Crack Divider Research Articles

Effect of microstructural inhomogeneity on mechanical anisotropy of Al–Cu–Li alloy plate

In order to reveal the relationship between microstructure inhomogeneity and mechanical anisotropy, the Al–Cu–Li alloy 100 mm thick plate for aerospace was evaluated. The grain structure, crystallographic texture, phase structure and fractography of different thickness positions and directions were observed by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The in-plane and through-thickness anisotropy were characterized by tensile properties, fracture toughness and hardness tests. The results demonstrate a greater in-plane anisotropy in yield strength at the same thickness position of the plate. The Brass texture component increases successively at T, T/4, and T/2 positions, while the Cu texture component exhibits minimal variation. The in-plane anisotropy mainly arises from the Brass texture. Through-thickness anisotropy in the same direction is primarily influenced by the diameter, number density, and volume fraction of the T1 strengthening phase. The average diameter and number density of the primary strengthening phase T1 within grains are highest at the T/2 position and lowest at the T position. The plate contains unrecrystallized, elongated pancake-like grains along the rolling direction. Fracture toughness tests conducted in the L-T and T-L directions predominantly reveal dimple-type transgranular shear fractures, while the S-L direction exhibits intergranular fracture as the primary characteristic. The crack divider delamination toughening mechanism is the main contributor to higher fracture toughness values in the L-T and T-L directions compared to those in the S-L direction. The results reveal the corresponding relationship between the microstructure inhomogeneity and the mechanical anisotropy of Al–Li alloy ultra-thick plates, and provide ideas for the adjustment of process parameters during processing deformation and the regulation of microstructure during heat treatment of large-scale Al–Li alloy thick plates, which has engineering application reference value.

Effects of thermal aging at 873 K on the impact properties of an ODS ferritic steel

This investigation presents the effects of long-term thermal aging on the impact properties of an ODS reduced activation ferritic steel. The ODS ferritic steel, with composition Fe–14Cr–2W–0.4Ti–0.3Y2O3 (wt. %), was manufactured by mechanical alloying, compacted by hot isostatic pressing and hot cross rolled. A 1273 K annealing treatment was applied, followed by thermal aging at 873 K for 2000 h in Ar, to evaluate the thermal stability of the alloy. Charpy impact testing was performed from 77 to 473 K to investigate the mechanical response of the material under dynamic loads. The ductile-to-brittle transition temperature (DBTT) showed a slight increase with values of 255 ± 12 K before aging, and 270 ± 20 K after aging. The aging treatment softened the steep character of the Charpy impact curves inducing a wider transition regime with mixed brittle and ductile fracture mechanisms. This behavior was confirmed by the analysis of Force vs. Displacement Charpy curves and fractographic observations. Hot cross rolling induced a crack divider geometry and secondary cracks formation by delamination. Ductile fracture was less evident after aging due to the redistribution and transformation of Cr-W-rich precipitates along grain boundaries. Electron Backscattered Diffraction highlighted the features of transgranular brittle cracks at temperatures below the DBTT and showed misoriented and deformed regions near the fracture surface at temperatures above the DBTT.

Failure Behaviour of Carbon-Epoxy Composite in Crack Arrester and Crack Divider Mode

Failure behaviour of a carbon-epoxy composite is studied both in crack arrester (CA) and crack divider(CD) mode by conducting 3-point bend test of un-notched (UN) and single edge pre-crack notched (N) (notch depth varied from 0.5 to 3.3 mm) specimens at various cross head speeds(CHS) such as 0.2, 50, 100, 200 and 500 mm/min. Influence of notch depth (ND) and CHS on the conditional fracture toughness(CFT) is understood using ANOVA. In both the mode, while the CFT values varies linearly with CHS, its values are observed to be lower at short ND, higher at medium ND and again lower at high ND. In both the modes at high ND (~3.3mm) opening mode/fibre breaking (mode I) failure occurs because of high crack tip stress concentration (as analysed from ANSYS 15.0) which cause fibre failure at the tip and corresponding Mode I fracture toughness was observed to be~20 MPa.m0.5. The composite also shows similar flexural strength both in CA mode (641+48) and in CD mode (634+49 MPa). In CD mode N specimen predominantly fails by mode I. While in CD mode the delamination (mode II) is only observed in UN specimens, in CA mode UN or small N specimen shows mode II failure and medium notched specimen shows mixed mode failure. In CA mode, number of delaminated layer is higher for the specimen tested at high CHS. In CA mode during charpy test (strain rate 103/s), the composite shows numerous delamination and absorbed much higher energy(~40J/cm2) than that in CD (~10J/cm2) mode where mode I failure is predominant. During impact test, lowering of test temperature to -40oC has very little effect on the impact energy since it does not significantly affect the mode of failure.

