Compressive Stress Profile Research Articles

Surface Hardness and Abrasion Threshold of Chemically Strengthened Soda-Lime Silicate Glasses After Steam Processing

Chemical strengthening by diffusive ion exchange (IOX) is a common method to improve the mechanical performance of glass products. However, the process of ion-stuffing is often associated with an increase of surface hardness and a decrease of the resistance to abrasive wear during scratching, even when the thickness of the exchanged layer is low. Autoclave steam-treatment presents a way to compensate the enhanced surface brittleness accompanying IOX. It causes a notable shift in the load threshold for microabrasion to more abrasion-resistant glasses. Subject to the specific processing parameters, the softening effect is constrained to a surface layer of less than 500 nm in thickness; therefore, the overall compressive stress profile is not affected and the advantages of IOX strengthening are retained. In turn, ion-stuffing by IOX counteracts severe autoclave corrosion of soda-lime silicate glasses, making them suitable for a combination of both processes.

Glass with hydration‐induced compressive stress profiles

AbstractA novel potassium phospho‐aluminosilicate composition is described that can be strengthened by water vapor to achieve deep compressive stress (CS) profiles. Water vapor treatment at (A) 85°C and 85% relative humidity for 40 days results in a CS of 389 ± 20 MPa and a compressive depth of layer (DOL) of 18 ± 2 μm. When treated at (B) 160°C and 0.1 MPa for 7 days, a CS of 245 ± 20 MPa and a DOL of 40 ± 2 μm is achieved. Glasses with hydration‐induced stress profiles can provide high retained strength following flaw introduction compared with ion‐exchanged soda‐lime silicate glass. Sample treatment B also has an exemplary Vickers indentation cracking threshold value greater than 20 kgf. The hydration profile determined by secondary ion mass spectrometry (SIMS) is shown to closely match the stress profile for these samples. SIMS analysis also shows that the depth of water enrichment correlates well with the depletion depth of phosphorus. The high tendency towards water‐induced strengthening for this new type of glass even enables self‐strengthening by the generation of a near‐surface CS profile following exposure to ambient conditions.

Thermal Shock Behavior of a Soda Lime Glass Treated by Ion Exchange

In this work, we have submitted a soda lime glass to thermochemical treatment by ion exchange in order to introduce compressive stresses in their surface. Subsequently, we studied the hot-cold thermal shock resistance of ion-exchange reinforced glass. A potassium ion from the molten salt bath replaces the sodium ion of the glass surface by a superficial diffusion process. We have determined the potassium diffusion profile by two techniques: X-ray fluorescence spectroscopy XRF and X-rays microanalysis coupled with Scattering Electronic Microscopy. In addition, for the same purpose, we used X-ray fluorescence spectroscopy. Then, we determined by micro-indentation the profile of the compressive stresses induced in the glass surface by the exchange of Na+ ions by larger ones of K+. The compressive stress profile induced by chemical tempering depends on potassium ion concentration. It reaches a maximum value of 988 MPa for a treatment done during 50 hours at 480°C. We have subjected the glass specimens treated by ion exchange to a hot-cold thermal shock technique; we realized the cooling by air jet at room temperature. The heat transfer coefficient was about 600 W/m2 °C (Biot number β = 0.3). We have determined the residual strengths of the ion exchange treated and thermal shocked specimens by a three-point bending test. The strength is improved; it passes from 90 MPa of the as received glass to 380 MPa for the chemically treated glass. We noticed that the critical temperature difference ΔTc, which caused the thermal shock fracture, depends on the glass samples states. Consequently, because of the thermochemical treatment, ΔTc has shifted to higher temperatures. Its value was 254°C for raw glass and reached 455°C for the case of reinforced glass by ion exchange at 480°C during 50 hours. The thermochemical ion exchange treatment improve the glass thermal shock resistance.

Effect of low-temperature surface hardening by carburization on the fatigue behavior of AISI 316L austenitic stainless steel

The influence of low-temperature gaseous carburization on the fatigue behavior of AISI 316 L austenitic stainless steel was investigated. Tension-compression axial fatigue tests were performed under ambient conditions on untreated and carburized AISI 316 L. The results show that the carburized AISI 316 L has a 22% higher endurance limit compared to untreated AISI 316 L. Fractography investigations with scanning electron microscope (SEM) reveal that for the untreated AISI 316 L fatigue cracks initiate at the surface regardless of the applied stress level. For the carburized AISI 316 L fatigue cracks initiate at the surface for relatively high-level stresses; for relatively low-level stresses fatigue cracks initiate at inclusions beyond the carburized case. After carburization, the ductility in the outmost 10 μm of the carburized case has significantly reduced, leading to micro-crack occurrence during fatigue tests and associated relaxation of compressive residual stress in this region. Beyond this surface-adjacent region, no evident stress relaxation occurs due to the enhanced yield strength of the carburized case. The enhanced fatigue performance is mainly ascribed to the compressive residual compressive stress profile introduced by the carbon-concentration profile over the case. Moreover, solid solution strengthening by interstitially dissolved carbon contributes to improve the fatigue performance.

