Miniature Specimen Testing Research Articles

P92 and 15CH2NMFA Steels – A Comparison of Fatigue Characteristics obtained on standard and miniature test specimens

In this paper, the results of the fatigue tests of the P92 and 15CH2NMFA steels are presented and discussed. The fatigue S – N curves were obtained by means of both standard and miniature test specimens. The Small Fatigue Test (SFT) specimens were manufactured partially by machining and also by waterjet cutting.The fatigue tests were performed on Amsler 10 HFP 5100 ZWICK//Roell high-frequency pulsator at room temperature and R = 0,1 and R = -1 cycle asymmetries. The results obtained on standard and SFT specimens are compared and in case of P92 steel are also compared the two methods of SFT specimens manufacturing.In case of 15CH2NMFA steel, the results of standard and SFT specimens are in a very good agreement both within the time limit fatigue range and the fatigue limit. After taking the stress concentration of SFT specimens into consideration, the results of traditional fatigue test samples and SFT samples are similar.In case of P92 steel, the leaning branch of S-N curves is much steeper for SFT specimens and the fatigue limits lower than it could be expected after the stress concentration is taken into account. The fatigue limits for equal cycle asymmetry show the same ratio of standard to SFT fatigue limits, for R = 0,1 it is 1,31, for R = 1 it is 1,32. This very good agreement can be used for the evaluation of fatigue behaviour of P92 steel by means of SFT specimens.The comparison of the two ways of SFT samples manufacturing revealed a scatter of fatigue test results, nevertheless the fatigue limits differ by 5 MPa only, so that even the influence of the manufacturing seems to be negligible.

A Novel Hybrid Heating Method for Mechanical Testing of Miniature Specimens at Elevated Temperature

A novel hybrid heating method which combines the conventional electric-resistance specimen heating with microcoil heating of specimen ends to achieve uniform heating over the gauge length is presented. Resistive heating of a miniature specimen develops a parabolic temperature profile with lowest temperature at the grip ends because of the heat loss to the gripper. Coil heating at the specimen ends compensates for this heat loss resulting in uniform temperature distribution over the central segment of the specimen. Thermo-electric finite element simulations were carried out to analyze the transient and steady temperature distribution in miniature specimens followed by experimental validation.

Miniature test techniques for life management of operating equipment

Life management of operating equipment is always a preferred way for maximising its operating life effectively and safely. Many novel test techniques, based on hardness or indentation methods, are used for assessing the material properties of the equipment to ensure its safe operation. The novel test techniques are correlation based indirect test methods to provide tentative material properties, which help in determination of residual life of the equipment. In comparison to these indirect test techniques, residual life estimate based on miniature test techniques are unique and precise as they give direct material data from the equipment. This is achieved by conducting destructive test of miniature test specimens, which are made from the material scooped out from the operating equipment. The critical issue is reliability and acceptability of these results from miniature test specimens in absence of any international standards. Considerable work is going on in this direction for making the miniature test techniques as standard test techniques, which will make it acceptable to all concerned enabling them to exploit many of its advantages. Even though various applications of miniature test techniques are found in literature, this paper discusses one of the important industrial applications of the technique which is in the field of life management of operating equipment. Due to lack of standardisation, there are many issues with the miniature test techniques, which must be addressed prior to its application in any area. This paper discusses the issues with some of the miniature test techniques, viz. tensile test, fatigue test and small punch test techniques. These issues have been addressed using experimental, analytical and metallurgical methods to make the techniques suitable for application in life management of equipment. At the end, scope of application of miniature test techniques in development of a new material, named as functionally graded materials (FGM), has been discussed.

