Influence of Dissipative Structures Formed by Impulsed Loads on the Processes of Deformation and Fracture

Volodymyr Hutsaylyuk1,a, Mykola Chausov2,b, Valentin Berezin2,c, Andriy Pylypenko 2,d , Katerina Volyanska 2,e

1 Faculty of Mechanical Engineering, Military University of Technology, 2 Gen. S Kaliskiego str., 00-908 Warsaw, Poland

2 Department of Mechanics, Strength of Materials and Construction, Technical Education and Research Institute, National University of Life and Environmental Sciences of Ukraine, 12 Geroiiv

Oborony, 03041, Kyiv, Ukraine

avhutsaylyuk@wat.edu.pl, bMich@nubip.edu.ua, cvalikb@yandex.ua, dandriy3pl@gmail.com, evo-kate@yandex.ua

Keywords: impulse loading, dissipative structures, mechanical properties, digital image correlation.

Abstract. By applying original experimental methodology and software system for non-contact

research of deformation field investigated conditions of DNP in the test setup of given stiffness.

Established that at the speed of additional impulse loading within 300-500 MN / s, specimens of

aluminum alloy 2024 - T3 and D16 exposed low cycle, high-frequency oscillations with a

frequency of 1.5 ... 2 KHz. This process of load with deformations speeds of the material 2 ... 60s-1

promotes plasticizing of aluminum alloys and delays in "neck forming.".

Introduction

Investigation of a nonlinear dynamics of non-equilibrium systems in modern materials has

considerable influence on dissemination of knowledge about the behaviour of materials [1]. This

study has scientific and practical importance. However, the experimental research of influence of

dynamic nonequilibrium processes (DNP) on the mechanical properties of construction materials of

different classes practically has not been carried out. This was due to the lack of appropriate

techniques and equipment for such research.

For the first time the authors have experimentally found that at realization of DNP in

mechanical systems due to additional impulse loads in materials are formed a thin-strip dissipative

structures. They are less durable and volumetrically related on the different scale levels [1,2,3].

At the further loading this leads to major change in mechanical properties of materials,

compared to the original, in particular the characteristics fracture toughness and impact strength

[4,5,6]. When applying the additional impulse loads on static tension, materials are exposed to low

cycle high-frequency oscillations with high levels of stress. This influences on the mechanism of

deformation.

External energy absorption by the material is happening irregularly in overall volume.

Probably the energy initially enters the zone less strong and weakened by material defects. There

also appearing effect of "tunneling", because the high-frequency oscillatory process creates barriers

on the path of dislocation motion. Thus, more - less uniform motion of dislocations under static

tension becomes significantly heterogeneous in dynamic non-equilibrium processes.

There is zone formation, with a much lower density of dislocations, less strong. Continuing

deformation of materials will be happening mainly in these areas. On the basis of experimental

researches the authors observed a fact of apparent absence of strengthening materials at DNP. On

this basis, it was concluded that the lack of strengthening is one of the main attributes of the

realization of dynamic non-equilibrium processes in the material.

The aim of this work is to study the conditions of DNP realization in a loading system of

specified stiffness and estimation of influence of newly created thin-band dissipative structures in a

sheet of aluminum alloys on further processes of deformation and fracture.

Key Engineering Materials Vols. 577-578 (2014) pp 273-276Online available since 2013/Sep/23 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.577-578.273

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Methodology of Research

For realization of the DNP in this paper a new methodology of mechanical tests is used, which was

developed by Professor Mykola Chausov with employees [1]. The additional force pulse load of

specified value is applied at the appropriate level of static deformation basic specimen material.

Load realized by the way of brittle fracture of two specimen-satellites of different cross section,

located parallel to the basic specimen.

The analysis of such complex process as DNP requires the use of equipment that provides

information about the level of deformation of the material and its velocity at any point on the

working part of the specimen. We used an original system for the analysis of deformation fields by

the method of digital image correlation (Digital image correlation) with the appropriate software.

The complex was equipped with a high-speed camera Phantom v711 [7].

For the purpose of this work we researched aluminum alloys D16 and 2024 - T3, which is

widely used in industry. Tests were carried out on a flat specimens with thickness of 3 mm and a

width of 10 and 25 mm. Data registration of high speed camera Phantom v711 in the

implementation of DNP was performed in the range of 15,000 measure / s ... 680 000 measure / s.

The research results

The original experimental results demonstrate the possibility of DNP by simple scheme, by the

single impulse loads. In this paper, was used an improved method of deformation diagrams

recording of materials during realization of DNP. Also were applied a special type strain gauges

EP-08-500BL-350, which were pasted directly on the working part of flat specimens and registered

not only elastic, but plastic deformation in the range of 20%. Speed of data recording for each

measuring channel was 20,000 measure / s.

