Residual Stresses In Specimens Research Articles

Comparative study of the effects of conventional shot peening and ultrasonic shot peening on very high cycle fatigue properties of GH4169 superalloy

This study investigates the effects of two different shot peening techniques, conventional shot peening (CSP) and ultrasonic shot peening (USP), on the very high cycle fatigue (VHCF) properties of the GH4169 superalloy. The study evaluates and compares surface roughness, X-ray diffraction patterns, microhardness, and residual stress in specimens treated with CSP and USP. The VHCF tests reveal that both CSP and USP treatments improve the VHCF properties of the GH4169 superalloy. However, while the USP-treated specimens have deeper work hardening and compress residual stress layers, their VHCF lives are significantly shorter than those of the CSP-treated specimens. Additionally, the study finds that the stress triaxiality, which inhibits dislocations at the crack tip, is higher closer to the center of the specimen. The transmission electron microscopy (TEM) results further confirm that the reduced dislocation density and increased stress concentration in the plastic zone of the high stress triaxiality region promote fatigue short crack propagation and reduce the fatigue properties of the material.

Mechanism of Residual Stress Generation in Quenched Fe-Ni Alloy Cylinders Using Simulated Strains-Based Approach*

Abstract To elucidate the mechanisms of residual stress generation in quenched steel parts has been a longstanding problem. Qualitative explanations for the changes in the stress distribution in quenched cylinders appeared in the 1930s when it became possible to measure residual stresses in specimens. The explanation at that time used the concept of thermal and transformation stresses, which is still included in current textbooks. This concept is referred to here as the estimated stresses-based approach. To simplify the explanation of stress generation due to the combined effects of temperature change and phase transformations, quenching experiments were devised using Fe-Ni alloys in which only martensitic transformation occurs. On the other hand, since heat treatment simulations provide strains resulting from thermal, phase transformation, plastic, transformation plastic, and creep phenomena, a method to elucidate the mechanism using these strains was devised in the early 2000s and named the simulated strains-based approach. This paper contrasts the estimated stresses-based and simulated strains-based approaches to the mechanism of residual stress generation in quenched Fe-Ni alloy cylinders and highlights the superiority of the latter.

The issue of residual stresses in additive technologies

The paper considers the issue of residual stresses in 3D-printed plastic models. Most additive technologies create residual stresses in products. Residual stresses occur in the printed material due to its expansion when heated and contraction when cooled. Residual stresses and their intensity depend on the printing technology and technique. The paper discusses the impact of printing techniques and various printing nozzle diameters and model shapes (rectangular, circular) on the occurrence of residual stresses in specimens. As part of the study, in transparent models, residual stresses were detected using a PPU-7 polarization-projection unit. Two series of six specimens each have been printed. The first and second series models had the shape of a parallelepiped and a disk, respectively. The frequency-division multiplexing technology was chosen. In the study, the models were manufactured from a polyethylene terephthalate-glycol plastic filament. This material has a high optical sensitivity. Nozzles of two diameters (0.4 and 1 mm) were used to print specimens. Shell-less and singleand double-shell specimens were printed. The dependence of residual stresses on the specimen shape, the printing nozzle diameter, and the model shell thickness has been estimated. The study is focused on finding a technique for printing models from plastic filament, completely free of residual stresses in the specimen material. This is dictated by the photoelasticity requirements for piezo-optical materials, including some transparent plastics used in 3D printing.

Исследование остаточных сварочных напряжений при подводной мокрой сварке низколегированной стали с применением магнитоанизотропного и рентгенодифрактометрического метода

The present study possibility of assessing residual welding stresses using magnetoanisotropic and X-ray diffractometric methods. Using the method of magnetic anisotropy approve of inhomogeneity of residual stresses in specimens with surfacing made by flux-cored wire and covered electrode in air and underwater has been revealed. The use of two nondestructive testing methods made it possible to determine and compare the values of residual welding stresses.

