Fasern

Reactive Systems and Fiber-Reinforced Composites 

In the characterization of materials, measuring the viscosity of natural-fiber-reinforced plastics and WPC poses a problem because of the fiber and particle geometries and the fact that the fibers may be very long. A special instrumented nozzle (in-line rheometer) allows these material data to be determined directly during injection molding and at high shear rates. 

In-line Rheometer for Natural-Fiber-Reinforced Plastics 

Rheologie

Messgeräte für Viskosität

Determining Viscosity Directly in the Injection Molding Process 

Although the first microdamage in a part occurs long before its total failure, the damage is extremely difficult to detect from outside. However, the Institut für Kunststofftechnik (IKT) is investigating a method using acoustic emission analysis to detect the onset of material-specific damage as precisely as possible. This permits an improved and more reliable  design of components. 

Acoustic Emission Analysis Predicts Damage and Material Failure 

Polycarbonat

Akustische Messgeräte

Schallemissionsprüfgeräte

The Sound of Materials 

Non-Destructive Series Testing of Large Fiber-Reinforced Components

“Region of Interest” (ROI) Computed Tomography for Local Analysis of the Fiber Orientation without Sampling

The local fiber orientation in composite materials can be analyzed non-destructively using microcomputed tomography. In practice, however, usually small samples are taken from larger components in order to avoid imaging errors. However, the ROI method allows parts of a size relevant to practice to be analyzed directly.

The local fiber orientation influences the mechanical properties of fiber-reinforced plastic (FRP) parts. It can be analyzed with the aid of 3D image data provided by microcomputed tomography (µCT). Although this method is non-destructive, samples with a few millimeters edge length are usually cut from larger parts in order to avoid imaging artifacts. Here, it is shown that images of sub-regions from large glass fiber-reinforced carriers, combined with orientation analysis of the 3D texture permit the µCT method to be actually applied non-destructively to parts of a size relevant to practice.

Fiber-composite components that are subject to high mechanical loads or safety-relevant must be reliably non-destructively tested, since their mechanical properties critically depend on their anisotropic microstructure, which varies within a part due to the production process. In general, glass and carbon fibers are stiffer than the surrounding plastic matrix and therefore have a strengthening effect in the fiber direction. The part behavior of fiber-reinforced plastic parts therefore depends crucially on the local volume density and the local orientation of the reinforcing fibers.

Spatial imaging with µCT provides this valuable microstructure information: the method generates three-dimensional volume images, whose intensities (grayscales) essentially reflect the X-ray absorption of the respective material component. Glass fibers consequently appear light-colored in a polymer matrix. For carbon fibers, the imaging contrast is low. If denser matrix polymers are used, the grayscale contrast is inverted, i. e. the matrix appears light colored compared to dark carbon fibers.

For analysis of the fiber orientation with voxel precision, 3D scans with voxel edge length Δx of 1/3 of the fiber thickness or finer have been used until now, i. e. for 10µm fiber thickness, that is ∆x ≤ 3.5 µm. Larger parts were cut up into small samples with edge lengths of a few millimeters in order to image them in high resolution in the cone-beam of the tomograph. These problems are overcome with region-of-interest CT (ROI-CT) combined with 3D texture-orientation analysis. Quantitative fiber-orientation analysis based on ROI-CT images is a non-destructive quality assurance method suitable for series testing.

The methodology is explained with the aid of a front carrier from a car engine compartment. The carrier of injection molded glass fiber-reinforced polypropylene (PP-LGF30) is approx. 90 cm long and 35 cm wide and serves to receive the hood lock, the hood buffer, the radiator and the front headlamp, and also contributes to impact protection. 30 % of the weight is accounted for by long glass fibers with 10 to 20 µm thickness and 10 to 15 mm length (before processing). The wall thickness of the structure is approx. 2 mm. Flow simulations of the manufacturing process formed the basis for choosing regions for 3D imaging with ROI-CT and fiber orientation analysis (Fig. 1).

