1,720,954 research outputs found
Fracture Toughness of Aerospace Structures: On the Use of Redundant Lattice Structures
No full textBoth experimental and numerical studies have shown that the crack-bridging is one of the most significant mechanism contributing to the fracture toughness of natural materials like nacre and bone. This study aims to explore if additive manufactured lattice structures can be used to bridge fatigue cracks as well, thereby improving the fracture toughness of aerospace structures. By using enlarged Selective Laster Sintered unit cells, the response under mix-mode loading of five different unit cells was studied. Numerical models were used to give further understanding of the stiffness, fatigue response, and crack bridging capability of a redundant lattice core.Aerospace EngineeringAerospace Structures & Material
Critical Parameters in Mode I Interlaminar Fracture Toughness Testing of Thermoplastic Composite Materials
No full textThe goal of the thesis (executed for Fokker Aerostructures) was to further the understanding of critical parameters in the DCB test method for thermoplastic composites. The influence of fiber bridging, specimen thickness, insert film thickness, crack length and crack opening rate are investigated by means of experimental DCB tests for thermoplastic C/PEKK specimens. Microscopic research is performed in order to link the mechanical behavior to the fractography in the C/PEKK material. The results show that the mode I fracture toughness is influenced by specimen thickness, insert film thickness and crack length as consequence of fiber bridging effects and plastic resin deformation, which can again be related to the tough resin properties and relatively poor fiber/matrix interfacial strength in thermoplastic composites.Aerospace EngineeringAerospace Structures and MaterialsStructural Integrity and Composite
The dangers of single-lap shear testing in understanding polymer composite welded joints
No full textSingle-lap shear (SLS) joints are straightforward to manufacture. This makes them especially attractive for testing polymer composite welded joints. Owing to local heating, which is characteristic of composite welding processes, the production of more geometrically intricate joints (such as double-lap or scarfed joints) or bigger joints (such as end-notched flexure or double cantilever beam) typically entails significant complexity in the design of the welding process. Testing of SLS joints is also uncomplicated and, even though, owing to mixed-mode loading and uneven stress distribution, it does not provide design values, it is widely acknowledged as a valuable tool for comparison. Even so, comparing different aspects of composite welded joints through their corresponding SLS strength values alone can be deceptive. This paper shows that comparison of different welding processes, adherend materials, process parameters or different types of joining techniques through SLS testing is only meaningful when strength values are combined with knowledge on other aspects of the joints such as joint mesostructure, failure modes and joint mechanics. This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.Accepted Author ManuscriptAerospace Structures & Computational MechanicsStructural Integrity & Composite
An investigation into the fatigue behavior of micro-cracks in resin rich regions of composite laminates produced by liquid infusion
No full textDamage is found in resin rich bead corner radii of RTM 6 epoxy-based composite ribs due to in-service thermal-mechanical loading after an aircraft inspection. The same types of damage are obtained through pure thermal cycling of a single bead. However, thermal cycling is a time-consuming process. Therefore, a faster way of damage investigation is required. A potential way to achieve this objective is by mechanically cycling a coupon specimen with simpler geometry than the bead. To investigate potential solutions to the problem a literature review is performed. The scope of the literature review covers several topics as follows. Laminate fatigue damage modes and their impact on the laminate is researched. It is discovered that it is typical for RTM 6 epoxy-based laminates to build-up high matrix residual tensile stresses after manufacturing. Several differences between thermal and mechanical cycling are discovered. It is found that the most common way to test composites is by the use of ASTM standards. However, there is little available information about the fatigue behavior of laminates with resin rich areas. Investigation of the fatigue behavior of laminates with resin rich areas is performed by the use of FEA and physical tests. Four specimen types labeled from A to D are manufactured. Type A is a dog-bone pure RTM 6 specimen. Types B to D are all composite specimens with the same in-plane geometry and different layups and manufacturing processes. All specimens are tested statically and in fatigue. In the fatigue testing session, fractography of the damage occurring at different test conditions for different specimen types is performed. In addition, two FE models