Thermal Residual Stress Relaxation Research Articles

Measurement of microscopic strain distributions of CFRP laminates with fiber discontinuities by sampling moiré method

Carbon fiber-reinforced plastics (CFRP) laminate structures manufactured using prepregs usually contain fiber discontinuities that result from the prepreg cutting for design and cost-efficiency. In this study, the sampling moiré method was used to measure the microscopic strain of unidirectional CFRP laminates with fiber discontinuities to investigate the deformation behavior that causes damage. In addition to the strain distributions of the laminate before and after cracking, the strain results due to thermal residual stress relaxation and residual plastic deformation during cracking were also obtained. The experimental results of this study were in good agreement with the finite element method results. Our results revealed that the sampling moiré method is an accurate method of measuring microscopic strain that can be used to evaluate the mechanical properties, damage behavior, residual strains in laminates with fiber discontinuities, and other different materials and structures.

In situ observation of the deformation and fracture of an alumina-alumina ceramic-matrix composite at elevated temperature using x-ray computed tomography

The deformation and fracture of a 0/90° Nextel 720 mullite fibre / alumina matrix ceramic-matrix composite material has been observed by in situ synchrotron x-ray computed micro-tomography at ambient temperature and at 1100 °C. A three-point loading configuration was used and samples were loaded monotonically until final fracture. The flexural strength was found to be unaffected by temperature. 3D visualisation of the microstructure showed fracture occurred by propagation of internal cracks at stresses close to the flexural strength. Shear fractures propagated across fibre bundles that were oriented perpendicular to the flexural stress at both room temperature and 1100 °C. At the higher temperature, an additional failure mode of delamination was observed, that may arise from relaxation of thermal residual stresses and creep.

The effect of heat treatment on tensile properties of 2D C/SiC composites

AbstractTwo‐dimensional (2D) carbon fiber reinforced silicon carbide (C/SiC) composites with different initial strength were prepared by chemical vapor infiltration (CVI). After tensile property testing, results exhibited that as the heat‐treatment temperature (HTT) increases to 1900°C, the tensile strength and toughness of the low strength specimen (LSS) increased by 110% and 530%, while the high strength specimen (HSS) increased by 5.4% and 550%, respectively. As observed from morphologies, the heat treatment increases the graphitization of the amorphous PyC interphase, which leads to the weakening of interfacial bonding strength (IBS). Meanwhile, the defects arising from heat treatment cause thermal residual stress relaxation. Therefore, the tensile strength and toughness of LSS with relatively high initial IBS increase significantly as HTT increases. For HSS with moderate initial IBS, the heat treatment slightly improves the tensile strength, but significantly improves the toughness. Consequently, the post‐heat‐treatment tensile properties of 2D C/SiC composites can be regulated by varying HTTs and different initial strength.

Heatproof Relaxation of Welding Stresses Openwork Designs of Large Size Ultrasonic Field

Designed experimental setup, and a prototype of the original devices for athermic relaxation of thermal residual stresses in openwork welded constructions of large dimensions for the rational use of energy high-frequency acoustic (ultrasonic) fields, conducted laboratory studies and pilot tests in real conditions of production. Ultrasonic relaxation when welding thin-walled constructions standard openwork rolled steel profiles 10 large dimensions (up to 3 x 10 m). It has been established that with a decrease in the time of ultrasonic relaxation, the efficiency of its use decreases somewhat. Therefore, in an industrial environment, it is advisable to use the duration of ultrasonic treatment in the range of 0.02-0.04 min per mm of weld length.

Thermal relaxation of residual stresses, arising in heat-resistant materials after application of laser shock processing technology

The study of thermal relaxation of residual stresses in heat-resistant steel 37Kh12N8G8MFB (GH2036) after the application of laser shock processing technology and subjected to annealing at temperatures up to 650°C. For estimating residual stresses and their relaxation from the action of high temperature a calculation model has been proposed. The high efficiency of the proposed technology for hardening the surface layers of thermally loaded material was shown.