Dispersible hydrogel force sensors reveal patterns of solid mechanical stress in multicellular spheroid cultures

Understanding how forces orchestrate tissue formation requires technologies to map internal tissue stress at cellular length scales. Here, we develop ultrasoft mechanosensors that visibly deform under less than 10 Pascals of cell-generated stress. By incorporating these mechanosensors into multicellular spheroids, we capture the patterns of internal stress that arise during spheroid formation. We experimentally demonstrate the spontaneous generation of a tensional ‘skin’, only a few cell layers thick, at the spheroid surface, which correlates with activation of mechanobiological signalling pathways, and balances a compressive stress profile within the tissue. These stresses develop through cell-driven mechanical compaction at the tissue periphery, and suggest that the tissue formation process plays a critically important role in specifying mechanobiological function. The broad applicability of this technique should ultimately provide a quantitative basis to design tissues that leverage the mechanical activity of constituent cells to evolve towards a desired form and function.

Effect of deep cold rolling on mechanical properties and microstructure of nickel-based superalloys

Mechanical surface treatments are performed on aerospace components for fatigue life enhancement by introducing beneficial compressive stress profiles and material strengthening. This paper studies the influence of Deep Cold Rolling (DCR) on the residual stress distribution, work hardening and the microstructure modification of two nickel-based superalloys, IN100 and RR1000. Two different diameters (6.3 mm and 12.6 mm) of the rolling ball inserts were investigated in this analysis. The hardness and residual stresses at the subsurface after DCR were analyzed along the rolling and transverse directions. Electron BackScatter Diffraction (EBSD) technique was used to characterize the microstructure of the samples both qualitatively and quantitatively. The degree of work hardening of fine grain RR1000 after DCR was characterized using full width at half-maximum (FWHM) of the X-ray diffraction peaks and Grain Orientation Spread (GOS) profiles acquired by EBSD characterization. The results clearly indicate that deep cold rolling introduces compressive residual stresses as deep as 1 mm, with significant work hardening at the subsurface in the coarse grain IN100. DCR resulted in the grain refinement, increase in low angle grain boundaries and clustering of dislocation density around the carbides in IN100. Depending on the ball diameter, DCR of RR1000 induced compressive residual stresses up to 700 µm and work hardening till the depths of 400 µm. Additionally, severe deformation of grains occurred near the rolling surface. The larger diameter of the rolling ball resulted in high degree of work hardening and better residual stress distribution deeper into the materials.

INFLUENCE OF ANNEALING ON THE DEPTH MICROSTRUCTURE OF THE SHOT PEENED DUPLEX STAINLESS STEEL AT ELEVATED TEMPERATURE

The depth profiles of residual stresses and lattice parameters in the surface layers of shot peened duplex stainless steel at elevated temperature were investigated utilizing X-ray diffraction analysis. At each deformation depth, residual stress distributions in both ferrite and austenite were studied by X-ray diffraction stress analysis which is performed on the basis of the sin[Formula: see text] method and the lattice parameters were explored by Rietveld method. The results reveal that difference changes of depth residual compressive stress profiles between ferrite and austenite under the same annealing condition are resulted from the diverse coefficient of thermal expansion, dislocation density, etc. for different phases in duplex stainless steel. The relaxations of depth residual stresses in austenite are more obvious than those in ferrite. The lattice parameters decrease in the surface layer with the extending of annealing time, however, they increase along the depth after annealing for 16[Formula: see text]min. The change of the depth lattice parameters can be ascribed to both thermal expansion and the relaxation of residual stress. The different changes of microstructure at elevated temperature between ferrite and austenite are discussed.

