A new etching technique for revealing the plastic deformation zone in an Al–Cu–Mg alloy

A new etching technique for revealing the plastic deformation zone in an Al–Cu–Mg alloy has been developed. The etching with the proposed etching agent was conducted on the deformed sample after being heated to 673 K for 3 h. With this etching technique, the plastic deformation zone was clearly observed even under low magnification. This was due to the change of microstructural characteristics in the plastic deformation zone after the heating process, in which there is significant precipitation of Al₂Cu and Mg₂Si, caused by the high energy arising from the severe deformation.     

Etching technique for revelation of plastic deformation zone in low carbon steel

Abstract

In this study, an etching technique to detect the localised plastic deformation behaviour in a low carbon steel was developed. With this technique, etching with Fry solution under ultrasonic vibration was carried out on samples plastically deformed and then heated at 550°C for a certain period of time. The plastic zone was revealed by different degrees of etching in the plastically deformed and non-deformed regions; the plastic zone was found to be only slightly etched, whereas the other region was deeply etched. From the surface offset after etching, the deformation zone was found to be observable even at low magnification, such as 10 times. As the heating duration increased, the plastic zone became clearer. The mechanism for such an etching reaction is discussed on the basis of electrochemical analysis.     

Dislocations produced in magnesium oxide crystals due to contact pressures developed by softer cones

An experimental investigation shows that dislocations, encompassing a predictable dislocated volume, are produced in MgO single crystals subjected to circular contact pressures due to cones of other solids which may be an order of magnitude softer. The mean contact pressure necessary to produce localized plastic flow, without fracture, is shown to be directly related to the critical resolved shear stress of the MgO. Dislocation etching is used to investigate the influence of cone hardness and normal loading on the nature of the deformed zone in the harder crystal and the results are discussed in terms of the deformation characteristics of the softer cones and the crystal.     

Ultrasonic Detection and Sizing of Plastic Zones Surrounding Fatigue Cracks

Abstract

The problem of detection and sizing of fatigue cracks that are closed due to the reverse plastic deformation is addressed in this paper. In particular, it is suggested that the zone of plastic deformation can be identified by measuring the changes in the wave speeds in the plastically deformed material. This is supported through experimental results, first by determining the changes in wave speed with monotonic plastic deformation and then by investigating the plastic zones near fatigue cracks. The characteristic changes in the wave speeds are interpreted in terms of the theory of wave propagation in nonlinearly deformed media and elastic plastic fracture mechanics.     

Ultrasonic detection and sizing of plastic zones surrounding fatigue cracks

The problem of detection and sizing of fatigue cracks that are closed due to the reverse plastic deformation is addressed in this paper. In particular, it is suggested that the zone of plastic deformation can be identified by measuring the changes in the wave speeds in the plastically deformed material. This is supported through experimental results, first by determining the changes in wave speed with monotonic plastic deformation and then by investigating the plastic zones near fatigue cracks. The characteristic changes in the wave speeds are interpreted in terms of the theory of wave propagation in nonlinearly deformed media and elastic plastic fracture mechanics.     

Phase transformations in zones of plastic deformation beneath the surface of impact fractures in steel N32T3

Impact tests are performed on steel N32T3 in the hardened and aged conditions. The mechanism of failure and martensitic transformation in zones of plastic deformation between the surface of fractures obtained over a wide temperature range is studied. It is shown that the failure mechanism for both hardened and aged steel in the stage of crack propagation depends weakly on test temperature. An increase in KCU and KCT impact strength for the steel with an increase in temperature is due to an increase in the work for crack generation and formation at the failure site of zone L whose microrelief is predominantly folded with elongated pits. Two zones of plastic deformation are detected by means of x-ray structural analysis beneath the surface of fractures in the hardened condition and one in the aged condition. It is established that within the limits of a highly deformed microzone for hardened steel the amount of martensite is constant in spite of presence of a deformation gradient, but in aged steel it decreases constantly. It is noted that for correct evaluation of the effect of phase transformations occurring in zones of plastic deformation on impact strength and failure mechanism for the steel it is necessary to consider local heating and the actual phase composition of the steel at the tip of a propagating crack.Translated from Problemy Prochnosti, No. 7, pp. 49–56, July, 1990.     

