Evaluation of ductile-brittle transition temperature of anisotropy material by small punch test with U-shaped notch

Miniature and standard specimens, were cut from the anisotropy materials with axial, central and radial directions to study the mechanical property. In the paper, main research focused on the small punch test (SPT) with un-notched and U-shaped notched specimen in low temperature. Through the small punch energy variation with temperature, the ductile-brittle transition temperature by the small punch test (TSP) can be determined. The results indicated that there was no obvious difference among three different directions in transformation temperature of SPT with un-notch specimens, and it cannot represent upper plateau impact energy of three different directions. And the SPT with U-shaped notched specimens can determine the differences of upper plateau fracture energy of three different directions. Therefore, SPT with U-shaped notch specimens was more useful to evaluate the material anisotropy.


Introduction
The small punch test (SPT) originated in the 1980s and was proposed by Baik [1] et al, by which various material properties could be obtained from a fairly small disk specimen with an almost nondestructive. The smallpunch testing method has been applied successfully to evaluate a variety of material mechanical properties, such as yield strength, rupture strength, ductile-brittle transition temperature, fracture toughness [2] and creep performance [3]. The index of material embrittlement commonly was represented by the ductile-brittle transition temperature by the small punch test (TSP), which is associated with standard ductile to brittle transition temperature by Charpy Impact test (TCVN) [4]. Due to the bidirectional force, the specimens were insensitive to the anisotropy by SPT. The mechanical properties of each anisotropic material cannot be obtained in different directions by SPT. This paper study the A350 flanged forging by changing the conventional pattern specimens of SPT, in order to effectively characterizing the anisotropic materials, which has certain guiding significance for evaluating the toughening brittle characteristics of anisotropic materials and also has a certain help for the improvement of the technology of small punch test in the future research.

Chemical composition and mechanical performance test
In order to compare the correlation with the small punch test, the chemical composition analysis, tensile test and impact test of the material must be carried out.

Chemical composition analysis
The test material in this paper is A350 flanged forging, and its chemical composition was shown in Table 1, which conformed to the ASTM A350 standard. Tensile test specimens were respectively carried along the axial direction L, circumferential direction C and radial direction R. According to Table 2, the mechanical properties on the materials of circumferential and radial basic are quite, while properties of axial direction on mechanical are the worst. The axial yield strength and tensile strength are slightly smaller, and the percentage elongation after fracture is obviously inadequate. In order to investigate the anisotropy of the impact toughness of A350 flanged forging, the flange should be sampled according to the fracture surface with different notch direction. For the impact samples, the axial direction (L), circumferential direction (C) and radial direction (R) are respectively used to the length direction of the impact sample. The Charpy impact test specimens sampling are shown in Figure 1, which cover the impact toughness of flange in all directions. Figure 2 showed that CVN curves at three different temperatures of A350 samples. The Boltzmann function was used to fitting the ductile-brittle transition temperature curve. The ductile-brittle transition temperature in the three different directions of the A350 material was determined: C-L direction TCVN=205.4K; R-L direction TCVN=193.8K; L-C direction TCVN=199.4K. From the ductile-brittle transition temperature, the result was R-L > L-C > C-L, which had a little difference. From the upper shelf impact energy, the result was C-L > R-L > L-C, which had obvious difference.

Disk samples of SPT
SPT samples are disks with a diameter of 10 mm and thickness of 0.5 ± 0.005 mm with a good finish (P1200). Figure 3 showed the specimen with U-shaped notch. Both surfaces are parallel. The specimen with U-shaped notch was processed by Electric discharge method on the basis of sample without gap with a diameter of 0.18 mm. The size of the sample with U-shaped notch was shown in Figure 4.
As shown in Figure 5, the samples of the small punch were carried out in three different directions of the A350 material respectively, and the directions were the same as that of the Charpy impact sample.
Finarelli [5] observed that in the process of the small punch test, the crack appears near the peak load point (Fm) and gradually expand, until the break. According to the GB for SPT [6], the fracture energy of small punch test specimen is abbreviated as ESP is commonly derived from integrating the load-displacement curve along the X axis with defining 80% of Fm point for integral upper limit. In the impact test, the relationship between impact energy and test temperature is more commonly used by Boltzmann function and hyperbolic tangent function [7]. Some studies showed that the former has a good correlation coefficient and a little error. Therefore, Boltzmann function is used to fit the relationship between the fracture energy of the small punch and the experimental temperature. 2. TSP is identified as the temperature of the average value of the peak energy and the lower shelf if the lower shelf can be appeared.
The first determination was used in this article. And the first half of the curve was fitted with Boltzmann function, and the second half was fitted with a straight line. The fracture energy fitting curve of three different directions was shown below. Figure 6. Temperature dependence of fracture energy for A350 material with disk specimens. Figure 6 showed the fracture fitting curves in three different directions with disk specimens. And the TSP of A350 material was determined by the energy curve method.
Axial direction: TSP=138.2K; Central direction: TSP=135.2K; Radial direction: TSP=139.2K. As shown in Figure 6, the curves in three different directions of the A350 materials are almost the same. And the upper shelf energy and TSP have a little deference in three different directions.

U-notched samples of SPT at low temperature
Due to conventional small punch test results cannot evaluate the variation of the upper shelf energy for anisotropic material, the small punch test at low temperature was carried out by using the U-notched shape samples.      Table 3 was the results of the T SP of small punch test and T CVN of Charpy impact test for the A350 material. With U-notched specimens, the differences of transition temperature among three different directions are larger than disk specimens. Especially the transition temperature of axial direction is much smaller than the other two different directions, which means the axial direction performance is best. In this case, the force of axial specimen is the combined force of radial and circumferential tensile stress, which is exactly the same as the tensile strength. The transition temperature obtained by small punch test with disk specimens can only be used in empirical correlations with the ductile-brittle transition temperature by the Charpy Impact test [11] and is unable to evaluate the upper shelf impact energy on impact test curves. However, the small punch test with the U-notched specimens can show the difference in the ductile-brittle transition temperature of anisotropic materials in different directions. 3. Discussion

1.
The results of conventional tensile test in three different directions of the A350 showed that the A350 has obvious anisotropy and the axial tensile properties is the worst, but the results of Charpy impact test showed that the difference in ductile-brittle transition temperature is a little among three different directions, while the upper shelf impact energy have obvious differences, which is the lowest in axial direction, and is consistent with the tensile properties.
2. The transition temperature and upper shelf impact energy obtained by small punch test and Charpy impact test are consistent in three different directions. So the conventional small punch test can be used to link the TSP and the TCVN.
3. The force of small punch specimens in three different directions is complex. The force of axial direction is the combined force of the radial direction and circumferential direction tensile stress. The force of radial direction is the resultant force of the axial direction and circumferential direction tensile stress. The force of circumferential direction is the combined force of the radial direction and axial direction tensile stress. According to the tensile results, the intensity in axial direction is weaker than the circumferential direction and the radial direction, so the axial sample has a good performance, and the axial TSP should be the lowest. The fracture energy of the sample decrease with the U-notched specimens and the tendency of decline is getting smaller. The fracture energy of the U-notch specimens shifted to the right with the temperature changing, and the transition temperature of the U-notch specimens is obviously changed.

Conclusion
1) The study for the anisotropy of A350 material found that the small punch test with disk specimens can be used to link the TSP and the TCVN.
2) The fracture energy of the U-notch specimens shifted to the right with the temperature changing, and the transition temperature of the U-notch specimens is obviously changed.
3) The small punch test with U-notched specimens can effectively reflect the differences of the impact energy in three different directions.