Small Punch Tensile Research Articles

Evaluation of effect of spheroidization heat treatment on mechanical properties via small punch test

Due to the long time high temperature thermal action, pearlite heat-resistant steel 15CrMo is prone to pearlite spheroidization, which reduces its strength and makes it difficult to meet the actual service requirements. Therefore, quantifying the effect of pearlite spheroidization is an important factor in evaluating the high temperature performance of 15CrMo steel. In this study, the mechanical properties of 15CrMo steel were tested by uniaxial tension (UT), small punch tensile (SPT) test; the microstructure of 15CrMo steel after spheroidization heat treatment (SHT) was obtained by image processing technique to obtain the geometric characteristic parameters of pearlite spheroidization phase (PSP); the influence of spheroidization characteristics and properties were analyzed by mathematical and statistical analysis, and the results showed that the PSP size had the greatest influence on the properties; based on this, the linear relationship models between the spheroidization parameters and the mechanical properties of UT and SPT results were established with high accuracy of prediction.

Machine learning method for estimating the defect-related mechanical properties of additive manufactured alloys

The mechanical properties of the additive manufactured (AM) nickel-based alloys: GH4169 and Hastelloy X were estimated via small punch tensile (SPT) tests. The surface defects of SPT samples were visualized and parameterized via processing specimen images. Statistical analysis and mathematical models indicated the high sensitivity to AM defects and test characteristic parameters, especially the size of AM defects and the thickness of the SPT specimen. Besides, the defects and test parameters were combined to estimate the mechanical properties of AM Ni-based alloys via machine learning (ML) models, which displayed higher accuracy compared with traditional models. Moreover, for solving the scarce data and improving accuracy, a data-generated model (DGM) was employed and proved with high reliability and accuracy.

Uncertainties in and recommendations to small punch tensile and creep tests for ductile materials

The objective of this paper is to clarify the complex nature which involves the interaction of several non-linear deformation mechanisms experienced in small punch tensile and small punch creep tests, and as a consequence, the associated difficulties in data interpretation. The paper starts with a highlight of some specific features experienced during small punch deformation, e.g., large deformation, high local plasticity, contact, the effect of initial plasticity, possible early cracking etc. and the consequences of them in data interpretation. Although several test standards have been available, uncertainties in data interpretation still exist and a universally accepted conversion method has not been available. On this basis, for the first time, critical comments on the uncertainties and applicability of the test methods in determining the tensile and creep properties and in high-temperature structural integrity assessment are addressed. Finally, recommendations on future developments are provided.

A solution for estimating high-temperature strength based on additive manufacturing characteristics

Large sizes and destructive items hindered standard uniaxial testing for in-service parts manufactured by laser additive manufacturing (LAM). Therefore, small-scale testing techniques, such as small punch tensile (SPT) testing, were widely employed to assess local mechanical characteristics. In this study, layer-by-layer fabrication of Hastelloy X was produced via LAM-assisted ultrasonic micro forging treatment (UMFT). The microstructure and fracture of components showed that UMFT successfully reduced LAM defects; the defect distribution characteristics of the sample blocks and SPT specimens were determined by the extreme value method (EVM); the effect of defect characteristics on strength was evaluated by Pearson correlation coefficient (PCC); the SPT strength estimation models based on the LAM characteristics were proposed using the surface defect distributions of SPT specimens. The results of various error assessments showed that the proposed models had better accuracy and stability than the classical SPT models; based on the distribution of defects in LAM blocks, the total strength estimation error of LAM Hastelloy X was within 2%.

Characterisation of deformation process and fracture mechanisms of P91 steel at 600 °C in small punch tensile testing

This paper investigates the microstructural evolution and fracture mechanism of the P91 steel during small punch tensile tests. Disc specimens, 8 mm in diameter and 0.5 mm in thickness, were tested in a small punch test rig at 600 °C using a constant displacement rate of 2 μm/s. Interrupted small punch tensile tests were performed to investigate the microstructural evolution in different deformation regimes. Deformed specimens were characterised by scanning electron microscopy and electron backscatter diffraction. Microstructure characterisation showed that block boundary alignment occurred in the plastic deformation direction, before achieving the maximum punch load. It was revealed that the martensitic structure was recovered when the tests progressed. The elongation of recovered grains occurred after achieving the maximum punch load. In addition, it was shown that the voids tended to nucleate along the recovered grain boundaries and became elongated to align with the plastic flow within the specimen.

The effect of oxidising thermal exposures on the fatigue properties of a polycrystalline powder metallurgy nickel-based superalloy

A number of thermal exposures were undertaken to determine the effect oxidation has on the fatigue lives of the nickel-based superalloy RR1000. The thermal exposures initially caused a considerable reduction in the fatigue lives with increasing oxidation. However, the longest exposure time resulted in an S-N curve that lay between the shortest and mid length exposure. A range of analyses were undertaken to determine the presence of any mechanisms that could have caused this unexpected life increase; vacuum thermal exposures, small punch tensile, nanoindentation, microscopy and transmission Kikuchi diffraction. It was found that few of the mechanisms investigated were likely to have an effect on the change in life with the exceptions being; reduction in dislocation movement and crack deflection as a result of a recrystallised zone at the surface, variation in hardness of different regions due to a γ′ depleted plastic zone and a hard ceramic oxide, and reduced initiations due to the presence of a ‘healing’ chromium oxide scale. Notch fatigue tests were performed to determine the importance of the findings to components in service and it was found that the notch acted as the cause of crack initiation, effectively mitigating against the effects of the oxidation damage.

