Automated Ball Indentation Research Articles

Understanding the fatigue response of small volume specimens through novel fatigue test methods – Experimental results and numerical simulation

This paper presents the current understanding relating to fatigue response of materials that are small in volume and tested through cyclic automated ball indentation and cyclic small punch test methods. The paper covers both the experimentation and numerical simulation aspects. Recently, researchers have proposed the hydraulic bulge test as another test method for fatigue life evaluation. All these test methods provide an indication of failure due to fatigue loading. For any given material, each one of the test methods causes its own state of stress (bi-axial or tri-axial), which is different from the uni-axial state of stress for a standard specimen, which makes the correlation of strain-life data between test methods to be difficult. In view of this, a numerical simulation of cyclic ball indentation, cyclic small punch test, and hydraulic bulge test has been carried out. The results of numerical simulation suggest that the mean stress for cyclic ABI and cyclic SPT tests are different, but the failure life data can be corrected using Goodman or similar mean stress correction methods.

Effect of post heat-treatment on the microstructure and mechanical properties of Hastelloy-X structures manufactured by laser based Directed Energy Deposition

This paper reports a systematic investigation of Hastelloy-X (Hast-X) structures built by Laser Directed Energy Deposition in as-built and post heat-treated conditions. The optical microscopy shows microstructures with fine dendrites in as-built condition due to higher cooling rate during deposition, while recrystallized equiaxed grains are observed after post heat-treatment (at 1177 °C) due to recrystallization. X-ray diffraction studies reveal nickel γ-matrix with variation in crystallite size and a peak shift after post heat-treatment, primarily due to change in surface residual stress. Surface topography reveals the reduction in average roughness with post heat-treatment. Further, the maximum compressive residual stress of 350 MPa and maximum tensile residual stress of 252 MPa are observed at the surfaces of post heat-treated and as-built samples, respectively. The average micro-hardness changed from 239 HV1.96N to 208 HV1.96N after post heat-treatment. Single Cycle Ball Indentation studies indicated increase in energy storage capacity by a factor of 1.55 after post heat-treatment. Automated Ball Indentation studies quantify yield strength and ultimate strength as 478 MPa and 765 MPa in as-built samples, while 393 MPa and 630 MPa in post heat-treated samples, respectively. Further, crack propagation studies indicate an improvement in the fatigue life after post heat-treatment, while the specific wear rate increased by a factor of 1.72 with increased delamination in heat-treated samples. Thus, post heat-treatment of Hast-X samples changes the material properties significantly.

Nondestructive measurements of flow properties of nanocrystalline Al-Cu-Ti alloy using Automated Ball Indentation (ABI) technique

The Stress-Strain Microprobe® (SSM) system and its Automated Ball Indentation® (ABI®) test technique were applied in the present study for the purpose of determining the flow properties of the alloy under study in a non-destructive manner. ABI tests were applied at 21 °C to the consolidated nanocrystalline Al-10 wt% Cu-5 wt% Ti alloy samples which were subjected to three and six hours of milling. The samples processed for six hours of milling were further tested at elevated temperatures of 200 °C and 400 °C. The results of the ABI tests of the alloy under study include the indentation load versus depth of penetration curves, true stress versus true-plastic-strain curves, determination of yield strength curves, ultimate tensile strength values, and ABI Brinell hardness number (BHN). X-Ray Diffraction (XRD) was applied to investigate the microstructure of the Al-10 wt% Cu-5 wt% Ti alloy subjected to different milling time before and after consolidation. XRD analysis includes identification of the main phases/elements present and calculating the crystallite size in the processed alloy for 3 and 6 h of milling before and after consolidation. Apreo Field Emission Scanning Electron Microscope was applied to characterize the size and morphology of the indentation area after conducting the ABI tests. In addition, the same technique was applied to reveal the elemental distribution in the indentation area of the tested samples.

