Automated monitoring of the crack propagation in mode I testing of thermoplastic composites by means of digital image correlation

One of the main challenges of the mode I double cantilever beam (DCB) test is the simultaneous determination of the applied load and displacement with the developing delamination length. The present work addresses this issue by side-view tracking the crack propagation by means of digital image correlation (DIC). Two different reduction methods were developed to determine the crack length from the DIC data. On the one hand, the crack tip position was defined by the high strain concentration in the immediate vicinity of the crack tip, and on the other hand, by crack tip opening displacement (CTOD). The data obtained enabled the calculation of the energy release rate of carbon fibre reinforced thermoplastic specimens with either run-arrest or stable crack extension. For reasons of comparability, top surface analysis (TSA), as recently reported, was also carried out. Following this approach, the crack propagation was tracked applying DIC to the top specimen surface. The methods developed showed a good correlation with both the standardised procedure and TSA. It was shown that DIC can be used as an alternative to the conventional optical measuring tools to follow the crack propagation in the mode I DCB test.     

Direct fracture toughness determination of a ductile epoxy polymer from digital image correlation measurements on a single edge notched bending sample

This work presents a combined experimental and numerical study on the fracture toughness behaviour of a ductile epoxy resin system. Quasi-static fracture tests using single edge notched bending (SENB) specimens were conducted under room temperature conditions. In addition, the digital image correlation technique was employed to experimentally map the full-field displacements and strains around the notch and crack tip, allowing direct calculation of the J-integral fracture toughness. The magnitude of fracture toughness was found to be 1.52 ± 0.03 kJ/m², showing good consistency with the results measured according to the standard analytical formulations. A numerical model of the single edge notch bending specimen was built to compute the local strain field around the crack tip, together with the fracture toughness parameter. Good agreement was confirmed for both the experimental J-integral fracture toughness and the local surface strains around the crack-tip from the digital image correlation based optical technique, compared to the results obtained by numerical simulation. The fracture surfaces of the samples were examined using an optical microscope to analyze the failed surface morphology and the corresponding failure mechanisms.     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

Strain determination of polymeric materials using digital image correlation

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.     

数字图像相关技术在应力应变测量中的发展与最新应用

在现代材料的应力应变测量中,数字图像相关测量法具有非接触、全场性、对实验条件要求低、精度高等优点,是一种有效、实用的应力应变测量方法,被广泛的应用于实验力学及其他学科领域。本文综述了数字图像相关技术在材料应力应变方面的发展;将数字图像相关技术与其它测量技术进行了比较;介绍了数字图像相关技术的系统结构与相关算法、搜索方法;最后列举了数字图像相关技术在材料应力应变中的最新应用和新产品。     

Experimental set-up for determination of the large-strain tensile behaviour of polymers at low temperatures

In this study, we present a method to determine the large-strain tensile behaviour of polymers at low temperatures using a purpose-built temperature chamber made of polycarbonate (PC). This chamber allows for several cameras during testing. In our case, two digital cameras were utilized to monitor the two perpendicular surfaces of the test sample. Subsequently, the pictures were analysed with digital image correlation (DIC) software to determine the strain field on the surface of the specimen. In addition, a thermal camera was used to monitor self-heating during loading. It is demonstrated that the PC chamber does not influence the stress-strain curve as determined by DIC. Applying this set-up, a semi-crystalline cross-linked low-density polyethylene (XLPE) under quasi-static tensile loading has been successfully analysed using DIC at four different temperatures (25 °C, 0 °C, −15 °C, −30 °C). At the lower temperatures, the conventional method of applying a spray-paint speckle failed due to embrittlement and cracking of the spray-paint speckle when the tensile specimen deformed. An alternative method was developed utilising white grease with a black powder added as contrast. The results show a strong increase in both the Young’s modulus and the flow stress for decreasing temperatures within the experimental range. We also observe that although the XLPE material is practically incompressible at room temperature, the volumetric strains reach a value of about 0.1 at the lower temperatures.     

