Papers by Keyword: Dynamic Fracture Toughness

Abstract: The absorbed impact energy KV and the dynamic fracture toughness K_Id of the low-alloyed steel OCHN3MFA were measured in the respective temperature ranges 〈–40, 90〉 and 〈–60, –20〉 °C. The values of impact energy in the range of 〈14.1, 21.3〉 J were obtained using Charpy V-notch samples after subtracting parasitic energies (friction in bearings, etc.). All these values corresponded to an extended transition region since the temperature –40 °C was still higher than the lower shelf impact-energy (shear lips present) and the temperature 90 °C was lower than the upper shelf (only 20 % of ductile morphology). Dynamic fracture toughness was determined using Charpy V‑notch samples with fatigue pre-cracks. The related K_Id-values were obtained in the range 〈55.8, 77.5〉 MPa.m^1/2. They were found to be valid linear-elastic fracture toughness and were somewhat higher than the previously reported static K_Ic-values. The study revealed that OCHN3MFA steel has sufficient resistance to dynamic fracture, particularly at low temperatures.

Abstract: Three point bending and impact tests with sub-sized Charpy specimens were performed on the JRQ reference steel for reactor pressure vessels. Quasi-static and dynamic fracture toughness data were calculated and the fracture behavior in the ductile to brittle transition region was evaluated within the frame of the master curve method (ASTM E1921). Specimens with shallow and deep cracks were studied and the respective influence of crack length and loading rate on the reference transition temperature was determined. The force-time curves of specimens with shallow cracks presented significantly smaller oscillations with respect to the absolute force, making the fracture toughness evaluation more accurate.

Abstract: In this work, Mode I dynamic fracture experiments are conducted on pre-cracked three point bending specimens by using modified split-Hopkinson pressure bar. Two sets of specimens with different initial textures are considered here: one set of the specimens are machined from a hot rolled AZ31B Mg alloy plate with a bigger grain size. The others are treated by four pass of equal channel angular pressing (ECAP) after they are cut from the initial material. They are with the finer grain size. Digital image correlation (DIC) technique is used to determine the strain contours around the crack tip and electron back scatter diffraction (EBSD) is employed to analyze the texture evolution after tests. It is found that the dynamic fracture toughness of finer grain specimen is higher than that of coarse grain specimen. The fracture toughness of both sets of specimens is enhanced by increasing the loading rates. Texture analysis shows the formation of tensile twinning in the ligament ahead of the crack tip in the coarse grain specimen but no sign in fine grain specimen. The brittle features e. g. cleavage planes and twinning lamellas are observed on the fracture surface of coarse grain specimen by scanning electron microscope (SEM). However, the relative ductile features such as micro-voids surrounding by tear ridges present on the fracture surface of fine grain specimen.

Abstract: The SHPB loading three-point bending specimen is a popular way for measuring the dynamic fracture toughness of the material. The wedge shapes on the loading end of the incident bar have necessary effects on the propagation of the stress wave. The dynamic mechanical response and the fracture toughness of the aluminum alloy were measured by SHPB with different wedge shapes, and the influences of wedge shape on determination of dynamic fracture toughness was analyzed in this study. The investigation result can be used to provide reference for the design of a new Hopkinson apparatus for dynamic fracture testing.

Abstract: The researchers and scientists have concluded that material dynamic fracture properties must be considered during the design stage of the modern structure. The dynamic stress intensity factor is very important in understanding of material dynamic behavior. Keeping in view the importance of the materials dynamic stress intensity factor: an efficient and reliable numerical-analytical procedure is developed for calculation of dynamic stress intensity factor. For this, three-dimensional model of a Modified Hopkinson Pressure Bar (MHPB) and a specimen is modeled and analyzed with the ANSYS software. Transient dynamic analysis technique is used for simulation of load-variations as a function of time. As an output of analysis, values of load point displacement and Crack Mouth Opening Displacement (CMOD) are obtained. These values are substituted into two different analytical formulas for calculation of a dynamic stress intensity factor. The results obtained are compared with previous published results, and a good agreement is found.

Abstract: This report aims to illustrate some multifaceted approaches for estimating fracture toughness of high-strength Ni-Cr-Mo low alloy steels in order to search for a suitable methodology for their rapid quality control in industrial production. The chemistry, microstructure, hardness, tensile and impact toughness properties of the selected AISI 4335 grade steels from different batches of commercial productions were assessed by standard methods. Measurement of plane strain fracture toughness (K_Ic), dynamic fracture toughness (K_Id) and fracture toughness using chevron notched bend bar specimens (K_Icv) have been done on the selected steels. The magnitudes of K_Ic depend on the amount of inclusions and tramp elements in different batches of production. The values of K_Id estimated by LEFM approach and K_Icv are in reasonably close agreement with K_Ic values but the latter methodology indicates the potential to be an alternate approach. Comparative assessments of K_Id and K_Icv values with respect to K_Ic values of the steels have been made using concepts based on fracture mechanics and inherent characteristics of test parameters.

Abstract: For design to be more safe, dynamic fracture toughness (DFT) of material needs to be determined. Compared to static loading, dynamic loading procedures are not well established. Calculation of DFT is complicated and costly. Failure process of structures is greatly influenced by dynamic loading. In the past only steel and cast iron were employed for structure design purposes but now many new materials such as (a) composite, (b) alloys (titanium, magnesium), (c) ceramic, (d) concrete, and (e) brittle materials are being used. DFT calculations of materials under dynamic loading have resulted in new theories and experimental techniques. In this paper a critical review of the developments for the calculation of DFT for the materials is presented.

Abstract: Some problems of testing rock dynamic parameters in the SHPB experiment were analyzed and the relevant measures were proposed, which included how to determining the fracture time and the dynamic load etc. Based on the dynamic fracture experiment in the SHPB system with flattened Brazilian disc made of marble, the strain gauge technique to get the accurate fracture time was applied, and the different pulse shaper were adopted to get more precise wave data. The effective test methods of determining rock dynamic fracture toughness were discussed.

Abstract: Based on the experimental and FEM analysis, it was discussed that the stress balance was not a necessary condition for determining rock dynamic fracture toughness accurately using holed-cracked flattened Brazilian disc. The dynamic stress distribution modes of disc sample with time change were analyzed from the early loading stage to continued loading process. It was proved that the fracture time may occur before the balance of stress was achieved. It was not necessary meet the condition of the stress balance using the experiment combined with numerical method. We could get dynamic fracture toughness, according to the dynamic stress intensity factor of the crack tip and fracture time.

Abstract: J-Integral is the main effective and commonly used tool for cracked elastic-plastic material resistance assessment. Determination of fracture toughness under impact loading conditions is related with problems of crack length measurement. Nevertheless, current experimental techniques restrict the specimen’s geometry taking into account span and height ratio, which is equal to four. Evaluation of fracture toughness estimation method which requires only experimental load-line displacement curve of single specimen is research object of dynamic fracture mechanics. This article proposes an approach of impact fracture toughness determination of elastic-plastic steel from single any size specimen test. Load-line displacement data obtained from three-point-bending tests of rectangular cross section specimens with V form single edge notch was used for J-integral calculation. Five series of specimens with different geometry were manufactured from ductile steel and tested.