Crack Propagation Energy Research Articles

Laser shock peening induced fatigue crack retardation in Ti-17 titanium alloy

Laser shock peening is an advanced surface treatment technique of great interest introducing beneficial compressive residual stress and further enhancing fatigue crack propagation resistance of metallic components. In this study, fatigue crack propagation and subsequent retardation of Ti-17 titanium alloy under laser shock peening are presented. Varying degrees of fatigue crack retardation were observed after peening with pulse energy of 20 J and 30 J. The fatigue life was increased up to 2.4 times that of the unpeened counterpart. The fatigue arrests were observed in the deceleration zone after peening, showing different angles with the fatigue crack path as the peening energy varied. The fatigue crack retardation mechanism based on the plastic zone size and crack propagation energy density drop at the crack tip was further discussed, and a crack tip energy density criterion was proposed to quantitatively understand the fatigue crack retardation.

Microstructure and impact properties of Ta-TaC core–shell rod-reinforced iron-based composite fabricated by in situ solid-phase diffusion

Tantalum-tantalum carbide (Ta-TaC) core–shell rod-reinforced iron-based composites were designed and fabricated by in situ solid-phase diffusion reaction at 1150 °C for 5, 10, 15, 20, and 80 min, respectively. Phase composition, microstructure, impact toughness, micro-hardness, and elastic modulus of the composite, morphologies and grain sizes of TaCs were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, impact test, and nano-indentation test. Furthermore, formation kinetics of TaC-Fe shell layer was analyzed by measuring the thickness of pure TaC layer as a function of heat treatment time and temperature. Results indicated the generation of dense and compact TaC layer around the Ta rod by in situ solid-phase diffusion reaction, and together they were designated as the Ta-TaC core–shell rod. Predominant features of the Ta-TaC core–shell rod included a gradient nanostructured TaC-Fe shell layer and a Ta core. Predominant phases of the Ta-TaC core–shell rod-reinforced iron-based composite were Ta, TaC, and α-Fe phases. Kinetic results of TaC-Fe shell layer exhibited parabolic relationship between the thickness of pure TaC layer and annealing time, and growth rate constant for pure TaC layer was estimated to be 6.25 × 10−8 cm2 s−1. TaC particles in TaC-Fe shell layer preferred a perfect cube morphology enclosed by {100} facets with minimized total surface free energy. Grain size of TaC increased gradually from about 150 to 500 nm with the increasing distance from the interface of Ta core/TaC-Fe shell layer to the substrate. The Ta-TaC core–shell rods play an important role in improving the mechanical properties of the composite. Micro-hardness of the TaC-Fe shell layer was in the range of 10.4–24.8 GPa, which is 3–8 times than that of the substrate due to the dense and compact TaC. Impact toughness of the composite was 100.6 J cm−2, which is markedly higher than that of the gray cast iron (7.8 J cm−2). Excellent impact toughness of the composite is mainly attributed to Ta core of Ta-TaC core–shell rod, which undergoes a large plastic deformation under external forces. Moreover, the compact TaC distributed around Ta rod reduced split action to the matrix, and the matrix absorbed large amount of crack propagation energy.

General mixed mode I/II failure criterion for composite materials based on matrix fracture properties

The available mixed mode I/II fracture failure criteria for orthotropic materials have been proposed mostly for cracks along fibers. The important fracture parameter in these criteria is the mode I fracture toughness that can be derived by utilizing standard tests. Whenever the crack is not along the fibers, the prediction of mode I fracture toughness is impossible due to mode mixity. Therefore, a comprehensive criterion that can predict the crack initiation instance has not been proposed so far. In this study, in line with the definition of equivalent fracture toughness for off-axis orthotropic materials, a general mixed mode I/II fracture failure criterion is proposed, which is independent of the orthotropic fracture properties. As per this criterion, it is assumed that the fracture occurs in the isotropic matrix and it is the effect of the fibers modeled as tensile or shear reinforcements. On the other hand, only elastic and fracture properties of the isotropic matrix are included in the proposed criterion, which can be derived easily by simple standard experimental tests. In providing this general criterion, the strain energy release rate approach was considered. The concept of crack initiation and propagation energy is employed for investigating the wasted energy in the fracture process zone due to toughening micro-mechanisms. Moreover, comprehensive experimental tests and the required finite element simulations are performed for validation of the proposed criterion.