Effects of diameter and preparation of round shaped tensile specimen on mechanical properties

The effect of specimen size and specimen preparation with round cross-sectional area on tensile properties in twinning-induced plasticity (TWIP) steel has been investigated. Round type tensile specimens were prepared with two ways: the one is lathe processed specimen and the other is mechanically and chemically polished specimen after lathe processing. And then, tensile tests with various specimen diameters from 2 mm to 13 mm at the fixed ratio of gage length to diameter of five were conducted. The polished specimens had similar yield strength (YS), tensile strength (TS), total elongation (TE), and reduction in area (RA) values regardless of specimen diameter. Whereas, lathe processed specimens had a variation in tensile properties with specimen size: YS increased and TE decreased with decreasing specimen size. The deformation twins was occurred in the surface layer of lathe processed specimens of undeformed and at the engineering strain of 10%, which is not related to the tensile stress during tensile test but to the torsional stress or compressive stress during specimen preparation by lathe process and torsional stress during tensile test. The tensile properties with specimen diameter were well explained using the concept of fraction of initially generated deformation twins during specimen preparation. Finally, it is recommended that bigger specimens should be used as much as possible to reduce the surface effect of specimens and experimental errors. For instance, the gage diameter of 5 mm was enough to ignore the effect of specimen preparation and size. Inversely, the surface residual stress in specimen needed to be removed before tensile test when small tensile specimen machined by lathe process was used.

Assessment of Residual Stresses in Specimens of Ferritic-Pearlitic Steel with Austenitic Cladding

A bimetallic reactor vessel material (steel 15Kh2MFA with austenitic cladding) is studied. The residual stresses arising in the bimetal after heat treatment are determined with the help of x-ray diffraction, acoustic and computer control techniques. The causes of the appearance of plastic strain in the specimens are assessed and the deflections and the residual stresses after quenching and annealing are determined.

Effect of shape memory alloy wires on high-velocity impact response of basalt fiber metal laminates

Fiber metal laminates are commonly used materials in industrial applications due to low density and excellent mechanical properties such as impact resistance. In this study, the effect of shape memory alloy wires on the impact properties of fiber metal laminates is evaluated and discussed. The volume fraction of wires and the pre-strain of wires were selected as the variables of the experiments. The samples were fabricated using hand lay-up method and their behavior was evaluated under high-velocity impact test. Based on the results, a significant change in the amount of absorbed energy was obtained using embedded wires. However, with increasing the volume fraction of wires, the absorbed energy decreases due to the discontinuity in mechanical properties of the specimens. On the other hand, applying pre-strain leads to residual stresses in samples. The observed residual stress in specimens with four embedded wires and 2% pre-strain level shows the most absorbed energy.

Influence of the phase composition and microstructure of plasma cladding Fe-Cr-Ni-C alloy coating on residual stress and crack formation

The Fe–Cr–Ni–C alloy coatings with different phases have been prepared by plasma cladding technique on high manganese austenitic steel preheated with different temperatures. The effect of the phase composition on crack formation, residual stress, and micro-hardness of Fe–Cr–Ni–C alloy coatings was investigated by scanning electron microscopy (SEM) with energy-dispersive spectrum (EDS), micro-hardness tester, infrared thermal imager, and residual stress tester. The results show that for Fe-based coating deposited on the substrate non-preheated, the transverse and perpendicular cracks are generated after the end of cladding, which belongs to transcrystalline fracture. The Fe–Cr–Ni–C alloy coatings are composed of the solid solution of austenitic and fishbone-like eutectic carbide, which was mainly (Cr, Fe)7C3. As the preheating temperature of substrate is 350 °C, granular metallic carbides with the diameter 1 μm are precipitated at grain boundaries and no cracks formed. The carbide phases are mainly (Cr, Fe)7C3 and (Cr, Fe)23C6 in cladding coatings. With the decreased rate of cooling, carbide precipitation phase changed from metastable (Cr, Fe)7C3 to stable (Cr, Fe)23C6 phase. The precipitation stable carbide phase has significance to prevent the formation of cracks and improves the micro-hardness of the Fe-based coating on the substrate preheated to 350 °C. The residual stress of specimen with substrates of non-preheating and preheating 250 and 350 °C is 234.2, 267.3, and 300.6 MPa, respectively. The maximum value of micro-hardness of coating with substrates of non-preheating is 986.1 HV, which is about six times than that of substrate.

Reduction Method for Residual Stress of Welded Joint Using Two Ultrasonic Vibrations with Different Frequencies

A reduction method of tensile residual stress using two ultrasonic vibrations with different frequencies is proposed. Advantage of the method to using one ultrasonic vibration is examined. Obtained results are summarized as follows. 1. Thick plate is buildup welded using ultrasonic vibration is used during welding in experiment. Residual stress of specimen using one ultrasonic vibration is reduced compared with that without ultrasonic vibration. Residual stress of specimen using two ultrasonic vibrations is reduced compared with that using one ultrasonic vibration. 2. Using two-dimensional model, residual stress is calculated by simulation method. Same results as experiment are obtained.

