Indentation Creep Studies of Cross-linked Glassy Polymer Films

1993 ◽  
Vol 308 ◽  
Author(s):  
Kevin M. O'Connor ◽  
Pamela A. Cleveland
Keyword(s):  
Plastic Strain ◽  
Strain Rate ◽  
Indentation Depth ◽  
Glassy Polymer ◽  
Equivalent Stress ◽  
Plastic Strain Rate ◽  
Indentation Creep ◽  
Characteristic Relaxation Time ◽  
Depth Sensing ◽  
Creep Flow

ABSTRACTIndentation creep testing was done on cross-linked glassy polymers based on polystyrene, specifically poly(styrene-co-divinylbenzene) (PS-DVB) and poly(divinylbenzene) (PDVB). The continuous depth-sensing capabilities of the Nano-Indenter II were used to measure the time-dependent response to indentation at constant applied load. The raw creep data in terms of indentation depth vs time showed that PDVB was about 20% more resistant to penetration than PS-DVB. A data analysis program was developed that converted the raw data to stress σ and plastic strain rate ε and generated the plastic flow curve that was observed to follow the power law . The stress exponent a for PS-DVB increased with applied loads between 1 and 27 mN and was generally larger in magnitude than the exponent for PDVB. When compared at equivalent stress and indentation depth, the plastic strain rate for PDVB was observed to be about 100 times slower than for PS-DVB. This was attributed to the higher degree of cross-linking increasing the characteristic relaxation time governing creep flow in these polymers.

PSEUDO-STEADY INDENTATION CREEP

2010 ◽  
Vol 24 (01n02) ◽  
pp. 227-237 ◽  
Author(s):  
HIDENARI TAKAGI ◽  
MING DAO ◽  
MASAMI FUJIWARA
Keyword(s):  
Strain Rate ◽  
Power Law ◽  
Equivalent Stress ◽  
Equivalent Plastic Strain ◽  
Tensile Creep ◽  
Plastic Strain Rate ◽  
Indentation Creep ◽  
Element Simulation ◽  
Representative Points ◽  
Indentation Strain

Theoretical analysis and computational modeling have been performed to carry out constant-indentation strain rate tests. Results show that both the indentation pressure and indentation strain rate become constant after a lapse of loading time. Moreover, the finite element simulation reveals that the contour-line patterns of equivalent stress and equivalent plastic stain rate underneath a conical indenter evolve with geometrical self-similarity corresponding to indenter displacement. This finding confirms that pseudo-steady indentation creep occurs in the region beneath the indenter. We define representative points in the underlying material as those with equivalent stress equal to the indentation pressure divided by a constraint coefficient of 3. During the pseudo-steady indentation creep of a power-law material, the equivalent plastic strain rate at these points is proportional to the indentation strain rate for compatibility. These results point out that a constitutive equation for the tensile creep of a power-law material can be predicted by constant-indentation strain rate tests.

An Experimental Study on the Effect of Prior Plastic Straining on Creep Behavior of 304 Stainless Steel

1993 ◽  
Vol 115 (2) ◽  
pp. 200-203 ◽  
Author(s):  
Z. Xia ◽  
F. Ellyin
Keyword(s):  
Stainless Steel ◽  
Plastic Strain ◽  
Strain Rate ◽  
Creep Behavior ◽  
Test Procedure ◽  
Constant Strain Rate ◽  
304 Stainless Steel ◽  
Creep Model ◽  
Plastic Strain Rate ◽  
Creep Tests

Constant strain-rate plastic straining followed by creep tests were conducted to investigate the effect of prior plastic straining on the subsequent creep behavior of 304 stainless steel at room temperature. The effects of plastic strain and plastic strain-rate were delineated by a specially designed test procedure, and it is found that both factors have a strong influence on the subsequent creep deformation. A creep model combining the two factors is then developed. The predictions of the model are in good agreement with the test results.

