Final Fracture Stage Research Articles

Cyclic plasticity behavior and deformation mechanism of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings

Fatigue performance and durability application have always been the key issues impeding the understanding of aero-engine pressure pipe due to its non-standardized shape, making tests more challenging. This paper constructs an optimized testing system to elucidate the plastic shakedown and ratcheting behavior of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings. Moreover, microstructural evolution's role in regulating the cyclic plasticity behavior is qualitatively established. Experimental results indicate that the pipe deformation exhibits compressive ratcheting behavior in plastic shakedown state, and the initial cyclic hardening followed by cyclic saturation occurs during the whole cyclic response. This corresponds to a decreasing ratcheting displacement rate followed by a steady state appears. Except for the first two similar deformation stages mentioned above, the abrupt cyclic softening and the resultant increasing ratcheting displacement rate are obtained at the final fracture stage of the pipe deformation in ratcheting state. A series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and acoustic emission (AE) are combined to reveal the underlying microstructural mechanisms during cyclic deformation. The volume fraction of austenite and ferrite and the average grain size change during different stages of plastic shakedown and ratcheting deformation. The synergistic effects of dislocation structure multiplication, destruction, and dissolution at different stages of plastic shakedown and ratcheting deformation dominate the cyclic plasticity behavior.

Microseismic Signal Characteristics of the Coal Failure Process under Weak-Energy and Low-Frequency Disturbance

In deep mining, “critical static stress + slight disturbance” is an important inducing form of coal mine rockburst disasters. In previous studies, the critical static stress has been shown to be consistent with the loading direction of a slight disturbance but cannot reflect all types of rockbursts. In addition, the research that uses microseismic (MS) signals to reflect the overall process and critical stages of coal failure and instability under weak-energy and low-frequency disturbance conditions is immature, and more information, such as the critical state, has not been fully revealed. The aims of this paper are to further elucidate the important role of weak-energy and low-frequency disturbances in the occurrence of rockburst disasters. First, briquette samples were prepared from the Tashan Coal Mine, which is severely affected by rockbursts, and their homogeneity was verified using ultrasonic longitudinal wave velocity. Second, the natural frequency of the coal sample specimens was measured using a testing system. Then, based on the self-developed static pressure loading system, dynamic and static combined loading test system and MS signal monitoring device, the MS signal characteristics during the process of coal body failure and instability were comprehensively analysed. Finally, a comparison was made between weak-energy and low-frequency disturbances and impact disturbances. The results are summarized as follows. (1) The longitudinal wave velocity test results reflect that the briquette samples prepared in the experiment have high homogeneity. The smaller the particle size is, the higher the density and moulding pressure, and the denser the sample. (2) The natural frequency of the briquette samples is between 30.79 Hz and 43.34 Hz, and most of them fluctuate at approximately 35 Hz. (3) During the static loading stage, the occurrence of more than three MS signals of larger magnitude in a continuous cluster is an important criterion for the critical failure of the samples. (4) The weak-energy and low-frequency disturbance actually leads to fatigue damage, and the briquette sample experiences three stages: the near-threshold stage, the high-speed expansion stage and the final fracture stage. The smaller the particle size of the coal sample, the denser the specimen, the stronger the amplitude and energy of the single effective MS signal formed during the destruction process, the longer the time duration of crack expansion from the near-threshold stage to the high-speed expansion stage, and the stronger the ability of the coal sample to resist weak-energy and low-frequency disturbances. This study may contribute to a more comprehensive understanding of the destabilization mechanism of coal bodies and MS signal characteristics under weak-energy and low-frequency disturbances and provide a reference for further research and discussion.

