Accepted Manuscript Estimating fracture toughness of various matrix structured ductile iron using circumferentially notched tensile bars Gülcan Toktaş, Alaaddin Toktaş PII: DOI: Reference:
S0013-7944(17)31168-2 https://doi.org/10.1016/j.engfracmech.2018.02.032 EFM 5890
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Engineering Fracture Mechanics
Received Date: Revised Date: Accepted Date:
3 November 2017 28 February 2018 28 February 2018
Please cite this article as: Toktaş, G., Toktaş, A., Estimating fracture toughness of various matrix structured ductile iron using circumferentially notched tensile bars, Engineering Fracture Mechanics (2018), doi: https://doi.org/ 10.1016/j.engfracmech.2018.02.032
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Estimating fracture toughness of various matrix structured ductile iron using circumferentially notched tensile bars Gülcan TOKTAŞ*1, Alaaddin TOKTAŞ2 1,2
Balıkesir University Department of Mechanical Engineering, 10145, Cagıs Campus, Balıkesir, TURKEY
Abstract In this experimental study, circumferentially notched tensile (CNT) specimens with various notch root radii of 0.05, 0.1, 0.16, 0.25, 0.4, and 0.8 mm were employed to investigate the influence of matrix structure on the fracture toughness of 1.03% Cu, 1.25% Ni, and 0.18% Mo alloyed ductile iron. The assayed matrix structures in this study were ferritic, pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic. The microstructures were obtained by several heat treatment processes. The microstructures and morphology of broken surfaces of CNT specimens were also examined by the optical and scanning electron microscopes. It was observed that for the steady-phased (ferritic, pearlitic/ferritic, and pearlitic) matrixes, the increasing rates of fracture toughness under the applications of different notch tip radii were lower than that of the tempered martensitic, lower and upper ausferritic matrixes. It was also inferred that lower values of fracture toughness were obtained by the applied rapid method.
Keywords: Matrix structure; notch tip radius; circumferentially notched tensile; fracture toughness; ductile iron
Fracture toughness is a material property that resists the propagation of a crackinduced from any pre-existing flaw. So the calculation of fracture toughness is a crucial aspect of any engineering design. If a material with low fracture toughness contains any crack, it may fail before yielding. On contrary, a tough material with high fracture toughness will yield even under the existence of a crack. All engineering materials except the carefully produced single crystals contain different imperfections or defects. Cracks may occur from the production methods or other sources. Inclusions or voids may form during the manufacturing processes or they may pre-exist in materials. Machining, cyclic loading, corrosion, and shrinkage in castings and weldings may also cause cracks. In this regard, design engineers should consider the damage tolerant design to circumvent these manufacturing and service induced damages to prevent the catastrophic failures, such as crashed aircrafts, collapsed bridges, burst boilers, and so on. Damage tolerance is the ability to resist fracture from the pre-existent damage for a given period of time and an essential attribute of components whose failure could result in disastrous result . Many parameters, such as materials (microstructures, dimensions), manufacturing, service environment (corrosion, temperature, moisture), and loading (single or triaxial stress, low or excessive stress, cyclic and creep stress) can affect the reliability of damage tolerance. The fracture toughness, one of the most significant material properties, should be taken into account during the damage tolerant design. The experimental measurement and standardization of fracture toughness play an imperative role in the application of fracture mechanics to structural integrity assessment, damage tolerant design, fitness-for-service evaluation, and residual stress analysis for different engineering components and structures . However, the testing of fracture toughness of metallic materials requires standard specimens that are relatively large and expensive to machine . Moreover, the specimens should be pre-cracked by fatigue, which is an expensive and time-consuming process. A rapid, cost-effective, and reliable technique for fracture toughness evaluation has 2
been proposed using circumferentially notched tensile (CNT) specimens . The advantages of CNT specimens can be listed as follows [5, 6]:
The plane strain condition can be obtained because the circumferential crack has no end in the plane stress region comparing the standard specimen geometries.
