RESUMEN
Combining carbon fiber reinforced polymers (CFRP) with steel offers the potential of utilizing the desired characteristics of both materials, such as specific strength/stiffness and fatigue strength of fiber reinforced polymers (FRP) and impact resistance of metals. Since in such hybrid laminates multiple material layers are combined, a gradual failure is likely that can lead to changes in mechanical properties. A failure of the metal partner leads to an increase in stress on the FRP, which under fatigue load results in increased self-heating of the FRP. Therefore, a suitable testing procedure is required and developed in this study, to enable a reproducible characterization of the mechanical properties under fatigue load. The resulting testing procedure, containing multiple frequency tests as well as load increase and constant amplitude tests, enabled characterization of the fatigue performance while never exceeding a testing induced change in temperature of 4 K. In addition to the development of the testing procedure, an insight into the manufacturing induced residual stresses occurring in such hybrid laminates, which impacts the load-bearing capacity, was established using finite element simulation. The gathered data and knowledge represents a basis for future in-depth investigations in the area of residual stress influence on the performance of hybrid laminates and highlights its importance, since not only the used testing procedure determines the measured fatigue performance.
RESUMEN
Microstructural responses to the mechanical load of polymers used in tissue engineering is notably important for qualification at in vivo testing, although insufficiently studied, especially regarding promising polycaprolactone (PCL). For further investigations, electrospun PCL scaffolds with different degrees of fiber alignment were produced, using two discrete relative drum collector velocities. Development and preparation of an adjusted sample geometry enabled in situ tensile testing in scanning electron microscopy. By analyzing the microstructure and the use of selected tracking techniques, it was possible to visualize and quantify fiber/fiber area displacements as well as local fractures of single PCL fibers, considering quasi-static tensile load and fiber alignment. The possibility of displacement determination using in situ scanning electron microscopy techniques for testing fibrous PCL scaffolds was introduced and quantified.
RESUMEN
Short glass fiber reinforced plastics (SGFRP) offer superior mechanical properties compared to polymers, while still also enabling almost unlimited geometric variations of components at large-scale production. PA6-GF30 represents one of the most used SGFRP for series components, but the impact of injection molding process parameters on the fatigue properties is still insufficiently investigated. In this study, various injection molding parameter configurations were investigated on PA6-GF30. To take the significant frequency dependency into account, tension-tension fatigue tests were performed using multiple amplitude tests, considering surface temperature-adjusted frequency to limit self-heating. The frequency adjustment leads to shorter testing durations as well as up to 20% higher lifetime under fatigue loading. A higher melt temperature and volume flow rate during injection molding lead to an increase of 16% regarding fatigue life. In situ X-ray microtomography analysis revealed that this result was attributed to a stronger fiber alignment with larger fiber lengths in the flow direction. Using digital volume correlation, differences of up to 100% in local strain values at the same stress level for different injection molding process parameters were identified. The results prove that the injection molding parameters have a high influence on the fatigue properties and thus offer a large optimization potential, e.g., with regard to the component design.
RESUMEN
The application of additive manufacturing changes from prototypes to series production. In order to fulfill all requirements of series production, the process and the material characteristics must be known. The machine operator of additive manufacturing systems is both a component and a material producer. Nevertheless, there is no standardized procedure for the manufacturing or testing of such materials. This includes the high degree of anisotropy of additively manufactured polymers via material extrusion. The interlayer bonding performance between two layers in the manufacturing direction z is the obvious weakness that needs to be improved. By optimizing this interlayer contact zone, the overall performance of the additively manufactured polymer is increased. This was achieved by process modification with an infrared preheating system (IPS) to keep the temperature of the interlayer contact zone above the glass transition temperature during the manufacturing process. Combining destructive and non-destructive testing methods, the process modification IPS was determined and evaluated by a systematic approach for characterizing the interlayer bonding performance. Thereby, tensile tests under quasi-static and cyclic loading were carried out on short carbon fiber-reinforced polyamide (SCFRP). In addition, micro-computed tomography and microscopic investigations were used to determine the process quality. The IPS increases the ultimate interlayer tensile strength by approx. 15% and shows a tendency to significantly improved the fatigue properties. Simultaneously, the analysis of the micro-computed tomography data shows a homogenization of the void distribution by using the IPS.