Effect of ultrasonic rolling on crack propagation behavior of Ti6Al4V titanium alloy laser welded joints

This study investigated the effect of ultrasonic rolling on the fatigue resistance of Ti6Al4V titanium alloy laser welded joints, with an emphasis on crack growth rate. Multiple analyses were conducted on samples subjected to different numbers of rolling passes, including analysis of surface roughness, morphology, cross-section structure, hardness, fatigue fracture, and residual stress. Ultrasonic rolling was found to considerably enhance the life cycle of the sample. One, three, and five rolling passes resulted in fatigue life increases of 17.14%, 46.38%, and 66.61%, respectively, when compared with untreated samples. Treated samples exhibited rough and intricate crack propagation paths on fracture surfaces, along with numerous secondary cracks. Fatigue striation width decreased by 48.6%, 62.5%, and 69.4% for samples receiving one, three, and five rolling passes, respectively. These fatigue striations, which were narrower and more densely distributed, indicate that ultrasonic rolling effectively increases fatigue life. The ultrasonic rolling treatment enhanced fatigue performance and reduced the rate of crack propagation due to changes in surface compressive residual stress, microstructure, microhardness, and improved surface quality. Crack retardation primarily occurred in initial stage of crack propagation and stable propagation stages, whereas inhibitory effects weakened during the final fracture stage due to superposition and offset of the higher driving force with compressive residual stress. In summary, the study demonstrates that treatment via ultrasonic rolling is an effective method for improving fatigue life and fracture toughness in Ti6Al4V titanium alloy laser-welded joints.

Material and Damage Characterization of the Elastoplastic Response of the EK4 Deep Drawing Steel

Although EK4 drawing steel is nowadays widely used to manufacture a great variety of parts, it exhibits a marked normal and planar anisotropy that can make it difficult to control the process during its forming. In order to achieve an accurate description of the elastoplastic material response in sheet forming operations, this work presents a detailed material and damage characterization of EK4 deep drawing steel through a two-step methodology involving both experiments and finite element simulations. Firstly, tensile tests on sheet samples cut along the rolling, diagonal and transverse directions were carried out. The corresponding measurements were used to calibrate the material parameters related to the following modeling approaches adopted in the present study: the Hollomon hardening law, the non-associated Hill-48 phenomenological constitutive model and the anisotropic Hosford-Coulomb ductile fracture criterion. Secondly, this characterization was assessed and validated in the numerical simulation of the technological Erichsen test in which the material is mainly subjected to a biaxial stress state. The obtained predictions show a good agreement when compared with the corresponding experimental measurements of the punch load–displacement curve and thickness radial profile at the final fracture stage of the sample.

The investigation of low-cycle fatigue crack propagation of 16 Mn eccentric bar based on the acoustic emission technique

Compared to the traditional cropping technology, low-cycle fatigue cropping can reduce load and improve cross-section quality. Geometric factors of V-shaped notch and processing parameters (such as load amplitude and load frequency) influence the cropping efficiency and section quality. By changing the factors, the sufficient efficiency of cropping and quality of cross-section get together. However, the influences of material defects, geometric parameters, loading frequency and other factors are different in each stage of the whole cropping process. If the effects of the parameters on the each stage of cropping process are clear, the higher productivity and the better cross-section quality will be obtained by applying suitable parameter on each stage of cropping process. To investigate the effects of eccentric ratio on each stage of low –cycle fatigue cropping process, a suitable monitoring method is needed. This study proposes acoustic emission (AE) technique to detect the low-cycle fatigue cropping process of 16 Mn eccentric bar (The bar prefabricated eccentric notches. The prefabricated notch improve the efficiency of cropping). The parameters of signals such as counts and kurtosis during the low–cycle fatigue process are obtained. According to the counts changes over time, the process of the 16 Mn metal bar cropping can be divided into three stages: the crack initiation stage, the crack propagation stage, and the fracture stage. Based on the cumulative counts, the eccentric ratio’s influence on the time of each cropping process stage is obtained. The time of the crack initiation stage and the final fracture stage doesn’t increase with the eccentric ratio. The time of the propagation stage influenced by the eccentric ratio greatly. Besides, the eccentric ratio’s influence on the cross-section quality is studied by using an advanced optical microscope system. The cross-section quality was influenced by the eccentric ratio significantly. The results of the paper indicate the acoustic emission (AE) monitoring technique is a useful method to detect the process of low-cycle fatigue cropping. Especially, it provide effective information to investigate the effects of the notch eccentric ratio during the low-cycle fatigue cropping process. The eccentric ratio’s effects on each stage of low-cycle fatigue cropping process offer guidance to improve low-stress fatigue cropping efficiency and cross-section quality.

