Key Engineering Materials Vol. 809

Abstract: The surface condition of carbon fibre reinforced plastic (CFRP) substrates is decisive to obtain high bond strength and lifetime of adhesively bonded parts. Those surfaces were adjusted in terms of their microscopic topography by means of peel plies and release foils. The subsequent surface treatment via atmospheric pressure plasma jet or vacuum blasting allowed the modification of the microscopic roughness as well as the surface chemistry. Those configuration were assessed using surface analytic methods as well as quasi-static and cyclic fracture tests on single lap shear specimens. The microscopic surface roughness, if at all, only showed a small influence on the bond strength. Despite release agent residues, fracture was found within the fiber-matrix interface, which caused difficulties in evaluating the effect of surface pretreatments on the adhesion strength. Fatigue tests revealed a lifetime reduction of uneven microscopic rough surfaces, which was assigned to stress concentrations at the tip of asperities. The crack propagation was accelerated in case of release agent residues. If surfaces were free of contaminations, no differences between microscopically smooth and slightly structured surfaces were found. Overall, fatigue testing on single lap shear specimens showed an increased sensitivity with regard to the assessment of surface morphology.

Abstract: Current elevators are mostly designed as rope-dependent elevators. Main components in the roping system are the deflection sheaves which are conventional manufactured of grey cast iron. Due to the high weight of cast iron sheaves there is a high potential for reducing mass, especially when regarding aspects of effort, safety and ergonomics while assembly and maintenance in the elevator shaft. Within the framework of a R&D co-operation the Chemnitz University of Technology and the AMB Oberlungwitz GmbH developed a fibre-reinforced plastic sheave that comply with the requirements of lightweight design. The technological basic approach to realize that development is compression molding of glass-mat-reinforced thermoplastics (GMT). The project includes the whole development-chain, consisting of part design, tool design, process chain arrangement, parameter studies as well as validation of specimen. In the course of the project appeared a high potential for improvement of the part properties by alternative design solutions. In this context current activities are focused on multi material design methods such as combining GMT with other thermoplastic prepregs. In this manner it is possible to equip every local area with the specific properties that are required. For example the ribs of the sheave, that receive highest values of stress, can be made of materials with continuous-fibre-reinforcement while the basic part body consists of GMT, which is long-fibre-reinforced. This method also enables to avoid process influenced effects like the segregation of the fibre-matrix distribution in GMT. Additionally the input of different materials offers chances to inlay non-preheated prepreg blanks into the compression mould, so that the amount of the preheated material volume can be reduced. In this way cycle times and also lost of temperature due to transfer times of the heated blanks to the mould can be reduced.

Abstract: Like all additive manufactured parts, FLM (fused-layer-modeling) components are characterized by a large number of joints between the similar and different materials. These joints can be divided into five categories:- Permanent cohesive bonds between single layers of the same thermoplastic material,- Permanent adhesive bonds between single layers of different thermoplastic materials- Temporary adhesive bonds between single layers of different thermoplastic materials,- Permanent adhesive bonds between a single layer of thermoplastic material and a non-thermoplastic base material,- Temporary adhesive bonds between a single layer of thermoplastic material and a non-thermoplastic base material.The first three types of bonds describe the binding process when parts are manufactured of base material, a second permanently bonded component or a support material that has good release properties. The last two types of connection describe the permanent connection between the printing part and a non-FLM-component or the temporary joint between the printed part and the build platform during the printing process. The resulting compounds can be characterized as material compounds.Since the FLM process is mainly used for design and hobby applications, the fundamentals of the bonding processes and their influence on the internal structure of the material are hardly described. Some information can be taken from the literature for welding plastics and the production of hybrid parts by injection molding. Due to the different pressure and temperature conditions as well as the very large surface-volume-ratio of the discharged strand occur significant differences in the following factors: melt flow, solidification time, formation of surface layers and wetting of the surfaces.The investigation and characterization of these effects as a function of external process parameters is an important constituent for the further development of the FLM process into an industrially suitable manufacturing process for thermoplastic components with very high lightweight and complex geometry in small series etc.

Abstract: In order to enhance lightweight potential and fabrication efficiency of structural parts from carbon fiber-reinforced plastics, the approach of locally tailored component properties has proven effective. For braided hollow profiles, however, technical possibilities to achieve process integrated adaption of a components’ mechanical properties are limited. Therefore, a novel braiding process approach for cycle-time-neutral adjustment of a triaxial braids filler yarn count is examined and its advantages for component design are illustrated. The main objective of this study is to evaluate the approaches applicability with regard to process stability and disturbance of product quality. Two consecutive test series are carried out to identify the most influential process variables and to link the variation of these variables with multiple criteria of process quality. A clear dependency between quality and braid angle, braid yarn tension as well as the number of filler yarns the approach is applied on is derived. On that basis a set of process variables for faultless application of the concept is pointed out.

