Deep Compressive Stresses Research Articles

Study on the mechanisms of time-dependent crack propagation and the gradual collapse of roadways in soft–hard composite strata

As mineral extraction extends to deeper strata, the creep deformation differences between soft and hard rocks are further amplified by high stress. This increases the demand for controlling the lifecycle deformation of soft–hard composite rock roadways (S-HRs). This paper investigates the time-dependent characteristics of soft–hard composite rock composed of mudstone and sandstone (M-SR) via laboratory experiments and the damage bond model previously proposed by the author. The results show that crack propagation in M-SR exhibits a clear time-dependent effect, with the creep damage of mudstone being greater than that of sandstone, and this trend increases over time. Over 90 % of microcracks during the first two creep stages develop and coalesce in the mudstone, whereas crack propagation in the sandstone is inhibited. This process plays a critical guiding role in the final failure mode of the M-SR. After excavation, a tensile stress zone forms around the S-HR, with more drastic changes occurring on the sidewalls. This zone expands over time, whereas the deep compressive stress in the roof and floor shifts toward the sidewalls, exacerbating the depth of sidewall failure and continuously inducing the flow of the rock mass into the roadway from the sidewalls. After 48 days, the deformation of the sidewalls increases by 281 %. Increasing the support of sidewalls is a viable approach to solve this issue, and the support range should extend beyond the stress concentration zone. Leveraging the feedback mechanism between the roof and the sidewalls helps reduce creep damage and deformation on the sidewalls.

Analysis of the effect of deep-profile compressive residual stress induced by laser peening on fatigue strength of 7075 aluminum alloy

The effect of laser peening-induced compressive residual stress on the fatigue strength improvement of A7075 specimens was investigated. Rotary bending fatigue tests and residual stress distribution measurements using the sequential polishing method were conducted on untreated, shot-peened, and laser-peened specimens. The results revealed that laser peening provides a deeper compressive stress and a deeper fracture origin, resulting in higher fatigue strength. Furthermore, controlling the laser parameters could extend the depth of the compressive stress layer by approximately 5 mm, indicating the possibility of applying laser peening to thicker materials to improve the fatigue strength.

Machining of additively manufactured alloy 718 in as-built and heat-treated condition: surface integrity and cutting tool wear

AbstractAdditive manufacturing (AM) using powder bed fusion is becoming a mature technology that offers great possibilities and design freedom for manufacturing of near net shape components. However, for many gas turbine and aerospace applications, machining is still required, which motivates further research on the machinability and work piece integrity of additive-manufactured superalloys. In this work, turning tests have been performed on components made with both Powder Bed Fusion for Laser Beam (PBF-LB) and Electron Beam (PBF-EB) in as-built and heat-treated conditions. The two AM processes and the respective heat-treatments have generated different microstructural features that have a great impact on both the tool wear and the work piece surface integrity. The results show that the PBF-EB components have relatively lower geometrical accuracy, a rough surface topography, a coarse microstructure with hard precipitates and low residual stresses after printing. Turning of the PBF-EB material results in high cutting tool wear, which induces moderate tensile surface stresses that are balanced by deep compressive stresses and a superficial deformed surface that is greater for the heat-treated material. In comparison, the PBF-LB components have a higher geometrical accuracy, a relatively smooth topography and a fine microstructure, but with high tensile stresses after printing. Machining of PBF-LB material resulted in higher tool wear for the heat-treated material, increase of 49%, and significantly higher tensile surface stresses followed by shallower compressive stresses below the surface compared to the PBF-EB materials, but with no superficially deformed surface. It is further observed an 87% higher tool wear for PBF-EB in as-built condition and 43% in the heat-treated condition compared to the PBF-LB material. These results show that the selection of cutting tools and cutting settings are critical, which requires the development of suitable machining parameters that are designed for the microstructure of the material.

Effect of spindle speed during ultrasonic rolling on surface integrity and fatigue performance of Ti6Al4V alloy

Ultrasonic rolling refines the surface grains of Ti6Al4V alloy and forms compressive stress. Spindle speed strongly affects the plastic deformation degree and then changes the surface roughness, microstructure and residual stress distribution. Compared with other spindle speeds, 200 rpm can obtain the lowest roughness of 0.049 μm, the deeper compressive stress field of ∼ 680 μm and greatest gains to the fatigue resistance of Ti6Al4V alloy. Excessive spindle speed will cause negative effects. High surface integrity, deep compressive stress field and surface nanocrystalline effect are considered to be important reasons for ultrasonic rolling to improve the fatigue performance of titanium alloy.

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.

FE simulation for stress distribution and surface deformation in Ti-6Al-4V induced by interaction of multi scale laser shock peening parameters

The article investigates the residual stress distribution and surface deformation induced with Laser Shock Peening (LSP) using Finite Element Method (FEM) model.LSP surface treatment is complex phenomenon with huge number of parameters interaction in nanosecond span of time to induce deep residual stress. ABAQUS Explicit Dynamic FEM is used with Johnson-Cook material model for analysing non linear constitutive behaviour of Ti6Al4V and compared with published experimental results. A fine correlation is observed for simulated model and literature results at particular peening condition. A Design of Experiment based simulation of LSP is performed to analyse the effect of Laser spot diameter, laser spot overlapping rate, Laser power density, Number of laser shots and laser pulse time on residual stress distribution and surface deformation. Fractional factorial design is adopted for designing the experiments with five factors having three levels each. Equal in levels of laser spot overlap, laser power density and number of shots will gradually increase in residual stress induced and huge surface deformation with respect to laser pulse duration. Random interactions of these parameter levels have two different type of results; deep compressive stress with minimum deformation and reversal of compressive to tensile stress with huge deformation on surface followed by deep compressive stress.

