Institute of Fundamental Technological Research

The critical monotonic strain of Ni-W and MoS2(Ti,W) coatings on steel substrates was studied. The idea of axisymmetric bending test (called here as coin bending test) limited to monitoring of the coating failure was used. Experiments revealed mechanism of the coating failure, as cracking initiated from coating surface defects and/or substrate was demonstrated using indentation technique. By pushing the center of the uncoated side of a circular plate, the axisymmetric stress state was generated in the coating. The stress components varied gradually from the greatest value in the center to the smallest value at the edge of the specimen. The changes of the sample surface as a result of loading were monitored step by step via optical microscopy.

Ni-W coating, MoS2(Ti.W) coating, steel substrate, fracture, coatings strength, axisymmetric bending test

The analysis of mechanical response of joined bi- or multi-layer systems is a typical problem for both geomechanics and composite technology. The elastic or visco-elastic layers interact through joining interfaces transferring stress state between layers and assuring structure integrity. The typical damage modes are related to progressive delamination at bonding interfaces, affected by distributed layer cracking. The present work is aimed to provide an analytical study of the stress state in a bi-layer system and of the progressive delamination process. The cohesive zone model is applied to simulate the interface response with shear stresses related to displacement discontinuities and to the specific fracture energies in shear mode. The following specific issues are discussed: delamination mode growth with the related critical and post-critical response of evaluation length of the process zone, scale effect of the critical stress. The analysis results can be applied to clarify the effect of material parameters on the damage process and to discuss experimental testing of epoxy joined ceramic elements, with specification of the connection strength, related to both the critical interface stress and the specific fracture energy.

Delamination, Cohesive zone, Shear lag, Damage process

The fatigue response of electrodeposited Ni-W on low carbon steel substrate was studied. The considered Ni-W coatings are a promising substitute for toxic hard chromium coatings which, according to EU directives (2000/53/WE, 2011/37/UE), must be eliminated. Specimens of pure substrate and those covered by coating were compared. Coated specimens exhibited lower fatigue resistance under higher stress amplitudes than their non-coated counterparts in stress-controlled fatigue experiments. However, at lower stress amplitudes, the fatigue strength of both specimen types was similar. The cyclic softening of the coatings was demonstrated using indentation techniques.

fatigue, hardness, indentation, low carbon steel substrate, Ni-W coatings, S-N curve

The simplified approach to the modelling of low cycle fatigue (LCF) of functionally graded materials (FGM) based on the continuum mechanics is presented. The fatigue damage model takes into account the mechanical part of the load and a constant service temperature. The concept of FGM as a particle-reinforced metal-matrix composite with gradual change of the reinforcement fraction is used. The FGM is considered as a material consisting of homogeneous layers containing different volume fractions of the reinforcement. The variation of the reinforcement fraction changes the material properties for each layer. The different material properties are obtained according to modified rule of mixture. Since the fatigue damage of metal matrix composites is strongly influenced by the inelastic deformation of the metallic matrix, the constitutive equations of LCF damage model are taken into consideration. The combined isotropic/ kinematic hardening model with linear behaviour of isotropic and kinematic parts of hardening is adopted. The damage scalar parameter is associated with the plastic energy dissipation which is used to update the material properties. The fatigue damage model presented in this paper is applied to the fatigue damage analysis of the cooling channel of thruster used in space shuttles and rockets.

Fatigue Damage, Low Cycle Fatigue, Functionally Graded Materials, Layers

The models of crack growth in mixed mode conditions are reviewed for the plane and three-dimensional (3D) states of stress. Both critical load value and crack path or surface growth are predicted by different criteria in terms of elastic singular stress states and T-stress component. Monotonic and cyclic loading induced crack growth is considered. The energy and critical plane criteria based on local or non-local measure of stress and strain are most useful in developing predictive crack growth simulation. The finite critical distance from the crack tip should be specified to provide averaged or local stress and strain states. The application of MK-criterion of crack growth expressed in terms of volumetric and deviatoric strain energies is presented for several specific cases of monotonic and cyclic loading. The concepts of smooth and rough crack surface are discussed with application to 3D crack surface growth.

