Change In Fracture Mechanism Research Articles

Structure–Property–Fracture Mechanism Correlation in Heat-Affected Zone of X100 Ferrite−Bainite Pipeline Steel

Structural performance of a weld joint primarily depends on the microstructural characteristics of heat-affected zone (HAZ). In this regard, the HAZ in X100 ferrite−bainite pipeline steel was studied by separating the HAZ into intercritically reheated coarse-grained (ICCG) HAZ containing and non-containing regions. These two regions were individually evaluated for Charpy impact toughness and characterized by electron back-scattered diffraction (EBSD). Low toughness of ~50 J was obtained when the notch of impact specimen encountered ICCGHAZ and high toughness of ~180 J when the notch did not contain ICCGHAZ. Fracture surface was ~60 pct brittle in the absence of ICCGHAZ, and 95 pct brittle (excluding shear lip) in the presence of ICCGHAZ in the impact tested samples. The underlying reason is the microstructure of ICCGHAZ consisted of granular bainite and upper bainite with necklace-type martensite−austenite (M−A) constituent along grain boundaries. The presence of necklace-type M-A constituent notably increases the susceptibility of cleavage microcrack nucleation. ICCGHAZ was found to be both the initiation site of the whole fracture and cleavage facet initiation site during brittle fracture propagation stage. Furthermore, the study of secondary microcracks beneath CGHAZ and ICCGHAZ through EBSD suggested that the fracture mechanism changes from nucleation-controlled in CGHAZ to propagation-controlled in ICCGHAZ because of the presence of necklace-type M-A constituent in ICCGHAZ. Both fracture mechanisms contribute to the poor toughness of the sample contained ICCGHAZ.

Effect of the Tool Penetration Depth in Friction Stir Spot Welding (FSSW) of Dissimilar Aluminum Alloys

In the last years Friction Stir Welding has produced a big impact on several industries due to its advantages. Particularly, automotive industry has developed a variant of the original process named Friction Stir Spot Welding (FSSW), having a strong influence on welding of thin sheets of aluminum alloys and dissimilar materials. Nevertheless, the knowledge about the influence of welding procedure on joint characteristics is scarce, mainly for dissimilar materials. The aim of this work was to study the effect of the tool penetration depth during welding, as well as, the relative position of the materials used in the superimposed joints, when AA5052-AA6063 sheets samples were welded. Tool penetration depths between 0.05 and 1.25mm were analyzed, for both materials positions. Macrostructural and dimension characterization, microhardness profiles and Peel tests (PT) were done for different conditions. The fracture loads in PT increased with the tool penetration depth for both material positions, being higher when AA6063 was the upper material. However, the tool penetration depth has a limit in the increasing of fracture load, for which the fracture mechanism changes from interfacial to circumferential.

Study of the fracture of ferritic ductile cast iron under different loading conditions

AbstractThis work is a continuation of the studies presented in a recent paper by the authors, where the fracture surfaces of pearlitic ductile cast iron under different loading conditions were exhaustively analysed. In this study, fracture surfaces of ferritic ductile cast iron (or ferritic spheroidal graphite cast iron) generated under impact, bending and fatigue loading conditions were characterised and compared. The fracture surfaces were characterised qualitatively and quantitatively from the observation under a scanning electron microscope. The fracture mechanisms in each case were identified. For impact tests, as test temperature increases, the dominant fracture mechanism changes from brittle to ductile. For bending tests, a fully ductile fracture micromechanism dominates the surface. In fatigue tests, the surface shows a mix of flat facets that appear to be cleavage facets and ductile striations, but the typical fatigue striations are not easily found on the fracture surface. Methodologies for the determination of the macroscopic direction of main crack propagation in both ductile and brittle failure modes are proposed. These allow identifying main crack propagation direction with good approximation.The results are potentially useful to identify the nature of loading conditions in a fractured specimen of ferritic spheroidal graphite cast iron. The authors believe that it is necessary to extend the methodologies proposed in samples with different geometry and size, before they can be used to provide additional information to the classical fractographic analysis.

