Formation Of Surface Cracks Research Articles

Stress distributions due to hydrogen concentrations in electrochemically charged and aged austenitic stainless steel

As a result of hydrogen concentration gradients in type austenitic stainless steels, formed during electrochemical charging and followed by hydrogen loss during aging, at room temperature, surface stresses were developed. These stresses were measured by X-ray technique and the crack formation thus induced could be studied using equilibrium stress equations. After various electrochemical charging and aging times, X-ray diffraction patterns obtained from samples indicated that the reflected and broadened diffraction peaks are the result of the formation of a non-uniform but continuous solid solution in the austenitic matrix. Since both hydrogen penetrations during charging and hydrogen release during aging are diffusion controlled processes and huge hydrogen concentration gradients in the thin surface layer, at depths comparable with the depth of X-ray penetration, are observed. The non-uniform hydrogen concentration in the austenitic matrix, results to the non-uniform expansion of the atomic microstructure and latter inevitably leads to the development of internal stresses. The internal stresses development formulae’s are very similar to those relating to non-uniform heating of the materials, where thermal stresses appear due to non-uniform expansion or contraction. The relevant well developed theory is applicable in our case of non-uniform hydrogen concentrations in a solid solution of electrochemically charged and aged austenitic matrix. A few cracks were present on the surface after some minutes of electrochemical charging and the severity of cracking increased as hydrogen was lost during subsequent aging. This is consistent with the expectation of high compressive stresses in the bulk of the specimen during charging and high tensile surface stresses (at the level of 1 × 10 11 Pa) during the aging process. These stresses can induce the formation of surface cracks during the aging process after electrochemical charging in the AISI 316 stainless steel. Electrochemical charging of AISI 316 stainless steel led to surface cracking that was both in the types transgranular and intergranular.

Experimental and theoretical studies of corrosion-induced mechanical properties degradation of aircraft 2024 aluminum alloy

In the present work, the influence of the corrosion exposure time on the mechanical properties degradation of the aluminum 2024-T3 alloy is studied. Tensile and fracture toughness mechanical tests had been carried out on pre-corroded specimens, exposed for different times to laboratory accelerated, exfoliation corrosion solution. The test results show that both the tensile mechanical properties as well as the fracture toughness exponentially decreased with increasing exposure time. A mechanical model has been devised to calculate the specimen’s remaining effective thickness after different corrosion exposure times. It has been shown that the decrease in the alloy’s fracture toughness is mainly associated with the thickness reduction degradation mechanism and a small reduction is attributed to hydrogen embrittlement. The hydrogen embrittlement degradation mechanism is saturated for exposure times up to 24 h. For higher exposure times, the prevailing degradation mechanism of alloy’s fracture toughness is the formation of corrosion-induced surface cracks and the resulting reduction of the effective thickness of the test specimen. An FE model has been developed for a fast calculation of the uncorroded alloy’s fracture toughness with good results. The model has also been exploited to predict the alloy’s fracture toughness degradation for the increasing exposure time to corrosive environment.

Improving the fatigue life of electro-discharge-machined SDK11 tool steel via the suppression of surface cracks

The present study performs an experimental investigation to identify the EDM processing parameters which suppress the formation of surface cracks in the machined surface of SKD11 tool steel specimens. In the EDM trials, the specimens are machined using pulse currents of 4 A, 16 A or 32 A with pulse-on durations of either 4 μs or 16 μs. The various specimens are then fatigue tested at loads ranging from 1470 to 2401 N in order to determine their respective fatigue lives. A polished SKD11 specimen is also fatigue tested for comparison purposes. Finally, the fracture surfaces are examined using scanning electron microscopy to examine the crack propagation characteristics. The results show that increasing the pulse current and reducing the pulse-on duration provides an effective means of suppressing the surface cracking phenomenon. Higher values of the pulse current and pulse-on duration are found to increase the average thickness of the recast layer. Overall, the present results show that the four specimens considered in the fatigue test can be ranked in order of reducing fatigue life as follows: (1) the polished specimen, (2) the specimen with a thin recast layer and no surface cracks, (3) the specimen with a thick recast layer and no surface cracks and (4) the specimen with surface cracks.

