Small Punch Specimens Research Articles

Use of multi-step loading small punch test to investigate the ductile-to-brittle transition behaviour of a thermally sprayed CoNiCrAlY coating

Small punch tests under multi-step loading conditions were performed to characterise the ductile-to-brittle transition behaviour of a high velocity oxy-fuel (HVOF) thermally sprayed CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y (wt%)) coating. Small punch specimens, 8mm in diameter and ~0.4mm in thickness, were tested between 21°C (RT) and 600°C. A 100 N load was applied in increments every 30mins as a step to investigate the coating deformation at different temperatures. The displacement and strain obtained from multi-step loading SPTs at each load increment were small below 500°C but a significant increase was noted at 600°C. The strain rate behaviour was more distinct at 600°C and large plastic deformation were shown, which is likely due to the ductile-to-brittle transition occurred between 500 and 600°C. Fractographic investigation revealed that the fracture surfaces at low temperatures exhibited flat features with isolated β-phase particles, indicating the inter-splat shearing and brittle failure, whereas the main fracture mode was dominated by extensive ductile tearing at 600°C.

Study of the energy for embrittlement damage initiation by SPT means. Estimation of KEAC in aggressive environments and rate considerations

There is large experience on the determination of crack propagation initiation parameters in SCC and HE scenarios by fracture mechanics tests. Although there are satisfying experiences, the estimation of a parameter in those situations by Small Punch means is in continuous research.In this work a new approximation for the parameter KEAC is presented. It is based on the energy for embrittlement damage initiation on notched Small Punch specimens in SCC and/or EH conditions. Likewise another key aspect is analyzed, as it is the punch rate for SPT in aggressive environments, aiming not to have an influence on propagation mechanisms. Finally, bases for future research in this field are established.

The use of deeply double edge notched small punch specimens for the determination of the essential work of fracture (EWF) parameters in polymer sheets

In recent decades, one of the most widely used techniques to characterize the fracture properties of polymer sheets under plane stress conditions has been the essential work of fracture (EWF). This method allows the testing of different pre-cracked specimens although those most commonly tested are the deeply double edge notched tensile specimens (DDEN-T). However, in situations in which the access to the material is limited, e.g. in service-exposed components, the use of smaller specimens is proposed: deeply double edge notched small punch specimens (DDEN-SP). In this paper the EWF theory is applied to the miniature test in order to obtain fracture parameters, and the viability of the method is discussed. These results are also compared to those obtained from standard specimens (DDEN-T).

Development of a small punch testing method to evaluate the creep property of high Cr ferritic steel: Part I—Effect of atmosphere on creep deformation behavior

In a preliminary study carried out for evaluating creep properties with the small punch (SP) creep testing method, creep tests were performed using reduced activation ferritic/martensitic steel (RAFMs) F82H specimens both in an argon atmosphere and in vacuum (less than 10−3Pa). The results showed that the rupture time was approximately 2.5 times longer in the argon atmosphere compared to that in the vacuum. Changes in the deflection rate with the deflection and time and changes in the minimum creep deflection rate with the rupture time were almost independent of the test atmosphere, although the minimum creep deflection rate decreased as the rupture time increased. After comparing the SP creep test results, which were sorted by using the Larson-Miller parameter, and the standard creep test results, the ratio (F/σ) between the load (F) in the SP creep test and the stress (σ) in the standard creep test was determined to be 2.1 and 2.3 for the argon atmosphere and vacuum, respectively. Consequently, it was found that the ratio, namely, the load/stress conversion coefficient, depended on the test atmosphere. This result implied that the stress and/or strain distribution on the SP specimen varied with the type of atmosphere. This change with the test atmosphere seemed to be closely associated with changes in the friction between the specimen and the ball.

Determination and Validation of Gurson-Tvergaard Model Parameters for Finite Element Simulation of Small Punch Test

Small punch test (SPT) is a miniature sample test technique which can evaluate in-service material properties with an almost non-destructive method. Since the deformation behavior of the small punch specimen is complicated, finite element simulation embedded with Gurson-Tvergaard (GTN) model is adopted to simulate elastic-plastic behaviour until fracture. Choosing the proper GTN parameters is crucial for the small punch simulation, which directly influence the precision of load-displacement curve obtained from simulation. In this paper, load-displacement curve is divided into five stages and the parameters identification process is done by adjusting the simulation curve with experimental load-displacement curve in different stages which controlled by separately parameters. The results show that the parameters determined based on this criterion are not unique. In order to validate the reliability of this method, specimen’s minimum thickness of cross-section after fracture was introduced as an extra criterion which turned out to be feasible. Load-displacement curves cannot serve as the only criterion to verify the GTN parameters.

