Single Specimen Method Research Articles

Evaluations of J-integral of nuclear graphite combining experimental and numerical methods

Three-point bending tests were performed on centrally notched graphite beams to study the fracture properties of nuclear graphite. Electronic speckle pattern interferometry (ESPI) was used to measure the full-field deformation of graphite beams. In order to understand crack growth in graphite, numerical simulations were performed using a combination of ABAQUS software and the extended finite element method (XFEM). The numerical results were compared with the experimental results and good agreement was observed, confirming the validity of the numerical models. For evaluation of the J-integral, single specimen method is recommended; however, its geometric factor is uncertain and not easily determined. Thus, the load–displacement curves of beams with similar geometry and different notch depths were computed using the numerical simulations and a formula was proposed to obtain the geometric factor. Based on the ESPI results, the J-integral for crack extension was obtained by numerical computations using the measured full-field displacements as boundary conditions. Furthermore, the fracture resistance (R-curves) of the graphite was evaluated experimentally and compared with that calculated by way of the empirical formulas. The results indicated that energy dissipation rose rapidly to approximately 137 N/m before the peak load, and then increased gradually until reaching a plateau of approximately 209 N/m, when the fracture process zone was fully developed. Finally, the fracture parameters, including the critical crack opening displacements and J–integral at the fracture initiation of the graphite beams, were evaluated.

Critical Damage and Microstructure Characteristics of Sandstone in the Three Gorges Reservoir Area by Single Specimen Method Under Different Stress Paths

The critical damage value of rock is an important parameter of stability analysis when engineering rock mass. Based on the previous single test method, this paper, combined with the deformation characteristics of rock residual phase under different stress paths, proposed a single specimen repeated loading method to measure the critical damage value of rock based on different damage variables. The paper also proposed to modify the critical damage value based on residual constitutive energy and to improve the definition of damage variables based on constitutive energy dissipation. According to the microstructure characteristics of the rock sample, the mechanism of the single specimen method to determine the critical value of rock sample damage is revealed. The results show that: 1) Comparing the results of triaxial failure test of rock under different stress paths and single specimen repeated loading test, the residual strength of rock sample is mainly controlled by the confining pressure of loading, which is not an obvious relationship with initial confining pressure and stress path; 2) The number of repeated compressions has little effect on the internal structure, particle flatness, and particle surface smoothness of their specimens, which is reflected at the macroscopic level that the residual strength of sandstone tends to achieve a stable value; and 3) The corrected critical damage value based on residual strain energy is closer to the theoretical value. Test methods in this paper can provide useful references for determination of critical damage value.

Mechanism of strengthening and toughening of a nanostructured styrene-butadiene based block copolymer by oligostyrene-modified montmorillonites

The influence of oligostyrene-modified montmorillonite (OS-MMT) on the morphological, mechanical and fracture mechanical behaviour of a nanostructured styrene-butadiene based block copolymer were studied by using electron microscopy (TEM, SEM), small angle X-ray scattering (SAXS) and uni-axial tensile testing. The crack toughness behaviour of the block copolymer nanocomposites was evaluated with the essential work of fracture approach using a pseudo single specimen method. A high degree of exfoliation of MMT vis-a-vis well distributed tactoids with gallery spacings of ca. 10 nm could be observed. PS-phase specific nano-confinement of silicate tactoids could also be confirmed as indicated by the shift in the glass transition temperature (Tg-PS) of the PS-rich phase to higher temperatures. The Young's modulus and yield stress of the block copolymer nanocomposite (BCP-NC) increased with oligostyrene-MMT content without any compromise in the ductility/toughness. A maximum in the resistance to crack propagation (βwP) at 3 wt.-% of oligostyrene-MMT accompanied by a change in deformation mechanism from homogenous plastic flow (drawing and micro-necking of microdomains) in the BCP-NC with 0-1 wt.-% of OS-MMT to a craze-like deformation (microvoid formation and stretching of polymer fibrils) for the BCP-NC with 3–5 wt.-% of OS-MMT. Above 5 wt.-% OS-MMT content a tough-to-brittle type transition attributed to the μm-sized aggregates in combination with an increased content of rigid PS phase controlling the fracture process as in the BCP-NC with 10 wt.-% nanofillers could be ascertained.Thus, our study demonstrates the possibilities of simultaneous strengthening and toughening of block copolymers by incorporation of modified layered silicates via tuning the dynamics of “intercalation-exfoliation” and by phase-specific nano-confinement of silicates in the BCP-NC.

