Residual Stress Concentration Research Articles

Investigation of micro-yield strength and coefficient of thermal expansion of Al–Cu–Mg–Li–Sc–Ag alloys with various contents of Li

Abstract

Newer vs. older CAD/CAM burs: Influence of bur experience on the fatigue behavior of adhesively cemented simplified lithium-disilicate glass-ceramic restorations

This study evaluated the effect of the CAD/CAM burs experience (newer vs older as consequence of the milling sequence) on fatigue failure load (FFL), number of cycles for failure (CFF), and survival rates of lithium disilicate glass-ceramic simplified restorations adhesively cemented to a dentin analogue substrate. Three sets of CAD/CAM burs were used to mill disc-shaped ceramic specimens (1 bur set – 18 milled discs with 10 mm diameter and 1.5 mm thickness), considering the bur experience as a result of the milling sequence to compose the study groups: G1-6 – discs obtained from the 1st to 6th milling of each bur set; G7-12 – specimens from the 7th to 12th milling; G13-18 – discs from the 13th to 18th. Discs of dentin analogue (G10, 10 mm diameter and 2.0 mm thickness) were made to serve as substrate (base material) and randomly assigned into pairs with the respective ceramic discs. Then, the ceramic discs were adhesively cemented onto the dentin analogue substrate, composing a three-layer specimen that mimics a monolithic restoration of a posterior tooth. Specimens were tested under stepwise fatigue approach: frequency = 20 Hz, 5000 cycles at maximum load of 400 N to accommodate the testing assembly, followed by incremental steps of 200 N with initial load ranging from 10 to 1000 N, to a maximum of 20,000 cycles/each step, until the occurrence of failure (radial crack). FFL and CFF were recorded at the end of the testing and subjected to statistical analysis. Supplementary roughness analysis of the milled surface was performed (n = 18) using a contact profilometer. Residual stress after milling and acid etching were accessed via X-ray Diffractometry analysis. FFL and CFF were not affected by increase on bur experience (no statistical differences among groups), despite that, it affected both Ra and Rz parameters (G1-6 had the smoothest surface). The residual stress concentration was negligible (milling did not induce residual stress concentration). It is concluded that the fatigue behavior of adhesively cemented lithium-disilicate glass-ceramic restorations was not influenced by CAD/CAM bur experience (newer vs older as consequence of the milling sequence), and so the residual stress concentration induced by milling was negligible.

Modeling of Residual Stress Distribution for Components Manufactured by Selective Laser Melting

In this paper, the selective laser melting (SLM) simulation analysis of components is carried out. The residual stress distribution of the formed part was predicted, and the influence of process parameters such as exposure time, laser power and laser scanning speed on the residual stress of the SLM formed part was analyzed. It was found that the residual stress concentration of the formed part was in the middle of the upper surface or the bottom surface. In addition, the laser power and the laser scanning speed have a great influence on the residual stress of the formed part. The results of this study lay a theoretical and experimental basis for the optimization of residual stress and quality control of SLM components.

Study of microstructural degradation of a failed pinion gear at a cement plant

Abstract The failed pinion gear assembly of a cement plant was analyzed by visual, residual stress, microstructure and mechanical property examination to determine the root cause of its failure. It is observed that the worn out region contains several surface micro cracks, especially closer to the tip (addendum) as compared to the root (dedendum) of the gear tooth profile. The surface analysis of the worn out regions showed depletion of protective chromium oxide layer at the surface and formation of iron oxide scales due to corrosion attack by lubricants, contaminants resulted in micro pitting and subsequent wear. Residual stress measurements showed tensile residual stresses at the tip (face) of the gear teeth and large compressive stresses at the root (flank) of the gear teeth. The cross-sectional microstructure examination revealed the damages due to rolling contact surface fatigue in the form of subsurface dark etched region bands and coarsened carbides in the surface. Apart from this, the difference in the tempered core microstructure below the case hardened layer between the flank and face regions lead to compressive residual stress in case of soft core and tensile stress in case of harder core respectively. The concentration of tensile residual stresses due to untempered core at the tip aided the micro-pitting and micro-cracking due to the rolling contact surface fatigue was responsible for the initiation of surface cracks and final failure of the gear.

Residual-stress-induced helical long period fiber gratings for sensing applications.

