Fundamental Investigations of Bond Behaviour of High-Strength Micro Steel Fibres in Ultra-High Performance Concrete under Cyclic Tensile Loading

The objective of the contribution is to understand the fatigue bond behaviour of brass-coated high-strength micro steel fibres embedded in ultra-high performance concrete (UHPC). The study contains experimental pullout tests with variating parameters like load amplitude, fibre orientation, and fibre-embedded length. The test results show that fibres are generally pulled out of the concrete under monotonic loading and rupture partly under cyclic tensile loading. The maximum tensile stress per fibre is approximately 1176 N/mm2, which is approximately one third of the fibre tensile strength (3576 N/mm2). The load-displacement curves under monotonic loading were transformed into a bond stress-slip relationship, which includes the effect of fibre orientation. The highest bond stress occurs for an orientation of 30° by approximately 10 N/mm2. Under cyclic loading, no rupture occurs for fibres with an orientation of 90° within 100,000 load changes. Established S/N-curves of 30°- and 45°-inclined fibres do not show fatigue resistance of more than 1,000,000 load cycles for each tested load amplitude. For the simulation of fibre pullout tests with three-dimensional FEM, a model was developed that describes the local debonding between micro steel fibre and the UHPC-matrix and captures the elastic and inelastic stress-deformation behaviour of the interface using plasticity theory and a damage formulation. The model for the bond zone includes transverse pressure-independent composite mechanisms, such as adhesion and micro-interlocking and transverse pressure-induced static and sliding friction. This allows one to represent the interaction of the coupled structures with the bond zone. The progressive cracking in the contact zone and associated effects on the fibre load-bearing capacity are the decisive factors concerning the failure of the bond zone. With the developed model, it is possible to make detailed statements regarding the stress-deformation state along the fibre length. The fatigue process of the fibre-matrix bond with respect to cyclic loading is presented and analysed in the paper.

Current Trends in Dissimilar Diffusion Bonding of Titanium Alloys to Stainless Steels, Aluminium and Magnesium

This article provides a comprehensive review of the advancements made in the diffusion bonding of titanium and its alloys to other advanced materials such as aluminium, stainless steel, and magnesium. This combination of advanced alloys has received considerable attention in different industries, including aerospace, petrochemical, and nuclear applications due to high specific strength, lightweight, corrosion resistance, and moderate to high mechanical properties. The mechanisms of bond formation are discussed based on the type of microstructures formed and the mechanical properties achieved. The scientific literature identifies various methods/processes for controlling the volume of intermetallic compounds formed within the joint regions, as well as ways of maximising the strength of the weld/joints. This paper discusses the relationship between weld/bond properties and bonding parameters such as time, temperature, surface roughness, pressures, interlayer composition, and thickness. The scientific literature also shows that the bonding mechanisms and microstructural evolution of the bond zone can be significantly affected by suitable optimization of the bonding parameters. Additionally, this is a method of maximising bond strength.

The Analytical Model of Stress Zone Formation of Ti4Al4V/AA1050/AA2519 Laminate Produced by Explosive Bonding

This paper contains an analytical description of the deformation of the upper layer AA2519/AA1050/Ti6Al4V laminate produced by an explosive bonding method. The basic parameters of the explosive welding process that influence the quality of the bonding are the detonation velocity of the explosive, the explosion energy, and the impact angle of the combined materials. The developed description uses the theory of elastodynamic character of materials deformation at the connection point due to local traction load. The presence of high pressure during joining was limited to the region where the plane surface moving with a constant subsonic velocity. An analytical description of the residual stresses distribution was also a performer. Results of analytical investigations were verified by structure examination of the bond zone. The work was supplemented by the chemical composition analysis of the base materials and a monotonic stretching test characterizing the basic mechanical properties of the produced laminate.

FE analysis of circular CFT columns considering bond-slip effect: A numerical formulation

In this paper, a numerical model for the analysis of bond-slip occurring in concrete filled steel tube (CFT) columns is introduced. Unlike the classical bond-link or bond-zone element using double nodes, the introduced model considers the bond-slip effect without taking double nodes by incorporation of the equivalent steel stiffness. Upon constructing the equations system on the basis of the force equilibrium and the compatibility condition at each nodal point, the deformations in a steel tube are determined, followed by evaluation of the bond-slip. Moreover, as a part of solving the equations system to evaluate the slip behavior, the mechanical properties for steel and bond-slip have been changed and updated through an iteration procedure. Finally, the validity of the introduced numerical model is verified by comparing the experimental data with the analytical results for CFT columns subjected to axial force and bending moment.

Proofs and Contradictions for Wave Formation Theories in Collision Welding

Joining processes initiated by collisions provide several advantages in comparison to conventional welding techniques. Although collision welding is already used industrially, some effects during the bond formation, like the occurrence of waves in the interface, have not been fully understood, yet. In order to describe these effects, a number of theories have been developed that can be divided into three categories by the governing mechanism: 1) indentation theory 2) stress wave theory and 3) fluid theories. However, none of these theories has so far been conclusively proved or disproved. A unique test rig allows specifically targeted examinations of the wave formation and the transition region from straight to wavy interface as well as the evolution of the waves. Within the presented investigation, the kinetic energy and the hardness as process and material parameters have been examined regarding their influence to wave formation. For a specific investigation of the overall deformation of the bond zone a parameter, bond length elongation Φ, was introduced to connect wave length and wave amplitude. The results of the experiments provide arguments for wave formation being caused by both stress waves and fluidic effects.

Stress-reduced Direct Composites for the Restoration of Structurally Compromised Teeth: Fiber Design According to the “Wallpapering” Technique

SUMMARY Purpose: The purpose of this work was to present a restoration technique based on an understanding of the biomechanical properties of the dentinoenamel complex (DEC) and the physical-mechanical properties of the resin-based composite including the stress generated from both polymerization shrinkage and occlusal forces. Technique Summary: The DEC is a functional interphase that provides crack tip shielding; the DEC should be preserved during restorative procedures. Dentists can design the strategic placement of restorative materials into the cavity to both resist the mode of failure and mimic the performance characteristics of the intact natural tooth. The term “wallpapering” describes a concept of covering the cavity walls with overlapping closely adapted pieces of Leno weaved ultra-high-molecular-weight polyethylene (LWUHMWPE) ribbons. The key for success is that the ribbons are adapted and polymerized as closely as possible against the contours of residual tooth substrate. The resulting thin bond line between the fibers and the tooth structure creates a “bond zone” that is more resistant to failing due to the intrinsic stress and energy absorbing mechanism of the LWUHMWPE ribbons. The formation of defects and voids, from which crack propagation may start, is also reduced. The fibers' tight adaptation to tooth structure allows a dramatic decrease of the composite volume between the tooth structure and the fiber, thus protecting the residual weakened walls from both the stress from polymerization shrinkage and the occlusal load. Conclusion: By using a similar approach, fiber-reinforced stress-reduced direct composite restorations may be performed in the restoration of structurally compromised vital and nonvital teeth.