Because of the radial symmetry, the microstructure of a material along the circumferential area is completely uniform.
Preparation of CNT specimen is easy and cheap.
Fracture toughness test is easy to perform.
CNT has a relatively small cross-section, and it makes possible to achieve quite high levels of loading by using moderate loads.
CNT specimen reduces material requirements.
Over the past years, many investigations have been performed to determine the fracture toughness of various materials using CNT specimens. Alaneme and Aluko  examined the fracture toughness of as-cast and age-hardened Al(6063)-SiC composites using CNT specimens and reported a reliable outcome. They utilized the relation D ≥ (KIC/σy)2 to test the validity of KIC values obtained from the experiment. Nath and Das  studied the effects of notch angles (45°, 60°, and 75°) and notch diameters (5, 6, and 4.2 mm) of CNT samples on the fracture toughness of asreceived and annealed medium carbon steel (0.5% C). They concluded that K IC decreased as the notch angle decreased (sharper notch) for a given notch diameter. It was also found that the samples with lower notch diameters showed higher K IC for the same notch angle. Dharne and Todkari  evaluated the fracture toughness of Al7075-T6 alloy by employing CNT samples of different specimen diameters (8, 9, and 10 mm) and notch angles (45°, 60°, and 75°). The plane strain fracture toughness of the tested alloy varied in the range of 25.5 to 26.1 MPa.m-1/2, which is a reported valid range obtained by the standard tests. It was also noticed that the values of fracture toughness increased along with the rising specimen diameters and notch angles. Bayram et al.  investigated the fracture toughness of various materials (dual phase steel, Al-Zn-Mg-Cu wrought alloy, and Al-Si cast alloy) using CNT specimens and indicated that CNT testing can be readily used for the rapid determination of fracture toughness of metallic materials.
According to the aforementioned literature survey, it is evident that no such works are available for the rapid determination of fracture toughness using CNT specimens in alloyed ductile iron, especially with various matrix structures. Moreover, the effect of notch radius is also less studied so far. The objectives of our present work are (a) to determine the effect of notch radius on the fracture toughness of Cu, Ni, and Mo alloyed ductile iron with various matrix structures, (b) to compare our results with the previous studies on standard specimens, and (c) to discuss the validity of the rapid fracture toughness testing.
The alloyed ductile iron was produced according to the methods described earlier  and its chemical composition is presented in Table 1. The microstructure and nodule appearance of as-cast ductile iron are shown in Figure 1. Bull’s eye structure with ferrite surrounding the graphite nodules is dominated in a pearlitic matrix. In order to achieve different matrix structures in ductile iron, six types of heat treatment methods were applied. At first, all specimens were homogenized at 927 °C for 7 h, then cooled to 500 °C in a furnace, and subsequently, transferred to room temperature to prevent the negative effects, such as segregations, residual stresses, etc., of the casting process. A fully ferritic matrix structure was obtained by the homogenization treatment, and one group of these specimens was left in this condition. All other heat treatments were applied to the homogenized specimens. Following the applied heat treatment methods, ferritic, pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic structures were obtained, and these structures are denoted as F, P/F, P, TM, LA, and UA throughout the paper, respectively (Table 2). The standard techniques were performed to prepare the metallographic sample. The metallographic etching was done with 2% nital and the microstructure of the sample was revealed using an Olympus BH2-UMA optical microscope. Pearlite volume fractions for P/F and P samples were determined based on the quantitative 4
metallography by point counting method as 76 and 94%, respectively. In order to identify the fracture mode, the fractographic examinations of the fractured surfaces of the CNT samples were performed using an LEO 1455(VP) scanning electron microscope (SEM). The CNT specimens were machined with an outer diameter of 10 mm, notch diameter of 8 mm, notch angle of 60°, and various notch radii (0.05, 0.1, 0.16, 0.25, 0.4, and 0.8 mm). The configuration of the CNT specimen is shown in Figure 2. After machining, the notch radii of specimens were measured by the aid of Mitutoyo profile projector with a magnification of 50X and a sensitivity of 0.005 mm. The circumferentially notched tensile tests were performed at room temperature on a universal tensile testing machine with a cross head speed of 1 mm/min. Five specimens were prepared and tested for each radius. The fracture toughness (KıC) values of the CNT specimens were calculated using the following equation [10,12]: (1) where Pf is the fracture load, D and d are the diameters of unnotched and notched sections, respectively.
