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In this paper, we use a deterministic multi-asperity model to investigate the elastic contact of rough spheres. Synthetic rough surfaces with controllable spectra were used to identify individual asperities, their locations and curvatures. The deterministic analysis enables to capture both particular deformation modes of individual rough surfaces and also statistical deformation regimes, which involve averaging over a big number of roughness realizations. Two regimes of contact area growth were identified: the Hertzian regime at light loads at the scale of a single asperity, and the linear regime at higher loads involving multiple contacting asperities. The transition between the regimes occurs at the load which depends on the second and the fourth spectral moments. It is shown that at light indentation the radius of circumference delimiting the contact area is always considerably larger than Hertzian contact radius. Therefore, it suggests that there is no scale separation in contact problems at light loads. In particular, the geometrical shape cannot be considered separately from the surface roughness at least for approaching greater than one standard roughness deviation.

Numerical simulations of quasi-static indentation and low velocity impact of low density polymethacrylimide (PMI) Rohacell 51 WF foam using indenters with different nose shapes (conical, truncated-conical, hemi-spherical and flat) were carried out using the finite element code LS-DYNA. A 2D axisymmetric model was generated. A strain-rate dependent material model and r-adaptive remeshing were used for low velocity impact simulations. Numerical predictions matched the available experimental data very well. Moreover, the predicted resistance force closely matched the empirical results. The results demonstrated the ability of the model to reproduce the deformation mechanisms of the penetration process of Rohacell 51 WF foam.

This paper represents the nanomechanical properties of various loading levels of montmorillonite clay filled polyester composites and randomly distributed jute fiber reinforced hybrid composites through Vickers micro-hardness test. The study of indentation fracture mechanics in polymer materials is a simple and cost-effective technique for the determination of fracture toughness. Ultrasonication technique was used to disperse the clay in the polyester matrix. The hand layup method was adapted to prepare the test specimens. Incorporation of 5wt.% montmorillonite clay into the polymer matrix results in an enhancement in hardness of 26.52% and the modulus of elasticity increased from 4205.21MPa for neat polyester to 5051.46MPa with the addition of clay. Fracture toughness was observed to depend on the montmorillonite clay content, and the maximum value was observed at 5wt.% nanoclay and 25wt.% jute fibers. The results show that the increase in the fiber content reduces the crack propagation in hybrid composites and increases the fracture toughness. To predict the crack size, the scanning electron microscope images are used.

Lead-free piezoelectric (Bi_1/2Na_1/2)TiO₃ (abbreviated as BNT) films were deposited on 0.2 mm thick pure titanium(Ti) substrates by a hydrothermal method. Scratch tests and Vickers indentation tests were performed to quantitatively assess the adhesion strength between BNT films and Ti substrates. Some of Ti substrates were pretreated by chemical polish and mechanical polish respectively prior to BNT film deposition with a view of investigating the effects of substrate surface pretreatments on the adhesion of BNT films. In the scratch test, the critical force was determined from the variations of the tangential force and the acoustic emission (AE) signals with the normal force. The scratch test results revealed that the chemical polish pretreatment effectively improved the adhesion of BNT films. In addition, the critical substrate strain inducing the adhesion failure of BNT films has been investigated by the Vickers indentation test combined with finite element analysis (FEM).

The force response of poroelastic materials including poroelastic gels to indentation is known to be time- and space-dependent (i.e., a function of indenter shape and size). Despite the complexity of the poroelastic response and in contrast to viscoelastic mechanics, poroelastic mechanics can be captured in terms of several intrinsic mechanical properties, such as elasticity, permeability, and Poisson ratio. While these intrinsic properties can be found from time-domain or frequency-domain master curves, indentation is usually conducted and analyzed only in the time domain using stress-relaxation or creep experiments. This paper advocates using frequency-domain analysis of poroelastic gels by reviewing and analyzing the relevant works of the literature. The analysis and methods, proposed here, enable researchers to characterize dynamic moduli of poroelastic gels in frequency domain using only a few experimental defining parameters. The authors have intentionally provided extensive details and background, to make this work useful for researchers who consider using frequency-domain analysis for the first time. This work reviews and explains the instantaneous elastic modulus, depicted over normalized time as a unifying and understandable set of master curves for time-domain stress relaxation tests on poroelastic gels for cylindrical, conical, and spherical indenters. The dynamic elastic modulus, depicted over normalized frequency, are derived symbolically and numerically and explained for the first time as master curves with simple transfer function in the frequency domain for presenting poroelastic mechanics of gels.

Residual stress plays an important role in the mechanical performance of engineering structures. In this paper, the indentation of pre-stressed strain hardening materials by a rigid sphere is simulated through the finite element method. It is shown that the mean contact pressure in the post-yield regime keeps continuously increasing with the indentation proceeding. Compared to the case without residual stress, compressive residual stress tends to increase the mean contact pressure at a given indentation depth, while tensile stress has an inverse effect. To quantitatively indicate the influence of residual stress, a nominal contact pressure is defined, which obtains its maximum value at a critical depth. The dependences of the maximum nominal pressure and the critical indentation depth on the residual stress and the material properties are determined explicitly through large numbers of simulations. Inspired by the nominal pressure deviation stemmed from residual stress, a new approach is proposed for measuring residual stress through spherical indentation tests.

Young's modulus and Poisson's ratio are the important parameters for representing the mechanical properties of soft tissue. Indentation test is an in vivo, non-invasive and convenient technique for measuring the tissue properties. In this chapter, two methods for the simultaneous estimation of Young's modulus and Poisson's ratio of soft tissues using indentation are presented. With the consideration of the finite deformation effect, the first method is based on the use of two sized indentors for conducting two different indentation tests, whereas the second one is only a single indentation test. Finite element (FE) analysis was used to demonstrate the feasibility of these two methods. The FE results were found to be comparable to the one shown in the previous literatures. It was revealed that finite deformation effect is of vital importance in the estimation of the Young's modulus and Poisson's ratio. Simultaneous estimation of these two parameters is necessary for achieving an accurate measurement of the Young's modulus.

A method that using the nano-scale blunt round tip of cone-shaped diamond tool to indent and pierce through the metal sheet at the bottom to form a pore even ultramicropore is proposed and studied in this paper. Firstly, Deform-3D software is used to simulate and analyze the copper sheet deformation during indenting process under different constraint conditions, and finding the bottom constraint condition suitable for the pore forming. Secondly, based on the constraint condition chosen, the model is simulated in two kinds of substrate material, aluminum and glass. Then the copper plastic flow characteristics, the quality of the pore-forming and the equivalent stress distribution are analyzed through simulation. Finally, a pore-forming experiment is done to test the simulation results, showing that the hard substrate is better for the pore-forming of copper sheet.