Mechanical properties and failure mechanisms of 3D six-directional braided composites at elevated and liquid nitrogen temperatures

3D six-directional braided carbon/epoxy composites are fabricated successfully, and their mechanical properties and failure mechanisms at room, elevated and liquid nitrogen temperatures are reported. The results show the temperature has an important influence on load-deflection curves and bending properties. All the curves are divided into three stages: initial linear elastic, nonlinear and final fracture stage. At room temperature, the linear characteristic is obvious and properties are excellent. The failure is a gradual process from external to internal due to good reinforcement of transverse yarns, and fibers are sheared and pulled out. At elevated temperatures, the curves decline successively and nonlinear features increase, with decrease of bending properties. The damage is matrix softening and plasticization, interface debonding. At liquid nitrogen temperature, zigzagged fluctuation feature increases and linear elastic characteristic strengthens. Bending properties have been improved significantly and failure is fibers shear fracture and formation of shear bands among fiber/matrix interface.

Experimental Techniques for Finite Shear Strain Measurement within Two Advanced High Strength Steels

There is a growing need to experimentally characterize local shear deformation in advanced high strength steels (AHSS) for the calibration of stress-state dependent fracture criteria and to better understand sheared edge cracking during secondary forming operations. Planar shear test specimens with digital image correlation (DIC) strain measurement are now commonly performed tests but may not be able to resolve the local strains during the final stage of fracture when the macroscopic shear band collapses to a micro-shear band with intense local strains. Studies of sheared edge stretching of AHSS have shown that the microstructure at the sheared edge experiences extreme local shear deformation with the shear-affected zone (SAZ) that can be much larger than the macroscopic strains reported using DIC on planar shear tests. In this work, two independent experimental techniques are proposed to characterize the residual strain distribution within the shear-affected zone for two AHSS grades with a similar strength level: a complex-phase (CP) steel, CP800, and a dual-phase (DP) steel, DP780. The first method uses finite strain theory to calculate the work-conjugate equivalent strain from grain rotations within the shear bands of interrupted in-plane shear tests. A comparison between the local DIC strain measurements and the grain rotation measurements were found to be in excellent agreement until just prior to failure. The second technique used micro-hardness measurements taken from the interrupted shear tests to develop correlations with the measured equivalent strain from the DIC system. The hardness and grain rotation techniques were then used to characterize the local strain distribution within the SAZ of hole expansion test specimens for punch clearances of 12% and 28%. Both methods provided similar strain distributions with the grain rotation method having the highest strain resolution. The residual strain field within the SAZ of both AHSS was found to be strongly dependent upon the punch clearance. Finally, a homogenization procedure was applied to the SAZ strain distributions to facilitate the length scale transition from the grain-level to length scales appropriate for finite-element modelling of sheet metal forming operations with sheared edges.

Hot Ductility of Cast High Nitrogen Fe-Cr-Mn-Ni Steel

Hot deformability and fracture of as-cast Fe-Cr-Mn-Ni stainless steel ingots with high nitrogen and high carbon contents were studied. Effective stress-strain curve indicates a decrease of the UTS from 510MPa to 90MPa with increase of temperatures from 600°C to 1200°C. Effective stress-strain curves exhibited typical work hardening until the final stage of fracture from 600°C up to 900°C. The fracture elongation decreased from 600°C with the increase of temperature up 850°C, but started to increase appreciably as the work softening becomes dominant in the stress-strain curves above 950°C, suggesting the increase of ductility above 950°C is associated with dynamic recrystallization. The high temperature deformability was enhanced above 1000°C by homogenization of the as-cast alloy due to the dissolution and redistribution of segregated particles, providing the homogeneous distribution of the nucleation site of dynamic recrystallization. . The presence of fatal crack at 1,250°C and the loss of hot ductility can be attributed to the partial melting in austenite grain boundaries at high temperatures.