Multiscale physical characterization of an outdoor-exposed polymeric coating system

Surface topography and gloss are two related properties affecting the appearance of a polymeric coating system. Upon exposure to ultraviolet (UV) radiation, the surface topography of a coating becomes more pronounced and, correspondingly, its gloss generally decreases. However, the surface factors affecting gloss and appearance are difficult to ascertain. In this article, atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM) measurements have been performed on an amine-cured epoxy coating system exposed to outdoor environments in Gaithersburg, Maryland. The formation of the protuberances is observed at the early degradation stages, followed by the appearance of circular pits as exposure continues. At long exposure times, the circular features enlarge and deepen, resulting in a rough surface topography and crack formation. Fourier Transform Infrared Spectroscopy (FTIR) study indicates that the oxidation and chain scission reactions are likely the origins of the surface morphological changes. The relationship between changes in surface roughness and gloss has been analyzed. The root mean square (RMS) roughness of the coating is related to nanoscale and microscale morphological changes in the surface of the coating as well as to the gloss retention. A near-linear dependence of RMS roughness with the measurement length scale (L) is found on a double logarithmic scale, i.e., RMS ∼ Lf. The scaling factor, f, decreases with exposure time. The relationship between surface topography, on nano- to microscales, and the macroscale optical properties such as gloss retention is discussed. Moreover, a recent development in using an angle-resolved light scattering technique for the measurement of the specular and off-specular reflectance of the UV-exposed specimens is also demonstrated, and the optical scattering data are compared to the gloss and the roughness results.

Secondary cooling of continuous-cast billet at OOO Uralmash-Engineering

modifying the thickness and mechanical properties of the ingot crust. However, this often leads to the formation and development of surface cracks, especially edge, reticular, and transverse cracks. Thus, by selecting different cooling rates, the temperature gradient and the temperature stress and strain at the outer surface and solidification front may be adjusted, and the brittleness range may be avoided. A compromise value must be sought here, since increasing the temperature gradients is favorable in terms of compressive stress and strain at the center of the billet but also tends to increase the tensile stress and strain at the outer surface. At most Russian metallurgical enterprises, quality requirements are formulated in terms of axial porosity, axial chemical inhomogeneity, internal cracks (perpendicular to the broad or narrow faces, axial cracks, pitlike cracks), and external cracks (longitudinal, transverse, edge, and reticular). The appearance of such cracks is due to complex factors, most notably the derivatives of the thermal stress and stress-strain state with respect to the mechanical loads. Manufacturers of continuous-casting machines employ different approaches to cooling.

Damage of beryllium surface under irradiation with a high-current electron beam

Radiation damage of a beryllium surface by a high-current electron beam at the GSEP-3 accelerator is considered. The degree of damage of beryllium samples has been determined. The temperature fields inside the sample and the distributions of thermal stresses have been calculated. The reasons for beryllium surface cracking formation have been found. The concentrations of point defects have been calculated. The possible reasons for an increase in microhardness of the irradiated beryllium surface layer are discussed.

Surface modification of Sylgard 184 polydimethylsiloxane by 254 nm excimer radiation and characterization by contact angle goniometry, infrared spectroscopy, atomic force and scanning electron microscopy

The modification of polydimethylsiloxane (PDMS) by narrow band 254 nm excimer radiation under a nitrogen atmosphere was characterized by contact angle goniometry, attenuated total reflectance infrared spectroscopy, atomic force and scanning electron microscopy. UV irradiation results in the formation of the carboxylic acids that influences the wettability of the surface. Continued exposure results in the formation of an inorganic surface (SiO x (1 < x < 2)) which hinders the ability to continually increase the wettability. The continuity of this inorganic layer is disrupted by the formation of surface cracks. These results have implications in the fabrication and chemical modification of microfluidic or micro-electro-mechanical systems.

