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811.
The thermodynamic setting for the formulation of the “crack-driving force” for a singular crack in conjunction with rate-independent material response is discussed. One key ingredient is the introduction of a fixed (absolute) configuration, relative to which both physical and (virtual) configurational and spatial changes can be described. Only quasistatic and isothermal conditions are considered in this paper. A variational framework is established for the rate of global energy dissipation (integrated over the whole material domain) due to the combined action of a (discrete) crack extension and continuum inelasticity, whereby the material time derivative of internal variables and the rate of crack extension are coupled. The classical assumption (previously adopted in the literature) is that there is no coupling, i.e. the internal variables are considered as fixed (material) fields just like an inhomogeneous material property. The other (extreme) assumption is that the internal variables fields are convected with the configurational motion due to the virtual crack extension. Both cases are investigated in this paper for a simple 2D example of an edge crack in a plate in a setting of small strains and hardening plasticity. In particular, we consider convergence issues from mesh refinement. 相似文献
812.
针对目前空气冲旋钻头钻进中牙齿频繁断裂、脱落以及磨损严重,导致破岩效率低、使用寿命短等问题,采用纳米改型技术,在硬质合金中掺杂纳米Al2O3制备出Al2O3/WC Co纳米/微米复合材料,对其性能、组织结构进行分析,对其耐磨性、耐冲击性进行实验研究,结果表明:加入3%的纳米Al2O3并添加适量的抑制剂和稀土元素能明显改善Al2O3/WC Co纳米/微米复合材料的组织结构、细化晶粒,复合材料的耐磨性和耐冲击性能得到大幅提高,较硬质合金YG8提高4~8倍。为大幅度提高钻头破岩效率,延长钻头使用寿命提供了可能性。 相似文献
813.
以醋酸钠为矿化剂, 通过水热合成法直接获得了较纯净的Material Institut Lavoisier-101(MIL-101). 样品经粉末X射线衍射、比表面积及孔径分布和热重分析表征, 并与以氢氟酸为矿化剂合成的MIL-101进行了比较. 结果表明, 以醋酸钠为矿化剂合成的MIL-101的性能优于以氢氟酸作为矿化剂合成的MIL-101, 并且样品经后续纯化处理后能进一步提高比表面积和孔容; 用非定域密度泛函法(NLDFT)分析计算得到了MIL-101的孔结构信息. 合成的MIL-101骨架能稳定至300 ℃以上, 其热稳定性略低于以氢氟酸作为矿化剂合成的MIL-101. 相似文献
814.
815.
李鹏 《西昌学院学报(自然科学版)》2009,23(1):49-52
在岩石工程的锚固过程中,影响锚杆加固效果有许多因素:锚杆材料、锚杆结构、锚固方式、岩体性质和锚固参数等。对其影响因素的研究可以为岩石工程围岩的有效防护和具体的施工提供很有价值的参考。研究表明加锚前后围岩稳定性的改变将是锚固效果的直接反映。 相似文献
816.
A novel method is suggested to deal with so-called uncertainty inverse problems (UIPs) which are a class of inverse problems with uncertainty in the system parameters of the forward model.
Interval which represents a closed bounded set of real numbers is used to model and characterize the uncertainty in our formulation,
and hence only the bounds of the uncertainty of the system parameters are needed. For a specific input vector, the possible
values of the outputs form an interval vector because of the uncertainty. An error function is defined using the measured interval vector of the outputs and those computed
using a forward model. The UIP is then formulated as an optimization problem which minimizes the error function. To improve
the optimization efficiency, an interval forward model is constructed based on the interval analysis method which can calculate
very efficiently the bounds of the outputs caused by the uncertainty of the system parameters. The present method is applied
to a complex inverse problem, namely material characterization of composite laminates using elastic waves. Uncertainty of
load is considered, and the hybrid numerical method (HNM) is used to compute the transient displacement responses. The engineering
constants of two kinds of laminates are successfully identified using the simulated measurements of the outputs. 相似文献
817.
Local strain data obtained throughout the entire weld region encompassing both the weld nugget and heat affected zones (HAZs)
are processed using two methodologies, uniform stress and virtual fields, to estimate specific heterogeneous material properties
throughout the weld zone. Results indicate that (a) the heterogeneous stress–strain behavior obtained by using a relatively
simple virtual fields model offers a theoretically sound approach for modeling stress–strain behavior in heterogeneous materials,
(b) the local stress–strain results obtained using both a uniform stress assumption and a simplified uniaxial virtual fields
model are in good agreement for strains ɛ
xx < 0.025, (c) the weld nugget region has a higher hardening coefficient, higher initial yield stress and a higher hardening
exponent, consistent with the fact that the steel weld is overmatched and (d) for ɛ
xx > 0.025, strain localization occurs in the HAZ region of the specimen, resulting in necking and structural effects that complicate
the extraction of local stress strain behavior using either of the relatively simple models.
相似文献
S. M. AdeebEmail: |
818.
Shear horizontal (SH) waves in free and clamped monoclinic plates with an arbitrary inhomogeneity across the plate are considered. Firstly, some general properties of dispersion spectra of, specifically, the SH waves are pointed out. Secondly, analytical and numerical modeling of the SH dispersion branches is presented. Closed-form estimations are compared with exact curves computed for a free plate with continuously varying properties. 相似文献
819.
The physical occurrence that crack surfaces are in contact at the compressive edges when a flat or a shell is subjected to a bending load has been recognized. This article presents a theoretical analysis of crack–face contact effect on the stress intensity factor of various shell structures such as spherical shell, cylindrical shell containing an axial crack, cylindrical shell containing a circumferential crack and shell with two non-zero curvatures, under a bending load. The formulation of the problem is based on the shear deformation theory, incorporating crack–face contact by introducing distributed force at the compressive edge. Material orthotropy is concerned in this analysis. Three-dimensional finite element analysis (FEA) is conduced to compare with the theoretical solution. It is found that due to curvature effect crack–face contact behavior in shells differs from that in flat plates, in that partial contact of crack surfaces may occur in shells, depending on the shell curvature and the nature of the bending load. Crack–face contact has significant influence on the stress intensity factor and it increases the membrane component but decreases the bending component. 相似文献
820.