The influence of the notch shape on the impact fracture of 17Mn1Si steel is investigated at different temperatures with the focus placed on the low-temperature behavior. An approach towards fracture characterization has been suggested based on the description of elastic-plastic deformation of impact loaded specimens on the stage of crack initiation and growth at ambient and lower temperatures. The analysis of the impact loading diagrams and fracture energy values for the pipe steel 17Mn1Si revealed the fracture mechanisms depending on the notch shape. It was found that the testing temperature reduction played a decisive role in plastic strain localization followed by dynamic fracture of the specimens with differently shaped notches. A classification of fracture macro- and microscopic mechanisms for differently notched specimens tested at different temperatures was proposed which enabled a self-consistent interpretation of impact test results.
A common tendency in transportation pipeline development, particularly for main gas and oil pipelines, is a gradual increase in their service life and performance [
Pipe steel structures are highly sensitive to the technological process of their manufacturing. Variations in technological variables result in considerable scatter of properties along the pipelines, increasing the probability of sudden damage and failure [
Currently available approaches to characterizing the base metal ductility allow estimating the dynamic crack initiation conditions that are crucial for the prevention of gas and oil pipeline failure [
Furthermore, analysis of the literature data [
The present paper is aimed at obtaining a deeper insight into the influence of the notch shape on the impact fracture of 17Mn1Si steel at different temperatures with a focus on the low-temperature fracture behavior.
Specimens for investigation were cut by spark erosion from a pipe steel sheet of 30 mm thickness produced by the Machine Building Factory of Yurga (Russia). A batch of specimens,
Impact test conditions for 17Mn1Si steel specimens.
Notch shape | Geometry | Notch tip radius, | Concentrator length, | Testing temperature, |
---|---|---|---|---|
U-notch | | 1.0 | 2.0 | −60−40 |
V-notch | | 0.25 | −20 | |
I-notch | | 0.1 | 0+20 |
Temperature dependence of impact toughness (a); impact diagrams in the load/displacement coordinates at test temperatures: 20 (b), 0 (c), −20 (d), −40 (e), and −60°C (f) for V-, U-, and I-notched specimens.
Typically, for structural steel, the test temperature dependence of impact toughness (Figure
It should be noted that the obtained temperature dependence of the impact toughness for specimens with sharp V- and I-notches almost coincides within the entire studied temperature range. Only at −20°C was the impact toughness of V-notched specimens slightly higher than that of I-notched specimens. On the whole, the impact toughness of U-notched specimens is about 3 times higher in the entire test temperature range than that of V-notched specimens (Figure
The obtained dynamic loading curves of the specimens corroborate their sensitivity to changes in the macrofracture localization conditions which are associated with the notch shape (Figure
The specimens of 17Mn1Si steel fracture in a ductile manner at test temperatures from 20°C to −20°C, which is evidenced by gradually ascending and descending parts of the loading curve (Figures
The shape of the loading diagram (Figures
The influence of the test temperature reduction for the I-notched specimens is similar to the influence described above for the V-notched specimens. At
The yield plateau in the impact diagrams of I-notched specimens is only slightly seen. This is indicative of local hardening processes on early deformation stages, which are accompanied by a decrease in the resistance of steel to strain localization in bending (which is quite undesirable for pipelines).
The impact fracture of notched 17Mn1Si steel specimens with the maximum notch tip radius is accompanied by considerable plastic deformation. The observed macroscopic deformation behavior of the material bears witness to the activation of relaxation processes, which leads to an increase in the height and width of the impact diagram in the entire studied temperature range.
As one can see, the maximum load value decreases at room temperature (Figure
From the viewpoint of experimental data application, one of the most important issues in postcritical deformation models is the determination of the point of transition to the postcritical stage of specimen deformation, that is, the stage of crack growth. Tables
Maximum load values on impact diagrams.
| | ||
---|---|---|---|
V-notch | U-notch | I-notch | |
−60 | | | |
−40 | | | |
−20 | | | |
0 | | | |
20 | | | |
Fracture energy of 17Mn1Si steel specimens with different notches under impact bending.
| Fracture energy, | ||||||||
---|---|---|---|---|---|---|---|---|---|
V-notch | U-notch | I-notch | |||||||
| | | | | | | | | |
−60 | 7.26 | 2.47 | 4.79 | 25.02 | 20.22 | 4.80 | 7.35 | 2.06 | 5,29 |
−40 | 15.82 | 4.86 | 10.96 | 35.53 | 7.91 | 27.62 | 14.47 | 3.36 | 11,11 |
−20 | 36.27 | 18.31 | 17.96 | 51.55 | 38.76 | 12.79 | 37.98 | 7.65 | 30,33 |
0 | 47.11 | 7.25 | 39.86 | 68.39 | 41.59 | 26.80 | 48.01 | 7.39 | 40,62 |
20 | 57.51 | 27.37 | 30.14 | 80.75 | 37.55 | 43.20 | 55.15 | 21.13 | 34,02 |
These data are very important for understanding the effect of stress stiffness on the crack resistance of 17Mn1Si steel because
Additionally, as is known, the total fracture energy of Charpy specimens (
Hashemi [
Results of CF estimation, calculated from (
Temperature dependence of the CF parameter for 17Mn1Si steel specimens with V-notch (a), U-notch (b), and I-notch (c).
It should be noted that the use of the CF parameter is pertinent in the presence of a large sharp defect in the pipe, or for pipes under long-term operation conditions, when
Considering the steel in the as-supplied state, the present results show that the material has sufficient ductility. However, the CF parameter of steel decreases during operation.