Mechanical properties of Ti-6Al-4V three-layer material

The paper presents the results of studies on the mechanical behavior under impact and tensile loading of a Ti-6Al-4V three-layer material obtained by diffusion bonding at 800°C. Diffusion bonding is the process that integrates one material with another by pressure at a temperature below the melting points of materials. Diffusion bonding of Ti-6Al-4V alloy is particularly interesting for aerospace structures, ballistic armor and sports equipment manufacturing. Predominantly single spherical pores are present in the joint zone; chains of pores are extremely rare. The static mechanical properties of the laminate are at the level of properties of monolithic material. The laminate is characterized by a different fracture resistance depending on the layer orientation relative to the direction of impact load action. The impact strength value of the laminate is minimum in the crack short-transverse orientation due to the lower energy of crack propagation. The energy of crack nucleation in the studied laminate is the same for all studied orientations of crack propagation. The results of the evaluation of the relative value of the impact strength showed that the laminate has the highest fracture resistance for the crack divider orientation. The studied layer orientations relative to the direction of impact load action are characteristic for the real diffusion-bonded structures.

Interlamellar boundaries govern cracking

Contrary to previous researches of cracking behaviors in lamellar materials, a novel phenomenon has been found in polysynthetic twinned (PST) TiAl single crystals that the specimens with crack divider (type II) own much higher toughness than those with crack arrester (type I). By combining microscopic characterization and theoretical calculation, we reveal that this contrary is caused by the coherency of interlamellar boundaries. For PST TiAl single crystal with coherent interlamellar boundaries, delamination in type I specimen is suppressed by the coherent boundaries and the crack tends to penetrate lamellae along the deformation twinning planes. While sessile dislocations produced in type II specimen hinder the further motion of other dislocations and induce the passivation at crack tip. As a result, the specimen with type II owns higher fracture toughness than that with type I. A toughness ratio, RT=Ttype I / Ttype II, is proposed to evaluate the cracking behavior with different levels of boundary coherency in lamellar materials. In previous studies, RT > 1 because their boundaries are not coherent, which promotes the delamination and crack tip passivation. The present work sheds light on the relationship of interlamellar boundaries and crack propagation, which provides a new perspective for the development of lamellar materials.

Retraction Note to: Artificial neural networks for prediction Charpy impact energy of Al6061/SiCp-laminated nanocomposites

In this work, Charpy impact energy of Al6061-SiCp nanocomposites produced by mechanical alloying has been modeled by artificial neural networks in both crack divider and crack arrester configurations. Al6061 powders were mixed by 2, 3, and 5 vol % of SiC nanoparticles and ball-milled for 45 min. Afterward, the produced powders were hot-pressed in aluminum cans and then were extruded to produce a dense bulk. Charpy impact specimens were prepared from the produced samples in layered form with different adhesives and thickness of the layers. To build the model, training, validating, and testing was performed using 171 pair input-target data. The used data in the multilayer feed-forward neural networks models were arranged in a format of six input parameters including the thickness of layers, the number of layers, the adhesive type, the crack tip configuration, the content of SiC nanoparticles, and the test trial number. The output parameter was Charpy impact energy of the nanocomposites. The training, validating, and testing results in the neural network models have shown a strong potential for predicting Charpy impact energy of Al6061/SiCp nanocomposites in the considered range of input-target values.

Retraction Note to: Application of artificial neural networks for analytical modeling of Charpy impact energy of functionally graded steels

In the present study, the Charpy impact energy of ferritic and austenitic functionally graded steel produced by electroslag remelting has been modeled in crack divider configuration. To produce functionally graded steels, two slices of plain carbon steel and austenitic stainless steels were spot welded and used as electroslag remelting electrode. Functionally graded steel containing graded layers of ferrite and austenite may be fabricated via diffusion of alloying elements during remelting stage. Vickers microhardness profile of the specimen has been obtained experimentally and modeled with artificial neural networks. To build the model for graded ferritic and austenitic steels, training, testing and validation using, respectively, 174 and 120 experimental data were conducted. According to the input parameters, in the neural networks model, the Vickers microhardness of each layer was predicted. A good fit equation that correlates the Vickers microhardness of each layer to its corresponding chemical composition was achieved by the optimized network for both ferritic and austenitic graded steels. Afterward, the Vickers microhardness of each layer in functionally graded steels was related to the yield stress of the corresponding layer and by assuming Holloman relation for stress–strain curve of each layer, the area under each stress–strain curve was acquired. Finally, by applying the rule of mixtures, Charpy impact energy of functionally graded steels in crack divider configuration was found through numerical method. The obtained results from the proposed model are in good agreement with those acquired from the experiments.