Influence of Thermal Aging on Tensile and Low Cycle Fatigue Behavior of Type 316LN Austenitic Stainless Steel Weld Joint

Influence of short-term thermal aging on the low-cycle fatigue (LCF) behavior of 316LN austenitic stainless steel weld joint with 0.07 wt pct N has been investigated. Prior thermal exposure was found to improve the fatigue life compared with the as-welded condition. Besides, the treatment also imparted a softening effect on the weld metal, leading to an increase in the ductility of the weld joint which had a bearing on the cyclic stress response. The degree of cyclic hardening was seen to increase after aging. Automated ball-indentation (ABI) technique was employed toward understanding the mechanical properties of individual zones across the weld joint. It was observed that the base metal takes most of the applied cyclic strain during LCF deformation in the as-welded condition. In the aged condition, however, the weld also participates in the cyclic deformation. The beneficial effect of thermal aging on cyclic life is attributed to a reduction in the severity of the metallurgical notch leading to a restoration of ductility of the weld region. The transformation of δ-ferrite to σ-phase during the aging treatment was found to influence the location of crack initiation. Fatigue cracks were found to initiate in the base metal region of the joint in most of the testing conditions. However, embrittlement in the weld metal caused a shift in the point of crack initiation with increasing strain amplitude under LCF.

High temperature tensile properties of 316LN stainless steel investigated using automated ball indentation technique

Type 316LN stainless steel (SS) is the principal structural material for the components of sodium cooled fast reactors operating under elevated temperature conditions. In order to assess the degradation in strength of service exposed components using a small specimen testing technique such as automated ball indentation (ABI), it is necessary to carry out prior detailed ABI studies on the virgin material. In this investigation, the tensile behaviour of as-received 316LN SS were investigated at several temperatures in the range 298–973 K using ABI technique. The load-depth of indentation data measured from ABI tests was analyzed using semi-empirical relationships to obtain the tensile properties. The yield stress and the flow curves were determined by correlating ABI results with corresponding uniaxial tensile test results. Trend curve for tensile strength with temperature, as estimated from ABI tests, exhibited a plateau region in the temperature around 823 K, similar to uniaxial tensile tests. The variations of strength coefficient, strain hardening exponent, yield ratio, hardness and uniform ductility with temperature were evaluated from ABI tests. The ABI technique was found to estimate the influence of temperature on tensile properties sensitively.

Tensile behavior in selective laser melting

By the nature of selective laser melting (SLM) additive manufacturing (AM), property variations are likely to arise in specific structures of parts. The problem here is that it is hard to predict exactly how properties will be affected or conversely, how the mechanical properties in local sections of the built parts can be intentionally changed. There is little known about how build geometry and grain structures unique to SLM affects the mechanical properties of SLM parts. To address this issue, various SLM manufactured parts, created using the Renishaw™ AM 250, were tested for significant variation in mechanical properties. Specifically, the use of both mini tensile tests and Automated Ball Indentation (ABI) is employed to incrementally test localized sections. ABI has the potential to greatly improve the ability to monitor properties. This research advances collective understanding of AM and leads to methods that assure property uniformity or intentional manipulation of mechanical properties.

Mechanical properties of bovine cortical bone based on the automated ball indentation technique and graphics processing method.

Cortical bone provides the main form of support in humans and other vertebrates against various forces. Thus, capturing its mechanical properties is important. In this study, the mechanical properties of cortical bone were investigated by using automated ball indentation and graphics processing at both the macroscopic and microstructural levels under dry conditions. First, all polished samples were photographed under a metallographic microscope, and the area ratio of the circumferential lamellae and osteons was calculated through the graphics processing method. Second, fully-computer-controlled automated ball indentation (ABI) tests were performed to explore the micro-mechanical properties of the cortical bone at room temperature and a constant indenter speed. The indentation defects were examined with a scanning electron microscope. Finally, the macroscopic mechanical properties of the cortical bone were estimated with the graphics processing method and mixture rule. Combining ABI and graphics processing proved to be an effective tool to obtaining the mechanical properties of the cortical bone, and the indenter size had a significant effect on the measurement. The methods presented in this paper provide an innovative approach to acquiring the macroscopic mechanical properties of cortical bone in a nondestructive manner.

