Experimental Study and Computer Simulation of Fracture Toughness of Ni Base Superalloy Using CT Specimen

Due to superior mechanical and metallurgical performance, Nickel-base Alloys 617 and 276 have been considered as structural material for used in complex and stochastic applications. Surface irregularities such as cracks in the material may be vulnerable to the structural integrity of an engineering component. Void growth behaviour is however analysed using crystal plasticity theory in nickel-based super alloys. Elastic-plastic fracture mechanics base single compact tension specimen has been used to determine the J1C value as a function of temperature of austenitic Alloy 617 and 276 for ductile crack growth behaviour. Crack formation is appropriately explained through crack nucleation based on the microstructural heterogeneity properties of the alloys. Alloy 617 showed a fair increased resistance to fracture as temperature increased from ambient to 5000C for duplicate testing, satisfying the EPFM criteria. Whereas the J1C values of Alloy 276 increased gradually with temperature up to 300° C and due to enhanced plasticity in the vicinity of 4000 C this alloy shows inconsistent value. Two-dimensional simulation of J-integral model of these nickel base super alloys at temperature range 1000C to 5000 C has been proposed. Particular focus is given on the load line displacement where crack propagation occurs during the loading phase only. Path independency of J-integral has been clearly demonstrated for both the alloys up to 3000 C employing finite element analysis meshing with 1922 quadrilateral 2D solid elements in ANSYS. Cracks are typically initiated in relation to the level of strain range. A higher strain range initiates cracks due to precipitate shearing, whereas a low strain range initiates cracks with oxidation reactions and carbide diffusion. The values of K1C and crack tip opening displacement for these alloys have been calculated based on the experimental data. Moreover, fracture morphology in the loading and unloading sequences near the crack tip has been analysed by SEM.


INTRODUCTION 1.Background
Due to a disparity in supply and demand, energy price is on the rise.Moreover, excessive use of fossil fuels produces pollutants.For these reasons, researchers are searching for alternative energy sources.Renewable energy is a promising sector as it is clean, available and cheap.Wind is a major source of renewable energy.Wind turbines are used to convert wind energy to electrical power.Currently, worldwide 40,000 MW power is produced from wind energy and it is predicted that the growth rate will be 30% in the next decade [1].Alloy 617 and 276 were found appropriate to be used as wind turbine blade material.These alloys are also suitable for heat exchanging applications, such as in Next Generation Nuclear Plants (NGNP) [2].Therefore, metallurgical characterization of these materials is necessary.Mechanical properties of a structure can be modified in many ways, for example; through the application of annealing, giving Alloy 617 resistance to degradation after long-term exposure [3].In this paper, the behavior of Alloys 617 and 276 in terms of the metallurgical and mechanical properties are characterized based on the fracture toughness, particularly for wind turbine blade applications along with other conventional polyester glass fiber materials.

Material Selection
To be operated in harsh environments such as desert areas where dust and sandstorms are more commonly to happen, turbine blade material should sustain with high impact forces and abrasion.Impact velocity is determined by wind flow velocity and the blade's speed of revolution.Due to the maximization of rotational speed at the leading edges of an aerofoil, an elastomeric material should be applied to prevent erosion.However, it is necessary to replace the tape frequently as they fail to satisfactorily absorb the concussion energy of the particulate matter.Hence, blade materials should be chosen carefully to prevent damages.Polyester-glass fibre blade material is suitable for high impact erosion.However, nickel-based superalloy has the potential to be used as it has a high tensile strength as well as corrosion resistance properties.
In 1970, Ni-base Alloy 617 was developed to be used in aerospace engineering as an advanced sheet material.The significant properties of this alloy are high strength, oxidation resistance at high temperature (980°C) as well as high resistance to both creep deformation and ruptures at temperatures up to 850 0 C [4].In addition, it has excellent metallurgical stability and low density, which provides a high strength-to-weight ratio.Due to its oxidation resistance characteristics, Alloy 617 along with Alloy 276 are used in the production process of Nitric Acid, heat treating baskets and reduction boats in molybdenum refinement as catalyst-grid support [5].Some other high-temperature applications of Alloy 617 are inducting, combustion cans, transition liners in gas-turbines, thermal energy storage capsules, manufacturing of retort furnace and gas cooled reactors.NASA has been using this Alloy material for heat shielding in space transportation systems.