Results and Discussion
Mechanical and Microstructural Properties. Figure 3 displays the microstructures of matrixes obtained by the applied heat treatment methods. The matrix structures of F, F/P, P, TM, LA, and UA are shown in Figure 3A-3F. Spheroidal graphites are embedded in a fully ferritic matrix due to the slow cooling rate after austenitization in the furnace (Figure 3A). Figure 3C represents the nearly fully pearlite structure in the matrix due to the high cooling rate (5 °C/min). Needle martensite with some amount of retained austenite is observed in Figure 3D. Figure 3E and F exhibit the needleshaped acicular ferrite (dark areas) and feathery ferrite with more blocky austenite (white areas) due to austempering at 300 °C and 365 °C, respectively.
prediction of volume percent of retained austenite was estimated by a series of
empirical equations, given in a recent study , as 14,5 and 29,7% for LA and UA structures , respectively. The nodule characteristics of each matrix are displayed in Table 3. The nodularity varies between 92–96% for the matrixes. The nodule diameter and nodule count range between 29–37 µm and 100–155 mm–2, respectively.
characteristics of each matrix have satisfying values for providing adequate mechanical properties. The tensile and hardness properties of the matrixes are extracted from the previous work  and shown in Table 4. The LA structure showed the best combination of strength, elongation, and hardness, and the TM structure manifested the highest proof and ultimate tensile strengths but the least elongation value. Increasing pearlite content in the matrix structures resulted in higher hardness, yield, and ultimate tensile strengths, it is an expected result because while ferrite softens, pearlite hardens and strengthens the matrix .
Fracture Toughness. The relationship between fracture toughness and notch tip radius is shown in Figure 4. The fracture toughness of the matrixes increased with respect to the rising notch radii of CNT specimens. The fracture of a loaded specimen starts from a small crack in front of the notch root. By sharpening the notch, the triaxiality of stresses at the notch root increases. For CNT specimens, additional stresses are also responsible for crack propagation; triaxial stress with concomitant radial, circumferential, and axial stresses cause more complex stress condition. These high stresses cause excessive local plastic deformations at the notch tip, thus expedite the crack growth by facilitating the hardening effect . The higher local stresses at the notch tip result in low fracture toughness of the sharper notch radius. The fracture toughness variations of the F, P, and P/F matrixes are parallel with each other, and no sign of rises is seen in their values with increasing notch root radius (Figure 4). The P and P/F matrixes have very close fracture toughness values for all notch radii, while the F matrix exhibits distinct low values than that of the other two. For instance, the fracture toughness of the F matrix is 24.8 MPa.m-1/2 for 0.05 mm 6
radius, but for the other two, it is nearly 31 MPa.m-1/2. Ferrite has higher ductility and lower strength than pearlite, therefore, higher accumulation of localized deformations occurs in ferrite phase compared to pearlite. It happens because the graphite nodules are typically embedded in the soft ferrite phase and act as the local stress raisers . Conversely, Fengzhen et al.  observed that the germination of cracks initiates in the pearlite cell near the boundary of pearlite and ferrite. Gonzaga et al.  also reported that higher pearlite contents in ductile iron led to small plastic deformation, fragility, and lower fracture toughness. The rise of fracture toughness values for the TM, LA, and UA matrixes are higher than that of the steady phased matrixes (F, F/P, and P), in other words, the bluntness of notch radius is more effective in TM, LA, and UA matrixes. The microstructural properties, such as type, shape, size, distribution, affect the stress distribution at the notch tip. The retained austenite can be attributed as a cause of high bluntness effect. The retained austenite can transform into martensite more easily in sharpnotched specimens due to high local stresses, hence, cracking initiates. On the other hand, the retained austenite can locally deform at blunt notches, thus it reduces the magnitude of stresses at the notch tips, prevents crack propagation, and enhances fracture toughness. For the low notch radii (0.05, 0.16, and 0.25 mm), the values of fracture