Microstructure and mechanical properties of new high strength beta-titanium alloy Ti-1300

The influence of aging treatment on microstructural characteristics and mechanical properties of Ti-1300 alloy was investigated in this work, and both the α/β and β solution treatment were introduced to study the relationship between microstructural characteristics and mechanical properties. Large quantities of strip shape primary α phases form during the α/β solution treatment, which not only increase the stability of β matrix, but also suppress the growth of secondary α phase during subsequent aging. In α/β solution plus aging condition, the alloy is composed of a mixture of primary α phase, secondary α phase and β phase, which results in a high strength above 1350 Mpa with elongation over 13.5%. With the increasing of aging temperature, the strength decreases and the plasticity increases, the primary α phase is also remarkably coarser in width than that in length, and which rejects the moving of the dislocation and restrains the size of prior β grains. When the alloy is under β solution plus aging condition, many secondary α phases with acicular shape around 80–200 nm in width precipitate in β matrix, and the alloy exhibits superior strength around 1640 Mpa, but the elongation decreases to about 4.5%. Fractography analysis shows that fracture mechanism changes from a transcrystalline fracture at the α/β solution treatment conditions to a mixture of intercrystalline and transcrystalline fracture at the β solution treatment condition.

Effects of corrosive environments on properties of pultruded GFRP plates

Glass Fiber Reinforced Polymer (GFRP) composites have low permeability and high corrosion resistance. In recent years, as the use of GFRP in applications requiring high corrosion resistance has gradually expanded, its long-term performance in highly corrosive environments has caught public attention worldwide. Nevertheless, there is only limited theoretical research and accumulated test data on the corrosion resistance of GFRP. This paper analyses the long-term performance of GFRP in highly corrosive environments. Samples cut from pultruded GFRP profiles were exposed to alkaline, saline, and acidic solutions of different concentrations at 60°C. Additionally, the effects of acidic solutions at high temperature (90°C) were investigated. Flexural properties, changes in the flexural fracture mechanism, hardness, the rate of weight gain, changes in appearance, and scanning electron microscopy (SEM) images of fracture sections were analyzed over periods of 7, 15, 30 and 90days. Results indicate that flexural strength decreases with increased exposure time to both acidic and alkaline solutions. Under conditions of high concentration acidity and high temperature acidity, respectively, flexural strength decreases even more drastically. Additionally, it was found that saturated NaCl solution has little effect on GFRP.

Effects of hydrogen on fracture toughness and fracture behaviour of SA508-III steel

Different quantities of H are transported into SA508-III steel by a high pressure thermal charging method. The fracture toughness and fracture behaviour of SA508-III steel with different H contents are investigated by means of three-point bending tests and the J integral method. The results show that the fracture toughness decreases with increasing H content. Based on the analysis of fracture surface, it is clear that with increasing H content, the fracture mechanism changes from normal microvoid coalescence fracture to a mixture of tearing and the dimple model. Because the carbides are strong traps for H, the H is easily transported in the form of Cottrell atmosphere to the carbides following the moving dislocations. Then, H will be enriched at the carbides, and the bonding force between carbides and matrix is decreased in the deforming process. When local H concentration reaches a critical value, microcracks will appear at the interface between the carbides and the matrix, leading to fracture. Moreover, the concentration of H increases continuously at the three-dimensional stress area of the crack tip. The movement and propagation of dislocation will be increased, and the initiation and growth of the crack will be accelerated, which reduces the resistance of crack propagation and decreases the fracture toughness.

Finite Element Modelling of the Temperature Dependent Low Cycle Fatigue Damage Mechanism of an AlSi12 Type Alloy

Aluminium cast alloys are used for engine components, such as pistons and cylinder heads. The micromechanical properties of an AlSi12 cast alloy under monotonic and cyclic loadings are investigated. Therefore a microstructure-based two dimensional finite element model is generated. The characteristic shape of primary precipitates is analyzed and translated into an artificial microstructure. The quality of the generated microstructure is evaluated based on the stress distribution along the primary particle boundaries. The effect of the temperature dependent material behavior of the aluminium matrix is studied with respect to the resulting stress distribution along the particle boundaries. The results are discussed in terms of a possible change of fracture mechanisms from a brittle type fracture at low temperatures to an increasingly ductile fracture at high temperatures.

Fracture Toughness and Fatigue Behavior of T7451 Al-Zn-Mg-Cu Alloy Thick Plate

The mechanical property, fracture toughness and fatigue behavior of T7451 Al-Zn-Mg-Cu alloy thick plates in different orientations and with various thicknesses were investigated by means of tensile, fatigue and plain strain fracture toughness testing. And the microstructures and fracture morphologies were analyzed with optical microscopy and scanning electron microscopy. The results showed that the samples in longitudinal (L)-transversal (T) orientation possessed better mechanical property, fracture toughness and fatigue resistance than that in T-L orientation. Fractography and optical microanalysis clearly demonstrated that the feature of recrystallized grains is the decisive factor for this anisotropy. On the other hand, values of strength and fracture toughness decreased with the increase of plate thickness, but their fatigue crack growth rate became slower. Combined with the fractography analysis, the increase of recrystallization degree and the coarser grains in the thicker plate should be the main reason for the detrimental to the strength and toughness properties since the main fracture mechanism changes from ductile transgranular fracture to intergranular failure. However, these coarse recrystallized grains play an advantageous role for fatigue resistance from crack deflection and closure perspectives.

