The components of energy release rate for interfacial cracks

Interfacial crack growth is inherently mixed mode in nature and mode-mixity must be defined clearly in order to characterize it. Mode   and mode   strain energy release rates for an interfacial crack in bimaterial system were analytically derived by the virtual crack closure technique. It is shown that the energy release rate for mode   and mode   do not converge due to the presence of violent oscillatory near tip behavior. However, the total energy release rate is well-defined.     

3D finite element analysis of stress distributions and strain energy release rates for adhesive bonded flat composite lap shear joints having pre-existing delaminations

The rate of propagation of embedded delamination in the strap adherend of lap shear joint (LSJ) made of carbon/epoxy composites has been evaluated employing three-dimensional non-linear finite elements. The delamination has been presumed to pre-exist in the thin resin layer between the first and second plies of the strap adherend. The inter-laminar peel and shear stress distributions have been studied in details and are seen to be predominantly three-dimensional in nature. The components of strain energy release rate (SERR) corresponding to the opening, sliding and cross sliding modes of delamination are significantly different at the two fronts of the embedded delamination. The sequential release of multi-point constraint (MPC) finite elements in the vicinity of the delamination fronts enables to simulate the growth of the delamination at either ends. This simulation procedure can be utilized effectively for evaluation of the status of the structural integrity of the bonded joints.     

Numerical calculation of energy release rates by virtual crack closure technique

A seamless analysis of material behavior incorporating complex geometry and crack- tip modeling is one of greatly interesting topics in engineering and computational fracture mechanics fields. However, there are still large gaps between the industrial applications and fundamental academic studies due to a time consuming detailed modeling. In order to resolve this problem, a numerical method to calculate an energy release rate by virtual crack closure technique was proposed in this paper. Both free mesh method and finite element method have been utilized and, thereafter, robust local and global elements for various geometries and boundary conditions were generated. A validity of the proposed method has been demonstrated through a series of fracture mechanics analyses without tedious crack-tip meshing.     

Quantification of wear,strain and heat energy consumption rates for sliding steel ball against diamond-like carbon coatings

Abstract

In this work, we evaluated the different energy consumption rates associated with the total frictional energy for a ball sliding on a flat surface. The energy generated by the sliding two bodies in contact is dissipated into the materials in various forms. The wear consumption energy for a steel ball against a diamond-like carbon surface was evaluated by the wear coefficient of the wear volume–energy input equation. The strain energy generated in the steel ball as a result of being made to slide under a certain load was calculated using the Hertzian theory. The chemical reaction energy was estimated based on iron oxidation. Finally, the frictional energy dissipated as heat was obtained by subtracting the wear and the strain energies from the total frictional energy.     

Energy release rates for various defects under different loading conditions

It is well known that the energy release rate associated with translation, rotation and self-similar expansion of defects in solids are expressed by the path-independent integrals J, L, and M, respectively. It is shown that these integrals for a crack or a circular hole may be obtained by first considering an elliptical cavity and then performing a limiting process. This obviates dealing with singularities at the crack tip. The energy release rates for these defects under various mechanical, thermal and electromechanical loading conditions are calculated.     

Clustering of decomposed strain signal energy for durability classification

Journal of Mechanical Science and Technology - This paper presents clustering of automotive spring fatigue life for failure classification based on K-means approach. For safety promotion of buses,...     

A simple method to calculate mode components of strain energy release rate of free-edge delaminations in composite laminates

A simple method, which calculates the mode components of the strain energy release rate of free-edge delaminations in the laminates, is proposed. The interlaminar stresses are evaluated as an interface moment and interface shear forces that are obtained from the equilibrium equations at the interface between the adjacent layers. Deformation of an edge-delaminated laminate is calculated by using a generalized quasi-three dimensional classical lamination theory developed by the author. The analysis provides closed-form expressions for the three components of the strain energy release rale. The analyses are performed on [+30/−30/90]s laminates subjected to uniaxial extension, with free-edge delaminations located symmetrically and asymmetrically with respect to the laminate midplane. Comparison of the results with a finite element solution using the virtual crack closure technique shows good agreement. The simple nature of this method makes it suitable for primary design analysis for the delaminations of composite laminates.     

残余应力应变释放系数的有限元分析

通过建立16Mn残余应力盲孔法有限元分析模型,模拟出孔边应力、应变的变化情况.揭示了残余应力引起的孔边应力、应变变化情况与应变释放系数A、B值之间的关系,并用孔边应力、应变的有限元运算结果计算出A、B值.通过应变计中点应变法、应变计中点应力法和平均应变法三种方法去标定和修正应变释放系数,通过对三种方法的比较表明,三种方法的计算结果与实验结果相符.且计算精度较高.证实了残余应力有限元模拟的准确性和A、B值标定和修正方法的多样性.     

