Fatal Crack Research Articles

High temperature VHCF of a 3rd generation Ni-based single crystal superalloy with different casting pore sizes

Very high cycle fatigue (VHCF) of a Ni-based single crystal superalloy processed by high-rate solidification (HRS), liquid metal cooling (LMC) and HRS plus hot isostatic pressing (HIP), respectively were studied at 1000 °C, 20 kHz and R = -1. For HRS and LMC, the fatal cracks mainly initiate from casting pores surrounded by a rough zone. γ′-rotation as the main feature within the rough zone is related to the operation of {111}〈110〉 and {111}〈112〉 slip systems. HRS+HIP specimens mainly fail from the surface and they present the best VHCF property. LMC process could increase the VHCF durability like HIP treatment, especially at very low loading conditions.

Transition from internal to surface crack initiation of a single-crystal superalloy in the very-high-cycle fatigue regime at 1100 °C

Transition from internal to surface crack initiation is controlled by oxidation assisted fatigue-crack process in the very-high-cycle fatigue regime. Between 760 and 1000 °C, single crack initiation site is associated with internal casting defect, followed by a crystallographic Stage I propagation. By contrast, multiple surface crack initiation sites appear at 1100 °C, as the consequence of internal oxide penetration. The fatal crack follows a Mode I propagation and oxygen can diffuse into the material along the crack path. γ′-phase depletion appears surrounding the oxidised and cracked regions, while localised rafting can occur close to the crack tip.

VHCF life of AM1 Ni-based single crystal superalloy after pre-deformation

VHCF properties of the Ni-based single crystal superalloy AM1 with pre-deformation have been investigated at 1000 °C. Pre-deformed material did not present life debit under fully reversed conditions (R = −1). Pre-deformation decreased VHCF lifetime under high mean stress conditions (R = 0.5) with a shift from internal to mode I surface crack initiation. The pre-deformed material has microstructure coarsened bands lying on the former {111} slip planes, facilitating microstructure shearing during VHCF at R = 0.5. Slip traces increased surface roughness, causing a surface recrystallization and initiation of the fatal crack from the surface. Rejuvenation heat treatment after pre-deformation can restore the microstructure and the fatigue life.

Failure analysis of H13 steel die for high pressure die casting Al alloy

High pressure die casting (HPDC) die is operated at severe conditions such as high pressure, high temperature and corrosion, which limit its service life. Analysis of failure mechanism of the die is beneficial to guide die design and choose of materials so as to improve the die life. A failed H13 steel die which was used to manufacture a 5G component via Al alloy HPDC, has been investigated using a complementary range of techniques such as X-ray diffractometer (XRD), Optical microscopy (OM), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM). The results show that erosion and cracking are two main failure modes. Furthermore, the fatal cracking occurs around fillet radius and hole/insert, which is correlated with high local stresses.

Change of Carrying Shaft Innovative Capacity in The Process of Growth of Fatal Cracks

In the paper, the processes of changing the technical state of the crankshaft due to deformation and fatigue failure are considered. The defect method is presented, which allows to estimate the degree of fatigue damage of the crankshaft and to choose the way of restoration of the crankshafts

Role of microstructure in plastic deformation and crack propagation behaviour of an α/β titanium alloy

The deformation and crack propagation behaviour have been comparatively investigated in a titanium alloy with bimodal microstructure (BM) and bilamellar microstructure (BLM). The results show that higher fracture toughness (58 MPa m1/2) was obtained for the BLM than that BM one (45 MPa m1/2). The deformed microstructure, crack initiation and propagation and surface damage morphology were observed by SEM and EBSD. Fatal cracks easily nucleated at the larger primary α phase, especially at the equiaxed αe/β interface and primary α lath (αl), and often grow along a straight line, which accelerated the crack propagation and leaded to non tortuous crack path in BM. For the BLM, slip hardly transferred the αl/βtrans interface due to the secondary α(αs) precipitation in the β lamellas. Cracks initiated at the boundaries of α colony and propagated with a zig-zag way, which leaded to a high branch and fluctuation of the crack of BLM and generated a relatively superior fracture toughness performance. These results indicate that a small amount of αl colony surrounded with βtrans matrix is beneficial to the improvement of fracture toughness, which can provide a good theoretical support to tailor the microstructure and mechanical performance of titanium alloys.