Experimental characterization of dynamic behaviour of gelatin-based material using DIC

In this paper, the dynamic response of gelatin-based soft material under impact loading is investigated. The dynamic tests are principally performed by the classical SHPB (Split Hopkinson Pressure Bars) technique. However, due to the very low mechanical impedance of the specimen compared with the Hopkinson bars, the feeble impact forces are measured by highly sensitive piezoelectric polyvinylidene fluoride (PVDF) pressure sensors instead of SHPB measurement system. The PVDF pressure sensors are placed on the interfaces between the specimen and the bars. During the impact test, the non-equilibrium stress state and inhomogeneous strain fields are developed in the specimen; a digital image correlation (DIC) technique is proposed to identify the inhomogeneous displacement fields using high speed photography. A non-parametric approach based on the DIC technique is developed to deduce the transient stress fields in the longitudinal and transverse directions from the displacement fields measured by DIC. The validation of the calculated stress fields is performed by comparing them with the stress measurements from the PVDF pressure sensor at the bottom end of the specimen. Furthermore, stress-strain response is carried out using this approach throughout the specimen. It is clearly shown that the average highest strain rate varies with position in the specimen. This lead to multiple stress-strain relations determined at different strain rates by only one impact test. The significant strain rate sensitivity is observed at the tested rate range from 81/s to 269/s. Under compression loading, the axial stress state is developed as a simple compression only in the central part of the specimen due to the friction at the interfaces between the specimen and the bars. According to the calculated results based on movement of “long waves”, the region of the simple compression stress state in the central part of the specimen is localized. It is observed that the axial stress is much more important than the transverse stress in the central part and this confirms the assumption of uni-axial compression stress state in the specimen.     

Characterizing the constitutive response and energy absorption of rigid polymeric foams subjected to intermediate-velocity impact

As an optimum energy-absorbing material system, polymeric foams are needed to dissipate the kinetic energy of an impact, while maintaining the impact force transferred to the protected object at a low level. Therefore, it is crucial to accurately characterize the load bearing and energy dissipation performance of foams at high strain rate loading conditions. There are certain challenges faced in the accurate measurement of the deformation response of foams due to their low mechanical impedance. In the present work, a non-parametric method is successfully implemented to enable the accurate assessment of the compressive constitutive response of rigid polymeric foams subjected to impact loading conditions. The method is based on stereovision high speed photography in conjunction with 3D digital image correlation, and allows for accurate evaluation of inertia stresses developed within the specimen during deformation time. Full-field distributions of stress, strain and strain rate are used to extract the local constitutive response of the material at any given location along the specimen axis. In addition, the effective energy absorbed by the material is calculated. Finally, results obtained from the proposed non-parametric analysis are compared with data obtained from conventional test procedures.     

弹性体共混改性聚丙烯的增韧机理

阐述了以聚丙烯（ＰＰ）／高密度聚乙烯（ＨＤＰＥ）为复合基体，苯乙烯－丁二烯－苯乙烯（ＳＢＳ）为增韧剂经三元共混所得的性能优异的一类新材料．从三个层次（形貌结构转变、宏观力学响应和裂尖过程区演化）系统地探讨了其增韧机理．结果表明由形貌结构控制和对早期体膨胀变形抑制可造成裂尖平面应变区的超钝化从而达到增韧．     

Influencing parameters on measurement accuracy in dynamic mechanical analysis of thermoplastic polymers and their composites

Long-term predictions of material properties such as stiffness and creep resistance are important in many engineering applications and require high reliability and accuracy. This is especially true for polymer materials and their composites as their viscoelastic nature results in time-dependent material behaviour and any measurement uncertainties or errors amplify in long-term predictions. To measure this behaviour at smallest loadings, Dynamic Mechanical Analysis (DMA) is frequently declared as an ideal method. However, the measurement accuracy and repeatability of this method is strongly influenced by (i) the testing fixture and corresponding loading mode, (ii) the sample preparation and (iii) the plotting scale to interpret the test results. In this study, relevant experimental parameters were found for DMA and a proper procedure was designed, which was then applied to measure the viscoelastic behaviour of a highly temperature and creep resistant thermoplastic polymer (polyethersulfone) and of a highly graphite filled polypropylene composite. In combination with finite element simulations and in-situ strain measurements by digital image correlation (DIC), the main influences on measurement accuracy of three-point-bending DMA were identified and subsequently used to determine measurement guidelines. Using these guidelines, DMA measurements allow quantitative determination of the viscoelastic response for rigid polymer and composite materials.     