Thermal mismatch stress relaxation and dislocation transformation of 45%SiCp/Al composites by continuous diode laser heating

The purpose of the study was to investigate the effect of laser beam scanning on residual stress relaxation and metallographic structure evolution of 45%SiCp/Al composites with an increase in the temperature. Based on the theory of elasticity and equivalent inclusions, the residual stresses distribution of individual component in 45%SiCp/Al composites with an increase in the temperature are obtained. The results demonstrated that the speed of stress relaxation for matrix and interface was faster with the temperature variation from 25 to 300 °C comparing with the temperature variation from 300 to 500 °C. The dislocation configurations and morphologies of second precipitated phases with laser treatment were characterized using the optical microscopy and transmission electron microscopy. It is found that the thermal residual stress relaxation mechanism with an increase in the temperature was caused due to the evolution of dislocation density and dislocation structure arrangements. A physical model of dislocation and grain evolutionary process with an increase in the temperature was proposed, and the effects of laser scanning on precipitated phase was analyzed in view of the interaction mechanism between the mobile dislocations and the precipitated phase. The nanoindentation test was also carried out to analyse the effect of residual stress and dislocations configuration on the mechanical response of individual component.

Thermal relaxation of residual stress in shot-peened CNT/Al–Mg–Si alloy composites

A carbon nanotubes (CNTs)-reinforced Al–Mg–Si alloy composite was made and shot peened. After shot peening, a compressive residual stress field was generated in shot peened layer. The change law of residual stress and microhardness with annealing time at elevated temperatures (150°C, 200°C, and 250°C) was investigated. An Al–Mg–Si alloy without CNT addition is prepared in the same process to compare with the CNTs/Al–Mg–Si alloy composite to verify the effect of CNTs on residual stress relaxation. Results showed that for the both materials, macro and micro residual stresses in the shot-peened layers decreased firstly due to thermal recovery and then trend to be stable during annealing process. Increasing the annealing temperature could accelerate the speed of stress release and increase the decrement rate. Specially, the addition of CNTs could improve the thermal stability of the macro residual stress created by shot peening.

A low cycle fatigue life model for a shot peened gas turbine disc alloy

Turbine disks in gas turbines are subjected to cyclic load at high temperature, making, especially the fir tree type blade attachments, susceptible to fatigue. Shot peening of the fir tree attachments may be used to increase the fatigue life by introducing compressive residual stresses. In the current study, both polished and shot peened notched specimens made from alloy 718 were subjected to low cycle fatigue at 450–550 °C. The shot peening generally increased the fatigue life, although the effect diminished for high loads. It was shown that the effect of shot peening could be handled as mean stress effects in a life model based on a Smith–Watson–Topper (SWT) type parameter, σmaxΔε/2. A material model which captured the mean stress was set up to get the SWT parameter at the notch root. It was shown that thermal relaxation of residual stresses and initial strain hardening from cold work could be excluded from the finite element analysis used to establish the mean stress; this since the plasticity in the first cycle dominated the plastic deformation of the specimen. Overall, the SWT-based life model worked satisfactorily. However, the prediction of correct mean stresses at 550 °C proved somewhat difficult as the degree of mean stress relaxation at this temperature varies widely in available literature data.

Investigation on thermal relaxation of residual stresses induced in deep cold rolling of Ti–6Al–4V alloy

In the present study, a finite element model has been developed to simulate the deep-cold rolling process on Ti-6Al-4V specimens and the following short term exposure of the treated components to elevated temperature. The developed model can be effectively used to predict the residual stress profiles induced by the process at room temperature and the following residual stress relaxation at the elevated temperature. In the present study, the thermal relaxation stage is performed using a viscoplastic model which couples the creep and plasticity deformation mechanisms to predict the state of residual stresses at the elevated temperature. For this purpose, a new set of hyperbolic creep law coefficients are identified in order to describe the primary creep at 450 °C. The accuracy of the developed finite element model to predict residual stresses is validated by comparison with the experimental data available in the literature. It has been shown that the finite element predictions correlate well with experimental results with error generally less than 10%. The developed model can be effectively utilized for parametric studies to understand the effect of different process parameters on the induced residual stresses without performing expensive experimental tests.

Effect of oxidation and residual stress on mechanical properties of SiC seal coated C/SiC composite

The effect of oxidation and thermal residual stress on mechanical properties of SiC seal coated C/SiC composite at ambient temperature and high temperature were studied. The oxidation of SiC seal coated C/SiC composite at 1300 and 1500°C resulted in carbon fibres burn area near through thickness micro cracks in the SiC seal coating. With the increase in exposure time, the formation of SiO2 layer in SiC matrix near carbon fibres burns area was found. Residual mechanical properties of SiC seal coated C/SiC composite after exposure in air show significant degradation. First time, a continuous measurement of Young's modulus with temperature of C/SiC composite was carried out using an impulse excitation technique. The effect of relaxation of thermal residual stress on mechanical properties was observed with the help of continuous measurement of Young's modulus as a function of temperature in an inert atmosphere.