Mechanical properties of REBaCuO single-grain bulk fabricated by the infiltration growth technique

In order to investigate the mechanical properties and fracture mechanisms of (Gd,Dy)BaCuO dense bulk material fabricated by the infiltration growth technique, tensile tests were carried out for specimens cut from the as-grown bulk material. The average tensile strength value of the (Gd,Dy)BaCuO dense bulk material fabricated by the infiltration growth technique was 44 MPa, which is superior to that of a porous bulk material fabricated through melt-processing in a 50% O2 atmosphere (37 MPa), and comparable to that of a dense bulk material fabricated through melt-processing in 100% O2 (45 MPa) that have been reported elsewhere. Through the observations of the flow-like patterns formed by the crack propagations on the fracture surfaces, the fatal crack initiation site was identified. One extraordinarily large pore, segregated secondary phase particles and some inclusion that mainly consists of Pt were observed around the crack initiation site on the fracture surfaces of the (Gd,Dy)BaCuO dense bulk material.

Effects of Combined Salt-Damage Resistant Agent on the Shrinkage, Chloride Penetration and Chemical Erosion of Mortar

Reinforced concrete structures located on coastal landfill frequently adjoin sea-water environment, and are exposed to sea water and humid environment during construction. Particularly, in the case of large-scale structures like dams, their drying shrinkage is accompanied by fatal cracking, and thus chlorine ion penetration becomes easier. The present study develops a salt damage-resistant agent (SRA) to which aluminum salts, oligomer condensate, and amino alcohol derivatives with the alkyl group are applied as binding inducers. SRA performs the roles of reducing the drying shrinkage of cement composites, binding chlorine ions, and preventing erosion by sulfate ions. This study tests and evaluates its resistance to degradation factors that may occur to structures constructed on coastal landfill and so on. As a result of evaluating shrinkage cracking properties by performing the restrained shrinkage cracking test, SRC showed the shrinkage reduction compared with BSC. As for the performance of resistance to chlorine ion and the chemical sulfate erosion rate, SRC showed the highest resistance performance, followed by BSC and OPC, regardless of the concentration of aqueous solutions for immersion. In addition, as for the rate of mortar weight change by sulfate erosion, the SRA-intermixed SRC mixture showed a weight reduction rate at the level of 1/3 of BSC and 1/6 of OPC, respectively.

Tensile and fatigue behavior of electron beam welded TC17 titanium alloy joint

The tensile fracture was originated from the strain concentration in equiaxed grain region adjacent to the fusion center, where a lower micro-hardness occurred when compared to the columnar grain region in the fusion zone. A higher stress level was more potential to fabricate a fatal fatigue crack initiating from the intrinsic metallic deficiencies. At a lower stress of 180 MPa, fatigue crack appeared to initiate from the intergranular welding pores other than the intrinsic metallic deficiencies. Furthermore, intergranular welding pores induced fatal crack initiation became the major fatigue crack initiation mechanisms when the fatigue stress amplitude down to 140 MPa.

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.

A preliminary study on the potentialities of the Circular Semi-Ring test

The potentialities of the Circular Semi-Ring test, as an alternative method for the indirect determination of the tensile strength of brittle materials, are discussed. The study is motivated by the fact that the outcomes of the familiar Brazilian-disc test are quite often criticized as providing erroneous results for the tensile strength, especially in case the origin of the fatal crack leading to the specimen’s fracture is not located at the disc’s center. The study described here focuses on the analytic determination of the displacement field developed in a circular semi-ring, assuming that it is simultaneously subjected to both compression forces and bending moments. The analysis is implemented by taking advantage of the complex potentials technique as it was formulated by Muskhelishvili. The solution derived is then validated taking advantage of the data for the respective displacement field obtained experimentally with the aid of the Digital Image Correlation technique.