are created. The first model is of the bead. The second model is a harmonized model applicable to all composite specimen types with required layup readjustments for each specimen type. It is discovered by FEA that the maximum principal stress in the resin rich area is perpendicular to the matrix cracks in the bead and the composite specimens. It is also discovered that the maximum principal stress cycle of the matrix at the resin rich layer interface with the fabric is similar in the bead and the specimens. The similar fatigue parameters are the R-ratio and the stress amplitude. The parameter similarity suggests they could potentially drive the resin rich layer fatigue damage initiation. Moreover, a positive through-the-thickness stress gradient is discovered, which suggests the cracks are likely to initiate bellow the resin rich layer surface. This hypothesis is further supported by fractographic observations of cracks not reaching the laminate free surface. Static and fatigue tests are performed. The static test provides the UTS of all specimen types, based on which cyclic load levels are selected. In the specimen fatigue tests several results are observed. In the first place, damage similar to the bead damage is found, namely cracks and delamination. In the second place, the damage is observed to penetrate through the laminate thickness and to be dependent upon the laminate compaction. However, this penetration depth dependency on the compaction might be influenced by the second curing cycle of specimen type C, in which the damage was observed. Finally, reduction in the specimen stiffness is observed due to fatigue damage accumulation in time. Based on the results recommendations are formulated. For design purposes resin rich area formation should be avoided both inside and outside the laminate. If their formation is inevitable, at least the laminate should be kept well compacted and the resin rich area location should be kept only at the surface. For future research, two topics are identified as requiring such. First, is the laminate stiffness reduction. Second, is the influence of the second curing cycle of the well compacted specimen type C.Aerospace EngineeringAerospace Structures & Material
The effect of stress ratio on the fatigue behavior of additively manufactured porous biomaterials
No full textMeta-biomaterials are porous biomaterials created by additive manufacturing techniques such as Selective Laser Melting. These materials are built up form a repeating unit cell, resulting in a porous structure that can be applied for bone implants or prosthetics. The mechanical properties of these meta-biomaterials can be tailored by variations in unit cell type and strut thickness, resulting in different porosity values. This makes it possible to create a material with mechanical properties similar to bone, which prevents negative side effects of conventional bone implants such as stress shielding. The fatigue behavior of these meta-biomaterials has been studied before, but only at a single stress ratio of R=0.1 This study investigates the fatigue behavior at different stress ratios which result in an increased mean stress. A cylindrical porous structure that is built up from a diamond unit cell is tested at stress ratios of R=0.1, R=0.3, R=0.5, R=0.7 and R=0.8. Two samples types are made of Ti-6Al-4V ELI powder, resulting in a theoretical porosity of 80% and 10%. Also an experimental DIC method is developed, to visualize the deformation behavior during the fatigue tests.Materials Engineering and ApplicationsMaterials Science & EngineeringMechanical, Maritime and Materials Engineerin
Disbond arrest in fibre metal laminates
No full textIn the development of materials used for constructing aircraft, progress has been made in the production, certification and implementation of materials such as Carbon Fibre Reinforced Polymer (CFRP) and Glass Laminate Aluminium Reinforced Epoxy (GLARE). The state-of-the-art in joining techniques however, has not changed much: fastening is still the current used method for high load transfer (HLT) joints. Unfortunately, fastening is associated with high fatigue sensitivity due to the need for holes in the joined materials, causing stress concentrations and a highly concentrated load introduction. Using bonding for HLT joints would alleviate this fatigue sensitivity by continuous load transfer over the bonded area. Currently, certification of these bonded major loadpath joints is only allowed if the design of bond-line secures a limitation of the maximum disbond size originating from manufacturing anomalies or in-service impact events. In order to meet this requirement, disbond arrest features (DAFs) need to be implemented in the bond-line. As a spin-off from the BOPACS (Boltless Assembling of Primary Aerospace Composite Structures) project, where fasteners are used for disbond arrest, the transferability of this design strategy in GLARE is investigated. The fatigue sensitivity of the metallic bonded surface in GLARE in combination with the implemented fasteners inside the bonded area prove to affect the disbond arrest performance to a large extent. A unique combination of fatigue crack growth (FCG), adherend delamination and adhesive disbonding is observed in the tested configurations, detrimental to the DAFs intended use. In the process, a qualitative in-situ disbond monitoring system is developed and verified. Additionally, the root causes of the observed failure mode and the subsequent damage progression are identified. By identification of the one-off damage progression characteristics, new DAF designs are proposed for the certification of bonded HLT joints in GLARE.Aerospace EngineeringAerospace Structures and Material
