Neural networks for tip correction of spherical indentation curves from bulk metals and thin metal films

Spherical indentation is widely used to determine a variety of important mechanical properties from small volumes. However, the available nanoindenter tips mostly deviate from the perfect spherical shape making the application of analysis methods developed for perfect spheres uncertain. In this paper, neural network-based methods are presented that are used to correct force-depth curves measured with such indenter tips. Finite element simulations for imperfect and perfect spherical tips with varying material behaviour are used to train the neural networks, which solve the inverse problem of mapping the true tip shape and the measured force-depth curve to one that corresponds to a perfect spherical indenter. Solutions are provided for bulk materials and thin films. The method has been verified experimentally on nanocrystalline nickel and a copper film on a titanium substrate for different spherical tips.     

Light structured deposition (1): Material properties

An original light-assisted approach for optical-material deposition is introduced, and preliminary results concerning the properties of the deposits, in the case of As–S binary glass alloys, are reported for the first time. Such approach uses a continuous-wave laser source and on-axis geometry, to grow a semiconducting glass alloy onto a transparent substrate under the concurrent actuation of a given light-intensity distribution. Significant differences are observed when the material is prepared by using this technique, in comparison with those reported for similar alloys prepared by both physical- and chemical-vapor based techniques.     

Nanoindentation study of size effect and loading rate effect on mechanical properties of a thin film metallic glass Cu49.3Zr50.7

A binary metallic glass (MG) Cu_49.3Zr_50.7 in the form of thin film was successfully grown on a Si (1 0 0) substrate by magnetron sputtering. The mechanical properties, specifically, hardness and modulus at various peak loads and loading rates were characterized through instrumented nanoindentation. Unlike other metallic glasses showing an indentation size effect (ISE), the composition of this study does not have an ISE, which is phenomenologically the result of the negligible length scale according to the strain gradient plasticity model. The proportional specimen resistance model is applicable to the load-displacement behaviors and suggests that the frictional effect is too small to contribute to the ISE. The occurrence of plasticity depends on loading rates and can be delayed so that the displacement during the load holding segment increases logarithmically. In addition, the hardness and modulus are both dependent on the loading rates as well, i.e., they increase as the loading rate increases up to 0.1 mN/s and then hold constant, which is independent of creep time (≤100 s). These loading-rate-dependent behaviors are interpreted as the result of viscoelastic effect rather than free volume kinetics.     

Atomistic study of deposition process of Al thin film on Cu substrate

In this paper we report molecular dynamics based atomistic simulations of deposition process of Al atoms onto Cu substrate and following nanoindentation process on that nanostructured material. Effects of incident energy on the morphology of deposited thin film and mechanical property of this nanostructured material are emphasized. The results reveal that the morphology of growing film is layer-by-layer-like at incident energy of 0.1-10 eV. The epitaxy mode of film growth is observed at incident energy below 1 eV, but film-mixing mode commences when incident energy increase to 10 eV accompanying with increased disorder of film structure, which improves quality of deposited thin film. Following indentation studies indicate deposited thin films pose lower stiffness than single crystal Al due to considerable amount of defects existed in them, but Cu substrate is strengthened by the interface generated from lattice mismatch between deposited Al thin film and Cu substrate.     

The effect of indentation sequence on rock breakages: A study based on laboratory and numerical tests

Rock may response differently to external loads applied in different sequences. Thus, we conducted indentation tests to investigate the effect of the indentation sequence on rock breakages. Sequential indentations, consuming less indentation energy, usually resulted in larger and deeper grooves and then led to lower specific energies. Thus, we conclude that sequential indentations occur instead of simultaneous indentations form larger grooves with the same indentation energy. To further validate this conclusion, we performed a series of numerical tests. The numerical analysis of stress evolution shows that, for simultaneous indentations, the propagation of an internal crack from an inner rim restrained the propagation of the other internal crack from the other inner rim. However, the chipping pattern varied for sequential indentations. In the first indentation process, an internal crack, initiating from an inner rim, is usually connected with an internal crack caused by the second indentation. The deflection angles of the internal cracks for the sequential indentations were smaller because of the lower compressive stress in the horizontal direction. Then, these smaller deflection angles led to larger chips.     

