Influence of material properties on dynamic fracture toughness of steels

Recent studies of plastic enclave formation at running brittle cracks were extended to account for the influence of crack tip boundary conditions on the temperature at which the enclaves start to develop. The En 2A and three other steels were used in the analysis. It was found that this temperature depends very strongly both on the magnitude and on the distribution of the stresses in the discrete crack tip zone. This suggests that the onset of enclave formation and the rate of their growth are governed by the balance of two sets of material characteristics. The first set consists of at least two parameters describing the microscopic fracture resistance which promotes enclave formation. The second set includes the macroscopic yield and flow properties which may make enclave formation more difficult in higher strength steels.These findings are related to the dynamic or crack arrest fracture toughness which is found to be derived from two different sources. One is connected with the microscopic plastic deformation of the fracturing metal in the crack tip zone and is present at all temperatures. The other is the result of enclave formation, it is present only at higher temperatures and is responsible for the energy transition. In contrast to the case of crack initiation, the dynamic fracture toughness depends not only on the microscopic fracture strength or strain but on the complete stress-displacement relationship of the weakened material which is governed by the microscopic fracture mechanism at the tip of a running crack. It is noted that the present results can be expected to be valid for all steels which fracture in the cleavage or quasi-cleavage modes.     

A THEORETICAL AND EXPERIMENTAL STUDY OF ENVIRONMENTAL HYDROGEN-ASSISTED SHORT FATIGUE CRACK GROWTH IN AN Al-Li ALLOY

Abstract— The initiation and propagation of fatigue cracks in an Al-Li 8090 alloy in a vapour environment of 0.6 M NaCl solution was investigated. A severe degradation of the resistance to short crack growth was exhibited. Preliminary work carried out to establish the susceptibility of the material to hydrogen embrittlement demonstrated a close correlation between the deformation mode of this alloy and hydrogen absorption. The combination of highly localized slip and highly localized hydrogen fugacity creates a high susceptibility to hydrogen-assisted crack growth.
On the basis of current micro-mechanical models, it is suggested that hydrogen trapping induces a reduction of the friction stress acting in the crack tip plastic zone. Consequently, enhanced plasticity at the crack tip due to the decrease in friction stress leads to an increase in crack growth rate.
An exact solution for a surface crack in a semi-infinite plane is obtained based on a dislocation crack model. Using this solution a computer method is developed to calculate the time-dependent short crack growth rate and fatigue lifetime. Both solutions show good correspondence with the experimental results.     

Numerical modeling of fracture coalescence in a model rock material

The crack pattern, as well as crack initiation, -propagation and -coalescence observed in experiments on gypsum specimens with pre-existing fractures in uniaxial, biaxial, and tensile loading are satisfactorily predicted with the numerical model presented in this paper. This was achieved with a new stress-based crack initiation criterion which is incorporated in FROCK, a Hybridized Indirect Boundary Element method first developed by Chan et al. (1990). The basic formulation of FROCK is described, and the code verified for both open and closed pre-existing fractures either with only friction or with friction and cohesion. The new initiation criterion requires only three material properties: σ_crit, the critical strength of the material in tension; τ_crit, the critical strength of the material in shear; r₀, the size of the plastic zone. The three parameters can be determined with the results from only one test. Predictions using this model are compared with experiments on gypsum specimens with pre-existing fractures loaded in uniaxial and biaxial compression performed by the authors. Specifically, wing crack and shear crack initiation, crack propagation, coalescence stress and -type as well as the crack pattern up to coalescence can be modeled. The model can also duplicate experimental results in compression and tension obtained by other researchers. These results show that stress-based criteria can be effectively used in modeling crack initiation and crack coalescence. This revised version was published online in August 2006 with corrections to the Cover Date.     