We demonstrate a high-efficiency grating fabrication system, which can be used to inscribe a high-quality helical long period fiber grating (HLPFG) on single-mode fiber by means of hydrogen-oxygen flame. Such the HLPFG can be produced in enormous quantities with a uniform grating parameters and good reproducibility of grating inscription. Possible mechanisms for refractive index modulation in the HLPFG can be attributed to residual stress concentration by solidifying the periodic twisting stress under a fused status of optical fiber. Moreover, the HLPFG exhibits an excellence performance of high temperature sensing with a high sensitivity of ~132.8 pm/°C and a measuring range from room temperature to 900 °C. Comparing to the traditional LPFG fabricated by CO2 laser or arc discharge technique, the HLPFG has a low the bending and tensile strain sensitivity of 1.94 nm/(1/m) and 1.41 pm/με, respectively. So the proposed HLPFG could have a great potential in special applications as optical high-temperature sensors.

Vacuum brazing of the Cf/C composite and Ni base superalloy using MBF 20 filler

Carbon fiber reinforced carbon matrix (Cf/C) composite and Ni base superalloy were brazed in vacuum using MBF 20 filler. By varying brazing temperatures and holding times, the effects of brazing parameters on the microstructure as well as its evolution and mechanical properties of the joints were investigated. The results show that the microstructural evolution of joints experiences three stages. With the rising of brazing process parameters, the dissolution of Cf/C turns strong, leading to the formation of rough composite/filler interface and the increasing of amount of Cr7C3 and MC complex metal carbides. The residual stress concentration in joint changes consequently. The maximum shear strength of the joint can reach 35 MPa (which is up to 92% of the shear strength of Cf/C substrate), 15 MPa and 9 MPa at room temperature, 800 °C and 1000 °C, respectively. The fracture initiates and propagates in Cf/C composite.

Properties of injection-molded poly (l-co-d,l-lactic acid) using different melt temperatures and stress concentrator in the specimen geometry

This paper investigates the morphology and mechanical properties of poly (l-co-d,l-lactic acid) (PLDLA) specimens injection-molded using different melt temperatures and stress concentrator in the specimen geometry. The values for the tensile strength and ultimate strain increased with a reduction in the melt injection temperature for unnotched specimens. The notched specimens molded using low and high melt injection temperatures showed similar values for the tensile strength compared with the unnotched specimens. The fracture surfaces of the notched and unnotched specimens molded at the low melt temperature showed the characteristic of ductile failure presenting fibrillation and displacement in the oriented skin layer. A restriction in the chain rotation and conformation due to the oriented skin layer explains the less viscous behavior observed in the dynamic mechanical analysis for low injection temperature specimens. The photoelastic analysis indicated a birefringence along the specimens manufactured using the low melt injection temperature, suggesting the existence of residual stress due to filling phase and rapid solidification. On the other hand, specimens injected using the high temperature showed residual stress concentration near the gate due to the packing effect of holding pressure. The enthalpic relaxation peak at the glass transition (Tg) observed in the differential scanning calorimetry analysis confirmed the existence of significant residual stress in all PLDLA specimens, especially when injected using the low temperature.

Effect of corrosion and hydrogen embrittlement on microstructure and mechanical properties of mild steel

This paper investigates the effect of corrosion on degradation of mechanical properties of G250 mild steel. Corrosion tests are conducted by immersing steel into hydrochloride acid solutions, with acidity ranging from 0.00001 M to 3 M. Both macro analysis and micro analysis are undertaken to investigate the mechanism of corrosion induced degradation of mechanical properties. Intergranular corrosion is found for mild steel during corrosion. Results indicate that there is considerable reduction in mechanical properties due to corrosion, which is attributed to a combined effect of hydrogen embrittlement, changes in element composition, grain size and phase composition, and the residual stress concentration at corrosion pits.