Die Impulshärtung von Stahl

The Pulse Hardening of Steel . With pulse hardening of steel thin surface layers are heated to austeniting temperature for a short period through energy pulses and then rapidly cooled through the heat absorbing surrounding material. The present paper describes the energetic processes during heat treatment by rapid localized heating with electron and laser beams, current pulses, friction pulses, and plastic deformation of the crystal lattice. It offers a view on the austeniting processes during rapid localized heating and reports on the properties of the zones hardened by such methods. Transformations and dissolutions are similar to those of long time austeniting. Some special features: increasing inhomogenity of the structure of hypereutectoid steels with decreasing austeniting times, in some cases a reduction of the grain size, a somewhat increased hardness due to the extremely high cooling rates. The white etching areas develop from plastically deformed austenite after cooling, their properties correspond to those of an ausformed structure. In principle the residual stresses of all rehardening zones are identical: the compressive stresses of the hardened zones are followed by areas with extreme tensile stresses.     

Low-Temperature Internal Friction and Thermal Conductivity of Plastically Deformed, High-Purity Monocrystalline Niobium

The low-temperature internal friction Q ^–1 and thermal conductivity of plastically deformed, high-purity niobium monocrystals have been investigated and compared with measurements on an amorphous SiO₂ (a-SiO₂) specimen. After plastic deformation at intermediate temperatures, an approximately temperature independent internal friction Q ^–1 was observed with a magnitude comparable to that of the a-SiO₂ specimen. Plastic deformation at low temperatures leads to an internal friction Q ^–1 with a considerably smaller magnitude. In the temperature range between about 0.3 and 1.5K, the lattice thermal conductivity k of the deformed specimens decreases with increasing deformation. It is, however, nearly independent of the amount of deformation at the lowest temperatures investigated. In this temperature regime, the lattice thermal conductivity of the specimens varies proportional to T ³ and has a magnitude as would be expected for an undeformed sample. Additional heat release experiments on an undeformed sample clearly show no long-time energy relaxation effects. We conclude that the defects introduced by plastic deformation cannot be described with the tunneling model which had been proposed to describe the low temperature elastic and thermal properties of amorphous solids. The phonon scattering mechanisms observed in deformed niobium are tentatively related to the dynamic interaction of phonons with geometrical kinks in dislocations.     

Structural changes in plastic deformation zones with impact loading of metastable austenitic steels

Structural changes are studied in zones of plastic deformation for two metastable steels with dynamic loading in impact strength experiments. On the basis of the results of x-ray structural analysis and fractographic studies it is established that within the limits of the plastic deformation zone the amount of martensitic phase is constant, but it increases at the boundary of a strongly deformed microzone of plastic deformation.Translated from Problemy Prochnosti, No. 9, pp. 72–75, September, 1991.     

A study on spot heating induced fatigue crack growth retardation

The propagation of a growing fatigue crack can be effectively retarded by heating a spot near the crack tip (under zero stress condition). Spot heating to a subcritical temperature and at a precise location modifies the crack growth behaviour in a way, more or less, similar to specimens subjected to an overload spike. It is observed that the magnitude of spot heating induced crack growth retardation increases with increase in spot temperature. It is also observed that the crack growth behaviour is influenced by the position of the heating spot and there exists an optimum position of hot spot that produces maximum retardation in fatigue crack growth rate. The plastic zone length due to spot heating has been estimated using experimental data. It is found that the plastic zone length due to spot heating increases exponentially with increase in spot temperature. The Wheeler model for crack growth retardation has been modified by introducing a plastic zone correction factor λ. The values of λ and the shaping exponent, m, in the Wheeler model have been obtained for different spot heating temperatures.     

