Papers by Keyword: Stress Relaxation

Abstract: Sandwich panels are widely used in engineering applications due to their advantageous combination of lightweight and high strength. However, their long-term mechanical performance under repeated loading highly depends on the internal lattice structure. This study experimentally investigates the behavior of SLA-manufactured sandwich panels with different lattice geometries under cyclic loading conditions. Various lattice configurations were designed and subjected to repeated compressive loads while monitoring stress relaxation and mechanical deformation over time. The results demonstrate that lattice geometry significantly affects load-bearing capacity and energy dissipation. Notably, structures incorporating vertical support members exhibited higher energy absorption, whereas those without vertical supports, such as M4 and M5, showed improved resistance to stress relaxation over multiple cycles. Furthermore, while all specimens experienced load reductions after 25 cycles, the magnitude of these reductions varied based on the lattice configuration, with M3 exhibiting the highest load decay. These findings contribute to optimizing lattice-based sandwich structures for enhanced durability and mechanical efficiency in engineering applications.

Abstract: Stress relaxation is one of the methods used to characterize polymer foam materials. Stress relaxation data are valuable and provide important information, as they can prevent failure or unsafe usage of these materials under different loads. The aim of this study is to introduce the artificial neural network (ANN) technique for predicting the stress relaxation of polymer foam over time. The neural network model was constructed with relaxation time, stress, and strain as input parameters, and normalized stress relaxation as the output. The results demonstrate that the ANN model achieved highly accurate predictions for stress.

Abstract: Copper alloys have high thermal conductivity, relatively high mechanical strength, and toughness over a wide range of temperature; hence they are highly sorted for complex structural applications that required extreme heat flux under load. Creep of materials is classically associated with time-dependent plasticity under a constant stress/load at an elevated temperature, often greater than the absolute melting temperature. This research is aimed to study the evaluation of stress and creep rate in copper, identifying the mechanisms at which copper can easily be exposed to stress and creep deformations in structures. A 12 mm diameter copper rod with the composition of 52.05 % CuO and 30.26 % SnO₂ was procured locally. Samples from the procured rod were heat treated to 650^°C for 30 minutes and cooled in the still air as well as inside the furnace. Creep test was carried out at 760uC with a constant load corresponding to an initial stress (between 1.5 MPa and 350 MPa) and stress relation was carried out on a 98 kN capacity stress relaxation frame (from 350 MPa to 300 MPa). Rockwell hardness test and metallographic analysis (at 200 mm) were also conducted on the heat treated and unheated control samples. It was established that heat treatment reduced the hardness property of stress relaxed copper, accelerated the stress relaxation process up to 60 %, speed up both primary creep rate and the tertiary creep rate as well altered the linear creep pattern and behaviour.

Abstract: Materials resistance to edge failure during sheet metal flanging operations is known as stretch-flangeability. It is one of the important concerns in the current automotive sector. Stretch flangeability of sheet metal is estimated by hole expansion test and commonly it is represented by hole expansion ratio (HER). The objective of the present work is to comprehend the hole expansion deformation behavior of DP600 steel. Firstly, finite element analysis was performed to understand the stress state at the edge during the hole expansion test. Thereafter, the effect of stress relaxation was studied by conducting hole expansion tests (HET) in monotonic and interrupted mode. Considerable improvement in the HER was observed. The HER was found to increase with the pre-strain. The combined effect of friction and stress relaxation played a crucial role in delaying the edge failure, resulting in the enhanced HER.

Abstract: Vibration stress relief (VSR) and thermal stress relief (TSR) are important method to eliminate the residual stress of structural parts. The thermal vibratory stress relief (TVSR) is a new method to decrease and homogenize the residual stress. Based on the stress relaxation tests and the equivalent vibration equation of modal analysis, the creep constitutive model and the bilinear isotropic hardening plasticity material model (BISO) are combined to establish the numerical simulation model of TVSR of 7075 aluminum alloy ring part. The simulation results show that four different initial blank residual stress levels are obtained after quenching process, and the residual stress elimination and homogenization effect of TSR and TVSR is better than that of VSR. TVSR has a better effect on both residual stress elimination and homogenization, and the residual stress relief rate can reach more than 20%.

Abstract: 17-7PH stainless steel is high age-hardening property due to precipitate NiAl intermetallics by aging heat treatment after the deformation induced martensitic transformation by cold working. In this study, the effect of aging conditions on stress relaxation behavior of 17-7PH stainless steel was investigated, and the mechanism of the stress relaxation was discussed. The 0.2% proof stress after aging at 753K for 180s-18ks is about 450MPa, and then decreases after 18ks. On the other hand, the stress relaxation ratio decreases by long time aging at 753K. The dislocation density of 17-7PH decreases by long time aging at 753K. The formation of NiAl clusters around 5nm by 3D-AP analysis is observed in 17-7PH aged at 753K for 1.8ks. It is suggested that the reduction of the stress relaxation ratio after long time aging at 753K is caused by NiAl clusters and decreasing mobile dislocation density.

Abstract: The relaxation properties of GH4169 alloy were studied contrastively at temperatures ranging from 600 ^oC to 700 °C and initial stress ranging from 550 MPa to 850 MPa. The relationship between the microstructure and relaxation behavior was evaluated using transmission electron microscopy techniques. It was found that the relaxation limit and relaxation stability of the alloy decreased obviously with the increase of temperature. Further investigations show that the relaxation behavior is mainly depend on both precipitate characteristics and its interaction with dislocations. The alloy with higher strength lever has more excellent stress relaxation stability, because of the inhibition of a large number subgrains on dislocations motion.

Abstract: This paper presents the results of the investigation of stress relaxation at the Si-SiO₂ interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been shown, on the basis of EPR, XPS, C-V, and deflection data, that the mechanism of stress relaxation depends on the oxidation conditions: temperature, cooling rate and oxide thickness. In the Si-SiO₂-Si₃N₄ system the stress relaxation occurs due to the difference in the thermal expansion coefficient of SiO₂ and Si₃N₄ material. With an appropriate choice of oxidation conditions compressive stresses in SiO₂ and tensile stresses in Si are almost equal and stress can be reduced considerably at the interface.

Abstract: The capillary-porous structure of cement stone and concrete is known to be mainly characterized by the presence of two major structural components – crystalline and gel-like products in the form of calcium sub-microcrystals of variable composition. The environmental factors cause changes in the structural-mechanical properties of the concrete structure which can be divided into “internal” and “external” ones. The number of internal properties includes those that change the relative content of the structural components of concrete – its composition, hardening conditions, and etc. The external factors include those that act directly on the already formed structure of concrete in operating conditions. These include mechanical, physical, chemical and other factors that affect the properties of the structure over time.

Abstract: The paper provides the comparison of vibromechanical technology for stabilization of bearing rings and thermal tempering on the following parameters: performance, energy consumption, and magnitude of residual stresses after treatment. To assess the effectiveness of residual stress relaxation through the usage of vibromechanical energy, there were conducted experimental studies on a specially designed and manufactured prototype of the equipment. The results of experimental studies show that the energy consumption during vibromechanical stabilization is ten times less, and performance is several times higher than during the thermal tempering. Moreover, vibromechanical stabilization allows more effective residual stresses relaxation. The costs for capital investment can be reduced, as the cost of equipment for vibromechanical stabilization is ten times less than the cost of electric furnaces.