SPD Processed Materials Mechanical Properties Determination with the Use of Miniature Specimens

The main reason why new technologies and treatment procedure are being developed is to attain special mechanical properties. However, these developments are nowadays done on a small material volume either using some laboratory simulators, applying sever plastic deformation procedures or chemical composition screening for multicomponent alloys development by laser or electron beam melting. In all these application a small volume of the material assessed is available and standard procedures for crucial mechanical properties determinations are not applicable. Thus small size techniques should be applied. There has been extensively used small punch test technique (SPT) for those cases in recent years. This technique is mainly based on the evaluation using correlation between standard and SPT tests for considered material. In cases when insufficient material volume is available, those correlations cannot be established and thus comparative evaluation only can be carried out. This kind of evaluation is insufficient for the contemporary purposes, when full material potential is to be utilized. Therefore, procedures providing results directly comparable with standard specimens are being developed. Fundamental properties are those determined from tensile tests. The current paper is presenting application of developed miniature tensile test specimen method to materials after SPD processes. Quasi static properties determination is shown here for Magnesium and Titanium alloys for ECAP and Rotary Swaging SPD techniques. The results obtained from testing can be used not only for a direct material properties assessment and comparison, but also as input data for FEM codes, significantly increasing the materials considered application potential assessment.

Quasi-static Tensile and Compressive Behavior of Nanocrystalline Tantalum based on Miniature Specimen Testing—Part I: Materials Processing and Microstructure

Grain size reduction of metals into ultrafine-grained (UFG, grain size 100 nm < d < 1000 nm) and nanocrystalline (NC, d < 100 nm) regimes results in considerable increase in strength along with other changes in mechanical behavior such as vanishing strain hardening and limited ductility. Severe plastic deformation (SPD) has been among the favored technologies for the fabrication of UFG/NC metals. Primary past research efforts on SPD UFG/NC metals have been focused on easy-to-work metals, especially face-centered cubic metals such as copper, nickel, etc., and the limited efforts on body-centered cubic metals have mainly focused on high strain rate behavior where these metals are shown to deform via adiabatic shear bands. Except for the work on Fe, only a few papers can be found associated with UFG/NC refractory metals. In the first part of the present work (Part I), high-pressure torsion (HPT) is used to process UFG/NC tantalum, a typical refractory metal. The microstructure of the HPT disk as a function of radial location as well as orientation will be examined. In the subsequent part (Part II), the location-specific mechanical behavior will be presented and discussed. It is suggested that refractory metals such as Ta are ideal to employ SPD technology for microstructure refinement because of the extremely high melting point and relatively good workability.

Quasi-static Tensile and Compressive Behavior of Nanocrystalline Tantalum Based on Miniature Specimen Testing—Part II: Mechanical Properties

In Part I of this work (this issue), we presented the microstructure of tantalum processed by high-pressure torsion (HPT). In this part, we will present results based on site-specific micro-mechanical testing. The experimental techniques were used due to the intrinsic microstructure gradient associated with HPT processing. The primary objective is to explore the grain size effect on the quasi-static mechanical properties of HPT processed tantalum with ultrafine grained (UFG, grain size d 100 nm) and nanocrystalline (NC, d < 100 nm) microstructure. Two distinct deformation modes are observed, i.e. a homogeneous (non-shearing) region and a localized (shear banding) region. Transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) show that the shear bands form by grain rotation. Comparing d in these two regions to the mechanism proposed in the literature shows that reduced d in the shear banding region is more susceptible to localized shearing via grain rotation. This work unifies, or at least further substantiates, the notion that body-centered cubic metals with UFG/NC microstructure tend to have localized shear band even under quasi-static uniaxial compression.

Special Testing Equipment and Validation of Measurement Methodologies for High Temperature Low Cycle Fatigue Testing of Miniature Metallic Specimens

A technique for high temperature low cycle fatigue testing of metallic materials has been developed, to determine fatigue behaviour through the testing of miniature specimens. The miniature specimen geometry was specifically designed, such that it could be manufactured from a small volume of material removed by chain-drilling extraction. An extensometry method to measure and control strain at the specimen shoulders during testing was adopted. This was undertaken to minimise the deleterious contact effects that can occur via extensometry attached at the gauge length of specimens, hence leading to premature failure and inaccurate fatigue data. By the application of this technique, the high temperature low cycle fatigue behaviour of 2.25Cr-1Mo steel was successfully characterised at 540 °C, under a fully reversed strain-controlled regime. The fatigue properties of the steel obtained from testing miniature specimens were shown to correlate well with existing literature for the material under comparable conditions, as determined by the testing of conventional standard-sized specimens.

