3.1. Experimental materials

Material GOST 15Ch2NMFA were delivered in a form of rod with diameter of 130 mm and length 150 mm. At first, three 2 T-CT specimens were produced in R-C orientation (according to the standard ASTM E399-09 [3]). Technical drawing of 2 T-CT specimens is depicted in Figure 10. Broken halves of the 2 T-CT specimens were subsequently used for production of the other specimens (tensile test specimens, Charpy specimens, etc.).

Material EN X5CrNi18-10 (AISI 304) was delivered in the form of hot rolled rod with quadratic cross section of dimensions 60 <sup>30</sup> 400 mm<sup>3</sup> . All specimens were produced in T-L orientation according to standard [3].

Material Ti6Al4V was investigated in the form of bar with dimension of 10 <sup>20</sup> 100 mm<sup>3</sup> . Designation of specimen orientation was done according to the standard [25]. Where the first letter represents the direction normal to the crack plane and second latter represents the expected direction of crack extension. Orientation and its designation of the specimens in the prism are depicted in Figure 11.

### 3.2. Tensile tests

Seok et al. [21] investigated effect of specimen configurations using 0.5–2 T CT and further specimens with constant width (101.6 mm) but different thickness plus specimens with same thickness but different width. Therefore, the effect of plane size, specimen size and thickness could be investigated. Moreover, the effect of the crack length and side grooves was discussed as well. The resulting J-R curve increased with increasing plane size, though there is a difference of increasing amount according to the material states, base or weld metal and stainless or carbon steel. The resulting J-R curves decreased with increasing crack length and showed that the effect of the crack length was significant. However, relatively weak influence was observed from the change of the specimen thickness and size. It was also observed that the J-R curve decreased by applying the side grooves and the effect of side groove was related to material properties.

Figure 9. Results obtained at room temperature from mini-CT and 1 T-CT specimens of 18MND5 steel [24].

fracture toughness determination in the upper shelf region. As a general conclusion, in these investigations it was observed that mini-CT specimens consistently and systematically underestimate elastic-plastic fracture toughness as measured from 1 T-CT specimens, in terms of both ductile initiation and tearing resistance. Figure 9 shows an example of such a behavior

between data measured from mini-CT and 1 T-CT specimen; below this threshold, mini-CT

Examples of fracture toughness test with the use of miniature test specimens are going to be presented in this chapter. Results obtained on three experimental materials are shown here.

) applicability for

, no significant deviation was observed

Lucon et al. [22–24] investigated mini-CT specimen (10 <sup>10</sup> 4.15 mm<sup>3</sup>

could therefore provide a reliable measurement of the material's toughness.

and also shows that, below approximately J = 200 kJ/m<sup>2</sup>

154 Contact and Fracture Mechanics

3. Small size experimental specimen testing

Tensile test were carried out on standard- and miniature-sized specimens at room temperature under quasi-static loading conditions for demonstration of comparable results obtained with the use of miniaturized specimens. Tests were following procedure according to standard (ISO CSN EN 6892-1) in the case of the full-size specimen testing. Testing procedure based on

Figure 10. 2 T-CT specimen geometry.

mechanical extensometer for strain measurement. The M-TT specimens (Figure 12b) were tested with the use of small-sized linear drive-based testing system with capacity of 5 kN. Strain in the course of the M-TT test was measured using DIC system ARAMIS by GOM. Prior to tests, strain calibration with certified calibration blocks was performed. An appropriate pattern was applied on the specimen surface for the strain measurement by DIC system. M-TTs were done with constant crosshead velocity of 0.25 mm/min and 1 mm/min for the "standard" geometry. Three to five specimens were tested per batch. Specimens' dimensions were measured prior to tests and after tests in order to evaluate tensile testspecific parameters. Summarized test records obtained for the materials investigated are shown in Figures 13–15. Averaged test results for each material investigated are shown in

Fracture Toughness Determination with the Use of Miniaturized Specimens

http://dx.doi.org/10.5772/intechopen.73093

157

Figure 13. Tensile test results, material 15CH2NMFA, geometry: standard and miniaturized.

Figure 14. Tensile test results, material AISI304, geometry: standard and miniaturized.

Table 4–6.