Influences of different dimensional carbon-based nanofillers on fracture and fatigue resistance of natural rubber composites

The dimensions of reinforcing filler is a key factor in influencing the fracture and fatigue of rubbers. Here, the fracture and fatigue resistance of natural rubber (NR) filled with different dimensional carbon-based fillers including zero-dimensional spherical carbon black (CB), one-dimensional fibrous carbon nanotubes (CNTs) and two-dimensional planar graphene oxide (GO) were explored. To obtain equal hardness, a control indicator in the rubber industry, the amounts of CB, CNTs, and GO were 10.7 vol%, 1.2 vol%, and 1.6 vol%, respectively. J-integral and dynamic fatigue tests revealed that NR filled with CB exhibited the best quasi-static fracture resistance and dynamic crack growth resistance. The much higher hysteresis loss of NR filled with CNTs weakened its fatigue resistance. The planar GO played a limited role in preventing crack growth. Furthermore, digital image correlation revealed that NR filled with CB had the highest strain amplification level and area at the crack tip, which dissipated the most local input energy and then improved the fracture and fatigue performance.     

Impact testing of polymeric foam using Hopkinson bars and digital image analysis

The present investigation aims at testing polymeric foam under impact loading using large diameter nylon Hopkinson bars and optical field measurements. Accurate average stress-strain relations can be obtained when soft large diameter polymeric pressure bars and the appropriate data processing are used. However, as there are generally no homogeneous strain and stress fields for polymeric foams, an optical field observation is needed. In contrast to quasi-static tests where the digital image correlation (DIC) measurement is commonly used, technical difficulties still remain for the reliable use of DIC under impact conditions. In this paper, an accurate synchronization method based on the displacement measurement of the end of pressure bars (calculated by a robust DIC algorithm) is preferred to conventional MCDL box time synchronization. Also, the bar end displacement measurement offers a complementary calibration method for the tension/strain conversion coefficient. Strain fields are obtained for tests on foam sample at impact velocities up to 20 m/s. The localized strain fields permit better understanding of the observed stress plateau from SHPB results. The relevance of the present method for establishing mechanical response of polymeric foam is then demonstrated.     

On the strain measurement and stiffness calculation of carbon fibre reinforced composites under quasi-static tensile and tension-tension fatigue loads

The applicability of different strain measurement techniques for carbon/epoxy laminates under quasi-static tensile and tension-tension fatigue loads was studied. Strain gauges, mechanical extensometers, digital image correlation and 2 D camera systems were applied on laminates tested at angles of 0°, 45°, 60°, 90° and ±45°. In addition, displacements recorded by the servo-hydraulic piston were monitored and compared to local strain measurement techniques. Representative examples that illustrate characteristics and limits of each technique in quasi-static and fatigue tests are discussed. Influences of the respective method of strain measurement, the specimen surface, fibre direction and processes in the specimens during tests on the recorded stress-strain behaviour and on the calculated stiffness are presented. Recommendations for accurate strain measurement of anisotropic laminates based on the results are made.     

Plasticity in polymeric honeycombs made by photo-polymerization and nozzle based 3D-printing

Study of the plastic deformation in polymeric honeycombs can pave the way for understanding the deformation localization in more complex cellular structures, which have received progressive attention in the past few years. This study compares the strain localization in deforming honeycombs made by two cost-effective 3D-printing technologies. Hexagonal honeycombs and their unit cell models were 3D-printed by both PolyJet™, using a photo-crosslinkable polymer, and fused deposition modeling (FDM) using a thermoplastic material. The state of the art digital image correlation (DIC) technique was employed as the experimental route in order to calculate the strain field during the deformation of manufactured parts. It was found that DIC is an effective tool to study the localization in 3D-printed honeycomb struts. Moreover, in comparison with FDM, PolyJet technology provides more homogeneous strain distributions in struts. In addition, FDM decreases the maximal strains generated on the side layers of the honeycomb struts. Accordingly, the ligament damage under plastic deformation can be postponed and the energy absorption capability of the product can be improved when PolyJet technology is utilized.     

Biaxial tensile behavior and strength of architectural fabric membranes

The utilization of composite fabric membrane materials for large-span membrane structures has attracted considerable attention in recent decades due to enhanced material properties. Biaxial mechanical properties with respect to real engineering applications are essential and indispensable in comparison with uniaxial ones. This study focuses on true biaxial characteristics of a typical polyvinylidene fluoride (PVDF)-coated polyester membrane material in terms of stress-strain characteristics and breaking criteria.The true stress-strain curves obtained from an experimental study, i.e. seven loading ratios on the basis of symmetry and typical conditions, are investigated with digital image correlation method. The interpolation of these curves in combination of least square method achieves a three-dimensional strain surface as a function of warp and weft strains, which is useful to assess reasonable structural behavior. A new breaking criteria intended for architectural fabric membrane is proposed in analogy to Tsai-Hill, Yeh-Stratton and Norris failure criteria. The basic constants in the criteria are determined using experimental results. A comparative analysis between available uniaxial and biaxial criteria shows that the new criteria can cover all criteria due to the fact that biaxial mechanical properties are larger than uniaxial ones. Furthermore, a similar but glued specimen is employed to compare welded specimens. It is obtained that observations, values and curve tendency are similar, demonstrating the suitability of using new specimens to identify true biaxial properties.     