Engineering Framework to Utilize Miniaturized Charpy Type SE(B) Specimens to Predict Jc of Full Sized Specimens

This paper introduces our experience of using miniature Charpy type SE(B) specimen in obtaining fracture toughness Jc of a material in the ductile to brittle transition temperature (DBTT) region. Width W x thickness B of 2 x 2 mm, 3 x 3 mm and 10 x 10 mm were chosen as miniature specimens and 25 x 25 mm were chosen as full sized specimen. 0.55% carbon steel JIS S55C, whose tensile to yield stress ratio σTS/σYS was equal to 1.8 was chosen as a material to simulate a degraded (embrittled) material in the DBTT region. Focus was placed on whether cleavage fracture could be predicted for these miniaturized specimens. Another focus was placed on whether the Jc of full sized specimen is predictable from the test results of the miniature sized specimens, in case cleavage fracture were observed. The results showed that the modified Ritch-Knott-Rice (RKR) failure criterion (which predicts the onset of cleavage fracture when the crack opening stress measured at 4 times the crack-tip opening displacement exceeds this σ22c) could predict whether cleavage fracture would occur or not. Another finding was that, in case cleavage fracture was observed though, the critical value σ22c in the modified RKR failure criterion was independent of specimen size, and thus, Jc of the full sized specimen is predictable from the miniature specimen test results, though M = (W-a)σYS/Jc was smaller than ASTM E1921 requirement of 30. Here, a and σYS are crack length and yield strength, respectively.

Assessment of Creep Strain Distribution Across Base Metal of 316LN Austenitic Stainless Steel Weld Joint by an EBSD-Based Parameter

Electron backscatter diffraction (EBSD) analysis has been used to estimate the accumulated strain in base metal region of 316LN austenitic stainless steel weld joints, creep tested at 923 K (650 °C), and at stresses of 175 and 225 MPa. The variation in strength of weld metal, heat-affected zone (HAZ), and base metal-induced stress and strain gradients across the weld joint under creep exposure. Finite element analysis (FEA) of von-Mises stress distribution across the joint has been carried out on incorporating strength of different constituents of the joint, derived by miniature specimen testing techniques. The FEA simulations revealed preferential accumulation of von-Mises stress in the base metal region near to HAZ. The variation in accumulated plastic strain across the base metal has been estimated using a ‘crystal deformation’ (Cd) parameter which quantifies the orientation spread within a grain. This parameter was obtained by EBSD analysis carried out using a scanning electron microscope. The trend in variation of accumulated plastic strain across the base metal accounted well with the von-Mises stress variation, which causes plastic deformation. The plastic strain in the base metal in both the stress levels was found to accumulate preferentially near to the HAZ and reduced steadily toward the ridge at the end of specimen. Transmission electron microscopic study has been carried out to substantiate the findings of the EBSD investigation.

Experimental Investigation of Mechanical Behavior of Materials by Small Punch Testing of Miniature Disk Specimens

A procedure and experimental equipment are developed to perform small punch testing of miniature disk specimens. The procedure is experimentally tested on specimens of 45 steel. A comparative analysis is made of the known procedures for determining the yield stress from the small punch load–displacement curves for miniature disk specimens.

Mechanical properties of Zr–2.5%Nb pressure tube material subjected to heat treatments in α + β phase field

Heat treatment studies from α+β phase field were carried out on Zr–2.5wt%Nb alloy pressure tube (PT) material using quenching dilatometry. The effects of cooling rate and soaking temperature on the nature of β phase transformation and the morphological changes in the transformed products were studied using electron microscopy. The mechanical property variations in these heat-treated dilatometry specimens were studied using miniature specimen testing techniques such as Small Punch Test (SPT) and the Automated Ball Indentation (ABI) tests at room temperature. The effects of cooling rate and the soak temperature on the mechanical properties has been correlated with the microstructural features such as size and distribution of transformed products and their morphological characteristics. The present study has established that SPT and ABI techniques are sensitive to the microstructural changes resulting from different thermomechanical treatment and hence can be adopted to characterise the mechanical properties of pressure tube at various stages of manufacturing.