A model for predicting crack initiation in forged M3:2 tool steel under high cycle fatigue

AISI type M3 class 2 tool steel (or in German designation DIN: HS6-5-3 tool steel) is most commonly used in tooling industry, and also in some engine parts. Those components are usually subjected to cyclic stresses and mostly fail by fatigue. Fatigue crack initiation in this material occupies large fraction of total lifetime and strongly depends on microstructural features of primary and eutectic carbides, such as shape, shape ratio, volume fraction, the distribution of carbides as well as load ratio. To model fatigue initiation mechanisms of forged M3:2 tool steel, McDowell’s model was modified and developed for different length-scales. For fatigue crack formation and short crack growth, a hierarchical approach was used and the life time of these stages were estimated based on the local cyclic plasticity. Through this relation the effect of microstructural features on both fatigue crack formation and short crack growth in the material were identified. The results of the proposed model have explicitly reflected the influence of microstructural features on both fatigue crack formation and propagation in forged M3:2 tool steel. Moreover, the model can be used for improving the fatigue resistance of a tool steel component.     

Debonding of a thin rubberised and fibre-reinforced cement-based repairs: Analytical and experimental study

An analytical approach for the prediction of debonding initiation between a rubberised cement-based overlay and old concrete substrate under monotonous mechanical loading was applied. Based on the linear elastic fracture mechanics, a model has been developed taking into account the interlocking between two crack surfaces in the overlay. Assuming that the debonding initiation just occurs after the crack cutting the overlay layer reaches the overlay–substrate interface, the stress intensity factor of the debonding tip can be calculated, allowing prediction of stress fields near the interface debonding tip. Then with a criterion of debonding initiation and propagation depending on the interface tensile strength, the load associated could be determined and might be interesting for the design of thin bonded cement-based overlays. The adequateness of this analytical approach was verified by both experimental data and finite element calculations. It has been used to show the relevance of a cement-based material with low modulus of elasticity combined with a high residual post crack strength to achieve sustainable repairs.     

Experiment and Simulation of Breakage of Particle Compounds under Compressive Loading

ABSTRACT

Two-dimensional finite element (FE) compressive stress analyses were carried out on the particle compound material to understand the stress pattern distributions before cracking. FE analysis was followed by discrete element (DE) simulation. A study of the crack propagating mechanism in a particle was represented by a model material that typifies pellets of high-strength pressed agglomerate building materials. For this, concrete spheres of strength category B35 (compressive strength 35 N/mm²) were used. It was observed that the ring tensile stress is responsible for the crack initiation in the spherical particle compounds.     

Fracture R-curve characterization of toughened epoxy adhesives

The R-curve behavior of two different rubber-toughened epoxy adhesives was measured as a function of the mode ratio. A bilinear model was used to characterize the fracture resistance (R-curve) behavior from crack initiation to steady-state crack propagation. Experiments showed that the model parameters depended strongly on the loading mode ratio and the adhesive bondline thickness, but were largely independent of the crack initiation geometry. The results are relevant to the prediction of the crack initiation load and ultimate strength of adhesive joints having relatively short overlap lengths such that a steady-state damage zone cannot develop prior to rupture.     

Creep crack initiation and growth in AISI 316 (N) weld – FEM and experiments

Abstract

There are two aspects of the creep crack growth behaviour, namely, the crack initiation and the crack propagation. An incubation period is often observed prior to the onset of creep crack growth. In this study, creep crack initiation and propagation in pre-cracked compact tension (CT) specimens of a 316 (N) stainless steel weld at T = 550 and 625°C under static loading is investigated. Both the crack initiation time and the crack growth rate are measured as a function of fracture parameter C*. It is shown that it is possible to correlate the creep crack initiation time with the C* parameter. It is also shown that the creep crack growth rate can be correlated with the C* integral. Additionally, finite element analyses by using the ANSYS software have been performed at one test condition (T=625°C) in order to estimate numerically the crack mouth opening displacement rate history for a propagating crack using the node release technique. When the FEM results are compared with the experimental data, the results show a very satisfactory prediction capability.     