Earlier, we studied Charpy specimens with in-service fatigue cracks [
The impact toughness values of 17Mn1Si steel determined by taking into account the total fracture energy of Charpy specimens are given in Table
Impact toughness test results for 17Mn1Si steel.
| Impact toughness, J/cm2 | ||
---|---|---|---|
V-notch | U-notch | I-notch | |
−60 | | | |
−40 | | | |
−20 | | | |
0 | | | |
20 | | | |
In the way of discussion, the authors would like to highlight a few features in the data shown in Tables
The mentioned “variations” are most probably related to the influence of the testing temperature as well as of the stress-strain state in the loaded specimens. From the microscale fracture mechanism point of view, it should be underlined that irreversible deformations under quasi-brittle fracture are rather low while fracture energy is predominantly determined by the stress stiffness parameters. Under ductile failure, the decisive role is related to accumulated plastic strains [
As has been said above, there is a correlation between the amount of plastic strains and external view of pipe fracture surfaces in full-scale air pressurization tests. Similar correlations were observed in the laboratory conditions on Charpy specimens [
The impact loading develops a strain field ahead of the notch tip [
Optical images of the shear lips in specimens with V-notch (a), U-notch (b), and I-notch (c) for the test temperature
Analysis of laboratory and full-scale air pressurization test data has shown that the capability to arrest ductile fracture is determined by the volume and intensity of plastic deformation ahead of the propagating crack tip [
Macroscopic aspects of fracture surface of specimens impact tested at different temperatures. Fracture zones are labelled as (I) crack initiation, (II) crack growth, (III) shear lips, and (IV) final rupture.
SEM fracture surfaces of crack initiation zone in specimens with different-shaped notches after impact bending test at different temperatures.
The fracture of V-notched specimens is similar to that observed at The fracture surface in U-notched specimens is formed with the appearance of delamination regions [ The fracture surface in I-notched specimens has brittle (quasi-brittle) “faceted” fracture relief (Figure
The fracture surface in V-notched specimens exhibits a river-like pattern (Figure The fracture mechanism for U-notched specimens is complex (Figure The fracture mechanism for I-notched specimens can be associated with quasi-cleavage (Figure
Fracture micromechanisms for impact bending test specimens are classified in Table
Fracture micromechanisms in the crack initiation zone for specimens with different-shaped concentrators.
Test temperature | Notch shape | ||
---|---|---|---|
V | U | I | |
| Ductile fracture with macrodimples of size 40–80 | Ductile fracture with macro- and microdimples | Ductile fracture with macrodimples of size ~40 |
| Ductile fracture with macro- and microdimples | Ductile-brittle delaminations | Facet fracture, ductile-brittle fracture |
| Brittle fracture with river-like fracture pattern | Ductile-brittle fracture with facets + microdimples | Brittle fracture with partial local microscopically ductile ridges |
The fracture surface in V-notched specimens consists of ductile dimples (Figure The crack propagation zone in U-notched specimens also has dimples of ductile origin: shear dimples (Figure The fracture surface in I-notched specimens is formed by the ductile mechanism, but the observed dimples are shallow, the so-called “flat-bottomed” dimples (Figure
Fracture micromechanisms in the crack propagation zone for specimens with different-shaped notches.
Test temperature | Notch shape | ||
---|---|---|---|
V | U | I | |
| Dimpled ductile fracture | ||
| Brittle (flat cleavage facets) | Brittle (cleavage facets) | Brittle (cleavage facets + local microdimples) |
| Brittle fracture with cleavage facets | Brittle fracture with cleavage facets and local microcracks | Brittle fracture with cleavage facets |
Fracture surfaces corresponding to the crack propagation zone in the specimens tested in impact bending at different temperatures.
Thus, we have shown that the notch shape strongly affects the morphology of the formed ductile fracture dimples.
The fracture surface in V-notched specimens is characterized by the presence of brittle structural elements, such as cleavage facets, terraces, and individual regions with quasi-ductile microrelief, and by zones of local microdeformation (Figure The quasi-cleavage fracture mechanism in U- and I-notched specimens also includes the nucleation of transgranular cleavage microcracks in the bulk of structural elements and subsequent coalescence of neighboring brittle microcracks with rupture of ductile (plastic) metal ligaments between them. Part of these cracks do not have enough energy for propagation and remained as voids or delaminations (Figures
The fracture surface in V- and I-notched specimens is formed by crystalline brittle fracture along cleavage planes with facet formation on the surface (Figures The U-notched specimens exhibit relatively “flat” brittle fracture facets. Periphery regions of quasi-cleavage facets have well-defined contours that bear witness to the absence of microplastic strains in these metal volumes (Figure
An approach towards fracture characterization has been suggested based on the description of elastic-plastic deformation of impact loaded specimens on the stage of crack initiation and growth at ambient and lower temperatures. The analysis of the shape of impact loading diagrams and energy fracture values for impact loaded specimens of pipe 17Mn1Si steel revealed the fracture mechanisms of this steel depending on the notch shape.
It was found that the test temperature reduction plays the decisive role in plastic strain localization and subsequent impact fracture of specimens with different-shaped notches. This is reflected in localization of deformation processes, decrease in crack propagation energy, and “degradation” of shear lips.
The present work represents a logical extension of the approach that utilizes the shape of shear lips as an informative feature (both qualitative and quantitative) in fracture diagnostics. A classification of fracture macro- and microscopic mechanisms for differently notched specimens tested at different temperatures was proposed, which enabled self-consistent interpretation of impact test results.
The authors declare that they have no competing interests.
This work was performed in the framework of fundamental research projects of the Russian State Academies of Sciences (2013–2020), with a partial support of RFBR Grant no. 15-08-05818_a, and Project of the Headquarter of the Russian Academy of Sciences on Artic Research. The authors are grateful to the Shared Use Center “Nanotech” of ISPMS SB RAS for the assistance in fractographic investigations.