Manuel de Freitas

Thermoset fibre-reinforced composites are widely used in high-end industries, but a growing demand for more sustainable and recyclable alternatives conveyed the research efforts towards thermoplastics. To expand their usage, new approaches to their manufacture and mechanical performance must be tackled and tailored to each engineering challenge. The present study designed, manufactured and tested advanced multi-layer laminated composites of thermoplastic polypropylene prepreg reinforced with continuous woven fibreglass with interlayer toughening through thermoplastic polyurethane elastomer (TPU) layers manufactured by fused filament fabrication. The manufacturing process was iteratively optimized, resulting in successful adhesion between layers. Three composite configurations were produced: baseline glass fibre polypropylene (GFPP) prepreg and two multi-layer composites, with solid and honeycomb structured TPU layers. Thermal and mechanical analyses were conducted with both the polyurethane elastomer and the manufactured laminates. Tensile testing was conducted on additively manufactured polyurethane elastomer specimens, while laminated composites were tested in three-point bending. The results demonstrated the potential of the developed laminates. TPU multi-layer laminates exhibit higher thermal stability compared to the baseline GFPP prepreg-based composites. The addition of elastomeric layers decreases the flexural modulus but increases the ability to sustain plastic deformation. Multi-layer laminate composites presenting honeycomb TPU layers exhibit improved geometric and mechanical consistency, lower delamination and fibre breakage, and a high elastic recoverability after testing.

Thermoset fibre-reinforced composites are increasingly used in high-end industries such as aerospace or autosport. As the demand for more sustainable materials strengthens, thermoplastics have emerged as one key focal point due to their potential recyclability, and for more efficient and sustainable applications. To change, replace, or expand the use of thermoplastic composites, new approaches to their manufacture and mechanical performance must be tackled and tailored to each application, structure or engineering challenge. The present study is focused on design, manufacture and test of advanced multi-layer laminated composites made by thermoplastic polypropylene (PP) prepreg matrix reinforced with continuous woven fibreglass. The composite laminate was subjected to interlayer toughening through thermoplastic polyurethane elastomer (TPU) layers manufactured by Fused Filament Fabrication (FFF). The manufacturing process was iteratively optimized, resulting in successful adhesion between layers. Three composite configurations were produced: baseline Glass Fiber Prepreg with Polypropylene (GFPP); and two multi-layer composites, GFPP with solid TPU layers (GFPP-TPU) and GFPP with honeycomb structured TPU layers (GFPP-TPU-HC). Thermogravimetric and Differential Thermal Analysis were also conducted on the specimens. TPU layers contributed to increase the thermal decomposition temperature of the specimens in which it is included. Specifically, the GFPP-TPU and GFPP-TPU-HC composites exhibited higher thermal stability compared to the baseline GFPP, attributed to the stabilizing effect of the TPU layers. Tensile testing was conducted on FFF additively manufactured TPU specimens, while all laminated composites were tested in three-point bending. The results demonstrated the potential of the developed composites. The addition of solid TPU layers to GFPP decreased the flexural modulus but limited the plastic deformation in the specimens that did not undergo delamination during the testing. In contrast, GFPP-TPU-HC exhibited improved geometric consistency and, on average, a less variable and increased flexural modulus.

The primary goal of this systematic literature review is to identify, characterize, and make a comprehensive analysis of the available research on the fatigue strength of additively manufactured (AMed) metal materials when subjected to loading cycles that exceed the preconceptualized fatigue limit ( cycles) and the very high cycle fatigue (VHCF) regime. Considering the inherent complexity and magnitude of influential variables present in AM metal processes, this paper explores in‐depth the relevant conclusions taken by theoretical/experimental studies and their respective results of several AMed materials in the VHCF regime. The present review focuses on key research topics of metal AM fatigue strength, such as crack initiation and failure mechanisms, examining the influence of microstructure and defects, the effect of the input process parameters, postprocessing methods, and the influence of testing conditions on fatigue strength.

Fatigue damage has special relevance on the lifespan of mechanical components and structures, as it takes responsibility for the majority of registered structural failures. Most fatigue testing today still uses uniaxial loads, nonetheless it is generally recognized that multiaxial stresses occur in many full-scale structures. Nowadays, the growing need for greater lifespans forced the study of material behavior under what is nowadays very high cycle fatigue (VHCF) regime, fatigue research beyond 10E07 up to 10E10 cycles. However, most classic fatigue machines are often very costly, both in terms of time and energy, for such an elevated number of cycles. The high costs of equipment and time to conduct such experiments have seen a massive improvement with the introduction of high frequency ultrasonic machines. Presently, it is already possible to characterize materials VHCF response under axial and multiaxial loading conditions in a fraction of the time. Ultrasonic testing machines were first introduced for simpler uniaxial loading and more recently adapted for multiaxial loading conditions. The present work reviews the current background of ultrasonic fatigue testing machines working at 20-kHz frequency, with emphasis on multiaxial fatigue and VHCF. A review of a series of fatigue tests are presented, respectively, axial, shear, axial/shear, and in-plane bi-axial fatigue tests with different combining dimension specimens allowing different shear/axial stress ratios. Special attention will be put into the performance of multiaxial classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low-cost piezoelectric transducers. By comparing all so far developed and successfully reached multiaxial specimens and respective methodologies, two base approaches were possible to be established. When designing new specimens or ultrasonic machines, or both, one of the two methods must be followed. To have a well-comprehended methodology and successful design approach will undoubtedly guide future multiaxial ultrasonic method ideas.

