Toughness Anisotropy Research Articles

Ultra-strong and damage tolerant metallic bulk materials: A lesson from nanostructured pearlitic steel wires.

Structural materials used for safety critical applications require high strength and simultaneously high resistance against crack growth, referred to as damage tolerance. However, the two properties typically exclude each other and research efforts towards ever stronger materials are hampered by drastic loss of fracture resistance. Therefore, future development of novel ultra-strong bulk materials requires a fundamental understanding of the toughness determining mechanisms. As model material we use today’s strongest metallic bulk material, namely, a nanostructured pearlitic steel wire, and measured the fracture toughness on micron-sized specimens in different crack growth directions and found an unexpected strong anisotropy in the fracture resistance. Along the wire axis the material reveals ultra-high strength combined with so far unprecedented damage tolerance. We attribute this excellent property combination to the anisotropy in the fracture toughness inducing a high propensity for micro-crack formation parallel to the wire axis. This effect causes a local crack tip stress relaxation and enables the high fracture toughness without being detrimental to the material’s strength.

Mechanical anisotropy in Ca-treated and ultra-low sulphur HSLA-100 steel

A few elongated MnS inclusions were observed in ultra-low sulphur HSLA-100 steel which significantly decreased the yield strength (YS), ultimate tensile strength and notch toughness of short transverse direction. The slight anisotropies of YS (≈5 MPa) and notch toughness (≈15 J) between longitudinal and transverse directions are correlated to the presence of texture components in the vicinity of {113}<110> and {554}<225>. In the case of anisotropy between longitudinal and short transverse directions, it is found that recognising the contribution of each effective factor in the creation of strength anisotropy is not possible. However, the most primary effective factor in the creation of remarkable notch toughness anisotropy (≈70 Pct), especially at −85°C, is related to the existence of elongated MnS inclusions.

Modelling of the fracture toughness anisotropy in fiber reinforced concrete

Steel fiber reinforced concrete is potentially very promising material with unique properties, which currently is widely used in some applications, such as floors and concrete pavements. However, lack of robust and reliable models of fiber reinforced concrete fracture limits its application as structural material. In this work a numerical model is proposed for predicting the crack growth in fiber reinforced concrete. The mixing of the steel fibers with the concrete usually creates nonuniform fibers distribution with more fibers oriented in horizontal direction, than in vertical. Simple numerical models of fiber reinforced concrete require a priori knowledge of the crack growth direction in order to take into account bridging action of the fibers, which depends on the fibers orientation. In proposed model user defined elements are used to calculate the bridging force during the course of the analysis when the crack starts to grow. Cohesive elements were used to model the crack propagation in the concrete matrix. In cohesive zone model the cohesive elements are embedded between all solid elements to simulate the arbitrary crack path. The bridging effect of the fibers are modeled as nonlinear springs, where the stiffness of the springs is defined from experimentally measured pull-out force and the angle between the fiber and crack opening direction.

Effect of MnS inclusion and crystallographic texture on anisotropy in Charpy impact toughness of low carbon ferritic steel

The combined effect of microstructure and crystallographic texture on the anisotropy in Charpy impact toughness has been studied in two hot-rolled low-carbon steel plates having different sulphur contents: 0.03wt% S in high-sulphur, HS, steel and 0.01wt% S in low-sulphur, LS, steel. Sub-size Charpy impact specimens were prepared from the rolled plates along 0°, 30°, 60° and 90° to the rolling direction and tested over the temperature range of +40°C to −196°C. Microstructure, inclusions and crystallographic texture have been characterized on the plane parallel to the fracture plane of each sample. The variation in upper shelf energy (USE) was more severe (∼69%) in HS steel due to the presence of coarse and elongated MnS inclusions. Crystallographic texture, especially, higher fraction of alpha- to gamma-fibre texture and stronger cube texture resulted in stronger variation in ductile to brittle transition temperature (DBTT) in LS steel (by ∼26°C). Increase in the fraction of {001} planes of the crystals on the fracture plane of the sample increased the DBTT by helping the cleavage crack propagation. On the other hand, increase in the fraction of {110} planes of the crystals on the shear planes (inclined at 45° to fracture plane) increased the USE by promoting the plastic deformation.