Impact toughness of a 10% Cr steel with high boron and low nitrogen contents

The effect of temperature on the impact toughness and fracture behavior of a 10% Cr-2% W-0.7% Mo-3% Co-0.05% Nb-0.2% V-0.008% B-0.003% N (all in wt%) steel was studied. The ductile-brittle transition (DBT) occurs at 10 °C. At the DBT temperature (DBTT), the onset of unstable crack propagation occurs at the maximum load. The embrittlement is attributed to the onset of unstable crack propagation at stresses below the general yielding. A decrease in nitrogen content and an increase in boron content lead to the formation of chains of closely spaced M23C6 carbides at lath boundaries. The formation of numerous voids at carbide-matrix boundaries results in the appearance of crack with critical dimension after stable crack propagation over a very short distance. As a result, the energy of the stable crack propagation is low and the embrittlement takes place at relatively high temperature.

Fracture toughness and damage development in limpet shells

Fracture toughness is an important property for many biological materials, but it can be difficult to obtain accurate and relevant values of toughness in such materials owing to complexities of geometry, material anisotropy, etc. Here we present the results of the first ever attempt to describe and measure cracking and fracture toughness in the shells of limpets. Three different experiments were devised. Firstly, small single-edge-notched bend specimens were machined, enabling us to measure KIC for through-thickness cracks growing in the circumferential direction in the shell walls, giving a value of 0.98 MPa√m. Secondly, radial notches were cut into intact shells which were loaded in compression through the apex. Failure occurred by crack propagation from the notch roots, and finite element analysis (FEA) was used to obtain critical K values. However the analysis gave a surprisingly high toughness value and the results were very sensitive to test variables, especially friction. The experiment demonstrated the remarkable resistance of shells to this kind of damage, but could not be used to measure KIC. Thirdly, impact tests were carried out to create internal damage in the form of delamination cracks. This allowed us to estimate toughness in terms of a crack propagation energy GIC for these cracks of 146 J/m2, equivalent to a KIC of 2.59 MPa√m. Scanning electron microscopy showed that the delamination cracks had much smoother fracture surfaces than those from the through-thickness cracks, however they displayed a regular structure of folds or pleats at the 100 nm scale which may act to hinder crack face movements during shear/compression loading as occurs under impact, which is a common cause of damage for these shells in their natural surroundings.

The dynamic cleavage energy of brittle single crystals

Crack initiation in brittle materials is usually dictated by the energy required to create two new surfaces. This energy is known as Griffith barrier and equals twice the free and relaxed surface energy, 2γs. This value is usually taken as the cleavage energy for crack propagation as well. We investigated, experimentally, the fundamentals of cracks dynamics in brittle single crystals, with emphasis on the cleavage energy at initiation and during propagation. Silicon single crystal, which has a diamond cubic symmetry, served as a model material. The experiments were performed using the Coefficient of Thermal Expansion Mismatch method (CTEM), developed in our laboratory in recent years. The CTEM method possesses the advantage of controlling the quasi-static energy release rate (ERR) gradient to the crack front, dG0/da, denoted here Θ.The cleavage energies at initiation and propagation of dynamic cracks on two Low-Energy Cleavage Systems (LECSs) of brittle silicon crystal, (110)[11¯0] and (111)[112¯], were evaluated by comparing the experimental energy–speed relationship with the theoretical one, manifested by Freund theory for cracks dynamics, known as Freund equation of motion. The ERR, G0, was evaluated using quasi-static, anisotropic Finite Element Analysis (FEA), and the crack speed was measured using Potential Drop Technique (PDT).The experiments revealed that the energy required to initiate and propagate a crack is governed by Θ. When Θ > 0.7 J/m2mm, the cleavage energy at initiation is higher than 2γs and linearly increase with increasing Θ for both LECSs of silicon. Surprisingly, the cleavage energy for initiation and propagation remain constant during the event of fracture for a prescribed Θ, meaning, it is not crack speed dependent. Moreover, we show that all the variables participating dynamic cleavage are linearly dependent on Θ, which provides the evidences that the ERR gradient is the controlling variable governs cracks dynamics.Finally, we suggest that the physics of the above fundamental behavior is laying on the way crack initiate and propagate at the atomistic scale, i.e., the bond breaking mechanisms in form of kinks.