Influence of the notch shape of pre-notched small punch specimens on the creep failure time

Nowadays, there are standards for determining the creep-fracture properties of a material. However, a sufficient amount of material to be tested is usually required, something that is not always possible or convenient. In certain cases where not enough material is available for carrying out conventional tests to determine these properties of the material to be analysed, there are now several non-standard tests that can achieve this purpose. One of them, the Small Punch Creep Test (SPCT), basically consists of punching, under a constant load, a miniature specimen in which the sides of the specimen are clamped between two dies. One of the greatest challenges at present is to obtain the creep-fracture properties of a material from this type of test using pre-notched specimens. To achieve this initial notch in the SPCT specimen prior to creep-fracture testing, there are several techniques which are being used at present. The main objective of this paper is to analyse the differences between these techniques, taking into account the shape of the pre-notch obtained and the stress distribution at the pre-notch tip during the test. In this way, it is possible to determine which of them is the most appropriate for estimating the creep-fracture properties of the material used.

Comparative Study on the Determination of Creep Properties by Using Different Types of Small Specimens

The application of small specimen creep test techniques in determination of creep properties of materials in-service has been increasing. There exists a number of small specimen creep testing techniques, such as sub-size uniaxial specimen, small punch specimen, shear punch specimen, impression specimen, three-point bending specimen, cantilever beam specimen, beam specimen with fixed constraint, circular ring specimen and tow-bar specimen. The diversity in specimen types and dimensions will affect the evaluation of creep properties. It is hence necessary to make a comparison among different small specimen creep tests. A comprehensive review has been made on the effect of the specimen dimension, stress state, conversion equation, creep properties, strain sensitivity and other influence factors. The results show that different small specimens result in different creep curves. Influenced by many factors, the small punch specimen has a very complicated data interpretation. With a large equivalent gauge length, the cantilever beam and circular ring specimens can obtain rather accurate creep data. Large deformation effect exists for the three-point bending, cantilever beam and circular ring specimens, which can be controlled by the critical load approach. Because of the low frictional sensitivity, the beam specimen with fixed constraint can obtain creep rupture data of high accuracy. A code of practice is then recommended for selecting a proper small specimen to evaluate the creep properties for in-service components.

Influence of biaxial pre-deformation on fracture properties using pre-notched small punch specimens

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 in those cases where there is not enough material to identify these properties from standard tests, such as the 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 the fracture properties using the P-SPT on a material previously subjected to a biaxial deformation by the Marciniak stretching test. The obtained results show a linear decreasing fracture toughness tendency under the influence of biaxial pre-strain in plane stress conditions.

Very High Plastic Strain Zones in 304 Stainless Steel Small Punch Specimen Loaded at RT by Martensite Formation and Recrystallization Technique

Very high plastic strain zones with equivalent plastic strain above 0.2, PZ0.2 and above 0.5, PZ0.5 in 304 stainless steel small punch specimens loaded at RT to various level were observed and measured by martensite formation and recrystallization technique, respectively. It is found that both the very high plastic zones are formed ,at middle stage of the small punch test, at first near the outer surface region of the specimen where the loading ball is contacted to the specimen. The zones extend with increasing load toward the inner surface. Thus the contact area part of the specimen with the ball causes a significant strain gradient through thickness. This will be due to the constraint of the plastic deformation near the contact region by the friction force.

High-temperature small punch test for mechanical characterization of a nickel-base super alloy

A testing device is presented, which allows the mechanical testing of miniaturized specimens at temperatures up to 1273K. Small punch specimens made from a thermally sprayed nickel-base super alloy are tested at different temperatures and under different atmospheres. The tests are accompanied by finite element simulations. The sequential programming algorithm is used to determine the constitutive material parameters by minimizing the difference between simulated and experimentally obtained small punch force–displacement curves. These findings are compared to results received by high-temperature tensile tests on standard-sized samples.