Determination of J-integral of asphalt concrete based on SC(B) test configuration and image analysis

The J-integral of asphalt concrete is usually measured with multiple specimens used for fracture tests because crack growth is not usually monitored during the fracture test. This research uses an industry digital image camera to monitor the crack growth during the Semi-Circular Bend [SC(B)] fracture test for asphalt concrete. The crack length extension during the SC(B) fracture test is then calculated after testing by an image analysis method. Therefore, a single specimen method based on the SC(B) test configuration and image analysis is proposed to determine the J-integral of the asphalt concrete. The effects of test temperature, Nominal Maximum Aggregate Size (NMAS), binder type and loading rate on J-integral of asphalt concrete are investigated. The same factors’ effects on fracture energy of asphalt concrete are also studied to detect the difference between the J-integral and fracture energy in quantifying fracture resistance of asphalt concrete. The following conclusions are made in this study: (1) J-integral can be measured using the SC(B) test configuration and image analysis method. (2) Test temperature and NMAS are the two significant factors which impact the J-integral of asphalt concrete. (3) The effect of NMAS on J-integral has the opposite trend compared to the effect of NMAS on fracture energy. (4) The polymer modification of binder and the loading rate affect the J-integral, but the effects are not statistically significant.

The effects of initial crack length on fracture characterization of rubbers using the J-Integral approach

The effects of initial crack length on the mode I fracture properties of crosslinked elastomers were investigated. A J-integral approach was implemented using single-specimen test procedures. Quasi-static fracture tests were carried out on filled and unfilled acrylonitrile-butadiene rubbers with various pre-cut (initial) crack lengths, a0, at a fixed rate of extension, using single edge notch rectangular bar specimens. Data from these experiments were analyzed and crack resistance curves (R curves) were obtained. A detailed analysis of the single-specimen method was carried out to establish correlation with the fracture properties derived from the more involved multi-specimen method. The weak dependence of geometry factor on rubber composition and displacement (in the neighborhood of crack initiation displacement) was verified. The crack initiation resistance and tearing modulus of the materials were calculated based on the R curve for each initial crack length. Fracture properties of lightly cracked specimens (a0/W<0.4, where W is the specimen width), including crack initiation resistance, strain at crack initiation, tearing modulus and maximum load and extension before fracture, were found to show significant variations with a0. These variations are characterized and discussed for the filled and unfilled rubber materials.

Comparison of J-integral methods for the characterization of tough polypropylene grades close to the glass transition temperature

The fracture mechanical characterization of very tough Polypropylene at room temperature is of great interest for many technical applications. Precision and validity of obtained values can significantly be influenced by notching and crack length determination, which has thoroughly been shown in literature. Additionally, the proximity to the glass transition temperature can induce further uncertainties.The aim of this study is to compare different J-integral methods of elastic plastic fracture mechanics to determine J-R curves for tested materials. Methods consist of the classical multi-specimen, a single specimen method (load separation) and a statistical approach (Weibull). Specific data reduction procedures based on physical principles were applied to reduce scattering and achieve reliable values. In this way, a reliable fracture mechanics characterization of Polypropylene at room temperature was achieved with a critical verification of the used methods. Finally, the determined fracture toughness value showed a good agreement with the surface texture of inspected fracture surfaces.

Fracture characterization of hyperelastic polyacrylamide hydrogels

This work aims at investigating the fracture behaviour of polyacrylamide hydrogels, which have been chosen for their ideal hyperelastic response and their widespread engineering and biomedical applications. The applicability of the energy separation principle to this class of hydrogels is analysed. This principle allows identifying the fracture initiation without visual monitoring of the specimen and to calibrate the geometry factor required to estimate the fracture toughness. Such innovative approach is compared to established methods: (i) single specimen methods, based on the determination of the energy release rate, for the calculation of fracture toughness; (ii) a multi-specimen methodology for the calibration of the geometry factor. Results show that the energy separation principle is valid for this class of hydrogels, and it can provide a toughness parameter corresponding to a small crack advancement.

