Study on ring compression test using physical modelling and FE simulation

This work presents the differences found when using two types of methodology (continuous and incremental) in the application of load-lubricant during ring compression tests. Friction calibration maps have been obtained by conducting ring compression tests to flat rings of aluminium alloy UNS A97075-T6 with molybdenum disulphide (MoS2) as lubricant. Accordingly, in order to determine the particular values of the Coulomb friction coefficient, the problem was simulated using an elasto-plastic model with the finite element code Abaqus/Standard. A friction coefficients map from experiments and finite element simulation is drawn to compare the friction coefficient values derived from the continuous and incremental procedure. A comparison with experiments done with graphite powder as lubricant is also presented. Results of this investigation show that friction curves are affected by the methodology of load-lubricant application, and some guidelines about the use of these maps depending on the actual process conditions are provided.

Frictional conditions at the workpiece–die interface are critical in metal forming, as significant plastic deformation generates heat that affects lubricant performance. Understanding lubricant behavior, especially its influence on friction under elevated temperatures, is essential for optimizing forming processes and meeting ecological demands. While the conventional ring compression test evaluates friction through inner diameter changes, it becomes unreliable when friction is transient. In this study, a warm ring compression test incorporating an in situ measurement system is proposed to evaluate the transient frictional behavior of lubricants under temperature rise due to plastic deformation. Results show that at T = 50 °C and 150 °C, the friction coefficient increases notably with the compression ratio, whereas at T = 100 °C, it remains relatively stable. This stability is likely due to the optimal performance of the chlorinated base lubricant at 100 °C, where boundary lubrication is most effective. At T = 50 °C, the additive activation is insufficient, and at T = 150 °C, thermal degradation may reduce its effectiveness. Finite element simulations using the transient friction coefficient reproduce the deformed ring cross-section with high accuracy, while those using constant friction values show less agreement.

We present a seminumerical simulation method called SIRFEM, which enables the efficient prediction of high-frequency transducer outputs. In particular, this is important for acoustic microscopy where the specimen under investigation is immersed in a coupling fluid. Conventional finite-element (FE) simulations for such applications would consume too much computational power due to the required spatial and temporal discretization, especially for the coupling fluid between ultrasonic transducer and specimen. However, FE simulations are in most cases essential to consider the mode conversion at and inside the solid specimen as well as the wave propagation in its interior. SIRFEM reduces the computational effort of pure FE simulations by treating only the solid specimen and a small part of the fluid layer with FE. The propagation in the coupling fluid from transducer to specimen and back is processed by the so-called spatial impulse response (SIR). Through this hybrid approach, the number of elements as well as the number of time steps for the FE simulation can be reduced significantly, as it is presented for an axis-symmetric setup. Three B-mode images of a plane 2-D setup-computed at a transducer center frequency of 20MHz-show that SIRFEM is, furthermore, able to predict reflections at inner structures as well as multiple reflections between those structures and the specimen's surface. For the purpose of a pure 2-D setup, the SIR of a curved-line transducer is derived and compared to the response function of a cylindrically focused aperture of negligible extend in the third spatial dimension.

This article describes the use of compression tests, namely, cylindrical compression, ring compression, and plane-strain compression tests at elevated temperatures. It discusses the effects of the temperature, strain rate, and deformation heating on metals during the cylindrical compression test, with the help of flow curves. The article illustrates the testing apparatus used in the cylindrical compression test. It describes the issues regarding friction and temperature, and strain-rate control with proper test equipment and experimental planning during the ring compression test and plane-strain compression test. The article also reviews the testing conditions, procedures, and advantages of hot plane-strain compression test.

Self-piercing riveting (SPR) has been widely used in automobile industry, and the strength prediction of SPR joints always attracts the attention of researchers. In this work, a prediction method of the cross-tension strength of SPR joints was proposed on the basis of finite element (FE) simulation and extreme gradient boosting decision tree (XGBoost) algorithm. An FE model of SPR process was established to simulate the plastic deformations of rivet and substrate materials and verified in terms of cross-sectional dimensions of SPR joints. The residual mechanical field from SPR process simulation was imported into a 2D FE model for the cross-tension testing simulation of SPR joints, and cross-tension strengths from FE simulation show a good consistence with the experiment result. Based on the verified FE model, the mechanical properties and thickness of substrate materials were varied and then used for FE simulation to obtain cross-tension strengths of a number of SPR joints, which were used to train the regression model based on the XGBoost algorithm in order to achieve prediction for cross-tension strength of SPR joints. Results show that the cross-tension strengths of SPR steel/aluminum joints could be successfully predicted by the XGBoost regression model with a respective error less than 7.6% compared to experimental values.

