A study on the efficiency of a capped hardening elasto-plastic model for soft clays

It is well known that the deformation and stress-resistant characteristics of fine-grained soils, especially soft clays, are significantly influenced by the soil softness. It is therefore very important to employ a model which can accurately simulate the effects of this phenomenon. A constitutive model must be able to create a balance among stress paths, the number of parameters, the process of parameter determination, and finally, the simplicity of the computational calculations.The current study investigates the performance of a two-yield surface (cone and cap yield surface) model for soft soils. The efficiency of the cap yield surface has been studied as well. The model has been calibrated by employing the data derived from previous researches for Bangkok clay. The incorporated data have been obtained from the results of CD triaxial, CU triaxial, and oedometer tests. The proposed method for the model calibration can accurately predict the triaxial test results and oedometer test stress path simultaneously. This method for predicting the soil behavior is based on the main soil characteristics taken from common soil mechanics tests. It can be widely employed for engineering practices, especially when complicated soil behavior is encountered.     

Integration of Production Steps on a Single Equipment

The article describes a new method for integrated processing which combines high-energy heating by high frequency currents (HEH HFC) and abrasive grinding. Research clearly showed that implementation of integrated processing may lead to 2 … 2.5-fold increasing of the processing performance. It was established experimentally that the proposed integrated treatment results in increasing of the microhardness and the level of residual compressive stresses in the surface layer. This increases the contact–fatigue strength of steel parts up to 10% … 16%.     

镁质晶须对油井水泥石力学性能的影响

为提高油井水泥石强度,选用镁质晶须MBW为增强剂,研究了MBW在不同掺量下对水泥浆体的基本性能及水泥石力学性能的影响,并通过XRD、MIP及SEM等表征方法进行了探讨。结果表明,油井水泥中掺入1%~5%的镁质晶须后,其浆体密度及凝结时间基本不变;流动度随晶须掺量的增加而降低,通过加入减阻剂USZ来改善;水泥石的抗压强度、抗折强度随晶须掺量的增加得到明显提高,当晶须掺量为4%时,80℃下水泥石1、3、7和28 d抗压强度比净浆水泥石分别提高8.3%、15.4%、21.5%和18.8%,抗折强度分别提高9.4%、19.4%、13.1%和0.3%。镁质晶须属于惰性微填料,可填充水泥石内部的有害孔隙,增加水泥石密实度,同时作为微纤维,在水泥石内部通过桥连、拔出、剥离、折断等作用机制起到对水泥石增强增韧的双重功效。     

Feasibility of laser surface treatment of pearlitic and bainitic ductile irons for hot rolls

The effects of laser surface treatment on the microstructure, crackability and stresses generated on laser hardened layers produced in several ductile cast iron materials were investigated. Two kinds of alloys having pearlitic (SGP) and acicular (SGA) matrix microstructures were selected. Hardened layers with thicknesses ranging from 1.5 to 2.5 mm were obtained by means of laser remelting (LSRm) or laser hardening (LSH). Thermal stresses generated upon laser processing have been estimated by a simple thermal model. For energy densities delivered onto the material at above 40 J/mm², extensive cracking was developed in SGA and SGP irons due to the contribution of thermal stresses. By lowering the energy density, crack formation was avoided in SGP irons only. At low energy densities, crack formation is controlled by the generation of transformational stresses due to excessive austenite retention. An increase of the surface temperature or the alloying content gave rise to an increase of the retained austenite and the formation of lower bainite at the remelted zone and the heat affected zones, respectively. K_C fracture toughness of Fe₃C carbides embedded in pearlitic and acicular matrixes was measured by means of the nano-indentation technique. Fracture toughness of cementite in SGP irons was slightly higher than in SGA irons, which can help to reduce the crackability of LSH layers.     

Multi-response optimization of post-fire performance of strain hardening cementitious composite

This paper presents results of an experimental program conducted to optimize the post-fire performance of Strain Hardening Cementititous Composites (SHCC) using Taguchi approach with utility concept. The experiments were first undertaken by determining nine SHCC mixes using a standard L9 (3⁴) orthogonal array of four parameters and each parameter with three levels. The four parameters of SHCC mixes included fly-ash/binder ratio, sand/binder ratio, water/binder ratio and fiber proportions. The responses of SHCC to be optimized were tensile strain capacity, compressive strength and post-fire compressive strength after subjected to 200 °C, 400 °C, 600 °C and 800 °C of isothermal heating. Utility concept was introduced to simplify the multi-response problem into mono-response question together with Taguchi method. The role of different parameters on the composite responses of SHCC was examined. Furthermore, an optimal SHCC mix to maximize multi-responses was determined based on statistical analysis and validated by additional confirmation tests.     