toughness for TM structure were little higher than that of the F structure and increased more rapidly with the rising notch radius. The TM structure showed the highest increase rate of toughness among all matrixes. The fracture toughness was obtained as 26.8 MPa.m–1/2 for 0.05 mm radius and this value was increased to 44.1 MPa.m–1/2 for the notch radius of 0.8 mm, thus showing an increment of 69%. While the LA structure showed the highest fracture toughness, the UA displayed the lowest values for all radii except 0.4 and 0.8 mm. The stage II or second reaction during austempering is considered as the main reason for low fracture toughness values of UA structure. High carbon austenite decomposes further into ferrite and carbide during the second reaction. Hard carbides make materials brittle and degrade the mechanical properties .
Ductility is not a determinant for fracture toughness itself. Although the F matrixed iron has the second elongation value among all matrixes (Table 4), it manifests the lowest fracture values. It can be concluded that the combination of high ductility and high strength leads to higher fracture toughness. The high fracture toughness of LA structure can be attributed to optimum ferrite grain size and austenite volume fraction obtained by austempering at 300 °C. Rao et al.  revealed that the ferrite grain size and volume fraction of high carbon austenite has a great effect on the fracture toughness of austempered ductile iron. While fine ferrite obtained at low austempering temperatures increased the fracture toughness, high carbon austenite fraction obtained at high austempering temperatures also enhanced it. So the optimum value of maximum fracture toughness was observed at a certain tempering temperature (300 °C).
The Validity of the Method. The fracture toughnesses were found in the range of 23–44 MPa.m–1/2 for all matrixes and radii in the present study. These values varied between 23–32 MPa.m–1/2 for the sharpest notch tip radius (0.05 mm). Table 5 represents the fracture toughness values of ductile irons which were extracted from previous studies performed on standard specimens. Due to the proximity to standard sample, the values of fracture toughness at sharpest notch radius are comparable with the standard test results. In general, lower fracture toughness values were obtained by the non-standard easy machined CNT specimens in the current study. Ductile irons with the austempered structures have higher fracture toughness compared to the steady matrix phases, such as ferritic, pearlitic, or combination of both (Table 5). In the present study, the LA structure (austempered at 300 °C) exhibited the highest values for each notch radius. While the fracture toughnesses of F, P/F, and P structures ranged in 26–38 MPa.m–1/2 in the previous works [21, 22], they varied between 24–32 MPa.m–1/2 for 0.05 mm radius in our study. Durmuş et al.  found the KıC values of F, FP, PF, and P matrixes lied in the range of 24–29 MPa.m–1/2 for 0.01 mm radius of CNT specimens. It is well known that any mechanical property of a material depends on many parameters. For example, a small change in the amount of any element can produce a significant difference in overall property of an alloy. Manufacturing processes, heat 8
or mechanical treatments also influence greatly the mechanical and microstructural properties of a material. So the low toughness values found in our study can be attributed to (i) the prior heat treatment processes, especially the homogenization treatment applied to all specimens and (ii) the distinct alloying additions. Fractography. The SEM images of the broken CNT samples (0.05 mm notch radius) are shown in Figure 5. Figure 5A exhibits a typical ductile fracture in the fully ferritic matrix by providing evidence of dimples formed due to the growth and coalescence of microvoids. Pearlitic matrix has a mixed type of fracture, rarely ductile with dimple formation and mostly brittle with some flat surfaces of cleavage (Figure 5B). Since pearlite is harder than ferrite, the plastic zone in front of the crack tip is small, and so the crack can propagate by cleavage . More dimple formation caused a ductile fracture in the LA matrix, it happens due to the highest toughness, ductility, and strength values of this structure (Table 4). Both TM and UA structures display clear brittle fracture in parallel with their low ductility (Figure 5C and E). The graphitematrix interface cracks and micro-matrix cracks were found in these two structures more apparently.