Plastic work approach for surface defect prediction in the hot bar rolling process

In this study, a numerical technique was developed to study the effect of processing parameters on instability of the steel during five passes hot bar rolling process. For this a processing map developed based on the plastic work approach was used with finite element analyses to judge occurrence of instability during the process. The effect of roll geometry and processing temperature on hot rolled bars was examined with the help of a parametric study by employing the numerical technique developed. Based on this parametric study, the improved roll pass design and initial processing temperature were determined, respectively, for reducing flow instability during the process. The laboratory rolling mill was used to validate the numerical results. It was found that with the increase of the initial temperature in the process the less flow instability was observed compared to the change of the roll geometry introduced at each pass under the present investigation condition. It was found that the current numerical technique implemented will be useful in identifying the important parameters governing the formation of surface cracks in the multi-stage hot bar rolling process.

Neural network analysis of the influence of chemical composition on surface cracking during hot rolling of AISI D2 tool steel

The reasons for the formation of surface cracks during the hot rolling of tool steels are not well understood. However, we know that apart from the parameters of the thermo-mechanical processing, the chemical composition of the tool steel has a big influence on the formation of these surface cracks. The majority of examinations of the hot deformability (appearance of surface cracks) of various steel grades made so far were limited to studying the influence of a minor number of chemical elements on the formation of the surface cracks, where the databases were based on laboratory tests. This paper proposes a new approach to the study of hot workability by analysing crack formation during the hot rolling of AISI D2 tool steel. The database was formed on the results from the surface cracking of rolling stock in an industrial rolling process and the rolling stock’s chemical composition. The analysis of the spatial influence was performed with CAE neural networks, and included an analysis of the influence of carbon and carbide-forming elements, of manganese and sulphur, copper, tin, aluminium, etc. The results of the analyses revealed a new understanding of the influences, and thus also the possibility to reduce the amount of surface cracking if the chemical concentrations of the elements were to be closer to the exactly determined values, or closer to the more exactly determined ratios.

Optimizing substrate and intermediate layers geometry to reduce internal thermal stresses and prevent surface crack formation in 2-D multilayered ceramic coatings

In order to avoid the formation of normal cracks in the external layer of multilayered ceramic coatings during cooling, determination of the residual thermal stress field is essential. Concerning the effect of the intermediate layers (transition zone) on a possible reduction of stress in the external layer, it has been shown that in the case of a symmetrical (or constrained in-plane strain) system, reduction only occurs over a critical substrate thickness. In order to investigate the effect of the substrate geometry, the residual thermal stress field has also been determined in other configurations such as an asymmetrical arrangement (one side coating) allowing bending and a concentric-cylinder structure. In these last two configurations, a strong stress reduction is obtained. For practical applications such as gas turbine ceramic components, these results may provide guidelines for an optimum design of the geometry of substrate and intermediate layers of multilayered ceramic coatings.

The Study of Inner Surface Crack Formation of Seamless Modified 9Cr-1Mo Tube Rolled on Mandrel Mill and Its Application

The hot workability of modified 9Cr-1Mo, as a grade of heat resistant steels, is inferior to that of low-alloy steel, so the inner surface crack (ISC) easily occurs in seamless boiler tubes produced by the Mandrel Mill under improper rolling conditions. With the aid of FEM, the metal flow status during the rolling process was analyzed in 140mm 8-stand mandrel mill of Bao Steel. Both the metallographic shape and size of the ISC together with the result from the simulation show that the ISC of seamless tube forms at the elongation stage of shell. The mechanism of the ISC was discussed. With its initiation in stand No.1 and No.2 due to poor hot workability of modified 9Cr-1Mo steel, the ISC develops in subsequent passes. Based upon the mechanism devised was a special roll pass system which substantially upgraded the yield of qualified products.