Udarna žilavost osnovnog materijala i metala šava spoja zavarenog FSW postupkom od aluminijumske legure EN AW 7049A T652 FSW

The paper analyses the impact of the tool geometry on the friction stir welding (FSW) method on fracture toughness values of the base metal (BM) and weld metal (WM) of a butt welded joints of a high strength aluminium alloy. Values of fracture toughness KJIc were obtained using single-edge notched bend (SENB) specimens with fatigue precrack sampled from the BM and TMAZ (thermomechanically affected zone). Single specimen method was used according to ASTM E 1820 and parameters of elastic-plastic fracture mechanics were determined (CTOD crack tip opening displacement and the J integral). Although the welding tools had different values of cone angle (α = 2.5° - 10°) and the variable length of the pin (h = 5.1 mm - 5.4 mm) at a constant ratio of the number of tool revolutions and the speed of welding, the largest value of fracture toughness has the retreating side of the weld, then the advancing side of the WM, while the BM has the lowest value. This is the effect of recrystallization process in the retreating side of the WM due to combined rotating-straight forward motion of the tool during which the softened material is repeatedly transformed from one side to the other side around the centre of the tool.

Estimation of Fracture Toughness &lt;i&gt;J&lt;/i&gt;&lt;sub&gt;IC&lt;/sub&gt; by Miniature Specimen Hydraulic Bulge Test

The fracture toughness tests and a new miniature specimen technology named hydraulic bulge test (HBT) of 3Cr1Mo1/4V at four service time were carried out. Four J-R resistance curves by single-specimen method with one inch CT specimens were obtained to compute the JIC. Different definitions of equivalent fracture strain according to the section morphologies of HBT testing specimens were compared, and fracture energy of miniature specimens with three different thicknesses (0.4mm, 0.5mm and 0.6mm) were also calculated. Results showed that the typical HBT load-deflection curve can be divided into four sections like SPT curve. Equivalent fracture strain and fracture energy EHB can be chosen as two fracture parameters for the HBT specimen. Ductile fracture toughness JIC can be related approximately linearly to both the equivalent fracture strain and fracture energy EHB.

Fracture toughness of type 316LN stainless steel welded joints

The fracture toughness of two types 316LN welded joints in different orientations at 300K and 623K in air was investigated. The tests were conducted using single specimen method, where J-R curves and fracture toughness values (J0.2) were obtained. Microstructure of the specimens were characterized before and after the test. Results show that 2GT welded joint exhibits higher toughness as compared to 5GT welded joint, and orientation has slight effect on the fracture toughness. Moreover, the fracture toughness drops remarkably with the increase of temperature.

Study on deformation-induced damage evolution for Inconel718 superalloy with the use of innovative single-specimen method

An innovative method for investigation of deformation-induced damage of elastic-plastic materials is proposed. A static tension test, performed on a specimen with a variable cross-section gage part enabled analysis of degradation of the material structure for all stages of permanent deformation. Modified Johnson model has been used to quantify damage. Analysis of the damage of the specimen surface as well as observations of the damage induced inside the gage part of the specimen has been performed using SEM observations. Debonding at the interface between a hard inclusion and a ductile matrix has been found to be responsible for initiation of cracks on the specimen surface as well as inside the gage part of the specimen. Analysis of the subsequent void growth has been performed. Surface cracks are associated with plastic deformation resulting in an increase of the surface roughness. Variations of the specimen surface roughness have been found to be in good correlation with the damage parameter. This correlation enables the use of surface roughness as the relative damage indicator for the investigated material and deformation mode.

Elastic–plastic fracture toughness and rising JR-curve behavior of cortical bone is partially protected from irradiation–sterilization-induced degradation by ribose protectant

