Tensile fracture of an adhesive joint: the role of crack length and of material mismatch

The tensile strength of an adhesive joint is predicted for a centre-cracked elastic layer, sandwiched between elastic substrates, and subjected to remote tensile stress. A tensile cohesive plastic zone, of Dugdale type, is placed at each crack tip, and the cohesive zone is characterised by a finite strength and a finite toughness. An analytical theory of the fracture strength is developed (and validated by finite element simulations). The macroscopic strength of the adhesive joint is determined as a function of the relative magnitude of crack length, layer thickness, plastic zone size, specimen width and elastic modulus mismatch between layer and substrates. Fracture maps are constructed to reveal competing regimes of behaviour. The maps span the full range of behaviour from a perfectly brittle response (with no crack tip plasticity) to full plastic collapse. When the sum of crack length and cohesive zone length is less than 0.3 times the layer height, the effect of elastic mismatch between substrate and adhesive layer has only a minor influence upon the macroscopic fracture strength. For this case, the cracked adhesive layer behaves as a centre-crack in an infinite solid made from adhesive, and a transition from toughness control to strength control occurs when the crack length is comparable to that of the cohesive zone length. Alternatively, when the sum of crack length and cohesive zone length exceeds 0.3 times the layer height, the elastic mismatch plays a major role; again there is a transition from toughness control to strength control, but it occurs at a ratio of crack length to layer thickness that depends upon both the elastic mismatch and the ratio of cohesive zone length to layer height. The study also highlights the importance of a structural length scale in the form of layer height times modulus mismatch: this scale is on the order of 1 metre when the layer height equals one millimetre and the elastic modulus of the substrate is one thousand times that of the adhesive layer. The in-plane structural dimensions (including crack length) must exceed this structural dimension in order for a remote K-field to exist within the substrate. Experimental validation of the cohesive zone approach is achieved by measuring the sensitivity of fracture strength to crack length and layer height for a centre-cracked strip made from cellulose acetate layer, sandwiched between aluminium alloy substrates.Financial support from the Engineering and Physical Sciences Research Council (UK) award 1611305, the ERC MULTILAT grant 669764, and SABIC are gratefully acknowledged

Pair-breaking quantum phase transition in superconducting nanowires

A quantum phase transition (QPT) between distinct ground states of matter is a wide-spread phenomenon in nature, yet there are only a few experimentally accessible systems where the microscopic mechanism of the transition can be tested and understood. These cases are unique and form the experimentally established foundation for our understanding of quantum critical phenomena. Here we report the discovery that a magnetic-field-driven QPT in superconducting nanowires - a prototypical 1d-system - can be fully explained by the critical theory of pair-breaking transitions characterized by a correlation length exponent $\nu \approx 1$ and dynamic critical exponent $z \approx 2$. We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross sectional area, and microscopic parameters of the nanowire material. At the critical field, the conductivity follows a $T^{(d-2)/z}$ dependence predicted by phenomenological scaling theories and more recently obtained within a holographic framework. Our work uncovers the microscopic processes governing the transition: The pair-breaking effect of the magnetic field on interacting Cooper pairs overdamped by their coupling to electronic degrees of freedom. It also reveals the universal character of continuous quantum phase transitions.Comment: 22 pages, 5 figure

Simulation of the many-body dynamical quantum Hall effect in an optical lattice

We propose an experimental scheme to simulate the many-body dynamical quantum Hall effect with ultra-cold bosonic atoms in a one-dimensional optical lattice. We first show that the required model Hamiltonian of a spin-1/2 Heisenberg chain with an effective magnetic field and tunable parameters can be realized in this system. For dynamical response to ramping the external fields, the quantized plateaus emerge in the Berry curvature of the interacting atomic spin chain as a function of the effective spin-exchange interaction. The quantization of this response in the parameter space with the interaction-induced topological transition characterizes the many-body dynamical quantum Hall effect. Furthermore, we demonstrate that this phenomenon can be observed in practical cold-atom experiments with numerical simulations.Comment: 8 pages, 3 figures; accepted in Quantum Information Processin

Holographic Vitrification

We establish the existence of stable and metastable stationary black hole bound states at finite temperature and chemical potentials in global and planar four-dimensional asymptotically anti-de Sitter space. We determine a number of features of their holographic duals and argue they represent structural glasses. We map out their thermodynamic landscape in the probe approximation, and show their relaxation dynamics exhibits logarithmic aging, with aging rates determined by the distribution of barriers.Comment: 100 pages, 25 figure