Solder joints in automotive electronic assemblies are exposed to thermomechanical and vibrational loads. Usually, passive thermal cycling results in thermomechanical loads in the low strain rate plastic and creep regime of the solder alloy. In the case of vibrational loads, high strain rate plastic deformations in solder interconnections are expected. In order to investigate the deformation and failure behavior of the solder material in the high strain rate regime, we performed several high cycle fatigue (HCF) experiments on standardized specimens of a SnAgCu alloy under varying mean stresses and ambient temperatures. As a first step, high cycle fatigue tests with a frequency of 40 Hz at room temperature have been performed. From a statistical evaluation of the number of cycles to failure at different stress amplitudes and zero mean stress, we obtained a high cycle fatigue Woehler curve. Subsequently, the mean stress and temperature levels were changed, and the load frequency has been kept constant. The aim of this high cycle fatigue Woehler experiments was to explore the temperature and mean stress effects on the fatigue performance of the solder alloy. In order to assess the reliability of a solder ball grid array (BGA) under vibrational loading by means of finite elements (FE) simulations, a viscoplastic material model is calibrated based on the experimentally observed stress-strain material behavior in the HCF measurement. FE simulations using a fatigue material model were used to address the lifetime of the BGA solder joints under vibration during electrodynamic shaker testing on board level. The FE-based lifetime prognosis is discussed and compared to experimental statistical failure data of real solder joints obtained from electrodynamic shaker testing.

Quantifying the efficacy of protective eyewear in pediatric soccer-induced retinal injury

Determination of the hoop fracture properties of unirradiated hydrogen-charged nuclear fuel cladding from ring compression tests

Due to the material deformation process in drilling are quite complicated, research using finite element simulation on drilling is limited and not widely reported. To investigate drilling process, a coupled thermo-mechanical finite element model of drilling is developed. Several key technologies, such as material constitutive model, material failure law, contact and friction law, have been implemented to improve the accuracy of finite element simulation. Using this finite element model, drilling process of Ti6Al4V alloy is predicted. Comparing the predicted cutting forces with the measured forces shows the finite element model is reasonable. Analyses show that the distributions of Misses stress and temperature during drilling process all increase at first, and get maximal value when drilling is in steady state, then decrease gradually till the hole is drilled through. These results confirm the capability of finite element simulation in predicting drilling process and selecting optimal tool and cutting parameters.

Reinforcing cement-based materials by nanomaterials has attracted extensive attention over the past decades. It has been reported that the mechanical properties of cement-based materials can be greatly improved by incorporating a small amount of nanomaterials. However, experimental and theoretical works have all focused on the enhancement of cement-based materials subjected to static loading. To the author’s knowledge, no studies have reported on the mechanical properties of nanomaterial- reinforced cement-based composites under high strain rate loadings. This study investigates the reinforcing effect of graphene oxide (GO) on cement mortar under high strain loadings. GO is an emerging nano-scale candidate for enhancing cement-based materials, which is valued for its two-dimensional geometry, superior mechanical properties and large surface area. Tensile splitting and compression tests were conducted on GO-reinforced cement mortar under both static and high strain rate loadings. The high strain rate testing was performed using a Split Hopkinson Pressure Bar (SHPB) apparatus. Static experiments showed that incorporation of GO (of 0.02 wt% of cement) improved the tensile splitting and compression strength of plain cement mortar by 8.9% and 10.8%, respectively, and there was little difference between the 0.02 wt% and 0.04 wt% GO addition. The SHPB tests showed that the reinforcing effect of GO under high strain rate is directly related to the number of cracks. In tensile splitting tests, because the major crack is always localized in the centre of the specimen regardless of the strain rate, the reinforcing effects of GO on cement mortar were the same for different strain rates. In compression tests, however, because the cracks were diffused and there were more cracks under high strain rates, the reinforcing effect of GO was also more significant under higher strain rates. To understand the reinforcing mechanisms of the nanomaterials, the key is to determine the interaction properties between the nano-scale reinforcements and the cement matrix. However, understanding of the governing forces in the reinforcement’s pull-out behaviour at such a small scale is still very limited. The present study is the first to identify the governing force during the pull-out of atomically thin two dimensional (2D) nanosheets (e.g. graphene and its derivative GO) and to develop corresponding theoretical models. In molecular dynamic (MD) simulations, friction was found to make negligible contribution to the pull-out force because of the lack of asperities on atomically thin materials (ATM). The pull-out force was revealed to be governed by a “crack plane adhesion”. Unlike frictional pull-out, crack plane adhesion produces a pull-out force independent of the embedded length of ATM. The magnitude of pull-out force and its affecting factors were investigated by MD simulations. On the basis of crack plane adhesion, the relations between the pull-out force and the pull-out displacement for ATMs was formulated. Furthermore, a new theoretical model was developed to predict the crack bridging stress (σB) of 2D ATMs. The magnitude of maximum σB was estimated to be 3MPa for 0.2 wt% of GO in cement matrix, which was used in the finite element simulations as discussed herein. The fundamental theory and new model proposed here serve as theoretical support for understanding and development of ATM-reinforced composite materials. Finite element simulations for static and SHPB compression tests were conducted using a micromechanical model, the microplane model. First, the microplane model was examined by comparative study with the commonly used concrete damaged plasticity model. Then finite element simulation models of static and SHPB compression tests were built in ABAQUS. For simulation of the static test, by including the calculated crack bridging stress into microplane model, the experimental compressive strength of GO-reinforced cement mortar could be accurately simulated, which validated the calculation of the crack bridging stress by the proposed model in Chapter 4 and the effectiveness of the microplane model to simulate the static behaviour of GO-reinforced cement-based composites. For simulation of the SHPB test, the microplane model with a logarithmic relation between the dynamic increase factor (DIF) and the strain rate to account for the strain rate effect worked for the simulations of plain cement mortar samples. However, for simulation of the GO-reinforced mortar samples, the simulation results give a significant underestimation compared with the experimental results under higher strain rates.