Study of Residual Stress Relaxation Mechanics Using a Micro-indent Method

The aim of this study is to show the feasibility and accuracy of a micro-indent method to comprehensively evaluate the relaxation mechanics of residual stress in specimens of rolled aluminium alloy. This micro-indent method was adapted to implement four annealing treatments. The residual displacements were measured using a high-accuracy measuring machine, which enables to decrease the absolute error down to ±300 nm. This study presents an innovative data analysis using Mohr's circles, which allowed to study all directions of the in- plane residual stress for different relaxation times. The results revealed that the relaxation process finishes when the residual stress relief is completed in some preferential directions and then, a brief stress recovery phase begins.

Increase in Strength of Partially Stabilized Zirconia After Shot Peening

The effects of shot peening (SP) on the strength of partially stabilized zirconia (PSZ) were studied. The compressive residual stress, apparent fracture toughness (K C), and bending strength values of specimens subjected to SP were investigated. Results of x-ray diffraction analyses showed that SP introduced large compressive residual stress in specimens. As a result, the K C and bending strength values of specimens having semi-elliptical pre-cracks on their surfaces increased significantly. Shot-peened specimens having surface pre-cracks with lengths less than 140 µm exhibited strength comparable to that of smooth specimens and fractured outside the pre-crack zone, indicating that the pre-cracks were rendered harmless by SP. Thus, the introduction of a compressive residual stress by SP is an effective technique for increasing the strength of PSZ.

Characterization of materials' elasticity and yield strength through micro-/nano-indentation testing with a cylindrical flat-tip indenter

Abstract

Experimental Study on Residual Stress in Titanium Alloy Laser Additive Manufacturing

The residual stress in laser additive manufacturing titanium alloy sample was measured using indentation stress measurement method. The residual stress variation formulas was fitted with the major process parameters such as laser power, scanning speed, the powder feed rate etc.. It was studied that the influence of processing layers and scanning corner on the specimen residual stress. The results show that the specimen residual stress increases first and then decreases with the increase of processing layers, and the maximum appears in the fiftieth layer, in addition, the residual stress in the side of corner sample is mainly pressure stress, the maximum appearing in the 150°scanning angle, the minimum appearing in the 120°scanning angle. Therefore, it can reduce the overall sample residual stress effectively by an obtuse angle scanning trajectory.

Determination of Remnant Residual Stresses in Fracture Toughness Specimens Extracted From Large Components

Abstract: Material fracture toughness data are required to undertake fitness‐for‐service assessments of engineering components containing cracks. Calculations of crack driving force in the component are compared with material fracture toughness values to assess the likelihood of subsequent failure. Experimental measurements of fracture toughness are usually made on small specimens extracted from a larger ‘parent’ component following strict experimental guidelines, formulated to ensure measured toughness values in the fracture specimens are appropriate for use in the full‐size component. Implicit in this procedure is the assumption that the extracted fracture specimens contain no residual stresses, with any residual stresses in the full‐size component being accounted for in the crack driving force calculation. This paper considers a recent conjecture within the structural integrity community that the extracted fracture specimens may themselves contain a residual stress field which may influence measurements of fracture toughness. This could potentially lead to a degree of ‘double accounting’, i.e. the effect of residual stresses may be included in both the material toughness and the crack driving force. This, in turn, could lead to unnecessary conservatism in safety assessments. To explore this conjecture, the results of numerical modelling and neutron diffraction measurements of residual stresses in fracture specimens extracted from two different welded parent components are presented. One of the components is significantly larger than the extracted specimens, with the other being marginally larger than the extracted specimens. Results confirm the intuitive expectation that the residual stresses in specimens extracted from much larger components are negligible, whereas if the dimensions of the extracted specimens are comparable with the larger component then significant residual stresses may remain.

