Miniature Specimen Testing Research Articles

Utilization of High-Temperature miniaturized testing to Assist in life management for gas turbine Blades: An Overview

Safely operating gas turbine blades in aero-engines or industrial gas turbines under severe thermo-mechanical conditions always poses a continuous challenge. There is an urgent need for more reliable life prediction tools, necessitating earlier and more rigorous assessments of in-service damage. This paper introduces a systematic examination of high-temperature miniature specimen testing method within a practical life management framework. To improve current practices in assessing the condition of service-aged gas turbine blades, a proactive utilization of miniature specimen testing is recommended for early in-service assessment within the component lifecycle and integrated into a comprehensive life assessment/management framework. The paper outlines a structured approach to life management, combining miniature specimen sampling and creep and fatigue testing methods. This forms the basis for a novel, holistic life assessment methodology, which is proposed in this paper, incorporating the innovative use of high-temperature miniature specimen testing.

On comparison of fracture toughness of irradiated and thermally aged decommissioned nuclear weld metals with miniature specimen test technique

This paper investigates the fracture toughness of thermally aged and irradiated weld metals extracted from head and beltline regions of a decommissioned nuclear reactor pressure vessel. Miniature compact tension specimens are fabricated from the thermally aged reactor pressure vessel head weld as well from the circumferential and axial beltline welds having fluences of 2.90 × 1016n/cm2 and 7.94 × 1017n/cm2, respectively. Master curve approach in accordance with the ASTM E1921 is applied to determine the reference temperature T0 and it is found to be − 113.5 °C, −104.4 °C, and − 89.3 °C for the reactor pressure vessel head, axial beltline, and circumferential beltline welds, respectively. In addition, the multimodal reference temperature Tm to assess the homogeneity of welds as well as key curve analysis for the quality assurance of testing are also provided. Fractographic analysis identified variations in crack initiation sites and microstructural inhomogeneities, particularly in the irradiated beltline welds. Overall, fracture mechanical testing of the materials demonstrated that miniature compact tension specimens are effective for characterizing the fracture toughness of limited surveillance weld materials, revealing differences due to irradiation and weld location.

Semi-automatic miniature specimen testing method to characterize the plasticity and fracture properties of metals

Miniature-testing is essential to characterize the plastic response of materials for which the extraction of conventional full-size specimens is impossible. Examples are weld regions with high property gradients or electronic circuit components. Here, we present the design of a novel testing system for miniature specimens. After inserting the specimens with wedge-shaped shoulders, the system carries out the clamping, initial geometry scanning, and mechanical testing with digital image correlation in an automated manner. The automation prevents the accidental damaging of the miniature specimens during manual handling and adds to the robustness of the overall procedure. It is shown that the experiments with miniature specimens extracted from DP600 steel and aluminum 7075-T6 are highly reproduceable. More importantly, they lead to the same plasticity and fracture properties as experiments on conventional ten-times larger full-size specimens. A correction procedure accounting for the surface roughness, white layer and heat affect zone induced through wire EDM is provided. In addition to developing and validating the new experimental technique, it is applied to characterize a 22MnB4 rod material and stainless steel 316 L foil.

Comprehensive Correlations for Small Punch Test Response Post-Processing Toward Preserved Mechanical Strength Estimation

Abstract The small punch test (SPT) approach is a miniature specimen testing technique to estimate the preserved mechanical strength of an in-service component to check its fitness for service. The SPT results are summarized in form of force-specimen deflection, (F − u) and force-punch displacement, (F − v) response. There are many standards published in an attempt to define a universally accepted approach for SPT-aided mechanical characterization. However, it was recognized that such standards were not concerned to practice a consistent approach while SPT response measurement and strength estimation toward outlining proclaimed best-fitting correlations. This paper narrates limitations caused by known inconsistent practices and proposed comprehensive correlations for accurate strength estimation for metallic materials which are exposed to 100–2000 MPa strengths.