An experimental and numerical study of a CoNiCrAlY coating using miniature specimen testing techniques

Small punch tensile (SPT) tests have been carried out for a CoNiCrAlY coating at room temperature (RT), 500 °C and 700 °C. Highly re-producible results have been obtained for SPT at 700 °C while the formation of early cracking at lower temperatures tends to compromise the repeatability of the tests. An alternative novel miniature specimen testing method has been developed and used for the miniature tensile tests of the CoNiCrAlY coating in the same temperature range. Good agreements have been achieved between the SPT and miniature tensile testing results regarding the ductile to brittle transition temperature (DBTT). Excellent repeatability has been achieved over the full range of the temperature for the miniature tensile tests. An inverse approach has been developed and used to extract the mechanical properties of the material from the miniature tensile tests, using a temperature-dependent Johnson-Cook model. Finite element (FE) modelling of the SPT tests, including damage evolution, has been carried out using the extracted material properties to give comparable predictions of the SPT testing results, such as the load-displacement curves and the approximate locations of the fracture failures.

The influence of microstructure on the ductile to brittle transition and fracture behaviour of HVOF NiCoCrAlY coatings determined via small punch tensile testing

The development of new thermal barrier coatings (TBC's) capable of increasing the efficiency of gas-powered turbines requires an understanding of how the tensile behaviour and ductile to brittle transition temperature (DBTT) of MCrAlY bond coats are influenced by the coating microstructure. In this study, small punch tensile (SPT) tests were conducted on two high velocity oxy-fuel (HVOF) NiCoCrAlY coatings. Both coatings, referred to as BC1 and BC2, comprised a BCC β-NiAl matrix with FCC γ-Ni and TCP σ-Cr2Co secondary phases. Coating BC2 also contained FCC γ′-Ni3Al. Small punch tensile (SPT) tests were conducted on the coatings between RT and 750 °C. The DBTT's of coatings BC1 and BC2 were found to be 600–700 °C and 650–750 °C respectively. Lower phase fractions of γ-Ni were shown to increase the DBTT. The main mode of crack propagation in both coatings was intergranular fracture along the grain boundaries of differing phases. Schematic models were used to demonstrate the change in tensile behaviour across the DBTT and explain the influence of the coating microstructure on the fracture behaviour of both coatings.

High Temperature Deformation Mechanisms in a DLD Nickel Superalloy.

The realisation of employing Additive Layer Manufacturing (ALM) technologies to produce components in the aerospace industry is significantly increasing. This can be attributed to their ability to offer the near-net shape fabrication of fully dense components with a high potential for geometrical optimisation, all of which contribute to subsequent reductions in material wastage and component weight. However, the influence of this manufacturing route on the properties of aerospace alloys must first be fully understood before being actively applied in-service. Specimens from the nickel superalloy C263 have been manufactured using Powder Bed Direct Laser Deposition (PB-DLD), each with unique post-processing conditions. These variables include two build orientations, vertical and horizontal, and two different heat treatments. The effects of build orientation and post-process heat treatments on the materials’ mechanical properties have been assessed with the Small Punch Tensile (SPT) test technique, a practical test method given the limited availability of PB-DLD consolidated material. SPT testing was also conducted on a cast C263 variant to compare with PB-DLD derivatives. At both room and elevated temperature conditions, differences in mechanical performances arose between each material variant. This was found to be instigated by microstructural variations exposed through microscopic and Energy Dispersive X-ray Spectroscopy (EDS) analysis. SPT results were also compared with available uniaxial tensile data in terms of SPT peak and yield load against uniaxial ultimate tensile and yield strength.

Small Punch Testing of Powder Bed Direct Laser Deposits

Additive Layer Manufacturing (ALM) technologies, such as Powder Bed Direct Laser Deposition (PB-DLD), have gained increasing popularity within the aerospace industry due to the advantages they hold over conventional processing routes. Among the advantages are the ability to produce more sophisticated cross-sectional geometries, a decrease in production lead times and an improvement to the buy-to-fly ratio. However, build quality and microstructural characteristics have a dependency on the process variables such as build direction. In order to understand the influence of grain size and build orientation on tensile behaviour, the Small Punch Tensile (SPT) testing technique has been applied to variants of the nickel based superalloy C263, manufactured using the PB-DLD method. The test technique utilises miniaturised samples, requiring only small volumes of material and is therefore a desirable test method to employ. SPT testing has characterised the mechanical properties between vertically and horizontally built PB-DLD C263 in comparison with the cast material derivative. Differences in mechanical performance between each variant have been revealed and found to be associated with microstructural variations. The deformation behaviour across each material variant have been exposed by interrupted tests. SPT results have also been accompanied by fractography, fracture energy calculations along with comparisons with uniaxial data.

The Application of the Small Punch Tensile Test to Evaluate the Ductile to Brittle Transition of a Thermally Sprayed CoNiCrAlY Coating

Thermally sprayed MCrAlY bond coats are important elements of thermal barrier coating (TBC) systems which are applied to the surface of gas turbine components to protect them in high temperature environments. Knowledge of their mechanical properties is essential in preventing TBC failure which can have catastrophic consequences. However, limited data on modulus, strength and ductility are available for such coatings. In this work, the ductile to brittle transition behaviour of a CoNiCrAlY coating has been investigated via the small punch tensile test (SPTT). Displacement controlled tests were carried out on free standing coatings at room temperature (RT) and between 400-750 °C at a rate of 1 μms-1. At low temperatures there was evidence of elastic-brittle behaviour and at high temperatures there was clear evidence of yielding and plastic deformation. The ductile to brittle transition temperature was found to be between 500-750 °C. The yield stress ranged from 1000-1500 MPa below 600 °C to less than 500 MPa above 650 °C. The elastic modulus was found to be approximately 200-230 GPa at 500 °C and 55 GPa above 700 °C. At room temperature the fracture surface showed flat, smooth features indicating brittle failure whereas at 700 °C there was evidence of ductile tearing.