Full length article
Application of a nanosecond laser pulse to evaluate dynamic hardness under ultra-high strain rate

https://doi.org/10.1016/j.optlastec.2015.10.011Get rights and content

Highlights

  • A new method for dynamic hardness measurement has been developed.

  • A laser pulse results in plastic deformation with repeatable geometry.

  • No thermal effects of laser pulses on surface of materials were stated.

  • Nanosecond laser pulses with an energy of 1 J can be used to evaluate dynamic hardness.

  • Values of the hardness HDL of materials can be compared with each other.

Abstract

The paper presents results of experimental tests of plastic metals deformation generated by a shock wave induced by laser pulse. Tests were carried out on the Nd:YAG laser with a wavelength of 1064 nm and the laser pulse of 10 ns duration. The shock wave generate by the laser pulse was used to induced local plastic deformation of the material surface. The study examined the possibility of application the process to develop a new method of measuring the dynamic hardness of materials under ultra-high strain rate. It has been shown that the shock wave induced by the laser pulse with an energy of 0.35–1.22 J causes a repeatable plastic deformation of surface of commercially available metals and alloys without thermal effects on the surfaces. Based on the knowledge of an imprint geometry, it is possible to evaluate the dynamic hardness of materials at strain rate in the range of 107 s−1.

Introduction

Knowledge of the behaviour of materials and their properties under conditions of dynamic deformation is essential for proper designing of machines and equipment and for predicting their behaviour in manufacturing and operating processes. Ultra-high-speed deformations occur in the processes of friction and machining of materials, as well as, operation of components used in many fields of technology. Properties of materials under conditions of dynamic deformation significantly differ from those in static conditions. They depend on the speed of deformation, microstructure of the material and temperature. Strain rate substantially affects the strength properties of construction materials. Sensitivity to the strain rate considerably increases at strain rates above 10 s−1 of most metals and alloys [1], [2], [3]. With the increase in the strain rate the yield stress raises. It has been shown [4] that the sensitivity to the strain rate grows linearly with the increasing strain rate for steel in the range of strain rate of 10–104 s−1. Other material properties such as tensile strength, uniform or total elongation also dependent on the deformation rate [5]. Therefore, in order to determine properties of materials under different conditions, experimental tests are needed in the widest possible range of the strain rate.

Evaluation of the plastic properties of materials and layers deformed at the high strain rate is a serious experimental issue [1], [3]. There are several methods for material testing at high speed deformation such as the Split Hopkinson Pressure Bar (SHPB) method, miniaturised direct impact test, shock methods, explosive methods, but all are complex and destructive [2].

Mechanical properties at high strain rate may be tested also by means of dynamic hardness testers. Strain rate in these devices is typically in the range of 103 s−1. Dynamic hardness measurements are based on a dynamic action of an indenter on the surface of the tested materials. These methods are not standardized and the measurement results should be treated as approximate. Recently, the reports on development of new methods for measuring the dynamic hardness using a high velocity gas gun have occurred. The strain rate in these methods is about 1500–2200 s−1 [6], [7]. The results of the dynamic hardness measurements based on the Vickers method are presented in [6]. The obtained hardness values for different materials varied from a few to several percent relative to the hardness values in static conditions. Studies of the dynamic hardness indicate the presence of analogous relationships between hardness and other mechanical properties, e.g. yield point or ultimate strength, as for the respective static characteristics [5], [8]. Dynamic hardness measurements are non-destructive, fast and less complex compared to other methods of material testing in considered conditions. They allow indirectly, determining other material properties, e.g. the yield point [7], [8].

The study of processes occurring in materials at very high speed rate is possible using nanosecond laser pulses [9], [10]. A shock wave is generated for this purpose. As a result of the laser radiation on material plasma and a pressure wave are generated. Nanosecond high power pulses and specially selected absorption layer as well as constrained layers, allow obtaining a wave pressure from a few to several GPa [9]. The absorption layer should ensure a proper conduct of the development and expansion of the plasma cloud without impact of thermal effects on the tested material. The proper propagation of the pressure wave to the material is provided by the use of a constrained layer. It reduces scattering of the pressure wave. Medium with suitably selected impedance is used as the constrained layer, typically it is water or quartz glass [9], [11], [12]. When the constrained layer is applied, there is a growth of the amplitude of the pressure pulse, improving its shape, shortening the rise time and decay time, and improving the repeatability of these characteristics. In the case of liquids, it also increases the speed of the cooling process at the interface of phases, which reduces the thermal diffusion of the tested material [11]. The described process has been used for many years for example for surface treatment of metals and alloys, laser shot peening – LSP [13], [14], [15]. LSP is a mechanical process without thermal effect.

In the 1990s, attempts to use the pressure wave generated by a laser pulse to testing the adhesion of thin layers obtained by PVD and CVD methods were made (laser spallation technique). Tensile stresses that caused separation of the layer from the surface at the interface of the material/layer phases have been studied [2]. In these methods, adhesion of layers is calculated based on the pressure at which the shock wave acts on the tested material, its speed of propagation in the material and geometry of debonding of the layer. The accuracy of the method depends mainly on the accuracy of pressure measurements and the propagation velocity of the shock wave. Due to very short time of process, from several to tens of nanoseconds, advanced measurement techniques are required [16], [17].