Figure 11. Orientation and designation of the specimens for the material Ti6Al4V produced SLM AM technology (Z = building direction).

standard developed in [26, 27] was employed for mini tensile test (M-TT) specimens. Specimen geometry used for the current investigations is displayed in Figure 13. Full size specimens (Figure 12c) were tested with the use of electromechanical testing system Zwick Z250 with

Figure 12. Tensile test specimen geometries. (a) Comparison of the standard and miniature tensile test specimens. (b) Dimensions of mini tensile test (M-TT) specimen. (c) Standard size specimen.

mechanical extensometer for strain measurement. The M-TT specimens (Figure 12b) were tested with the use of small-sized linear drive-based testing system with capacity of 5 kN. Strain in the course of the M-TT test was measured using DIC system ARAMIS by GOM. Prior to tests, strain calibration with certified calibration blocks was performed. An appropriate pattern was applied on the specimen surface for the strain measurement by DIC system. M-TTs were done with constant crosshead velocity of 0.25 mm/min and 1 mm/min for the "standard" geometry. Three to five specimens were tested per batch. Specimens' dimensions were measured prior to tests and after tests in order to evaluate tensile testspecific parameters. Summarized test records obtained for the materials investigated are shown in Figures 13–15. Averaged test results for each material investigated are shown in Table 4–6.

Figure 13. Tensile test results, material 15CH2NMFA, geometry: standard and miniaturized.

standard developed in [26, 27] was employed for mini tensile test (M-TT) specimens. Specimen geometry used for the current investigations is displayed in Figure 13. Full size specimens (Figure 12c) were tested with the use of electromechanical testing system Zwick Z250 with

Figure 12. Tensile test specimen geometries. (a) Comparison of the standard and miniature tensile test specimens. (b)

Dimensions of mini tensile test (M-TT) specimen. (c) Standard size specimen.

Figure 11. Orientation and designation of the specimens for the material Ti6Al4V produced SLM AM technology

(Z = building direction).

156 Contact and Fracture Mechanics

Figure 14. Tensile test results, material AISI304, geometry: standard and miniaturized.

3.3. Fracture toughness measurements

covering most of them.

utilized for brittle and ductile fracture description.

Based on theoretical and experimental analyses of possible fracture toughness specimen downsizing, several geometries were proposed as it was discussed before. Demonstration of the fracture toughness property measurement with the use of miniaturize specimens is shown here on samples of several geometries here. The geometries employed here are miniature compact tension specimen (0.16 T-CT) (Figure 16) and miniature Charpy specimens (half Charpy specimen typically 4 3 22, KLST); see Figure 17. These specimens' geometries are

Fracture Toughness Determination with the Use of Miniaturized Specimens

http://dx.doi.org/10.5772/intechopen.73093

159

As the input data for the fracture toughness tests are used, results of tensile tests for precracking parameter determination and subsequent evaluation of validity limits and J-R curves. The effect of the temperature on fracture toughness is known for many years. It is a question of the material if it will exhibit sharp or gentle change. As it was mentioned above, the fracture toughness-temperature dependency can be divided in several regions, and the current tests are

Figure 16. Miniature compact tension specimen (0.16 T-CT): (a) front face geometry and (b) the "top and bottom" geometry.

Figure 17. Miniature Charpy specimen (KLST): (a) geometry 4 <sup>3</sup> 22 mm3 and (b) geometry 4 <sup>2</sup> 20 mm<sup>3</sup>

.

Figure 15. Tensile test results, material AM Ti6Al4V, geometry: M-TT.


Table 4. Tensile test results, material AISI 304, geometry: standard and miniaturized.


Table 5. Tensile test results, material 15CH2NMFA, geometry: standard and miniaturized.


Table 6. Tensile test results, material AM Ti6Al4V, geometry: miniaturized.

### 3.3. Fracture toughness measurements

Figure 15. Tensile test results, material AM Ti6Al4V, geometry: M-TT.

158 Contact and Fracture Mechanics

Table 4. Tensile test results, material AISI 304, geometry: standard and miniaturized.

Table 5. Tensile test results, material 15CH2NMFA, geometry: standard and miniaturized.

Table 6. Tensile test results, material AM Ti6Al4V, geometry: miniaturized.