ObjectiveThis study tested the hypothesis that pre-treating cortical bone with ribose would protect the rising fracture resistance curve behavior and crack initiation fracture toughness of both bovine and human cortical bone from the degrading effects of γ-irradiation sterilization. Materials and methodsA ribose pre-treatment (1.8 M for bovine, and 1.2 M for human, in PBS at 60 °C for 24 h) was applied to single-edge notched bending fracture specimens prior to sterilization with a 33 kGy dose of γ-irradiation. Fracture resistance curves were generated with a single specimen method using an optical crack length measurement technique. The effect of the treatment on overall fracture resistance behavior, crack initiation fracture toughness, and tearing modulus was compared with non-irradiated and conventionally irradiation sterilized controls. Hydrothermal isometric tension testing was used to examine collagen network connectivity and thermal stability to explore relationships between collagen network quality and fracture resistance. ResultsThe ribose pre-treatment successfully protected the crack growth initiation fracture toughness of bovine and human bone by 32% and 63%, respectively. The rising JR-curve behavior was also partially protected. Furthermore, collagen connectivity and thermal stability followed similar patterns to those displayed by fracture toughness. ConclusionsThis paper demonstrates that the fracture toughness of irradiation-sterilized bone tissue can be partially protected with a ribose pre-treatment. This new approach shows potential for the production and clinical application of sterilized allografts with improved mechanical performance and durability.

The Use of the Normalization Data Reduction Technique for Measuring Upper Shelf Toughness Under Impact Loading Rates

Abstract The multiple-specimen method (low-blow or stop-block tests) is the conventional approach for measuring the upper shelf fracture toughness of steels under impact loading rates, using fatigue precracked Charpy (PCC) specimens tested with an instrumented pendulum machine. The method is fairly simple but requires a relatively large number of specimens. As an alternative, the single-specimen method denominated normalization data reduction (NDR) is based on the analysis of the instrumented force/displacement trace and does not need dedicated instrumentation for the measurement of crack extension during the test. In this study, the NDR technique was applied to low-blow tests performed at different temperatures on two reactor pressure vessel (RPV) steels, 20MnMoNi55 and A553B Cl. 1. Analyses were performed after applying a double fitting approach to the raw test data, which allows selecting a limited set of force/displacement data which are representative of the whole instrumented trace. Results show that the NDR technique provides acceptable accuracy in terms of both ductile crack initiation and resistance to crack propagation (tearing modulus).

Crack Tunnelling and Crack Tip Opening Angle in Drop-Weight Tear Test Specimens

Interrupted drop-weight tear tests (DWTT) of typical low-strength and high-strength pipe steels were performed at quasi-static and impact loading rates, to quantitatively characterize crack tunnelling and crack-tip opening angle (CTOA). Crack front tunnelling was observed in all interrupted DWTT specimens, but the extent of tunnelling varied with the steel grade and the loading rate. CTOA values measured optically at the surface were higher than those measured at a cross-section close to mid-thickness. This is attributed to the effect of crack tunnelling, i.e. the effect of constraint, on CTOA. CTOAs derived from load vs. load-line displacement, using the simplified single-specimen method, are in good agreement with values measured at the mid-thickness.

Measurement and analysis of crack tip opening angle in pipeline steels

The quasi-static tearing tests were conducted to analyze the crack propagation process and fracture mechanism in X70 and X80 pipeline steels. The crack tip opening angle (CTOA) was measured by a single specimen method using modified double-cantilever beam (MDCB) specimen. Some influence factors of CTOA toughness were discussed including specimen thickness, precrack types, loading rate and crack tunneling. Experimental results showed that critical CTOA values increase with increasing specimen thickness, which indicated an opposite trend in comparison with the earlier aerospace aluminum alloy studies. The crack tunneling transformed from “triangular shape” to “parabolic shape”, as specimen thickness increased.

Evaluation of the ASTM and ISO J Initiation Procedures by Applying the Unloading Compliance Technique to Reactor Pressure Vessel Steels

Other than the brittle failure, the ductile behavior of the aged nuclear reactor pressure vessel (RPV) steels is also of interest for the integrity assessment and evaluation of the irradiation response. The fracture toughness of high toughness materials like RPV steels can be characterized by a J-R curve. Since the RPV steel material available for testing purposes, like surveillance specimens, is limited, the single specimen method is used for the J-R curve determination. In this study, J-R curves were measured on Charpy size SE(B) and 1T-C(T) specimens of different RPV steels in the unirradiated and irradiated conditions. It was observed that despite the available sophisticated instrumentation and strict implementation of the recommended test procedures, the J-initiation value for all the different material specimens tested could not be ascertained according to the test standards ASTM E1820 and ISO 12 135. For Charpy size SE(B) specimens, it was found that though valid JIC/J0.2BL values could be obtained in irradiated conditions, in un-irradiated conditions, especially for high toughness RPV steels, it was not possible. The evaluation showed that the aoq fit of the ASTM standard compensates uncertainties in the initial J-Δa value resulting in reliable and more number of qualified test results. But these uncertainties strongly influence the A parameters of the ISO fit and the J0.2BL(B) value. Additionally, in the ISO evaluation the lower offset of the first exclusion line and a higher slope results in lower J0.2BL values compared to the ASTM analysis. Furthermore, for the two specimen geometries the course of J-R curves up to the JQ value was similar even for high toughness materials, but the lower specimen size was disqualified due to the lower prescribed Jlimit. Similarly, the J-R curves for un-irradiated and irradiated condition had a similar course up to the J0.2BL value, even for extremely high irradiation induced embrittlement.