Numerical investigation of deformed layout and fatigue life of automotive braking hose to the helix angle of fabric braided layers

The fabric braided rubber hose used in the automotive hydraulic braking system exhibits the anisotropic large deformation while its movable end is moving along the cyclic path during the steering and bump/rebound motions of vehicle. The complicated large deformation may cause not only the interference with other neighboring parts but also the durability problem resulting in the fatal micro cracking. In this regard, the design of high-durable braking hose with the interference-free layout becomes a hot issue in the automotive industry. However, it has been traditionally relied on the cost-/time-consuming trial and error experiments because the computational methodology for the large deformation and fatigue evaluation of the braking hose with the fabric braided layers in complex micro structure pattern has not been fully developed. Furthermore, the hose deformed layout and fatigue life are significantly influenced by the helix angle of braided layers, so that their characteristic investigation becomes an important research subject. As a preliminary step for the optimum design of high-durable and interference-free braking hose, this study investigates the hose deformed layout and the fatigue life cycle with respect to the helix angle of braided layers.

Fatigue behaviour of age-hardened Cu-Ni-Si alloy and effect of sporadic discontinuous precipitates on fatigue strength

Microstructure and fatigue strength of Cu-Ni-Si alloy with high solute concentrations were studied. The maximum hardness of this alloy was obtained by 0.5 h aging (500°C) and the matrix was strengthened by nano-size δ-Ni2Si precipitates. In addition, some parts of grains transformed into discontinuous precipitates which bring a detrimental effect on mechanical properties, producing a sporadic distribution of discontinuous precipitates. The fatal cracks, which led to the fracture of the specimen, were initiated at the grain boundaries, not at discontinuous precipitation phases. The physical background of fatigue damage is discussed in light of the role of microstructure on the behaviour of fatigue cracks.

Evaluation of peak-free electromechanical piezo-impedance and electromagnetic contact sensing using metamaterial surface plasmons for load monitoring

Continuous structural health monitoring (SHM) and delayed SHM techniques can be contact/ contactless, surface bonded/embedded, wired/wireless and active/passive actuator-sensor systems which transfer the recorded condition of the structure to the base station almost instantaneously or with time delay respectively. The time between fatal crack initiation and its propagation leading to the collapse of key infrastructures such as aerospace, nuclear facilities, oil and gas is mostly short. Timely discovery of structural problem depends heavily on the scanning period in well-established techniques like piezoelectric (PZT) based electromechanical impedance (EMI) technique. This often takes much scanning time due to the acquisition of resonant structural peaks at all frequencies in the considered bandwidth; thus poses a challenge for its implementation in practice. On the other hand, recently developed strain sensors based on metamaterials and their breeds such as nested split-ring resonators, localized surface plasmons (LSP), etc, employ measurement of reflected or transmitted signal, with super-fast scanning in the order of at most 1/100th of the time taken by the EMI technique. This paper articulates faster measurements by reducing unnecessary resonant structural peaks and focusing on rapid monitoring using PZT and metamaterial plasmons. Our research adopted wired PZT and wireless LSP communications with impedance analyser and vector network analyser respectively. We present integrated and complementary nature of these techniques, which can be processed rapidly for key infrastructures with great effectiveness. This integration can result in both continuous and delayed SHM techniques based on time or frequency or both domains.

Relationship between Grain Growth and Formation of Fracture Surface of Ultrafine Grained Cu in High-Cycle Fatigue

Fatigue tests were conducted on specimens of ultrafine grained copper produced by equal channel angular pressing. The growth behavior of a fatal crack was monitored successively. The morphological features of the crack growth paths and fracture surfaces were influenced by variation of grain size and damaged areas of the crack tip, depending on the stage of progress of the fatigue damage. The change in fracture surface was discussed by considering the interrelation between the reversible plastic zone size at the crack tip and the microstructure evolved during cyclic stressing.

Influence of heat treatments on the stress corrosion cracking susceptibility of 7075 aluminum wires in NaCl solutions

The influence of heat treatments on the stress corrosion cracking susceptibility (SCC) of industrially drawn 7075 wires was investigated in 2 wt% NaCl solution at a pH of 3. The T4 treated material showed the highest susceptibility, whereas the T6 treated alloy showed a reduced sensitivity to SCC. Over-aging was found to reduce SCC susceptibility most effectively. The high susceptibility of the T4 treated material was proposed to be a result of the strong intergranular corrosion attack during testing, reducing the loaded cross-section of the tensile specimen and increasing the stress level. The fatal crack formation is proposed to be a result of hydrogen embrittlement by the evolved hydrogen.