Hardening and ductility loss of RAFM steel CLF-1 irradiated with high-energy heavy ions

Irradiation induced hardening and ductility loss of a Chinese reduced-activation ferritic / martensitic steel CLF-1 is evaluated by utilizing the high-energy heavy ion irradiation and the subsequent miniature specimen test technique. Specimens were irradiated with high energy 14N6+ and 56Fe17+ ions supplied by a cyclotron to generate a quasi uniform atomic displacement damage plateau from surface to a depth about 25 μm. Three successively increasing damage levels (0.05, 0.1 and 0.2 dpa) were approached at a low temperature around 223 K. The small punch test at room temperature showed that the ductility loss can be observed at the lowest damage level and increases with the increasing damage level, similar to the variation trend of yield stress with irradiation dose. The TEM observation revealed that the average size and number density of irradiation-induced defect clusters also increased with displacement damage. Assuming that the hardening is caused by the self-interstitial-type dislocation loops, an estimation by combining the numerical calculation of the number density / mean diameter of dislocation loops with the dispersion barrier hardening model yields a similar dose dependence of the irradiation hardening.

On the Corrosion Fatigue of Magnesium Alloys Aimed at Biomedical Applications: New Insights from the Influence of Testing Frequency and Surface Modification of the Alloy ZK60

Magnesium alloys are contemporary candidates for many structural applications of which medical applications, such as bioresorbable implants, are of significant interest to the community and a challenge to materials scientists. The generally poor resistance of magnesium alloys to environmentally assisted fracture, resulting, in particular, in faster-than-desired bio-corrosion degradation in body fluids, strongly impedes their broad uptake in clinical practice. Since temporary structures implanted to support osteosynthesis or healing tissues may experience variable loading, the resistance to bio-corrosion fatigue is a critical issue that has yet to be understood in order to maintain the structural integrity and to prevent the premature failure of implants. In the present communication, we address several aspects of the corrosion fatigue behaviour of magnesium alloys, using the popular commercial ZK60 Mg-Zn-Zr alloy as a representative example. Specifically, the effects of the testing frequency, surface roughness and metallic coatings are discussed in conjunction with the fatigue fractography after the testing of miniature specimens in air and simulated body fluid. It is demonstrated that accelerated environmentally assisted degradation under cyclic loading occurs due to a complicated interplay between corrosion damage, stress corrosion cracking and cyclic loads. The occurrence of corrosion fatigue in Mg alloys is exaggerated by the significant sensitivity to the testing frequency. The fatigue life or strength reduced remarkably with a decrease in the test frequency.

Non-contact temperature control and stereo digital image correlation for high-temperature testing of miniature tubular specimens.

Component failures very often occur due to high temperature and multiaxial stress states arising at critical component locations. To imitate such loading conditions, a multiaxial miniature testing system (MMTS) with axial, torsional, and internal pressurization capabilities for high-temperature testing of miniature tubular specimens has been developed. Among many challenges of developing the MMTS, uniform heating, temperature measurement and control, and surface strain measurement on a miniature tubular specimen at high temperatures have significant difficulties. This paper addresses two significant challenges: first, the development of a non-contact temperature control system using infrared thermography to uniformly heat a miniature specimen of 1 mm outer diameter (OD), and second, the development of a stereo digital image correlation (stereo-DIC) setup for strain measurement on the miniature specimen subjected to high temperature. The developed control system maintains the test temperature through a closed feedback loop and employs a fail-safe mechanism to protect the MMTS load frame components against unanticipated temperature rises. The thermocouple wire-size effect on the measured temperature was examined for three different wire sizes: 0.05, 0.25, and 0.5 mm for accurate emissivity determination required for infrared thermography. Emissivities of the specimen surface at different high temperatures were experimentally determined. Inherent error analysis of the developed high-temperature stereo-DIC setup showed acceptable strain measurement uncertainty. The effectiveness of the developed non-contact temperature control system and high-temperature stereo-DIC setup has been verified by performing tensile testing of a 1 mm OD specimen at 500 °C.