Numerical modelling of damage behaviour of laser-hybrid welds

The effect of laser-hybrid welds on deformation and failure behaviour of fracture mechanics specimens is investigated in order to provide quantitative prediction of damage tolerance and residual strength. The simulation of crack initiation and crack extension in hybrid welds is performed by applying GTN damage model. The identification of damage parameters requires combined numerical and experimental analyses. The tendency to crack path deviation during crack growth depends strongly on the constraint development at the interface between base and weld metal. In order to quantify the influence of local stress state on the crack path deviation, the initial crack location is varied. Finally, the results from fracture mechanics tests are compared to real component, beam-column-connection, with respect to fracture resistance.     

A numerical analysis of failure characteristics of ductile layers in laminated composites

In this study, the failure of the ductile layers from collinear, multiple and delaminating cracks that occur in laminated composite systems was studied using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. The results indicate that, in laminated composites, void nucleation and growth ahead of the cracks occur at a much faster rate because of evolution of much higher stress values in the interface region. Except for short crack extensions, collinear and multiple cracks develop crack resistance curves similar to that seen for a crack in the ductile layer material as a homogenous isotropic cases. For delaminating crack cases, the fracture behaviour is strongly influenced by the delamination length. The resistance of the ductile layers to crack extension can be significantly reduced by short delamination lengths; however, for large delamination lengths the resistance to crack extension becomes greater than that seen for the ductile material. The results also show that, if the crack tip is at the interface, similar maximum stress values develop in the ductile layers as in the fracture test of the same ductile material, suggesting that ductile–brittle fracture transition behaviour of the ductile layers is dependent upon the extent of the cracks in the brittle layers and fracture characteristics of the brittle layers.     

Influence of protective coatings on damage mechanisms and lifetime of Alloy 247 DS in thermo-mechanical fatigue tests

Abstract

The degradation process and lifetime of the directionally solidified Ni-base superalloy Alloy 247 DS was investigated under out-of-phase thermo-mechanical fatigue (OP-TMF) loading in relation to the properties and microstructure of oxidation protection coatings. The influence of two coating systems applied by low pressure plasma spraying (i) MCrAlY coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl-topcoat was studied.

The MCrAlY coating did not cause significant changes in failure behaviour and TMF life of the superalloy, whereas duplex coating reduced TMF life significantly because of accelerated fatigue crack initiation and propagation into the substrate material. Due to much lower fracture strains of the duplex coating than of MCrAlY coating the crack formation in the duplex coatings was already observed at very early stages of lifetime. Furthermore, crack propagation was found to be significantly affected by crack branching at the interface between coating and substrate alloy.     

Crack propagation of CCT foam specimen under low strain rate fatigue

The objective of this study is to investigate the effect of holes on the low strain rate fatigue properties of the nickel foam material and to understand the lifetime of this material which is subjected to the repeated loads. Failures of foam materials under single and repeated loads analogous to fatigue are essential to designers and users in military and aerospace structures. The material failure induced by repeated low strain rate loading becomes a critical issue because of significant loss of stiffness and compressive strength in the foam material. Testing methods to study low strain rate (that is, strain rate) fatigue are quite numerous; no single standard testing procedure is defined for studying the low strain rate fatigue property of a material. The increasing application of foam material in aerospace structures, owing to high specific stiffness and strength has attracted a great concern about the high sensitivity to low strain rate damage introduced during manufacture or in service, and the effects of such damage on structural degradation. To investigate this issue, this study sets up an experimental procedure to determine the low strain rate fatigue properties of nickel foam material. This study performs both experimental and numerical investigations to catch the low strain rate fatigue behavior of nickel foam with open-cell type. The experiments are conducted by rod up and down at the strain rate fatigue of loading. The crack length at the specific cycles are measured experimentally by taking pictures with a paper ruler attached on the surface of specimen and these values are apply to the computer simulations as crack seam model. The simulation result of stress intensity factors are compared with a well known theoretical calculation. Design life and probability of failure or survival at specified life can be calculated so that the fatigue life of nickel core material subjected to repeated low strain rate loading is predicted.     