基于应变能约束的梁截面拓扑优化

根据工程实际应用中最关注的最优设计和合理经济尺寸的问题,建立了以梁截面为设计变量,以结构总重量极小化为目标,考虑应变能约束条件的梁结构拓扑优化设计数学模型,并结合ESO方法和单元应变能分析,讨论了受弯曲载荷作用下的梁的截面形式拓扑优化,建立其参数化模型,最后利用ANSYS的优化模块对梁的合理截面参数进行了优化设计.结果...     

Strain energy release maximization model for evolution of recrystallization textures

In 1995, the author advanced a model for the evolution of recrystallization texture, in which the absolute maximum internal stress direction due to dislocations generated during deformation or fabrication in the deformed material is aligned with the minimum Young's modulus direction in recrystallized grains, whereby the energy release during recrystallization can be maximized. This comes from the fact that the material concerned does not macroscopically change its shape and volume during recrystallization, and so the recrystallization is a displacement-controlled process. This strain energy release maximization model originates from the presumption that the stored energy due to dislocations is the major driving force for the recrystallization. The absolute maximum internal stress direction may be obtained from the operating slip systems, which are related to the deformation mode and texture. If one slip system is activated, the absolute maximum normal stress direction is parallel to the slip direction, or the Burgers vector direction. If more than one slip system is activated, the absolute maximum normal stress direction can be determined by the vector sum of related slip directions, by taking into account their contribution to slip. This paper reviews recrystallization textures of plastically deformed metals, based on the SERM model.     

Measurement of strain rate sensitivity of aluminium foams for energy dissipation

In this paper the structural performance of aluminium alloy foams have been investigated under both static and dynamic compression loads. Three foam typologies (M-PORE, CYMAT, SCHUNK) in a wide range of density (from 0.14 to 0.75 g/cm³), made by means of different process-routes (melt gas injection, powder metallurgy, investment casting) have been analysed. Foams microstructural characterization has been carried out through morphometric measurements by means of Scanning Electron Microscopy (SEM) and Computed Tomography (CT) and subsequent digital image processing in order to determine average cells size and cell distributions on different section planes. The experimental study aims to assess the strain rate sensitivity and energy absorption capability of commercially available metal foams and to point out the correlation between the mechanical behaviour and the physical and geometrical properties of the foam. It has been found that the specific energy dissipation of foams with similar density can be quite different: for the same volume of foam, average values of 1770, 1780 and 5590 J/kg at 50% nominal compression have been measured on M-PORE (0.19 g/cm³), CYMAT (0.28 g/cm³) and SCHUNK (0.28 g/cm³) foams, respectively. Impact tests showed that the dependence of the plateau stress on strain rate could be considered negligible for M-PORE and CYMAT foams while it is quite remarkable for SCHUNK foams. Moreover, it was found that the peak stress of CYMAT foams has a quite large sensitivity on the loading rate.     

Simplified hot rolling load calculations incorporating material strain rates

Hot rolling theory is briefly reviewed, and the use of a simplified method for calculating roll separating forces, which incorporates an approximation for the variation of flow stress as a function of strain rate, is discussed. Calculations of rolling loads for different samples and different conditions of strip inlet temperature, roll surface temperature, and strip reduction are included in an attempt to reproduce measured values. Agreement between these calculations and measured values for these different samples and conditions ranged from 12 to 33%.     

Plastic bulging of sheet metals at high strain rates

The biaxial bulge test is a material test for sheet metals to evaluate formability and determine the flow stress diagram. Due to the biaxial state of stress induced in this test, the maximum achievable strain before fracture is much larger than in the uniaxial tensile test. A new dynamic bulge testing technique is simulated and analyzed in this study which can be performed on a conventional split Hopkinson pressure bar (SHPB) system to evaluate the strain-rate dependent strength of material at high impact velocities. Polyurethane rubber as pressure carrying medium is used to bulge the OFHC copper sheet. The use of hyperelastic rubber instead of fluid as a pressure medium makes the bulge test simple and easy to perform. The input bar of SHPB is used to apply and measure the bulging pressure. The finite element simulation using ABAQUS/explicit and analytical analysis are compared and show good correlation with each other. The results clearly show that as the strain-rate increases, the strength of the OFHC copper increases. From the study, a robust method to determine the material behavior under dynamically biaxial deformation conditions has been developed.     

On reconstruction of cracks with interacting edges

A method for the reconstruction of the configuration of cracks is suggested based on finding heat sources that model the interaction between cracks edges (such as friction and collapse) in solids exposed to external ultrasound. The input information for reconstruction is a stationary temperature field on a solid boundary. The functional of nonreciprocity was constructed; the study of this functional made it possible to convert the problem of determining a cracks parameters into a few transcendental equations; explicit formulas were obtained for small cracks. The results of computational experiments on reconstruction of the parameters of a straight crack are considered.Translated from Defektoskopiya, Vol. 40, No. 10, 2004, pp. 62–69.Original Russian Text Copyright © 2004 by Vatulyan, Solovev.     

