Thomas Keller

Related publications (346)

Design of fibre-polymer composite structures – European Technical Specification: Fatigue and detailing

Thomas Keller

2022

Mechanical Properties of FRP Materials at Elevated Temperature. Definition of a Temperature Conversion Factor for Design in Service Conditions

Mário Alexandre De Jesus Garrido, Thomas Keller

One of the main concerns about the use of fibre reinforced polymer (FRP) materials in civil engineering structural applications is the potential reduction of their stiffness- and strength-related properties when exposed to elevated temperature. The first part of this paper presents an assessment of existing design guidelines for FRP structures, regarding the recommendations they provide about the reduction of FRP mechanical properties at elevated temperature. The second part of this paper presents a survey of test data available in the literature concerning mechanical tests at elevated temperature in FRP materials produced with different fibres, resins, shapes and manufacturing methods. The results of the survey show that the provisions available in existing design guidelines are not always accurate; moreover, in some cases, they are even non-conservative. Based on this assessment, the third part of the paper presents a method to define a temperature conversion factor for design purposes, consistent with the partial factor method of the Eurocodes, which was calibrated with the results included in the database. The method proposed takes into account not only the maximum service temperature experienced by the FRP material, but also its glass transition temperature; the method considers also the type of mechanical property, namely if it is either fibre- or matrix-dominated. The conversion factors obtained applying the method proposed presented a good agreement with the test data available in the literature.

SPRINGER-VERLAG SINGAPORE PTE LTD2022

Rate dependent adhesive modeling of ductile adhesive composite joints

Ghazaleh Eslami, Thomas Keller

2022

Static and fatigue behavior of bonded, bolted and hybrid FRP joints

Thomas Keller, Lulu Liu

2022

Thermophysical and thermomechanical properties of basalt-phenolic FRP rebars under high temperature

Thomas Keller, Ting Li, Jiahui Shen, Hongwei Zhu

An experimental investigation was conducted on the thermophysical and thermomechanical properties of phenolic-basalt fiber-reinforced polymer (P-BFRP) rebars subjected to high temperature. As a comparison, vinylBFRP (V-BFRP) and epoxy-BFRP (E-BFRP) rebars were also investigated within the same program. The mass variation of all BFRP rebar types was similar in air atmosphere while P-BFRP rebars decomposed more slowly in nitrogen atmosphere. All BFRP rebar types were found to have a similar specific heat and thermal conductivity up to 350 degrees C. P-BFRP rebars exhibited a much higher glass transition temperature compared with the other two types of BFRP rebars. Three failure modes could be differentiated for all BFRP rebar types according to three temperature ranges, the latter were shifted to higher values for P-BFRP rebars. No reduction in tensile strength occurred in P-BFRP rebars up to 300 degrees C, while V-BFRP and E-BFRP rebars experienced a significant degradation of tensile strength at around 100 degrees C already. This advantage of P-BFRP rebars was mainly caused by the much higher Tg which delayed the initiation of interfacial bonding failure up to 300 degrees C. The elastic modulus of all BFRP rebar types exhibited a similar degradation, which was caused by progressive fiber and fiber bundle failure due to non-uniform stress distributions.

ELSEVIER SCI LTD2022

A novel fatigue life prediction methodology based on energy dissipation in viscoelastic materials

Ghazaleh Eslami, Thomas Keller

This paper introduces a new fatigue life prediction methodology for viscoelastic materials in the tension-tension fatigue loading region. The model was established based on the total amount of energy dissipated during fatigue loading, and offers two main advantages with respect to existing models in the literature, i.e. it considers the creep effect on fatigue behavior and requires less input data. The model was applied to three different materials an angle-ply glass/epoxy fiber-reinforced polymer composite, a cross-ply glass/epoxy fiber-reinforced polymer composite, and an epoxy adhesive - to cover a wide range of structural viscoelastic materials used in the industry. It was observed that the model predicted the fatigue life of the studied materials well at different stress ratios including those close to 1.0 where the creep effect was considerable. The model was used to plot constant life diagrams (CLDs) by considering the cyclic-creep interaction to counter the lack of accuracy of existing models at high stress ratios. A new definition for the cyclic-creep interaction was also proposed, which suggests that the participation of the creep and cyclic parts in the cyclic-creep interaction is equal to the total amount of energy dissipated by each. Accordingly, the proposed model was employed to simulate cyclic-creep interaction and determine the cyclic- and creep-dominated regions in CLDs.

