Jan Klusák

A Comparison of Two Direct Methods of Generalized Stress Intensity Factor Calculations of Bi-Material Notches

The study of bi-material notches is becoming a topical problem as they can model geometrical or material discontinuities efficiently. Assessing the conditions for crack initiation in bimaterial notches makes it necessary to calculate the generalized stress intensity factors H. In contrast to the determination of the K factor for a crack in an isotropic homogeneous medium, for the ascertainment of a generalized stress intensity factor (GSIF) there is no procedure incorporated in the calculation systems. The calculation of these fracture mechanics parameters is not trivial and requires certain experience. Nevertheless, the accuracy of the H-factor calculation directly influences the reliability of the assessment of the singular stress concentrators. Direct methods of the estimation of H factors usually require choosing the length parameter entering into the calculation. Two types of direct methods of calculating the GSIFs are presented, tested and mutually compared. Recommendations for reliable estimation of H factors are suggested.

Study of the Stress Distribution Around an Orthotropic Bi-Material Notch Tip

Knowledge of the stress distribution is the first and necessary step for the reliable assessment of construction with a geometrical or material discontinuity. General geometry and orthotropic material characteristics of both material components lead to singular stress distribution with general stress singularity exponents different from ½. For the final stress field determination both analytical and numerical approaches are utilised. The results of the theoretical approaches are compared to results from finite element method.

Experimental Investigation of the Influence of the Bond Conditions on the Shear Bond Strength between Steel and Self-Compacting Concrete Using Push-Out Tests

Steel-concrete joints are often provided with welded shear studs. However, stress concentrations are induced in the structure due to the welding. Moreover, a reduction in toughness and ductility of the steel and a decreased fatigue endurance of the construction is observed. In this paper the shear bond strength between steel and ultra-high performance concrete (UHPC) without mechanical shear connectors is evaluated through push-out tests. The test samples consist of two sandblasted steel plates with a thickness of 10 mm and a concrete core. The connection between steel and concrete is obtained by a 2-component epoxy resin. Test samples with a smooth adhesive layer are compared with those with an epoxy layer, which is applied with a toothed paddle and/or gritted with small aggregates. In this research, specimens prepared with river gravel, crushed stone, and steel grit are compared and also two different epoxy resins are used. During the tests, the ultimate shear force is recorded as well as the slip between steel and concrete. All test specimens exhibited a concrete-adhesive or concrete failure. Furthermore, test results show that the use of a more fluid epoxy resin improves the anchorage of the gritted aggregates in the adhesive layer, resulting in higher shear bond stresses. No significant difference is found between specimens, gritted with river gravel or crushed stone. Applying the adhesive layer with the toothed paddle in horizontal direction slightly improves the bond behaviour. Finally, the experimental results of the test members with a smooth epoxy layer without gritted aggregates, provide test data for a fracture mechanics approach, which uses a 2D numerical model of the test specimen, composed of steel, epoxy resin, and concrete.

A Bi-Material Wedge – a Model for the Prediction of Failure Initiation at Shape and Material Discontinuities

Geometrical and material discontinuities in constructions lead to singular stress concentrations and consequently to a crack initiation. The model of a bi-material wedge makes it possible to analyse such construction points to assess their stability. The presented approach is based on the knowledge of the strain energy density factor distribution in the concentrator vicinity.

Modelling of interfacial transition zone effect on resistance to crack propagation in fine-grained cement-based composites

In this paper, the attention is paid to investigation of the importance of the interfacial transition zone (ITZ) in selected fine-grained cement-based composites for the global fracture behaviour. This is a region of cement paste around the aggregate particles which specific features could have significant impact on the final behaviour of cement composites with a crack tip nearby this interface under applied tension. The aim of this work is to show the basic interface microstructure by scanning electron microscopy (SEM) done by MIRA3 TESCAN and to analyse the behaviour of such composite by numerical modelling. Numerical studies assume two different ITZ thicknesses taken from SEM analysis. A simplified cracked geometry (consisting of three phases – matrix, ITZ, and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix–ITZ interface. ITZ’s modulus of elasticity is taken from generalized self-consistent scheme. A few conclusions are discussed based on comparison of the average values of the opening stress ahead of the crack tip with their critical values. The analyses dealing with the effect of ITZ’s properties on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

Conditions for Crack Initiation in an Orthotropic Bi-Material Notch

In the present contribution the joint of two orthotropic materials is investigated as the singular stress concentrator and it is modelled as an orthotropic bi-material notch. Such places in constructions are usually responsible for crack initiation and consequently for the final failure of the construction. Within the paper the parametric study of the crack initiation conditions is presented for specific geometry and material combinations and for varying loading conditions. The determination of the crack initiation direction and critical loading conditions is a necessary step in the reliable design of structures with singular stress concentrations.

Crack Propagation from Bi-Material Notches – Matched Asymptotic Procedure

The methods based on the properties of the two-state integrals allow one to calculate the amplitude of singular and the other terms of the Williams’ asymptotic expansion. The paper is focused on the use of the Y-integral, whose application is conditioned by the knowledge of the so-called auxiliary solution of the solved problem. On the other hand, the Y-integral can be applied to the analysis of the problems with various geometries, e.g. the analysis of the bi-material notches. The application of the Y-integral can be also extended to the matched asymptotic procedure, which allows one to predict the behavior of the cracked notches or following crack growth near the bi-material interfaces.

Failure Conditions from Push Out Tests of a Steel-Concrete Joint: Fracture Mechanics Approach

In order to evaluate the shear bond strength of a steel-concrete joint using an epoxy adhesive interlayer, push-out tests were carried out. The test samples consisted of two sandblasted steel plates and a self-compacting concrete sample, with the epoxy layer applied on the steel plates and gritted with granulates. During testing, an external force was applied to the concrete core and continuously recorded. To investigate the failure mechanism in detail, a fracture mechanics approach is required. In this paper theoretical-numerical assessment of the push-out test is performed. Regarding the finite element calculations, the locations suitable for failure initiation match bi-material (steel-concrete) notches. The most dangerous locations are evaluated from a generalized linear elastic fracture mechanics point of view. The critical load corresponding to the conditions of failure initiation is estimated and compared with the experimental results.

Case Criterion of Crack Onset in Orthotropic Bi-Material Notches

A bi-material notch composed of two orthotropic parts is considered. The stresses and displacements are expressed using the Stroh-Eshelby-Lekhnitskii formalism for plane elasticity. The potential direction of crack initiation is determined from the maximum mean value of the tangential stress or the local minimum of the mean value of the generalized strain energy density factor in both materials [1, 2]. The matched asymptotic procedure is introduced to derive the change of potential energy for the debonding crack and the crack initiated in the determined direction [3].

On the Crack Initiated from the Bi-Material Notch Tip

The bi-material notch composed of two orthotropic parts is considered. The radial and tangential stresses and strain energy density is expressed using the Stroh-Eshelby-Lekhnitskii formalism for the plane elasticity. The potential direction of the crack initiation is determined from the maximum mean value of the tangential stresses and local minimum of the mean value of the generalized strain energy density factor in both materials. Matched asymptotic procedure is used to derive the change of potential energy for the debonding crack and the crack initiated in the determined direction.