Experimental study on impact resistance and mechanical characteristics of low-temperature sintered Al2O3–ZrO2 composites with anti-sandwich structure

Al2O3–ZrO2 composites with anti-sandwich structure prepared by two-step pressing method were sintered at a low temperature of 1540º°C in air. As a reference, Al2O3–ZrO2 monolithic composites without this structure were also fabricated using the same process. In the study, relative density, fracture strength, fracture toughness, residual stress and especially impact energy of different composites were tested. Furthermore, interface and fracture morphologies were observed. The results indicated that current process was available in preparing Al2O3–ZrO2 composites based on the requirement of low-temperature sintering. Al2O3–ZrO2 composites with the anti-sandwich structure exhibited both better impact resistance and other mechanical properties than those without the special structure. The better properties resulted mainly from the anti-sandwich structure, which led to a change of fracture mechanism: from priority of intergranular fracture to that of transgranular fracture. S1 sample with the compositions of 95wt.% Al2O3+ZrO2 in the surface layer and 85wt.% Al2O3+ZrO2 in the center layer exhibited the best properties in the whole series, where its fracture strength, fracture toughness and impact energy were 577MPa, 10.56MPa·m1/2 and over1.4J, respectively.

Microstructure and properties of composite materials based on UHMWPE after mechanical activation

In this work microstructure and mechanical properties of UHMWPE composites filled with Al2O3 nanoparticles and microspheres after mechanical activation were studied. Ultra-fine particles of filler are distributed along the boundaries of UHMWPE “grains” and the filling level above 3wt.% leads to forming of a solid layer on the surface of polymer particles, which leads to difficulty in self-diffusion of UHMWPE macromolecules during compaction and, consequently, the change of fracture mechanism. Shock-shear forces during mechanical activation lead to the formation of a metastable monoclinic phase from orthorhombic, breaking chemical bonds. According to proposed model mechanical activation leads to the formation of low molecular weight polymer chains which help to increase the rate of interdiffusion between the borders of the polymer particles and lead to better sintering of UHMWPE particles and higher mechanical properties.

Material Damages and Life Time of Solids under a Cyclic Contact

A two-dimensional theoretical model is proposed for investigation of the fracture processes and assessing contact durability of solids subjected to cyclic contact. The model is based on the step-by-step calculation of fatigue crack propagation paths in the contact region which includes the criteria of local fracture of materials under complex stress-strain state, characteristics of fatigue crack growth resistance of materials and also presupposes the possible change of fracture mechanisms (mode II – mode I fracture mechanisms). Within the frames of the model the peculiarities of formation of such typical contact fatigue damages like pits and spalls in rolling bodies and edge cracks growth in the elements of fretting couples under conditions of sliding/sticking between them are investigated. Examples of assessing the life time by damages formation in the contact region are presented.

Influence of Features of Interphase Boundaries on Mechanical Properties and Fracture Pattern in Metal–Ceramic Composites

The results of a theoretical study on the influence of strength of interphase boundaries in metal–ceramic composite on macroscopical characteristics of composite response such as strength, deformation capacity, fracture energy and fracture pattern are presented. The study was conducted by means of computer-aided simulation by means of movable cellular automaton method taking account of a developed “mesoscopical” structural model of particle-reinforced composite. The strength of interphase boundaries is found to be a key structural factor determining not only the strength properties of metal–ceramic composite, but also the pattern and rate of fracture. The principles for achievement of the high-strength values of particle/binder interfaces in the metal–ceramic composition due to the formation of the wide transition zones (areas of variable chemical composition) at the interphase boundaries are discussed. Simulation results confirm that such transition zones provide a change in fracture mechanism and make the achievement of a high-strength and a high deformation capacity of metal–ceramic composite possible.