If the µCT scanner has a suitable configuration (long source-detector distance), small sample sections from large parts can also be measured with fine three-dimensional scanning without the need to cut regions out of the part. The MetRIC µCT scanner from Fraunhofer EZRT is designed especially for such ROI-CT. It has a microfocus transmission tube and an X-ray flat-panel detector. The construction of the scanner allows high-precision positioning of the sample and measurement regions (regions of interest) within the sample. The extremely large focus-detector distance (up to 3.3 m) as well as 10 motorized degrees of freedom permit voxel edge lengths up to 3 µm, also in parts several centimeters thick. Artifacts that may occur in the volume-image reconstruction due to the ROI geometry are avoided by continuous sample rotation and by window functions during filtering of the projection images.

The computed CT- image represents the entire measurement region as a set of voxels. Each voxel corresponds to a cubic volume element in the region of interest. Correspondingly, individual fibers in the reconstructed CT-image can only be identified as individual image objects in the case of low fiber-volume contents and with very high local resolution. In the case of glass fibers, the fiber component can be easily distinguished from the matrix. For this material, image processing algorithms have been developed over the last decade that compute a preferred spatial direction (the direction of the fiber axis) from the grayscales of the individual voxels of the fiber component. For a detailed description of the voxel-precise methods and a comparison see [1]; for an overview of the different algorithm classes for global, locally averaged or voxel-precise fiber orientation analysis, see [2].

The algorithm applied here utilizes the Hessian matrix of the 2nd partial derivatives of the voxel grayscales in the immediate neighborhood of a pixel: as a local fiber-direction, the algorithm finds the one that is least curved in the grayscale histogram, i. e. the eigenvector of the smallest eigenvalue of the Hessian matrix. Uninformative values from matrix voxels are hidden. The 2nd order orientation tensor is subsequently derived. For this analysis, it was necessary to resolve the fiber diameter of 10 to 20 µm with at least three voxels. With a coarser resolution, the measurement results are actually shifted slightly toward isotropy; nevertheless, for the carrier investigated here, the orientation tensor could still be determined very accurately, even with voxel edge lengths of 45 µm, i. e. much coarser than the fiber thickness (Table 1). Figures 1 and 2 show ROI scans and analysis results.

With the aid of the 3D images (ROI-CT with high resolution was used here), the actual local microstructure can be compared with that predicted by simulation of the flow behavior in the part as a whole. For three partial regions, the 2nd order orientation tensors simulated with Moldflow were compared with the tensor components determined from the ROI-CT. In the dual domain mesh, the entire carrier has 170,000 elements with an edge length of approx. 2 mm. This 2.5D mesh only has surface information, while a 3D mesh contains information, or data, for the entire part thickness. For the simulation, the rotational-diffusion model with default settings was used. The CT data were subdivided into regions of comparable size, but cubic.

Figure 3 shows the diagonal elements of the simulated orientation tensors determined by image analysis. The components in the sheet plane agree very well qualitatively; though quantitatively, the image analysis shows greater anisotropy than predicted by the simulation. The strongest deviation can be seen for the component perpendicular to the thickness direction, with conspicuous peaks in the simulated values for the central layer. These features do not occur in the ROI-CT measurement.

Using a standard component from automotive engineering, it has been demonstrated that ROI-CT can generate 3D image data for large FRP components with a quality adequate for a detailed local fiber-orientation analysis. The prerequisite for this is that the sample can be positioned in the tomograph with a sufficiently large number of degrees of freedom and extensive movement paths.

It was also shown that, even with voxel sizes well above the fiber diameter, local orientations can be (at least qualitatively) analyzed. The results for voxel edge lengths between 45 µm and 3 µm are consistent, the flow direction was always identified as the main fiber direction, but also the typical incorrect orientation of the central layer was found at each of the resolutions investigated. With coarser resolutions, however, the local structural information is smeared, which distorts the results of the orientation analysis toward isotropy. However, comparisons of the results for voxel edge lengths 45 µm, 21 µm and 10 µm also show that it is not the voxel edge length alone that decides how good the orientation can be measured using a 3D image. The signal-to-noise ratio (interpreted three dimensionally, see [3]) plays just as much a role as the nature of the material.

