Small Punch Specimens Research Articles

Determination of the Gurson-Tvergaard-Needleman damage model parameters for simulating small punch tests of heat-resistant alloys

The small punch (SP) test is utilized to assess the mechanical characteristics and damage progression of heat-resistant alloys. The inverse finite element analysis method incorporating SP tests is a parameter identification method based on adjusting the accuracy of the simulated load-displacement curves. In this paper, the elastoplastic parameters of the Hollomon model and the damage parameters of the Gurson-Tvergaard-Needleman (GTN) model are determined based on the undamaged and damaged stages of the load-displacement curves, respectively. The whole stress-strain curves of the tested materials are then built using the results of finite element simulations of the tensile specimens of ZG15Cr2Mo1, P91, 316H, and Hastelloy X at room and elevated temperatures. Comparison with uniaxial tensile tests indicates that the simulated stress-strain curves closely resemble the experimental data from the tensile testing. In addition, the simulated damage evolution characteristics of the SP specimens are consistent with the mechanical model based on the actual deformation behavior. It is possible to comprehend the damage evolution process by analyzing the SP specimens’ stress and strain change characteristics.

Evaluation of Mixed-Mode Stress Intensity Factors Using a Small Punch Specimen with a Circular Crack: Preliminary Experiment and an Estimation of Stress Intensity Factors

Evaluation of Mixed-Mode Stress Intensity Factors Using a Small Punch Specimen with a Circular Crack: Preliminary Experiment and an Estimation of Stress Intensity Factors

Determination on the fracture toughness of the welded joints of X80 pipeline steels based on small punch test

For the sake of design and integrity assessment of the fracture toughness of high-grade pipelines, it is necessary to carry out accurate fracture toughness assessment of the welded joints. The study of more concise and effective fracture toughness testing methods has important theoretical and engineering value. At present, the fracture control of girth welding joints is mainly based on J integral, CTOD and CTOA. However, it is hard to carry out standard fracture toughness tests at some local narrow areas of the welded joints such as the heat affected zone for the difficulty of sampling. On this basis, this paper aims to propose a method to determine the fracture initiation toughness JIC of the welded joints of high-grade pipeline steels based on the micro-sample test method - small punch test. Firstly, small punch specimen with through-thickness notch was designed. Secondly, the finite element model of the notched specimen was established, and verified by the notched specimen tests of X80 steel. Furthermore, the elastic–plastic material properties of different zones of the welded joint were obtained by the empirical correlation method of small punch test and further used for the determination of J contour integral. The GTN damage parameters were calibrated by the three-point bending tests and further used for the determination of the crack initiation displacement. Finally, the fracture initiation toughness JIC(SPT) of different zones of the welded joint were obtained by the SPT method and verified by the three-point bending test. The results show that the fracture initiation toughness JIC(SPT) of each region of the welded joint obtained by notched SPT specimen were all less than the SENB test results. But there is a good empirical correlation between the two results so that the SPT method can be further modified by establishing an empirical correlation model. The SPT method has good engineering application prospect for the accurately characterization of the different distribution of fracture toughness in different areas of the welded joints.

A novel theoretical model for obtaining Norton's law of creep materials using different small specimens

The test method for evaluating the creep properties of a material using small specimens mostly relies on an empirical correlation equation derived through uniaxial tests, and it is difficult to obtain a unified theoretical model for creep specimens with different configurations. We propose a novel and unified theoretical model that describes the creep displacement–time curves based on the energy density equivalent principle, which can be expressed as h = h*B(P/A*)vt. A simple expression of the model parameters related to sizes was established for cantilever beam, ring compression, and flat small punch specimens using finite element analysis (FEA). Forward and reverse verification using FEA and experimental validation of the model using small specimens of a zinc alloy (ZA27) at 27°C were conducted. The results revealed that the Norton's law parameters predicted using the new creep model are in good agreement with the results obtained using the uniaxial creep test. The new model does not require an empirical correlation between the displacement and uniaxial reference equivalent strain and has a uniform characterization for four types of specimens with different configurations and sizes.