Fatigue crack growth retardation inconel 600

The effect of single-cycle overloads on the subsequent fatigue crack growth behavior of Inconel 600 is studied. Overloads ranging from 10 to 50% are applied to a sample undergoing baseline fatigue crack growth at constant Δ$\text{K}$. In all cases, the crack growth rate increases slightly immediately after the overload and then decreases rapidly to a minimum value before later returning to the pre-overload value. The plastic zone size, affected crack length and the crack growth increment at minimum crack growth rate, $\text{a}\text{?}$, are measured for each overload.The affected crack length is considerably larger than the overload plastic zone size for overloads greater than 20%. Consequently, although the minimum crack growth rate occurs within the plane stress overload plastic zone, the effect of the overload extends well beyond the overload region.Within the overload plastic zone, contact occurs between the crack faces due to the excessive deformation produced during the overload cycle. The size of the contact region agrees very well with the overload plastic zone size. Beyond the overload region, Δ$\text{K}{}_{\mathrm{eff}}$ remains less than the applied Δ$\text{K}$ for some time due to the wedge action of the plastically deformed overload region, delaying recovery of the pre-overload crack growth rate. The crack growth rate recovers only after the crack grows out of the region of influence of the wedge.     

Effect of dynamic plastic deformation on microstructure and annealing behaviour of modified 9Cr−1Mo steel

The effect of dynamic plastic deformation on the microstructure of a modified 9Cr?1Mo steel has been investigated in comparison with the effect of quasi-static compression. It is found that the boundary spacing after dynamic plastic deformation is smaller and the hardness is higher than those after quasi-static compression. The microstructure after dynamic plastic deformation is however less stable than the microstructure after quasi-static compression. Annealing at 675 and 700°C leads to structural coarsening and recrystallisation in each sample, but with recrystallisation occurring faster in the sample annealed after dynamic plastic deformation. The lower thermal stability of the microstructure produced by dynamic plastic deformation is attributed to a higher driving force for recrystallisation in the dynamically deformed material.     

Analysis of fatigue crack tip plasticity in Fe-2.6Si

Saurface roughness analysis, etch-pit, and recrystallization techniques were employed to characterize the plastic deformation zones around fatigue cracks in Fe-2.6wt% Si. Both surface and sub-surface cross-sections are evaluated. The plastic zone size changes significantly in the surface layers. It is correlated with energy release rate, J, because of the elastoplastic nature of crack propagation and a linear correlation is found. The plastic deformation experienced inside the plastic zone is analysed and is reported in terms of equivalent tensile strains. The material in the immediate vicinity of the crack tip experiences large plastic strains under a steep gradient. This region, however, occupies a very small fraction of the plastic zone, the rest of which is deformed to less than 4% plastic strain. In the light of the experimental data, an attempt is made to identify potential sources for the variation in measured plastic zone parameters reported in the literature.     

Numerical study on void growth in rate and temperature dependent solids

This paper is concerned about void growth and associated deformation models in porous visco-plastic solids under conditions similar to those found in highly stressed regions ahead of a crack. A plane-strain unit cell containing an initially circular void is examined to simulate the stress states during dynamic fracture of a metal. Two proportional loading rates are prescribed in the two directions of the cell and their ratio is called the “strain biaxiality” expressed in a monotonic relation with stress triaxility. Finite element analysis is performed for the effective stress–strain curves of the porous solids during void growth for a range of initial porosities, strain biaxialities, strain rates and thermal softening coefficients. Numerical results show that the void evolution and the associated non-uniform deformation depend in a complex fashion on these factors. The local zone of high stress concentration which emanates from the void spreads out in the cell to trigger non-uniform deformation and plastic yielding. Subsequently, a small zone with intense plastic strain and heating either expands smoothly near the growing voids or propagates in a specific direction determined by its interaction with the boundary conditions of the cell such as strain biaxility. At low strain biaxiality and for small voids, formation and propagation of zones with intense plastic strain and heating is localized. However, high strain biaxiality leads to rapid uniform expansion of small voids as observed experimentally. It is found that the intense heating zone follows the zone of high plastic strain concentration and diffuses with imposed strain. Thermal softening which reduces the overall stress can be neglected at the early stage of void growth, but it is magnified past the peak stress by accelerating the void growth. But in the long term, the void growth rate is insensitive to thermal softening coefficient. Increasing strain rates can promote void growth and the rate of which tends to be proportional to the eventual strain rate.     

