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Relationships Between Dynamic Fracture Toughness and Charpy Impact Test

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Relationships Between Dynamic Fracture Toughness and Charpy Impact Test Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 Relationships between dynamic fracture toughness and charpy impact test M. Biel Golaska^ & L. Golaski^ ^Foundary Research Institute, Cracow, Poland ^Kielce University of Technology, Kielce, Poland ABSTRACT The dynamic fracture toughness test and Charpy impact test for cast steel in the temperature range from +20°C to -60°C were made. The empirical formulas where correlation between critical value of J integral under dynamic loading conditions and Charpy impact test for samples with different notch geometry at different loading rate were determined. Thus, the dynamic fracture toughness of cast steel from the results of Charpy impact test was estimated. The critical values of dynamic J-integral were used to calculate admissible length of defects in bogie frames and cross bars. INTRODUCTION In the last years a number of failures of rolling stocks components operated at low temperatures were noticed. Fractures started at casting defects in bogie frames and cross bars made of cast steel. In such large castings it is difficult to avoid the defects, therefore it is necessary to estimate the admissible flaw size in these elements. Cracking took place mainly at low temperatures when railway cars Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 496 Structures under Shock and Impact were dynamically charged with stones. In this case in the considered elements the strain rate is 2 significant, achieving the value of 10 1/s, while under ordinary service conditions the strain rate is _2 estimated to be 10 1/s. As mechanical properties depend on temperature and loading rate, to estimate the admissible defect size, the dynamic fracture toughness should be determined at low temparatures which are met during operating of railway cars. In this paper to determine the admissible length of defects, the fracture toughness under dynamic loading was tested. This was done using the method of J-integral employing multi specimen technique. However, the applied method is rather complicated and requires instrumented Charpy pendulum. This equipment is not available in a simple industrial laboratory which carries out acceptance tests of castings on the base of Charpy impact test only. For this reason to estimate the dynamic fracture toughness experimental relationships between the results of Charpy impact test and the critical value of dynamic J integral at low temperature were evaluated. These results made it possible to predict an approximate value of the dynamic fracture toughness and admissible size of defects on the basis of Charpy impact tests only. In the seventies some papers were published [1], where the relationships between fracture toughness Kj and Charpy impact resistance KCV were estalished. However the results referred to two different physical criteria (loading for Ky and energy for Charpy impact test), both values were measured at different loading rates on samples with different geometries. In this paper the relationships between fracture toughness and Charpy impact tests were also determined. However, both tested values J^ and Charpy impact resistance were based on the energy criterion. They were tested at the same loading rates using samples with the same geometries. Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 Structures under Shock and Impact 497 EXPERIMENTAL PROCEDURE AND RESULTS The Purpose of Investigations One of the aims of the investigations was to establish a mathematical relationship between the results of dynamic fracture toughness and the results of Charpy impact resistance tests in temperature range from +20 to -60°C. The second aim of this work was calculation of admissible length of defect basing on the results of fracture toughness. Mater i a 1 The chemical composition of the tested cast steel is listed in table 1. Table 1. The chemical composition of the cast steel in weight per cent c Si Mn P S 0.4T 1.38 0.025 0.025 0.25 The cast steel was normalized at the temperature of 890°C and next annealed at a temperature of 600°C for 2 hours. This grade of cast steel was applied for bogie frames and cross bars shown in Figs 1 and 2. 