ELSEVIER SCI LTD2021

Design and foldability of Miura-based cylindrical origami structures

Yuntong Du, Thomas Keller

A methodology for designing cylindrical origami with different patterns and different cross-sectional shapes is presented. Planar and cylindrical origami with curved-crease patterns are designed and proved to be foldable and developable regardless of the type of crease line. Specifically, cylindrical origami structures with arc-Miura pattern and circular cross section, arc pattern and circular cross section, arc-Miura pattern and triangular cross section, arc pattern and triangular cross section are designed. The relationships between the two-dimensional (2D) crease lines in a sheet and three-dimensional (3D) spatial origami structures are derived. Fabrications of cylindrical origami by a folding process are performed to validate the design theory for 2D crease lines. This work paves the way for applications for cylindrical origami in engineering, such as energy absorption.

ELSEVIER SCI LTD2021

This paper aims to present an experimental investigation on the behavior of web-flange junctions (WFJs) rotational stiffness of pultruded fiber-reinforced polymer composites (FRP). Channels and I-sections were tested using a simple set-up, which was developed in order to experimentally characterize the junctions in a direct manner. The Digital Image Correlation (DIC) technique was used, allowing overall deflections and relative rotations between web and flange to be monitored. The WFJs' imperfections were analyzed through an optical microscope and correlated with the cracks' formation. Further, damage thresholds are identified using available stress equations for curved composite members and lower bound functions are proposed to simulate the junction stiffness retention. Finally, two Equations are developed in order to analytically predict pultruded junctions' rotational stiffness per unit of width. In general, the theoretical and experimental results agreed fairly well, with a maximum difference of 24% for I-sections and 38% for channels.

ELSEVIER SCI LTD2021

Numerical modeling of two-dimensional delamination growth in composite laminates with in-plane isotropy

Thomas Keller, Anastasios Vassilopoulos, Congzhe Wang

The two-dimensional (2D) delamination growth in fiber-reinforced polymer (FRP) laminates with in-plane isotropy under Mode I loading condition was numerically investigated using finite element analyses. Two sizes of plate models were developed, focusing on different fracture stages. Cohesive elements were employed to simulate the fracture behavior in the presence of large-scale bridging (LSB). The influences of the pre-crack shape/area, loading zone shape/area and fracture resistance were parametrically studied. It was found that either a flatter pre-crack shape or a flatter loading zone shape could result in higher initial structural stiffness and less uniform distribution of the strain energy release rate (SERR) along the pre-crack perimeter during crack initiation and early propagation. However, they had only a minor effect on the stiffness after full fiber bridging development in all directions. The plates finally achieved constant stiffness, which increased linearly with the fracture resistance. The final crack shape was dependent on the loading zone shape and area, but the effects were relatively weak.

2021

A folding fabrication method inspired by origami for carbon fiber-reinforced composites to fabricate three-dimensional structures is presented. PMI foams serve as substrates, making it possible to manufacture this kind of origami-inspired composite sandwich materials in a single-step process. Analytical models are proposed to provide upper and lower bounds of the out-of-plane compressive strength of the origami-inspired composite sandwich materials. Finite element analysis (FEA) and experiments are conducted to characterize the compressive behaviors including the deformation history, failure mode, strength, etc., and verify the validity of the analytical models. By incorporating PMI foams, the out-of-plane compressive failure mode of the composite sandwich material at low relative density changes from buckling to wrinkling or crushing, thus greatly increasing the strength. Additionally, the energy absorption capacity is significantly enhanced due to the PMI foams and their interaction with the composite origami core. This work creates a new method to fold composites into lightweight sandwich materials with high strength and high energy absorption, paving the way for their applications in transport, energy and communication.

ELSEVIER SCI LTD2021