A review on the strain rate dependency of the dynamic viscoplastic response of FCC metals

The response of structures and materials subject to impulsive loads remains a field of intense research. The dynamic loading and temperature increase affect the material’s mechanical/failure response. For example, strains due to explosive blast will increase at rates from 10² to 10⁴ s⁻¹, leading to regimes of elastic/plastic wave propagation, plane stress and adiabatic deformations. Few constitutive models consider high strain rate effects, however some constitutive approaches that were developed and tested at low strain rate regimes will also be addressed here due to their relevance. Specific reference will be made to strain rate regimes close to 10⁴ s⁻¹, where shock waves may develop. The paper focuses on constitutive models for polycrystalline face-centred-cubic (FCC) metals since their behaviour under high strain rate regimes is not yet fully understood mostly due to path loading dependency. Reference is also made to aluminium alloys since they are widely used in virtually all fields of industry and in armour and protective structures and systems. A basic review of the main theoretical aspects that constitute the basis for most of the constitutive models described is also presented and the main features of each model are thoroughly discussed.     

Comparison of three real time techniques for the measurement of dynamic fracture initiation toughness in metals

Three techniques for measuring dynamic stress intensity factor time histories of dynamically loaded stationary mode-I cracks are compared as applied to dynamically loaded pre-cracked 6Al-4V titanium alloy specimens. The three techniques are crack opening displacement (COD), dynamic strain gage measurement, and coherent gradient sensing (CGS). The stress intensity factor histories are inferred from each measurement technique and are used to obtain the critical dynamic initiation toughness as a function of loading rate (). There are significant differences in implementation and information obtained from each of the three measurement techniques, though for the tests performed all are found to yield very similar results.     

Comparison of dynamic initiation toughness measured by strain gauges and the shadow optical method

The deformation and fracture behaviour in dynamically loaded Charpy specimens of BS11 rail steel were studied by two different measuring techniques. The transient records obtained from straingauges were compared with results obtained simultaneously by the shadowoptical method. Good agreement was obtained when measuring and comparing the stress intensity factor K in the initial stages of deformation behaviour prior to crack initiation. During fast fracture, the shadow optical method indicated the influence of dynamic effects on crack propagation.     

A review of deformation and fatigue of metals at small size scales

Mechanical devices are being introduced whose size scale is well below that of conventional mechanical test specimens. The smallest devices have sizes in the nanometer range, though a good proportion of structural devices are of the micrometer scale. Development of these products raises the question of how their mechanical behaviour and reliability may be predicted. Conventional macroscopic test data can be used, but these are obtained using specimens whose size is much larger than the devices themselves. There is a risk that performance predictions will be inaccurate, due to the existence of size effects. This paper covers small size scale testing in metallic specimens and devices, concentrating on free‐standing specimens. To begin, some examples of micro‐scale devices are given. Fabrication methods for small metallic devices are then briefly described. This is followed by a review of experimental observations of mechanical properties in various metallic materials at the micro‐scale, highlighting the differences in results from different research groups and the gaps in our current knowledge. A section on computational and predictive modelling is included, in recognition of the role of modelling in device design and testing. Overall, the findings are that size effects are common, particularly in crystalline samples when the grain size is similar to one or more of the specimen dimensions. However, observations of size effects differ between studies and mechanical properties can vary widely, even for the same type of material. As a consequence, the relationships between specific device processing methods, specimen size and material properties must be adequately understood to ensure successful performance.     

Numerical simulations of fracture initiation in ductile solids under mode I, dynamic loading

In this paper, an overview of some recent computational studies by the authors on ductile crack initiation under mode I, dynamic loading is presented. In these studies, a large deformation finite element procedure is employed along with the viscoplastic version of the Gurson constitutive model that accounts for the micro-mechanical processes of void nucleation, growth and coalescence. A three-point bend fracture specimen subjected to impact, and a single edge notched specimen loaded by a tensile stress pulse are analysed. Several loading rates are simulated by varying the impact speed or the rise time and magnitude of the stress pulse. A simple model involving a semi-circular notch with a pre-nucleated circular hole situated ahead of it is considered. The growth of the hole and its interaction with the notch tip, which leads to plastic strain and porosity localization in the ligament connecting them, is simulated. The role of strain-rate dependence on ductile crack initiation at high loading rates, and the specimen geometry effect on the variation of dynamic fracture toughness with loading rate are investigated.     

A note on the arrangement of strain gauges and the sensitivity of strain measurement

A discussion for investigating the yield surface of metal is described. The paper also presents the discussions of the accuracy of strain measurements and the arrangement of three–element 45° strain gauges on a thin–walled tube under combined loadings.     

A method for determining dynamic stress intensity factors from cod measurement at the notch mouth in dynamic tear testing

A formula is derived for determining dynamic stress intensity factors directly from crack mouth opening displacements in dynamic tear test specimen. The results obtained by the present estimation method for stationary as well as propagating cracks agree excellently with those directly obtained through a highly accurate moving-singularity finite element method. The present method can also be applied for other types of specimen which have a relatively short edge crack without any loading on the crack surface. The present simple estimation method should be of great value in the experimental measurement of dynamic stress-intensity factors for propagating cracks in (opaque) structural steel dynamic tear test specimens.     

Full-field measurement technique and its application to the analysis of materials behaviour under plane strain mode

The purpose of the paper is to provide a comprehensive experimental and numerical analysis of one of the encountered and critical state modes in sheet metal forming processes. The study is carried out with the help of the full-field measurement techniques. In order to confer some generality to the proposed work, several materials and different specimen shapes are considered that exhibit more or less homogeneous strain field. The proposed experimental study of the plane strain test is completed by a preliminary identification of the material parameters for non-linear behaviour at finite strains, using heterogeneous strain field.     