2550 Zones of stress concentration Fig 1. Cross bar of bogie Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 498 Structures under Shock and Impact Zones of stress concentration Fig 2. Bogie frame The Strength Properties The tensile tests were performed and the proof stress a , ultimate tensile stress UTS, elongation Ap- and reduction of area Z were determined. The results obtained are given in Table 2. Table 2. The Strength Properties of Tested Material Mater ial UTS Z *y *5 [MPa] [MPa] [%] [%] 54T. 0 34. 0 54. 6 L20G 352.0 Charpy Impact Resistance Tests The tests were made on Charpy pendulum. Specimens of the dimensions 10*10*55 mm and different notch shapes were applied. The U-shape, V-shape and Y-shape notches were used. The Y-shape notch in samples were made from samples with V-notch by fatigue precracking The Charpy impact tests were made at temperatures of +20,0,-20,-30,-40,-50 and -60°C at loading rate 5 m/sec. Only one set of Y-notch samples were loaded at 2 m/sec. The Charpy impact energies KCU, KCV and KCY versus temperature are shown in Table 3 and in Fig.3. The denotation KCY^ concerns Charpy impact resistance for samples with Y-shape notch loaded at 2 Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 Structures under Shock and Impact 499 m/s , while the denotation KCY^ concerns this type of samples loaded at 5 m/s. The data inserted in Table 3 concern loading rate 5 m/s. Table 3. The results of Charpy impact resistance tests Temperatu U-notch V-notch Y-notch re [J/crn^] [J/crn^] [J/cm^] [ °C] + 20 109.31 T8.88 51. 66 0 62. 74 45. 36 32.04 -20 52.11 30. 24 18.63 -30 45. 77 29.83 16.02 -40 47. 00 19.20 17. 35 -50 46.39 26.36 16.99 -60 50. 2T 20.02 15. 03 -60 -MD -20 0 Temperature, °C Fig 3. Charpy impact resistance versus temperature Dynamic Fracture Tests The fracture tests were made on samples of 10*10*55mm with V-shape notch and fatigue precrack. The Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 500 Structures under Shock and Impact dynamic fracture tests were made on the stand which included instrumented hammer for impact testing, a storage osciloscope and microcomputer. The hammer consists of an instrumented pendulum equiped with a strain gauge for measure the load, an optical deflection measurement system to measure the bending of the specimens and a stop-block. The details of the stand are given in [2]. The tested material revealed a significant plasticity that is why the application of a single specimen method described in draft standard [3] was impossible. The samples were tested at +20,0,-20,-30,-40 -50 and -60°C. The loading velocity was equal to 2 m/s. The fracture toughness was determined using the stop-block and multiple specimen technique. This method is described in [4]. The critical values of J-integral versus temperature are shown in table 4 and in Fig.4. Table 4. Dynamic fracture toughness and admissible defects lengths Tempera- Length of Length of tura Jld admissible admissible embedded surface defect a^ defect a^ [°C] [kJ/m^] for a=o\ , for a=o\ [mm] y [mm] y + 20 115. 46 11.00 9.00 0 146. OT 13.00 11.00 -20 117. 91 11. 00 9.00 -30 12.97 1. 20 1.00 -40 32.16 3.00 2.00 -50 17. 21 1.50 1.30 -GO 18.76 1. 60 1. 40 Calculation of the Admissible Flaw Length The results of critical J-integral values in different temperatures were using to calculate the admissible flaw length in tested elements . Two types of elliptical defects were assumed; embedded and Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 Structures under Shock and Impact 501 surface with the length to depth ratio which was equal 5. 160 120 : 80 40 0 -60 -40 -20 0 Temperature ,°C Fig 4. Critical values of dynamic J-integral versus temperature For this purpose the following formula has been used: (1) (1-v*) Y n where : Jy^- the critical J-integral value, E - Young's modulus, v Poisson's ratio, a applied stress equal proof stress, Y - coefficient depending on the shape of the element being considered n - safety factor being equal to 10 To estimate the admissible defect length, it was assumed that due to the residual stresses the stress level locally can reach the level of proof stress. Therefore to calculation the admissible defect stress level which equals to proof stress was taken. The results of the admissible flaw length at various temperatures are presented in Table 4 and on the bar chart (Fig. 5). Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509 502 Structures under Shock and Impact 10 o _c ~CT -50 -20 +20 Temperature , ° C DH Embedded flaw d Surface defect Fig 5. Admissible defect lengths at various temperatures DISCUSSION The results of Charpy impact tests for three notch geometries and the results of dynamic fracture tests were used to derive the equations in which the dependences of KCU, KCV, KCYg and KCY on temperature are given.
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