Deformation field in deep flat punch indentation and the persistence of dead-metal zones

The indentation of metal by a flat punch is a model system for forming processes and intimately linked with hardness testing. Here, we perform first-in-class, high-fidelity finite element (FE) simulations in an arbitrary Lagrangian–Eulerian (ALE) framework to study the deformation field in deep punch indentation of annealed copper. The use of ALE allows indentation depth to punch width ratios as high as 1.6, while the use of Lagrangian tracer particles reveals pathlines of material transport. Field quantities such as the plastic strain, strain rate and velocity are obtained at high resolution. A low-strain, dead-metal zone (DMZ) that is stationary with respect to the indenter forms immediately below the punch. Crucially, it is found that DMZs are unavoidable in deep punch indentation, forming at the outset and irrespective of the coefficient of friction. However, the area of this zone shrinks as the indentation progresses at a rate that is inversely related to the friction. The simulations thus explain why Prandtl’s view of punch indentation, which incorporates DMZs, is physically more accurate than Hill’s view. The computations successfully reproduce the strain field inhomogeneity seen in recent in situ imaging experiments. While DMZ formation is impervious to the hardening model used, Zerilli–Armstrong hardening provides more accurate indentation force estimates than Johnson–Cook hardening. Lastly, the residual impression and factors affecting its shape are studied. The sides of impressed metal are never vertical, but at an inclination to it. Methods to modify such features, of potential interest in metal forming, are discussed briefly.     

SiC/Al2O3纳米复合材料压痕残余应力压谱法测量研究

利用压力荧光光谱法研究了5％ SiC／Al2O3纳米复合材料压痕周围残余应力的大小及其分布规律。结果发现在压痕区及压痕周围10μm的应力区范围中,荧光R-线明显增宽,而且压应力与离压痕中心距显著相关。通过获得的R双峰(R1和R2荧光光谱)频率变化,计算了压痕区及其周围复杂应力场中的等静压力。结果表明：沿着压痕区主对角线方向和边界垂直平分线方向,其应力分布存在十分相似的变化规律。同时发现在压痕周围存在着对称残余应力区。     

Elastic-plastic contact mechanics of indentations accounting for phase transformations

A contact mechanics model is developed which takes into account possible phase transformations in materials induced by hydrostatic and shear stresses associated with indentation. The proposed model allows prediction of the average thickness and approximate shape of the phase transformation zone in semiconductors and ceramics under various types of diamond indenters. The results of theoretical calculation are in good agreement with the available experimental data.     

Improving spatio-temporal resolution of infrared images to detect thermal activity of defect at the surface of inorganic glass

This paper proposes a noise suppression methodology to improve the spatio-temporal resolution of infrared images. The methodology is divided in two steps. The first one consists in removing the noise from the temporal signal at each pixel. Three basic temporal filters are considered for this purpose: average filter, cost function minimization (FIT) and short time Fast Fourier Transform approach (STFFT). But while this step effectively reduces the temporal signal noise at each pixel, the infrared images may still appear noisy. This is due to a random distribution of a residual offset value of pixels signal. Hence in the second step, the residual offset is identified by considering thermal images for which no mechanical loading is applied. In this case, the temperature variation field is homogeneous and the value of temperature variation at each pixel is theoretically equal to zero. The method is first tested on synthetic images built from infrared computer-generated images combined with experimental noise. The results demonstrate that this approach permits to keep the spatial resolution of infrared images equal to 1 pixel. The methodology is then applied to characterize thermal activity of a defect at the surface of inorganic glass submitted to cyclic mechanical loading. The three basic temporal filters are quantitatively compared and contrasted. Results obtained demonstrate that, contrarily to a basic spatio-temporal approach, the denoising method proposed is suitable to characterize low thermal activity combined to strong spatial gradients induced by cyclic heterogeneous deformations.     

Nanomechanical testing of ODS steels irradiated with 1 MeV/amu heavy ions

Development of oxide dispersion strengthened (ODS) steels, as candidates for fuel claddings for Gen IV nuclear reactors, requires a comprehensive study of their behaviour under operating conditions. In this work, 1.2 MeV/amu Kr and Xe irradiation was used to simulate fission fragment impact. New irradiation approaches with respect to a non-homogeneous damage profile under ion irradiation were proposed. Hardness profiles of irradiated ODS steels were obtained by continuous stiffness measurements with subsequent analysis of size effects according to the Nix–Gao model. It was found, that heavy ion irradiation leads to hardness saturation in ODS steels in a damage dose range of 0.1–1 dpa. Observed hardening is about 20% and is not connected with the radiation stability of Y–Ti–O and Y–Al–O oxide particles in ODS steels as it was studied by TEM.