Loading history and structure of fracture of material

The bearing capacity of the structured materials and features of their fracture depend on local reaction of elements of structure on loading history. The initial structure of material can be modified in the course of prefracture at the initial stage of loading such that at further deformation, including deformation with the changed ratio of loadings on axes, this induced structure determines the scenario of fracture of material and its strength. The effective strength of heterogeneous materials and scenarios of their fracture are influenced by the level of the intermediate main stress. At compression of a cracked body lack of significant reaction to stresses along the sliding plane is characteristic for initial microcracks (areas of sliding). In a porous body such effect is absent. Therefore at tests with the proportional mode of loading according to the Karman or Böker schemes the effective measured strength can differ.To estimate an influence of a type of initial structural elements – pores and microcracks - on a final phase of fracture at uniaxial compression within a plane problem, an approach based on their equivalence to action of some concentrated forces is used. If the main crack is formed by joining a system of microcracks – the sliding areas, the movement on which causes the wedging forces, then weakening of the action in the process of the main crack growth is small. In case of a porous structure an influence of pores, remote from the crack tip, on the effective stress intensity factor KI in the process of crack sizes increasing weakens much faster. Therefore in a porous body development of echelons of rather short cracks joining small number of pores (N ~ 2-3) is energetically preferable, while occurring of the main cracks is characteristic for a cracked body at a final stage of fracture at compression. For a body with microcracks in which after coalescence of cracks from two and more concentrators the stress intensity factors for the formed defect become larger than it was at the moment of microcracks coalescence, the choice of the fracture scenario – development of the main fault occurs at the initial stage of fracture.In the porous body, forming the cracks with an influence of pores, favorable conditions for initiation of fracture in the new source of fracture at the small size of cracks are only created near the crack tips. The induced structure of fracture arising at partial unloading also influences the fracture mechanism at the subsequent loading. Development of a system of the feathering of cracks in the vicinity of the main fault also belongs to such structures. The last can be used as a method for the main crack arresting.

In situ observations of transgranular crack propagation in high-manganese steel

Crack propagation in high-Mn steel was investigated using in situ transmission electron microscopy . Preferential slips developed in the early stages of deformation on {1 1 1}, followed by the formation of a crack, which propagated along the pre-developed slip traces. A sharp corner was observed at the crack tip along two adjoining {1 1 1} planes. Surface ripples developed when the tip of the crack crossed pre-existing twins on the crack path, which was frequently observed in the surface during the final stage of fracture.

Effects of initial δ phase on hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy

Uniaxial tensile tests of a typical Ni-based superalloy are conducted under the deformation temperature range of 920–1010°C and strain rate range of 0.01–0.001s−1. The effects of initial δ phase (Ni3Nb) on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The results show that: (1) For the studied Ni-based superalloy with a large amount of δ phase, the flow stress curves are composed of three distinct stages, i.e., work hardening stage, flow softening stage and the final fracture stage. (2) The initial δ phase has significant effects on the deformation behaviors of the studied superalloy. δ phase can cause the obvious work hardening at the beginning of hot deformation, and then accelerates the flow softening by promoting the dynamic recrystallization with further straining. With the increase of initial δ phase, the strain rate sensitivity coefficient decreases firstly and then increases. (3) The combined effects of localized necking and microvoid coalescence cause the final fracture of specimens. The increase of initial δ phase increases the density of nucleus for the formation of microvoids, and promotes the nucleation and coalescence of microvoids.

Acoustic emission and fatigue damage induced in plasma-sprayed hydroxyapatite coating layers

In order to improve the adhesive strength of hydroxyapatite (HAp) coatings, grit blasting with Al2O3 powder and then wet blasting with HAp/Ti mixed powders was carried out on a commercially pure Ti (cp-Ti) substrate. Subsequently, an HAp/Ti bond coat layer and HAp top coat layer were deposited by plasma spraying. Fatigue tests of the HAp-coated specimens were carried out under four-point bending. Acoustic emission (AE) signals during the entire fatigue test were monitored to investigate the fatigue cracking behavior of the HAp-coated specimens. The HAp-coated specimens could survive up to 107 cycles without spallation of the HAp coating layers at the stress amplitude of 120 MPa. The HAp-coated specimens without HAp/Ti bond coat layer showed shorter fatigue life and easy crack nucleation compared to the HAp-coated specimens with HAp/Ti bond coat layer. The delamination and spallation of the HAp top coat with HAp/Ti bond coat on cp-Ti was not observed until the crack propagated into the cp-Ti during the final fracture stage of the fatigue cycle. Therefore, the HAp/Ti bond coat layer was found to greatly improve the fatigue damage resistance of the HAp coating layer. Three stages of the fatigue failure behavior of the HAp top coat with HAp/Ti bond coat on a cp-Ti substrate can be clearly estimated by the AE monitoring technique. These stages are cracks nucleating and propagating in the coating layer, cracks propagating in the substrate, and cracks propagating unstably to final fracture.