Specimen E YS UTS Elu El RA

AISI304\_standard Avg. 167.6 316.2 657.1 49.1 63.7 82.5

AISI304\_miniaturized Avg. 141.9 340.7 679.1 49.3 62.0 75.7

Specimen E YS UTS Elu El RA

15CH2NMFA\_standard Avg. 195.4 502.0 647.9 8.0 20.8 70.1

15CH2NMFA\_miniaturized Avg. 159.8 503.4 655,1 6.3 16.2 66.0

Specimen E YS UTS Elu El RA

B27\_ZYS\_miniaturized Avg. 114.6 927.3 1000.7 5.5 10.8 43.3

GPa MPa MPa % % %

GPa MPa MPa % % %

GPa MPa MPa % % %

St. dev. 9.5 21.4 13,1 1.1 1.4 0.4

St. dev. 27.9 11.7 8,3 0.3 1.4 5.2

St. dev. 2.0 20.2 17.1 0.8 3,4 2.1

St. dev. 16.9 27.6 6.8 1.2 2.1 1.8

St. dev. 8.1 9.9 4.8 1.3 3.0 1.7

Based on theoretical and experimental analyses of possible fracture toughness specimen downsizing, several geometries were proposed as it was discussed before. Demonstration of the fracture toughness property measurement with the use of miniaturize specimens is shown here on samples of several geometries here. The geometries employed here are miniature compact tension specimen (0.16 T-CT) (Figure 16) and miniature Charpy specimens (half Charpy specimen typically 4 3 22, KLST); see Figure 17. These specimens' geometries are utilized for brittle and ductile fracture description.

As the input data for the fracture toughness tests are used, results of tensile tests for precracking parameter determination and subsequent evaluation of validity limits and J-R curves.

The effect of the temperature on fracture toughness is known for many years. It is a question of the material if it will exhibit sharp or gentle change. As it was mentioned above, the fracture toughness-temperature dependency can be divided in several regions, and the current tests are covering most of them.

Figure 16. Miniature compact tension specimen (0.16 T-CT): (a) front face geometry and (b) the "top and bottom" geometry.

Figure 17. Miniature Charpy specimen (KLST): (a) geometry 4 <sup>3</sup> 22 mm3 and (b) geometry 4 <sup>2</sup> 20 mm<sup>3</sup> .

### 3.4. Testing in the transition region

Master Curve concept according to the standard ASTM 1921-17a [5] was applied on material 15CH2NMFA. The aim of this investigation was to show the shift of the reference temperature T0 with regard to the geometry of the specimen and size of the specimen. Therefore, compact tension specimens of different sizes (2 T-CT, 1 T-CT and 0.16 T-CT) and three-point bend specimens' geometries (CVN, standard Charpy specimen 10 � <sup>10</sup> � 55 mm3 , and miniaturized Charpy specimen, KLST (3 � <sup>4</sup> � 22 mm<sup>3</sup> )) were produced.

Pre-crack of all specimens was done on magnetic resonance testing machine RUMUL; the initial crack size was 0.5 W with the final stress intensity factor of 16 MPa.m0.5. After precracking, 20% side groves were introduced. The final tests were performed on servohydraulic testing machine MTS 810 with load capacity of 250 kN (in a case of 2 T-CT and 1 T-CT specimens) and servo-hydraulic testing machine Instron with load capacity 80 kN (in a case of CVN, KLST specimens), respectively. Both machines were equipped with environmental chamber for cooling of the specimens. In all cases specimens were held on testing temperature for 15 min before the tests. Deformation of the specimens was measured by means of COD extensometer on the load-line position. Testing setup for KLST samples is depicted in Figure 18.

The first estimation of the T0 was done according to the (7) presented in [5]. For this purpose ten standard Charpy specimens were produced, and value TK28J = �34.7 J was determined. The estimated reference temperature T0 was evaluated according to the (7) as �52.7�C. This estimation provides reference temperature with standard deviation of 15�C [5]:

$$T\_0 = T\mathcal{K}\_{28\text{\textquotedblleft}} - 18^\circ \mathcal{C} \tag{7}$$

It is clear from Table 7 that evaluated reference temperature using the KLST specimens and 2 T-CT specimens does not fulfill the validity criteria ∑rini > 1, and these reference tempera-

Fracture Toughness Determination with the Use of Miniaturized Specimens

http://dx.doi.org/10.5772/intechopen.73093

161

) from material 15CH2NMFA.