Optimized stereo digital image correlation setup for miniature round specimen: framework development and implementation

For the advancement of micro- and nano-technologies, multiaxial material testing at a small length scale is imperative. A novel multiaxial miniature testing system (MMTS) is under development for testing a tubular specimen of outer diameter (OD) as small as 1 mm. Because of the small specimen size of MMTS, stereo-Digital Image Correlation (DIC) is the preferred strain measurement technique. Although theoretically, stereo-DIC is length-scale independent, the implementation of stereo-DIC, particularly for miniature testing, faces experimental setup related challenges. For this reason, although stereo-DIC is strongly recommended over 2D DIC, researchers are sometimes compelled to use the latter. It is shown in the present study that the experimental setup related difficulties, particularly for miniature round specimen testing, can be overcome by a systematic development of a mathematical framework for stereo-DIC implementation. This framework addresses all setup decisions concerning stereo-DIC implementation, such as selections of stereo angle, speckle size, camera position, camera sensor size, lens focal length, dimensions of camera and lens bodies, calibration grid size, etc., as well as stereo-DIC analysis parameters, such as subset and step size. Besides serving the need of building a stereo-DIC setup for MMTS, since the developed mathematical framework treats all stereo-DIC setup decisions as variables, it can be used to develop an optimized stereo-DIC setup for any application. Examples of two general cases are reported. Since this general framework serves as a tool to solve the stereo-DIC experimental setup related challenges, the developed framework will contribute to the wider adoption of stereo-DIC over 2D DIC.

Indenter misalignment in impression creep test: Uncertainty, correction and recommendation

The impression creep test (ICT) method, as a miniature specimen test technique, has been used extensively to determine the in-service creep properties of power plant components. However, the experiment results of the ICT can be affected by some uncertainties associated with the inaccuracies of measurement, which have not been studied before. This paper presents some results of finite element analyses, to evaluate the effect of indenter misalignment on the results of the ICT, such as the steady-state creep deformation rate and the conversion parameters. The results obtained have shown that the angular misalignments of the indenter around horizontal directions have a relatively significant influence on the conversion parameters, and the misalignment around vertical direction has a much less significant effect. Empirical formulas were developed for the possible correction of the conversion factors. Finally, some recommendations on the control of the indenter and specimen alignment were given.

Pore-affected fatigue life scattering and prediction of additively manufactured Inconel 718: An investigation based on miniature specimen testing and machine learning approach

Fatigue life scattering and prediction of Inconel 718 fabricated by selective laser melting were investigated using miniature specimen tests combined with statistical method and machine learning algorithms. The relationship between pore features and fatigue life of the selective laser melting-fabricated specimens was analyzed statistically. The results show that the increase in the size and/or the number of the pores in the specimens, and/or the decrease in the distance from a pore center to the specimen surface degraded the fatigue life. The machine learning and statistical analysis results reveal that the fatigue life are most closely related to the location of the pores compared with the size and the number of pores in the specimens. The finding may provide a potential way to get high-throughput statistical data helping in evaluating defect-dominated scattering and prediction of fatigue life of additive manufactured metallic parts using miniature specimen testing assisted by the machine learning approach.

Evaluation of tensile properties of various regions of creep exposed dissimilar weld joint using miniature specimen testing

Strength variations in various regions of a dissimilar weld joint made between 316LN stainless steel (SS) and P91 steel after creep exposure at 550 °C and stress of 200 MPa was estimated using automated ball indentation (ABI) test. The ABI results were used to derive equivalent uniaxial tensile flow behaviour using established correlation factors. A comparison of the results obtained in the creep tested and post weld heat treated (PWHT) samples showed loss of strength in the P91 steel side. It was observed that, creep exposure under this testing condition strengthened the 316LN SS side of the weld joint. ABI derived results showed that, the inter critical-heat affected zone (IC-HAZ) of P91 steel is the weakest region across the weld after creep exposure. The narrow IC-HAZ created a strain constraint which caused the specimen to fail in the P91 steel base metal region.

Microstructural investigation and mechanical properties evaluation using miniature specimen testing of various constituents of dissimilar weld joint

Microstructural evolution in P91 (modified 9Cr–1Mo) steel and 316LN Stainless Steel (SS) bi-metallic weld joints fabricated by arc welding technique using Inconel-182 electrode has been studied in as-welded and post weld heat treated (PWHT) conditions. The PWHT brought about significant changes in the microstructure which altered the strength gradient especially in the ferritic steel side. The differences in weld thermal cycling which occurred in various regions of the P91 steel side caused M23C6 precipitates to evolve in varying sizes. The size variation in these precipitates was the paramount cause for the variation in strength in this region which could be probed by miniature specimen testing. Using this testing it could be established that PWHT resulted in offsetting the strength variation in the HAZ of the ferritic steel side. It could also be understood that PWHT generates a weak region in the P91 side making it most susceptible to damage.

