scholarly journals Viscoelastic effects during unloading in depth-sensing indentation

2002 ◽  
Vol 17 (10) ◽  
pp. 2604-2610 ◽  
Author(s):  
A. H. W. Ngan ◽  
B. Tang
Keyword(s):  
Linear Viscoelasticity ◽  
Room Temperature ◽  
Peak Load ◽  
Displacement Curve ◽  
Linear Variation ◽  
Depth Sensing ◽  
Viscosity Parameter ◽  
Viscoelastic Effects ◽  
Unloading Rate ◽  
Rate Plot

With polypropylene as a prototype viscoelastic material at room temperature, it was found that a “nose” may appear in the unloading segment of the load–displacement curve during nanoindentation when the holding time at peak load is short and/or the unloading rate is small, and when the peak load is high enough. The load at which the nose appears was also found to decrease linearly with decreasing unloading rate. A linear viscoelasticity analysis was performed to interpret this effect. The analysis predicts a linear variation between the nose load and the unloading rate, and the slope of such a linear variation is also shown to be proportional to the viscosity parameter of the material. Thus, by measuring the slope of the nose-load versus unloading rate plot at a given temperature, the viscosity parameter of the specimen can be found. This is a new way of measuring the viscosity parameter of a material in addition to the existing method of force modulation and noting the frequency response of the displacement.

Indentation responses of time-dependent films on stiff substrates

2004 ◽  
Vol 19 (8) ◽  
pp. 2487-2497 ◽  
Author(s):  
Michelle L. Oyen ◽  
Robert F. Cook ◽  
John A. Emerson ◽  
Neville R. Moody
Keyword(s):  
Peak Load ◽  
Elastic Response ◽  
Depth Sensing ◽  
Indentation Tests ◽  
Unloading Rate ◽  
Loading And Unloading ◽  
Load Displacement ◽  
Film Substrate ◽  
Single Set ◽  
Substrate Influence

A viscous-elastic-plastic indentation model was extended to a thin-film system, including the effect of stiffening due to a substrate of greater modulus. The system model includes a total of five material parameters: three for the film response (modulus, hardness, and time constant), one for the substrate response (modulus), and one representing the length-scale associated with the film-substrate interface. The substrate influence is incorporated into the elastic response of the film through a depth-weighted elastic modulus (based on a series sum of film and substrate contributions). Constant loading- and unloading-rate depth-sensing indentation tests were performed on polymer films on glass or metal substrates. Evidence of substrate influence was examined by normalization of the load-displacement traces. Comparisons were made between the model and experiments for indentation tests at different peak load levels and with varying degrees of substrate influence. A single set of five parameters was sufficient to characterize and predict the experimental load-displacement data over a large range of peak load levels and corresponding degrees of substrate influence.

Critical concentration of Mg addition for plastic instabilities in Al–Mg alloys

2000 ◽  
Vol 15 (5) ◽  
pp. 1037-1040 ◽  
Author(s):  
N. Q. Chinh ◽  
F. Csikor ◽  
Zs. Kovács ◽  
J. Lendvai
Keyword(s):  
Room Temperature ◽  
Loading Rate ◽  
Critical Concentration ◽  
Solute Concentration ◽  
Mg Alloys ◽  
Depth Sensing ◽  
Solute Content ◽  
Mg Addition ◽  
Constant Loading Rate ◽  
Al Mg Alloys

Plastic instabilities were investigated by the depth-sensing microhardness test in binary high-purity Al–Mg alloys with different Mg contents. During the tests the applied load was increased from 0 to 2000 mN at constant loading rate. The instabilities appeared as characteristic steps in the load–depth curves during indentation. It was shown that the occurrence and development of the plastic instabilities depend strongly on the solute content. Furthermore, the plastic instabilities occurred only when the solute concentration was larger than a critical value, C0. From room-temperature tests on Al–Mg alloys, C0 was found to be 0.86 wt% Mg. The critical concentration, which is necessary to get plastic instabilities, was also interpreted theoretically.