Competing Indentation Deformation Mechanisms in Glass Using Different Strengthening Methods

Chemical strengthening via ion exchange, thermal tempering, and lamination are proven techniques for strengthening of oxide glasses. For each of these techniques, the strengthening mechanism is conventionally ascribed to the linear superposition of the compressive stress profile on the glass surface. However, in this work we use molecular dynamics simulations to reveal the underlying indentation deformation mechanism beyond the simple linear superposition of compressive and indentation stresses. In particular, the plastic zone can be dramatically different from the commonly assumed hemispherical shape, which leads to a completely different stress field and resulting crack system. We show that the indentation-induced fracture is controlled by two competing mechanisms: the compressive stress itself and a potential reduction in free volume that can increase the driving force for crack formation. Chemical strengthening via ion exchange tends to escalate the competition between these two effects, while thermal tempering tends to reduce it. Lamination of glasses with differential thermal expansion falls in between. The crack system also depends on the indenter geometry and the loading stage, i.e., loading vs. after unloading. It is observed that combining thermal tempering or high free volume content with ion exchange or lamination can impart a relatively high compressive stress and reduce the driving force for crack formation. Therefore, such a combined approach might offer the best overall crack resistance for oxide glasses.

Chemical Strengthening of Alkali‐Free Glass via Pressure Vessel Ion Exchange

Ion exchange is a popular technique for chemically strengthening alkali‐containing glass articles, such as Corning® Gorilla® Glass. The ion exchange process is based on a replacement of small alkali ions in the glass with larger alkali ions from a molten salt bath through inter‐diffusion. As the larger alkali ions from the salt bath supplant the smaller alkali ions in the glass, a compressive stress profile is generated near the surface of the glass, which increases its strength and damage resistance. However, certain applications of high‐tech glasses require alkali‐free environments, such as glasses used as substrates for flat panel display applications. In this paper, we report the first successful chemical strengthening of an alkali‐free glass. This is achieved via an aqueous ion exchange of barium salts under high pressure and temperature. X‐ray photoelectron spectroscopy reveals that Ba2+ replaces both Ca2+ and B3+ in the glass, producing surface compressive stress values near 200 MPa. This technology may enable chemical strengthening for a wide range of applications, including flat panel display substrates.

Fatigue crack growth behaviour in the LCF regime in a shot peened steam turbine blade material

In this study, short fatigue crack initiation and early growth behaviour under low cycle fatigue conditions was investigated in a shot peened low pressure steam turbine blade material. Four different surface conditions of notched samples have been considered: polished, ground, T0 (industry applied shot peened process) and T1 (a less intense shot peened process). Fatigue crack aspect ratio (a/c) evolution in the early stages of crack growth in polished and shot peened cases was found to be quite different: the former was more microstructure dependent (e.g. stringer initiation) while the crack morphology in the shot peened cases was more related to the shot peening process (i.e. surface roughness, position with respect to the compressive stress and strain hardening profiles). Under similar strain range conditions, the beneficial effect of shot peening (in the T0 condition) was retained even at a high strain level (Δε11=0.68%), Nf,ground<Nf,T1<Nf,polished<Nf,T0. The a/c evolution effects were incorporated in K-evaluations and used in calculating da/dN from surface replica data. Apparent residual stress (based on crack driving force ΔK difference) was applied to describe the benefit of shot peening and was seen to extend significantly below the measured residual stress profile, indicating the importance of the strain hardening layer and stress redistribution effects during crack growth.

Molecular dynamics simulations of ion-exchanged glass

Chemically strengthened glass is one of the key enablers of the modern consumer electronics industry. Molecular dynamics simulations offer an opportunity to build enhanced fundamental understanding of the ion exchange process used for chemical strengthening. In this work, we apply molecular modeling techniques to investigate several aspects of ion-exchanged glasses that are either inaccessible through experiment or too difficult to probe through experiments alone. In particular, we use atomistic modeling to study the dependence of the linear network dilation coefficient on the concentration of alkali ions exchanged. We also calculate the evolution of elastic moduli as a result of the ion exchange process. Both of these quantities—the network dilation coefficient and the Young's modulus—are critical parameters for determining the final compressive stress profile of the chemically strengthened glass. Finally, we revisit the issue of structural differences between as-melted and ion-exchanged glasses in terms of local atomic number density around alkali ions. The results of this work point to the unique structures attainable through ion exchange that cannot be achieved through melt-quenching alone.

Degradation of ion-exchange strengthened glasses due to surface stress relaxation

Ion-exchange is a popular method to strengthen alkali-containing glasses. However, the experimentally observed residual compressive stress profiles often deviate from the theoretical profiles: a subsurface compression maximum is often observed, while monotonically decreasing stress with depth is expected. It is suggested here that this discrepancy can be accounted for by considering a surface stress relaxation process that takes place at a faster rate than bulk stress relaxation in the presence of water. This suggestion is based upon experimental data indicating the presence of water impurity in the molten salt used for ion-exchange strengthening of glasses. Recent work characterizing surface stress relaxation in oxide glass fibers as a diffusion controlled process aided by molecular water diffusion was applied to better explain the development of a subsurface compressive maximum within an accelerated timeframe. By combining the new surface stress relaxation diffusion mechanism with the well-established ion-exchange mechanism, a mathematical model predicting a subsurface maximum stress is developed with a good agreement to published data.

