Identification of Material Damage Model Parameters: an Inverse Approach Using Digital Image Processing

A reliable prediction of ductile failure in metals is still a wide-open matter of research. Several models are available in the literature, ranging from empirical criteria, porosity-based models and continuum damage mechanics (CDM). One major issue is the accurate identification of parameters which describe material behavior. For some damage models, parameter identification is more or less straightforward, being possible to perform experiments for their evaluation. For the others, direct calibration from laboratory tests is not possible, so that the approach of inverse methods is required for a proper identification. In material model calibration, the inverse approach consists in a non-linear iterative fitting of a parameter-dependent load–displacement curve (coming from a FEM simulation) on the experimental specimen response. The test is usually a tensile test on a round-notched cylindrical bar. The present paper shows a novel inverse procedure aimed to estimate the material parameters of the Gurson–Tvergaard–Needleman (GTN) porosity-based plastic damage model by means of experimental data collected using image analysis. The use of digital image processing allows to substitute the load–displacement curve with other global quantities resulting from the measuring of specimen profile during loading. The advantage of this analysis is that more data are available for calibration thus allowing a greater level of confidence and accuracy in model parameter evaluation.     

水泥砂浆在主动围压下的动态力学性能

水泥砂浆等脆性材料的力学性能与所受围压的大小密切相关。为了研究水泥砂浆在围压下的动态力学性能,研制了适于分立式Hopkinson压杆加载的主动围压装置,最大预加载主动围压大于30 MPa。实验得到了水泥砂浆在不同围压、不同应变率下的轴向应力应变曲线,发现材料在围压作用下抗压强度和韧性大大提高并且整体进入了伪塑性,而材料的应变率效应也是显著的。     

基于声发射参量的不同深度灰岩塑性破坏识别研究

对某矿深部灰岩进行单轴压缩试验,测得试件破坏全过程的应力、应变和声发射参数。通过分析应力、应变和声发射参数之间的关系,可根据声发射振铃计数率的变化情况,将全过程分为三个阶段:第一阶段处于孔隙裂隙压密阶段,振铃计数率较多;第二阶段处于稳定变形阶段,声发射处于平静期;第三阶段岩石进入塑性变形阶段,振铃计数率明显增加。研究发现,深度越深,第一阶段终止时的应力水平越高;将声发射累计能率与累计振铃计数率的比值定义为能振比λ,λ的变化规律一定程度上反映出试件内部裂隙发展情况。研究λ的变化规律能够为识别岩体失稳提供依据,提高声发射技术监测预报的准确性。     

Non-linear Model for Wood Saturation

The paper presents a non-linear model of saturation with fluid of anisotropic capillary porous bodies and the results of experimental investigations of wood saturation with methacrylate. The obtained experimental curves illustrating the distribution of methacrylate in wood samples allow the estimation of material coefficients and verification of the theoretical model. The theoretical model is developed based on the balance equations of mass, momentum, and energy, and the thermodynamics of irreversible processes. The non-linear differential equation, describing the distribution of methacrylate content in wood and its evolution during the saturation is solved numerically. The theoretical curves obtained on the basis of both linear and non-linear models are compared with the experimental data and better agreement between them for the non-linear model is stated.     

非线性系统参数辩识的一种频域模型

本文基于对非线性系统的可分离性假设，将非线性弹性力和阻尼力分别分解为物理坐标下各点间相对位移和相对速度的幂级数函数，导出了一般多自由度非线性系统在恒幅激励下的广义频率响应函数与输入输出之间的迭代关系式，提出了非线性系统中基本线性部分的概念，进而了一种在实验条件下的系统物理参数辩识方法。     

Flat-jack tests and inverse analysis for the identification of stress states and elastic properties in concrete dams