Effect of Treatment Area on Residual Stress and Fatigue in Laser Peened Aluminum Sheets

Two 2.0-mm-thick aluminum sheets were laser peened and the resulting residual stresses were measured using incremental hole drilling, surface X-ray diffraction, and synchrotron X-ray diffraction techniques. Laser peening was applied to two samples using the same laser peening parameters, but one of the samples has a larger peened area. The aim of this research was to discover the effect of peen area on residual stress, for application in aerospace structures for fatigue life enhancement. It was found that a larger peened area has higher and deeper compressive stresses in the crack-opening direction, leading to greater enhancement of fatigue life.

Simulation and Measurement of Through–Wall Residual Stresses in a Structural Weld Overlaid Pressurizer Nozzle

Full structural weld overlays (FSWOLs) have been used extensively as a repair/mitigation technique for primary water stress corrosion cracking in pressurizer nozzle dissimilar metal (DM) welds. To support an approved FSWOL design and safety submission for British Energy pressurized water reactor (PWR) nozzles, an in-depth evaluation was performed to assess the effects of a FSWOL on the through wall residual stress distribution in safety/relief pressurizer nozzles. Two safety/relief pressurizer nozzle mockups were fabricated based on British Energy’s PWR nozzle design. One mockup included the nozzle to safe-end DM weld and the safe-end to stainless steel weld, while the second mockup included the DM weld, the stainless steel weld, and a Westinghouse designed structural weld overlay. The mockups were fabricated utilizing materials and techniques that represented the plant specific nozzles as closely as possible and detailed welding parameters were recorded during fabrication. All welds were subsequently nondestructively evaluated (NDE). A thorough review of the detailed fabrication records and the NDE results was performed and several circumferential positions were selected on each mockup for subsequent residual stress measurement. The through wall residual stress profiles were experimentally measured through the DM weld centerline at the selected circumferential positions using both the deep-hole drilling (DHD) and incremental deep-hole drilling (iDHD) measurement techniques. In addition to experimental residual stress measurements, the through-wall residual stress profiles were simulated using a 2D axisymmetric ansys™ finite element (FE) model. The model utilized the application of temperature constraints on the weld elements to simulate the thermal welding cycle which greatly simplified the simulation as compared with detailed heat source modeling methods. Kinematic strain hardening was used for material modeling of the weld and base metals. A range of residual weld stress profiles was calculated by varying the time at which the temperature constraints were applied to the model. The simulation results were compared with the measurement results. It was found that the effects of the FSWOL were principally threefold. Specifically, the FSWOL causes a much deeper compressive stress field, i.e., the overlay shifts tension out toward the outside diameter (OD) surface. Furthermore, the FSWOL reduces tension in the underlying dissimilar metal weld, and finally, the FSWOL causes higher peak compressive and tensile residual stresses, both of which move deeper into the nozzle wall after the overlay is applied. Relatively good agreement was observed between the FE results and the measurements results.

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.

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.

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.

An analysis of the residual stresses generated in Inconel 718™ when turning

Inconel 718 is one of a family of nickel based superalloys that are used extensively by the aerospace industry in the hot sections of gas turbine engines. The literature detailing the effects of varying operating parameters on tool life when machining nickel based superalloys is comprehensive, however, relatively little of this data refers to their effects on machined workpiece surface integrity and residual stress generation. Greater knowledge of the effects of operating parameters on surface integrity is critical to the acceptance of new cutting tool materials, tool geometries and strategies. The paper initially reviews prior work on the surface integrity achieved when turning Inconel 718. Following on from this a series of experiments evaluating the effects of varying cutting tool material, geometry, wear level and operating parameters are detailed. The results show that the largest influence on surface integrity was tool wear. Cutting with a worn tool resulted in greater microstructural deformation, microhardness changes and high surface tensile stresses. High tensile stresses were also formed in the surface layer when cutting with a coated tool, while cutting with an uncoated tungsten carbide insert at the same operating parameters produced deep compressive stresses beneath a reduced tensile layer.

Selection of Materials for High-Speed Motor Rotors

We present strength and fatigue characteristics of materials suitable for high-speed motor rotors used in centrifugal compressors. We established that laser shock peening efficiently produces deep compressive stresses and significantly decreases the fatigue crack growth rate in titanium alloys. For titanium alloys of all types considered, the microstructure of the heat-affected zone after laser treatment significantly differs from the microstructure of the original alloy. Grain coarsening occurs both in unalloyed titanium and in near-alpha, alpha–beta, near-beta, and metastable alloys. For practical applications, titanium alloys require additional heat treatment. In particular, aging and mechanical treatment are used for increasing strength characteristics.