In the current study, the effects of notch tip radius and matrix structure on the fracture toughness of Cu, Ni, and Mo alloyed ductile iron were investigated by the circumferentially notched tensile specimens. The following conclusions can be drawn from our findings:
The fracture toughness variations of the F, P, and P/F matrixes with respect to different notch root radii were parallel with each other. The F matrix showed lower fracture toughness than that of the other two.
The increasing rates of fracture toughness by the application of different notch tip radii for the steady-phased matrixes (F, F/P, and P) were lower than that of the tempered martensite, lower and upper ausferritic matrixes.
The TM structure manifested the highest increase rate of fracture toughness among all matrixes. The rate was increased by 69% from the sharpest (0.05 mm) to the bluntest (0.8 mm) notch radius.
While LA exhibited highest fracture toughness among all matrix structures for all notch radii, UA showed the lowest values excepting 0.4 and 0.8 mm radii.
Lower toughness values (23–44 MPa.m–1/2) were obtained by the nonstandard easy machined CNT specimens. It proves more reliability of our data compared to the values obtained from the standard samples in previous studies.
The F and LA structures displayed the highest ductility. The ductile fracture with the dimple formation was the main fracture mechanism for these two.
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Table 1: Chemical composition of ductile iron (wt.%) C:
Table 2: Sample identification and the parameters for heat treatment Austenitizing Sample temperature/time,
Tempering and/or cooling conditions
identification T/ t 925°C – 500°C furnace cooling Ferritic (F)
925 °C /7 h 500°C – RT air cooling
Cooling to 660°C with 2.4°C /min, 900 °C /1 h
then air cooling Cooling to 650
°C with 5°C /min,
900 °C /1 h then air cooling
Oil quenching, tempering at 400°C for 900 °C /1 h
1 h then air cooling
Austempering at 300°C for 1 h, then 900 °C /1 h
Austempering at 365°C for 1 h, then 900 °C /1 h
Table 3: Nodule characteristics of matrix structures Matrix
Area fraction Nodule
count (mm- of nodule (%)
Table 4: Tensile and hardness properties of matrix structures Matrix
Table 5: Fracture toughness of ductile iron obtained by the standard samples and methods in earlier studies.
Lower and upper
Unalloyed and 1Cu
Lower and upper
1.03 Ni, 0.52 Cu
0.30Mo, 0.30Cr Unalloyed, 0.471Mo, 1.570Ni alloyed
1.5Ni, 0.3Mo, 0.5Cu alloyed
CT standard ASTM E399 ASTM E399
standard CT standard
Ausferritic (by conventional and twostep austempering)
1.03 Ni, 0.52Cu alloyed
Ferritic (by long
ASTM E1820–09 CT standard
Figure 1: Microstructure of as-cast ductile iron: (A) unetched and (B) etched samples
Figure 2: The geometry and dimensions of a CNT specimen
Figure 3: Matrix structures a) F, b) P/F, c) P, d) TM, e) LA and f) UA
Figure 4: Fracture toughness of the matrix structures by the notch tip radii
Figure 5: SEM images of fracture surfaces of a) F, b) P, c) TM, d) LA and e) UA matrixes
Lower fracture toughness values were obtained by the non-standard esay machined circumferential notched tensile specimens.
The steady-phased matrixes showed lower increasing rates of fracture toughness by the notch tip radii than that of the other matrixes.
Ductility is not a determinant of high fracture toughness itself. The ferritic matrix with the highest ductility displayed almost the worst fracture toughness among all matrixes.