In this paper an experimental-numerical method, centred on flat-jack tests, is presented for the identification of local stress states and possibly deteriorated elastic properties of concrete in existing dams. It is shown herein that the synergistic combination of new pattern of flat-jack experiment, computer simulation of the test (by conventional finite elements) and inverse analysis allows to exploit experimental data more effectively than by the traditional procedure, and to achieve more information on material properties. In fact, at suitably chosen locations on the free surface of the monitored dam, all the components of local (plane) stress state and the elastic moduli in two orthogonal directions (including shear stiffness) can be estimated by the proposed method. The inverse problem in point is formulated as a sequence of two parameter identifications, i.e. as a bilevel (in the sense of Stackelberg) mathematical programming problem. The solution in a stochastic context is achieved by means of a modified Bayes technique, allowing to obtain, in a “batch” (non sequential) way, parameter estimates endowed with a covariance matrix which quantifies their degrees of confidence and correlations. Dedicated to the memory of Professor Giulio Ceradini.     

聚丙烯短微纤维增强水泥冲击特性研究

基于唐志平等提出的剪切波跟踪技术(SWT),对聚丙烯短微纤维增强水泥(FCEM)和素水泥(CEM)进行了冲击速度76～506m/s、倾斜角为10的斜撞击试验。研究结果表明,两种材料的Hugoniot冲击绝热线均有四个明显的临界点A、B、C、D,分别对应材料的Hugoniot弹性限、剪切卸载波(S-)消失、孔洞崩塌点和颗粒材料的再压缩点。其中临界点B,在过去的研究工作中尚未被揭示,只有采用剪切波跟踪技术对S-波进行跟踪测量,才能最终确定。对剪切强度的分析结果表明,采用剪切波,尤其是卸载剪切波来探测脆性材料内部动态损伤非常有效。     

基于扰动状态概念的软岩蠕变本构模型与试验验证

软岩材料的蠕变过程从初始加载阶段开始即容易产生塑性变形,这与硬岩存在较大差异,而传统的元件模型仅在应力超过某一阈值后才开始描述其粘塑性行为。通过分析岩石加卸载时效变形过程中弹塑性状态的演变规律,以扰动状态理论为基础,确定了以塑性变形为变量的扰动因子函数,并通过塑性变形随时间的变化特征进一步建立了以时间为自变量的蠕变扰动因子演化方程。在此基础上,针对泥质页岩的蠕变变形过程,选取Burgers和Bingham模型分别描述扰动状态理论中的相对完整状态和完全调整状态,并选取蠕变扰动因子为权重函数建立了基于扰动状态理论的蠕变本构模型。通过泥质页岩的室内蠕变特性试验对模型进行有效性检验表明,理论曲线与实测曲线的逼近程度较高,蠕变扰动函数能随时间的发展持续调整相对完整状态向完全调整状态转换的过程,较高应力状态下软岩进入完全调整状态的速率较快,相对传统模型该模型具有更好的非线性及阶段协调性,可较好地描述软岩的初始、稳定和加速蠕变阶段。     

几种水机常用金属材料的冲蚀磨损性能研究

研究了在水力机械中普遍应用的铸铁(HT200)、中碳结构铸钢(40#)、不锈钢(1Cr18Ni9Ti)和高铬铸铁(KmTBCr26)4种材料的冲蚀磨损规律,得出了材料的冲蚀磨损率随磨损时间、冲蚀速度、磨粒浓度和粒径变化的规律,同时根据观察材料冲蚀磨损表面形貌探讨了其冲蚀磨损机理.结果表明:材料的冲蚀磨损性能受到材料基本机械性能的控制,其中高硬度材料的抗冲蚀性能较好,在相同试验条件下其磨损率由大到小排列顺序为HT200＞40#＞1Cr18Ni9Ti＞KmTBCr26;4种材料的冲蚀磨损率随磨粒粒径、浆料浓度和磨粒冲击速度的变化规律基本一致,当磨粒粒径小于425 μm时,磨粒粒径对KmTBCr26和1Cr18Ni9Ti的磨损率影响很小;金属材料的组织结构对材料的冲蚀磨损性能和机理影响较大,其中KmTBCr26和1Cr18Ni9Ti的冲蚀磨损机理以脆性断裂、破裂及颗粒脱落为主,而HT200和40#的冲蚀磨损机理以选择性切削式冲蚀磨损为主.     

