This study presents a new criterion (MMFC2) for predicting the forming limit curve (FLC) of sheet metal. The strain path evolution of a critical element examined in a uniaxial tensile test is ...elaborated by incorporating the results of experimental measurement, finite element simulation, and theoretical prediction via the Modified Maximum Force Criterion (MMFC). A scaling factor is introduced to mimic the theoretical evaluation with the simulated one. It is believed that the rotation of the principal axes of the theoretically considering material point, which is initially co-axial with the external load coordinate, implicates the macro track of the strain path change. Furthermore, an optimal event of the second derivative of the axial rotations is proposed to indicate the strain localization and formulate the FLC. The performance of the proposed criterion is compared with that of the original MMFC in predicting the FLC of three automotive sheet metals, of which all related data were published in the Benchmark of Numisheet 2014 conference. The use of three different hardening laws and three yield functions is examined in the analogy. The comparison reveals that the results of MMFC2 are more sensitive to the employed constitutive model than that of MMFC. Furthermore, the proposed MMFC2 presents concordant results with the experimental data. Nakajima tests are conducted for CR4 mild steel sheets to validate the capacity of the proposed criterion. Well agreement between the experimentally measured data and theoretical prediction based on the Yld2k yield function verifies its usefulness in practice.
•New equation for governing the strain path change after the initiation of diffuse neck is presented.•New criterion for strain localization is proposed based on the acceleration of the principal axis rotation.•The proposed criterion is applied to predict the FLC of three automotive materials using published data.•Experimental tests were conducted for CR4 sheet to validate the predictability of the proposed criterion.•For CR4 sheet, the performance of the proposed criterion is comparable to that of MK model but superior in computational time.
Fiber pullout tests have been frequently performed to determine the interfacial properties of fiber-reinforced composites. However, traditional experimental approaches and numerical investigations ...are restrained by being both labor-intensive and time-consuming. Hence, an accurate and effectual prediction of the interfacial properties is of paramount importance for composite design and tailoring. This work for the first time presents machine learning-assisted models to determine the interfacial properties based on previous micro-bond tests. Through a comparison between the pullout test results and prediction results, the effectiveness of the proposed model in the prediction of the interfacial shear strength and the maximum force is verified. The relationship between influencing attributes and interfacial properties can be reliably captured. It can be referred from the mean impact value analysis of the proposed models that the interfacial properties are significantly dependent on the fiber’s diameters. This work reveals that gradient boosting regressor (GBR) and artificial neural networks (ANN) exhibit adequate generalization and interpretation abilities. Besides, both ANN and GBR, with small datasets, have tremendous potential for a wide array of applications in predicting the shear resistance properties in fiber-reinforced composites.
•New formulation for strain hardening function is presented.•A procedure for identifying parameters of proposed hardening model is developed based on curve fitting method. In this procedure, stress ...and strain data obtained from standard uniaxial tensile test were utilized.•New method is presented for estimating the level of limited strain in the plane strain mode, FLC0. Based on this method, influence of post-necking prediction of hardening law on forming limit curve of aluminium sheet is discussed.
This study clarified the influence of the post-necking prediction of the hardening law on the theoretical forming limit curve (FLC) of aluminium sheets subjected to punch-stretching tests. A procedure was developed to identify the parameters of a recently developed hardening model (Kim–Tuan hardening model) based on the curve fitting method. Subsequently, this model was used to capture the post-necking behaviours of two aluminium alloy sheets (AL5052-O and AL6016-T4), which were compared with those of various other hardening models, including the Swift model, Voce model, Hockett–Sherby model, Ghosh model, and a linear combination of the Swift and Voce models. These hardening models, with their extrapolations for post-necking prediction, were employed to analytically calculate the FLCs of the tested materials based on the modified maximum force criterion (MMFC). Furthermore, a simple and effective method was found to estimate the level of limited strain in the plane strain mode, namely FLC0. The proposed method was used to clarify the influence of the hardening law and yield function on the level of the computed FLC. The results indicated that the flow curves predicted by the Kim–Tuan hardening model effectively matched the experimental data obtained from uniaxial tensile tests, while their extrapolations provided intermediate predictions of the post-necking behaviour between the flow curves of the Swift and Voce models. In addition, the proposed hardening model improved the accuracy of the computed FLC for the studied materials.
Display omitted
•An evolving non-associated Hill48 plasticity model was formulated.•The model accurately accounts for the anisotropic hardening as well as the r-value evolution.•Under non-associated flow rule, both ...the yield function and flow potential determine the occurrence of localisation.•The evolving feature of the anisotropic properties contributes to the good prediction of the forming limits.