Effect of Thermal cycles and Dimensions of the Geometry on Residual stress of the Alumina-Kovar Joint

Finite element method is employed to determine the effect of variation of residual stress with dimension and the stress generated under its working condition along the Kovar. 3 different dimensions of Alumina-Kovar joint with height to diameter ratio of 3/10, using TiCuSil as a filler material. Transient Structural Analysis is carried out for three different dimensions (diameter × height) (i) 60mm × 20mm (Geometry 1) (ii) 90mm × 20mm (Geometry 2) (iii) 120mm × 20mm (Geometry 3). A comparative study has been carried out between the residual stresses developed in the brazed joint that have undergone 5 thermal cycles subsequent to brazing and that between the brazed joint. The heating and cooling rates from the brazed temperature is 10°C/up to room temperature. The brazing temperature and holding time considered for the analysis are 900°C and 10 minutes. Representative Volume Element (RVE) model is used for simulation. Sparse Matrix Direct Solver method is used to evaluate the results, using Augmented Lagrange method formulation in the contact region. All the simulations are performed in ANSYS Workbench 15.0, using solver target Mechanical APDL. From, the above simulations it is observed high concentration of residual stress is observed along the filler region i.e. in between Alumina and Kovar, as a result of difference in coefficient of thermal expansion between Alumina and Kovar. The residual stress decreases with increasing dimensions of the geometry and upon application of thermal cycles, subsequent to brazing.

Modeling and simulation of weld residual stresses and ultrasonic impact treatment of welded joints

Most structures are fabricated using welded joints because of its low cost, structural strength and geometric flexibility. Welding is considered a highly complex metallurgical process that results in irregular geometries, material imperfections/flaws and tensile residual stresses. High tensile residual stresses and stress concentrations resulting from the weld process have a significant impact on fatigue life of structures, and thus a topic of great concern in product design. Ultrasonic impact treatment (UIT) is regarded as one of the most effective post welding treatment techniques to enhance the fatigue performance of welded structures. The UIT aims to introduce fatigue-beneficial compressive stresses by plastically deforming the weld toe and reduce stress concentrations by modifying local weld geometries. In this study, 3D modeling and simulation using finite element (FE) method has been performed to simulate welding process and numerical modeling of the UIT process to predict weld residual stress distribution of butt and T weld joints. The predicted numerical results under as-welded and UIT treatment conditions were compared to present weld residual stress improvements. Compared results shows that the UIT has potential applications on the fatigue design of welded structures, can lead to lighter structures and products, in which structures can be down-sized and optimized to reduce weights.

Experimental and numerical analysis of TIG-dressing applied to a steel weldment

Fatigue strength of welded joints is controlled above all by notch defects such as weld toe and weld root that act as stress concentrators. This is the reason why over the last years different post-weld fatigue improvement techniques have been developed with the main propose of reducing geometrical discontinuities. Among these, TIG-dressing is the most used because of its simplicity and effectiveness in lowering the residual stress concentration. By this process the weld toe is re-melted to provide a smoother transition between the plate and the weld crown and to beneficially modify the residual stress redistribution. However, because of the intrinsic difficulty to numerically simulate the TIG-dressing process due to the high coupling between fluid and mechanical analysis, the effects of a weld toe remelting in terms of residual stress redistribution is hardly quantified in literature. The present paper is aimed to analyse the influence of TIG-dressing process on metallurgical and mechanical properties of a steel T-joint. Finally, a numerical model, recently published in literature, was used to quantified the residual stress redistribution.

The role of maleic anhydride functionalized graphene oxide in improving the interfacial properties of carbon fibre/bismaleimide composites

AbstractThis work aims to assess the effect of maleic anhydride functionalized graphene oxide (MAH‐f‐GO) on the interfacial properties of carbon fibre/bismaleimide (BMI) composites by experimental and finite element (FE) methods. Transverse fibre bundle (TFB) specimens with different contents of MAH‐f‐GO nanoparticles were manufactured to investigate the interfacial strength of the carbon fibre/BMI composites. The fracture surface of the TFB specimens was examined by scanning electron microscopy to observe the morphologies of the fibre − matrix interface. The coefficient of thermal expansion, cure shrinkage and elastic modulus were measured and included in the FE simulation. An FE analysis model was established to simulate the thermal residual stress distribution around the carbon fibre and to estimate the interfacial bonding strength of the TFB specimens. The combination of experimental and FE analysis results indicated that the addition of MAH‐f‐GO nanoparticles noticeably reduced the concentration of residual stress at the fibre − matrix interface and enhanced the interfacial properties of the carbon fibre/BMI composites.© 2017 Society of Chemical Industry

Surface integrity generated with peripheral milling and the effect on low-cycle fatigue performance of aeronautic titanium alloy Ti-6Al-4V