Application of a n-Phase Model to the Diffusion Coefficient of Chloride in Mortar

The determination of the chloride diffusion coefficient of a concrete is needed to help the prediction of the service life of concrete structure. In this paper, we propose first a critical review of models for chloride diffusion coefficients already used in literature at different scales and then we develop an analytical model, which takes into account the characteristics of the different phases of concrete. These materials are treated as a three-phase composite, consisting of a cement continuous phase, of an aggregates dispersed phase and of an interface transition zone. Chloride diffusion coefficient using an n-layered inclusion-based micromechanical modeling is predicted. The details of calculations are summarized hereafter and experimental measurements obtained on mortars are compared with predicted results.     

运动副摩擦参数的识别方法研究

采用粘性 库仑摩擦模型建立了单自由度回转机械系统运动微分方程,分别从时域和频域角度提出了两种识别运动副摩擦参数的方法。仿真结果表明,两种方法在较高信噪比下均能有效地辨识出库仑摩擦和粘性摩擦参数,其中频域法比时域法具有更高的参数估计精度。对电机驱动的单自由度转动机械系统实验装置的运动副建立了相应的摩擦模型,实验结果表明:在不同试验条件下两种方法计算得到的库仑和粘性摩擦参数具有较好重复性,从而验证了本文参数辨识方法的正确性。     

基于科学计量学的中国实验力学学科研究水平的国际化比较

本文采用科学计量学方法,对中国实验力学的研究水平进行评估。首先介绍科学计量学方法及评价指标,采用关键词及目标期刊构造相关文献集合;然后对中国实验力学学科的整体产出情况进行国际化比较,并根据中国实验力学学科发展特色,从光测力学、微纳米实验力学、专门化实验力学三个领域分析中国实验力学学科的产出优势及发展趋势,同时给出实验力学相关文献集合的学科分布以及5种重要期刊。结果表明,中国实验力学学科的研究论文产出能力位居全球第二,其影响力水平位居全球第五。     

Identification of the Parameters of Van der Pol Oscillators

International Applied Mechanics - The problem of identifying the parameters of a dynamic system that describes the oscillations of coupled van der Pol oscillators is considered. A method for...     

膨胀石墨燃爆剂JWL状态方程参数拟合

用LS-DYNA对膨胀石墨燃爆剂爆炸过程进行数值模拟时,需要提供膨胀石墨燃爆剂的JWL状态方程的参数。把凝聚炸药等熵线物态方程作为目标方程,通过差分进化法拟合得出膨胀石墨燃爆剂JWL状态方程的参数。通过圆筒实验对所求得的参数进行验证,结果表明,理论值与实验值最大误差不超过3.3%,能够满足膨胀石墨燃爆剂爆炸模拟研究的需要。     

纯钽动态本构关系的实验研究

利用材料试验机和分离式霍布金森压杆实验装置测量了纯钽在较宽温度(-100～550℃)和应变率(10-5～103s-1)范围内的应力 应变曲线,该曲线表明钽的流动应力对温度和应变率都很敏感。利用所测得的应力 应变曲线拟合了钽在不同应变率和不同温度下的Johnson Cook与Zerilli Armstrong本构方程,Zeril li Armstrong本构方程与实验数据基本吻合,Johnson Cook本构方程与实验数据吻合得不太好。对Johnson Cook本构方程中的应变率强化项作了修正,修正后的Johnson Cook本构方程与实验数据吻合比修正前的好。     

岩石拉压蠕变试验仪研制及应用

为了探索岩石在周期性恒定拉伸、压缩荷载作用下的蠕变行为,结合杠杆原理,设计研制了一种岩石杠杆式拉伸、压缩蠕变试验仪。该仪器具有挂重质量小、可方便切换拉、压荷载等特点。首先,通过标定好的数显式拉压荷重传感器对该试验仪拉伸、压缩荷载进行了校正,得出试验仪压缩、拉伸荷载杠杆扩力比分别为81.29倍与59.46倍,挂重质量与施加在岩样荷载呈线性关系(压缩作用下线性关系相关系数R2=0.99975,拉伸作用下R2=0.9991),荷载施加稳定。最后,采用该试验仪对红砂岩进行了单轴恒定拉、压循环荷载下的蠕变试验,探讨了受荷岩样拉压蠕变规律。上述成果丰富了岩石蠕变测试与研究内容,有助于岩石力学试验测试的发展。     