Specimen Number of specimen T0/T0q ri.ni Diff.T0

1 T-CT 6 41,3 1 23,7 0.16 T-CT 9 62,6 1,5 2,4 CVN 14 73,1 2,33 8,1 KLST 14 51,8 0,56 13,2 2 T-CT 3 27,9 0,5 37,1 All specimens 46 65,0 5,9 —

C — C

tures can be taken as provisional reference temperature T0Q.

Figure 19. Example of crack size measurement.

Figure 18. KLST specimens (4 <sup>3</sup> 22 mm3

Table 7. Summarization of Master Curve results, material 15CH2NMFA.

For measurement and calculation of reference temperature T0, the multi-temperature approach was applied. All measured data were censored through crack front criterion defined in (8):

$$K\_{lc(limit)} = \sqrt{\frac{Eb\_0 \sigma\_{YS}}{30(1 - \upsilon^2)}}\tag{8}$$

where E is Young modulus, b0 = W-a0 (a0 = initial crack size), σYS is yield strength and v is Poisson ratio. For the final evaluation, all fracture toughness results were recalculated to KJC\_1T using Eq. (3).

Measured data which fulfill the limit stated in (8) were marked as ri = 1. If evaluated facture toughness values exceed limit (8) value, they were marked as ri = 0, respectively.

Crack lengths were measured through area measurement method. Example of fracture area measurement is presented in Figure 19.

Summarization of the reference temperature T0 determination is shown in Table 7. Master Curve with all measured data is depicted in Figure 20.

It is clear from Table 7 that evaluated reference temperature using the KLST specimens and 2 T-CT specimens does not fulfill the validity criteria ∑rini > 1, and these reference temperatures can be taken as provisional reference temperature T0Q.

Figure 18. KLST specimens (4 <sup>3</sup> 22 mm3 ) from material 15CH2NMFA.

Figure 19. Example of crack size measurement.

3.4. Testing in the transition region

160 Contact and Fracture Mechanics

Charpy specimen, KLST (3 � <sup>4</sup> � 22 mm<sup>3</sup>

depicted in Figure 18.

15�C [5]:

using Eq. (3).

measurement is presented in Figure 19.

Curve with all measured data is depicted in Figure 20.

Master Curve concept according to the standard ASTM 1921-17a [5] was applied on material 15CH2NMFA. The aim of this investigation was to show the shift of the reference temperature T0 with regard to the geometry of the specimen and size of the specimen. Therefore, compact tension specimens of different sizes (2 T-CT, 1 T-CT and 0.16 T-CT) and three-point bend

Pre-crack of all specimens was done on magnetic resonance testing machine RUMUL; the initial crack size was 0.5 W with the final stress intensity factor of 16 MPa.m0.5. After precracking, 20% side groves were introduced. The final tests were performed on servohydraulic testing machine MTS 810 with load capacity of 250 kN (in a case of 2 T-CT and 1 T-CT specimens) and servo-hydraulic testing machine Instron with load capacity 80 kN (in a case of CVN, KLST specimens), respectively. Both machines were equipped with environmental chamber for cooling of the specimens. In all cases specimens were held on testing temperature for 15 min before the tests. Deformation of the specimens was measured by means of COD extensometer on the load-line position. Testing setup for KLST samples is

The first estimation of the T0 was done according to the (7) presented in [5]. For this purpose ten standard Charpy specimens were produced, and value TK28J = �34.7 J was determined. The estimated reference temperature T0 was evaluated according to the (7) as �52.7�C. This estimation provides reference temperature with standard deviation of

<sup>T</sup><sup>0</sup> <sup>¼</sup> TK28<sup>J</sup> � <sup>18</sup>�

For measurement and calculation of reference temperature T0, the multi-temperature approach was applied. All measured data were censored through crack front criterion defined in (8):

s

where E is Young modulus, b0 = W-a0 (a0 = initial crack size), σYS is yield strength and v is Poisson ratio. For the final evaluation, all fracture toughness results were recalculated to KJC\_1T

Measured data which fulfill the limit stated in (8) were marked as ri = 1. If evaluated facture

Crack lengths were measured through area measurement method. Example of fracture area

Summarization of the reference temperature T0 determination is shown in Table 7. Master

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Eb0σYS 30 1 � v<sup>2</sup> ð Þ

KJc limit ð Þ ¼

toughness values exceed limit (8) value, they were marked as ri = 0, respectively.