Experimental and numerical investigations on the characterisation and prediction of cold formability of a ferritic steel sheet were performed in this study. Tensile tests and Nakajima tests were conducted for the plasticity characterisation and the forming limit diagram determination, respectively. For the plasticity behaviour description, an evolving non-associated Hill48 anisotropic plasticity model was formulated to accurately characterise the anisotropy evolution under monotonic loading, including anisotropic hardening as well as the r-value evolution. The detailed model parameter calibration procedure was also demonstrated. Eventually the model was applied to the forming limits prediction in conjunction with the modified maximum force criterion. A systematic and detailed study was performed addressing the impacts of the evolving and non-associated characteristics of the model formulation on the forming limits prediction by comparing the proposed model with the classical ones. Both the plasticity model and its application to the formability prediction were validated by the experimental results.
Display omitted
Abstract
Bimodal atomic force microscopy (AFM) uses two eigenfrequencies to map nanomechanical properties with high spatial and temporal resolution. To reliably map surface properties and to ...understand the links between experimental observables, energy dissipation, and viscoelastic properties considering the effects of nonconservative interaction forces is essential. To avoid damaging the sample, the maximum force between the tip and the surface and the maximum indentation of the tip into the sample needs to be controlled. In this work, we use both experiments and simulations to study how viscoelastic properties affect the cantilever response in bimodal AFM. We simulate the tip-sample interaction force, indentation, and energy dissipation for samples with different viscous properties. Under the tested operating conditions, we observe that more energy is dissipated in the higher eigenmode. The larger higher eigenmode free amplitude increases the energy dissipation in both eigenmodes. The larger energy dissipation increases the contrast of the bimodal AFM dissipation map. The simulations are cross-compared with experiments and similar trends are observed. This work is important for understanding and optimizing bimodal AFM measurements on samples with significant viscoelastic responses, such as cells, tissues, and polymers.
•Considère criterion must be enforced in the calibration of the hardening law.•The hardening rate can significantly influence the Nakazima FLC process-corrections.•Shear tests are a viable ...alternative to bulge tests for the hardening response to large strains.•In-plane formability can be accurately predicted using the MMFC model without calibration.
The present study investigates the use of both Marciniak and Nakazima tests to generate the forming limit curves (FLCs) for a power-law hardening dual phase steel, DP980, and an aluminum-magnesium alloy, AA5182, that exhibits dynamic hardening and saturation-type behavior. The recently proposed methodology of Min et al. (2016) to compensate for the so-called process effects of non-linear strain paths (NLSP) and contact pressure was evaluated and applied to the Nakazima FLCs to enable comparison with the Marciniak FLCs and the formability predictions of the Modified Maximum Force Criterion (MMFC) of Hora et al. (1994, 2013) for in-plane stretching. An emphasis was placed upon the experimental determination of the constitutive model to plastic strains in excess of 0.5 using tensile and shear tests. A flexible variant of the Hockett-Sherby (1975) hardening model for large strain levels along with constraints upon the hardening model calibration are proposed. It is demonstrated that the choice of hardening model, test data, and the calibration procedure can have a marked influence on the Nakazima process corrections and the predicted FLC using the MMFC model. An extension to the Linear Best Fit (LBF) limit strain detection methodology by Volk and Hora (2011) was developed to account for bend severity and the material hardening response and is compared with the ISO 12004-2 limit strains. If the hardening model is accurately calibrated to large strain levels, the analytical MMFC predictions were in excellent agreement with the process-corrected Nakazima and Marciniak FLCs for DP980. The results for the AA5182 were found to be strongly dependent upon the choice of hardening model that influences both the MMFC and the contact pressure correction.
Lightweight components and materials based on the joining of dissimilar materials have attracted wide attention in aerospace field. To obtain the optimized micromorphology and interfacial ...microstructure of the brazed joints, a new method of Au layer deposition on TiAl base metal was proposed to assist the brazing of TiAl plate to GH3536 thin-walled structure. Significantly, the design of Au layer affected the brazing fillet, dissolution of GH3536 core and the evolution of interface, and then the maximum force under tensile loading of the hetero-thin-walled structure. The effect of Au layer thickness on the interfacial microstructure and mechanical properties was discussed in detail. Upon the deposition of an Au layer with a concentration of 0.2 wt% onto the TiAl plate, the load-bearing capacity of the brazed thin-walled structure was observed to undergo a significant enhancement. Specifically, the maximum force sustained under tensile loading was measured to be 391 N, thereby exhibiting a remarkable increase of 117 % relative to that of the Au-free system.