Anisotropy in the impact toughness of selective laser melted Ti–6Al–4V alloy

Apparent anisotropies have been found in the mechanical properties of selective laser melted Ti–6Al–4V alloy. This study investigated the influences of the vertical and horizontal building directions on the impact toughness of selective laser melted Ti–6Al–4V, and a mechanism for the anisotropy in toughness is proposed. Disc-shaped building defects were clearly observed, even though the relative density was as high as 99.5%. The directionality of these defects reduces the load-bearing cross-section of a vertical specimen relative to that of a horizontal specimen; consequently, the impact energy of a horizontal specimen is 96% higher than that of a vertical specimen.

Crack path predictions and experiments in plane structures considering anisotropic properties and material interfaces

In many engineering applications special requirements are directed to a material's fracture behavior and the prediction of crack paths. Especially if the material exhibits anisotropic elastic properties or fracture toughnesses, e.g. in textured or composite materials, the simulation of crack paths is challenging. Here, the application of path independent interaction integrals (I-integrals), J-, L- and M-integrals is beneficial for an accurate crack tip loading analysis. Numerical tools for the calculation of loading quantities using these path-invariant integrals are implemented into the commercial finite element (FE)-code ABAQUS. Global approaches of the integrals are convenient considering crack tips approaching other crack faces, internal boundaries or material interfaces. Curved crack faces require special treatment with respect to integration contours. Numerical crack paths are predicted based on FE calculations of the boundary value problem in connection with an intelligent adaptive re-meshing algorithm. Considering fracture toughness anisotropy and accounting for inelastic effects due to small plastic zones in the crack tip region, the numerically predicted crack paths of different types of specimens with material interfaces and internal boundaries are compared to subcritically grown paths obtained from experiments.

Microstructure characterization, mechanical properties and toughening mechanism of TiB2-containing conventional cast TiAl-based alloy

Effects of TiB2 addition on microstructure and mechanical properties of Ti–48Al–2Cr–2Nb+(0.72,1.62)wt% TiB2 alloys fabricated by the induction skull melting (ISM) process were investigated. Results showed that the TiB2-induced microstructure was characterized by randomly orientated fully lamellar colonies and both the colony size and lamellae spacing were refined (100μm and ~185nm, respectively) by TiB2 addition. The borides were identified to be TiB2 with plate, needle and block morphologies, determined by different growth stages during solidification. At room temperature and 700°C, the TiB2-containing alloys exhibit non-deteriorated fracture toughness and superior tensile properties than that of the as-cast and heat-treated matrix alloys. Furthermore, the fracture toughness anisotropy was eliminated due to the randomly orientated lamellar microstructure induced by TiB2 addition. The fine TiB2 particles with special morphology (plate and needle) and the easy-to-deform ligament bridges induced by the refined microstructure can account for the notable fracture toughness of the studied TiB2-containing alloys. The main toughening mechanism was analyzed and discussed in light of the microstructure characterization, size and morphology of borides and the deformed ligament bridges.

Extended isogeometric dynamic and static fracture analysis for cracks in piezoelectric materials using NURBS