In-depth deformation of InP under a Vickers indentor

(001) surfaces of InP single crystals have been deformed at room temperature by a Vickers indentor submitted to a load of 1 N. The indents formed were observed by transmission electron microscopy (TEM) in cross-sectional views obtained by the focused-ion beam (FIB) technique. The structure of the plastic zone has been investigated and the different modes of plastic deformation involved during the deformation have been analysed. The flow stress of InP was estimated and compared to the reported values in the literature.     

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.     

Influence of monotonic and cyclic predeformation on fatigue crack propagation of high-strength aluminum alloys

Regarding the plastically deformed volume ahead of a propagating fatigue crack two types of plastic zones can be distinguished, the K_max-zone which is generated during the load increase upon maximum loading in a cycle and where monotonie tensile deformations occur, and the Δ K-zone, where the crack tip material is reversed plastically deformed.

In order to achieve similar deformation structures as they are present within these two types of plastic zones a predeformation was applied on the bulk material. The deformation structure of the K_max-zone was simulated by defined cold rolling and the cyclic predeformation structure by reversed cyclic plastic deformation. The crack propagation behaviour of the monotonically and cyclically predeformed material and of the completely non-predeformed material was observed. The tests showed that predeformation influenced the crack propagation rate: an increase in the predeformation led to an increase in the crack propagation rate.

In tests with variations in the loading conditions, e.g. a change from a high to a low loading level, considerable sequence effects were observed. The tests with predeformed and non-predeformed materials showed less retardation for the monotonically as well as for the cyclically predeformed material. The ranges in the crack lengths where retardation occurred after the decrease in the loading level, were smaller in the predeformed material. The test results give a clear indication of the significance of the plastic zone behaviour regarding sequence effects after variations in the loading conditions.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.     

Deformation and fracture of styrene-acrylonitril copolymer - rubber blends: Microscopy studies of deformation zones

A styrene-acrylonitril copolymer (SAN) was toughened by SAN-grafted polybutadiene core-shell rubber particles. Notched tensile specimens were fractured with a tensile speed ranging from 10-4 to 10 m s-1. The deformation processes close to the fracture surface were studied by means of transmission electron microscopy. A marked difference in the structure of the deformation zone was observed between low speed (10-3 m s-1) and high speed (≥1 m s-1) deformed samples. At low tensile speed the structure of the deformation zone correlated closely with fracture mechanics theory. When the tensile speed was increased the deformation zone had a layered structure. In the zone 400–1.5 μm below the fracture surface the deformation structure was similar to that at low speed. In the layer 1.5–0.5 μm from the fracture surface the rubber particles were strongly deformed, but no cavities or crazes could be observed. Directly next to the fracture surface the high speed deformation zone showed a small layer (0.5 μm) where all the deformation had vanished. It is suggested that due to high strain-rate plasticity at the crack tip a temperature rise occurs which is high enough to cause complete relaxation of the deformation in this layer. Therefore, locally the glass transition temperature of the matrix material was reached. The interaction between thermal effects and deformation processes at the crack tip is discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Transverse impact of free–free square aluminum beams: An experimental–numerical investigation

A combined experimental–numerical analysis was performed to model transverse impact of free–free square aluminum beams loaded at different locations along their length. The applied impact load was obtained from tests carried out on a single Hopkinson pressure bar. The 3D elastic–plastic numerical simulations show that the plastic deformation, adjacent to the impact location, is due to combined dominant bending and stretching modes. Most of the plastic deformation is confined to the impact zone but some partial additional plastic hinges are observed to develop. The plastic strain magnitude and distribution near the impact zone are similar for all tested impact locations, but higher for the more symmetrical impacts. The conversion of impact energy into kinetic, elastic strain energy and plastic dissipation work is characterized for various impact locations along the beam. It is observed that symmetrical impact results in higher plastic dissipation and lower kinetic energy as opposed to unsymmetrical impact. Between 52% and 76% of the applied energy is converted into plastic dissipation energy.