Under strain control tests and cyclic loading, extruded magnesium alloys exhibit a special mechanism of plastic deformation (“twinning” and “de-twining”). As a result, magnesium alloys exhibit an asymmetric material behavior that cannot be fully characterized with the typical numerical tools used for steels or aluminum alloys. In this sense, a new phenomenological model, called hypo-strain, has been developed to correctly predict the cyclic stress–strain evolution of magnesium alloys. On this basis, this work aims to accurately describe the local cyclic elastic–plastic behavior of AZ31B-F magnesium alloy under multiaxial cyclic loading with Abaqus incremental plasticity. The phenomenological hypo-strain model was implemented in the UMAT subroutine of Abaqus/Standard to be used as a design tool for mechanical design. To evaluate this phenomenological approach, the results were correlated with the uniaxial and multiaxial proportional and non-proportional experimental tests. In addition, the estimates were also correlated with the Armstrong–Frederick nonlinear kinematic hardening model. The results show a good correlation between the experiments and the phenomenological hypo strain approach. The model and its implementation were validated in the strain range studied.

In this work, the magnesium alloy AZ31BF subjected to proportional and non-proportional loads has been studied. For this purpose, a series of experimental multiaxial fatigue tests were carried out according to the ASTM E466 protocol. The main objective was to determine the relationship between the multiaxial fatigue strength of this alloy under these two different types of loading. The results showed that the AZ31BF magnesium alloy has different fatigue strengths depending on the loading type. Based on these results, it was found that the ratio between proportional and non-proportional damage in AZ31BF magnesium alloy varies depending on the number of loading cycles. To represent this variation, parameter Y was used to modulate the non-proportional damage of AZ31BF. In this way, two Y functions were considered, one for the normal stress component and the other for the shear stress component. The results obtained for the non-proportional parameter Y are of particular interest since the multiaxial fatigue models do not distinguish between these two types of loading when evaluating fatigue life. In this sense, the results of this study can be used in these models to overcome this limitation.

Ultrasonic fatigue testing (UFT) is a recent fatigue methodology that applies resonance to achieve very high cyclic load frequencies. Its primary purpose is to study fatigue in the very high cycle fatigue regime. In this study, an UFT machine was used to conduct multiaxial tension/torsion fatigue tests at a 20‐kHz frequency. The objective was to reach a reliable multiaxial fatigue test and methodology by modifying the geometry of the specimen, enabling also different shear to axial stress ratios. The study conducted numerical and experimental modal behavior analysis of the ultrasonic setup and specimens focused on achieving a consistent modal deformed shape and associating the measured displacement to the induced stresses thermographic imaging, laser and rosette strain gauges measurements were carried out to validate the results. A good numerical to experimental agreement was achieved. Tension/torsion fatigue test series were conducted to three combining dimension specimens with different shear/axial stress ratios. The crack initiation angles and fracture surfaces were analyzed and compared among each other and with tension–compression and pure torsion ultrasonic tested specimens. The results prove a clear trend in crack propagation details with the increase of the induced stress ratio.

In this paper, we investigate the cyclic response of AISI 303 stainless steel subjected to non-proportional loads with different amplitude ratios between shear stresses and normal stresses. Based on the experiments, a relationship between the proportional reference load and a varied range of non-proportional loads was established. To achieve this objective, an experimental program was implemented to evaluate the non-proportional parameter Y. Then, the evolution of this parameter was analyzed with the number of cycles to failure and with the ratio between shear and normal stresses, finally, the evolution of the non-proportional parameter Y was mapped by two functions. The results show that the non-proportional response of the AISI 303 can be estimated using the two functions obtained. This allows the estimation of the relationship between non-proportional and proportional stresses as a function of the number of cycles to failure together with the relationship between shear and normal stresses. The results obtained have direct application in the evaluation of accumulated damage, assessed in real-time, resulting from variable amplitude loading spectra. This is of particular interest for the evaluation of structural health monitoring of structures and mechanical components.