Mechanism of Internal Surface Crack Formation of Seamless Modified 9Cr-1Mo Steel Tube Rolled by Mandrel Mill and Its Application

Abstract The hot workability of modified 9Cr-1Mo, as a grade of heat resistant steels, is inferior to that of low-alloy steel, so the inner surface crack (ISC) easily occurs in seamless boiler tubes produced by the Mandrel Mill under improper rolling conditions. With the aid of FEM, the metal flow status during the rolling process was analyzed in 140mm 8-stand mandrel mill of Bao Steel. Both the metallographic shape and size of the ISC together with the result from the simulation show that the ISC of seamless tube forms at the elongation stage of shell. The mechanism of the ISC was discussed. With its initiation in stand No.1 and No.2 due to poor hot workability of modified 9Cr-1Mo steel, the ISC develops in subsequent passes. Based upon the mechanism devised was a special roll pass system which substantially upgraded the yield of qualified products.

A Review of wheel wear and rolling contact fatigue

Railway wheels lives are limited by wear and the formation of rolling contact fatigue (RCF) damage. This review describes how the forces that cause wear and RCF on wheels are generated. It is demonstrated that shear stresses at and just below the surface are driven by forces generated during steady-state curving. Repeated applications of plastic shear stresses causes shear deformation of the material at and just below the wheel's surface. This can eventually lead to the formation and growth of surface cracks. Depending on the contact conditions, the surface cracks can develop into either wear or RCF. Tangential and normal forces, contact pressure, creepage, and the presence of fluid are important factors in determining the amount of wear and RCF. High strength wheel steels have been developed to withstand severe service loads. New materials and treatments are being developed to extend wheel life. The references quoted in this review show that RCF and wear of rails and wheels have been the subject of investigation for over 25 years. Several well-developed models exist for predicting wheel and rail wear. New models are emerging for rail RCF. These need to be further developed to be applicable to wheels.

Effect of cathodic hydrogenation on the mechanical properties of AISI 304 stainless steel nitrided by ion implantation, glow discharge and plasma immersion ion implantation

Hydrogen embrittlement in austenitic stainless steels is restricted to the surface due to the low hydrogen diffusion in austenitic structures. The effect of three different nitriding processes: ion implantation (II), plasma immersion ion implantation (PI3) and glow discharge (GD), on the mechanical and structural properties of cathodically hydrogenated AISI 304 stainless steel were studied in the present work. Cathodic hydrogenation was made on untreated and nitrided samples. Surface microstructure after nitriding and hydrogenation was investigated by X-ray diffraction. Mechanical properties were measured by instrumented indentation. Surface crack formation and hardness decrease was observed in non-nitrided samples after cathodic hydrogenation. Hardness of nitrided samples decreases after hydrogen degassing but still has values higher than untreated samples. Comparative analysis of nitriding processes and working conditions indicated that glow discharge plasma nitriding process at 400°C or 450°C is the most adequate to avoid crack formation in steel surface after cathodic hydrogenation.

An Improved Process Design for the Hot Backward Extrusion of Ti-6Al-4V Tubes Using a Finite Element Method and Continuum Instability Criterion

The hot backward extrusion process of Ti-6Al-4V tubes was designed by finite element (FE) simulation and experimental validations. In the hot backward extrusion of Ti-6Al-4V alloy, the main problems to emerge were surface cracks and sticking between the punch and the workpiece. The punch designs and back draft angles were investigated with a common hot backward extrusion condition. Then, based on a relatively good die design, the optimum process conditions were suggested. The design criteria were to achieve uniform distributions of strain and temperature and to produce a defect-free final product. As a model for predicting the forming defects, the continuum instability criterion developed by Ziegler was coupled to the internal variables generated from FEM simulation. Experimental observations were carried out on the extruded Ti-6Al-4V tubes to validate the model for predicting the forming defects. The die-chilling and friction greatly influenced the deformation mode of the tube and the formation of surface cracks. The formation of defects in the extruded tube was attributed to the non-uniform distribution of strain, strain rate, and temperature in the extruded tubes at the given experiment conditions.