To address the challenge of identifying damage in the hangers and bridge deck systems of long-span suspension bridges, this paper proposes a non-contact monitoring method based on video image recognition. This method extracts structural vibration displacement responses through video acquisition and image analysis, and combined with the strain mode change rate index, it achieves damage localization, type identification, and severity assessment. The principle of extracting displacement time-history data from video images is first elaborated, and MATLAB-based computational code is developed, including pixel tracking and time-history curve generation methods. The eigensystem realization algorithm is used to identify displacement mode shapes, which are then converted into strain mode shapes via the central difference method. The strain mode change rate and its deviation rate are proposed as damage indicators: under undamaged conditions, the curve is smooth; at damage locations, peaks appear; the distribution range of peaks can distinguish between hanger damage and bridge deck cracks; the deviation rate quantifies damage severity. The feasibility of the method is validated through finite element simulations and physical model experiments. The results show that hanger damage causes broad peaks, while bridge deck cracks present narrow peaks; the deviation rate increases monotonically with damage severity. Applied to an in-service suspension bridge, the method successfully identified hanger bending and weld cracking, with assessment results consistent with on-site inspections. This study demonstrates that the strain mode change rate analysis based on video images enables damage identification without prior knowledge of the structural health state, relying solely on the damaged state response. Offering advantages such as non-contact measurement, full-field monitoring, and no need for sensor deployment, it provides a new technical approach for the long-term monitoring of suspension bridge hanger systems.

Investigating fault slip in a model of an underground gas storage facility

To measure the flow stress of metals under forming conditions, i.e. at large strains, high strain rates and elevated temperatures, a method based on the ring compression test is proposed. In this method, a ring-shaped specimen is compressed between two flat tools with and without lubrication. The coefficient of friction between the specimen and the tools is determined from the change in inner diameter, and then the load and reduction in height measured in the test are converted into the average flow stress and average equivalent strain by taking the friction into account. The calibration curves for determining the coefficient of friction, the average equivalent strain and the average flow stress are derived from the results of rigid-plastic finite element calculation. This method is simple to carry out and does not require control of friction during compression. Ring compression tests for certain kinds of lubricants, materials and strain rates are carried out. The method is confirmed to provide flow curves within an error of 5%, and is effective especially for measuring the flow stress of heat-resistant materials such as Ti alloys and Ni base superalloys which are formed at elevated temperatures. As an example, the flow curves of Ti-6Al-4V alloy measured by the proposed method are demonstrated.

The evaluation of the tribological conditions, among which the friction by means of simple models are questionable because the influence of superficial roughness and strength of materials, among other factors, is not considered. This paper presents a detailed analysis on the influence of surface roughness on friction at the specimen/die interface. The studies are based on experimental data obtained by the ring compression test, which they were compared with results of simulations and the calibration curves of friction modeled in finite element software. A series of ring compression tests was performed to obtain the friction coefficient for various ranges of surface roughness of the specimen. Key words: ring compression test, surface roughness, friction coefficient, numerical simulation.

This study investigates the mechanical properties and microstructural characteristics of explosion-welded Al/Cu two-layer strips during the ECAR process using finite element simulations and experiments. Furthermore, the explosion-welded Al/Cu two-layer strip was examined with various configurations to determine the effects of die angle and friction coefficient on the distributions of effective strain and stress. In addition, the experimental and computational approaches were utilized to explore the ECAR process in two routes, RA and RC, for three directions. Finite element simulations were run and validated using experimental data. The findings show that the ECAR method is effective to enhance the mechanical characteristics of the strips. Moreover, the obtained results represent that the ECAR process reduced the size of the grains. Furthermore, it was shown that by increasing the number of routes in the ECAR process, the stress values were increased. However, the ductility of the two-layer strip was decreased. According to the current study, the route and the number of routes used in the ECAR process have a considerable effect on the strength of the two-layer strip.