New technologies of manufacturing non-rigid parts made of titanium alloys and stainless steels

The article considers a new approach to solving a topical scientific problem of providing dimensional stabilization of non-rigid aircraft parts made of titanium alloys and stainless steels. It is known that a significant loss of accuracy and a change in the spatial orientation of surfaces of non-rigid parts are associated with the formation of undesirable technological residual stresses in the surface layers. The authors of the work propose a method of reducing the magnitude of residual stresses in the surface layer of cylindrical parts by ultrasonic treatment with an indenter having strip contact with the workpiece. We present the results of experimental research aimed at eliminating the lengthy operation of thermal relaxation of residual stresses in the process of manufacturing non-rigid parts made of titanium alloys and stainless steels through the rational use of technological heredity phenomena and ultrasonic field energy. The developed method is highly efficient and can be used to stabilize dimensional accuracy of non-rigid and thin-walled parts made of hard-to-machine materials.

High‐temperature tensile strength and fracture behavior of ZrB2‐SiC‐graphite composite

The tensile behavior of ZrB2‐SiC‐graphite composite was investigated from room temperature to 1800°C. Results showed that tensile strength was 134.18 MPa at room temperature, decreasing to 50.34 MPa at 1800°C. A brittle‐ductile transition temperature (1300°C) of ZrB2‐SiC‐graphite composite was deduced from experimental results. Furthermore, the effect of temperature on the fracture behavior of ZrB2‐SiC‐graphite composite was further discussed by microstructure observations, which showed that tensile strength was controlled by the relaxation of thermal residual stress below 1300°C, and was affected by the plastic flow during 1300°C and 1400°C. At higher temperature, the tensile strength was dominated by the changes of microstructures.

Relaxation of Thermal Residual Stress in Laser Irradiated Fused Silica by Annealing Process

To relax the surface residual stress of fused silica lens or windows irradiated by CO2 laser, it was proposed to treat it at high temperature above glass strain temperature in the traditional annealing process. However it is a time and energy consuming process, and a distortion will be introduced during the heat treatment. To deal with these problems, annealing temperatures lower than glass strain temperature were applied to the annealing process and a new scheme was designed in this paper. An numerical model was built to simulate the laser induced residual stress and optimize the stress variation in the annealing process. The surface stress of fused silica can be relaxed and deformation induced by heat treating can be ignored.

Laser shock processing on Ni-based superalloy K417 and its effect on thermal relaxation of residual stress

The compressive residual stress fields at ambient temperature and stress relaxation at elevated temperatures of nickel-based alloy K417 by laser shock processing (LSP) were investigated by the finite element analysis and experimental method. A temperature and strain rate-dependent plasticity model of Johnson–Cook (JC) material model was utilized to investigate the residual stress evolution under LSP and thermal loads. The residual stress distributions of original and processed specimens were measured by the X-ray diffraction method. Zener–Wert–Avrami function was employed to study the thermal relaxation process of LSP K417, and the simulated results have a significant agreement with the experimental data. Specifically, most of stress relaxation occurs before 30 min of exposure, and the relaxation amplitude increases with the rise of applied temperature. The activation enthalpy of the relaxation process for LSP K417 obtained from simulation and experiment are determined to be 2.86 and 2.85 eV respectively.

Laser peening without coating induced phase transformation and thermal relaxation of residual stresses in AISI 321 steel

Abstract Laser peening without coating (LPwC) was performed on austenitic stainless steel AISI 321 with a power density of 5.97 GW cm − 2 and pulse densities of 1600, 2500 and 3900 pulses cm − 2 . A large compressive residual stress resulted for 1600 pulses cm − 2 . Surface roughness was found to have increased to a relatively smaller extent with lesser number of valleys or peaks. Micro-hardness increased after the process with the depth of hardened layer extending up to 900 μm. LPwC induced martensitic phase transformation (about 18% in volume fraction) was observed near treated surface. Microstructures confirmed the phase transformation effect. The grains were not refined as a result of single LPwC scan. Thermal relaxation of residual stresses at 700 °C for 2 h were 41% in longitudinal and 140% in transverse (relative to LPwC scanning) directions. The influential factors on stress relaxation were discussed. No significant reduction in hardness was seen after thermal treatment. Followed by the thermal treatment, fully austenitic phase was recovered.