The fragment of a deformation diagram of a specimen of aluminum alloy 2024 - T3 with

width of 10 mm directly present the process or realization of dynamic non-equilibrium process in

the alloy (Fig. 1a). For the same fragment there is presented a dependence of deformation of the

alloy in respect to time (Fig. 1b).

a b

Fig.1. Fragments of the diagrams in the process of realization of DNP in alloy 2024-T3

(Pimp-78,9kN): a - deformations diagram; b - dependence of the deformation of the alloy in respect

to time.

As a result of experimental research it was shown, that the main parameter that specifies the

DNP in material is the speed of additional pulse loading in the mechanical system. During

realization of additional pulse loading with speed within 300-500 MN / s, researched specimens are

274 Advances in Fracture and Damage Mechanics XII

exposed to the low cycle high-frequency oscillations with a frequency of 1.5 ... 2 KHz (see Fig. 1a).

This initiates the formation of thin - band, volumetric-related at different scale levels, dissipative

material structures. Their presence contributes to the plasticizing of the material during later load.

The evolution of the deformations field on the surface of specimen with width of 25mm

aluminum alloy D16 with DNP registered using complex analysis of deformation fields by digital

image correlation shown in Fig. 2. Analysis of the research discovered substantial heterogeneity of

deformations field during realization DNP. The presence of two stages of the process; low-and

high-speed, that are responsible for the formation and development of dissipative structures in the

aluminum alloys 2024-T3 and D16 with DNP. The average speed of deformation in the first stage

of the process of formation of dissipative structures in local areas is relatively low, about 2 ... 3 s-1

.

The second stage of the process, when there is propagation of newly created dissipative structure by

volume of the specimen, a sharp increase in the speed of deformation in local areas to 50 ... 60 s-1

was noted.

a b c d

Fig. 2. The evolution of the deformation field in time on the surface of the alloy D16 during

realization of DNP from the moment of impulse energy input: a - t = 0 ms; b - t = 0,36 ms; c - t = 4

ms; d - t = 51 ms.

Significant heterogeneity of plastic deformation on the surface of the specimen in the

process of realization of DNP associated with the heterogeneity of the formation of dissipative thin-

band structures in the volume of the material. This is largely causes to change the mechanical

properties of alloys, especially plasticity, at the later static load. Materials receive reserves of

plasticity, which may manifest as an increase in the total plasticity of alloys at further loading, and

as in delaying the process of "neck forming."

Results of the comparison of deformations diagrams fragments of alloy D16, width of 25mm,

with "clear” static tension and at the complex mode of load "static tension - dynamic non-

equilibrium process - repeated static tension" are presented on Figure 3.

Fig. 3. Fragments of alloy D16 deformations diagrams at different modes of loading: 1 - DNP +

static tension, 2 - static tension.

Key Engineering Materials Vols. 577-578 275

A clear tendency for plasticization of alloy is evident after realization of the DNP. (Fig. 3)

According to the test results of a series of specimens (40 pieces) 25mm width, with additional

impulse load Pimp = 87 ± 12 kN, was found that there is a delay after the DNP process "neck

forming." Aluminum alloy 2024 - T3 and D16 under further static stretching, demonstrate a delay

of 3 ... 10%. The level of local plastic deformation at the points of the working part of specimen,

where the failure of the material occurs, also increases by 3 ... 6%. Alloy D16 after realization of

DNP characterizes stable drop in strength at 10 ... 50MPa.

Conclusions:

1. Aluminum alloys 2024 - T3 and D16 demonstrate a high sensitivity to the realization of

dynamic non-equilibrium processes. The consequence of DNP realization in the material is the

formation of thin-band dissipative structures, that contribute to plasticization of the material

during later load.

2. Determined the parameters under which realization of DNP is possible in the appropriate test

setup:

─ additional pulse loading speed within 300-500 MN / s;

─ oscillation frequency of specimens 1.5 ... 2 KHz. The corresponding rate of deformations

process specimens of DNP is 2 ... 60 s-1

.

3. Found that at the repeated tensile alloys after realization of DNP a significant plasticization

occurs with delayed "neck forming."

Acknowledgments

The authors gratefully acknowledge the funding by National Science Centre of Poland under the

grant No. N501 056740.

References

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Advances in Fracture and Damage Mechanics XII 10.4028/www.scientific.net/KEM.577-578 Influence of Dissipative Structures Formed by Impulsed Loads on the Processes of Deformation and

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