子结构物理参数的一种识别方法

本文基于复模态综合和优化技术,提出一种子结构物理参数识别方程。该方法利用复模态综合在广义坐标下建立物理参数识别方程,然后将物理参数识别化为优化问题求解。给出了一台钻床的子结构物理参数识别实例来说明方法的有效性.     

汽车液压阻尼器的四段线性化模型

通过对现有某车用液压阻尼器的实验研究和车辆悬架系统动力学仿真研究，发现这种阻尼器的动力学特性存在复杂的非线性行为。为准确描述这类阻尼器的动力学特性和进行汽车悬架系统的动力学仿真研究，本文根据阻尼器的实验数据，提出一种新型四段线性化阻尼器模型，并建立其相应的数学模型。通过汽车悬架系统的动力学仿真研究表明，采用四段线性化阻尼器模型，汽车悬架系统簧载质量在不同路面不平度函数作用下的加速度响应和均方根加速度响应随频率变化不会出现大的波动。与等效线性化阻尼器模型相比，四段线性化阻尼器模型，能较好降低汽车悬架系统簧载质量的加速度响应和均方根加速度响应，有利于提高汽车乘坐舒适性和行驶稳定性，并为新型阻尼器设计提供一个新的思路。     

Some considerations on the experimental determination of moments of inertia

The experimental determination of the moments of inertia of rigid bodies, which is a common practice in many fields of technology, is usually performed using torsional or multifilar pendula. Different experimental techniques, based on the study of the periodic or non-periodic motion of the test object on which known forces act, are described and critically analysed. In particular, the effect of damping and, in the case of devices which can be assimilated to a pendulum, that of the non-linearity due to the finite amplitude of the motion, are studied in detail.The equations of motion of the multifilar pendulum are studied in detail to assess how the assumptions usually considered affect the accuracy of the results. An example of experimental determination of the moment of inertia of a flywheel, employing the trifilar pendulum currently used at the Laboratory of the Mechanics Department of Politecnico di Torino, shows how the theoretical considerations apply to a practical case.

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Sommario La determinazione sperimentale dei momenti d'inerzia di corpi rigidi, prassi consueta in molti settori della tecnologia, si avvale normalmente dell'utilizzo di pendoli di torsione o di pendoli multifilari. Nel presente lavoro vengono descritti ed esaminati criticamente vari metodi, basati sullo studio di moti periodici o non periodici dell'oggetto in prova sottoposto a forze note. Viene studiato in particolare l'effetto dello smorzamento e, nel caso di metodi basati su dispositivi assimilabili ad un pendolo, quello delle nonlinearità dovute all'ampiezza delle oscillazioni sui risultati della misura.Vengono studiate in dettaglio le equazioni del moto del pendolo multifilare allo scopo di valutare in quale misura le ipotesi semplificative normalmente accettate influiscano sulla precisione dei risultati. Un esempio di misura sperimentale effettuata mediante il pendolo trifilare correntemente utilizzato presso il laboratorio del Dipartimento di Meccanica del Politecnico di Torino conclude il lavoro e permette di verificare le conclusioni teoriche in un caso concreto.

     

Stress analysis of weldments by holographic moiré and the finite element method

The research work presented in this paper deals with the determination of the stress concentration at the root of the fillet of a weldment on a T steel specimen. A finite element model was developed for the specimen. Measurements were carried out using the holographic moiré technique. Strain gages were added to evaluate extrapolation techniques proposed in the literature. Good agreement was found between the finite element results and the optically measured values. The strain gage extrapolation technique yielded very low values.