The influence of heat input on the toughness and fracture mechanism of surface weld metal

Surface welding is a way to extend the exploitation life of damaged parts and constructions and the heat input has a major influence on the weldment properties. In this paper is shown the influence of the heat input on the toughness and the fracture mechanism of the surface welded joint. Surface welding of high carbon steel with self shielded wire was conducted with three different heat inputs (6kJ/cm, 10 kJ/cm and 16 kJ/cm). Total impact energy, crack initiation and crack propagation energy were estimated at room temperature, -20oC and -40oC. Fracture analysis of fractured surfaces was also conducted and it has been found that increasing of heat input leads to an increase of share of transgranular brittle fracture, what is in complete accordance with the obtained energy values. Based on all obtained results, the optimum value of heat input for welding procedure applied was defined.

Microstructural and fracture analysis of microalloyed steel weld metal

In this paper is presented behavior of microalloyed steel weld metal with niobium during impact testing, as well as microstructural characterization. Impact tests were done on instrumented Charpy pendulum at room temperature, at -400C and -550C. At room temperature, crack propagation energy is much higher than crack initiation energy, while at -400C and -550C crack growth energy is lower than crack initiation energy. Fractographic investigation of the fracture surfaces has shown that at room temperature ductile trans-granular fracture is dominant, with a small amount of brittle trans-granular fracture. At lower temperature, the share of brittle fracture is increased, and inter-granular brittle fracture becomes dominant.

Investigation of the transverse compressive and buckling strength of aluminium grid reinforced hybrid GFRP composite

This work is concerned with the effect of addition of aluminium grid in Glass Fibre Reinforced Polymer Matrix Composite (GFRP) on the transverse compressive and buckling (axial) compressive strength of the composite. The 5-layered cylindrical composite consisting of alternate layers of bidirectional E-glass fibres and aluminium grid in an epoxy matrix, were fabricated using hand lay-up technique and subjected to transverse compressive and axial compressive (buckling) loads. Non-linear stress – strain response was achieved in transverse and axial loading conditions due to the yielding of the aluminium fibres. Fractography studies revealed that the brittle failure of the glass fibres and the yielding of aluminium fibres resulted in fibre rotation and shear distortion that helped to dissipate the crack propagation energy thereby increasing the strain to failure. This method may be effectively used to arrest the crack propagation that might develop as leakage cracks in composite pipes, enabling the hybrid composite to be used for underground pipelines in chemical and offshore platform applications

Ensuring combination of strength, ductility and toughness in medium-manganese steel through optimization of nano-scale metastable austenite

Medium-Mn steel was innovatively designed in terms of alloying elements for medium to heavy steel plates to obtain combination of high strength, good ductility, and excellent low temperature toughness. Martensite was the dominant microstructure in the directly quenched steel plate because of high hardenability obtained by the addition of 5.6wt% Mn. Three intercritical annealing processes were adopted to ensure transformation-induced-plasticity (TRIP) effect through optimization of the volume fraction, morphology, and C and Mn-enriched reversed austenite. On annealing, the dislocation density of martensite plate was decreased because of recovery and recrystallization, and metastable austenite film nucleated at the interface. Both thermal and mechanical stability of austenite decreased with increase in annealing temperature. The high yield strength of 840MPa, good elongation after fracture of 24.3%, and excellent toughness at −60°C of 130.3J was obtained at intermediate annealing temperature of 650°C, and the volume fraction of reversed austenite at room temperature and −80°C was 22% and 17%, respectively. The reversed austenite exhibited different stability with increased tensile strain, leading to improved ductility and delayed necking. Moreover, the crack initiation energy and crack propagation energy were increased via dynamic stress partitioning and relaxation together with the transformation of metastable austenite. Thus, significant degree of TRIP effect induced by moderate stability of austenite played an important role in governing mechanical properties.

Influence of intercritical tempering temperature on impact toughness of a quenched and tempered medium-Mn steel: Intercritical tempering versus traditional tempering

The influence of intercritical tempering temperature on impact toughness of quenched and tempered 0.05C-5.42Mn medium-Mn steel was studied and compared with traditional tempering. The experimental steel had high hardenability because of high Mn-content. Lath-like α'-martensite without retained austenite was obtained over a wide range of quenching rate of 0.5–30°C/s, and the quenched steel showed high strength but low impact toughness. On intercritical tempering at 625°C and 665°C, the impact toughness was enhanced, as compared to traditional tempering at 570°C. The reversed austenite enriched with Mn and C formed between the martensite laths was the underlying reason for the increased absorbed crack propagation energy, and the ductile-brittle transition temperature (DBTT) was reduced because of increased stability of reversed austenite. Compared to the steel tempered at 625°C, the steel tempered at 665°C contained more reversed austenite, but the reversed austenite was less stable because of reduced enrichment of Mn and C. The enrichment or depletion of Mn and C in austenite and martensite was thermodynamically studied by DICTRA. In striking contrast to the steels tempered between 625 and 665°C, twinned martensite was formed in the steel tempered at high temperature of 700°C, and the steel exhibited impact toughness lower than the quenched steel.