Compressive stress profiles of chemically strengthened glass after exposure to high voltage electric fields

The ion exchange process for chemical strengthening of glass involves an inter-diffusion of alkali ions between the glass and a molten salt bath. In most commercial glasses, this comprises an exchange of Na+ ions in the glass for larger K+ ions from the salt bath. The stuffing of K+ ions into sites previously occupied by Na+ in the glass results in the formation of a compressive stress profile in the glass. In the absence of stress relaxation, this compressive stress profile largely follows the concentration profile of the K+ ions. Any subsequent migration of K+ ions within the glass would result in a change in this stress profile, which could lead to different mechanical behaviors of the glass. In particular, a reduction of the surface compressive stress could lead to a compromise in the retained strength of the glass, making it more susceptible to failure. Recent work has shown that chemically strengthened glass can be used as an effective substrate material for organic thin film transistors (TFTs), since the temperatures involved with organic TFT deposition are low enough to avoid any compromise in the compressive stress profile. This opens the possibility of fabricating high strength organic TFT displays. However, the question remains as to whether the exposure of the glass to an electric field may lead to the diffusion of the alkali ions and a corresponding alteration of the stress profile, which could compromise the strength of the organic TFT device. In this paper, we demonstrate that there is no change in the stress profile of Corning® Gorilla® Glass 3 after subjecting the glass to much higher voltage dc fields compared to the maximum field that would be exhibited in an organic TFT device. The stress profile is modified only after treating the glass at sufficiently high temperature, where alkali migration becomes thermally activated.

High-speed camera study of Stage III crack propagation in chemically strengthened glass

Thermally or chemically strengthened glass is more resistant to damage and breakage compared to non-strengthened glass. Both strengthening mechanisms are based on incorporation of a compressive stress profile in the surface of the glass, which must be balanced by an equivalent amount of integrated tensile stress in the interior of the glass. This tensile stress is believed to affect the kinetics of Stage III crack propagation upon fracture of the sample. In this study, we use a high-speed camera to perform direct measurement of the kinetics of Stage III fracture in a strengthened glass sample. Data including crack propagation speed, crack bifurcation distance, and bifurcation angles are collected at a rate of 500,000 frames per second and then characterized. The authors believe that these data will provide a foundation for understanding the physics of Stage III fracture in strengthened glass samples.

Fatigue Crack Retardation in LSP and Sp Treated Aluminium Specimens

Highly loaded aircraft components have to fulfill strict fatigue and damage tolerance requirements. For some components besides the crack initiation mainly the fatigue crack propagation behavior is the main design criteria. To improve the crack propagation behavior of a component several methods are known or have been described in literature. For thin aircraft panels i.e. the application of crenellations [1] or bonded doublers [2, 3] can be a solution. For thick structures mainly the introduction of compressive residual stresses is beneficial. In this paper the potential of compressive residual stresses obtained by Laser Shock Peening (LSP) and Shot Peening (SP) is investigated. By means of Laser Shock Peening the residual compressive stress field can extend much deeper below the treated surface than that produced by conventional Shot Peening (i.e. with steel or ceramic balls) [4, 5]. The effect of such deep compressive stress profile results in a significantly higher benefit in fatigue behavior after Laser Shock Peening or after the combination of Laser Shock Peening and Shot Peening on top. The measurement of residual stresses as a depth profile has been performed by incremental hole drilling (ICHD) and contour method. Finally crack propagation tests have been carried out to validate the process technology approach.

Effect of chain architecture on the compression behavior of nanoscale polyethylene particles

Polymeric particles with controlled internal molecular architectures play an important role as constituents in many composite materials for a number of emerging applications. In this study, classical molecular dynamics techniques are employed to predict the effect of chain architecture on the compression behavior of nanoscale polyethylene particles subjected to simulated flat-punch testing. Cross-linked, branched, and linear polyethylene chain architectures are each studied in the simulations. Results indicate that chain architecture has a significant influence on the mechanical properties of polyethylene nanoparticles, with the network configuration exhibiting higher compressive strengths than the branched and linear architectures. These findings are verified with simulations of bulk polyethylene. The compressive stress versus strain profiles of particles show four distinct regimes, differing with that of experimental micron-sized particles. The results of this study indicate that the mechanical response of polyethylene nanoparticles can be custom-tailored for specific applications by changing the molecular architecture.