On the Indentation Modulus of Sintered Materials

2013 ◽  
Vol 586 ◽  
pp. 190-193
Author(s):  
Miriam Kupková ◽  
Martin Kupka
Keyword(s):  
Experimental Data ◽  
Indentation Size Effect ◽  
Displacement Curve ◽  
Indentation Modulus ◽  
Indentation Size ◽  
Depth Sensing ◽  
Nano Indentation ◽  
Individual Grains ◽  
Theoretical Results ◽  
Sintered Materials

When the depth-sensing (nano)indentation is applied to sintered samples, measured properties, which are expected to represent the material of an individual grain, seem to depend on the overall porosity of the macroscopic sample. To understand such a result, it is assumed that while the nanoindenter penetrates into the surface grain and probes the properties of its material, the grain itself serves as another, larger indenter indenting the rest of sample and probing the properties that represent the bulk of material rather than individual grains. Load vs. displacement curve reflects the synergetic response of these two “indenters” and so it contains information about the sample’s mechanical properties at both microscopic and macroscopic scales. Obtained theoretical results agree qualitatively with the experimental data (the dependence of the indentation modulus on the porosity of sample; the indentation size effect).

The Series K Criterion of Unstable Fracture of Concrete and its Application

2010 ◽  
Vol 152-153 ◽  
pp. 1878-1883
Author(s):  
Ai Min Deng ◽  
Dao Yuan Xu
Keyword(s):  
Crack Length ◽  
Peak Load ◽  
Unstable Fracture ◽  
Displacement Curve ◽  
Three Gorges Project ◽  
Concrete Fracture ◽  
Nonlinear Characteristics ◽  
The Three Gorges ◽  
Fracture Tests ◽  
The Three Gorges Project

The knowledge from extensive investigations showed that the non-linearity of the load and loading point displacement curve is mainly associated with crack propagation. Based on the linear asymptotic superposition assumption, the critical effective crack length is evaluated by analyzing the nonlinear characteristics of the load and loading point displacement curves of a series of the fracture tests. Furthermore, the unstable fracture toughness is determined by inserting the peak load and the critical effective crack length into a formula developed from LEFM. Based on the calculated results, the series K criterion of unstable fracture of concrete fracture is established. At last, an engineering example of cracking analysis of the upstream lock-head foundation of the ship-lift of the Three Gorges Project is given.

scholarly journals Nano-Indentation Response of Ultrahigh Molecular Weight Polyethylene (UHMWPE): A Detailed Analysis

Polymers ◽  
2020 ◽  
Vol 12 (4) ◽  
pp. 795
Author(s):  
Tanveer Iqbal ◽  
S. S. Camargo ◽  
Saima Yasin ◽  
Ujala Farooq ◽  
Ahmad Shakeel
Keyword(s):  
Molecular Weight ◽  
Strain Rate ◽  
Peak Load ◽  
Ultrahigh Molecular Weight Polyethylene ◽  
Holding Time ◽  
Creep Model ◽  
Depth Sensing ◽  
Nano Indentation ◽  
Wide Range ◽  
Ultrahigh Molecular Weight

Nano-indentation, a depth sensing technique, is a useful and exciting tool to investigate the surface mechanical properties of a wide range of materials, particularly polymers. Knowledge of the influence of experimental conditions employed during nano-indentation on the resultant nano-mechanical response is very important for the successful design of engineering components with appropriate surface properties. In this work, nano-indentation experiments were carried out by selecting various values of frequency, amplitude, contact depth, strain rate, holding time, and peak load. The results showed a significant effect of amplitude, frequency, and strain rate on the hardness and modulus of the considered polymer, ultrahigh molecular weight polyethylene (UHMWPE). Load-displacement curves showed a shift towards the lower indentation depths along with an increase in peak load by increasing the indentation amplitude or strain rate. The results also revealed the strong dependence of hardness and modulus on the holding time. The experimental data of creep depth as a function of holding time was successfully fitted with a logarithmic creep model (R2 ≥ 0.98). In order to remove the creeping effect and the nose problem, recommended holding times were proposed for the investigated polymer as a function of different applied loads.