ABSTRACTMachining-induced surface integrity has an important effect on reliability and service life of the components used in the aerospace industry where titanium alloy Ti-6Al-4V is widely applied. Characterisation of machining-induced surface integrity and revealing its effect on fatigue life are conducive to structural fatigue life optimisation design. In the present study, surface topography, residual stress, microstructure and micro-hardness were first characterised in peripheral milling of titanium alloy Ti-6Al-4V. Then, low-cycle fatigue performances of machined specimens were investigated on the basis of the tension-tension tests. Finally, the effects of surface integrity factors (stress concentration factor, residual stress and micro-hardness) on fatigue performances were discussed. Results show that stress concentration can reduce the fatigue life while increasing the residual compressive stress, and micro-hardness is beneficial to prolonging the fatigue life, but when the surface material of the specimen is subjected to plastic deformation due to yield, the residual stress on the surface is relaxed, and the effect on the fatigue performance is disappeared. Under the condition of residual stress relaxation, the stress concentration factor is the main factor to determine the low-cycle fatigue life of titanium alloy Ti-6Al-4V. While for the specimens with no residual stress relaxation, micro-hardness was the key factor to affect the fatigue life, followed by residual stress and stress concentration factor, respectively.

Uncovering the local inelastic interactions during manufacture of ductile cast iron: How the substructure of the graphite particles can induce residual stress concentrations in the matrix

Recent X-ray diffraction (XRD) measurements have revealed that plastic deformation and a residual elastic strain field can be present around the graphite particles in ductile cast iron after manufacturing, probably due to some local mismatch in thermal contraction. However, as only one component of the elastic strain tensor could be obtained from the XRD data, the shape and magnitude of the associated residual stress field have remained unknown. To compensate for this and to provide theoretical insight into this unexplored topic, a combined experimental-numerical approach is presented in this paper. First, a material equivalent to the ductile cast iron matrix is manufactured and subjected to dilatometric and high-temperature tensile tests. Subsequently, a two-scale hierarchical top-down model is devised, calibrated on the basis of the collected data and used to simulate the interaction between the graphite particles and the matrix during manufacturing of the industrial part considered in the XRD study. The model indicates that, besides the viscoplastic deformation of the matrix, the effect of the inelastic deformation of the graphite has to be considered to explain the magnitude of the XRD strain. Moreover, the model shows that the large elastic strain perturbations recorded with XRD close to the graphite–matrix interface are not artifacts due to e.g. sharp gradients in chemical composition, but correspond to residual stress concentrations induced by the conical sectors forming the internal structure of the graphite particles. In contrast to common belief, these results thus suggest that ductile cast iron parts cannot be considered, in general, as stress-free at the microstructural scale.

16.29: Determination of the residual stress distribution of steel bridge components by modelling the welding process

ABSTRACTWelding operations are typically used for the connections in steel bridges. Several locations with residual stress concentrations exist within the welded parts due to the welding itself, but also because of the influence of geometric discontinuities on the flow of the stresses. This can result in fatigue failure. The knowledge of residual stresses is essential to determine the effect of the residual stresses on the fatigue life behaviour of the welded components [1].In this paper, the residual stresses of two flat plates of construction steel S235 welded together are determined by modelling the welding process. The temperatures during the welding operations are calculated in a thermal analysis, taking into account the thermal properties and behaviour of the steel. The heat input during welding is implemented on the model by defining a thermal flux for the different time steps representing the passage of the welding torch. A double ellipsoidal volume heat source is used to describe the heat flow through the material for a moving heat source. The distribution of the thermal flux is considered, taking into account the thermal material properties of steel as a function of temperature. After the calculation of the temperatures introduced during the welding process in the thermal analysis, a subsequent mechanical analysis is performed. The temperatures are introduced as a time dependent load onto the model. The elements of the model in the fusion zone of the welding are activated based on their reached temperature. The finite element modelling predicts the residual stresses induced by the welding procedure which can later be verified through comparison with measured residual stress distributions.

Effect of O2/Ar flow ratio on Ga and Al co-doped ZnO thin films by rf sputtering for optoelectronic device fabrication

Ga and Al co-doped ZnO thin films were prepared on glass substrates by radio frequency magnetron sputtering and the effect of O2/Ar flow ratio on their physical properties was investigated. The films exhibited a hexagonal wurtzite structure with a predominant (002) peak intensity and their crystallinity improved with O2/Ar flow ratio. Raman spectroscopy confirmed the films’ wurtzite structure and revealed a decrease in residual tensile stress and charge carrier concentration with increasing O2/Ar flow ratio. Low surface roughness resulted in high optical transmittances around 85–90% in the visible region. The Urbach energy decreased with increasing O2/Ar flow ratio indicating a reduction in structural disorders. This was consistent with Raman spectroscopy and X-ray diffraction analysis. The least electrical resistivity (2.6×101 Ωcm) and highest figure of merit (8.8×10−6Ω−1) for films prepared with oxygen admittance were obtained at 0.667 O2/Ar flow ratio, indicating their suitability in optoelectronic device fabrication.