Superplasticity in a multi-directionally forged Mg–Li–Zn alloy

Superplastic behavior of the Mg–8Li–1Zn alloy, processed by extrusion at 573 K followed by multi-directional forging (MDF) at 423 K, was studied using shear punch testing (SPT) of miniature specimens at various temperatures and strain rates. Microstructural analysis indicated moderate grain refinement, reducing the gran size from 8 μm in the initial as-extruded condition to 4 μm after 8 passes of MDF. Processing by MDF not only refined the grain structure but also provided a more homogeneous microstructure and a larger fraction of high-angle grain boundaries. SPT was performed at shear strain rates in the range of 3.3 × 10−3–1.3 × 10−1 s−1 and temperatures in the range of 498–573 K. The strain rate sensitivity (SRS) index (m-value), obtained from the slope of the central region of the sigmoidal shear stress–shear strain rate curves, was as high as 0.51 for the MDF processed material at 548 K. This is in contrast to the as-extruded material that exhibited a linear behavior with a maximum m-value of only 0.29 at the same temperature. For the MDF processed material, the activation energy of 61 kJ mol−1, which is close to 67 kJ mol−1 for the grain boundary diffusion in β-Li, and the high m-value of 0.51 suggest that the prevailing deformation mechanism is grain boundary sliding (GBS) controlled by grain boundary diffusion.

The Essential Work of Fracture parameters for 3D printed polymer sheets

Additive manufacturing is becoming increasingly popular in academia and industry. Accordingly, there has been a growing interest in characterizing 3D printed samples to determine their structural integrity behaviour. We employ the Essential Work of Fracture (EWF) to investigate the mechanical response of polymer sheets obtained through additive manufacturing. Our goal is twofold; first, we aim at gaining insight into the role of fibre reinforcement on the fracture resistance of additively manufactured polymer sheets. Deeply double-edge notched tensile (DDEN-T) tests are conducted on four different polymers: Onyx, a crystalline, nylon-reinforced polymer, and three standard polymers used in additive manufacturing – PLA, PP and ABS. Results show that fibre-reinforcement translates into a notable increase in fracture resistance, with the fracture energy of Onyx being an order of magnitude higher than that reported for non-reinforced polymers. On the other hand, we propose the use of a miniature test specimen, the deeply double-edge notched small punch specimens (DDEN-SP), to characterize the mechanical response using a limited amount of material. The results obtained exhibit good alignment with the DDEN-T data, suggesting the suitability of the DDEN-SP test for measuring fracture properties of additively manufactured polymers in a cost-effective manner.

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.

A Setup for Complex Investigation of Mechanical Characteristics of Structural Materials for NPP Equipment

The complex experimental equipment has been developed and manufactured in compliance with the existing standards for measuring hardness and strength characteristics via the method of instrumented indentation, as well as for short-term static uniaxial tensile tests of structural materials and small punch testing of miniature disk specimens in the macrorange of loads of 2–10,000 N. This allows one to obtain the necessary characteristics of hardness and strength in relation to the current state of materials of the main equipment at NPP and thereby minimize the volume of material used for the production of specimens on the same setup employing various normative techniques of both destructive and non-destructive inspections. The equipment is designed in the form of replaceable units: indentation, tension, and punching of miniature disk specimens, which are installed depending on the test method. As a result, there is a unified assembly that makes it possible to reduce the effect of instrument errors in the validity of the data obtained using different test methods and facilitate its modification according to the other tests. The developed experimental equipment has been tested using the segment of metal of the double ball pipe wall ∅990 × 70 mm (the basic metal is 10GN2MFA steel and the deposited material is 08Kh19N10G2B steel) of the Du850 main circulation pipeline of NPP from WWER-1000 reactor pressure vessel. The mechanical characteristics obtained by the methods of indirect control were compared with those obtained from standard tensile tests. At the same time, the deviation of the values of yield strength and ultimate strength obtained employing the indirect control methods from the values obtained in the standard tensile testing is within the limits recommended by the normative documents.