The comparison with the flow simulation shows qualitatively good agreement, especially for the dominant tensor component in the flow direction. Image analysis using CT images is consequently very highly suitable for validating spatio-temporal simulations of injection molding processes. ROI-CT thus has the potential to become the standard tool for local fiber-orientation analysis of GRP parts. Comparable investigations on CRP samples are also highly promising.

[1] Wirjadi, O.; Schladitz, K.; Easwaran, P.; Ohser, J.: Estimating fibre direction distributions of reinforced composites from tomographic images. Image Anal. Stereol. 35(3) (2016), pp. 167–179.

[2] Baranowski, T.; Dobrovolskij, D.; Dremel, K.; Hölzing, A.; Lohfink, G.; Schladitz, K.; Zabler, S.: Local fiber orientation from X-ray region-of-interest computed tomography of large fiber reinforced composite components. Composites Science and Technology 183 (2019), 107786.

[3] Ullherr, M.; Zabler, S.: SNR Spectra as a Quantitative Model for Image Quality in Polychromatic X-Ray Imaging. arXiv and Nucl. Instr. Meth. Phys. Res. A 943, (2019) 162385.

Fraunhofer ITWM  

https://www.itwm.fraunhofer.de

Fraunhofer-Institut für Integrierte Schaltungen IIS Entwicklungszentrum Röntgentechnik

https://www.iis.fraunhofer.de

Montaplast GmbH  

https://www.montaplast.com

Ausgabenartikel

The (halved) front carrier from an auto engine compartment used for elucidating the measurement method, with four region of interest marked with crosses. The green area is shown in detail in Figure 1

Table 1. Diagonal components of the 2nd order orientation tensor, measured in 3D images of the region of interest with different resolutions

Fig. 1. Volume rendering of the region marked green in the Title figure and cross-sections from the 3D images of region of interest A3 with highest and lowest resolution

Fig. 2. Diagonal components of the 2nd order orientation tensor measured from the CT images. Sections through the 3D images in the xz-direction in the coordinate system from Figure 2 are shown (y: injection direction, z: thickness direction)

Fig. 3. Diagonal components of the 2nd order orientation tensor, comparison of the orientation predicted by injection molding simulation (orange) and the orientation measured in the 3D images with highest resolution (green). The position of the regions used in the simulation is marked red in the volume rendering

Name - Hauptartikel

Dr.-Ing. Thomas Baranowski supervises the research activities in the field of process and crash simulation for fiber composite parts in the “Chassis Materials & CAE” team at the Ford Research and Innovation Center Aachen, Germany.

Dascha Dobrovolskij is a mathematician with a doctorate in material sciences, and currently works in a project in the field of 3D analysis of microscopic image data.

Dr. Kilian Dremel received his doctorate at the department of X-ray microscopy at JMU Würzburg, Germany, since then he has worked for the NanoCT Systems of the Fraunhofer Development Center for X-Ray Technology (EZRT) in Würzburg.

Dr. Astrid Hölzing has been responsible for the applications field of fiber composite materials in the NanoCT Systems Group of the Fraunhofer EZRT since 2014.

Dr.-Ing. Günter W. Lohfink has been Head of Product Development at Montaplast GmbH in Morsbach, Germany since 2014.

Dr. Katja Schladitz has worked in the image processing department in the field of 3D image analysis at the Fraunhofer Institute of Industrial Mathematics ITWM in Kaiserslautern, Germany, since 1999; <a href="mailto:katja.schladitz@itwm.fraunhofer.de">katja.schladitz@itwm.fraunhofer.de</a>

Dr. habil. Simon Zabler has headed the NanoCT systems group of Fraunhofer EZRT since 2011.