The mammals of Angola

Scientific investigations on the mammals of Angola started over 150 years ago, but information remains scarce and scattered, with only one recent published account. Here we provide a synthesis of the mammals of Angola based on a thorough survey of primary and grey literature, as well as recent unpublished records. We present a short history of mammal research, and provide brief information on each species known to occur in the country. Particular attention is given to endemic and near endemic species. We also provide a zoogeographic outline and information on the conservation of Angolan mammals. We found confirmed records for 291 native species, most of which from the orders Rodentia (85), Chiroptera (73), Carnivora (39), and Cetartiodactyla (33). There is a large number of endemic and near endemic species, most of which are rodents or bats. The large diversity of species is favoured by the wide range of habitats with contrasting environmental conditions, while endemism tends to be associated with unique physiographic settings such as the Angolan Escarpment. The mammal fauna of Angola includes 2 Critically Endangered, 2 Endangered, 11 Vulnerable, and 14 Near-Threatened species at the global scale. There are also 12 data deficient species, most of which are endemics or near endemics to the countryinfo:eu-repo/semantics/publishedVersio

Failure of a pre-cracked epoxy sandwich layer in shear

Adhesive joints are frequently used in automotive, maritime and construction applications, yet joint reliability remains a concern. The purpose of this study is to develop a fracture mechanics methodology for the failure of an elastic-brittle lap-shear joint comprising a thick adhesive layer (an epoxy, of thickness on the order of 10 mm) sandwiched between thick steel adherends. This configuration is of practical application to ship-building, such as the bonding of a superstructure to the underlying hull. A modified thick-adherend shear test (TAST) is designed, and specimens are fabricated, with a range of interfacial pre-crack length and a range of adhesive layer thickness. The failure of samples with no pre-crack is governed by a critical value of corner singularity, whereas the failure of samples with long pre-cracks is governed by a critical value of interfacial stress intensity factor. Predictions for the dependence of failure load upon layer thickness and pre-crack length are obtained by analysing the elastic stress state for both a corner singularity and for an interfacial crack. Both the experimental results and the theoretical framework are useful in the design and fabrication of reliable lap-shear joints that comprise a thick elastic adhesive layer

Tensile fracture of an adhesive joint: the role of crack length and of material mismatch

The tensile strength of an adhesive joint is predicted for a centre-cracked elastic layer, sandwiched between elastic substrates, and subjected to remote tensile stress. A tensile cohesive plastic zone, of Dugdale type, is placed at each crack tip, and the cohesive zone is characterised by a finite strength and a finite toughness. An analytical theory of the fracture strength is developed (and validated by finite element simulations). The macroscopic strength of the adhesive joint is determined as a function of the relative magnitude of crack length, layer thickness, plastic zone size, specimen width and elastic modulus mismatch between layer and substrates. Fracture maps are constructed to reveal competing regimes of behaviour. The maps span the full range of behaviour from a perfectly brittle response (with no crack tip plasticity) to full plastic collapse. When the sum of crack length and cohesive zone length is less than 0.3 times the layer height, the effect of elastic mismatch between substrate and adhesive layer has only a minor influence upon the macroscopic fracture strength. For this case, the cracked adhesive layer behaves as a centre-crack in an infinite solid made from adhesive, and a transition from toughness control to strength control occurs when the crack length is comparable to that of the cohesive zone length. Alternatively, when the sum of crack length and cohesive zone length exceeds 0.3 times the layer height, the elastic mismatch plays a major role; again there is a transition from toughness control to strength control, but it occurs at a ratio of crack length to layer thickness that depends upon both the elastic mismatch and the ratio of cohesive zone length to layer height. The study also highlights the importance of a structural length scale in the form of layer height times modulus mismatch: this scale is on the order of 1 metre when the layer height equals one millimetre and the elastic modulus of the substrate is one thousand times that of the adhesive layer. The in-plane structural dimensions (including crack length) must exceed this structural dimension in order for a remote K-field to exist within the substrate. Experimental validation of the cohesive zone approach is achieved by measuring the sensitivity of fracture strength to crack length and layer height for a centre-cracked strip made from cellulose acetate layer, sandwiched between aluminium alloy substrates