A novel methodology for determining creep crack initiation and growth properties using FEM with notched small punch specimens

Determination of material properties on creep crack initiation (CCI) and creep crack growth (CCG) using small punch tests is still a virgin field to be exploited. Spanish and Czech researchers have carried out pioneering work and put forward an approach to correlate the notched specimen with the standard CT specimen. The advantage is that, the formulas of C*-integral for CT specimen are available and can be used directly. However, building a big data pool with both SP and CT tests within the creep region is a daunting task and hence it is worthwhile to derive C*-integral formulas for notched specimens themselves. The research is novel in that it proposes a methodology to derive the key parameter C*-integral for notched small punch specimens directly without supporting known CT tests. A 3D finite element model (FEM) for through-wall notched specimen with different crack lengths is build, and a general formulas for stress intensity factor (SIF) K, and limit load PL are obtained to calculate C*-integral for notched specimen. Experiments on notched specimens are carried out for stainless steel Sanicro 25 at temperature 700 °C. By using general formulas derived in this study, the key parameter C*-integral is obtained. The relation between C*-integral and the creep initiation time ti can be drawn as a straight line in a double logarithmic coordinate. The relation between C*-integral and the creep crack rate ȧ appears to follow the same tendency. Material constants for CCI and CCG can be derived according to the power law using least square fitting. The proposed method is not fully developed and further validating experiments will be necessary to complete the research.

Fully plastic J-integral and C* equations for small punch test specimen with a surface crack

Small punch (SP) creep test specimen with an elliptical surface crack was suggested for use with creep crack growth testing. Estimation equations for fracture parameters are needed. K-equation was derived previously for the crack in the SP specimen.In this study, estimation equations for J and C* were derived. 3D elastic-plastic finite element analyses were carried out to determine J value along the front of the surface cracks of the same geometries as were employed for the K-estimation. The dimensionless function, h1 was determined for various crack locations according to the EPRI scheme. The plastic limit load solution was also determined. The C*-equation of the cracked SP specimen was derived based on the Jp solution using the analogy between the secondary creep constitutive law and the Ramberg-Osgood plasticity law. The C*-equation may be able to be used to characterize the creep crack growth behavior under the extensive creep condition of the cracked SP specimen.

Evaluation of Transition Temperature in Reactor Pressure Vessel Steels 6using the Fracture Energy Transition Curve from a Small Punch Test

The small punch (SP) test is one of the small specimen test techniques, and standardization of the SP test method for evaluating the mechanical properties of metallic materials is in progress. In this study, the impact transition temperature of reactor pressure vessel steels (RPV) in nuclear power plants was estimated using the draft standard SP test method. The SP fracture energy (ESP) and normalized SP fracture energy (ENSP) of the RPV steels were evaluated at various temperatures, and their transition curves were derived and compared to the transition curve in the Charpy V notch (CVN) test. The SP transition region appeared at a much lower temperature range than that of the CVN owing to the size and notch effect. Ductile brittle transition temperature (DBTT) in the SP transition curve showed a linear relationship with DBTT and T41J in the CVN transition curve. The ductile to brittle transition behaviors of SP specimens were analyzed using fractographs and compared with the transition curves in ESP and ENSP. ENSP started to decrease at the temperature at which the SP ductile to brittle transition behavior occurred, and this means that the ENSP transition curves were in good agreement with transition behavior in the SP test. However, the ESP transition curves did not match transition behavior. Using DBTT in the ENSP transition curve is appropriate to estimate the CVNDBTT and T41J.

