Understanding the Interdependence of Penetration Depth and Deformation on Nanoindentation of Nanoporous Silver

A silver-based nanoporous material was produced by dealloying (selective chemical etching) of an Ag38.75Cu38.75Si22.5 crystalline alloy. Composed of connected ligaments, this material was imaged using a scanning electron microscope (SEM) and focused ion-beam (FIB) scanning electron microscope tomography. Its mechanical behavior was evaluated using nanoindentation and found to be heterogeneous, with density variation over a length scale of a few tens of nanometers, similar to the indent size. This technique proved relevant to the investigation of a material’s mechanical strength, as well as to how its behavior related to the material’s microstructure. The hardness is recorded as a function of the indent depth and a phenomenological description based on strain gradient and densification kinetic was proposed to describe the resultant depth dependence.

A High Resolution Automated Prestressing Wire Indent Profiling System for Verification of Wire-Concrete Mix Compatibility

It is well-known that the geometrical characteristics of the indents on prestressing wire used in the manufacture of prestressed concrete railroad ties affect the magnitude of the transfer length. In particular, it has been shown that such parameters as indent depth, indent volume and indent sidewall angle all affect transfer length, with indent volume being a major influence. Previous research has shown that the larger the indent volume, the shorter the transfer length. For full load bearing capacity, it is important that the transfer length not exceed the distance to the rail seat. Consequently, transfer length has been identified as a key diagnostic parameter for evaluating the load bearing capability of prestressed concrete railroad crossties. Furthermore, it has been proposed for use as a valuable quality control parameter. Ongoing research, as well as previously published research results, also indicates that the geometry of the prestressing wire indents plays a major role in the formation of cracking. This is particularly important in the manufacture of concrete ties intended for high speed rail applications. Cracking and debonding of prestressing wires associated with ties in service can result in severe splitting and complete tie failure. It is therefore not sufficient to guarantee a safe transfer length alone, without consideration of the cracking propensity. The wire specifications in standard ASTM A881 are intended to promote quality prestressed railroad tie behavior; however, the detailed causes of cracking and splitting, and the specific indent features that are responsible, are not well-known from a quantitative perspective. Until recently, inspection of prestressing wire indent properties consisted of sampling a few indents from a small segment of wire, providing very limited statistical information on wire indent properties. To address this deficiency, a high-resolution automated non-contact optical wire indent scanning system has been developed for completely and rapidly characterizing all relevant indent geometrical parameters. The system is capable of measuring large segments of wire to yield statistically significant samples of all relevant indent parameters including indent depth, indent width, indent sidewall angle, indent pitch, and indent volume. The current state-of-the-art in this system development, along with some new insights based on recent indent scanning results, will be presented. This system represents a valuable tool to aid in identifying the key indent geometrical features related to cracking. The overall goal is to quickly assess critical indent parameters, so as to ensure high-quality bond and eliminate in-track tie splitting failures.

Effect of Artificial Defects on the Very High Cycle Fatigue Behavior of 316L Stainless Steel

Widely used for structural materials in nuclear engineering, 316L austenitic stainless steel undergoes very high cycle fatigue (VHCF) throughout its service life. Since defects caused by service conditions are unavoidable in many engineering components during service life, the effects should be properly understood. In the present study, the effect of surface defects on the VHCF behavior were investigated on solution annealed (SA) and cold-worked (CW) 316L. Surface defects were artificially created using indentation. The VHCF test was conducted using an ultrasonic fatigue testing system. The results showed that the fatigue crack initiation was independent of the indent with the applied range of depth in this research. Furthermore, the critical depth of the indent was evaluated based on an empirical formula (Murakami’s model). In the case of SA 316L, the VHCF strength was not affected when the indent depth was less than 40 μm, which is consistent with the value obtained from the empirical formula. In the case of 20% CW 316L, the VHCF strength was not affected when the indent depth was less than 80 μm. The experimental results, i.e., the critical depth of the indent, were much larger than the results obtained from the empirical formula, and might have been caused by the plastic deformation, residual stress, and probable deformation-induced martensite transition around the indent.