Verformungsverhalten und rechnerische Abschätzung der Ermüdungslebensdauer metallischer Werkstoffe unter mehrachsig nichtproportionaler Beanspruchung

Deformation behaviour and numerical fatigue lifetime prediction of metallic materials under multiaxial nonproportional loading The development and evaluation of a model for lifetime prediction under multiaxial nonproportional loading is the aim of the current research project. It is assumed that the technical crack initiation life is consumed by short crack growth. This phenomenon is described using a fracture mechanics based approach. Herein, the effective cyclic J‐integral is used as crack tip parameter. Crack opening levels and J‐integral values are calculated applying approximation formulas. A plasticity model that is based on the Jiang model [Jia93] and extended to describe nonproportional hardening is used to predict the deformation behaviour. Experimental investigations on tubular and notched specimens with a wide range of different loading spectra serve for the verification of the model and for the identification of damage mechanisms.     

A NEW MODEL FOR THE NUMERICAL DETERMINATION OF PITTING RESISTANCE OF GEAR TEETH FLANKS

Abstract— A numerical model for determining the pitting resistance of gear teeth flanks is presented in this paper. The model considers the material fatigue process leading to pitting, i.e. the conditions required for crack initiation and then simulation of fatigue crack propagation. The theory of dislocation motion on persistent slip bands is used to describe the process of crack initiation, where the microstructure of a material plays a crucial role. The simulation of crack growth takes into account both short crack growth, where the modified Bilby, Cottrell and Swinden model is used for simulation of dislocation motion, and long crack growth, where the theory of linear elastic fracture mechanics is applied. The stress field in the contact area of meshing spur gear teeth and the functional relationship between the stress intensity factor and crack length are determined by the finite element method. For numerical simulations of crack initiation and crack propagation in the contact area of spur gear teeth, an equivalent model of two cylinders is used. On the basis of numerical results, and with consideration of some particular material parameters, the service life of gear teeth flanks is estimated. The developed model is applied to a real spur gear pair, which is also experimentally tested. The comparison of numerical and experimental results shows good agreement and it can be concluded that the developed model is appropriate for determining the pitting resistance of gear teeth flanks.     

A Continuum Mechanics Approach for Crack Initiation and Crack Growth Predictions

Very often, different approaches are used for crack initiation and crack growth predictions. The current article introduces a recently developed approach that can be used for the predictions of both crack initiation and crack propagation. A basic assumption is that both crack nucleation and crack growth are governed by the same fatigue damage mechanisms and a single fatigue damage criterion can model both stages. A rule is that any material point fails to form a fresh crack if the total accumulated fatigue damage reaches a limit. For crack initiation predictions, the stresses and strains are obtained either directly from experiments or though a numerical analysis. For the prediction of crack growth, the approach consists of two steps. Elastic‐plastic stress analysis is conducted to obtain the detailed stress‐strain responses. A general fatigue criterion is used to predict fatigue crack growth. Compact specimens made of 1070 steel were experimentally tested under constant amplitude loading with different R‐ratios and the overloading influence. The capability of the approach to predict both crack initiation and the crack growth under these loading conditions was demonstrated by comparing the predictions with the experimental observations.     

Experimental and numerical investigation on crack initiation of fretting fatigue of dovetail

The present study investigated the fretting fatigue crack initiation of dovetail structure based on experimental observation and multiple axial criteria. Two typical critical plane approaches of the Smith‐Watson‐Topper (SWT) and the Fatemi and Socie (FS) model were used to predict the crack initiation location, orientation angle, and fatigue life. The results indicate that both SWT and FS models predict consistent results with the experiment in crack initiation location. Regarding the crack initiation angle, FS model shows good agreement with the experimental observation, but SWT model exhibits a large difference. The two models give conservative results in fretting fatigue life. In view of this, the theory of critical distance (TCD) was incorporated into the SWT and the FS models. It shows that both the TCD‐SWT and the TCD‐FS predict fatigue lives within a scatter band of 2. It suggests that introducing the TCD into the critical plane model can greatly reduce the conservatism of the prediction. Furthermore, the prediction has less dependence on specific models.     