Retraction Note to: Modeling ductile to brittle transition temperature of functionally graded steels by fuzzy logic

In this article, a model based on fuzzy logic (FL) for predicting ductile to brittle transition temperature of functionally graded steels in both crack divider and crack arrester configurations has been presented. Functionally graded steels containing graded ferritic and austenitic regions together with bainite and martensite intermediate layers were produced by electroslag remelting. For purpose of building the model, training and testing using experimental results from 140 specimens produced from two basic composites were conducted. The used data as inputs in FL models are arranged in a format of six input parameters that cover the FGS type, the crack tip configuration, the thickness of graded ferritic region, the thickness of graded austenitic region, the distance of the notch from bainite or martensite intermediate layer, and temperature. According to these input parameters, in the FL, the ductile to brittle transition temperature of each FGS specimen was predicted. It has been found that FL model will be valid within the ranges of variables. The training and testing results in the FL model have shown a strong potential for predicting the ductile to brittle transition temperature of each FGS specimen.

Characteristics of the mechanical behavior of Ti–6Al–4V multilayer laminate under impact loading

Multilayer laminates comprising thirteen layers of Ti–6Al–4V alloy were produced by diffusion bonding. The influence of the processing conditions on the microstructure and mechanical properties of laminates were determined. The multilayer laminates were tested at room and liquid-nitrogen temperatures under impact bending tests with recording of dynamic loading diagrams. The influence of the interfaces pores on the total energy of fracture, the energy of crack initiation and the energy of crack propagation was analyzed. The impact strength value of the laminate decreased by 1.7 times for the crack divider orientation, and it increased by 14 times for the crack arrester orientation with the relative length of pores increasing from 1 to 30%.

Fracture toughness of a lamellar orientation-controlled TiAl-based alloy processed by either one-step or two-step compression at high temperature

In this work, the effects of lamellae orientation and the distribution of fine grains and fine lamellar colonies on the fracture toughness of orientation-controlled Ti-43 mol%Al alloy were examined. Fracture toughness was determined by three-point bending tests following the introduction of a chevron notch, and thermomechanical processing to control lamellar orientation consisted of high temperature compression in the α single-phase and (α + γ) two-phase regions. Materials processed via one-step compression in the α single-phase region exhibited tilting of the lamellae interface by approximately 20–30° relative to the compression plane. Further compression in the (α + γ) two-phase region (termed two-step compression) arranged the lamellae interface nearly parallel to the compression plane. Fine grains were also observed around the orientation-controlled lamellar colonies, and were converted into fine lamellar colonies during heat treatment following processing in the (α + γ) two-phase region. The proportion of fine grains decreased with an increase in the true strain rate during compression in the (α + γ) two-phase region. Specimens processed by one-step compression had fracture toughness values lower than those of two-step compression specimens. In addition, specimens processed using two-step compression and exhibiting a crack arrester orientation had higher fracture toughness values than those with a crack divider orientation. The fine grains and colonies formed at the lamellar colony boundaries were found not to affect the fracture toughness of specimens with either of these crack orientations. However, decreased quantities of fine grains and colonies may increase the toughness of specimens for which crack propagation occurs in conjunction with crack arrester orientation.

Short review: Potential impact of delamination cracks on fracture toughness of structural materials

The current energy policy envisages extended lifetime for the current nuclear power plants (GEN II NPP). This policy imposes a large research effort to understand the ageing of power plant components. In this goal, it is necessary to improve knowledge about safety, reliability and components’ integrity for more than forty years of operation. In Central and Eastern Europe, the majority of NPPs are VVER types, where some of the components are produced from austenitic steel 08Ch18N10T. Irradiated 08Ch18N10T may exhibit brittle behavior, namely delamination cracks are found in some cases on the fracture surface of irradiated 08Ch18N10T with elongated ?-ferrite. Delamination cracks have also been observed on the fracture surface of high-strength steels or aluminum-lithium alloys. This article presents a state-of-the art review to provide a detailed analysis of the influence of delamination cracks on the toughness of metal alloys. In general, the delamination cracks are present in metal alloys having a high texture and microstructure anisotropy. Three types of delamination cracks have been observed and are classified as crack arrester delamination, crack divider delamination and crack splitting delamination. The microscopy characterization, 3D fracture theories and computational studies explaining possible causes and effects of delamination cracks on the mechanical properties of metal alloys are presented.