Fracture criteria, T-stress, Fatigue crack growth, 3D crack, Smooth and rough cracks, Crack path, Critical plane, Non-local models

The new criterion determining the angle of crack propagation in mixed mode loading conditions is proposed. Both monotonic and cyclic loading are considered for homogeneous and bimaterial systems. For bimaterial it is assumed that either the crack does not touch or simply crosses the interface during its growth. The angle of the crack propagation corresponds with the direction where the ratio of the dissipation energy generated in the plastic processes to that generated in the decohesion processes reaches its smallest value. The criterion assumes also that the fracture crack propagation follows the direction for which the distortional component of the strain energy density, T D calculated along the line of the constant dilatation energy density T C V around the crack tip reaches its minimum value. The constant value of T C V is associated with the ultimate tensile stress σ C. The criterion involves the T -stress term that represents a constant stress parallel to the main crack orientation. A critical length of the radius of the decohesion zone determines the fracture condition. In the article a numerical method based on the singular integral technique involving the distribution of edge dislocations to represent the actual crack is proposed. The resulting singular integral equations with Cauchy-type singularities are solved numerically. The method allows us to determine the stress intensity factors (SIFs) and T-stress term at the tips of the propagating crack. The results show very good agreement with the numerical data found in the literature. The new equation to determine the fatigue crack growth rate is also proposed and used to trace the fatigue crack path in the homogeneous and bimaterial systems.

SIFs, fracture criteria, fatigue crack growth rate, bimaterial, T -stress, singular integral equations, interface

Praca dotyczy płaskiego zagadnienia wzrostu szczeliny zmęczeniowej w bimateriale powstałym na skutek idealnego zespolenia dwóch materiałów sprężystych. Proponowana metoda umożliwia określenie wzrostu takiej szczeliny, dowolnie usytuowanej w bimateriale. Oparta jest ona na istniejących modelach matematycznych rozwijanych w ramach mechaniki pękania i dających podstawy do wyznaczenia współczynników intensywności naprężeń dla szczelin w bimateriale. Metoda ta pozwala ocenić, dla każdego kolejnego cyklu obciążenia, kierunek propagacji i przyrost długości szczeliny, a w konsekwencji jej drogę i długość będące wynikiem procesu zmęczeniowego. Do tego celu wykorzystany jest dyslokacyjny model szczeliny, który prowadzi do układu osobliwych równań całkowych. Do rozwiązania tego układu równań zaproponowano efektywny algorytm numeryczny.

The crack growth rules are usually specified in terms of the potential energy release or of non local stress or strain criteria. In the present work, the criteria based on the critical plane concept are first discussed and expressed in terms of averaged stress developed in the crack front domain. The transition from tensile to shear fracture is predicted for varying mode mixity ratio.

mixed mode crack growth, critical plane, energy conditions, brittle fracture, ductile fracture, SIF, strain energy density, LEFM

Most structural components contain inhomogeneities such as hardened surface layers or coatings, welded or bonded elements, etc. The crack growth mode then depends not only on applied loading and structure geometry but also on material inhomogeneity. In the presentation the analysis of crack growth mode will be discussed for two cases: a bi-material structure with two materials of different thermo-elastic and plastic properties connected along the interface and a structure with introduced interlayer of different material. The simplified approach is presented, namely the MK -criterion [1,2] based on the linear elastic fracture mechanics (LEFM). In the criterion it is assumed that crack growth follows the direction of minimum distortion energy density at a distance corresponding to specified value of dilatation energy. For two materials, A and B the crack growth from A to B or along the interface AB may occur depending on relative critical stress and fracture energy values. For a three-phases structure, various modes of crack growth are possible.

SIF, Strain energy density, LEFM, Fracture, Bi-material, Layers, Fatigue crack growth

W pracy rozważono możliwość odchylania się szczeliny dla przypadku gdy KI>0, KII≥0 w dwuosiowym stanie obciążenia. Do analizy wykorzystano dwa kryteria liniowej sprężystej mechaniki pękania (LSMP) uwzględniając efekt naprężenia T: klasyczne kryterium maksymalnych naprężeń obwodowych (MNO) oraz energetyczne kryterium MK [7,8,9]. Przedstawiono po równanie obu kryteriów w odniesieniu do niestabilnego kierunku p ropagacji szczeliny, definiowanego w pracy jako odchylenie się ścieżki propagacji od płaszczyzny początkowej lokalizacji szczeliny. Przedstawiono również porównanie przypadków granicznych z wykorzystaniem modeli wzrostu początkowej szczeliny.