Characterization of Loading Rate Effects on the Interactions between Crack Growth and Inclusions in Cementitious Material

The microcapsule-enabled cementitious material is an appealing building material and it has been attracting increasing research interest. By considering microcapsules as dissimilar inclusions in the material, this paper employs the discrete element method (DEM) to study the effects of loading rates on the fracturing behavior of cementitious specimens containing the inclusion and the crack. The numerical model was first developed and validated based on experimental results. It is then used to systematically study the initiation, the propagation and the coalescence of cracks in inclusion-enabled cementitious materials. The study reveals that the crack propagation speed, the first crack initiation stress, the coalescence stress, the compressive strength and the ultimate strain increase with the loading rate. The initiation position, the propagation direction, the cracking length and the type of the initiated cracks are influenced by the loading rates. Two new crack coalescence patterns are observed. It is easier to cause the coalescence between the circular void and a propagating crack at a slow loading rate than at a fast loading rate.     

A phenomenological model of fatigue macrocrack initiation near stress concentrators

Fatigue macrocrack initiation is considered to be a two-parameter process. It is governed by the local or strain amplitude, and a certain linear parameter of the material. Corresponding parameters have been proposed, i.e. the local stress range Δσ*y and a characteristic distance d *, the prefracture zone size. The formation of this zone is conditioned by a decrease in yield strength within the material’s surface layers, microstructure, loading amplitude, cyclic strain hardening and environment. The value of d * is estimated experimentally by several methods and is assumed to be a certain material constant, independent of both notch and specimen geometry. At the prefracture zone boundary, a major barrier exists that retards the growth of a physically small fatigue crack. The moment when the physically small crack overcomes the prefracture zone boundary is assumed to be a quantitative criterion, a_{i = d *}, for the micro- to macrocrack transition. The proposed relationships, Δσ*y versus N_i , and d * versus N_i , can be used as a basis for the establishment of the materials resistance to macrocrack initiation.     

Fast time-scale average for a mesoscopic high cycle fatigue criterion

The paper discusses the lifetime prediction of structures in high-cycle fatigue based on the two-scale fatigue criteria of Dang Van type and several of its extensions in finite lifetime regime. The main assumptions for this criteria are (i) the material is polycrystalline and undergoes localised plasticity in one of the misoriented grains and (ii) crack initiation arises as a consequence of cumulated plasticity in this grain.The novelty of the presented approach is twofold. On the one hand a generalisation of mesoscopic plasticity model is presented, on the other a fast time scale average is introduced for tracking the cyclic material behaviour and the subsequent evolution of damage. The tracking method is based on the split between a quick quasi-periodic response of the system to the cyclic load and a slow evolution of the internal hardening and damage parameters of the material at the mesoscopic scale. The proposed method can be extended to a large class of local material behaviours involving not only plasticity, but also crack and damage evolution.The paper proposes a simplified plasticity-based model for the mesoscopic material behaviour and presents a comparison between predicted and experimental lifetimes. The results are discussed in terms of prediction capabilities and also in terms of the identification procedure of parameters of the mesoscopic model.     

A methodology for fatigue crack growth and residual strength prediction with applications to aircraft fuselages

The FRANC3D/STAGS software system has been developed to model curvilinear crack growth in aircraft fuselages. Simulations of fatigue crack growth have been reported previously (Potyondy et al. 1995). This paper presents two enhancements to this system. One is the generalization of the representation of cracks that allows the system to represent realistic damaged structures more accurately. With this capability, parameters that may affect the trajectory of a fatigue crack are studied. Results are compared with measurements from a full-scale test. The second enhancement is to model stable tearing for residual strength prediction. A stable tearing simulation along a crack path that captures the material nonlinearities inherent at the crack tip is performed. The CTOA (Crack Tip Opening Angle) is used as a crack growth criterion to characterize the fracture process under conditions of general yielding. Residual strength of cracked structures is predicted.