Residual Stress Profile of Ultra High Strength Low Alloy Steel Induced by High Speed Milling Process

In order to improve the surface quality of ultra high strength low alloy steel work-pieces produced by high-speed face milling process, 23-1 factorial design experiment was con-ducted and the residual stress profiles within the surface and subsurface layer of work-pieces were measured. Corresponding empirical models for the residual stress profile were presented and the effects of cutting parameters (cutting velocity, feed per tooth, depth of cut) on characteristics of the residual stress profile were studied. Results show that: with the range of cutting parameters tested, the compressive residual stress profile would be induced below the work-pieces’ surfaces machined by high speed face milling process. Feed per tooth has the critical influence on the characteristics of the compressive stress profile, and the mechanism of residual stress generation will be different when feed per tooth changes in high speed machining process. To obtain higher compressive stress and deeper compressive stress profile depth, larger feed rate and depth of cut are required.

Stress and Stress Intensity Factor Near Notches in Thick Cylinders

This work investigates the impact of semi-elliptical notches (erosion grooves) at the bore of pressurized and autofrettaged thick cylinders. It provides a robust yet rapid method to determine the rapidly varying stress in the near-notch region prior to crack development and crack tip stress intensity factor after a crack develops at the notch root. The procedure involves a sequence of asymptotic solutions and adjustments. A superposition is presented for the stress concentration factor (SCF) of a small edge notch in a pressurized cylinder with bore pressure infiltrating and acting upon the notch surface. A procedure for adjusting this SCF to account for a varying pre-existing stress field is described. This provides accurate predictions for notch depths of up to 15% of wall thickness. Stress variation beyond the notch root is determined by scaling analytic solutions. Solution accuracy appears to be approximately 5%. Stress profiles were used to calculate stress intensity factor (SIF) for cracks emanating from the notch root and deep into the wall. There are notable differences between SIF behavior in the pressurized tube and in the autofrettaged tube. The main reason for this difference is that compressive reyielding near the notch disrupts the residual compressive stress profile over an extended distance. This leads to the conclusion that a crack originating from a notch in an autofrettaged tube exhibits a much higher cyclic SIF range during pressurization than the same length crack originating from the bore. This will cause higher fatigue crack growth rates and shorter fatigue lifetimes.

Numerical and Experimental Analysis in the Manipulation of the Mechanical Properties for Enhancing the Mechanical Resistance of a Material

The evolution and development of mankind has been partly possible thanks to the transformation of diverse types ofmaterials and the manipulation of their mechanical properties. This work is focused on numerical and experimentalevaluations of the improvement of the mechanical resistance of one material (AISI 316L) through the application ofstrain hardening and a residual stress field induction. Additionally, the state of the stresses in the component isdetermined by the application of the Crack Compliance Method, a destructive method based on the FractureMechanics Theory. The relevance of the work lies on the implementation of a new methodology which can be used toimprove the mechanical resistance of the component by altering the state of the mechanical properties of thismaterial. This research also demonstrates that strain hardening and induction of a residual stress field must beperformed carefully or it could result in a component susceptible to failure. In this respect, bending tests are proposedto provide tensile and compressive stress profiles and to corroborate previous history loading on the material

Laser shock peening to improve the fatigue resistance of AA7050 components

PurposeLaser shock peening (LSP) is a process capable of introducing compressive residual stresses into a metallic component. The residual compressive stress field can extend deeper below the treated surface than that produced by conventional shot peening (SP). The effect of such deep compressive stress profile is expected to result in a significantly greater benefit in fatigue resistance after LSP compared to SP. The purpose of this paper is to examine this further.Design/methodology/approachResidual stress profiles have been determined by X‐ray diffraction and incremental centre hole drilling. They have been correlated with the respective LSP process parameters and the obtained fatigue behavior.FindingsA significant improvement of the fatigue life was found for an R ratio of 0.1. SP leads to a fatigue improvement of about 15 percent. For the same specimen geometry, a fatigue life improvement of about 25‐35 percent, depending on the load level, can be obtained after LSP. However, not only for the positive R ratio, where it is quite obvious, but also for the negative R ratios, R=−1 and −3, an increase of the fatigue life is generated by SP and LSP.Originality/valueA shown LSP has a high potential for extending the service life of metallic components at the design stage, but it may also be possible to apply this technique to in‐service aircraft to extend the service goals of existing structures.