Cu-8.6wt%Al 삽입금속을 사용한 페라이트계 스테인리스강의 아크 브레이징 접합부의 미세조직과 인장성질

Microstructures and tensile properties in arc brazed joints of ferritic stainless steel, 429EM using Cu-8.6%Al insert metal was investigated as function of brazing current. The brazing speed was fixed at 800mm/min and brazing current varied in the range of 80A to 120A. The initial phase of filler metal was Cu single phase. However, the insert metal structures of brazed joints was composed of Cu matrix and intermetallic compound such as γ1(Al4Cu9), and flower-shape Fe-Cr. The fraction of γ1(Al4Cu9) phase was similar with 80A and 100A brazing currents while that of brazed with 120A was decreased. On the other hand, the fraction of Fe-Cr phase increased with increasing of the brazing current. A reaction layer at the base metal/insert metal interface was observed and this reaction layer was thickened with increasing of the brazing current. In the brazed joints with the current lower than 100A, crack was grew up along the interface which was perpendicular to the tensile stress, and then, passed through the insert metal in the final stage of fracture. As the brazing current increased to 120A, fracture occurred at the base metal.

Fracture initiation revealed by variations in the fatigue fracture morphologies of PA 66 and PET fibers

The tensile and fatigue fracture morphologies of polyamide 66 (PA 66) and polystyrene (PET) fibers are well known [1]. Tensile, and also creep, failure in both fibers is similar and generally involves crack initiation at or near the fiber surface, followed by slow crack propagation, as shown in Fig. 1 [2]. Tensile failure occurs when the remaining cross section can no longer support the induced stress and rapid failure occurs, giving the type of failure morphology shown in Fig. 2. Crack propagation under conditions which induce fatigue in these fibers is markedly different from tensile failure. In both fibers, crack initiation is, usually, as in monotonic tension, at or near the surface. However, under fatigue conditions the crack propagation is deviated so as to run at a slight angle to the fiber axis, gradually reducing the load bearing cross section [3]. Complementary broken ends of a PA 66 fiber broken in fatigue are shown in Fig. 3. It can be seen from Fig. 3 that the one broken end consists of a long tongue of material, the free end of which is the point of crack initiation. It can be noted that the tongue is always curved inwards towards the fiber axis, suggesting a residual compressive stress in the surface of the formally intact fiber. A closer inspection of the tongue shows that the fatigue fracture surface is concave, with respect to the fiber surface and not convex as would be expected if a pealing mechanism was driving crack propagation [4]. The complimentary end reveals the reduction of the load bearing section and both areas of final failure show that the final stage of fracture is by the tensile process shown above. Fatigue failure in PET fibers occurs under similar cyclic conditions to those which produce fatigue failure in PA 66, but the angle of crack penetration is smaller so that longer tongues are generally seen and final failure occurs often behind the fatigue crack tip by a creep process. In both tensile and fatigue failure the process of crack initiation is poorly understood, though some fracture morphologies seem to indicate the role of a thin surface layer on some fibers or the presence of an irregularity at the point of initiation. The present study has identified several unusual fracture morphologies, which have been induced under conditions which were known to provoke fatigue failure. The slower crack propagation which occurs during the fatigue process has allowed the causes of the initiation of some cracks to be determined. It is thought that these observations may be of relevance in the understanding of crack initiation in both tensile and fatigue failure. Figure 1 Slow crack growth from the surface of a PA 66 fiber under monotonic tensile loading.