Influence of some Test Parameters on Automated Ball Indentation Test Results

Automated Ball Indentation (ABI) technique has the potential to evaluate tensile and fracture properties of materials by testing a small volume of material. ABI testing involves various parameters such as surface finish of test specimen, maximum depth of indentation, number of loading-unloading cycles and material of indenter. As ABI test is yet to be standardized, it is necessary to assess and understand the consequences of the various test parameters. In this investigation, the influence of the above test parameters on the tensile properties of 316LN SS has been investigated at 298 K. It was found that by increasing the maximum depth of indentation, number of loading-unloading cycles and surface finish of test specimen at the cost of time and efforts, ABI results were not altered significantly. The selection of indenter material has to be done judiciously, according to test temperature.

Evaluation of strength property variations across 9Cr-1Mo steel weld joints using automated ball indentation (ABI) technique

The variations of strength properties across 9Cr-1Mo steel weld joints fabricated by different arc welding processes such as shielded metal arc welding (SMAW), tungsten inert gas (TIG) and activated tungsten inert gas (A-TIG) have been evaluated employing automatic ball indentation (ABI) technique. ABI tests were conducted at 298K across various zones of the weld joints comprising of base metal, weld metal, heat affected zone (HAZ) and intercritical HAZ (ICHAZ) regions. The flow curves obtained from ABI tests were correlated with corresponding conventional tensile test results. In general, the tensile strength decreased systematically across the weld joint from weld metal to base metal. Inter critical HAZ exhibited the least strength implying that it is the weakest zone. The incomplete phase transformation in the ICHAZ during weld thermal cycle caused the softening. The A-TIG weld metal exhibited higher UTS and strain hardening values due to higher carbon in the martensite. The strain hardening exponent exhibited only slight variation across the various regions of the weld joints. A-TIG weld joint exhibited higher weld metal and HAZ strength, marginally higher UTS to YS ratio in the weld metal and HAZ compared to that of the other two processes. Hence, among the three welding processes chosen, A-TIG welding process is found to be superior in producing a 9Cr-1Mo steel weld joint with better strength properties.

Study of Fatigue Properties of Materials through Cyclic Automated Ball Indentation and Cyclic Small Punch Test Methods

One of the important inputs while estimating the remaining life of critical components is the fatigue property of materials. Fatigue data, in the form of stress vs. cycles to failure (or) strain vs. cycles to failure (or) fatigue crack growth rate data is used to predict the residual life. Material’s fatigue property degrades with time and usage; hence, it is appropriate to use the current properties for remaining life assessment. Often the quantity of material available for generating fatigue data is limited, especially, if the material is scooped out of existing component of a power plant. Further, fatigue response being probabilistic in nature, requires multiple specimens to be tested at any given stress/strain levels. This has prompted us to develop test procedures to determine the fatigue data of materials from a limited volume of material. This paper presents the results of cyclic ball indentation test method as well as cyclic small punch test method that is used to generate the fatigue data at different stress levels. There are several fine details relating to these test techniques – viz., establishing a equivalent damage criteria for failure life with standard LCF/HCF test specimens. The influence of one of the variables, viz., friction at the specimen-tool interface of a small punch test is investigated through numerical simulation and the results are presented here.

Comparison of irradiated 15Kh2MFA material mechanical properties using conventional testing methods and innovative approach of small punch testing (SPT) and automated ball indentation (ABIT)

THIS PAPER HAS BEEN REMOVED FOR LEGAL REASONS. This paper has been removed with the agreement of the conference organizers.