Abstract: This work is part of a publicly funded project called ReffiMaL (resource efficient material solutions for power electronics), which aims to substitute electroplated Nickel (Ni) as contact material in power electronic modules. The baseplates of these power electronic modules are based on the metal matrix composite material AlSiC, which needs to be coated to become solderable. Today, it is state-of-the-art technology to coat the baseplate with electroplated Ni to form an adhesive layer to the system solder. In this paper we present a performance comparison of physical vapor deposited (PVD) Ni and electroplated Ni. The main advantage of PVD Ni is a significant reduction of layer thickness compared to the electroplating process. Second advantage of PVD Ni is the limitation of the deposition to areas that get soldered, in contrast to a non-selective electroplated coating. When deposited by PVD at room temperature, Ni exhibits columnar growth patterns, whereas electroplated Ni tends to form a laminar layer. The columnar growth leads to an increase in interface area affecting phase formation behavior. To compare both adhesion layers, we investigate the phase formation after soldering with a Sn based soft solder-copper composite material. The baseplates are reflow-soldered at different temperatures and process times. Temperature varied between 270°C and 400°C. The corresponding process time ranged from 10 to 40 minutes. We inspect the samples optically to determine the phase formation. Intermetallic phase (IMP) composition is evaluated using energy dispersive X-ray analysis (EDX). ReffiMaL is funded by the German Federal Ministry of Education and Research (BMBF).

Abstract: With the increasing utilization of fiber-reinforced polymers in various areas of application such as aviation and automotive, the need for efficient production methods appropriate for these materials also gains importance. The widespread use of metallic fasteners in conjunction with differential design leads to stress concentrations when applied to FRPs and prevents the full utilization of their mechanical properties. In an effort to develop an appropriate joining technology that also contributes to reducing tooling costs, co-curing of thermoset FRP laminates is analyzed in the present work. To gain insight into the feasibility of the use of pre-cured laminates in co-curing processes, laminates of varying degree of cure are joined with uncured laminates in a co-curing process and the mechanical properties of the resulting laminates are investigated experimentally. Based on these experimental results, an interdependence between bonding properties and the degree of cure of the pre-cured laminate can be shown.

Abstract: The temperature- and time-dependent penetration of surface structures is examined in thermal joining between polypropylene and aluminum. Experimental and numerical investigations were carried out for spot joints in order to describe the main effects on structure penetration. Further investigations were performed in a half-section setup to gain information directly from the joining zone. The thermal expansion of the thermoplastic material as well as the temperature distribution in the melting layer were identified as key parameters for structure filling on the metal surface.

Abstract: Generating serial components via additive manufacturing (AM) a deep understanding of process-related characteristics is necessary. The extrusion-based AM called fused layer manufacturing (FLM), also known as fused deposition modeling (FDM™) or fused filament fabrication (FFF) is an AM process for producing serial components. Improving mechanical properties of AM parts is done by adding fibers in the raw material to reinforce the polymer. The study aims to create a more detailed comprehension of FLM and process-related characteristics with their influence on the composite.Thereby, a short carbon fiber-reinforced polyamide (CarbonX™ Nylon, 3DXTECH, USA) with 12.5 wt.‑% fiber content, 7 μm fiber diameter, and 150 to 400 µm fiber length distribution was investigated. To separate process-related characteristics of FLM, reference specimens were fabricated via injection molding (IM) with single-batch material. For the mechanical characterization, quasi-static tensile tests were carried out in accordance to DIN 527‑2. Quality assessment including void content and void distribution was performed via micro-computed tomography (CT).The mechanical characterization clarifies effects on mechanical properties depending on process-related characteristics of FLM. CT scans show higher void contents of FLM specimens compared to IM specimens and void orientation dependent on printing direction. FLM shows process-related characteristics which generally strengthen mechanical properties of polymers. Nevertheless, tensile strength of FLM specimens decrease by more than 28% compared to quasi-homogenous IM specimens.

Abstract: A couple of research projects could demonstrate the adhesive-free bonding of metal and polymer foils very well in the past. The remaining issues on the road to industrial usage of this technology focus on higher process velocities, quality management and process behaviour during long-term usage. For this purpose, a new research project was initiated to concentrate on the maximum achievable adhesion between the two bonding partners at line speeds up to 10 m/min

Abstract: A constant challenge for the design and operation of CFRP primary structures is their sensitivity towards impact loading. This can lead to the formation of externally invisible delaminations which endanger the structural integrity. In practice, this circumstance is encountered with elaborate inspections or conservative design. Structural Heath Monitoring (SHM) systems offer the potential for permanent monitoring and represent an alternative approach that has drawn more attention in the last decade. The biggest barriers to market entry for this technology are system costs and reliability. This study is dedicated to these two points with the development of a low-cost system with which representative acoustic emission sources can be located reliably in a complex CFRP structure. The implementation is carried out using acoustic emission analysis, which represents a promising solution for the integral monitoring of primary structures. It is based on the detection of acoustic waves that are released during crack initiation and growth and propagate over large areas in thin-walled structures as Lamb waves. The challenges of source localization in thin-walled CFRP structures lie in the consideration of wave dispersion, anisotropic material properties, variable component geometry and interfaces. In this thesis, this complexity is captured by training a neural network. For this purpose, artificial sources are used which imitate acoustic emissions of typical damaging events in the material in frequency and mode content. The demonstrating component is an omega profile equipped with a network of piezoelectric sensors that is designed for reliable localization within a defined window. Signal processing takes place on a single-board computer which, together with a digital oscilloscope, completes the measurement chain. The system represents a modular, low-cost approach that can be transferred to other applications by adapting the hardware and training.