Scope
An examination on tensile deformation of Alloy 617 and 276 under extensive variety of temperatures beginning from ambient to 1000°C had been completed by numerous agents [6].Surface irregularities impact the basic integrity of metallic components.Control variables of development rate of surface irregularities and voids comprise of stress triaxiality, the underlying void volume fraction, lode parameters, the crystallographic orientation, the initiated slip system and the level of elastic anisotropy [7].Variable stacking conditions are forced on these segments in temperamental desert conditions and dust storms.Elastic-Plastic-Fracture-mechanics (EPFM) was applied to appraise crack durability (fracture toughness) of this alloy at various temperatures [8], which happen contingent upon microstructural heterogeneities and the effect on behaviour-elastic anisotropy, morphology and crystallography with the idea of the enforced loading [9].Numerical analysis has been finished by business programming ANSYS utilizing Finite element technique, in which conduct of crack deformations are delineated based microscopic and macroscopic structures in impact temperature reliance of yield strength [10].In an investigation, specimens were tried by scanning electron microscopy (SEM) to decide the morphology of deficiency.

EXPERIMENTATION 2.1 Test Material
Alloy 617 was custom-melted at the Huntington Alloys Corporation, West Virginia by vacuum-induction-melting (VIM).Rectangular and round bars were fabricated using forging and hot-rolling.After the hot-rolling operation, the thickness of rectangular bars was reduced by cold-rolling.However, residual stresses were developed from hot and cold rolling operations, which were relieved further by thermal treatment.This thermal treatment consisted of solution-annealing at 2150°F (1175°C) for variable time periods depending on the thickness of the processed bars.It produces large sized austenitic grains in alloys.After heat treatment by solution-annealing at 1163°C (2125°F), rapid cooling was done which provides a fully austenitic microstructure.Table 1 and 2 provides the chemical composition and room temperature tensile properties of the material, respectively.Fracture toughness (J1C) of the alloy was determined by a compact-tension (CT) specimen of 25.4 mm thickness with a straight through notch.In the machining process, the longitudinal rolling direction was normal to the crack plane.Figure 1.describes the configuration and pictorial view of the test specimen.

Test Procedure
Test specimens and experimental setup were done as per the procedure prescribed by the ASTM Designation E 313-1989 [12].Initially, test specimens were designed to evaluate the plane strain fracture toughness [11] based on the linear-elastic fracture-mechanics (LEFM) concept for Alloy 617 [12].to comply with the LEFM criteria significantly ticker 20" compact tension (CT) specimens were desired and from practical standpoint it was not feasible at all.Specimens cost, machine grip, test machine max, pulling capacity and furnace chamber dimensions would not permit this thick specimen to test.Thus, to evaluate the fracture toughness in terms of J1C, 1 inch thick multiple CT specimens were tested using elastic-plastic-fracture-mechanics (EPFM) concept.For high strength superalloys like Alloy 617, two types of testing methods exist, namely single-specimen technique and multiple-specimen technique.Five specimens were needed for multiple specimen technique but only one specimen was required for single specimen technique to determine J1C value in a particular test temperature.All tests were conducted using single specimen technique at temperature ranging from ambient to 500 0 c.Instron testing machine (max capacity of 100 kN) and J1C fracture toughness software [13] provided by Instron Corporation was used to calculate and validate the fracture toughness value in terms of J1C .The compact tension specimen was pre-cracked to an approximate of 3 mm at room temperature using a load ratio(R) of 0.1 and frequency of 1 Hz.Maximum load for pre-cracking was maintained at 20 KN and number of cycle for pre-crack tanged from 55000 to 70000.Just after pre-cracking, CT specimens was subjected to 30 loading and unloading cycles to measure load line displacement (LLD) or crack opening displacement (COD).Maximum travel distance of a high temperature knife-edge extensometer was kept ±2  to measure the gap between the precracked specimens.Experimental setup and a standard load versus LLD plot is shown in the Figure 2 and 3 respectively.The shaded area in Figure 3(b) under the curve represents the energy (J-integral) required to cause each increment of crack length.Fracture toughness value in terms of J-integral for each small area was calculated using the following equations.The J-Integral value for each area was calculated using Equations 1-3, given below [12,14].CT Specimen

Extensometer
The combined elastic and plastic value of J then plotted against crack extension as shown in Figure 4 Loading compliance principle was used to evaluate each sequence of crack extension (  ).The following compliance equation 4 was used for   calculation [12].