Indent Depth in the Clay Backing for Ceramic Armor

Numerical models were conducted to study the load/momentum/energy transfer to the clay and the clay response when it was used as backing behind hard and soft armor during ballistic experiments. Tie-break contacts were used to explicitly model the delamination in the composite panels. Yarn level models accounting for woven structures were used to model the soft armor. A rate-dependent material model with different responses under compression and tension, developed previously from impact tests, was used to model the clay. The clay indent depth was correlated to the momentum and kinetic energy transferred to the clay for with and without soft armor between the hard armor and clay backing.

Are elastic moduli of biological cells depth dependent or not? Another explanation using a contact mechanics model with surface tension

Contrary to the existing reports that the apparent elastic modulus of a cell depends strongly on the indent depth in many AFM indentation experiments, we present a contact model with surface effects, and show that the actual elastic modulus of cell materials could be independent of the indent depth if surface tension is taken into account.

Micro/Nanocontact Between a Rigid Ellipsoid and an Elastic Substrate With Surface Tension

For micro/nanosized contact problems, the influence of surface tension becomes prominent. Based on the solution of a point force acting on an elastic half space with surface tension, we formulate the contact between a rigid ellipsoid and an elastic substrate. The corresponding singular integral equation is solved numerically by using the Gauss–Chebyshev quadrature formula. When the size of contact region is comparable with the elastocapillary length, surface tension significantly alters the distribution of contact pressure and decreases the contact area and indent depth, compared to the classical Hertzian prediction. We generalize the explicit expression of the equivalent contact radius, the indent depth, and the eccentricity of contact ellipse with respect to the external load, which provides the fundament for analyzing nanoindentation tests and contact of rough surfaces.

Compression of Hyperelastic Cells at Finite Deformation with Surface Energy

In this paper, the compression of an isolated cell by two rigid indenters is analyzed. The neo-Hookean model is employed to characterize the hyperelastic behavior of biological cells. Owing to the greatly increased ratio between surface energy density and elastic modulus, surface energy plays important roles in the mechanical performance of biological cells. Using the dimensional analysis method and a finite element approach incorporating surface energy, we study the elastic compression of hyperelastic cells at finite deformation and give the explicit relations of contact radius and indent depth depending on compressive load. Our results reveal that surface energy obviously influences both the local deformation and the overall responses of hyperelastic cells at finite deformation. The obtained results are useful to determine the elastic properties of biological cells from indent-depth curves accurately.

Indentation-induced two-way shape memory surfaces

A method is described for the creation of surfaces with cyclically reversible topographical form. Using spherical and cylindrical indenters applied to NiTi shape-memory alloys, an indentation-planarization technique is shown to result in a two-way shape memory effect that can drive flat-to-wavy surface transitions on changing temperature. First, it is shown that deep spherical indents, made in martensitic NiTi, exhibit pronounced two-way cyclic depth changes. After planarization, these two-way cyclic depth changes are converted to reversible surface protrusions, or “exdents.” Both indent depth changes and cyclic exdent amplitudes can be related to the existence of a subsurface deformation zone in which indentation has resulted in plastic strains beyond that which can be accomplished by martensite detwinning reactions. Cylindrical indentation leads to two-way displacements that are about twice as large as that for the spherical case. This is shown to be due to the larger deformation zone under cylindrical indents, as measured by incremental grinding experiments.

Simulation of Nano- and Micro-Indentation Behavior of Bone via a Plastic-Damage Model

Damage results in a loss of material continuity, which distinguishes it from other types of material degradation. The loss of continuity can have an adverse effect on mechanical properties, and may be manifested in the form of cracks and/or voids. Bone tissue, as a composite material, contains voids and other non-homogeneities that are naturally occurring and distinct from damage. However, when subjected to mechanical loading, such as indentation, further damage accumulation may occur. Figure 1 shows a cross-section of a bovine cortical bone specimen after high-load conical indentation to a depth of 300 μm, resulting in a large permanently deformed region. Nanoindentation, using a Berkovich tip at 10 mN maximum load, was then performed at numerous locations within three defined damage “zones”. Zone 1 is adjacent to the bottom of the indent, defined at 25% of the maximum indent depth. Zones 2 and 3 extend further away, both scaled as a function of the indentation depth, d. Figure 2 shows the variation in Young’s modulus in the three damage zones, averaged over approximately 25 indents per zone. The data suggest that local changes in mechanical properties may occur as a result of compaction of voids or cracks. The purpose of this work, therefore, is to investigate the application of a plastic-damage model for simulation of bone nano- and micro-scale indentation behavior.