Load-displacement behavior for double-edge cracked plate of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) has been employed in many engineering applications, mainly due to its special properties such as high electrical resistivity, high melting temperature, chemical inertness, corrosion resistance and very low friction. Although there are many works on PTFE, very few attempts have been made to understand the fracture behavior of this material. For this reason, the load-displacement behavior of double-edge cracked specimens of PTFE was examined and modeled and is reported in this paper. Specimens were tested under monotonic tensile load in quasi-static conditions at constant temperature. Images of the region around the crack were captured with a high-resolution camera and then processed by digital image correlation to obtain the displacement fields. Using these data, values of crack tip opening displacement and crack extension were estimated. To model the behavior of PTFE, a constitutive phenomenological model based on saturation and power law expressions combined with a damage evolution equation is proposed. The predictions are in good agreement with the experimental data.     

A test method for the determination of the stress cracking effects of liquid environments on thermoplastics by measurement of critical strain

The stress cracking effect of liquids on thermoplastic materials can be quantified by measuring the critical strain to cause cracking or crazing. The critical strain is ideally defined as the value of applied strain, for a given material and liquid combination, below which no cracking or crazing occurs. A method is described for the determination of critical strain using a simple straining jig. A strain gauge extensometer attached directly to the specimen is used to accurately monitor the applied strain while visual observation of the sample is used to record the time to crack or craze.Critical strain values measured by this technique are quoted for various alcohols and ketones in contact with polycarbonate and compared with literature values.     

In situ multiscale study of deformation heterogeneities in polylactide-based materials upon drawing: Influence of initial crystallinity and plasticization

This work aims at identifying defects called deformation heterogeneities developing in polylactide (PLA)-based materials upon drawing at room temperature. The influence of the initial crystallinity and of the plasticization methodology (physical blending vs. reactive blending) on the type of defect is also investigated. Defects are characterized in situ by (a) calculating the volume strain from digital image correlation (DIC), (b) measuring their surface density from optical microscopy, and (c) assessing their scattering invariant from small-angle X-ray scattering. Complementary structural analyses are done by microcomputed X-ray tomography and atomic force microscopy. Drawing is accompanied by crazing in the case of low-crystalline PLA, cracking in the case of annealed PLA, no defect in the case of plasticized PLA by physical blending, and shear bands and cracking in the case of plasticized PLA by reactive blending. These observations are discussed based on the initial structural features of the materials. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1452–1468     

Ex situ and in situ characterization of stress corrosion cracking of nickel-base alloys at high temperature

Recent ex situ and in situ characterization of the major factors influencing primary water stress corrosion cracking (PWSCC) and outside diameter stress corrosion cracking (ODSCC) of nickel-base alloys of Alloy 600 and Alloy 690 as the structural materials in pressurized water reactors (PWRs) was introduced to understand cracking mechanisms. In the primary water environment of PWRs, the effect of stress on PWSCC was analyzed using the in situ direct-current potential drop method combined with electrochemical noise measurement. The compositional and microstructural changes around a crack tip including the oxide film formed during crack propagation were evaluated by using ex situ microscopic methods with energy-dispersive spectroscopy (EDS). The equivalent local strain was also evaluated by using the electron backscatter diffraction method as ex situ technique. With the aid of ex situ and in situ characterization, it was proposed that the intergranular precipitation of Cr oxides ahead of the crack tip by O penetration along the grain boundary that make the “oxidized grain boundary” can significantly increase the susceptibility to PWSCC propagation of the alloys. In the secondary water environment of PWRs, the properties of oxide film formed on the surface of the alloys were characterized as a function of the Pb content as impurity by using in situ methods of potentiodynamic polarization measurement and electrochemical impedance spectroscopy and ex situ techniques of EDS and X-ray photoelectron spectroscopy. From these ex situ and in situ characterizations, the degradation model of the passivity of the surface oxide film by Pb incorporation was proposed to understand the ODSCC mechanism.