Prediction of cleavage fracture in ferritic steels by small punch tests with a modified Weibull stress model

In this paper, conventional small punch (SP) tests are carried out to determine the SP ductile-to-brittle transition temperature, TSP. The center holed small punch (CHSP) specimens are tested at TSP to determine the Weibull parameters with two different Weibull stress model. The relationship between temperature and the Weibull stress scale parameter, σu, is investigated. The result showed that a unique set of Weibull parameters for CHSP specimens and 0.4T SENB specimens is available by using the modified model not the original model. And the Weibull stress scale parameter, σu, is a power function of temperature.

Estimation of the Fracture Toughness of Structural Steels by Means of the CTOD Evaluation on Notched Small Punch Specimens

By means of the experimental analysis of two structural steels, one with a ductile behaviour and the other one with brittle behaviour, this paper compares the results obtained by means of small punch test (SPT) performed on notched samples with those obtained in standard fracture toughness tests, in order to obtain a relationship between them and to analyse the suitability of the SPT for estimating the fracture toughness. With the aim of analysing the evolution of the defect tip opening displacement (δSPT) during the test, loading was interrupted at different levels of punch displacement (d) and these specimens were analysed under SEM.In the case of the ductile steel, δSPT at the onset of crack growth was larger than the value of CTOD obtained in conventional fracture tests, but this could be expected due to the lower constraint condition of the SPT. By the other hand, the brittle steel exhibits ductile fracture micromechanisms in the SPT, so fracture mechanisms have changed from the standard to the small punch test, due to the different load conditions. Relationship between d and δSPT seems to be material independent.

Recrystallization Technique of Plastic and High Strain Zones and Fracture in Small Punch Specimen Loaded at Rt and 77K of 304 Stainless Steel

Plastic zone with equivalent plastic strain ε above 0.02 PZ0.02 and that above 0.12 PZ0.12 in 304 stainless steel small punch specimen loaded at RT and 77K to various level to fracture were measured by the recrystallization technique. Martensite transformation was also checked magnetically. It is found that the behaviour of both PZ0.02 and PZ 0.12 is at first stretching mode, then stretching and bending mode and then stretching mode. PZ0.02 and PZ0.12 propagated to the clamped region of the specimen.

Evaluation of fracture toughness by notched small punch tests with Weibull stress method

The fracture toughness obtained by pre-notched (pre-cracked) small punch specimens is much higher than that obtained by conventional single edge notch bending (SENB) or by compact tension specimens. In this paper, an equivalent Weibull stress method has been introduced. The tests with 0.4T SENB specimens and linear notched small punch specimens were carried out at 77K. At this temperature (on lower shelf), the results of both test methods show scatter but not significant. The loads at fracture of both types of specimen were used to determine the Weibull stress parameter, m, and the Weibull stress, σw, with finite element method. Base on the assumption that Weibull stress parameters are material constants, the m which could meet a condition that σw0.4TSENB=σwLNSP was regarded as Weibull stress parameter, m. The critical J integral of 1T SENB JcITSENB at 77K has been estimated with m and σw deduced by 0.4T SENB and LNSP specimens.

Introduction of a new notched specimen geometry to determine fracture properties by small punch testing

The small punch (SP) testing technique can be used to assess fracture properties from a very small volume of material. However, the correlation of fracture parameters obtained from SP testing with those produced using standard methods is non-trivial, not only due to the size effect, but also due to the absence of a notch in most existing types of SP specimens. In the present study, a novel SP specimen design is proposed, with a sharp circular notch, leading to the development of a near plane strain condition during the SP test. The new geometry is examined through SP testing of P91 steel, as well as finite element analysis. Optical and scanning electron microscopy is used to study the crack propagation from the notch tip in both ductile and brittle fracture modes.

Creep Deformation and Fracture Behaviors of High Cr Ferritic Steel Welded Joint by Rectangular SP Specimens