Laminating; the best way to improve Charpy impact energy of nanocomposites

An Al–TiC nanocomposite powder with different percentages of TiC nanoparticles, was synthesized via high energy ball milling. With increasing the TiC nanoparticles content, the crystallite size began to decrease, which led to an increase in the hardness of the produced nanocomposite. An Al–TiC laminated nanocomposite was then fabricated and the effect of process parameters on its Charpy impact energy (CIE) in both crack divider and crack arrester configurations was investigated. Comparison between the experimental results shows that for each of the crack divider and crack arrester configurations, the CIE of the laminated nanocomposite is much higher than that of monolithic one. We also reported an accurate estimate of the process parameters in order to maximize the Charpy impact energy in Al–TiC laminated nanocomposite. Accordingly, 99 datasets were collected from the experiments and then by a modeling algorithm called gene expression programing (GEP), a mathematical relation between the CIE and process parameters was developed. Afterwards by an optimization algorithm called Biogeography Based Optimization (BBO), the process parameters were optimized in order to achieve maximum CIE. Experiments were performed at the optimized parameters to prove the validity of the analysis which shows the potential application of these calculations and analysis in materials engineering.

Toughness enhancement in architecturally modified Al6061-5 vol.% SiCp laminated composites

Damage tolerance improvement in discontinuously reinforced aluminum matrix composites has been examined through both extrinsic and intrinsic approaches by researchers. In this study, extrinsic mechanism was considered in the form of architectural modification in laminates comprising two exterior layers of Al6061-5 vol.% SiCp and an interlayer of Al1050. Hot roll bonding was utilized to fabricate laminates since interfacial adhesion of layers was controlled by means of rolling strain. The interfacial strength and fracture resistance of specimens were examined by shear and three-point bending test, respectively. Achievements demonstrated that the quasi-static toughness of laminates tested in crack divider orientation was greater than that of monolithic samples. Also, it was revealed that the initiation, propagation, and total toughness were influenced by interfacial adhesion. In the other words, interfacial bonding played a major role in energy absorption during fracture. Enhancement of the interfacial adhesion by an increment in rolling strain elevated the energy consumed for emergence and growth of debonded area.

Toughness prediction in functionally graded Al6061/SiCp composites produced by roll-bonding

Functionally graded aluminum matrix composites (FGAMC) are new advanced materials with promising applications due to their unique characteristics in which composite nature is combined with graded structure. Different architectures of Al6061/SiCp composite laminates were fabricated by successive hot roll-bonding. For FGAMCs, two composite layers as outer strips and a layer of Al1050 as interlayer were applied. To investigate laminate toughness, the quasi-static three-point bending test was conducted in the crack divider orientation. Genetic programming as a soft computing technique was implemented to find mathematical correlations between architectural parameters and experimentally obtained results. Doing this, reliable data were achieved and randomly divided into 63 training sets and 12 testing sets. Input parameters of “specimen thickness”, “SiC content in composite layers”, “rolling strain”, “maximum achieved stress”, and “strain in utmost stress”, as well as, “total strain of specimen” were utilized for modeling. Models of different types each used 8 independent parameters, wholly and/or partially, were proposed. In order to introduce the best model, four criteria of “higher regression”, “minimum errors”, “minimum subtraction of errors summation” and “minimum sum of absolute subtractions of predicted and experimented toughness” were considered. Based on different evaluations, a model which used 5 inputs, as well as, 30 chromosomes, 8 head sizes, 2 genes, addition as linking function, RRSE as fitness function while implemented 5 different mathematical operators was selected. The study showed that predicted results and experimented data had good conformity while all models had reliable applicability for predicting fracture tolerance of aluminum matrix composites with graded structures under quasi-static bending condition.

Toughness Enhancement in Roll-Bonded Al6061-15 vol.% SiC Laminates via Controlled Interfacial Delamination

Researchers have examined different approaches to improve damage tolerance of discontinuously reinforced aluminum (DRA). In this study, three-layer DRA laminates containing two exterior layers of Al6061-15 vol.% SiCp and an interlayer of Al1050 were fabricated by hot roll bonding. Interfacial adhesion between the layers was controlled by means of rolling stain. The results of shear test revealed that, the bonding strength of laminates was influenced by number of rolling passes. Considering this effect, the role of interfacial bonding on the toughness of laminates was studied under three-point bending in the crack divider orientation. The quasi-static toughness of the laminates was greater than that of the monolithic DRA. Plastic deformation of the ductile interlayer and interfacial delamination were found as the major sources of energy absorption in this fracture process. It was shown that interfacial adhesion in these laminate does not alter the initiation energy in quasi-static test. Propagation energy under same loading condition, however, illustrated significant sensitivity to the interfacial bonding. The results of the current study reveal that improving the interfacial adhesion by means of rolling strain eliminates the ease of plastic deformation of the ductile interlayer and thus reduces the contribution of this mechanism in quasi-static toughness of the laminate.

