Influence of the vibrotreatment of structural elements with residual stresses on their damping ability

We investigate the influence of the vibrotreatment of annular elements made of St. 3 steel on the kinetics of residual stresses and dissipation of energy in the material. By using the characteristic features of changes in the decrement of oscillations of the material and residual stresses, we substantiated the possibility of control over the completion of vibrotreatment by the time of stabilization of the characteristic of dissipation of energy in the material. The established decrease in the decrement of oscillations shows that specimens undergo plastic deformation in the course of this process. Translated from Problemy Prochnosti, No. 5, pp. 113–121, September–October, 1997.     

Dynamic mechanical response of polyetheretherketone (PEEK) exposed to cyclic loads in the high stress tensile regime

Tensile fatigue tests of PEEK at high load levels were carried out and analyzed regarding the dynamic mechanic behavior of the material during the tests. The experiments were conducted in the high stress tensile regime of PEEK. After a short period of unsteady temperature increase distinct material changes were observed during the tests at medium and high load levels. The storage modulus increased continuously while the loss modulus, loss factor (tan δ) and the dissipation energy rate decreased continuously. It was concluded that the accumulation of residual stresses because of partially irreversible deformation causes this effect.     

On the Efficiency of Using Damping Characteristics of Structural Components for Damage Diagnostics

Using the generalized relationship between energy dissipation in a fatigue crack and the range of the stress intensity factor, which was derived earlier, the author has performed an analytical study of the influence of various factors on the logarithmic decrement of vibrations of a cantilever beam with an edge crack and a surface semielliptical crack. In particular, it is demonstrated that an increase in the vibration damping characteristic of a beam due to the crack initiation and growth can serve as an efficient damage diagnostics method for structural components in case such components, when intact, have a fairly low level of dissipation properties.     

Increased fracture toughness of ceramics by energy-dissipative mechanisms

A theoretical model for the fracture toughness of ceramics is developed which takes into account such energy-dissipative mechanisms as stress-induced microcracking or phase transformation. To establish the general fracture criterion, a Griffith-type energy balance is employed. This energy balance comprises the elastic energy, the fracture surface work consumed in the process zone at the crack tip, the energy dissipated in the dissipation zone and the energy stored by residual stresses. Stress-induced microcracking is considered in more detail. An expression for the dependence of the fracture toughness on the density of microcracks, the amount of residual stresses caused by thermal expansion mismatch between the ceramic matrix and small particles embedded in it and the volume fraction of these particles is derived. The final results are used to state conditions necessary for the fracture toughness to be increased. The theory agrees well with experimental results taken from literature (alumina with zirconia particles).     

振动时效在铝合金厚板应力消减中的局限与应用

运用振动时效对材料应力消减的作用,结合铝合金厚板残余应力消减后的分布及效果,认为振动时效对厚板结构残余应力消减作用有限。而运用位错理论和最小能量法分析,发现振动时效对材料因内部微结构差异而形成应力集中区域的应力松弛释放有所贡献,在一定激振频率和时效时间条件下,完全可以造成材料局部的微屈服,减小了微结构间残余应力水平,从而对板内应力的均匀化过程起到一定效果。借助实验对厚板振动时效前后表面应力强度和板形尺寸稳定性对比发现,振动时效对稳定板形和消减大梯度表面残余应力具有显著作用。     

内嵌碟簧型自复位防屈曲支撑性能试验及其恢复力模型研究

为解决防屈曲支撑在大震作用下产生较大残余变形的问题,该文提出了一种内嵌碟簧型自复位防屈曲支撑,阐述了该支撑的构造和工作原理。该新型支撑由防屈曲耗能系统和自复位系统并联组成,通过控制碟簧预压力和核心单元屈服承载力的组合,可得到工程结构控制所需的滞回特性曲线。在此基础上,设计制作了3个不同工况的内嵌碟簧型自复位防屈曲支撑,开展拟静力试验,分析其破坏特征,探讨了其残余位移和滞回耗能等特性,并建立其力-位移滞回关系恢复力分析模型。试验结果表明:内嵌碟簧型自复位防屈曲支撑不仅保留了防屈曲支撑稳定良好的耗能能力,而且能较好地控制残余变形,建立的恢复力分析模型与试验结果吻合较好,是一种有效地控制残余变形且性能稳定的结构减震元件。     

残余应力对金属疲劳强度的影响

残余应力对光滑试样高周疲劳极限的影响可以用Goodman关系来描述，但必须要得到残余应力作用系数m、合理地提取残余应力的表征值和区分开其它因素的影响。残余应力对缺口疲劳极限的作用大于对光滑试样的作用，是由于残余应力也存在应力集中现象，而且不易衰减。残余应力的应力集中系数不仅与缺口几何因素有关，还与材料特性有关。试验研究还表明，表层残余压应力对于承受轴向载荷且疲劳残纹萌生于表面的零件也十分有益。     