Modification of small punch (SP) creep test was investigated to develop a new procedure for evaluating the creep property and damage of welded joint of high Cr ferritic steels by a miniaturized specimen. Disk and square-types specimens are usually clamped between the upper and lower dies in the isotropic manner in the conventional SP creep test, whereas the specimen used in this study was rectangular (10 × 6 × 0.50t mm) and was clamped at both ends of the specimen. This newly proposed SP creep test was applied to the welded joint of P122 steel pipe, which had been actually used in a fossil power plant for a long time. The specimens were removed from the mid section of the thickness of pipe so that the HAZ was located on the center of specimen perpendicular and parallel to the longitudinal direction of specimen respectively. In addition, the rectangular specimen consisting of only base metal and the conventional SP test specimen (10 × 10 × 0.50t mm) with the HAZ were also subjected to the SP creep test for comparisons. Experimental results revealed that, by arranging the HAZ perpendicular to the longitudinal direction, the creep rupture strength decreased significantly reflecting the lower strength of the fine grain HAZ (FGHAZ). This decrease in creep strength was attributable to the acceleration of initiation and growth of cavity and/or microcrack at the FGHAZ. Consequently, it is indicated that the present modified SP creep testing technique has a high potential as a tool for evaluating the susceptibility to Type IV cracking and/or the creep damage of welded joints.

Some considerations on specimen types for small sample creep tests

Commonly used small creep specimen types (i.e. conventional sub-size uniaxial specimens, impression creep specimens and small punch specimens) and test methods, are briefly described, including their capabilities and the methods used to process the data from these tests. The performances of these test types are compared and their relative advantages and disadvantages, for specific practical applications, are assessed. An alternative, novel, “more flexible”, small ring-type specimen test method is described. A significant advantage of the ring-type specimen is that it has a much greater equivalent gauge length (EGL), compared with those of other small specimen types. This specimen type allows small strains to be related to relatively large deformations; it is particularly suitable for creep tests at relatively low equivalent uniaxial stresses. Possible future exploitation of the small specimen test techniques is briefly addressed.

Assessment of tensile strength using small punch test for transversely isotropic aluminum 2024 alloy produced by equal channel angular pressing

The microstructures of metallic materials become extremely refined and their mechanical strengths exhibit considerable anisotropy when subjected to severe plastic deformation by equal channel angular pressing (ECAP). The small punch (SP) testing method was employed as a substitute for the standard tension test to assess the anisotropic tensile strength of materials produced by ECAP. SP testing is a viable alternative to standard tension tests because ECAP samples are usually limited to approximately 10 mm in length in the transverse direction perpendicular to the extrusion (longitudinal) direction of the ECAP process and the dimensions of the SP test specimen are 10 mm × 10 mm × 0.5 mm. The SP tests were performed using specimens of over-aged aluminum 2024 alloy subjected to three ECAP passes at 150 °C and extracted by wire cutting along the longitudinal and transverse axes. The failure surfaces of SP specimens after testing showed that failure was caused by shear deformation and that ECAP-processed materials displayed anisotropic strength. Based on these results, a method is proposed to assess tensile strength along both the transverse and the longitudinal axes of ECAP materials exhibiting anisotropic strength. The validity of the proposed method was verified through a comparison of its performance evaluating plastic flow strength with measurements obtained by tension tests along the longitudinal axis.

Three-dimensional numerical simulation on plastic damage in small punch specimen of Zirconium

Small punch test (SPT) technique was used to evaluate the mechanical properties of Zirconium in this paper. The dimension of the disc specimen is φ 10 × 0.5 mm. Plastic damage in small punch specimen of Zirconium was investigated both experimentally and numerically, because it has great influence on small punch specimen. In order to simulate the plastic damage in the small punch specimen of Zirconium, the 3D finite element model incorporated with Gurson–Tvergaard–Needleman (GTN) plastic damage constitutive equation was established. Void growth and initiation of ductile crack of the small punch specimen were predicted. Results show that damage occurs on the bottom side of the specimen and grows across the specimen until complete failure, which has good agreement with the observation in the experiment.

Ductile Damage Analysis for Small Punch Specimens of Type 304 Stainless Steel Based on GTN Model

Abstract Based on small punch (SP) tests on Type 304 stainless steel at room temperature, a finite element (FE) model with Gurson-Tvergaard-Needleman (GTN) constitutive equations was established to analyze the ductile damage in SP specimens. Influences of the friction coefficient between SP specimen and steel ball and element size to the FE results were discussed. The evolution of void volume fraction (VVF) with time along different paths and the relation between VVF and equivalent plastic strain (PEEQ) under constant punching velocity were analyzed further by using the FE model. Good agreements between experimental results and simulation data were proved by load-displacement curves and failure locations. The numerical results show that initial crack occurs at the bottom surface about 0.86 mm away from the centre and damage localization phenomenon is extraordinarily severe in the SP specimen, and PEEQ has great effect on the evolutional rate of VVF.