J1C values
The validity criteria set by the ASTM Designation E 813-1989 was maintained in the J1C testing procedure.JQ values were obtained from it.In Figure 6., average J1C values are shown for temperature ranging from 30°C to 300°C.It is observed that J1C values decrease with increasing temperature.The effect is higher for temperature up to 100°C (156 to 103 kJ/m 2 ).However, from 200°C to 300°C, the decrement is insignificant (88 to 86 kJ/m 2 ). Figure 7. displays a load vs. LLD plot and a J-Integral vs. Δa plot is shown in Figure 8.The fracture toughness values are estimated using the J integral approach.Table 3. shows equivalent K1C and the CTOD values for Alloy 617 and Alloy 276.These values match with open literature.Further, the calculated δ values for alloy 617 were very close to a range in CTOD values (0.1 to 0.2) for an adequately tough material [17].

Tearing modulus values
The ASTM designation E 8-01 was used to establish the Young's modulus for Alloy 617.The tearing modulus for this alloy varies with temperature and is represented in a semi-logarithmic scale [18].By averaging the ultimate tensile strength and yield strength the flow stress is obtained with the highest uncertainty between all conditions.Based on recent research [19] large stretch zone corresponds to large tearing moduli.The furnace setback limited the study to be conducted at a specific temperature range (up to 500 0 C); however, the tearing modulus maintained to remain close in these temperature constraints resulting, in accurate results corresponding with the study.The values obtained show a high accuracy with the conducted study [19].A total of 20 paths were defined around the crack tip for J Integral estimation in all simulations.The J Integral values for alloy 617 and alloy 276 at varying temperatures are shown in Fig. 13 and Fig. 14 respectively.It was observed that as the temperature increases the J values do not converge in the near field paths due to an excessive drop in the yield stress and enhanced plasticity.An increase in J values was recorded for increasing temperatures [20] as shown in Fig. 14.That also points out to the fact that the path independency of the J integral could not be maintained at higher plasticity [21].This is shown in Fig. 14 where the J values do not converge easily to a single value but show a path dependency at 400 o C. Path independency for J integral was observed even until 500 o C for the alloy 617 but as the temperature reaches 400 o C for alloy 276 the J values show path dependence.Cracking deformation behaviour in nickel-based superalloys with stress and strain characteristics are correlated by finite element analysis of microscopic and macroscopic structures [10].

Fractographic Evaluations
Figure 15 indicates the SEM micrographs of alloy 617 that were sampled and experimented at room temperature.Three different sections are illustrated by the micrographs each causing a change in morphology.Fatigue precracking results in mild striations in one region; however, the vivid striations are caused by loading and unloading sequences and the last region shows dimples on the surface which result from fast rupture.Changes in microstructure, as well as applied strain range due to high temperatures, causes a decline in fatigue life in Alloy 617 [22].

Figure 5 :
Figure 5: Determination of JQ Using J-Integral versus Δa Plot

3. 4
Finite element analysis of experimental workHalf CT specimen shown in Figure.10 has meshed with 1922 quadrilateral 2D solid elements in ANSYS.A load of 30 kN was applied at the inside surface of the specimen hole as shown with arrows.30 kN load was distributed equally over 15 nodes with each node subjected to 2 kN force.A symmetric boundary condition is shown for the half specimen.Principal stress plots for alloy 276 at room temperature and at 400 o C are shown in Figures11 and 12respectively.As expected the plastic zone size at 400 o C is larger compared to the plastic zone size at 30 o C.

Figure 11 :Figure 12 :Figure 13 :
Figure 11: Principal stress distribution in CT specimen for Alloy 276 at 30 o C Figure 12: Principal stress distribution in CT specimen for Alloy 276 at 400 o C

Figure14: J Figure 15 :
Figure14: J Integral variation with increasing temperature for alloy 276