•The erosion behavior of thin-walled structure could be adjusted with Au layer.•The micromorphology of the brazed joint could be adjusted with Au layer.•The optimal tensile strength of thin-walled structure was achieved at 391 N.
Determination of sheet metal formability, which is commonly evaluated through a forming limit curve (FLC), is an essential task for part designs in the automotive engineering community. This study ...provides a coherent comparison among four theoretical FLC criteria including Swift's instability, Hill's localized neck, Storen-Rice bifurcation analysis, and Hora's MMFC. Within these criteria, closed-form solutions for FLC are available for fast and expensiveness explicitly estimating the forming limit diagram (FLD) of sheet metals. According to their closed-form solutions, an empirical suggestion is proposed to improve the accuracy of the theoretical FLC. Quantitative comparisons are then made to evaluate the performance of these criteria for numerous aluminum alloys and steel sheets which are frequently used in automotive industries. Based on the comparisons, the advantages and limitations of each model are discussed from a physical point of view, which appears to be a useful suggestion for selecting a proper theoretical model to evaluate the FLD of sheet metals without conducting experimental tests.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
It has been accepted that the formability of a sheet metal with a moderate ductility can be limited by not only localized necking (LN), but also ductile fracture (DF). In this study, a theoretical ...prediction framework is developed for a comprehensive formability characterization, in which forming limit curves at LN (FLCN) and DF (FLCF) are elaborately correlated by considering strain path evolution. A dual-phase steel material (DP590 sheet metal) is selected with a series of DF and Nakajima tests performed. A newly proposed DF model (uncoupled type) is calibrated by implementing a hybrid experiment-simulation method in line with the DF tests, which are designed to achieve DF under distinct stress states, such as simple shear (SS), uniaxial tension (UT), plane strain tension (PST), and balanced biaxial tension (BBT). The resulting three dimensional (3D) fracture surface demonstrates a good agreement with the tested data. The modified maximum force criterion (MMFC) is selected for the theoretical identification of strain path evolution. The calibrated MMFC results in a FLCN exhibiting a certain level of underestimation as compared to the tested data in the range of positive minor strain. The MMFC is improved (iMMFC) by incorporating with an initial strain path-based function for a higher accuracy in characterizing evolutive strain paths; the resulting FLCN is observed to have a better performance than that of MMFC. Theoretical FLCFs are determined by the integral of ductile damage increment (defined by the DF model calibrated) over each identified strain path from UT to BBT. After considering strain path evolution, different deformation stages are added into the finalized forming limit diagrams including both FLCF and FLCN. It is found that the FLCF based on iMMFC model demonstrates acceptable deviations as compared to all the tested cracking data. Moreover, this FLCF intersects with the corresponding theoretical FLCN as load path approaches from UT to BBT, representing a fracture pattern transition from a LN band-accompanied DF to the one without LN band; this prediction is further validated by experimental observations. The FLCF based on MMFC fails to predict this transition behavior. The current study confirms the presence of a competition between LN-induced failure and DF-induced failure for sheet metals. Moreover, all of these findings advance the insight into the importance of performing a DF prediction aside of a FLCN prediction, especially for the case where a sheet metal with a moderate ductility shows a fracture pattern transition behavior.
Display omitted
•A resistance function to strain path change is introduced into the MMFC.•A prediction framework for FLCF is proposed with strain path evolution considered.•FLCFs are built with different deformation stages highlighted.•The framework shows a competition between different failure mechanisms.•The experimentally observed fracture pattern transition is successfully predicted.
The purpose of this study was to investigate the concordance of isokinetic bilateral strength differences of the knee extensors in single- and multi-joint movement tasks. One hundred and nineteen ...male athletes performed isokinetic legpresses at 0.1 m/s and 0.7 m/s as well as isokinetic knee extensions at 60°/s and 180°/s. Bilateral differences and directed bilateral differences (sign indicating the direction of the difference) were calculated for all measurements. Bland-Altman-Plots were plotted to investigate if the different conditions detect bilateral differences of the same magnitude. Additionally, concordance correlations for the directed bilateral differences of the different tests were calculated to investigate magnitude and direction. The results indicate poor to fair concordance between the bilateral differences in the legpress conditions as well as between single- and multi-joint tasks. The single-joint knee extensions displayed a moderate level of agreement. Bilateral strength differences in isokinetic movement tasks are dependent on movement velocity and the nature of the task (single- or multi-joint movement) in the lower extremities. Both the value and the direction of the strength differences show no clear pattern across the investigated measurements and cannot be used interchangeably. Therefore, to assess interlimb strength balance, multiple different tests should be performed.
PDF
