Elsevier

Polymer Testing

Volume 29, Issue 3, May 2010, Pages 407-416
Polymer Testing

Test method
Strain determination of polymeric materials using digital image correlation

https://doi.org/10.1016/j.polymertesting.2010.01.005Get rights and content

Abstract

Application of digital image correlation (DIC) to polymeric materials has been proven to be a powerful tool for non-contact strain measurement. In this paper the limits of accuracy of this optical strain measurement system under different environmental conditions were investigated, and the technique was applied to the characterization of polypropylene (PP) and PP composites (PP-C) in the pre- and post-yield regimes. As regards accuracy, a fine speckle pattern and a light intensity just below overexposure provided best results. While vibrations related to the operation of the test machine were of minor influence in reducing the strain measurement accuracy, more pronounced effects were found for the operation of the temperature chamber. In characterizing the transverse strain behavior of PP-C, DIC results exhibited smaller values compared to transverse strains determined utilizing a mechanical clip-on extensometer. The latter effect is attributed to viscoelastic creep indentation of the extensometer pins, which mechanically interact with the specimen via the clip-on spring forces of the extensometer, into the surface. For the DIC system, it could be shown that it allows for the proper strain determination both in the pre- and post-yield regimes, and in terms of longitudinal and transverse strains as well as in terms of global average and local strains.

Introduction

In addition to traditional extensometry, optical strain measurement devices have been increasingly applied in recent years for various materials to characterize their mechanical behavior [1], [2], [3], [4], [5]. In contrast to clip-on or contact extensometers, which are mechanically attached to the test specimen, optical measurement devices operate contactless. Optical techniques are particularly suitable for soft polymeric materials, as local stress concentrations arising from the indentation of the specimen and the weight of an attached mechanical extensometer are entirely avoided.

Neglecting devices using coherent light sources (referred to as laser extensometers), in principle, two optical strain measurement systems can be distinguished: devices with a fixed gauge length measuring the strain between two edges or marks on the test sample, commonly referred to as video extensometers [6], and full-field strain analysis (FFSA) systems referred to as digital image correlation (DIC) [7]. A detailed literature survey of the history of photogrammetry and DIC systems can be found in [8]. In general, DIC is based on the principle of comparing speckle pattern structures on the surface of the deformed and the undeformed sample or between any two deformation states. For this purpose, a virtual grid of subsets of a selected size and shape, consisting of certain pixel gray value distributions, is superimposed on the preexisting or artificially sprayed-on surface pattern and followed during deformation by an optical camera system. In this manner, information on the in-plane local strain distribution is gained without assuming the constitutive behavior of the material a priori. Furthermore, this method is independent of specimen geometry and can also be applied to complex parts and geometries [9], [10], [11], [12], [13] to gain information on the deformation behavior of components in real service.

For traditional extensometry, the limits of resolution and accuracy are well known and can easily be determined. In the case of optical measurement devices the situation is more complicated, since resolution and accuracy depend on the whole measurement system including the objective, the camera and the light system. The number of pixels per mm, which represent an important characteristic as to the resolution of optical systems, strongly depends on the distance between camera and the specimen. Of specific importance, environmental conditions such as vibrations of the test equipment or glass panels between specimen and camera when testing at non-ambient temperatures may have a significant influence on the strain accuracy obtained. Moreover, out-of-plane movements of the specimen affect the apparent size of the specimen and, thus, may also alter the strain result in 2D measurements. Finally, the range of the depth of sharpness is limited and determines the operating distance to the measured object if significant out-of-plane movements occur.

Knowledge of the intrinsic material behavior of polymeric materials from the small strain range up to ultimate failure is of crucial importance for developing adequate material laws for numerical modeling and for a deeper understanding of the microscopic deformation mechanisms. Up to the yield point, polymers deform relatively homogeneously, therefore good agreement between results of mechanical extensometers and DIC systems may be expected. At large strains, in the yield and post-yield regimes, inhomogeneous deformations are observed in many polymers [14], which are not adequately recorded by traditional extensometry. However, for a proper calculation of true strain and true stress in the highly deformed neck, the local longitudinal strain as well as the local transverse strain values are needed.

As DIC provides a full description of longitudinal and transverse strain distributions also in the highly strained neck region of the specimen, true strain and true stress as well as Poisson's ratio and volume strain can be calculated, also providing information on the local strain history at each position of the specimen surface. Thus, DIC allows for determination of modulus and Poisson's ratio in the pre-yield regime, and true stress–strain relationships in the post-yield regime.

The objectives of this paper are (1) to analyze the effects of various parameters related to the DIC test set-up (light intensity, camera shutter time, speckle pattern structure, machine and temperature chamber vibrations) on the accuracy of the overall longitudinal tensile strain result (average value for the standardized gauge length of 50 mm), (2) to characterize the pre- and post-yield regime of polypropylene (PP) under tension, and (3) to compare the results obtained by DIC with those obtained by a conventional mechanical transverse extensometer.

Section snippets

Materials and experimental

A PP homopolymer (PP) and a particulate filled PP (PP-C) commercial grade with the same homopolymer as matrix were selected as model materials for all experiments. The materials were manufactured and delivered by Borealis Polyolefine GmbH Linz (Linz, Austria) as injection molded tensile specimens according to ISO 3167, type B.

The tensile tests were performed on an electro-mechanical universal test machine of the type Instron 5500 (Instron LTD; High Wycombe, UK). Unless otherwise indicated in

Effect of test set-up parameters

Among the test set-up parameters potentially affecting the quality of test results, the influence of light intensity, camera shutter time, speckle pattern structure, machine and temperature chamber vibrations were investigated in more detail. In general, when investigating the effect of one of these parameters, all other parameters were kept constant and set at the default value defined in preliminary experiments. Default values were maximum light just avoiding overexposure, a shutter time of 13

Conclusions

The objective of this paper was to identify the effect of test set-up parameters on the accuracy of a DIC system, to compare results of this system with those obtained by conventional extensometer techniques, and to determine the strain field in an inhomogeneously deforming polymer in the post-yield regime. Tests were performed on a neat PP and a particulate filled PP composite.

To evaluate the strain result of the DIC system in terms of strain accuracy, two quality indicators (mean value of SSD

Acknowledgements

The research work of this paper was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the Kplus-program of the Austrian Ministry of Traffic, Innovation and Technology with contributions by the University of Leoben and Borealis Polyolefine GmbH. The PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria.

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