An Assessment of the Method Used to Determine Activation Parameters in L12 Alloys

1998 ◽  
Vol 552 ◽  
Author(s):  
B. Matterstock ◽  
G. Saada ◽  
J. Bonneville ◽  
J. L Martin
Keyword(s):  
Mechanical Properties ◽  
Theoretical Analysis ◽  
Plastic Strain ◽  
Strain Rate ◽  
Characteristic Time ◽  
Constant Strain Rate ◽  
Activation Parameters ◽  
Plastic Strain Rate ◽  
Clear Indication ◽  
Load Relaxation

ABSTRACTThe characterisation of dislocation mechanisms in connection with macroscopic mechanical properties are usually performed through transient tests, such as strain-rate jumps, load relaxations or creep experiments. The present paper includes a careful and complete theoretical analysis of the relaxation and the creep kinetics. We experimentally show that the plastic strain-rate is continuous at the transition between constant strain-rate conditions and both load relaxation and creep test. The product of the plastic strain-rate at the onset of the transient test () with the characteristic time (tk) of the transient is found to be independent of , as theoretically expected. This is a clear indication that the assumptions underlying the theoretical analysis are relevant.

Calculation of Material Flow in Orthogonal Cutting by Using Streamline Model

2009 ◽  
Vol 407-408 ◽  
pp. 490-493 ◽  
Author(s):  
Xue Feng Bi ◽  
Gautier List ◽  
Yong Xian Liu
Keyword(s):  
Plastic Strain ◽  
Strain Rate ◽  
Line Shape ◽  
Material Flow ◽  
General Equation ◽  
Flow Line ◽  
Orthogonal Cutting ◽  
Plastic Strain Rate ◽  
Complex Variation ◽  
Streamline Method

The streamline method was used to investigate the plastic strain rate in machining. The streamline function presented in this paper is a general equation with three parameters controlling the complex variation of flow line shape. Velocity and deformation field were obtained by streamline analysis. The validation of this model was conducted by comparing with other experimental results published. It shows that the streamline model presented in the paper can be applied to the evaluation of strain rate in machining.

scholarly journals How to Realize Volume Conservation During Finite Plastic Deformation

2017 ◽  
Vol 84 (11) ◽  
Author(s):  
Heling Wang ◽  
Dong-Jie Jiang ◽  
Li-Yuan Zhang ◽  
Bin Liu
Keyword(s):  
Plastic Deformation ◽  
Plastic Strain ◽  
Strain Rate ◽  
Plastic Strain Rate ◽  
Strain Rate Tensor ◽  
Cauchy Stress ◽  
Volume Conservation ◽  
Tangential Stiffness ◽  
Manufacture Process ◽  
Universal Approach

Volume conservation during plastic deformation is the most important feature and should be realized in elastoplastic theories. However, it is found in this paper that an elastoplastic theory is not volume conserved if it improperly sets an arbitrary plastic strain rate tensor to be deviatoric. We discuss how to rigorously realize volume conservation in finite strain regime, especially when the unloading stress free configuration is not adopted in the elastoplastic theories. An accurate condition of volume conservation is first clarified and used in this paper that the density of a volume element after the applied loads are completely removed should be identical to that of the initial stress free states. For the elastoplastic theories that adopt the unloading stress free configuration (i.e., the intermediate configuration), the accurate condition of volume conservation is satisfied only if specific definitions of the plastic strain rate are used among many other different definitions. For the elastoplastic theories that do not adopt the unloading stress free configuration, it is even more difficult to realize volume conservation as the information of the stress free configuration lacks. To find a universal approach of realizing volume conservation for elastoplastic theories whether or not adopt the unloading stress free configuration, we propose a single assumption that the density of material only depends on the trace of the Cauchy stress by using their objectivities. Two strategies are further discussed to satisfy the accurate condition of volume conservation: directly and slightly revising the tangential stiffness tensor or using a properly chosen stress/strain measure and elastic compliance tensor. They are implemented into existing elastoplastic theories, and the volume conservation is demonstrated by both theoretical proof and numerical examples. The potential application of the proposed theories is a better simulation of manufacture process such as metal forming.

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