Abstract Accurate numerical modeling of multifield piezoelectric materials is challenging because of the inherent electro-mechanical coupling effect and material anisotropic behaviors. The modeling becomes even more difficult especially for problems with non-smooth solutions like crack under dynamic loading. We present in this paper an extension of the extended isogeometric analysis (XIGA) for simulation of two-dimensional fracture mechanics problems in piezoelectric materials under dynamic and static coupled electromechanical loads. The discretization of problem domain is based on basis functions generated from NURBS, which are used for both geometric description and approximation of solution field variables. To capture the discontinuity across the crack-faces and the singularity at the crack-tip, the isogeometric approximation is locally enriched by discontinuous Heaviside function and asymptotic crack-tip branch functions. The sixfold enrichment functions particularly suitable for electromechanical crack-tip singularity of piezoelectric materials are used. To evaluate the generalized fracture parameters, a domain-form of electromechanical interaction integral is employed. For dynamic analysis, the implicit time integration scheme considering inertial effect is taken. Five numerical examples for single and mixed-modes of impermeable cracks are considered and the generalized fracture parameters under dynamic and static loads are analyzed. The accuracy and effectiveness of the proposed XIGA are illustrated through numerical investigations of the generalized dynamic and static fracture parameters. Numerical results are validated against the reference solutions derived from the boundary element methods. The effects of some numerical aspect ratios on generalized fracture parameters are also investigated. Additionally, we present some numerical results of quasi-static crack propagation in piezoelectric solids using the developed XIGA, taking fracture toughness anisotropy of polarized electroelastic materials into account, and employing the maximum modified hoop stress intensity factor criterion for predicting the growing direction of crack.

Toughness improvement in hot rolled HSLA steel plates through asymmetric rolling

Asymmetric hot rolling and conventional hot rolling were applied to a HSLA steel in order to improve the impact toughness of thick-gauge hot rolled plates. The asymmetric rolling condition was introduced by applying mismatched roll diameters. The diameter ratio between big and small rolls was 1.25. The rolling temperatures were between 900°C and 1150°C. The results show that asymmetric rolling produced a smaller average grain size at the centre layer of the hot rolled plate than symmetric rolling and brought a through thickness texture change. The impact toughness was increased and the toughness anisotropy was reduced in the asymmetric hot rolled plate in the ductile to brittle transition temperature range. The improved impact toughness in the asymmetric hot rolled plate is related to the fine grain structure and the lower toughness anisotropy is related to the reduced α-fibre texture.

Microstructure and microtexture assessment of delamination phenomena in charpy impact tested specimens

In this article the delamination phenomena, which occurs in the X70 steel during fracture process from the Charpy impact tests, was investigated. Microstructure, microtexture and Taylor factor map analyses were performed by scanning electron microscopy, light microscopy and electron backscatter diffraction, respectively. In a cross-section of two fractured surface regions namely, in the perpendicular and parallel regions to the propagation fracture direction where delamination occurred, it was possible to notice that delamination showed a key role in the anisotropy of impact toughness between the L-T and T-L orientations. The results also revealed that the cause of the delamination can be attributed to the presence of microstructural banding and elongated ferrite grains aligned in a rolling direction. The presence of (100)[011] and (111)[110] crystallographic orientations, with Taylor factors close to 2.7 and 4.5, respectively, were identified in the delamination region. They contribute to the occurrence of cleavage delamination during the fracture process.

Anisotropy in Mechanical Properties and Fracture Behavior of an Oxide Dispersion Fe20Cr5Al Alloy

Anisotropy of fracture toughness and fracture behavior of Fe20Cr5Al oxide dispersion-strengthened alloy has been investigated by means of compression tests, hardness tests, and wedge splitting test. The results show a small effect of the compression direction on yield strength (YS) and strain hardening. The YS is minimum for longitudinal direction and maximum for the tangential direction. The transverse plastic strain ratio is similar for tangential and longitudinal directions but very different from that in normal direction. Hardness depends on the indentation plane; it is lower for any plane parallel to the L-T plane and of similar magnitude for the other orthogonal planes, i.e., the L-S and T-S planes. Macroscopically, two failure modes have been observed after wedge-splitting tests, those of LS and TS specimens in which fracture deviates along one or two branches normal to the notch plane, and those of LT, TL, SL, and ST specimens in which fracture propagates along the notch plane. Besides LT and TL specimens present delaminations parallel to L-T plane. Both, the fracture surface of branching cracks and that of the delaminations, show an intergranular brittle fracture appearance. It is proposed that the main cause of the delamination and crack branching is the alignment in the mesoscopic scale of the ultrafine grains structure which is enhanced by the 〈110〉-texture of the material and by the presence in the grain boundaries of both yttria dispersoids and impurity contaminations. An elastoplastic finite element analysis was performed to study what stress state is the cause of the branches and delaminations. It is concluded that the normal to the crack branches and/or the shear stress components could determine the crack bifurcation mechanism, whereas the delamination it seems that it is controlled by the magnitude of the stress component normal to the delamination plane.

