Partial Unloading Method Research Articles

Stress-strain relationship of translucent nanocrystalline Gadolinium Zirconate ceramic with grain size below 10 nm using nanoindentation

Translucent nanocrystalline Gadolinium Zirconate (Gd2Zr2O7) ceramic with grain size below 10 nm was successfully fabricated using high pressure sintering technology at 5 GPa and low temperature of 550 °C. The bilinear compressive stress-strain relationship of this sample was firstly extracted from nanoindentation procedure based on the partial unloading method. It indicated that the plastic deformation did not originate from dislocation sliding and was controlled by the grain boundary sliding. Moreover, the compressive yield strength, σy=7.8 GPa of the translucent nanocrystalline Gd2Zr2O7 ceramic was also determined by the intersection of the theoretical elastic line and the indentation compressive stress-strain line.

On the identification of an effective cross section for a cruciform specimen

AbstractBiaxial tensile testing of sheet metals is becoming increasingly popular for sheet metal forming. Determining equivalent stresses in biaxial tensile specimens is more complicated than in conventional uniaxial tensile specimens. In the present study, we compare four different approaches to calculate effective stresses during biaxial tensile loading of a cruciform specimen: (a) partial unloading method, where stresses are determined based on force–strain curves; (b) identification with uniaxial tensile testing; (c) an analysis of equivalent biaxial tests; and (d) numerical simulations. Considering experimental results for an AA1050 aluminium alloy and for a low‐carbon steel DC06, we show that, for the cruciform sample studied here, two methods do not yield physically reasonable results: The uniaxial approach does not properly take into account the effect of transverse loading, and the equivalent biaxial approach exhibits uncertainties in strain measurement data. The most comprehensible approach is the numerical method, because it also yields detailed information about the local stress and strain states. The numerical results are in excellent agreement with the partial unloading method in terms of the initial flow stress and of effective stress–strain curves for strains up to 0.02, with both methods predicting a similar effective cross section of 18.0 mm2 for the considered specimen.

Interfacial electro-mechanical behaviour at rough surfaces

In a range of energy systems, interfacial characteristics at the finest length scales strongly impact overall system performance, including cycle life, electrical power loss, and storage capacity. In this letter, we experimentally investigate the influence of surface topology on interfacial electro-mechanical properties, including contact stiffness and electrical conductance at rough surfaces under varying compressive stresses. We consider different rough surfaces modified through polishing and/or sand blasting. The measured normal contact stiffness, obtained through nanoindentation employing a partial unloading method, is shown to exhibit power law scaling with normal pressure, with the exponent of this relationship closely correlated to the fractal dimension of the surfaces. The electrical contact resistance at interfaces, measured using a controlled current method, revealed that the measured resistance is affected by testing current, mechanical loading, and surface topology. At a constant applied current, the electrical resistance as a function of applied normal stress is found to follow a power law within a certain range, the exponent of which is closely linked to surface topology. The correlation between stress-dependent electrical contact and normal contact stiffness is discussed based on simple scaling arguments. This study provides a first-order investigation connecting interfacial mechanical and electrical behaviour, applicable to studies of multiple components in energy systems.

The Role of Surface Structure in Normal Contact Stiffness

The effects of roughness and fractality on the normal contact stiffness of rough surfaces were investigated by considering samples of isotropically roughened aluminium. Surface features of samples were altered by polishing and by five surface mechanical treatments using different sized particles. Surface topology was characterised by interferometry-based profilometry and electron microscopy. Subsequently, the normal contact stiffness was evaluated through flat-tipped diamond nanoindentation tests employing the partial unloading method to isolate elastic deformation. Three indenter tips of various sizes were utilised in order to gain results across a wide range of stress levels. We focus on establishing relationships between interfacial stiffness and roughness descriptors, combined with the effects of the fractal dimension of surfaces over various length scales. The experimental results show that the observed contact stiffness is a power-law function of the normal force with the exponent of this relationship closely correlated to surfaces’ values of fractal dimension, yielding corresponding correlation coefficients above 90 %. A relatively weak correlation coefficient of 60 % was found between the exponent and surfaces’ RMS roughness values. The RMS roughness mainly contributes to the magnitude of the contact stiffness, when surfaces have similar fractal structures at a given loading, with a correlation coefficient of −95 %. These findings from this work can be served as the experimental basis for modelling contact stiffness on various rough surfaces.

Determination of reversible and irreversible contributions to the polarization and strain response of soft PZT using the partial unloading method

Experimental work is aimed at systematically investigating the non-linear ferroelectric and ferroelastic behavior of a commercially available soft lead zirconate titanate (PZT) material. The fast partial unloading method is used to measure the material properties of unpoled soft PZT under pure electric field and of initially pre-poled soft PZT under compressive stress loading. In the first experiment using unpoled PZT, the evolution of piezoelectric constants and dielectric permittivity is determined as a function of electric field. It is found that the piezoelectric constants and dielectric permittivity depend on the electric field history. The results are used to separate the reversible strain and polarization from the irreversible ones caused by domain switching. In the second experiment using initially pre-poled PZT, it is found that the strain response is significantly dependent on the stress loading rate. The elastic moduli and piezoelectric coefficients are evaluated with respect to the compressive stress history. The measured longitudinal and transverse irreversible strains change significantly during both loading and unloading processes. An attempt is made to discuss the use of irreversible strain and irreversible polarization as internal variables for constitutive modeling. This investigation provides valuable information for modeling to predict the performance and for improving the reliability of piezoelectric devices.