)) were produced.

, and miniaturized

C (7)

(8)

specimens' geometries (CVN, standard Charpy specimen 10 � <sup>10</sup> � 55 mm3


Table 7. Summarization of Master Curve results, material 15CH2NMFA.

3.5. Testing in the ductile region

tion line according to (9):

depicted in Figure 23.

unloading compliance method of measurement.

Testing in ductile region was done according the standard ASTM 1820-17 [18] where concept of J-R curve was applied with in order to evaluate the crack initiation and propagation of material AISI 304 and material Ti6Al4V produced by AM technology. It compared J-R measured using unloading compliance method (for CT specimen) and multiple specimen method (for three-point-bend specimens); simultaneous size of the specimens was taken into the account. Compact tension specimen (1 T-CT and 0.16 T-CT) was compared with standard

Specimens were pre-cracked at first up to the final initial crack size 0.5 W with the final stress intensity factor of 16 MPa.m0.5. After the pre-cracking 20% side groves were introduced, and magnetic resonance machine RUMUL was used for pre-cracking. Testing of 1 T-CT was carried out on servo-hydraulic testing machine MTS 810 with load capacity 250 kN. J-R curve tests of CVN, KLST and 0.16 T-CT specimens were carried out on servo-hydraulic testing machine Instron with the load capacity of 80 kN. In the scope of the multiple testing procedures, specimens were heat tinted after the tests and consequently cooled down in liquid nitrogen. The cooled specimens were then broken, and crack sizes were measured through area measurement method; see in Figure 19. J-R curve evaluation was done with the slope of construc-

Results of the J-R testing are present in Figures 21–23 and summarized in Table 8. Summarization of J-R curve tests on material Ti6Al4V produced by Additive Manufacturing technology is in Table 9. Comparison of J-R curves obtained for 0.16 T–CT and KLST (4 � <sup>2</sup> � 20 mm<sup>3</sup>

Figure 23. Comparison of J-R curves; material, AM Ti6Al4V; geometry of the specimens, 0.16 T-CTvs. KLST (4� <sup>2</sup> � 20 mm3

J ¼ 2σYSΔa (9)

Fracture Toughness Determination with the Use of Miniaturized Specimens

http://dx.doi.org/10.5772/intechopen.73093

) is

163

);

Charpy specimen (CVN) and miniaturized Charpy specimens (KLST).

Figure 20. Comparison of the Master Curve evaluation for various samples' geometries.

Figure 21. Comparison of J-R curves; material AISI 304; geometry of the specimens, 1 T-CT vs. 0.16 T-CT; unloading compliance method of measurement.

Figure 22. Comparison of J-R curves, material AISI 304, geometry of the specimens: 1 T-CT (unloading compliance method) vs. CVN and KLST (multiple specimen method).

#### 3.5. Testing in the ductile region

Figure 20. Comparison of the Master Curve evaluation for various samples' geometries.

compliance method of measurement.

162 Contact and Fracture Mechanics

method) vs. CVN and KLST (multiple specimen method).

Figure 21. Comparison of J-R curves; material AISI 304; geometry of the specimens, 1 T-CT vs. 0.16 T-CT; unloading

Figure 22. Comparison of J-R curves, material AISI 304, geometry of the specimens: 1 T-CT (unloading compliance

Testing in ductile region was done according the standard ASTM 1820-17 [18] where concept of J-R curve was applied with in order to evaluate the crack initiation and propagation of material AISI 304 and material Ti6Al4V produced by AM technology. It compared J-R measured using unloading compliance method (for CT specimen) and multiple specimen method (for three-point-bend specimens); simultaneous size of the specimens was taken into the account. Compact tension specimen (1 T-CT and 0.16 T-CT) was compared with standard Charpy specimen (CVN) and miniaturized Charpy specimens (KLST).