The Student Flight Data Recorder – Building a culture of learning from failure
No full textThis paper presents the results of a collaborative project initiated by first year teaching staff and study counsellors within the Aerospace Engineering Bachelor’s programme at Delft University of Technology aimed at tackling the challenge of stimulating critical self-reflection and coping with failure. This project took the concept of a study planner and reflection journal and turned it into a symbol synonymous with learning from failure – the aircraft Flight Data Recorder. This symbol was combined with animated storytelling to introduce and explain the purpose and function of the Student Flight Data Recorder (SFDR), after which the usage of the resource was scaffolded by student mentors. Acknowledging that some students would not feel compelled to use a resource that was not required, a moment of intervention was offered at the completion of the first academic quarter after the first round of final exams. Overall, the project team has observed that the project has created more awareness and discussion about these topics within the student population. The next step in the project is to add an educational researcher to the project team with the intent of carrying out quantitative research into the effectiveness of the tool.Structural Integrity & Composite
Energy Absorption of Additively Manufactured Lattices: On biomimetic abstraction of structural principles toward increased energy absorption in lattice structures
No full textCellular solids are characteristically excellent energy absorbers due to their capacity to store large amounts of energy through compression. A bioinspired design approach identified the Pomelo fruit (Citrus Maximus) as a biological role model for energy absorption. Fruit peels’ biologically most important functions lies in the need to provide protection of the seeds from mechanical damage or other negative environmental influences. This qualifies such biological structures as role models for the development of novel structures that protect commodities from damage. The main structural principle extracted from the Pomelo is a density graded strategy. With maturing additive manufacturing (AM) technologies allowing unprecedented control over structural topology, controlled lattice structures can now be investigated . Using a class of AM, selective laser sintering (SLS), lattice structures were fabricated out of polypropylene and subjected to static and dynamic compressive loadings. The lattices were designed to determine the influence of the biomimetic density grading, cell shape and cell size in 3 distinct lattice configurations. Quasi-static simple compression results are compared to analytical micro-mechanical models, finite element method simulations and digital image correlation. Dynamic impact data is assessed using high-speed camera images and evaluated with an analytical momentum analysis. Data analysis is discussed and it is concluded that the density grading strategy beneficially influences the energy absorption. This is attributed to a combination of local plasticity manipulation and higher densification strains in static regime. In dynamic tests, the collapse initiation trigger led to controlled, more gradual collapse with lower corresponding loads.Aerospace EngineeringAerospace Structures & Material
Mechanical Behaviour of PA2241FR used in Selective Laser Sintering
No full textThis research focuses on the possible causes of variation in the tensile strength and stiffness of witness specimens that are used in the manufacturing of aerospace grade selective laser sintered parts. Using specimens and data obtained from Materialise of the PA2241FR samples, the crystallinity and porosity of the samples was determined. The effect of the flame-retardant additive was also investigated. A relationship was found between the degree of crystallinity and the tensile strength, and between the porosity and the tensile strength. Unfortunately, no relationship could be found between the crystallinity and stiffness. The effect of the FR-additive could not be tested using NMR and as a result no data on the influence was available. When analysing the other factors, they seemed to indicate the significance of an unknown parameter on the mechanical properties and especially on the stiffness. A variation of the halogenated flame-retardant additive seems to fit this effect.Aerospace Engineerin
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