Effects of Tempering Temperature and Mo/Ni on Microstructures and Properties of Lath Martensitic Wear-Resistant Steels

The tempering behavior was experimentally studied in lath martensitic wear-resistant steels with various Mo/Ni contents after tempering at different temperatures from 200 to 600 °C. It is shown that a good combination of hardness (HV) (420 – 450) and – 20 °C impact toughness (38 – 70 J) can be obtained after quenching and tempering at 200 – 250 °C. The microstructure at this temperature is lath structure with rod-like and/or flake-like e-carbide with about 10 nm in width and 100 nm in length in the matrix, and the fracture mechanism is quasi-cleavage fracture combining with ductile fracture. Tempering at temperature from 300 to 400 °C results in the primary quasi-cleavage fracture due to the carbide transformation from resolved retained austenite and impurity segregation between laths or blocks. However, when the tempering temperature is higher than 500 °C, the hardness (HV) is lower than 330 and the fracture mechanism changes to ductile fracture due to the spheroidization and coarsening of cementite. Additions of Mo and Ni have no significant effects on the carbides morphologies at low tempering temperatures, but improve the resistance to softening and embrittling for steels when tempered at above 350 °C.

EFFECT OF SINTERING ATMOSPHERE ON MICROSTRUCTURE, PROPERTIES AND FRACTURE OF Cr-Mn SINTERED STEELS

The effect of sintering at 1120°C and 1250°C in 5%H 2 +95%N 2 and pure N 2 atmosphere on microstructure, mechanical properties and fracture mechanisms of the Fe-(Cr)-(Mn)-(Mo)-C sintered steels was studied. Also sintering in air, as well using an idea of a „self-cleaning” effect of manganese was investigated. The results of investigations of mechanical properties show that the investigated alloys belong to medium strength steels with tensile and yield strengths of up to 957 and 562 MPa, respectively. The advantage of investigated steels is they have a higher hardness, up to 473 HV30. Following microstructural research, the change of fracture mechanisms from predominantly interparticle ductile mode with shallow dimples to mixture of cleavage/intergranular modes was identified for low carbon (~0.15%C) and high carbon (~0.7%C) for both levels of Cr content. The effect of the “self-cleaning” of air atmosphere with optimal addition of ferromanganese lumps reflected in strength properties comparable with N 2 and H 2 +N 2 atmosphere has been identified as a relatively positive.

Solidification behaviour and mechanical properties of ductile iron castings with varying thickness

Achieving the desired mechanical properties in thin wall ductile iron castings poured in industrial production foundries is a challenge. In this work, the effect of copper addition (up to 0·74%) and melt processing (Ba based stream inoculation) on the matrix structure, mechanical properties and fracture behaviour of ductile iron castings with varying section thickness (4–16 mm) were investigated in a regular jobbing foundry. It was possible to obtain 80% pearlitic structure without carbides in 4 mm sections, giving a tensile strength of 658 MPa with 2·5% ductility and 264 Brinell hardness. The solidification behaviour represented by the cooling curves helped in checking the effectiveness of the melt treatment by observing the amount of undercooling. Fractography studies of tensile specimens showed the change in fracture mechanisms due to increase in copper content on fracture paths. The increased amount of pearlite in the matrix exhibited brittle fracture with river pattern.

Microstructure and Mechanical Properties of Stainless Maraging Steel Laser Weldments after Aging at Different Temperatures

The Fe-Cr-Ni-Mo-Ti(0Cr13Ni7MoTi)stainless maraging steel laser weldments have been investigated after post-weld ageing at different temperatures.The post-weld ageing treatments are performed in a temperature range of 420 ℃ to 580 ℃.Metallographic characterization of laser weldment reveals three zones,i.e.fusion zone,heat affected zone(HAZ) and unaffected parent material zone.Martensite is the predominant phase in the three zones when the ageing temperature is lower than 500 ℃,and large amount of reverted austenite occur in the three zones on ageing at above 540 ℃,this lead to hardness decrease in fusion zone and HAZ.After aging treatments,the hardness in three regions varies in different ways,low hardness appears in the HAZ or unaffected parent material zone.The tensile tests reveal that fracture location occurred in low hardness region and tensile properties are optimum after ageing at 460 ℃.Tensile fracture mechanism changes from brittle fracture to the ductile fracture under given aging treatments conditions.

Development of Fatigue Crack Propagation Technique for Freestanding Nano-Films

We developed an experimental technique for evaluating fatigue crack propagation properties of freestanding nano-films and conducted tensile and fatigue crack propagation experiments for about 500-nm-thick freestanding copper (Cu) films. We employed a sacrificial etching method for fabricating freestanding metallic nano-film specimens. A piezoelectric-actuator with long stroke and a load cell for small load were used for applying cyclic loading to nano-films. We developed original jigs for handling and aligning the nano-film specimens. Focused ion beam (FIB) was employed to introduce a single side-edge-notch in the nano-film specimens for fatigue crack propagation experiments. The results of tensile experiments revealed that the nano-films have resistance to plastic deformation comparable to cold-rolled Cu bulk, but have lower ductility. The results of fatigue crack propagation experiments revealed that a fatigue crack stably propagates in the nano-film by a uniaxial cyclic loading with constant maximum stress before unstable fracture. The fatigue crack propagation rate da/dN of the Cu nano-films is higher than that of Cu bulk counterpart in higher stress intensity factor range, ΔK. Fatigue crack started to propagate from the notch tip below the threshold value of Cu bulk with coarse grains, and the Cu nano-films show similar fatigue crack propagation properties to Cu bulk with ultrafine grains in lower ΔK. Morphology of the fracture surface transited from microstructure-sensitive rough surface to microstructure-insensitive ductile surface or chisel point fracture as fatigue crack propagated. This suggests that the dominant fracture mechanism changes from the accumulation of cyclic deformation to the monotonic tensile-dominant fracture.