Notch Impact Behavior of Oxide-Dispersion-Strengthened (ODS) Fe20Cr5Al Alloy

In this article, tensile tests as well as LS and LT notched Charpy impact tests were performed at the temperature range between 77 K (−196 °C) and 473 K (200 °C) on an oxide-dispersion-strengthened (ODS) Fe20Cr6Al0.5Y2O3 hot-rolled tube. The absorbed energy values in the range of high temperatures of LS notched specimens are considerably higher than those of LT notched specimens; however, such values tend to converge as temperature increases. Ductile fracture on the normal planes to RD with delaminations parallel to the tube surface was observed in the temperature range between room temperature (RT) and 473 K (200 °C). Delaminations of crack divider type were observed in LT specimens, whereas delaminations of crack arrester type were observed in LS specimens. The yttria particles in the grain boundaries and the transverse plastic anisotropy are the possible reasons why the delaminations were parallel to the tube surface.

Influence of notch orientation and temperature on the impact behavior of a dual hardness steel composite

The need for materials with good characteristics for critical applications, such as in defense and aerospace, has led to the development of new metallic materials. In the present work the impact fracture behavior of a dual hardness steel composite was studied in two geometries, “crack arrester” and “crack divider”. The composite was produced by forging and rolling followed by treatments of quenching and tempering and then annealing. The composite was characterized by optical microscopy as well as hardness, microhardness, tensile and impact tests. The failure mechanisms in impact at different temperatures were analyzed by scanning electron microscopy. The results showed that the mechanical behavior was significantly affected by the heat treating processes, with higher impact resistance associated with the crack arrester orientation. These results are analyzed in terms of the state of stress at the crack tip.

Paper multilayer with a fracture toughness of steel

We demonstrate in this paper that commercially available printing paper can reach very high fracture toughness, comparable to that of steel, simply due to a special arrangement of the paper sheets with respect to the crack. Fracture mechanics experiments are conducted on single sheets of paper as well as on multilayer specimens in crack divider and crack arrester configuration. It is demonstrated that an arrangement in crack arrester configuration leads to an increase of the fracture toughness by a factor ten. An explanation of the effect is given and the transferability to other materials is discussed.

Simulation of critical J-integral in functionally graded steels components weakened by U and blunt V-shape notches under Charpy test

In the present study, the failure resistance of the dual-layer austenitic–martensitic functionally graded steel components weakened by blunt U and V-shape notches has been modeled by finite element method. The mentioned materials contain graded austenitic region were produced through electro slag remelting process by setting the plain carbon and austenitic stainless steel with appropriate arrangements as initial electrode. The failure resistance of functionally graded materials (FGMs) specimens has been obtained experimentally in the form of crack divider configuration for the following values of the notch opening angle: 0<2α<90° by analysis load–displacement curves acquired from Charpy tests instrumented with Hopkinson pressure bar. Cubic elements were joined together to build the standard Charpy specimen in finite element software ABAQUS environment. The data used for each cubic element in finite element modeling was the corresponding stress–strain curve obtained considering predicted load–displacement curve of that layer under Charpy test loading. After applying the impact loading, a sound compatibility between experimental results and those obtained from simulation was observed.

Predicting Charpy impact energy of Al6061/SiC p laminated nanocomposites in crack divider and crack arrester forms

Charpy impact energy of the produced Al6061–SiCp laminated nanocomposites by mechanical alloying was modeled by adaptive neuro-fuzzy interfacial systems (ANFIS) in both crack divider and crack arrester configurations. The model was constructed by training, validating and testing of 171 gathered input–target data. The thickness of layers, the number of layers, the adhesive type, the crack tip configuration and the content of SiC nanoparticles were five independent input parameters utilized for modeling. The output parameter was Charpy impact energy of the nanocomposites. The performance of the proposed models was evaluated by absolute fraction of variance, the absolute percentage error and the root mean square error and the best values of 0.9945, 3.521 and 8.224, respectively acquired for them. The results introduced ANFIS as an influential tool for predicting the Charpy impact energy of the considered Al6061–SiCp laminate nanocomposites. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.