不同应变率下45钢的层裂研究

本文在利用轻气炮加载技术，确定击靶速度为500m／s左右的非对称碰撞情况下，利用VISAR测试技术，测得样品后表面的的自由面速度历史，并通过在靶前加不同阻抗的铝和有机玻璃(PMMA)来改变材料的加载应变率，其应变率范围在10^4-10^5／s情况下，测得45钢的层裂强度受应变率的影响．     

对膨胀壳体材料失稳的一种简化处理

提出一种简化的塑性失稳处理并被作为高应变率拉伸下材料破坏的粗估判据。这种准则假设局部化失稳在如下条件下发生：应变及应变率硬化率恰被几何及热软化率平衡。指出了经验本构方程及本征本构方程对膨胀断裂应变的影响。通过算例也说明了热软化壳体在内压作用下的破裂过程。     

Modeling Creep and Continuous Fracture Process Zones in Spatial Prismatic Bodies

The semianalytic finite-element method is used to develop a method for solving problems of creep and continuous fracture of complex spatial bodies. The method allows modeling the variation of the stress-strain state during creep, accompanying accumulation of dispersed microdamages, and development of macroscopic effects—continuous fracture process zones. The growth of a continuous fracture process zone is modeled. A criterion is formulated for determination of the applicability limits of continuum damage mechanics. The method is exemplified by the problem of deformation and continuous fracture of a gas turbine blade __________ Translated from Prikladnaya Mekhanika, Vol. 41, No. 9, pp. 70–86, September 2005.     

Elasto-plastic analysis of influences of bond deformability on the mechanical behavior of fiber networks

Micromechanical modeling of three-dimensional (3D) fiber networks is performed by reducing the web structure to fiber segments and fiber–fiber bonds to explore the influence of fiber–fiber bond deformability on fiber network’s elasto-plasticity. The fiber segment between every two adjacent bonds is described by a Timoshenko beam element, while fiber–fiber bonds are taken as a different two-node element due to the extremely large height-to-span ratio. This overcomes the rigid-bond assumption employed by most previous network models, providing the feasibility to build the relationship between bonding condition and global mechanical properties, resulting in a 3D network structure and able to accommodate curled fibers. Both fibers and bonds are assumed to be elasto-plastic and described by the bilinear kinematic hardening model. Deformation of the network with load increasing is simulated by the Newton–Raphson method. Numerical tensile tests show reasonable results and agree qualitatively with experiments. The influence of bond deformability on the mechanical behavior of the network is discussed in detail.     

Creep of axisymmetrically loaded plates with allowance for damage accumulation in their material

The Rabotnov kinetic creep theory was used to calculate the stress-strain state and damage accumulation in the material of axisymmetrically loaded circular and ring plates at any time before the beginning of fracture. It is shown that the solution of the problem can be reduced to solving the same problem under the assumption of steady-state creep of the material. The unsteady creep problem is solved by multiplying the known solution of the steady-state creep problem by certain functions of the coordinates and time, which are determined from a corresponding system of equations. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 157–168, September–October, 2006.     

Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650°C.     

The investigation of the metal damage at the creep by the method of electrical resistance measuring

In this paper the method of damage measurement of metal structure at the creep is proposed. In contrast to other methods, it allows the measurement of this damage to be carried out in the process of creep test without unloading and cooling of specimens. Experimental damage curves during creep are obtained as a result of test data processing by the suggested method. The analysis of these curves leads to a conclusion that the material damage at repture is monotonically decreasing function of the applied stress. This conclusion is an experimental verification of the theoretical result, obtained earlier. Visiting scholar (1988–1989) at South China University of Technology     

Determining the Thermoviscoplastic State of Shells of Revolution Subject to Creep Damage

A technique is developed for determining the thermoviscoplastic state of shells of revolution with allowance for creep damage. The technique is based on the hypotheses of rectilinear element and the theory of deformation along paths of small curvature. The equivalent stress appearing in the kinetic equations of damage and creep is determined using a creep-rupture criterion that accounts for the stress mode and the level of irreversible strains. The technique is tested by determining the thermoviscoplastic state and time to failure of tubular specimens under a tensile force and a torque     