Study on Cooling Performance of Eject Quenching by Mixture of Nitrogen-Spray Water

Though gas quenching has several advantages, such as minimal environmental impact, little deformation, low energy cost, clean products, and ability to control the cooling locally and temporally for best product properties, its cooling performance is less than that of oil and water. If gas is mixed with spray water, the mixture will has favorable cooling performance. The cooling performance of eject quenching by mixture of Nitrogen-spray water is studied experimentally. Alloy steel 9SiCr is specimen, and it is quenched by a certain proportion of mixture gas of Nitrogen and spray water under normal pressure and high velocity, and the continual cooling curve is determined. The experimental results show that it can improve the cooling effect of quenching medium on the one hand; on the other hand, it can decrease the deformation and residual stress of specimen. The results also show that cooling performance of the mixture and cooling velocity can be enhanced by increasing gas pressure when the mixture ratio is fixed, and it takes short time to cool to room temperature. The cooling performance of the mixture and cooling velocity can be enhanced by increasing the mixture ratio under same pressure, and it also takes short time to cool to the room temperature. The continual cooling curve is determined, which gives the information of cooling performance of eject quenching by mixture of Nitrogen-spray water under Normal pressure changed with the pressure and the mixture ratio of Nitrogen and spray water.

A residual stress evaluation in laser welded lap joint with hole drilling method

Residual stresses deteriorate strength of materials affecting the quality of industrial products. A removal or reduction of the residual stresses is an essential procedure in successful engineering component development. Effective and convenient methods are necessary for detection and evaluation of residual stresses. In this paper Hole Drilling Method is chosen for identification and a quantitative determination of the residual stresses in specimens of two groups under different laser welding speed conditions, 4.1m/min and 5.1m/min. The lap joints welded with STS 301L sheet of two different speeds were investigated along the welding line at two locations, the middle and the end location in the heat affected zone (HAZ). The identification of HAZ is carried out by taking hardness values from weld centerline to the raw material. Based on the experimental results and analysis, it is found that higher welding speed reduces the residual stresses. Also, the end location is found to be higher residual stress area compared with middle location due to the convective boundary condition. The residual stresses decrease as the depth increases from the top surface due to the lower heat input to depth increments.

Explanation of the origin of quench distortion and residual stress in specimens using computer simulation

Measurements of distortion and internal residual stresses after quenching in specimens (e.g., cylinders, disks, rings) have been performed for many years. As a result, patterns of distortion and stress distribution have been identified in the typical shapes, steels and cooling conditions. These experimental data were examined by pioneers of the heat treatment simulation for verifying their computer programs in the 1970s and 1980s. However, early researchers did not fully explain the mechanism of distortion and residual stress generation based on their simulated results. A way to explain the generation mechanism using simulated results, especially distributions of all types of strains, has been recently developed and successfully applied to some experimental works by the authors of this paper. Its concept is described briefly, and its applications for some specimens are summarised.

Development of residual stress and damage in thermal barrier coatings

Photo-luminescence piezo-spectroscopy through the electron beam deposited (EBPVD) yttria-stabilised zirconia (YSZ) thermal barrier coating has been used to study the evolution of residual stress due to the alumina thermally grown oxide (TGO) formed on Pt–Al bond coats on CMSX-4 substrates. The coated specimens were thermally cycled between room temperature and 1150 °C, holding for 1 h at the top temperature. The experiments explored the effects of two different bond coat surface finishes (normal and polished) and thickness of YSZ from 70 to 270 μm. In most cases the mean TGO stress was found to gradually decrease with thermal cycling in agreement with previously published results. This is mainly due to rumpling increasing the waviness of the TGO and hence the ability to relieve stress by bending. For the same reason, the TGO residual stress in specimens with polished bond coats was higher initially than in those with a normal surface finish. Stress mapping showed that there are regions, approximately 100 μm in size, where the stress is particularly low (< 1 GPa compressive) and these were attributed to local TGO damage or delamination. The incidence of these low stress “events” increased markedly just before coating spallation. The failure was in the TGO, or in the YSZ close to the TGO, probably as a result of weakening in this region by the damage induced by rumpling and driven by the stored energy in the YSZ.

Residual stresses in specimens of VT3-1 alloy with titanium-nitride coatings

Residual stresses in specimens of VT3-1 alloy with titanium-nitride coatings

Residual Stresses Induced by Laser FreeForm Fabrication

In this paper we report our attempts to use the neutron and X-ray diffraction techniques to characterize residual stresses in specimens manufactured by laser FreeForm technique. The aim of our work has been to understand how residua stresses develop during forming and their possible correlation with the microstructure, in particular the pore density which varies with the laser scan speed. The specimens under investigation were built by layer-sintering a ferritic-steel powder with a Cu-based-alloy powder as the binder material. Limited results have been obtained for the ferrite steel phase and are presented in the paper.