The Role of Retained Austenite on the Mechanical Properties of a Low Carbon 3Mn-1.5Ni Steel

The present studies focus on the correlation between retained austenite characteristics and the cryogenic temperature Charpy impact toughness, strength, and plasticity. The steels with different volume fractions and stabilities of retained austenite were prepared by quenching followed by intercritical heat treatment, and the microstructure was characterized using scanning electron microscope, electron back-scattered diffraction, and X-ray diffraction. The grain size, dislocation density, crack initiation energy, and crack propagation energy were quantified. It has been demonstrated that the volume fraction of retained austenite plays a significant role in the reduction of the measured yield strength and the effect of tempered martensite/ferrite matrix on cryogenic temperature impact toughness can be assumed to be similar due to the similar grain size, dislocation density and element content in solution for different heat treatments. It was found that the stability of retained austenite plays a determining role in the increase of cryogenic temperature impact toughness. Furthermore, the dependence of the crack propagation energy on retained austenite is much greater than that of the crack initiation energy. Generally, an excellent UTS × TEL does not produce good cryogenic temperature impact toughness.

Nanostructured Steel Susceptibility to Sulfide Stress Cracking

The susceptibility of a high strength low alloy nanostructured steel to sulfide cracking resistance (SSCC) is assessed. Notch Tensile Slow Strain Rate (NTSSRT) method is used to evaluate the threshold stress intensity values (KISSCC)). The objective is to understand the effect of the microstructure on the susceptibility to SSCC, the control condition is a conventional low alloy carbon steel with a tempered martensite micro structure. The tests were performed in a brine with 1% NaCl buffered at pH 4.5 with H2S gas with 1% to 100% (mol %) balance N2 at room conditions. The nanostructured steel results of the KISSCC parameter are: 107 Ksi in0.5 at 1% H2S to 42 Ksi in0.5 at 100% H2S. The control condition exhibited 22 Ksi in0.5 at 100% H2S. The evaluation of the relative crack propagation energy suggest that the nanostructured ferrite grains exhibit a much lower susceptibility to SSCC than the tempered martensitic microstructure

Determination of the fracture behaviour of MgO-refractories using multi-cycle wedge splitting test and digital image correlation

Refractories with reduced brittleness show a pronounced deviation from linear elastic behaviour and an enhanced thermal shock resistance. This paper aims to study the influence of microstructure on the fracture behaviour of magnesia refractories. The wedge splitting test(WST), which enables stable crack propagation for quasi-brittle materials, was used to identify the fracture behaviour and evaluate the energy dissipation. The evaluation of the crack lengths of the magnesia and magnesia spinel materials during the entire cyclic WST is based on the localized strain evaluated using the digital image correlation (DIC). A significant fracture process zone develops in the magnesia spinel material. The relationship between the dissipated energy and the actual crack length, which was used to characterize the crack growth resistance, was determined. The refractory materials that showed reduced brittleness consume a small amount of energy for fracture initiation but a large amount of energy for further crack propagation.

Fracture and Crack Propagation Behavior of 560 MPa Microalloyed Pipeline Steel under Different Cooling Schedules

Three kinds of pipeline steel with different microstructures were fabricated by varying cooling schedules during thermo-mechanical controlled processing (TMCP). Charpy impact property of the pipeline steels were obtained, and the fracture and crack-arrest mechanisms were further studied. The results indicated that the steels were classified into two kinds according to their microstructures, the mixture of acicular ferrite (AF), quasi-polygonal ferrite (QF), granular bainite (GB) and small fraction of degenerate pearlite (DP), and the mixed microstructure of AF and GB, respectively. The processed steel with microstructure of AF and GB exhibited more excellent low-temperature toughness and crack-arrest properties with upper shelf energy of ~281 J and energy transition temperature of ~-76°C. The mixed microstructure (AF + GB) possessing smaller effective grain size hindered the propagating of crack and consumed large amount of energy during fracture. The effective grain size of microstructure was the dominant factor controlling low-temperature toughness and crack-arrest properties of pipeline steel, which increased the high-angle boundary length per unit area and further increased the crack propagation energy during fracture.