Investigation of creep behavior of 316L stainless steel produced by selective laser melting with various processing parameters

Creep behavior of 316L stainless steel was experimentally investigated using small punch (SP) specimens produced by the selective laser melting (SLM) with various processing parameters. Five rectangular blocks were fabricated from gas atomized powder using the SLM. Various combinations of the processing parameters were used in the SLM process, such as the scanning speed and the laser power, when the energy density was kept as a constant. The scanning speed was increased from 336 to 784 mm/s, while the laser power was simultaneously increased from 165 to 385 W. The microstructural characteristics of each block were examined to verify the manufacturing conditions. Internal defects such as pores and un-melted powder were observed in the specimens fabricated when the low laser power was used. Small punch (SP) creep tests were carried out at 650 °C in the applied load range of 320–550 N. Creep-rupture life and Norton’s power law creep deformation constants were measured. The results showed that even though the energy density condition was same for all of the creep specimens, those with higher laser power showed better creep resistance. Since it was known that a higher scanning speed could deteriorate the creep resistance, the higher creep resistance under the high scanning speed condition was indebted to the higher laser power used. The observed creep behavior of the SLM blocks was explained in relation to the microstructures.

A novel methodology for estimating tensile properties in a small punch test employing in-situ DIC based deflection mapping

Determining tensile properties from small punch test is being pursued actively in the nuclear industry due to the limited volume of material such tests use compared with standard tests which can be critical when considering active or development samples. One of the crucial challenges in harnessing the full potential of this technique is formulating methodologies which correlate the small punch specimen’s deflection to equivalent uniaxial tensile properties. Existing approaches for correlation rely on deflection obtained from a single point on the small punch test specimen, used with empirical equations to make the correlation. However, the deflection and strain accumulation in a small punch specimen is highly heterogeneous and data from a single point does not represent the gross deformation evolving in the specimen. This data when used in conjunction with the empirical formulations for deriving equivalent uniaxial tensile properties, would not result in accurate identification of material properties. In this work we offer an alternative approach which uses the full field deflection of the specimen mapped through in-situ digital image correlation. The use of digital image correlation combined with inverse finite element analysis augments the existing method of material properties identification from single point deflection data thereby significantly improving the reliability of the measurements.

Investigating creep rupture and damage behavior of 41Fe-25.5Cr-23.5Ni alloy small punch creep specimens using a novel microstructure meshing approach

The Voronoi tessellation technique was used to model an idealized polycrystalline microstructure. This novel meshing approach was also implemented to study the crack initiation and growth behaviors of Sanicro25 alloy through small punch creep tests using a modified creep damage constitutive model. Furthermore, the influence of specimen thickness on crack fracture mechanics was examined. The suitability of this novel method was validated on the basis of the accuracy of simulation results. Transgranular fracture patterns were found at the primary stage, which were transformed into intergranular cracks as the creep time progressed. Cracks in small punch specimens were predominantly initiated on the outer surface, where high stress triaxiality occurred. Simultaneously, the high stress and stress triaxiality deviation controlled crack growth behaviors were determined by considering the various stress states.

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.

Application of the small punch test in combination with the master curve approach for the characterisation of the ductile to brittle transition region

The Master Curve approach allows the full characterisation of the ductile to brittle transition region (DBTR) of ferritic steels to be performed with a reduced number of tests. In this paper, the approach has been combined with the application of the small punch (SP) test. Modified SP specimens have been successfully employed to estimate the fracture toughness values of a pressure vessel steel and three structural steels. In addition, a methodology has been proposed, including a validity criterion for the performed tests. The estimated reference temperatures have been compared to the values obtained by means of full-scale conventional techniques. A unique simple relationship between both methodologies has been established for all the analysed materials. Therefore, this paper confirms the suitability of the small punch testing technique for the characterisation of the DBTR of several ferritic steels. It is a promising, simple and cost-effective test, which can be performed with simple equipment.

