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.

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.

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

In the "Clarity Fuel Cell" vehicle by Japanese automotive producer Honda, the rear bumper support is made of Tepex composite material based on polyamide (PA)6, and of Durethan (PA6) high-performance plastic. The plastic is supplied by Lanxess AG in Cologne, Germany, while the composite material comes from the company’s subsidiary Bond-Laminates. According to Honda company information, this is the first time a bumper support is applied that was produced in a serial hybrid molding process. The component is manufactured in the one-shot hybrid molding process, requiring only one single stage to process the engineering plastic and the composite material. The producer states this simplifies the production process, while reducing weights by approx. 50 % as compared to metal components. 

Fiber-Reinforced Bumper Carrier Made by One-Shot Hybrid Molding 

Polyamide

Kohlenstofffasern / -matten

Faserverarbeitung

Spritzgießen

Fasern

New Fuel Cell Vehicle Boosting Plastics  

Arkema and Lalou Multi have cooperated to develop and construct a boat made from recyclable thermoplastic composite materials. The hull and the bridge of the Mini 6.50 Arkema 3 “Innovation” prototype sailboat are made entirely from a recyclable thermoplastic composite, thanks to the Elium resin, infused with carbon fiber. 

Lightweight and Recyclable 

Thermoplaste

Boat Made from Recyclable Thermoplastic Composite Materials 

Current developments in lightweight construction intend to contribute to improving the structural integrity of hybrid components – without neglecting the energy and resource consumption this involves. A research consortium is concentrating on improving the production processes for integral form-fitting and cohesive material composites made from fiber-reinforced plastics and metals. Future applications in mass production intend to reduce CO₂ emissions in street traffic. 

New Mold and Processing Technologies Increase the Competitiveness of Eco-Efficient FRP-Metal Composite Hybrid Components  

Spritzgießsimulation

Hybrid-Spritzgießen (Metall / Kunststoff)

Hybridtechnik

Reliability Made Easy 

The high-quality mechanical characteristics of fiber-reinforced components stem from a load-adapted composition of polymermatrix and fibers. In order to remove the matrix material close to the surface, the Fraunhofer Institute for Laser Technology ILT developed a method that allows the outer layers of the surrounding polymer matrix to be removed without damaging the underlying fibers. 

Structuring Fiber-Reinforced Components and Removing their Coatings 

Werkzeuge für Faserverarbeitung

Technik Trend

kunststoffe-international.com

Ultrashort Pulsed Laser Radiation Removes Outer Layers 

Plastics materials and lightweight construction are closely connected with each other. The favorable ratio between strength and weight, as well as efficient generation by injection molding, including the freedom of design incurred, are only some of the benefits. However, the mechanical behavior of thermoplastics is very complex. This is why reliable characteristic values are necessary to dimension components with regard to respective load, and to utilize efficiently the potentials of lightweight construction.   

On the Track of Complex Material Behavior 

Prozesssimulation

Utilizing the Potentials of Plastics for Lightweight Construction 

Once known for rubbers, Lanxess has switched its focus in the last two years to engineering plastics, chemical intermediates and specialty chemicals. This transformation led to the founding of the joint venture, Arlanxeo, that will be celebrating its premiere at the K. The technical achievements are concentrated at present on the automotive industry and on electrical and electronics applications.  

Lanxess Has Undergone Reinvention and Is Presenting itself for the First Time with the  Joint Venture, Arlanxeo.  

Hochleistungskunststoffe

Kautschuk / Elastomere

Restructured and with a New Look 

A thermoplastic composite sheet based on TPU film reinforced with continuous carbon fibers has been developed for lightweight load-bearing structures. The orientation of the fiber plies in the thermoplastic composite sheet can be varied from unidirectional to multiaxial, and adapted to the load case. Economical and robust components can be fabricated using a laminate structure consisting of two thin outer layers of TPU composite sheet and a core of continuous glass fiber-reinforced polyamide 6.  

Thermoplastic Polyurethane Composite Sheet with Continuous Carbon Fiber Reinforcement 

Polyurethan

Halbzeuge und Zwischenprodukte

Ready for Large-Scale Production  

The mass production of fiber-reinforced polymer composites is based on the high reaction rate of modern resin systems. However, conventional approaches for the viscosity measurement require preparation time and therefore cause a blind spot at the beginning of the reaction. A process-oriented approach intends to close this gap.  

Process-Oriented Characterization of Fast-Curing Resin Systems 

Reaktionsharze

Resin Transfer Molding

Messgeräte für Viskosität

Viscosity of Fast-Curing Resin Systems 

For hard-soft combination with liquid silicone, polyolefins are a lower-cost alternative to engineering thermoplastics as carrier materials. To achieve a satisfactory bond between silicone and polyolefin, a multilayer film was trialed as an adhesion promoter. The material bond is produced directly during the injection molding cycle using an in-mold decoration film, so saving time and money. 

Polyolefins Modified for Use as Carrier Materials in Thermoplastic-Liquid Silicone Rubber Combination by In-Mold Decoration  

Polyethylen

Polypropylen

Silikone

Folien

An Incompatible Pair Stick Together 

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-Institut f. Wirtschafts- u. Technomathematik (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.