Some Inverse Problems of Deformation and Fracture of Physically Nonlinear Inhomogeneous Media

The following two types of physically nonlinear inhomogeneous media are considered: linear-elastic plane with nonlinear-elastic elliptic inclusions and linear-viscous plane with elliptic inclusions from a material that possesses nonlinear-creep properties. The problem is to determine infinitely distant loads that produce a required value of the principal shear stress (in the first case) or principal shear-strain rate (in the second case) for two arbitrary inclusions. Conditions for the existence of solutions of these problems for incompressible media under plane strains are obtained.     

Damage Accumulation in Aluminum Alloys under Plastic Deformation and Creep

The accumulation of local and bulk damage in D16 AT and 1201 T1 aluminum alloys used in aircraft engineering is studied. The local damage level is calculated from data of thermoactivation analysis of the residual life of D16 AT alloy specimens after preliminary plastic deformation. The bulk damage level is determined from the elastic-modulus defect by measuring the natural frequency of 1201 T1 alloy specimens. Life tests of the specimens were performed at constant tensile loads and elevated temperatures. The dependence of the local damage on preliminary plastic strain at room temperature is obtained. The residual life of the specimens is calculated with allowance for the damage to the material in the initial stage of failure and compared with experimental results. Data are given on the kinetics of bulk-damage accumulation in various test regimes. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 1, pp. 172–182, January–February, 2006.     

The influence of a rate sensitive effect of material for a propagating crack-tip field

The elastic-viscoplastic model proposed in [1] is used in this paper to analyze the quasi-statically growing crack field and dynamic propagating crack-tip field of mode III. By analysis, we obtained more reasonable results than those of an elastic-plastic material. When the effect of rate sensitivity of a material is considered, it is found that only the quasi-statically growing crack-tip field is the special case of dynamic propagating crack-tip field when Mach numberM approaches zero.     

Service life of pressurized vessels

A study is made of the dependence of the time of propagation of a fracture front in thick-walled vessels subjected to internal pressure on the wall thickness and the employed version of kinetic theory of creep. The stress intensity is taken to be the equivalent stress. Recommendations on using the results in computational applications are formulated. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 5, pp. 173–182, September–October, 2007.     

Nonlinear stochastic creep problem for an inhomogeneous plane with the damage to the material taken into account

An analytical method for the solution of two-dimensional nonlinear creep problems is developed using as an example the biaxial extension of a plane from a stochastically inhomogeneous material with damage accumulation and the third stage of creep taken into account. The governing creep relation is adopted in accordance with the energetic version of the nonlinear theory of viscous flow. The stochasticity of the material is defined by two random functions of coordinates. Formulas for calculating the stress variance are obtained. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 2, pp. 140–146, March–April, 2007.     

Asymptotics of stresses and strain rates near the tip of a transverse shear crack in a material whose behavior is described by a fractional-linear law

An approximate solution of the problem of determining the fields of stresses and strain rates due to creep near the tip of a transverse shear crack in a material whose behavior is described by a fractional-linear law of the theory of steady-state creep is given. It is shown that the strain rates have a singularity of the type ∼ r^−α near the crack tip; the order of singularity α changes discretely, depending on the polar angle, and takes the values 1, 2/3, and 1/2. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 1, pp. 165–176, January–February, 2009.     

Creep damage constitutive equations for multi-axial states of stress for 0.5Cr0.5Mo0.25V ferritic steel at 590°C

Multi-axial creep damage constitutive equations are developed for 0.5Cr0.5Mo0.25V ferritic steel at 590°C. It is aimed to overcome the deficiency of inconsistency in predicted and observed creep strains existed in previous formulations [Q. Xu, in: European Conference on Advances in Mechanical Behaviour, Plasticity and Damage, 7–9 November 2000, Tours, France; Creep Damage Mechanics Research Report, The School of Engineering, The University of Huddersfield, 2000; I.J. Perrin, D.R. Hayhurst, J. Strain Anal. 31 (4) (1996) 299]. In this paper, the lifetime and strain at failure are qualitatively validated in a wider range of states of stress by which the capability of producing consistent results is demonstrated.     