Effect of Pre-Strain on Small Punch Creep Test of 316L Stainless Steel at 373K

Creep does not only appear at high temperature, but also appears at low temperature for 316L stainless steel that threatens the safety of equipment. In this work, the creep behavior of as-received and pre-strained 316L stainless steel at 373K was investigated by uniaxial creep (UC) tests and small punch creep (SPC) tests. The parameters of power-law creep model were determined from stress dependence of UC tests. Then, the creep behavior of SPC test was analyzed by finite element (FE) simulation combined with power-law creep model. Comparing with experimental creep deflection, the results of FE simulation can reasonably reflect the creep behavior of as-received and pre-strained small punch specimens. Based on the comparison of as-received specimen and pre-strained specimen from UC test, SPC test and FE simulation, pre-strain significantly restrains creep behavior of 316L austenitic steel at 373K.

Pre-notched dog bone small punch specimens for the estimation of fracture properties

In recent years, the pre-notched or pre-cracked small punch test (P-SPT) has been successfully used to estimate the fracture properties of metallic materials for cases in which there is not sufficient material to identify these properties from standard tests, such as CT or SENB specimens. The P-SPT basically consists of deforming a pre-notched miniature specimen, whose edges are firmly gripped by a die, using a high strength punch. The novelty of this paper lies in the estimation of fracture properties using dog-bone-shaped specimens with different confinement levels. With these specimens, three confinement variations have been studied. The results obtained enable the establishment of a variation of fracture properties depending on the level of confinement of each miniature specimen and selection of the most appropriate confinement for this goal.

Application of the small punch test to evaluate the integrity of a cold spray titanium coating

Metal Cold Spray (MCS) is currently under evaluation for its suitability for aerospace applications. However, before this technology can be implemented into the jet engine, the mechanical performance and structural integrity of this novel process must be fully understood. Limited data is currently available to determine key materials properties given the discrete and transient nature of a MCS component. Furthermore, it is extremely challenging to produce uniaxial test coupons that are truly representative of the in-service geometry. As such, the small punch (SP) test offers an attractive alternative, since miniature disc SP specimens can be extracted from localised discrete locations. This paper will report the findings from an experimental collaborative programme of work currently being undertaken by Swansea University, Rolls-Royce Singapore and Nanyang Technological University Corp Lab to understand the contrasting modes of failure in a Ti-6Al-4V coating sprayed on to a Ti-6Al-4V substrate, which is expected to have properties akin to a forged variant. This will include a series of SP tests to assess the integrity and performance across the substrate, bond line and coating. Results will be supported by additional microstructural and fractographic investigations.

Determining the ultimate tensile strength of fuel cladding tubes by small punch testing

The Small Punch (SP) test with constant deflection rate is a miniature technique that can provide estimates on the material tensile properties. Linear correlations are usually used for relating the maximum force and displacement at maximum force, recorded during the SP test, to the ultimate tensile strength. Fitting coefficients used in the correlations are calibrated on data from flat SP specimens. SP test requires only a small amount of testing material which represents a clear benefit when irradiated samples have to be tested. Therefore, there is a considerable interest in using SP for testing fuel cladding material properties. In this study we show that the same correlation equations, albeit with adjusted fitting coefficients, can be used to estimate the ultimate tensile strength from tube SP specimens made out of P91 ferritic/martensitic and 15-15Ti austenitic stainless steel. The calculated fitting coefficients lead to reasonable estimates of the ultimate tensile strength at temperatures of up to 650 °C although the coefficients themselves have been computed at room temperature. The coefficients are more suited for assessing ductile materials as the models used for computing the coefficients do not take into account damage (degradation of the material stiffness) or crack initiation and propagation, observed during the SP tests of brittle material. Finally, using the calculated ratios of maximum forces and displacements at maximum forces, one can map the two values of a given curved SP test to the equivalent flat SP values.