Local and overall flow in composites predicted by micromechanics

Rate-dependent inelastic flow in metal matrix composites subjected to multiaxial stress states is quantified by flow surfaces, which are geometrically analogous to yield surfaces. The definition of flow is important because the most meaningful definition from a theoretical viewpoint, dissipation, is not measurable in the laboratory. Inelastic power is measurable, but differs from the dissipation due to residual stresses and evolution of the material state. Since experiments are necessary for development and validation of models, both definitions are important and considered here. The relationship between local flow in the matrix and overall flow of the composite is explored using finite element and generalized method of cells micromechanical analyses. The loci of flow surfaces in the axial–transverse and transverse–transverse stress planes are plotted. At the threshold, the overall flow surface is the intersection of all the local flow surfaces. Beyond the threshold, the intersection of all the local flow surfaces is smaller than the overall flow surface and differences between the dissipation and inelastic power are notable. Most importantly, the directions of the overall inelastic strain rate vectors are generally not normal to the overall surface of constant dissipation after the material state has begun to evolve. Thus, an associative macroscale continuum model will be, at best, approximate. Interestingly, local flow surfaces beyond the threshold are not necessarily convex when plotted in the overall stress plane. This is due to the existence of residual stresses. In addition, the generalized method of cells was found to accurately estimate the inner and outer envelopes of the local flow surface cluster with a surprisingly small number of subcells.     

Residual stress. Part 1 – Measurement techniques

Abstract

Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be very detrimental to the performance of a material or the life of a component. Alternatively, beneficial residual stresses can be introduced deliberately. Residual stresses are more difficult to predict than the in-service stresses on which they superimpose. For this reason, it is important to have reliable methods for the measurement of these stresses and to understand the level of information they can provide. In this paper, which is the first part of a two part overview, the effect of residual stresses on fatigue lifetimes and structural integrity are first summarised, followed by the definition and measurement of residual stresses. Different types of stress are characterised according to the characteristic length scale over which they self-equilibrate. By comparing this length to the gauge volume of each technique, the capability of a range of techniques is assessed. In the second part of the overview, the different nature and origins of residual stress for various classes of material are examined.     

Precursor gas effect on the mechanical and optical properties of ion beam deposited diamond-like carbon films

The effect of precursor gases on the diamond-like carbon (DLC) film deposition was investigated by the direct ion beam deposition method. DLC films were deposited using methane and benzene as the precursor gases. Ion energies for the deposition range from 100 to 700 eV were achieved by adjusting the beam voltage. The residual stresses, refractive indices and optical band gaps were compared at the same ion energy. We observed significant differences in residual stress and optical properties between these films. As in r.f. plasma-assisted CVD, the residual stresses of the films deposited from benzene show a characteristic behaviour of lower ion energy deposition than those deposited from methane. The present observations are discussed in terms of the difference in ion energy per carbon atom at the growth surface. We also observed that the Ar addition effect on the residual stress is strongly dependent on the precursor gases.     

Interfacial crack propagation in a thin viscoelastic film bonded to an elastic substrate

Edge decohesion along the interface of a thin viscoelastic film bonded to an elastic substrate under tensile residual stresses is considered. The tensile residual stress in the film is replaced by a combination of edge loads, and an explicit relation of strain energy with respect to time is obtained through simple beam analysis. The strain energy function is discretized into time steps which are assumed to be very small so that the dissipation effects over the time steps can be neglected. The energy release rate is then calculated using a Griffith type energy balance. An analytical model is developed to predict the crack growth and its velocity. Extent of crack growth along the interface is prediced based on a fracture criteria. The analytical predictions are compared with results from a viscoelastic finite element analysis.     

Energiedissipationshypothese zur Festigkeitsrechnung bei mehrachsiger Schwingbeanspruchung

Energy-dissipation hypothesis for strength calculation at multiaxial fatigue A method called energy dissipation hypothesis is developed to estimate the fatigue life for metallic materials under multiaxial, Stochastic loads. The energy dissipation is calculated for every cycle of the various deviatoric stresses and of the hydrostatic stress. The influence of the mean stress components on the material behaviour is modeld using a simple closed form solution, which needs neither to integrate different orientations nor to determine of a critical orientation. This paper the hypothesis is used for periodic load cases and compared with experimental data.     

Off-axis fatigue loading of steamed wood

Cyclical off-axis compression was applied to steamed Spruce wood in uniaxial strain under stress control. The strain level is a step function of stress level and test duration. Energy dissipation is negligible when the stress amplitude in relation to the greatest compressive stress is small, regardless of the applied stress level. There is a significant energy dissipation when the stress amplitude in relation to greatest applied compressive stress is large, regardless of stress level. Dynamic stiffness appearing during the dynamic loading strongly depends on applied stress and stress amplitude. Small-strain stiffness deteriorated during any dynamic loading experiment, apparently depending on the greatest compressive strain appearing during loading. Also plastic strain appears to depend on the strain appearing during loading. Thus stiffness decrement and plastic strain do not appear to reflect the variety of material reactions to fatigue treatments.     