Modeling of impact on concrete plates by use of the microplane approach

A numerical approach to model 3D impact scenarios on concrete structures is presented and validated by experimental investigations. Special focus is given to dynamic contact modeling of an impactor loading a concrete plate at very high velocity. In this context, the main aspect is the elimination of non-physical oscillations of the kinematic fields by using an updated Newmark integration scheme. Two Lagrangian multipliers are used for this update which successfully reduces the unphysical oscillations of the above mentioned fields. In a further step, the material behavior of concrete is described by the microplane model introducing a new rate-dependent non-local damage formulation. Using the equivalent strain as a non-local field, the method is able to model, in an adequate manner so that the crack pattern in concrete is represented by the damage evolution in case of transient problems. The accuracy is validated and proved by the help of benchmark simulations as well as the observed phenomena to experimental investigations.     

A Network Modeling Approach to Derive Unsaturated Hydraulic Properties of a Rough-Walled Fracture

The hydraulic properties of a rough-walled fracture in a limestone sample are estimated using a network model based on three-dimensional representations of the fracture apertures. Two different scenarios are considered: drainage of water out of a fracture and infiltration of water into a fracture. Besides capillary effects, the model takes into account an accessibility criterion (invasion percolation) and, in the case of infiltration, the rate dependence of the water movement. A hysteresis effect between drainage and imbibition hydraulic properties can be observed, which increases with increasing capillary number. The measured permeability is overestimated by 15% by the network model. In a sensitivity analysis the influence of the main fracture field characteristics (field size and fracture segment size in relation to correlation length) on the calculated hydraulic properties is investigated. Field size has an important influence on the inverse of the water/air entry value for imbibition, making it difficult to scale this parameter to other field sizes. A parameter analysis investigating the influence of the main fracture characteristics (mean fracture aperture, roughness, and correlation length) on the hydraulic properties shows that the mean fracture aperture is the most important fracture parameter influencing both strongly the saturated permeability K and . The effect of varying the variance and the correlation length on K and is much less than the influence of the mean fracture aperture. The effective permeability of the fracture is also calculated by the geometric mean K _g . Up to (log_e(K)) = 1, the discrepancy between K _g and K _n (network model result) is less than 15%. At larger correlation lengths (for a constant (log_e(K))), the discrepancy between K _g and K _n increases.     

Creep damage and life analysis of anisotropic materials

Summary  This paper provides a short survey of some recent advances in the mathematical modelling of materials behaviour under creep conditions. The tertiary creep phase is accompanied by the formation of microscopic cracks on the grain boundaries in such a way so that damage accumulation occurs. The paper is divided into three parts. Firstly, the damage state in a uniaxial tension specimen is discussed and the time to rupture calculated. The second part is concerned with the creep behaviour of materials in multiaxial stress. Because of its microscopic nature, damage generally has an anisotropic character even if the material was originally isotropic. The fissure's orientation and length cause anisotropic macroscopic behaviour. Therefore, damage in an isotropic or anisotropic material, which is in a state of multiaxial stress, can only be described in a tensorial form. Thus, tensorial constitutive and evolution equations have been developed. Some examples for practical use are discussed. Finally, some own experiments are mentioned which have been carried out in order to validate the mathematical modelling. Received 16 July 1999; accepted for publication 8 March 2000     

TNT炸药动静态变温三轴压缩力学性能的试验研究

介绍了一种高能材料动静态变温三轴压缩试验装置，在熔点以下温度，通过对国产单质炸药ＴＮＴ在中等应变速率（２－４／ｓ）及准静态（３×１０－３／ｓ）条件下三轴压缩力学性能的测试，得出ＴＮＴ的杨氏模量Ｅ、泊松比ν和屈服极限Ｙ，以及当加载速率和温度改变时这些参数的变化规律。并提出三轴应力状态下屈服极限的一种确定方法。该三轴压缩试验装置以及三轴压缩应力状态下力学参数的测试、分析、处理系统为研究高能材料力学性能提供了一套完整的技术和手段。