Size effect criteria on the small punch test for AISI 316L austenitic stainless steel

The miniature specimen test technique has been extensively studied for quantifying the properties of bulk materials with greatly reduced volume recently. In this paper small punch test (SPT) is used to evaluate effects of specimen thickness, grain size and thickness-to-grain size ratio (TGR) on mechanical properties of 25 samples of type 316L austenitic stainless steel with nearly the same crystallographic texture. Effective SPT yield and maximum loads were measured and correlated with yield and ultimate strengths of conventional tensile test. The results show that SPT is sensitive not only to grain size, but also to thickness and TGR. Size effects exist between SPT and standard-sized samples. The present work gives a size effect criterion of SPT for engineering application. In addition, the underlying mechanisms of these size effects are provided.

Significance and Mechanism of the Crack Initiation Process during Very High Cycle Fatigue of Duplex Stainless Steel

In the very high cycle fatigue (VHCF) regime, fatigue damage starts by the formation of slip bands within the softer austenite phase. By high energy X-ray diffraction it was shown that local austenite plasticity due to elastic/plastic anisotropy results in the gradual formation of residual stresses within the ferrite grains. Consequently, cracks are initiated either transgranularly or intergranularly at austenite/ferrite phase boundaries or ferrite/ferrite grain boundaries in the vicinity of the austenite slip bands. By means of ultrasonic fatigue testing of miniature specimens in combination with (i) in-situ observation, (ii) electron back- scatter diffraction, (iii) synchrotron computer tomography, and (vi) high-energy X-ray diffraction, the fatigue crack initiation process and the interactions of the cracks with the first microstructural barriers were quantified. By implementing the results in a numerical modeling approach, the experimental observation that the first barrier seems to limit the VHCF life was supported.

Use of Miniature Tensile Specimen for Measurement of Mechanical Properties

This paper gives a comparative study of methods used for evaluation of mechanical properties of materials using miniature and sub-size tensile test specimens. Aim of this paper is to establish the potential of miniature tensile tests which can be useful for life estimation of any in-service-equipment and for development of new materials. Both these applications intend to use very small amount of material for evaluation of the mechanical properties. Apart from various types of novel techniques developed worldwide, the evaluation of mechanical properties from a miniature tensile test has a greater advantage as it is a direct method of measurement of mechanical properties. The paper also discusses various challenges in fabrication of miniature tensile test specimens, testing methodologies and acceptance of the test results. A comparison with the test results from conventional size specimen has been done for establishing the suitability of the miniature test specimen. For establishing the geometrical design of the miniature test specimen and its behaviour over application of tensile load, the tests have been corroborated with a finite element based analysis.

Characterization of the creep properties of heat resistant 9–12% chromium steels by miniature specimen testing

A miniature specimen geometry is presented that allows to extract small samples for tensile creep tests directly from critical components of steam generators in power plants like e.g. superheater tubes. In this way, material can be tested which has been subjected to the full processing chain of the component. The specimens then exhibit all microstructural modifications that are present in the component after the shaping process and heat treatment, and representative mechanical properties are determined. Similarly, specimens can be extracted from pre-corroded test pieces or used components after service to determine the impact of complex aging/loading/oxidation conditions during service on the mechanical properties of the material. The applicability of the miniature specimen test method is demonstrated in a comparative creep study involving standard and miniature specimens of P91 tempered martensite ferritic steel. Test results indicate satisfactory accuracy/repeatability of the method. Comparison with large scale specimen data reveals an influence of specimen size on the obtained creep behavior. These size effects need to be considered for a correct interpretation of results from miniature specimen creep tests.

Determination of Mechanical Properties of Aged NPP Components Using Instrumented Hardness Testing and Other Miniature Specimen Testing Techniques

Analyses of instrumented hardness test and small punch test are done using experimental tests and finite element simulation. These methods can be used when some material properties are unknown and there is no way to cut larger pieces from the structure to determine them using standard size specimens.