Spherical Indentation Creep Following Ramp Loading

2004 ◽  
Vol 841 ◽  
Author(s):  
Michelle L. Oyen
Keyword(s):  
Contact Stiffness ◽  
Peak Load ◽  
Polymeric Materials ◽  
Spherical Indenter ◽  
Material Behavior ◽  
Spherical Indentation ◽  
Indentation Creep ◽  
Indentation Testing ◽  
Depth Sensing ◽  
Linear Viscoelastic Material

ABSTRACTDepth-sensing indentation testing is a common way to characterize the mechanical behavior of stiff, time-independent materials but presents both experimental and analytical challenges for compliant, time-dependent materials. Many of these experimental challenges can be overcome by using a spherical indenter tip with a radius substantially larger than the indentation depth, thus restricting deformation to viscoelastic (and not plastic) modes in glassy polymers and permitting large loads and contact stiffness to be generated in compliant elastomers. Elastic-viscoelastic correspondence was used to generate spherical indenter solutions for a number of indentation testing protocols including creep following loading at a constant rate and a multiple ramp-and-hold protocol to measure creep response at several loads (and depths) within the same test. The ramp-creep solution was recast as a modification to a step-load creep solution with a finite loading rate correction factor that is a dimensionless function of the ratio of experimental ramp time to the material time constant. Creep tests were performed with different loading rates and different peak load levels on glassy and rubbery polymeric materials. Experimental data are fit to the spherical indentation solutions to obtain elastic modulus and time-constants, and good agreement is found between the results and known modulus values. Emphasis is given to the use of multiple experiments (or multiple levels within a single experiment) to test the a priori assumption of linear viscoelastic material behavior used in the modeling.

A Simulation Strategy on the Quasi-Static Axial Crushing Process of Composite Tubes Based on LS-DYNA

2014 ◽  
Vol 1082 ◽  
pp. 351-358
Author(s):  
Zhi Jie Li ◽  
Guo Qing Yuan ◽  
David Hui ◽  
Zhe Min Jia
Keyword(s):  
Peak Load ◽  
Displacement Curve ◽  
Structural Failure ◽  
Composite Tubes ◽  
Material Density ◽  
Real Material ◽  
Simulation Strategy ◽  
Simulation Results ◽  
The Stability ◽  
The Given

Based on the code LS-DYNA, the simulation strategy on the quasi-static crushing process of composite tubes is studied by conducting two series of comparison simulations: one is at different virtual loading speeds with the real material density and the other is with different virtual material densities at the given loading speed. The simulation results are summarized as three types, namely mode-varying type (MVT), unstable type (UT) and stable type (ST), depending on the change of the structural failure mode and the occurrence of the load-fluctuating stage in the load-displacement curve. Analysis has shown that the steady load possesses the stability, the peak load has the convergence zone, and the thresholds of energy ratio can be adopted to evaluate the simulation results. In the end, the simulation strategy is proposed to makes it possible to acquire the simulation results with a higher query precision at a lower computing cost.

Effect of preload and shim types on the mechanical properties of composite-aluminium bolted joints

2021 ◽  
pp. 095440622110071
Author(s):  
Xuande Yue ◽  
Luling An ◽  
Zengtao Chen ◽  
Yuebo Cai ◽  
Chufan Wang
Keyword(s):  
Mechanical Properties ◽  
Composite Laminates ◽  
Damage Model ◽  
Peak Load ◽  
Lap Joints ◽  
Surface Strain ◽  
Displacement Curve ◽  
Progressive Damage ◽  
Single Lap Joints ◽  
Tensile Stiffness

