Relationships Between Dynamic Fracture Toughness and Charpy Impact Test

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

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.

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

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

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

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

Structures under Shock and Impact 501 surface with the length to depth ratio which was equal 5.

160

120 : 80

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).

502 Structures under Shock and Impact

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. The diagrams of Charpy impact tests and J- integral versus temperature for all the applied notch geometries were described with the help of the same exponential equations:

y = A exp BT (2)

These equations are as follows:

KCU = 5.54 e 0. 0093T (3)

KCV = 0.55 e 0.0164T (4)

°'015T KCYg= 0.52 e (5)

KCY%= 0.30 e 0-01TT (6)

(T)

Structures under Shock and Impact 503

where: T- temperature in Kelvin degrees

By dividing equation (T) by equations from (3) to

(6), the relation between Jj^ and KG are obtained. For example:

[kN/m] (8)

= 0,012 eO'OZTKCY [kN/m] (9)

Thus on the basis of Charpy impact test results the values of critical J-integral -J^ can be calculated. As it can be seen in Fig. 3, the results of Charpy impact energy test decrease significantly with the notch sharpening. The Charpy energy values for U-shape notch samples are by 40-50% higher than the values for V-shape notch samples, while the energy for precracked Y-shape samples is by 40-50% lower in comparison with the Charpy energy values for

V-shape notch samples. It is interesting to note that the increase in impact velocity from 2 to 5 m/s does not change significantly the impact energy at low temperatures.

However, it should be added that in spite of different notch geometries the shapes of Charpy impact. resistance curves versus temperature diagrams are similar. The diagrams plotted in function of temperature are almost parallel to each other. This is confirmed by the similar values of

coefficient B in equations 4, 5 and 6.

CONCLUSIONS

1. The results of the Charpy impact resistance tests performed on samples with different notch geometries have shown that the curves describing changes in energy with temperature are almost similar.

2. The increase of the loading velocity from 2 to 5 m/s has no essential influence on the Charpy impact

test results.

3. The sharpening of notch decreases the Charpy impact resistance values. The value of impact resistance for U-notch samples is about 50% higher than the same value for V-notch samples and the impact resistance for V-notch samples is about 50% higher than the same value for precracked Y-notch samples.

4. The critical value of the dynamic J-integral can be determined on the basis of the results of Charpy impact tests for all the examined notch geometries and two loading rates utilizing the empirical equat i ons.

5.The admissible flaw size in the narrow range of temperatures from -20 to -30°C decreases 10 times reaching the value of 1 mm at temperature -30°C. Therefore, below -20°C the probability of cracking of the tested elements increses significantly.

REFERENCES

1.Marandet B.,Sanz G. "Evaluation of the toughness of thick medium strength steels by using linear elastic fracture mechanics and correlations between K% and

CV", Revue de Metallurgies Vol.73, pp.359 - 384, 1976

Z.Biel Golaska M. "A test stand and method to determine the fracture toughness by dynamic loading".

Transactions of the Foundry Research Institute, Cracow, Vol.43, pp.1- 13, 1993.

3. ASTM E 24.03.03. Proposed Standard Method of Test for Instrumented Impact Testing of Precracked Charpy Specimens of Metal 1 ic Materials. Philadelphia 1980.

4.Biel Gotaska M. "A method of testing the dynamic fracture toughness of materials characterized by high p 1 ast ic i ty'' Meta 11 urgy and Foundry Fngineering,

Vol.19, pp.491 - 500, 1993.