소형펀치시험법을 이용한 알루미늄 6061-T6 Type-3 용기 라이너의 파괴인성 평가

Type-3 용기 라이너는 두께가 5 mm일뿐 아니라 곡률을 가지고 있기 때문에 파괴인성시험을 위한 표준형 시편의 제작에 어려움이 있다. 따라서 용기 라이너의 파괴인성을 평가하기 위해서는 소형시편 시험기법이 도입되어야 한다. 본 연구에서는 용기 라이너의 파괴인성을 평가하기 위해 소형펀치(small punch, SP)시험법을 도입하였다. SP시험으로부터 측정된 하중-변위 곡선 결과에서 라이너 재료는 연성이 소진하여 막신장 영역에서 파손되었다. 라이너 재료는 제작공정 특성에 기인하여 소성변형량이 방향에 따라 다름에도 불구하고 등방성 파괴양상을 보였다. SP시험 데이터를 사용하여 파괴인성을 평가한 결과 <TEX>$J_{Ic}=13.0kJ/m^2$</TEX> 이었다. 이 값은 동종재료의 파괴인성 값과 유사하다. 따라서 SP시험 데이터를 사용하여 평가한 파괴인성은 타당하며, 완성된 용기 라이너는 유효한 파괴인성 값을 유지하는 것으로 확인 되었다. Type-3 cylinder liner has a limitation of machining the standard specimen for fracture toughness test because it has approximately 5 mm in thickness as well as a curvature. Hence, it needs to be employed a miniature specimen test technique to evaluate fracture toughness of the cylinder liner. In this study, small punch (SP) test method was employed to evaluate fracture toughness of the cylinder liner. Load-displacement curve result measured from the SP test showed that the liner material was failed during membrane stretching in the general SP load-displacement curve. Additionally, it was shown that liner material was isotropic although the amount of plastic deformation was different depending on the direction due to manufacturing process characteristics. Fracture toughness, <TEX>$J_{Ic}$</TEX>, was evaluated using the SP test data. The value of fracture toughness obtained was <TEX>$13.0kJ/m^2$</TEX>. This value was similar to that of the same kind of materials. Therefore, the fracture toughness evaluated using the SP test data was reasonable.

Use of Miniature Specimen Test and Finite Element Method for Evaluation of Yield Strength and Fracture Toughness in Metallic Alloys

Abstract The present paper describes the method for evaluation of yield strength and fracture toughness of three materials, namely, linepipe steel, aluminum (Al) alloy, and gray cast iron using miniature specimen test. Conventional standard test methods consume more material and money and may damage the integrity of the in-service component. Miniature specimen tests overcome all these demerits. A method is outlined to predict the yield strength and fracture toughness from the miniature test through the empirical relations. The validation of results has been carried out by conducting standard tensile and fracture toughness tests according to ASTM standards. Further, the finite element method has been used to simulate the miniature test. A close agreement is observed between the results of the miniature test and standard tests.

Development of a shear strength test method for NITE–SiC joining material

In this study, the evaluation of shear strength for joined SiC ceramics was conducted by using a torsion test. Two specimen types based on F734-95 ASTM Standard and one proposed for miniature test specimen in this study were prepared. The effects of specimen size and geometry were investigated by using finite-element stress method (FEM) analysis. The specimens were joined by the NITE process with mixed powder of Al2O3, Y2O3, SiO2 and SiC as joining adhesive. Joining was carried out at 1800°C for 1h, under an applied pressure of 20MPa by hot pressing. The fracture surfaces after torsion test were observed by using optical microscopy.

A Critique on Evaluating Fatigue Crack Propagation Properties Using Miniature Specimens

There are situations where baseline fatigue crack propagation (FCP) data are needed but the available material for its evaluation is far from enough to fabricate full sized standard specimens. Miniature specimen testing techniques have been proposed but published data seems to be slightly but consistently slower under the same stress intensity than that from standard specimen results. In this work, an aluminum alloy and a steel, which have vastly different mechanical behaviors, are used to generate different stress states and premature crack closure to allow the validity of miniature specimen FCP data to be critically examined.