Nanomechanical properties and mineral concentration in articular calcified cartilage and subchondral bone.

We studied articular calcified cartilage (ACC) and the immediately subchondral bone (SCB) in normal and osteoarthritic human femoral heads. Thick slices of human normal reference post mortem (PM) and osteoarthritic (OA) femoral heads (age 55-89 years) were embedded in PMMA, micromilled, carbon coated and studied using quantitative backscattered electron (qBSE) imaging to determine variations in degree of mineralization. With exact microanatomical location, nanoindentation was performed on the same block faces in representative superior (more highly loaded) and medial regions of the joint surface. Using a partial unloading method, elastic modulus as a function of indenter penetration depth was determined using a spherical tipped diamond indenter. A pointed indenter was used to determine the tissue hardness in selected locations. The relationship between mineralization and indentation modulus was more distinct in ACC than in SCB, the latter having a higher matrix concentration with variable collagen orientation. In OA, the bulk of the measurements were coincident with those in the PM samples, although there was a greater range in the levels of mineralization and modulus in ACC. In OA, extremely hypermineralized ACC was found in ACC proper, especially in superior regions, and translocated into SCB and hyaline cartilage. The very highly mineralized cartilage fragments may function as a hard grinding abrasive, accelerating wear rates whether attached to or fragmented from the eburnated surfaces of OA ACC. Highly mineralized regions would also alter loading patterns and thereby contribute to further destruction of the joint tissues.

Increase of Delayed Failure Strength by Partial Unloading Method

The effect of two types of prestressing, i.e. the partial unloading and the perfect unloading, on the delayed failure strength of the specimens with various thickness and notch root radius has been investigated on JIS SNCM8 steel quenched and tempered at 200°C. Here the partial unloading means that the applied stress is raised to a maximum stress, σmax, then decreased to a testing stress σ, and the perfect unloading means that the stress is once decreased to zero from σmax and is again increased to a testing stress σ(<σmax).Both prestress methods, especially the partial unloading method, can largely increase the 100hr delayed failure strength regardless of specimen thickness and notch root radius if the unloading stress σa(=σmax-σ) is chosen appropriately. In order to raise the 100hr delayed failure strength by both prestress methods, it is necessary to increase the unloading stress to a critical value, (σa)c, which increases with increase of specimen thickness and decrease of notch root radius. The reason why the delayed failure strength is increased by both prestress methods can be explained by the decrease of surface stress at notch root, which will suppress the corrosion reaction and prevent the invasion of hydrogen atoms into the material.

Enhancement of delayed failure resistance by partial unloading method

The effects of two types of pre-stressing, i.e. partial unloading (0 →K max → K) and perfect unloading (0 → K max → 0 → K) on the delayed failure strength were investigated using pre-cracked specimens of JIS SNCM8 steel quenched and tempered, where K max is the maximum stress intensity factor at pre-stressing, and K is the stress intensity factor under which delayed failure test is carried out. Both pre-stress methods can markedly increase the delayed failure strength or the lower limit stress intensity factor K ISCC . The partial unloading method is superior to the perfect unloading method in each tempering condition (200 or 400°C) and in each environment (distilled water or 3% NaCl water). The reason why K ISCC is increased by each pre-stressing can be explained by the decrease of surface stress at crack tip, which will suppress the corrosion reaction and prevent the invasion of hydrogen atoms into the material.

部分除荷法による遅れ破壊き裂発生強さの向上

The effect of two types of prestressing, i.e. the partial unloading and the perfect unloading, on the delayed failure strength of the specimens with various thickness and notch root radius has been investigated on JIS SNCM8 steel quenched and tempered at 200°C. Here the partial unloading means that the applied stress is raised to a maximum stress, σmax, then decreased to a testing stress σ, and the perfect unloading means that the stress is once decreased to zero from σmax and is again increased to a testing stress σ(<σmax).Both prestress methods, especially the partial unloading method, can largely increase the 100hr delayed failure strength regardless of specimen thickness and notch root radius if the unloading stress σa(=σmax-σ) is chosen appropriately. In order to raise the 100hr delayed failure strength by both prestress methods, it is necessary to increase the unloading stress to a critical value, (σa)c, which increases with increase of specimen thickness and decrease of notch root radius. The reason why the delayed failure strength is increased by both prestress methods can be explained by the decrease of surface stress at notch root, which will suppress the corrosion reaction and prevent the invasion of hydrogen atoms into the material.