Using genetic algorithms to detect interfacial cracks on the basis of the thermal resistance of multilayer materials

To address the heat-conduction problem of multilayer materials for thermal contact resistance, this paper proposes an innovative solution: the finite element method. The direct heat-conduction problem can be regarded as a virtual element for thermal contact resistance. There is hardly any error between the result generated from the finite element method and those generated from the analytic method and the analog resistance method. For an inverse heat-conduction problem, on the basis of the consideration that air may enter to form the thermal resistance once a material cracks, GAMAS [1] (genetic algorithms based on migration and artificial selection) and the finite element method are used to simulate the transient temperature on the boundary and, plus a temperature measurement error, detect the location where the multilayer materials crack inversely and correctly. The text was submitted by the authors in English.     

A technique for obtaining compressive strength at high strain rates using short load cells

A technique for obtaining stress-strain data at high strain rates is given. The method involves use of a short load cell and a fibre optics displacement transducer. Computer aided analysis of the force-time data allows correction of the signal for the effect of the dynamic characteristic of the force measurement system. The method involves transformation to the frequency domain of the force data by means of a Fast Fourier Transform algorithm, and is shown to be able to make effective corrections for the dynamic ringing of the load cell and the phase lag of the recording system. The compression mode of deformation is used, lubrication being effected by means of a Teflon film. The results of preliminary tests performed on a medium carbon resulphurised steel are given.     

Mechanical characterization at intermediate strain rates for rate effects on an epoxy syntactic foam

An intermediate strain-rate mechanical testing technique was developed through proper modifications of a hydraulically driven loading frame (MTS 810) and a split Hopkinson pressure bar (SHPB). The modified MTS and SHPB were used to obtain valid stress–strain data for an epoxy syntactic foam at intermediate strain rates in the order from 10⁻¹ to 10² s⁻¹. Additionally, lower and higher strain-rate characterization of the foam material was conducted, such that the compressive stress–strain data of the syntactic epoxy foam were obtained at strain rates from 0.005 to 2150 s⁻¹ without any gap in the intermediate strain-rate range. The syntactic epoxy foam exhibited nonlinear strain-rate dependency of failure strength.     

The role of interfacial energy in the wear of polymeric journal bearings

Experimental and theoretical relationships between the adhesional energy and the wear rate of a polymer, in the form of a miniature journal bearing for a steel shaft, are presented. Formulae are derived that can be used to predict the wear rate of a polymeric material in such bearings.     

Tensile behaviour of threaded steel fasteners at elevated rates of strain

The tensile behaviour of threaded steel fasteners was studied experimentally at elevated rates of strain. Two testing techniques were used to perform the tests at strain-rates in the range from 10⁻³ to 1.9×10³ s⁻¹. The tests at low and medium strain-rates were performed in a servo-hydraulic testing machine, while tests at high strain-rates were achieved using a split-Hopkinson tension bar. All tests were carried out using a purpose-made fixture to ensure uniform test conditions, and to control the location at which failure by thread shearing could occur. The material tests and the threaded assembly tests showed approximately the same trend of an increased strength with increasing strain-rate. Owing to the strength ratio between the purpose-made fixture and the threaded fastener, two of three possible failure modes occurred during the performed tests, i.e. bolt breaking and bolt thread stripping. The length of the thread engagement, the grip length and the strain-rate had an influence on the failure mode. In addition, these parameters had varying effect on the strength and ductility of the threaded assembly. A modification of the Alexander [4] model was proposed, to predict the maximum load and mode of failure of threaded steel fasteners at high strain-rates. The modified model was in good agreement with the experimental results.     

风力机叶片原生缺陷转捩的能量释放机理研究

针对风力机叶片原生缺陷演化为裂纹进而扩展导致断裂的问题,分析细观缺陷在外载荷作用下转捩为宏观裂纹的能量释放定量关系明晰裂纹萌生机理。首先根据风力机叶片的载荷特点构造一个新的应力函数,基于正交异性复合材料基本公式求解原生缺陷层间开裂的应力强度因子、应力应变和位移分量,由此获得细观缺陷变形过程释放的塑性应变能;使用红外热像仪采集原生缺陷转捩过程的温度场并计算热能耗散量,基于不可逆热力学定律获得内储能随着疲劳周期的变化规律;最后,在万能试验机上对含有气泡和纤维断裂的叶片试件进行疲劳试验。结果表明,应用提出的应力函数的计算结果与试验结果误差较小,可作为细观缺陷变形时塑性应变能的计算依据。原生缺陷转捩为微小裂纹时,内储能的变化规律可作为判断缺陷类型和程度的依据。这项研究探索复合多层材料跨尺度的疲劳能量理论,有助于实现风电机组关键部件的全寿命周期监测。