Factors affecting the reliability of precision measurement of liquid helium temperature in a dynamic regulation and stabilization mode

A study has been made of anomalies detected during precision measurements of liquid helium temperature, using secondary thermometers in a dynamic mode. An explanation is suggested for the physical nature of these anomalies.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 5, pp. 817–824, November, 1989.     

Plane strain asymptotic fields for cracks terminating at the interface between elastic and pressure-sensitive dilatant materials

The plane strain asymptotic fields for cracks terminating at the interface between elastic and pressure-sensitive dilatant material are investigated in this paper. Applying the stress-strain relation for the pressure-sensitive dilatant material, we have obtained an exact asymptotic solution for the plane strain tip fields for two types of cracks, one of which lies in the pressure-sensitive dilatant material and the other in the elastic material and their tips touch both the bimaterial interface. In cases, numerical results show that the singularity and the angular variations of the fields obtained depend on the material hardening exponent n, the pressure sensitivity parameter μ and geometrical parameter λ. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the measurement of dynamic fracture toughnesses — a review of recent work

The influence of dynamic effects on test procedures for measuring the crack arrest toughness and the impact fracture toughness is analyzed. It is shown that these dynamic effects can be significant: For cracks at arrest the stress condition is still dynamic and not static although the crack velocity has become zero. Dynamic effects become small only for small crack velocities or small crack jumps or for specially designed specimens. It is shown that the stress intensity factor history for cracks under impact loading cannot be adequately derived from instrumented impact data via static evaluation procedures. Only for large times to failure, resulting for small impact velocities and/or ductile material behavior do static approaches represent acceptable approximations. Reliable crack arrest and impact fracture toughness data can only be obtained by evaluation procedures which take the dynamic effects into account, e.g. by utilizing the reduced dynamic effects crack arrest test specimen or by applying the concept of impact response curves.

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Résumé On a analysé l'influence des effets dynamiques sur les procédures d'essais pour mesurer la ténacité à l'arrêt d'une fissure et la ténacité à la rupture par impact. On montre que ces effets dynamiques peuvent être significatifs: pour des fissures qui s'arrêtent, les conditions d'essais sont toujours dynamiques et non statiques, bien que la vitesse de la fissure soit devenue égale à 0. Les effets dynamiques deviennent petits seulement pour de faibles vitesses de la fissure ou pour de petits resauts de la fissure ou encore dans le cas d'éprouvettes spécialement conçues. On montre que l'histoire du facteur d'intensité de contrainte pour des fissures soumises à une charge d'impact ne peut être convenablement déduite de données d'impact instrumenté évaluée selon une procédure statique. Ce n'est que pour de longues périodes de temps avant rupture, qui résultent de faible vitesse d'impact et/ou d'un comportement ductile du matériau que les approches statiques conduisent à des approximations satisfaisantes. Les données fiables de ténacité à l'arrêt d'une rupture ou à la rupture par choc ne peuvent être obtenues que par des procédures d'évaluation qui prennent en compte les effets dynamiques, par exemple en utilisant une éprouvette d'essai d'arrêt de rupture à effets dynamiques réduits ou en appliquant le concept des courbes réponses à l'impact.

     

A finite element analysis of plane strain dynamic crack growth at a ductile-brittle interface

In this work, steady, dynamic crack growth under plane strain, small-scale yielding conditions along a ductile-brittle interface is analysed using a finite element procedure. The ductile solid is taken to obey the J ₂ flow theory of plasticity with linear isotropic strain hardening, while the substrate is assumed to exhibit linear elastic behaviour. The objectives of this work are to establish the validity of an asymptotic solution for this problem which has been derived recently [12], and to examine the effect of changing the remote (elastic) mode-mixity on the near-tip fields. Also, the influence of crack speed on the stress fields and crack opening profiles near the propagating interface crack tip is assessed for various bi-material combinations. Finally, theoretical predictions are made for the variation of the dynamic fracture toughness with crack speed for crack growth under a predominantly tensile mode along ductile-brittle interfaces. Attention is focused on the effect of mismatch in stiffness and density of the constituent phases on the above aspects.     

板坯中心裂纹产生的原因及改进措施

通过对板坯中心裂纹的原因进行了分析，并介绍了在连铸机设备精度、拉速、钢水过热度、硫含量和Mn／S等方面采取的防治措施，有效地解决了板坯中心裂纹出现的问题。     

Fabrication of silicon nitride nanoceramics—Powder preparation and sintering: A review

Fine-grained silicon nitride ceramics were investigated mainly for their high-strain-rate plasticity. The preparation and densification of fine silicon nitride powder were reviewed. Commercial sub-micrometer powder was used as raw powder in the “as-received” state and then used after being ground and undergoing classification operation. Chemical vapor deposition and plasma processes were used for fabricating nanopowder because a further reduction in grain size caused by grinding had limitations. More recently, nanopowder has also been obtained by high-energy milling. This process in principle is the same as conventional planetary milling. For densification, primarily hot pressing was performed, although a similar process known as spark plasma sintering (SPS) has also recently been used. One of the advantages of SPS is its high heating rate. The high heating rate is advantageous because it reduces sintering time, achieving densification without grain growth. We prepared silicon nitride nanopowder by high-energy milling and then obtained nanoceramics by densifying the nanopowder by SPS.     

Estimation of the periods of initiation and propagation of creep-fatigue cracks in thin-walled structural elements

A computational model is proposed for the determination of the service life of structural elements with concentrators under the conditions fatigue and high-temperature creep. The model is based on the energy approach and the representation of the service life of a structural element as the sum of the periods of initiation and subcritical growth of creep-fatigue cracks.