PP/蒙脱土复合体系流变性能的研究

通过熔融共混的方法研究制备了不同蒙脱土用量的聚丙烯(PP)/蒙脱土复合材料.运用X射线衍射及SEM对复合材料片层分散和微观形貌进行了表征,测试了复合材料的动态流变性能和拉伸流变性能.结果表明:直接共混蒙脱土片层没有形成剥离结构,在亲水体系中团聚现象严重,复合体系与聚合物基体的动态频率扫描结果几乎一致;同一配比下,疏水性蒙脱土复合体系比亲水性蒙脱土复合体系应变硬化现象明显;而随着蒙脱土用量增加,拉伸速率的增大,复合体系应变硬化现象也越明显.     

内耗法研究退火温度对含钒BH钢中固溶C原子浓度的影响

采用内耗方法研究退火温度对含钒超低碳烘烤硬化钢间隙固溶C的影响。对试样进行不同温度盐浴退火处理后,通过内耗法测量Snoek峰来研究固溶C随退火温度的变化规律,并测量相应的BH值和晶粒尺寸变化规律。研究结果显示:含V超低碳烘烤硬化钢在750～810℃盐浴退火,随着退火温度的升高,Snoek峰先逐渐下降后略有升高,间隙固溶C含量亦先减小后增加;但随着退火温度的进一步提高,间隙固溶C的浓度增加,内耗峰又有所上升,此结果与BH值对比取得了较好的一致性。此外随着内耗测试频率的降低,内耗与固溶C含量的比例系数K也随之降低。     

An experimental and theoretical study on the prediction of forming limit diagrams using new BBC yield criteria and M–K analysis

In the present paper, a comprehensive study on the prediction of forming limit diagrams (FLDs) for an AA3003-O aluminium alloy is developed theoretically and experimentally. For obtaining the experimental FLDs, an out-of-plane formability test was performed based on the technique proposed by Ozturk and Lee [F. Ozturk, D. Lee, J. Mater. Process. Technol. 170 (2005) 247–253]. The classical Marciniak–Kuczynski (M–K) model and some new yield criteria are utilized to simulate the necking phenomenon and calculate the limit strains theoretically. The employed yield functions are: the BBC2000, BBC2002, and BBC2003 yield criteria proposed by Banabic et al. [D. Banabic, S.D. Comsa, T. Balan, in: Proceedings of the Cold Metal Forming 2000 Conference, Cluj-Napoca, 2000, p. 217; D. Banabic, T. Kuwabara, T. Balan, D.S. Comsa, D. Julean, Int. J. Mech. Sci. 45 (2003) 797–811; D. Banabic, H. Aretz, D.S. Comsa, L. Paraianu, Int. J. Plast. 21 (2005) 493–512]. To calibrate and determine each particular coefficients of performed yield functions an appropriate error-function is defined and minimized by a Newton algorithm. To compare the calculated yield stresses and r-values with experimental data a relative root mean square deviation method presented by Leacock [Alan G. Leacock, J. Mech. Phys. Solids 54 (2006) 425–444] is used. Work-hardening effects on the FLD are analyzed by using Swift and Voce hardening laws. The effect of yield surface on the prediction of numerical FLDs and the number of experimental anisotropy parameters on the accuracy of yield functions are also studied.     

Formability prediction of advanced high strength steels using constitutive models characterized by uniaxial and biaxial experiments

Sheet formability, as determined by the limiting dome height (LDH) test, was evaluated for DP and TRIP steel sheet samples. The LDH test was also predicted with finite element (FE) simulations using various constitutive models. Three yield functions, von Mises, Hill's 1948, and Yld2000-2d, were considered to examine the effect of the yield criterion on formability. The anisotropy parameters were determined from different experimental tests and their influences on LDH predictions were analyzed. For Hill's 1948 model, the coefficients were calculated either using the yield stresses or r-values measured in different tension directions. The anisotropy coefficients of the Yld2000-2d were determined using in-plane biaxial test data in addition to the conventional uniaxial test-based data. The stress-strain curves for hardening characterization were measured using uniaxial and bulge tests. The latter provides the flow stress over an extended strain range, compare with uniaxial tension, without showing instability. The constitutive models were implemented in a FE code with a user material subroutine. They were evaluated by comparing the experimental and predicted punch load–displacement and sheet thickness variations after forming in the LDH test. The results for this particular example demonstrated that the non-quadratic yield function and the hardening curve of the bulge test improve the prediction accuracy for sheet forming and formability analyzes significantly.