Influences of crystallography and delamination on anisotropy of Charpy impact toughness in API X100 pipeline steel

The anisotropic behavior of Charpy toughness has been investigated in API X100 pipeline steel, and the combination of crystallographic textures and delamination is considered to be the reason for the observed anisotropy in Charpy toughness. The crystallographic textures can contribute to anisotropy by increasing the probability of {001} cleavage planes parallel to the fracture surfaces of the Charpy impact specimens, and biasing available slip systems for ductile fracture. Thus, the Charpy impact toughness is worst at 45° to the rolling direction, because the grains with {001} cleavage planes parallel to the Charpy specimen fracture surfaces is much more at that orientation. The existence of much more {001} cleavage planes at the 45° orientation can lead to a greater probability of cleavage fracture compared with the other orientations at the same temperature. Delamination related to elongated grain structures and {001} grain planes parallel to the rolling plane also lead to anisotropy in Charpy properties. The deep groove shape cleavage crack induced by delamination in the upper shelf region is responsible for a decrease in upper shelf energy. Delamination as a part of the fracture energy absorption process in the ductile–brittle transition region also entails additional plasticity, resulting in an increase in impact energy. Therefore, delamination can also play a role in the anisotropy of Charpy toughness.

Toughness anisotropy in X70 and X80 linepipe steels

The anisotropy of mechanical properties in the context of steels used to fabricate large diameter pipes can be detrimental to the design and performance of the pipes. We examine here the anisotropy of strength and toughness, and with the help of detailed characterisation show that there is a complex interaction between the ability of the steel to delaminate on the rolling plane and the crystallographic texture, which promotes brittle fracture on the macroscopic fracture plane of Charpy specimens.

Crack Paths at Multiple-crack Systems in Anisotropic Structures: Simulation and Experiment

This paper is targeted on numerical methods for accurate crack tip loading analysis and crack path prediction. Those are based on finite element calculations of the boundary value problem. Applying path-independent integrals to curved cracks in order to accurately calculate the J-integral, energy release rate (ERR) or stress intensity factors (SIF) is still not state of the art. Contours which are not confined to the crack tip require special analytical preparation and numerical treatment to supply results which are sufficiently precise for reliable crack path prediction. Methods to improve the calculation of the J-integral and the interaction inte- gral (I-integral) are presented. In particular, the latter has never been applied to strongly curved cracks. Also, efficient methods for the loading analysis and crack growth simulation of multiple interacting cracks based on path-independent integrals are presented. The anisotropy of fracture toughness is taken into account being a crucial part of the numerical model. Experiments are carried out with specimens made of aluminum alloy Al-7075, comparing subcritically grown cracks with simulations.

異方性を有する鋼のへき開き裂伝播の結晶粒レベル3次元シミュレーション解析

The authors proposed a three dimensional model to explain microscopic behavior of cleavage crack propagation in steel, which was validated by experimental results, in the previous paper. Purpose of this paper is to explore further into microscopic cleavage crack propagation behavior in steels having anisotropy using the proposed model. Charpy impact testing and small size crack arrest testing were conducted for observing anisotropy of cleavage fracture toughness and it was confirmed that R direction has lower toughness than LT and TL directions. And simulation was also conducted using the data of crystal orientation, which was derived from Electron Back Scattering Diffraction analysis. The comparison between the experiment and the simulation showed good agreement and the lowest toughness in the R direction was explained by a concentration of {100} planes in that direction.

Effect of Boron Microadditives on the Anisotropy of Mechanical Properties of Flat Preforms from Titanium Alloys

The properties of deformed titanium pseudo-alpha-alloys of type PT3V with boron additives are investigated. It is shown that boron produces a positive action on the toughness and ductility of the alloys. The anisotropy of the impact toughness of the alloys is lowered due to a considerable decrease in the size of the colonies of α + β phases in the structure. Determination of the crystallographic orientation of the α-phase plates and of the metallographic orientation of the β-phase layers in flat preforms of titanium alloys has made it possible to index the habit planes between the α- and β-phases.