Secondary foundry alloy damage and particle fracture

The damage mechanisms of a hypoeutectic Al–Si–Cu foundry alloy, obtained through die casting, have been studied. Observations were made during tensile tests using a tensile test machine that was inserted into an scanning electron microscope. The microstructure features that are critical for damage were determined. The cleavage fracture of intermetallic particles was found to be the main process that limited the alloy ductility. Shrinkage cavities mainly play a role in the final fracture stage; the reduction in cross-section area induced by shrinkage cavities was assumed to be the main contribution of these defects to the fracture of the specimens. Attempts were made to determine the cleavage stress that initiated the fracture of the particles. Three-dimensional finite element computations were performed considering particular locations at the free surface of the observed fractured particles. The maximum principal stress was calculated for different locations of the particle with reference to the free surface. Different shapes and different elastic behaviour were studied. It was shown that the influence of a free surface on the stress level in a particle depends on its shape. The results were compared with those found in literature concerning the fracture strength of particles in Al alloys.

Characteristic S-N Property of High Carbon Chromium Bearing Steel under Axial Loading in Long Life Fatigue.

Fatigue properties of a bearing steel (SUJ 2) including long life region were experimentally examined under axial loading and discussed from viewpoints of fractography and fracture mechanics. Thus, the complicated S-N property in the wide life region was well explained as the duplex S-N characteristics superimposed two different S-N curves for surface induced fracture and interior inclusion induced fracture. Fish-eye was formed on the fracture surface of each specimen in the interior inclusion induced fracture. Fine granular area (FGA) was necessarily observed around the inclusion at the center of the fish-eye. Stress intensity factor ranges at FGA, fish-eye and final fracture stage were in good agreement with their corresponding levels in the crack propagation law confirmed by CT specimens.

Numerical analysis of the processes of deformation and fracture of closed cylindrical shells under internal blast loading

A model of damaged medium is proposed for the description of dynamic deformation and fracture of thin-walled structures. The model makes it possible not only to describe the stage of latent fracture (initiation, growth and coalescence of microdamages), but also to predict the dynamics of macrocrack growth, i.e., the final stage of fracture. The processes of deformation and fracture of closed cylindrical shells have been studied under unitary internal blast loading. The method of computer simulation has been used for this purpose. Some characteristic features have been observed which accompanied the process of shell fracture. The results of the calculations are compared with the data of full-scale experiment.

Prediction of time to rupture in the creep of cast heat-resistant steels according to conditions in the final stage of fracture of notched specimens

An analysis of the dependence of residual life on the initial solidity of a specimen cross section containing a crack shows that it is possible to predict the time to rupture of a material from the conditions of fracture during the final stage of creep. Theoretical relations are first presented to make such a prediction, and good agreement with data from standard tests is demonstrated on the basis of results of more than 50 calculations of time to rupture for steels 15Kh1M1FL and 20KhML.

Influence of Change of Relaxation Properties on Polymer Tensile Strength

Abstract A specific property of polymer materials—the capability of the chain micromolecules to change shape due to external forces—manifests itself at the final polymer fracture stage (division into parts) in that its structure differs significantly, as a rule, from the initial stage. The description of such processes should take into account not only the change in the structure, but also change in the relaxation characteristics of the material associated with it. The present paper is a review of the state of the problem and describes the means for determining the relaxation characteristics of polymer materials at the final fracture stage.

The coal gasification plant for the developing country

An attempt has been made to discuss various design aspects of the coal gasification plant for the developing country on effective utilization of coal as a fuel there, where coal is used at the moment as the energy raw material for local cement, lime and brick manufacturers. This includes economic exploitation, economic justification and the IGCC technology proving to be an effective method of coal utilization and environmentally safe and clean design. The present work suggests the material with higher resistance to minimise crack failure for the transportation, storage and utilization of hydrogen fuels desired there. For this the soft soldered composite laminates made from 4340 steel were tested under plane strain conditions in the environments of hydrogen especially at 700 torr. This all is supported by the fracture behaviour of the soft soldered composite laminates which remains same with and without hydrogen during final stage of fracture there.