Up to now, there is no information about tests on evaluation of the hardness of the materials using a laser pulse. However, the imprint size after plastic deformation caused by shock wave induced by laser pulse was investigated previously for example in works [18], [19]. Li et al. notice that laser peen texturing can be regarded as a new method for dynamic hardness measurement and showed that deeper imprints indicate the smaller dynamic hardness. Theoretical and experimental studies of plastic deformation of the surface of various materials induced by a single or multiple laser pulse have been conducted mainly in order to determine the hardening of the material or internal stresses after the LSP process [18], [20], [21]. Numerical simulations of surface deformation under a single laser pulse showed a homogeneous imprint in the place of the impact of the shock wave. The size of the zone of deformation depends on the laser pulse energy and material properties [13], [20]. The strain rate during the process has been estimated from 107 to 109 s−1 [22].

The paper presents the results of plastic deformation of the material generated by the single nanosecond laser pulse and attempts to use the results for evaluation of the hardness of the material under the ultra-high strain rate.

Section snippets

Material and methods

The study of plastic deformation induced by a nanosecond laser pulse was carried out for typical commercial metals and alloys: aluminium EN 1050 (99.5% Al), copper EN CW004A (99.9% Cu), stainless steel EN X5CrNi18-10 1.4301 (AISI 304) and two aluminium alloys AlSi12 ISO 3522-1984 and 6060 AlMgSi0.5 (0.35-0.6% Mg, 3–0.6% Si, 0.10–0.30% Fe plus Al). Round samples of a diameter of 25 mm were cut by WEDM method out of sheets 1 mm thick. Surface of the materials was electrolytically polished. Brinell

Microstructure of the deformed zone

The impact of laser pulse can affect the condition of the surface layer and microstructure of the materials and thus the interpretation of research results. Therefore, the microstructure study was carried out in the zone of plastic deformation.

Microstructure analysis of the materials was done on the top surface of the samples in the zone of plastic deformation. Even at the highest applied pulse energies, i.e. 1 and 1.22 J no changes in the microstructure of the tested materials was notice.

Discussion

The study showed that single laser pulses with energy of 0.35–1.22 J and time length of 10 ns allow to get required levels of pressure and reproducible test conditions. The size of plastically zone deformed obtained under tested conditions is suitable to assess the dynamic materials hardness HDL. For steel and materials with high hardness, pulse energy greater than 1 J is needed. The use of scanning profilometry allows determining the size of the plastic deformation zone on the material surface

Conclusions

  • 1.

    The nanosecond laser pulses with energy of 0.35–1.22 J, and the time length of 10 ns, generate repeatable plastic deformation of surface of commercially available metals and alloys.

  • 2.

    The tests did not show thermal effects of laser pulses on the surface of examined materials within the tested range of the pulse energy values.

  • 3.

    The proposed measurement system allows obtaining the proper level of pressure and reproducible measurement conditions necessary to induced plastic deformation required to assess

References (29)

  • J.Z. Malinowski et al.

    Miniaturized compression test at very high strain rate by direct impact

    Exp. Mech.

    (2007)
  • K. Tanaka, T. Nojima, Dynamic and static strength of steels, in: Proceedings of the Second Conference on the Mechanical...
  • G. Frommeyer et al.

    Supra-ductile and high-strength manganese-trip/twip steels for high energy absorption purposes

    ISIJ Int.

    (2003)
  • W. Kohlhofer et al.

    Dynamic hardness testing of metals

    Int. J. Press. Vessel. Pip.

    (1995)
  • Cited by (18)

    • Design and theoretical analysis of dynamic indentation experimental device

      2020, Materials Today Communications
      Citation Excerpt :

      Jun Lu et al. [12] studied the strain rate sensitivity of micro flow stress of oxygen free copper under dynamic loadings. Joanna Radziejewska [13] has studied the microscopic dynamic hardness and flow stress of various metal materials. Ben Peng Wang et al. [14] studied the dynamic hardness of metallic glass and the failure characteristics of materials under different strain rates.

    • Experimental investigation of shock wave pressure induced by a ns laser pulse under varying confined regimes

      2020, Optics and Lasers in Engineering
      Citation Excerpt :

      The use of more sophisticated, additional experimental VISAR determinations to analyze pressure loadings and elastic limits under shock conditions was shown to be a key point to improve simulations [23] or allowed, by a simple measurement of sample back free surface velocities to analyses shock wave propagation, and deduce the pressure versus time profiles Hugoniot conservation equations [24,25]. On the basis of laser shock waves, new diagnostic methods for the dynamic behaviour of a material and layer [26], as well as adhesion of thin films, could be developed [27]. Several practical techniques of measuring the adhesion of thin layers were described in literature.

    • Collective evolution of surface microcrack for compacted graphite iron under thermal fatigue with variable amplitude

      2019, International Journal of Fatigue
      Citation Excerpt :

      However, due to the limitations of induction heating, temperature change within milliseconds is hard to achieve, and the operating condition of combustion cycle cannot be simulated by the induction heating method. In recent years, pulsed laser has been applied as one of the ideal heating source for the thermal fatigue experiment of variable amplitude [23,24], due to its precise controllability for time and space. In comparison with the thermal fatigue test based on the application of continuous laser and beam transformation [25,26], the main point of pulsed laser is the temporal distribution of laser spot, whereas the former is focused on the spatial distribution of laser spot.

    • The research of micro pattern transferring on metallic foil via micro-energy ultraviolet pulse laser shock

      2018, Optics and Laser Technology
      Citation Excerpt :

      The plastic strain mainly concentrated on the edge of mold hole. The whole process of laser shock transferring completed in dozens of nanoseconds, and the aluminum foil formed with a high strain rate exceeding 106 s−1 [24,25]. By analyzing the simulation results and AFM images, Fig. 16(i) showed the schematic of surface roughness change in laser shock process.

    View all citing articles on Scopus

    The project was financed by the funds from the National Science Centre award based on decision number DEC-2013/09/B/ST8/03468.

    View full text