Fracture analysis of rubber‐like materials using global and local approaches: Initiation and propagation direction of a crack

AbstractIn this work, fracture of elastomers is analyzed using global and local approaches, combining experiments, analytical developments, and finite element calculations. The J‐integral is chosen as a global parameter characterizing crack initiation in such materials. Particular attention is paid to single specimen methods for measuring the fracture surface energy. More precisely, models developed in the literature are summarized and, because of the lack of accuracy of these models, an original pertinent expression of J is proposed by analogy with linear elastic fracture mechanics frameworks. However, the J‐integral is not able to predict the kinetic or the propagation direction of a crack. Thus, some local parameters are examined: principal strains, principal stresses, and the strain energy density (SED) factor. Results revealed that all these parameters represent reasonable indicators of the crack propagation direction in elastomers. Moreover, unlike maximal principal stress and SED factor, maximal principal strain seems to govern the crack initiation in this kind of materials. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Static fracture resistance of ultra high molecular weight polyethylene using the single specimen normalization method

Fracture of ultra high molecular weight polyethylene (UHMWPE) components used in total joint replacements is a clinical concern. UHMWPE materials exhibits stable crack growth under static loading, therefore, their fracture resistance is generally characterized using the J–R curve. The multiple specimen method recommended by ASTM for evaluation of the J–R curve for polymers is time and material intensive. In this study, the applicability of a single specimen method based on load normalization to predict J–R curves of UHMWPE materials is evaluated. The normalization method involves determination of a deformation function. In this study, the J–R curves obtained using a power law based deformation function and the LMN curve based deformation function were compared. The results support the use of the power law based deformation function when using the single specimen approach to predict J–R curves for UHMWPE materials.

Preparation of a Single Prostate Needle Biopsy Specimen for Histological Diagnosis and RNA Analysis

When a prostate needle biopsy specimen is used for molecular study, a second specimen from the same area as the first one has previously been required for pathological diagnosis. However, we developed a method for obtaining substances such as RNA and we also made pathological diagnosis possible from a single needle biopsy specimen, improving reliability. Prostate needle biopsy specimens from 118 patients were frozen in optimal cutting temperature compound. Tissue from the upper part of the frozen specimen was sectioned longitudinally for histopathological examination. The remainder of the specimen was placed in TRIzol reagent to extract total RNA for molecular biological investigation. Pathological diagnosis and total RNA extraction (1.6 to 32.7 microg) could be obtained from 1 needle biopsy specimen. When 2 specimens were obtained from the same area of the prostate, pathological diagnoses were discordant in 19% of the cases. We established a method for histopathological diagnosis in the prostate needle biopsy specimen used for molecular investigation. This single specimen method may facilitate molecular research in prostate cancer.

The change of fracture toughness of martensitic steels after irradiation in SINQ target-3

Small bend specimens of martensitic steels T91, F82H and Optimax were irradiated in SINQ target-3 up to about 9 dpa in a temperature range of 100–250 °C. The single specimen method was used to determine the J-integral values. The specimens were tested in a temperature range from room temperature to 250 °C, mostly at irradiation temperatures. The results showed that the fracture toughness of the martensitic steels decreased with increasing irradiation dose. Specimens tested at the irradiation temperatures and 250 °C indicated that the fracture toughness of all three martensitic steels remained above 100 MPa m 1/2. One T91 specimen irradiated to 4.3 dpa tested at room temperature was totally brittle and broke in the elastic deformation region. The fracture toughness value was only about 28 MPa m 1/2, within the lower shelf of the fracture toughness. There was no obvious difference in the fracture toughness of the steels after irradiation.