Spherical indentation of freestanding circular thin films in the membrane regime

We present theoretical and experimental results to describe the mechanics of indentation of a clamped circular membrane with a frictionless spherical indenter. Analytical expressions and numerical simulations are presented for the relationships between contact radius, finite indentation strains (and stresses), pre-stretch, loads and deflection. These closed-form solutions are contrasted with point-load models that neglect the contact size (i.e. classical Schwerin-type solutions), and lead to important differences in the indentation strain and load-deflection response. The accuracy of these closed form expressions is illustrated by comparisons with detailed numerical results and experiments on thin elastomer films. We show that the closed-form solutions can be used to extract mechanical properties from indentation testing of freestanding films, with important implications for developing new tests on nanoscale films and/or compliant materials such as polymers and biological substances.     

Measurement of residual stresses in polymeric parts by indentation method

An indentation method was studied as a means of measuring the residual stress in an injection molded polymeric specimen because destructive methods restrict the reuse of measured parts and it is not possible to apply them to small and complicated parts. The load-displacement curve was measured for indentation at stressed and non-stressed positions. Residual stress distribution of the injection molded part was calculated by comparing the load-displacement curve results with respect to the indentation depth. The residual stresses measured by the indentation method were reliable because they were in good agreement with numerical results and those measured by the hole drilling method. The indentation method can be utilized to measure the residual stresses in polymeric parts for practical applications, particularly for small or complicated parts.     

A method for predicting the internal motion resistance of rubber-tracked undercarriages,Pt. 2. A research on the motion resistance of road wheels

In spite of an increasing number of rubber-tracked crawlers, the literature provides few guidelines and calculation models suitable for minimizing their internal motion resistance. This article presents a model where the internal resistance of double-flanged road wheels for rubber-tracked vehicles is calculated as a sum of the losses resulting from the indentation of the wheels into the track surface and friction of the wheels against the track guide lugs. The model allows for vertical and lateral load of the wheels, the non-uniform distribution of the wheel pressure on the track, and the relationship between the friction coefficient and normal reaction force in the interface between the wheel and track guide lugs. The model has been verified by experiments. According to the results of model computations and experiments discussed in the article, the internal losses of a given rubber-tracked undercarriage might be reduced if: the road wheels are covered with a material that exhibits low friction coefficient and mechanical hysteresis, the vehicle suspension system features oscillating bogie wheels, the undercarriage is fitted with the largest possible number of road wheels, and the vehicle weight is evenly distributed to all of the road wheels.     

Indentation analysis of mechanical behaviour of torsion-processed single-crystal copper by crystal plasticity finite-element method modelling

A crystal plasticity finite-element (CPFE) constitutive model is developed to investigate the mechanical properties and microtexture evolution of torsion-processed single-crystal copper by indentation modelling. The mesh-to-mesh solution (mapping) is used during modelling to bridge the gap between two different processes (torsion and indentation) and integrate them. The parameters for CPFE modelling are validated by comparison with experimental observations. The numerical results indicate that linearly increased effective strain along the radius of the torsion-processed single-crystal copper workpiece may slightly improve its radical hardness and ductility, while grain refinement will enhance its radical hardness and ductility significantly.     

Toward glasses with better indentation cracking resistance

The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass surface under sharp contact loading and the extent to which these cracks develop is intimately related to the way the material attempts to relax the corresponding stress field. Two processes which are known to occur upon indentation are densification and isochoric shear flow. The contributions of both mechanisms were quantitatively assessed for glasses belonging to different chemical systems in previous papers ,  and . In the present study, indentation cracking maps are provided, which offer guidelines to the design of glasses with better surface damage resistance based on their elastic properties and hardness.     

An indentation technique for estimating the energy density as fracture toughness with Berkovich indenter for ductile bulk materials

A technique is proposed to estimate the energy density as fracture toughness for ductile bulk materials with an indentation system equipped with a Berkovich indenter based on the theory of plastic deformation energy transforming into the indentation energy of fracture. With progressive increase of penetration loads, the material damage is exhibited on the effective elastic modulus. A quadratic polynomial relationship between the plastic penetration depth and penetration load, and an approximate linear relationship between logarithmic plastic penetration depth and logarithmic effective elastic modulus are exhibited by indentation investigation with Berkovich indenter. The parameter of damage variable is proposed to determine the critical effective elastic modulus at the fracture point. And the strain energy density factor is calculated according to the equations of penetration load, plastic penetration depth and effective elastic modulus. The fracture toughness of aluminum alloy and stainless steel are evaluated by both indentation tests and K_IC fracture toughness tests. The predicted S_cr values of indentation tests are in good agreement with experimental results of CT tests.     

Some basic contact problems in couple stress elasticity

Indentation tests have long been a standard method for material characterization due to the fact that they provide an easy, inexpensive, non-destructive and objective method of evaluating basic properties from small volumes of materials. As the contact scales in such experiments reduce progressively (micro to nano-scales) the internal material lengths become important and their effect upon the macroscopic response cannot be ignored. In the present study, we derive general solutions for three basic two-dimensional (2D) plane-strain contact problems within the framework of the generalized continuum theory of couple-stress elasticity. This theory introduces characteristic material lengths in order to describe the pertinent scale effects that emerge from the underlying microstructure and has proved to be very effective for modeling microstructured materials. By using this theory, we initially study the problem of the indentation of a deformable elastic half-plane by a flat punch, then by a cylindrical indentor, and finally by a shallow wedge indentor. Our approach is based on singular integral equations which have resulted from a treatment of the mixed boundary value problems via integral transforms and generalized functions. The results show significant departure from the predictions of classical elasticity revealing that it is inadequate to analyze indentation problems in microstructured materials employing only classical contact mechanics.     

Separating viscoelasticity and poroelasticity of gels with diferent length and time scales简

Viscoelasticity and poroelasticity commonly coexist as time-dependent behaviors in polymer gels. Engineering applications often require knowledge of both behaviors separated; however, few methods exist to decouple viscoelastic and poroelastic properties of gels. We propose a method capable of separating viscoelasticity and poroelasticity of gels in various mechanical tests. The viscoelastic char- acteristic time and the poroelastic diffusivity of a gel define an intrinsic material length scale of the gel. The experimen- tal setup gives a sample length scale, over which the solvent migrates in the gel. By setting the sample length to be much larger or smaller than the material length, the viscoelasticity and poroelasticity of the gel will dominate at different time scales in a test. Therefore, the viscoelastic and poroelastic properties of the gel can be probed separately at different time scales of the test. We further validate the method by finite-element models and stress-relaxation experiments.