Cohesive strength of zirconia/molybdenum interfaces and grain boundaries in molybdenum: A comparative study

We present calculations within density functional theory on the thermodynamic stability and mechanical properties of t-ZrO2(001)/Mo(001) interfaces. The interfacial strength is evaluated by applying energy-based (work of separation) and stress-based (theoretical strength) criteria for different cleavage planes. The lowest energy for crack propagation is obtained for a cut creating a stoichiometric ZrO2(001) surface. Our results reveal that molybdenum grain boundaries contaminated with oxygen are less stable against brittle fracture than pure Mo GBs. Addition of Zr, however, can strengthen Mo grain boundaries that contain oxygen by forming an ultrathin zirconia film between Mo grains. The stress required to cleave an ultrathin zirconia film is equal to that required for a pure Mo GB.

Mathematical Models for Calculation of Crack Resistance of Composite Materials

Crack resistance is an important parameter determining the carrying capacity and durability of composite materials in which as a binding element apply adhesive compositions. Estimation of crack resistance is usually carried out using an energy criterion – specific energy of crack propagation. Existing mathematical models to determine the value of crack resistance do not always give reliable results, because they can contain several dependent variables. In the article the mathematical models of the two types for calculation of crack propagation energy, which includes only one independent parameter, which needs to be defined experimentally are received. For the mathematical model of type I on the experimental setup it is necessary to define value of bending moment at which the formation of crack. Use of mathematical model of II type does not demand experimental setup as independent parameter is the radius of curvature of the bent beams.

Experimental Research of Crack Resistance of Composite Materials

An adhesive composition is used as a binder in many composite materials. Crack resistance of an adhesive composition is the most important characteristic determining the bearing capacity of composite materials. To date, a significant number of methodological recommendations and standards have been developed for estimation of adhesive compositions bearing capacity in different types of loading forces exertions which, with certain assumptions, can be used to assess their crack resistance. Known testing procedure for crack resistance of adhesive composition was selected as a basic one. It includes a bending load in the form of loadings acting perpendicular to cantilever beams which is applied to the double-cantilever sample. Therewith a sample response and a corresponding change in length of the resulting crack in the adhesive compound layer are being recorded. This is followed by a determination of the specific energy of crack propagation which provides information about the value of crack resistance.

Improved mechanical properties of Al2O3 ceramic by in-suit generated Ti3SiC2 and TiC via hot pressing sintering

Abstract Al2O3 multi-phase composites with different volume fractions of SiC varying from 0 vol% to 30.0 vol% were fabricated by vacuum hot pressing sintering at 1600 °C under the pressure of 30 MPa for 2.0 h. The aim of this work was to investigate the effect of SiC content on the morphology and mechanical properties of the Al2O3 multi-phase composite. The results show that the addition of SiC and Ti can produce new strengthening and reinforcing phases include Ti3SiC2, TiC, Ti5Si3, which would hamper the migration of grain boundaries and promote sintering. The mechanical performances could reach the comprehensive optimal values for 20.0 vol% SiC, delamination and transgranular fracture being the major crack propagation energy dissipation mechanisms.

High-temperature oxidation behavior of plasma-sprayed ZrO2 modified La-Mo-Si composite coatings

Plasma-sprayed ZrO2-modified LaB6-MoSi2 coating (ZLM) was firstly fabricated on SiC pre-coated carbon/carbon (C/C) composites to obtain an improved oxidation resistance at 1773K involving long-term oxidation and short-term cyclic oxidation in an atmospheric environment. A crack-free structure in ZLM coating has been established with interlocking adhesion between the layers of coating. It is shown that the specimen with ZLM coating exhibits superior oxidation with the mass loss of 0.96% after 140h at 1773K and spallation resistance with the mass loss of 0.61% after 30 thermal cycles between 1773K and room temperature. These beneficial effects can be attributed to superior integrated structure of the ZLM coating provided by the continuous Zr-La-Si-O protective scale gave rise to the suppression of oxygen diffusion in ZLM coating. Moreover, the formation of “inlaid phases” in the compound glass layer is helpful to consume the crack propagation energy and restrict the spreading of it. Dual protection of the “inlaid structure” and Zr-La-Si-O compound glass film is responsible for the excellent oxidation resistance of the ZLM coating.