Creep crack growth in a Cr-Mo-V type steel: experimental observation and prediction

In this study, the creep crack growth (CCG) properties and fracture mechanism of a Cr-Mo-V steel at 566 C in compact tension (CT) specimens were investigated, and the CCG rate was predicted by using the NSW model. The results show that the CCG rate measured by CT specimens is much lower than that predicted by the NSW model under plane-strain state. This means that the NSW model prediction for the CCG rate of the steel is over-conservative. In addition, the CCG rate da/dt versus C measured by the experiments shows the piecewise linear relation on log-log scale instead of a single linear relation predicted by the NSW model. The main reasons for these results are that the actual creep fracture mechanism of the steel and the actual creep crack tip stress field in the CT specimens have not been fully captured in the NSW model. The experimental observation shows that the creep crack propagates in a discontinuous way (step by step) at meso-scale, and the cracks at micro-scale are usually formed by the growth and coalescence of voids on grain boundaries. The NSW model based on the creep ductility exhaustion approach may not correctly describe this creep fracture process. In addition, the opening stress and triaxial stress ahead of crack tips calculated by three-dimensional finite element method is lower than those predicted by the HRR stress field which is used in the NSW model under plane-strain state. The use of the high HRR stress field will cause high CCG rates. The change in the creep fracture mechanism at micro-scale in different ranges of C may cause the piecewise linear relation between the da/dt and C . Therefore, it is necessary to study the actual CCG mechanism in a wide range of C and the actual creep crack tip stress field to establish accurate CCG prediction models.

High temperature low cycle fatigue behaviour of hot isostatically pressed superalloy Udimet 720 LI

Nickel base superalloy Udimet 720 LI was processed through a powder metallurgy (P/M) hot isostatic pressing (HIP) route. The effects of this consolidation technique (HIPing) on low cycle fatigue (LCF) properties were evaluated and are reported in this paper. Total strain controlled LCF tests have been conducted in air at 550°C at a nominal strain rate of 6.67 × 10−3 s−1. The as-HIPed Udimet 720 Li exhibits stable behaviour followed by very mild cyclic hardening at total strain amplitudes ± 0.4–0.5%. However, cyclic hardening has been observed at total strain amplitudes ± 0.6–1.2%. The cyclic hardening is attributed to dynamic strain aging (DSA). The dual slope behaviour in a Coffin-Manson plot is attributed to a change in fracture mechanism.

Modeling of fatigue contact damages formation in rolling bodies and assessment of their lifetime

Abstract Within the framework of materials fracture mechanics the computational model for investigation of fracture processes and prediction of rolling bodies residual lifetime under their cyclic contact has been formulated. The step-by-step calculation of crack propagation paths using solutions of the singular integral equations of two-dimensional contact problems for solids with curvilinear cracks and also local fracture criteria under complex stress–strain state are the basis of the model; calculation takes into account the fatigue crack growth resistance characteristics of materials and tribojoints performance parameters. The algorithms of cracks propagation paths construction in the contact zone take into account a change of the stress–strain state, that is caused both by the cracks extension and a counterbody movement (load change) in a contact cycle. They also take into consideration the possible change of fracture mechanism in the cracks propagation process and friction between cracks faces. Taking into account the published experimental data on crack initiation and propagation under cyclic contact of solids, two criteria of material local fracture have been included in the calculation model: the criterion of generalized normal opening ( σ θ -criterion) and the criterion of generalized transverse shear. Conditions for evaluation of pitting particles sizes in dependence on the service parameters and characteristics of fatigue crack growth resistance have been formulated. Realization of the model is given for a wheel–rail pair under boundary lubrication in contact. Using the fatigue crack growth resistance characteristics of RSB 12 and 75KhGST (75ХГСТ) rail steels, their residual lifetime has been evaluated by pitting development. The corresponding curves of contact fatigue have been constructed.