Formation mechanism of inclined fatigue-cracks in ultrafine-grained Cu processed by equal channel angular pressing

The formation mechanism of inclined fatigue-cracks in ultrafine-grained Cu processed by equal channel angular pressing was studied by using a partially notched specimen in which a fatal natural crack was introduced, to a specific site of the smooth surface was feasible regardless of microstructural inhomogeneity caused by the processing. The crack growth direction depended on the location along the circumferential direction of the round bar specimen and on the applied stress amplitudes. The role of the microstructure and deformation mode at the crack-tip areas on the formation behavior of fatal cracks is discussed in terms of the microstructural evolution caused by cyclic stressing and the mixed-mode stress intensity factor. The in-plane shear mode deformation at the crack tip assisted the formation of the inclined crack paths and the unique crack face profile.

Hot Deformation and Cracking during Compression of 21-4N Steel

Hot compressive deformation and cracking behaviors of cast 21-4N steel were investigated over a temperature range between 900°C~1250°C. The strength of the specimen at the outer surface of ingot is greater than that in the inner part because of the initial rapid solidification and corresponding fine structure at the outer surface. The initial flow stress hump in the stress-strain curves can be attributed to the dynamic recrystallization, which requires the initial strain energy build-up before the activation of grain nucleation process. With increase of temperature the critical stain for dynamic recrystallization decreases and at high enough temperatures, no critical strain for the build-up of strain energy is needed. At temperatures between 1000°C and 1200°C, homogeneous deformation occurred and no cracks were observed even after 50% strain. The cracks observed below 1000°C could be associated with the more difficult plastic flow in the presence of carbides and nitrides. The presence of fatal crack at 1250°C and the loss of hot ductility can be attributed to the partial melting in austenite grain boundaries at high temperatures.

Loading capacity strengthening to cement-stabilised crushed gravel using reinforced wire mesh

Settlement and non-uniform deformation of frozen soil subgrade introduce fatal cracks on semi-rigid base asphalt pavements. This research aims to use reinforced wire mesh to reduce serious cracks on semi-rigid asphalt pavements in frozen regions. Three impact factors, diameter and interval as well as two types of wire mesh have been analysed by loading tests. Results have shown that after setting the reinforced wire mesh: (a) failure load and displacement of the cement-stabilised crushed gravel samples have been improved; (b) diameter and interval of the wire mesh have interactive impacts on reinforced sample load capacity; (c) the reinforced wire mesh can limit cracks by redistributing stress in the specimen; and (d) the multiple linear regression model well reflects the variation of loading properties; the results make sense from the statistics view as well. Results also represent that the strengthened wire mesh can help spread out the applied load, and increase the overall loading capacity of cement-stabilised crushed gravel. The results provided a reference for semi-rigid base asphalt pavement cracks caused by non-uniform deformation on frozen soil subgrade as well as guidelines for construction and maintenance in frozen regions.

An Analysis of the Slow Compression Breakage of Coal Using Microfocus X-Ray Computed Tomography

The degradation of coal and the production of coal fines during handling and transport is a serious problem in processes that depend on closely sized large particles. To minimize the production of fines, a fundamental understanding of coal breakage is required, so, to clarify the complex nature of coal breakage, a series of experiments was conducted to determine the influence of the internal physical coal structures on compression breakage characteristics. The structures investigated are the cleat and layered structure of coal and the mineral inclusions. Samples of uniform size and shape were prepared from a large block of South African Waterberg coal. The samples were analyzed nondestructively using microfocus x-ray computed tomography then wrapped in cling film and mechanically compressed while the pressure applied was measured. The virtual three-dimensional volume tomograms of the initial sample and the progeny were compared and the changes qualitatively analyzed. Conclusions were drawn as to where the fatal cracks initialized and how the cracks propagated. Particle-size distributions were done to quantify the extent of breakage versus the breakage strength of the sample. It was found that, of all the internal structures, the inherent crack distribution has the biggest influence on breakage and breakage patterns of coal.