A Model for Spontaneous Morphology Change in Electrodeposited Metal Films during Room-Temperature Aging

A stress-relief model was developed to explain the phenomenon of spontaneous morphology change observed during room-temperature aging of electrodeposited copper and gold films. The model is based on excess vacancies due to which as-deposited films are initially expanded. These are gradually annihilated at the film surface. Quantitative modeling of vacancy diffusion revealed a well-defined vacancy-depleted zone near the film surface. A tensile stress developed in the near-surface region, the relaxation of which occurred with the formation of surface cracks or islands. The model is general and can be applied to both single-crystal and polycrystalline materials.

Determination of changes in mass and volume of linseed oil during drying

Plates of stainless steel were coated with a thin film of seven different types of oxidative drying oils in order to follow the development charge in mass and volume during 42 days of curing. The plates were weighed in air and in water, and the volumes determined by applying Archimedes’ principle. All of the oils gained considerably in mass, with a maximum of 12% within seven days. The volume, however, decreased with large individual differences from one oil to another. A cold-pressed raw linseed oil decreased in volume by almost 15% whereas the volume of an alkali-refined linseed oil only decreased by a few percent. It is well known that linseed oil-based coatings are prone to crocodiling (surface crack formation), it is suggested that the severity of this phenomenon may be related to the volumetric reduction of the binder during curing. The described method is suggested as a simple means of screening linseed oil binders for formulation and development purposes.

Deformation associated with a slow moving landslide, Tuen Mun, Hong Kong, China

Landslides in Hong Kong are commonly thought of as being fast moving with complete detachment of materials. However, following a recent hill fire, extensive ground deformation was exposed, predominantly in the form of surface cracks, associated with a slow moving landslide in an area of natural terrain above Tuen Mun, western Hong Kong. Mapping of these structures determined their extent, form and kinematics. In addition, a limited ground investigation including inclinometer monitoring was carried out to determine the depth, extent and approximate timing of deformation.

Mechanistic Studies of High-Temperature Crack Initiation in Single Crystal Materials

Abstract In this work, the results of mechanistic studies of surface crack initiation due to the coalescence of microcracks from casting defects are presented. Two approaches are considered; namely, a continuum damage (CD) one and another where a microvoid is explicitly introduced in the vicinity of a notch. In both cases, a rate-dependent crystallographic theory is relied upon to describe the visco-plastic behavior of the single crystal. The numerical studies are conducted on a notched compact tension (CT) specimen with and without a single casting defect, idealized as a cylindrical void close to the notch surface. CD predictions of the formation of surface cracks under constant far-field loading, linked to the nucleation and coalescence of microcracks from internal porosities, are obtained from a recently proposed mechanistic anisotropic void growth model. In the explicit-void modeling approach, the time to crack initiation under constant load has been predicted using a strain-based failure criterion. Finite element analysis of the CT specimen revealed that, due to the strong localization of inelastic strain at the void, a microcrack will initiate in the vicinity of the void rather than at the notch surface. The numerical results have also shown that the time to crack initiation depends strongly on the applied load level and the void location. These results and those obtained from the CD predictions are compared and discussed. The applicability of a failure assessment approach, based on the linear elastic stress intensity factor to predict the crack initiation time under creep loading, is also considered.

Mechanisms of long-term decay of tension stiffening

Details of an extensive collaborative project to study tension stiffening have been recently published in a number of papers. Among other factors, the project investigated the loss of tension stiffening with time. It has been concluded that the reduction of tension stiffening to a constant long-term value is fairly rapid, being complete in 10–30 days. This paper explores the mechanisms that may operate to cause the reduction of tension stiffening with time. It is concluded that the loss is mainly attributed to cumulative damage resulting from either the formation of additional surface cracks or the formation or extension of internal cracks. It would appear that the final tension stiffening is only minimally dependent on concrete strength.