Investigation of initial yielding in the small punch creep test

Abstract Unlike the uniaxial creep test results, the interpretation of small punch (SP) test results is not straightforward due to the complex deformation behavior of SP specimens. Accordingly, the reliability of correlations of SP creep test results with corresponding uniaxial tests is still not clear. One crucial point is that the initial yielding at the beginning of an SP creep test results in the hardening of the material before creep sets in and is thought to affect the creep properties. In this study, to clarify this effect, first an accurate numerical model of the SP creep test has been developed following which this model was used to study the influence of test parameters such as specimen thickness, punch diameter and the radius of the receiving hole on the initial yielding. Varying these parameters changes the deformation mechanisms and thus has a strong effect on the deflection-time curves along with the amount of initial plastic deformation. It was observed that decreasing specimen thickness and the punch diameter increased the initial yielding. Conversely, increasing the radius of the receiving hole resulted in higher initial yielding.

Small punch test and simulation of HR3C steel

As the traditional uniaxial test requires substantial amount of cost and material usage for specimen, it is hardly applied to the direct diagnosis of structural integrity of in-service facilities. Small punch (SP) test has been, alternatively, introduced to procure material properties because SP test just needs a slice of material (10 × 10 × 0.5 mm) which can be easily obtained from real components without loss of integrity. In this study, for HR3C steel, SP tests were carried out and its three-dimensional finite element (FE) model was also developed to cross-validate the test results. Scanning electron microscope images for SP specimens were taken and compared with crack propagation simulation results generated by the extended finite element method. Furthermore, the minimum thickness change of specimen during the test was measured by several interruption tests. Configuration of SP test specimen was intensively observed with respect to punch displacement by both experiment and FE analysis, respectively.

Estimation of high‐temperature fracture parameters for small punch specimen with a surface crack

AbstractAn outline of a newly proposed methodology for evaluating creep crack growth (CCG) parameters using cracked small‐punch (SP) specimens is explained. Three‐dimensional finite element analyses were performed to calculate the stress intensity factor along the crack front for a surface crack formed at the centre of a SP specimen. Effects of crack ratio, (a/t); crack aspect ratio, (a/c); and thickness of the specimen, (t), on the fracture parameters were studied. It was observed that the minimum variation of K‐value along the crack front can be achieved when a/c was 0.50 except the location very near the intersection of the crack and free surface. This condition is similar to the case of constant K‐values along the crack front of the conventional compact tension specimen. Thus, it can be argued that the SP specimen with a surface crack is a suitable specimen geometry for CCG testing. The proposed CCG test method was found to be practically applicable for the crack geometry of 0.10 to 0.30 of a/t with constant aspect ratio of 0.50. An estimation of the K and Ct‐parameter under the small scale creep condition was derived. Future work for further development of the suggested CCG testing is discussed.

Enhanced mechanical behaviors of gradient nano-grained austenite stainless steel by means of ultrasonic impact treatment

The gradient nano-grained (GNG) structure was formed on the top layer of AISI 304 in the presence of coarse grains (CG) by means of ultrasonic impact treatment (UIT). The impact velocity was 5m/s and the coverage changed from 3000% to 9000% in order to obtain the microstructure GNG/CG materials. The tensile test and small punch test (SPT) were further carried out in order to investigate the mechanical behaviors of the GNG/CG 304 at the different stress states. The results indicated that the yield strength of GNG/CG 304 obtained by tensile test and SPT increased at the small expense of ductility. The stress triaxiality T played an important role in the deformation behavior of GNG/CG 304. An increased T-value in the region of biaxial stretching of small punch specimen (T=2/3) led to an improved ductility compared with that noted in the case of uniaxial tensile specimen. The strain rate sensitivity m of GNG/CG 304 was 0.0468 that was estimated to be 25-fold greater than the coarse material. The yield strength of GNG/CG 304 at 400°C was 1.6-fold greater than the coarse material at ambient temperature due to the thermal stability of the GNG layer. Thus, the GNG structure could improve the comprehensive mechanical performances of AISI 304 with a balance of strength and ductility.