A Singular Nonlinear History-Dependent Cohesive Zone Model: Is it Possible?

Summary A history-dependent cohesive zone model is considered in the linear elasticity framework with the cohesive stresses governed by the fracture condition formulated in terms of a nonlinear Abel-type integral operator. A possibility for the cohesive stresses to possess a weak singularity has been examined by utilizing asymptotic modeling approach. It has been shown that the balance of the leading-term asymptotic representations in the model equations is possible for nonsingular cohesive stresses only.

Adhesion of steel and fiberglass meshes with concrete

Introduction. To prevent the formation of cracks and destruction of masonry structures they are reinforced with meshes at the stage of construction. Compatibility of the reinforcing material and the main body of the structure is an important parameter for the efficient operation of the structure. The article examines the amount of adhesion of fiberglass and steel mesh elements with sand concrete, which is the binding of bricks. Materials and methods. Reinforced beams are tested for bending. Samples of beams consist of two halves connected by a rod in a stretched zone and uncoupled by a hinge in a compressed zone. Elements of steel and fiberglass grids with limited anchorage zones in concrete act as reinforcement. The values of rod slippage in concrete were fixed by a dial gauge. Results. According to the results of the test, the slipping forces of the rods were obtained and the cohesive stresses of the elements of steel and fiberglass grids with concrete were calculated, and diagrams were drawn. The value of adhesion with concrete fiberglass rods was greater compared with the adhesion of steel rods for all values of slip. Conclusions. Fiberglass meshes have a number of drawbacks to steel meshes, however, a large number of positive characteristics of fiberglass meshes, including adhesion with concrete, make them competitive with steel, and in many cases more preferable for use in structures. Keywords: coupling reinforcement with concrete, adhesion, brickwork, reinforcement of structures, masonry mesh, composite materials, steel mesh, composite mesh, innovation in construction/

A Multi-Scale Viscoelastic Cohesive Layer Model for Predicting Delamination in HTPMC

In this paper, a novel numerical-experimental methodology is outlined to predict delamination in pristine as well as isothermally aged (in air) polymer matrix composites. A rate-dependent viscoelastic cohesive layer model was implemented in an in-house test-bed finite element analysis (FEA) code to simulate the delamination initiation and propagation in unidirectional polymer composites before and after aging. This unified model is fully rate-dependent and does not require a pre-assigned traction-separation law. The actual shape of traction separation law depends on: (a) the strain rate via the viscoelastic constitutive relationship, (b) the degree of thermo-oxidative aging via the changes in the experimentally measured creep compliance due to oxidation, and (c) the evolution of the internal state variable defining the state of damage. To determine the model parameters, double cantilever beam (DCB) experiments were conducted on both pristine and isothermally aged IM-7/bismaleimide (BMI) composite specimens. The J-Integral approach was adapted to extract cohesive stresses near the crack tip. A principal-stretch dependent internal damage state variable defines the damage in the cohesive layer. Within the cohesive layer, pristine and cohesive stresses were compared to estimate the damage parameters. Once the damage parameters had been characterized, the test-bed FEA code employed a micromechanics based viscoelastic cohesive layer model to simulate interlaminar delamination. From a numerical stability standpoint, the viscous regularization effect of the viscoelastic constitutive equations in the cohesive layer helps mitigate numerical instabilities caused by elastic energy released due to crack growth, thereby enabling the FEA model to simulate the load-deflection response of the composite structure well beyond peak load. The present cohesive-layer based FEA model was able to accurately predict not only the macro level load-displacement curve, but also the micro level crack growth history in IM-7/BMI laminate before and after thermal aging, using only three parameters.

A Cohesive Zone Model to Simulate Fatigue Crack Propagation under High Pressure Gaseous Hydrogen

In this study we focus on the effect of hydrogen gas on the cracking resistance of metals. The main objective is to predict the fatigue crack propagation rates in the presence of hydrogen. For this purpose, a Cohesive Zone Model (CZM) dedicated to cracking under monotonic as well as cyclic loadings has been implemented in the ABAQUS finite element code. A specific traction-separation law, adapted to describe the gradual degradation of the cohesive stresses under cyclic loading, and sensitive to the presence of hydrogen is formulated. The coupling between mechanical behaviour and diffusion of hydrogen can be modelled using a coupled temperature - displacement calculation available in ABAQUS. The simulations are compared with fatigue crack propagation tests performed on a 15-5PH martensitic stainless steel. They show that while the proposed model is able to predict a lower resistance to cracking in presence of hydrogen, at this stage it cannot fully account for the detrimental effect induced by high pressure of gaseous hydrogen.

Advanced Natural Stitched Composite Materials in Skin-Stiffener of Wind Turbine Blade Structures

In this paper the failure behaviour of natural stitched composite materials in the skin-stiffener of wind turbine blade structures are investigated. For this study, the laminated composite beams were stitched using Flax yarns before curing process. Two stiffener structures of T-beam and Box-beam are studied in this paper. These specimens were tested under quasi-static loading condition to compare the failure resistance of adhesive and stitched bonding methods. Furthermore, the cohesive zone modelling (CZM) which is known as a variation in the cohesive stresses with the interfacial opening displacement along the localised fracture process zone is used to predict bonding failure in the skin-stiffener of wind turbine blade structures.

Finite Element Analysis of the Cohesive Zone across a Strength Mismatched Bimetallic Interface

When a Mode I crack in soft steel body grows and reaches near the perpendicular interface of ultra strong steel body, its cohesive zone penetrates into the interface body which influences the crack tip parameter. The paper presents finite element analysis of the cohesive zone across the interface of such elastically matched but strength mismatched bodies in linear elastic regime. Parent alloy steel (ASTM 4340) body and interface maraging steel (MDN 250) body are considered for analysis. The cohesive zone is modeled in accordance with the Dugdale criterion. J integral is evaluated over the path around the interface to examine the effect of cohesive stresses on the crack tip. The results are compared vis-à-vis those obtained from the theoretical model. The two are in very good agreement with each other.