GS10 SUS304ステンレス鋼圧延材の破壊強度異方性

Stainless steel is mainly used as structural materials under severe environment. Therefor clarifying the fracture strength of stainless steel is important. The purpose of this study is to clarify the fracture toughness anisotropy of rolled stainless steel. Inclusion distribution is influenced by the rolling process. The stable crack grows by coalescence of voids which occur as nuclear inclusions. Therefore the distribution of inclusions affects the fracture strength of the material. In this study rolled SUS304 stainless steel with the thickness of 30mm was used. Specimens in the crack direction of the six were produced from this material. The fracture toughness values of these specimens were evaluated by direct current potential difference method. As a result, the fracture toughness of this material showed anisotropy. The specimen of which crack plane was parallel to the rolling plane showed lowest fracture toughness value. This tendency was the same as other the rolled metal material having ductile properties. As a result the orientation relationship between crack plane and rolled surface affects the fracture toughness. In addition, in case the large dimples are present in the fracture surface, the fracture toughness values of the specimens were low.

Delaminations by Cleavage Cracking in Duplex Stainless Steels at Sub-zero Temperatures

Impact toughness testing was conducted on 10 and 30 mm plates of 2205 together with a 30 mm plate of LDX 2101® duplex stainless steel (DSS). The testing temperatures were between 153 K (−120 °C) and room temperature. Interrupted fracture toughness tests of the 10 mm plate and a 50 mm plate of 2205 were also performed. The conclusion from the fractographic investigation was that the delaminations that occur in hot-rolled DSSs were cleavage fractures. The toughness anisotropy can be explained by the cleavage fracture and the appearance of the microstructure. The result from the interrupted fracture toughness test revealed that the delaminations initiated prior to the maximum force plateau and propagated ahead of the stable crack growth during testing. Estimated upper limit for the fracture delamination initiation toughness at sub-zero temperatures for the 2205 base metal according to the crack-tip opening displacement method was 28 to 61 μm for the 10 mm plate, 70 to 106 μm for the 30 mm plate and below 100 μm for the 50 mm plate.

Nature of anisotropy of impact toughness of structural steels with ferrite-pearlite structure

The anisotropy of the impact toughness of low-alloy steels of various compositions and purities with a ferrite-pearlite structure has been investigated using samples of type 11 according to the Russian Standard GOST 9454-78. It has been established that the anisotropy coefficient of the impact toughness depends on the anisotropy coefficient of the work of crack propagation and is independent of the degree of striation of the ferrite-pearlite structure and the work for nucleation of the ductile crack.

Micromechanical model for the rate dependence of the fracture toughness anisotropy of Barre granite

Laboratory measurements of mode-I fracture toughness of Barre granite under a wide range of loading rates were carried out with an MTS machine and a split Hopkinson pressure bar (SHPB) system using the notched semi-circular bend (NSCB) specimen. The fracture toughness anisotropy was found to decrease with the increase of the loading rate. A micromechanics model is utilized in this work to understand this experimental observation, invoking crack–microcrack interactions. Two micromechanics models are constructed based on the microstructural investigation of Barre granite samples using the thin-section method. In both models, the rock material is assumed to be homogenous and isotropic. The main crack (i.e., the pre-crack in the NSCB specimen) and the closest microcracks are included in the numerical analysis. Numerical results show that stress shielding occurs in the model where the two microcracks form an acute angel with the main crack and the nominal fracture toughness is bigger than the intrinsic one, while stress amplification occurs in the model where the microcrack is collinear to the main crack and the nominal fracture toughness is smaller than the intrinsic one. Assuming that the intrinsic fracture toughness of the rock material has the usual loading rate dependency, we are able to reproduce the decreasing trend of the fracture toughness anisotropy as observed from experiments.