Charakterisierung des Ermüdungszustandes in zyklisch beanspruchten Schweißverbindungen durch röntgenographische und mikromagnetische Kenngrößen

Characterization of the Fatigue of Cyclically Loaded Welded Joints by X-Ray Diffraction and Micromagnetic Testing The fatigue behaviour of welded Joints is influenced by the state of the material, its microstructure and residual stresses. By means of the micromagnetic Barkhausen-noise testing method in connection with x-ray diffraction the behaviour of cyclically loaded welded joints of the steel S355 is investigated. Proved by the results of the present investigations, the macro-and micro-residual stresses, micromagnetic and strain characteristic values are strongly connected with each other. Plastic deformations in the HAZ and base material during cyclic loading were identifies by a significant relaxation of the micro-residual stresses and a characteristic change in the micro-residual stresses determined by X-ray profile analysis. Analogous to the growing plastic strain amplitude an advancing damage process in the material is accompained by a significant change in the micromagnetic parameters. As a conclusion the micromagnetic testing method can be suited for the identification of fatigue processes in the material before a final damager of the specimen.     

Investigation of the stress intensity factor changing on the hole crack subject to laser shock processing

Laser shock processing (LSP) is a new surface modification technology. The effect of the compressive residual stresses generated due to laser shock processing (LSP) on the stress intensity factor (SIF) of a through-the-thickness radial crack at the edge of the circular hole was investigated. The residual stresses around the hole induced by LSP were measured by using X-ray method. The relationship between the SIF and the residual stress was determined on the basis of the weight function theory in fracture mechanics. Crack propagation characteristics for such cracks subjected to the combination of the applied stress and residual stress were discussed. And the results showed that the compressive residual stress could lead to the decrement of the SIF. Moreover, the number of the laser shocks had an important influence on the SIF.     

The effect of residual thermal stresses on transverse cracking in cross-ply laminates: an application of the coupled criterion of the finite fracture mechanics

A model to predict transverse cracking in cross-ply laminates in the presence of residual thermal stresses is developed here. This model is based on the coupled criterion of the finite fracture mechanics. This criterion has been successfully used for different materials, structures and scales to predict crack initiation. It is based on two main hypotheses: (i) crack initiation occurs as a finite-length crack onset and (ii) the crack onset requires that both stress and energy criteria are fulfilled simultaneously. The present model is developed under the generalized-plane-strain hypotheses combining the results obtained using the laminate theory and a boundary element code. The present analysis shows that the residual thermal stresses affect both the stress and the energy criteria in the form of adding a residual elastic-strain to the strain imposed by external mechanical loads. An explicit expression for this residual elastic-strain is provided. For certain composite materials as carbon/epoxy the value of this residual elastic-strain is shown to be relatively large in comparison with the nominal critical transverse strain of the material. The comparison with experiments shows that considering the residual thermal stresses using the strategy proposed here improves drastically the accuracy of the model predictions.     

纤维增强复合材料身管阻尼特性分析

针对火炮发射时纤维增强复合材料身管的阻尼效应,根据粘弹性材料动态特性,给出了纤维增强复合材料能量耗散公式,采用基于有限元技术的模态应变能分析方法,对某口径复合材料身管进行了阻尼特性分析,获得了该复合材料身管在各阶固有频率下的应变能和阻尼损耗因子,为复合材料身管的动态特性分析提供了阻尼特性参数.     

Experimental Data Highlighting the Role of Surface Fracture Energy in Quasi-Static Confined Comminution

Since the pioneering work of Griffith, Linear Elastic Fracture Mechanics has been widely experimentally validated and successfully developed in solid mechanics modeling. However, recent theoretical models applying the energy balance found in Griffith theory specifically for quasi-static confined comminution have until now not been systematically confronted to experiments. In this study, we analyze data of compression tests on crushable sand, where grain breakage has been triggered by flooding the initially dry material at constant stresses. We consider a partition of the dissipation between surface fracture energy and the rearrangement of fragments and grains surrounding crushed particles. Our results show that the role of the surface fracture energy is stressdependent and that its influence becomes less significant at high stresses.     

Numerical predictions and experimental measurements of residual stresses in fatigue crack growth specimens

Controlling macro residual stress fields in a material while preserving a desired microstructure is often a challenging proposition. Processing techniques which induce or reduce residual stresses often also alter microstructural characteristics of the material through thermo-mechanical processes. A novel mechanical technique able to generate controlled residual stresses was developed. The method is based on a pin compression approach, and was used to produce well-controlled magnitudes and distributions of residual stresses in rectangular coupons and compact tension specimens typically used in fatigue crack growth testing. Residual stresses created through this method were first computationally modeled with finite element analysis, and then experimentally reproduced with various levels of pin compression. The magnitudes and distributions of residual stresses in experimental specimens were independently assessed with fracture mechanics methods and good correspondence was found between residual stresses produced using the pin compression and processing techniques. Fatigue crack growth data generated from specimens with low residual stresses, high residual stresses resulting from processing, and high residual stresses introduced through the new pin compression technique were compared and validated. The developed method is proposed to facilitate the acquisition and analysis of fatigue crack growth data generated in residual stresses, validate residual stress corrective models, and verify fatigue crack growth simulations and life predictions in the presence of residual stresses.