Recent Developments in Small Punch Testing: Interlaboratory Tests and Standardization

Small specimen testing techniques are developed mainly for characterization of the structural materials with a limited volume of material. One of the most used miniature specimen testing techniques is the small punch (SP) test which is based on the testing of thin disc shaped samples. From the operational point of view SP test can be considered as a non-destructive method, because the material volume cut from industrial component is not diminishing its structural integrity. This benefit can be significant for the use in nuclear power generation sector, as many components are considered as irreplaceable and conventional testing techniques are not applicable for the determination of current mechanical properties. Although SP testing has been used for more than 40 years, there are still many research and development subjects. The article describes the SP test method as a suitable supplement to classical methods for evaluating mechanical properties that are fundamental for the nuclear power plant integrity and operational lifetime assessment processes. Main part of the article is focused on the current status of ASTM and EN standardization processes that are essential for possible usage in nuclear power plant integrity assessment and other industrial areas as well. Paper also describes results of the ongoing interlaboratory study which task is to determine repeatability and reproducibility of results obtained from material testing.

Detecting void-induced scatter of fatigue life of selective laser melting-fabricated inconel 718 using miniature specimens

In order to detect the uncertainty of fatigue properties of additive manufacturing-fabricated Inconel 718 parts, by using miniature specimens, we statistically reveal void-induced fatigue data scatter. We found that the voids located at the specimen surface and/or near the surface not only reduced fatigue strength but also increased the degree of the fatigue data scatter, while such a trend could be lessened by the proper heat treatment. We expect that the miniature specimen testing method could effectively sample the voids with different distribution features, and thus quantitatively detect such defect-dominated uncertainty of fatigue properties of the additive manufacturing-fabricated parts.

Miniature specimens for fracture toughness evaluation of dental resin composites

ObjectiveMillimeter-scale (“miniature”) specimens enable in-situ evaluation of mechanical properties of engineering materials at reduced cost. Here three such specimens for measuring fracture toughness (KC) are developed and implemented to new dental materials. The latter include concurrent methacrylate-based and new ether-based resin composites designed to reduce polymerization stress and enhance service life in restored teeth. MethodsFracture toughness of four experimental and one commercial dental resin composites are evaluated using three-point bending (3PB), wedge double-cantilever-beam (WDCD) and edge chipping miniature test specimens. The values of KC were compared with those obtained following ISO standard method ISO6872: 2014. The stress intensity factor (K) for the 3PB and WDCB specimens was determined using linear fracture mechanics analyses made in conjunction with the Finite Element technique, with due consideration given to the finite width of pre-crack. ResultsAnalytic expressions for predicting KC were developed for all three miniature specimens. The width of pre-crack, generally neglected for conventional specimens, significantly affect K. Measured KC conclusively agree with those of commercial or well-studied materials as obtained using conventional specimens, with error bounded by 5–10 percent. SignificanceThe edge chipping test was successfully applied for the first time to non-brittle materials like dental resin composites. The miniature specimens developed will expedite the evaluation of fracture toughness of dental resin composites by saving materials and provide needed in-situ assessment capability. The chipping test which requires no introduction of initial crack and involves no use of elastic constants is especially suitable to functionally graded materials and in-situ study of restored teeth. The WDCB specimen enables stable crack growth, a useful trait in fatigue studies.

A two-material miniature specimen test method and the associated inverse approach for high temperature applications

A miniature two-material testing method has been developed for material property characterization. Elastic-plastic and creep damage models have been applied for a semi-analytical model and a finite element (FE) model to simulate the miniature specimen tensile and creep tests on a two-material system. An inverse optimization algorithm has been developed to extract the creep properties of the unknown material in the two-material system from the experimental results. Single-material miniature specimen tensile tests have been performed for an aluminum alloy at room temperature and 400 °C and full stage creep tests have been performed for a P91 steel at 650 °C. The miniature specimen testing results are comparable to the results from the corresponding conventional uniaxial tensile and creep tests on the same materials and under the same test conditions. This miniature specimen testing technique and the developed inverse method have the potential to become a new approach for determining the elastic-plastic and the full-stage creep properties until rupture for a two-material system at high temperature, e.g. a coating-substrate system.