The influence of both preload and the presence of shim types on the mechanical properties of composite-aluminium single-bolt, single-lap joints were studied in this paper. The load-displacement curve and surface strain field of joints in different shim types and preloads were obtained through tensile experiments. A progressive damage model was established using the UMAT subroutine in ABAQUS. A hybrid failure criterion and a linear continuous degradation model were used to describe the progressive damage of composite laminates. The results show that for joints with no shim and for those with various types of shims, the tensile stiffness, peak load and initial damage load could be reduced when the preload is insufficient or too large. Compared with joints with no shims or with peelable fibreglass shims, joints with liquid shims required a larger preload to achieve the best mechanical properties. As the proportion of peelable fibreglass shim increased, the tensile stiffness and peak load continued to increase in joints with a mixed shim of liquid and peelable fiberglass shim. Shims can serve as tension bearings, but have little effect on the initiation and development of bearing failure.

scholarly journals Accurate measurement of tip–sample contact size during nanoindentation of viscoelastic materials

2003 ◽  
Vol 18 (5) ◽  
pp. 1141-1148 ◽  
Author(s):  
B. Tang ◽  
A. H. W. Ngan
Keyword(s):  
Contact Area ◽  
Contact Stiffness ◽  
Amorphous Selenium ◽  
Area Measurement ◽  
Displacement Curve ◽  
Viscoelastic Materials ◽  
Depth Sensing ◽  
Depth Measurement ◽  
Reduced Modulus ◽  
Satisfactory Prediction

Polypropylene (PP) and amorphous selenium (a-Se) were used as prototype materials at room temperature to explore the problems that may exist in the accurate measurement of the reduced modulus of viscoelastic materials using depth-sensing nanoindentation. As has been reported previously by others, we observed that a “nose” in the load-displacement curve may occur during unloading, indicating significant creep effects at the onset of unloading. To accurately measure the elastic modulus in viscoelastic materials like PP or a-Se, both the contact stiffness and the contact area at the onset of unloading must be determined accurately. The issue of removing the influence of creep on the measurement of the contact stiffness using the Oliver-Pharr method has been addressed in a previous paper by Feng and Ngan. In this work, the effect of creep on contact-depth measurement is considered. Removal of creep effects in both contact stiffness and contact-area measurement leads to satisfactory prediction of the reduced moduli in PP and a-Se.

scholarly journals Axial Compressive Performance of a Composite Concrete-Filled GFRP Tube Square Column

2021 ◽  
Vol 11 (15) ◽  
pp. 6757
Author(s):  
Jiancheng Lu ◽  
Yujun Qi ◽  
Yifei Li ◽  
Xuxu Wang
Keyword(s):  
Bearing Capacity ◽  
Load Capacity ◽  
Peak Load ◽  
Strain Curve ◽  
Displacement Curve ◽  
Composite Column ◽  
Composite Columns ◽  
Compressive Performance ◽  
New Type ◽  
The Impact

A composite concrete-filled glass fiber reinforced polymer (GFRP) tube square column is a new type of composite column, where GFRP is externally wrapped over several GFRP square tubes to form a multicavity GFRP tube, and then concrete is poured inside. External GFRP wrapping methods can be divided into two types: entirely wrapped and strip-type wrapped methods. The former is superior to the latter in terms of performance under stress. However, difficulties are introduced in the construction process of the former, and substantial materials are required to wrap the entire structure. To examine the axial compressive performance for this new type of composite column and the impact of the wrapping method, we designed and fabricated one type of entirely wrapped composite column and two types of strip-type wrapped composite columns with clear spacings of 85 mm and 40 mm, respectively, and performed static axial compression tests. Through tests and numerical simulations, we obtained the failure mode, load–displacement curve, and load–strain curve of the specimen, and analyzed the impact of the externally wrapped GFRP on the mechanical behavior of the composite column. The results show that the composite column reached the peak load before the fracture of the GFRP tube fiber occurred, and the bearing capacity declined sharply to approximately 75% of the peak load after the fiber fractured, then entered a platform section, thereby displaying ductile failure. As the wrapped layers of GFRP strips increased, the load capacity of the specimen exhibited a linear growth tendency. Compared with the performance of the entirely wrapped method, the load capacity of the specimens in the W5040 group declined 9.8% on average, and the peak efficiency of the GFRP strips increased by 50%, thereby indicating that the use of appropriate GFRP layers and strip distance intervals can ensure the appropriate bearing capacity of composite columns and full utilization of GFRP material properties.

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