Specimens were pre-cracked at first up to the final initial crack size 0.5 W with the final stress intensity factor of 16 MPa.m0.5. After the pre-cracking 20% side groves were introduced, and magnetic resonance machine RUMUL was used for pre-cracking. Testing of 1 T-CT was carried out on servo-hydraulic testing machine MTS 810 with load capacity 250 kN. J-R curve tests of CVN, KLST and 0.16 T-CT specimens were carried out on servo-hydraulic testing machine Instron with the load capacity of 80 kN. In the scope of the multiple testing procedures, specimens were heat tinted after the tests and consequently cooled down in liquid nitrogen. The cooled specimens were then broken, and crack sizes were measured through area measurement method; see in Figure 19. J-R curve evaluation was done with the slope of construction line according to (9):

$$J = 2\sigma\_{YS}\Delta a\tag{9}$$

Results of the J-R testing are present in Figures 21–23 and summarized in Table 8. Summarization of J-R curve tests on material Ti6Al4V produced by Additive Manufacturing technology is in Table 9. Comparison of J-R curves obtained for 0.16 T–CT and KLST (4 � <sup>2</sup> � 20 mm<sup>3</sup> ) is depicted in Figure 23.

Figure 23. Comparison of J-R curves; material, AM Ti6Al4V; geometry of the specimens, 0.16 T-CTvs. KLST (4� <sup>2</sup> � 20 mm3 ); unloading compliance method of measurement.


investigated, and multiple specimens' as well as single specimens' approaches were applied. Standard-sized specimen results yielded very good agreement with the results achieved for subsized three-point bend specimens, while the mini-CT specimen yielded values of about 60% of those ones obtained for standard-sized specimens. In the case of Ti-alloy, due to very limited amount of the experimental material, typically, e.g., for AM parts, mini specimens were investigated only. Both considered specimens' geometries yielded repeatable results. However, the CT specimens yielded significantly lower fracture toughness values of about 40% of those obtained for three-point-bend specimens. Large difference between these specimens' geometries results is in agreement with other published studies and results presented here for the

Fracture Toughness Determination with the Use of Miniaturized Specimens

http://dx.doi.org/10.5772/intechopen.73093

165

The results obtained here point out the fact that there is currently no available general solution for size effect description in the fracture toughness determination approaches so far. Varying agreement is found for various materials. Therefore, for a reliable "size-independent" value determination, the material of the interest has to be investigated and size effect quantified. It seems that the J-integral-based assessment has rather limited reporting value and better description has to be established for size-independent fracture toughness evaluation in the upper shelf. Lower transition region is well described by the Master Curve approach including size effect in the evaluation. Generally considered, there is no need in all cases to obtain sizeindependent values. These can be the case such as property assessment of the components of small wall thickness, where plain strain condition is in reality not predominant. Cases when local property anisotropy is being evaluated, just ratio among different locations and/or orientations, are considered. Typical examples of the materials produced with small wall thickness exhibiting high ration of property anisotropy are materials and components produced by the additive manufacturing processes. In these cases, there is generally hardly any chance to obtain "size-independent material properties" due to the reason that if produced in different wall thicknesses, different properties are achieved, and thus considered wall thickness has to

As it can be seen in many cases, no real size-independent values are possible to achieve for the material, and thus small-size techniques are the only way to characterize the properties. These values are related just to the component and the process considered; however, valuable information are provided allowing component design and process optimization. Miniaturized specimen-based techniques for the fracture toughness determination were demonstrated here as a tool providing deeper insight into the material fracture behavior for better understanding of the material behavior in cases when limited amount of the experimental is available.

This chapter was created with support of the projects TH02020448S service life assessment with the use of miniaturized test specimens, 2017–2020, and Development of West-Bohemian Centre of Materials and Metallurgy No. LO1412, financed by the MEYS of the Czech Rep.

stainless steel.

be directly assessed.

Acknowledgements

(UC = Unloading Compliance; MS = Multiple specimen method)

Table 8. Summarization of average values of fracture toughness results, material AISI 304.


Table 9. Comparison of fracture toughness results; material AM Ti6Al4V; geometry, 0.16 T-CT vs. KLST (4 � <sup>2</sup> � 20 mm<sup>3</sup> ); unloading compliance method of measurement.