激光喷丸IN718 镍基合金残余应力高温松弛及晶粒演变特征

激光喷丸IN718 镍基合金残余应力高温松弛及晶粒演变特征

High temperature fracture behavior of ZrB2–SiC–graphite composite in vacuum and air

Hot pressed ZrB2–SiC–graphite ultra high temperature ceramic composites were prepared for the fracture toughness tests in vacuum up to 1800°C and in air up to 1300°C. Measured results show that a brittle–ductile transition temperature of ZrB2–SiC–graphite composite is deduced to be 1300°C, and ZrB2–SiC–graphite composite exhibits a linear behavior below 1300°C. Fracture toughness in vacuum linearly decreases with the increase of temperature below 1300°C, increases from 1300°C to 1600°C, and has a rapid reduction at 1800°C. The phenomenon can be attributed to the relaxation of thermal residual stress under 1300°C, the existence of plastic flow from 1300°C to 1600°C, and the degeneration of graphite flakes and cavitation formation at 1800°C. In comparison to vacuum, the fracture toughness in air improves by the healing effect of pre-oxidation below 1300°C. The microstructure observations further reveal the great effects of temperature and pre-oxidation on the fracture behavior of ZrB2–SiC–graphite composite.

Experimental and Finite Element Simulation Study of Thermal Relaxation of Residual Stresses in Laser Shock Peened IN718 SPF Superalloy

An integrated experimental and modeling/simulation approach was developed to investigate and secure a quantified knowledge of the impact of high temperature exposures on the stability of residual stresses in a laser shock peened (LSP) high temperature aero-engine alloy, IN718 SPF (super-plastically formed). Single dimple LSP and overlap LSP treatments were carried out utilizing a Nd:Glass laser (λ = 1.052 μm), and subsequent heat treatments on the LSP-treated coupons were conducted at different temperatures between 550 and 700 °C. A 3-D nonlinear finite element (FE) computational model and the rate-dependent Johnson-Cook material model were calibrated using the experimental results of residual stress from the single dimple LSP and thermal relaxation treatments, and were further extended to the overlap LSP treatment case. Both experimental and FE simulations show that: a) a high level of compressive residual stress (~700 MPa at surface) and residual stress depth (~0.4–0.6 mm) were achieved following LSP, and b) the overlap LSP treatment gave higher residual stress and greater depth. The magnitudes of the initial residual stress (and plastic strain), heating temperature and exposure time were identified as the key parameters controlling the thermal relaxation behavior. The stress relaxation mainly occurs initially before 20 min exposure and the extent of relaxation increases with an increase in temperature and a higher magnitude of the initial as-peened residual stress. In addition, in regions deeper than ~300 μm or after initial thermal exposure where the residual stress was lower than ~300 MPa, stress relaxation was found to be negligible. Kinetic analysis of the experimental thermal relaxation data based on Zener-Wert-Avrami model gave an activation enthalpy of 2.87 to 3.77 eV, which is near that reported in the literatures for volume and/or substitutional solute diffusion in Nickel. These results suggest that thermal relaxation of the LSP-induced residual stress occurs by a creep-like mechanism involving recovery, rearrangement and annihilation of dislocations by climb.

Dimensional stability of epoxy-based and cyanate-based carbon fiber-reinforced plastics

In this article, the dimensional stability of epoxy- and cyanate-based laminates is discussed, focusing on the thermal deformation, moisture-induced deformation, and deformation induced by relaxation of thermal residual stress. Each of the deformations was calculated independently based on the laminate theory. The material properties of the unidirectional laminates were obtained by conducting thermal mechanical analysis, moisture absorption tests, and tensile creep tests. These material properties were adopted to the laminate theory to predict the deformation of quasi-isotropic laminate, and it was calculated that each type of deformation induced micron-level dimensional instability. The moisture-induced deformation was an order of magnitude larger than that caused by the other factors. The validity of the calculations was confirmed by comparing the calculated results with the experimental ones. It is important to control moisture absorption even if cyanate resin, which has high moisture resistance, is used.

A finite element analysis of thermal relaxation of residual stress in laser shock processing Ni-based alloy GH4169

The residual stress induced by laser shock processing and the thermal relaxation behaviors of residual stress in Ni-based alloy GH4169 were investigated by means of three-dimensional nonlinear finite element analysis. To study the effect of different given exposure time and different temperatures on residual stress in laser shock processing Ni-based alloy GH4169, Johnson–Cook material model was used in order to account for the nonlinear constitutive behavior. The influence of heating temperature and exposure time on the stress thermal relaxation was studied. It was concluded that stress relaxation mainly occured during the initial period of exposure, and the degree of relaxation increased as the temperature risen. The results would provide a theoretical basis for controlling the laser shock processing and guiding subsequent experiments.