Effect of Cooling Rate on Hot Tearing Behavior of Mg-9Al-1Zn-0.8Ce Alloy

The influence of cooling rate (1.5, 0.3 and 0.1 °C/s) on the hot tearing susceptibility (HTS) of Mg-9Al-1Zn-0.8Ce alloy was investigated by taking advantage of numerical simulation and experimental methods. Filling and solidification processes were observed directly using AnyCasting software. The results demonstrated that the reduction of cooling rate increases the residual melt modulus, deteriorate strain and stress concentration at last stage of solidification, and decrease the hot tearing resistance of alloy finally. The maximum value of HTS was obtained at the average cooling rate of 0.1 °C/s owing to the coarse microstructures and bulk Al11Ce3. The minimum value of HTS appeared at the rate of 1.5 °C/s thanks to the finest microstructures and a large amount of eutectic. With the increase of cooling rate, hot tearing susceptibility of the alloy shows a rapid reduction at beginning, and a slow decline followed. Besides, morphology of fracture surface and distribution of secondary phase were further discussed.

A study of machined surface RS gradient distribution and cumulative on HSM ball end milling

Residual stress (RS) has been intensively studied as an important criterion for evaluation of the surface integrity of high-speed machining (HSM) in this article. Based on the elastoplastic material property, a valuable machining property of gradient distribution of the machining path to the machined surface with HSM ball end milling is observed via an experimental test. Furthermore, an experimental formula is deduced from HSM ball end milling on H13 die steel. A cumulative coefficient for this effect is obtained by the residual stress measurement data. This is one method of predicting the residual stress concentration zone using the formula and coefficient to calculate the residual stress along the machining path. Based on the result, a discussion is presented in different fields of elastoplasticity metal material.

Microstructure and composition homogeneity, tensile property, and underlying thermal physical mechanism of selective laser melting tool steel parts

This paper systematically investigated the crystallization thermodynamics and dynamic process within melt pool of 5CrNi4Mo steel fabricated by selective laser melting (SLM). The experimental results in conjunction with finite element analysis (FEA) demonstrated that the nucleation rate during SLM process was determined by the combined effects of supercooling degree and transfer capacity of atoms near solid/liquid interface; variant nucleation rate in different region of melt pool caused microstructure heterogeneity. Chemical compositions, including Cr, Ni and C, were observed to be homogeneously distributed due to the rapid solidification of the material. Specimens built along different orientation exhibited discrepant tensile properties due to the different deformation mode during loading. All the as-fabricated SLM-processed tensile specimens showed unfavorable ductility due to heterogeneous microstructures and residual stress concentration. After post-vacuum heat treatment, for horizontally built specimens, the elongation was significantly elevated from 5.6–9.7% and the toughness was enhanced form 63.68J/m3 to 134.12J/m3. The tensile strength increased marginally from 1576MPa to 1682MPa. These promotions were mainly caused by pronounced relief of intrinsic residual stress and recrystallization effect.

Experimental and analytical investigation of fatigue crack propagation of T‐welded joints considering the effect of boundary condition

AbstractT‐welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subjected to cyclic loading always occurs in T‐welded joints because of the metallurgical differences, tensile residual stress fields and stress concentrations. The former researches about T‐welded joints fatigue have in common that the boundary condition needs to be taken into account as an influencing parameter to predict the crack propagation during cyclic loading. In this paper, the crack growth behaviour in T‐welded joint processed with Q345D steel (Pingxiang Iron & Steel Co., Ltd, Hukou, Jiangxi Province in China) under the fatigue loading was analysed via analytical model and verified via experiment. The results show that the influence of boundary condition should be considered in T‐welded joint structure during crack propagation in weld toe area. The correction factor concerning the effect of boundary condition and modified Paris' equation was proposed according to the experimental results and verified by the following repeated experiments.