Mechanics of ultra-thin Fiber-Reinforced Polymer (FRP) composite layers for aerospace applications
Thin ply materials represent today one of the most promising composite material for advanced applications in the aerospace industry, thanks to the developments in the so-called spread tow technology. Due to its capacity of avoiding fibers' breakage and surface property loss, this technology has been applied on an industrial scale to produce extremely thin fiber-reinforced prepreg plies. Initially devoted to sports equipment, thin-ply laminates are recently attracting the interest of aerospace structural designers for use in mission-critical applications such as cryogenic tanks and reusable space launchers' frames. Experimental evidence collected on thin-ply laminates suggest that these composites are capable of delaying and even suppressing the propagation of transverse cracks and the onset of delamination. It thus seems to confirm a much earlier result on the influence of thickness and lay-up sequence on the strength and crack suppression behavior of FRP laminates, namely the existence of the so-called thin-ply effect and in-situ strength. In-situ observations point to debonding at the fiber/matrix interface as the primary mechanism to investigate in order to achieve a better understanding of the initiation of transverse cracking and its suppression through an improved laminate design. It is paramount to this end to understand the process of fiber/matrix debonding, kinking and coalescence, as well as the effect of ply thickness, fiber cluster size, material properties’ mismatch and thermal strains.
Loading rate effects on mode I, mode II and mixed mode I-II delamination in advanced CFRP
Delamination onset and propagation represent one of the most safety-critical failure modes in structures made of FRP laminates. Due to the widespread use of laminated composites in primary and secondary structures, the assessment of fracture behavior of FRP and the development of reliable analytical and numerical formulations for damage prediction are increasingly important themes to be addressed. Experimental procedures for the determination of fracture toughness have been developed for quasi-static conditions; while simulations have mainly dealt with quasi-static conditions for benchmarking and with impacts for prediction, which are diﬃcult to compare with experimental measurement. Nonetheless, most of the applications in which composites are adopted are characterized by dynamic conditions and loading-rate eﬀects. Thus, the study of loading-rate eﬀects on delamination onset and propagation assumes a signiﬁcant importance, although it has not been addressed much in the past. Loading-rate eﬀects on delamination were addressed in a threefold way in the present work. Experimental observations were carried out on UD specimens under mode I, mode II and mixed mode I-II loading and on specimens with a 0/90◦ interface at the delamination front under mode I loading. For all the diﬀerent lay-ups and loading conditions, four diﬀerent velocities have been tested: 1, 50, 250 and 500 [mm/min]. SEM analysis was also carried out on delaminated surfaces under mode I loading both in UD and in the 0/90◦ interface. In the range of velocities analyzed, no dependence seems to be present for mode I, both in the UD and in the 0/90◦ interface, for mode II and for mixed mode I-II; diﬀerences are in the order of experimental scatter. A code was developed in Mathematica environment for the analysis and post-processing of frames of delamination propagation gathered during experiments, allowing for the extraction of more information from experiments with the use of simple optical devices. Dynamic numerical simulations were conducted in the FEM software ABAQUS, modeling the experiments performed. For mode I, mode II and mixed mode I-II with UD specimens, 3D models have been developed with hexahedral brick elements and a single layer of cohesive elements. Variations have also been studied: the presence of a cohesive layer with randomly distributed interface strengths has been analyzed for mode I, while the inﬂuence of initial lever misalignment has been addressed for mixed mode. For mode I with specimens with 0/90◦ interface, a 3D model has been developed with shell elements and with ﬁve layers of cohesive elements and intra-laminar damage modeling with the use of the Hashin criterion. Numerical models show a clear dependence on loading rate for all loading modes and lay-ups, except mode II with UD specimen; the agreement with experiments is good for quasi-static conditions. Initial lever misalignment does not have any signiﬁcant inﬂuence. A simulation has also been performed of the cooling process following curing for the 0/90◦ laminated plate from which specimens were cut. Stress concentrations can be observed due to the mismatch in orientation between plies. Analytical models for DCB (mode I), ENF (mode II) and MMB (mixed mode I-II) specimens have been formulated on the base of the Euler-Bernoulli and Timoshenko beam.
Modeling complex patterns of crack propagation: branching and merging mechanisms
The development of accurate models and simulations is becoming increasingly important in fault detection and prevention techniques applied to a wide variety of engineered systems. The recent advances in measurement devices technology has provided the designer an incredible amount of data related to acoustic, thermal and optic behaviour of the material subject to crack generation and propagation mechanisms. The lack of a reliable model prevents a correct interpretation of these data and makes predictions of future evolution difficult to be performed. In this project, a different approach is considered: the kinematic behaviour is addressed under the assumption that the crack shape can be reconstructed through the application of a suitable and known set of deterministic rules. The aim is to propose an alternative approach to the development of a model for crack propagation by applying the NKS method. The problem firstly addressed is the simulation of mechanisms of crack branching and merging in 2D using a set of simple rules; the features of the resulting pattern is then considered and boundaries shapes, crack diffusion proper-ties, frequencies of branching and merging are studied.
Qualifying Course for University Teachers in Sweden
The primary target group of this course is employees and PhD students at Luleå University of Technology. The aim is that the participant will develop fundamental pedagogical proficiency for teaching in Higher Education generally and at LTU specifically. The participants will develop a readiness to constructively take part in discussions among colleagues on the topic of education and improvement in teaching within their own fields. The contributions should be based on scientific grounds and proven experience. Moreover, participants shall begin describing their basic pedagogical standpoints, both in view of their own development but also for purpose of the qualification needed for employment. The course also aims for participants to develop a curiosity on teaching and the basic competence needed for active participation in further opportunities for pedagogical development, i.e: courses, workshops, development projects etc... Finally, the course has the purpose of creating a network for organizational development in general and pedagogical development in particular at LTU.