Constitutive Equations and ANN Approach to Predict the Flow Stress of Ti-6Al-4V Alloy Based on ABI Tests

The flow behavior of Ti-6Al-4V alloy was studied by automated ball indentation (ABI) tests in a wide range of temperatures (293, 493, 693, and 873 K) and strain rates (10−6, 10−5, and 10−4 s−1). Based on the experimental true stress-plastic strain data derived from the ABI tests, the Johnson-Cook (JC), Khan-Huang-Liang (KHL) and modified Zerilli-Armstrong (ZA) constitutive models, as well as artificial neural network (ANN) methods, were employed to predict the flow behavior of Ti-6Al-4V. A comparative study was made on the reliability of the four models, and their predictability was evaluated in terms of correlation coefficient (R) and mean absolute percentage error. It is found that the flow stresses of Ti-6Al-4V alloy are more sensitive to temperature than strain rate under current experimental conditions. The predicted flow stresses obtained from JC model and KHL model show much better agreement with the experimental results than modified ZA model. Moreover, the ANN model is much more efficient and shows a higher accuracy in predicting the flow behavior of Ti-6Al-4V alloy than the constitutive equations.

Uporaba ABI tehnike za merjenje mehanskih lastnosti aluminijevih zlitin: vpliv pogojev toplotne obdelave na mehanske lastnosti zlitin

Effects of chemical composition and heat treatment on the microstructures and mechanical properties were investigated with automated ball-indentation tests, scanning and transmission electron microscopy, and energy-dispersive X-ray analysis. In this work, the automated ball-indentation (ABI) technique was compared with the standard mechanical tests. The ABI method is based on load-controlled multiple indentations into a polished surface by a spherical indenter. The indentation depth is progressively increased to the specified maximum limit with intermediate partial unloading. This technique allows us to measure the yield strength, the stress-strain curve, the strength coefficient and the strain-hardening exponent. For all the test materials and conditions, the ABI-derived results were in very good agreement with those obtained with conventional standard test methods. We analyzed the effect of heat treatment on the alloys with different chemical compositions. Heat treatment leads to changes in the mechanical properties of the alloys, which are the results of several processes.

Comparison of ABI Technique and Standard Methods in Measuring Mechanical Properties of Aluminium Al-aloys

Mechanical properties and chemical composition of aluminium alloys were investigated by automated ball indentation tests, scanning electron microscopy and energy dispersive X-ray analysis.In this work, Automated Ball Indentation (ABI) technique was compared with the standard mechanical tests. ABI method is based on the load controlled multiple indentations into a polished surface by a spherical indenter. The indentation depth is progressively increased to a maximum user specified limit with intermediate partial unloading. This technique allows to measure the yield strength, stress-strain curve, strength coefficient and strain hardening exponent.For all these test materials and conditions, the ABI derived results were in very good agreement with those obtained from conventional standard test methods.

Uporaba tehnike ABI za merjenje mehanskih lastnosti aluminijevih zlitin: vpliv kemijske sestave na mehanske lastnosti zlitin

The effects of the chemical composition on the microstructure and mechanical properties were investigated using automated ball-indentation tests, scanning electron microscopy and energy dispersive X-ray analysis. It was observed that the mechanical properties change with the presence of the eutectic and varying excess content of elements. In this work, the automated-ball-indentation (ABI) technique was compared with the standard mechanical tests. The ABI method is based on controlled multiple indentations into a polished surface by a spherical indenter under load. The indentation depth is progressively increased to the maximum, user-specified limit, with intermediate partial unloading. This technique allows to measure the yield strength, stress-strain curve, strength coefficient and strain-hardening exponent. For all these test materials and conditions, the ABI-derived results were in very good agreement with those obtained with the conventional, standard test methods.

Characterization of Reactor Pressure Vessel Steel by ABI Testing

Microstructure of the base metal, the multilayer welding seam and the two-layer cladding was characterized through the thickness of the WWER 440 reactor pressure vessel wall. Mechanical properties were determined by performing a series of instrumented indentations across the weld at room temperature. The results were treated by so-called automated ball indentation technique. Mechanical properties obtained by instrumented indentation from the local stress-strain behavior were compared with minimum values required by the standard.

Estimation of Tensile Properties of Pressure Vessel Steel Through Automated Ball Indentation and Small Punch Test

Extraction of mechanical properties of in-service materials through small specimen test techniques has become attractive in the recent years. Of the available small specimen test techniques, automated ball indentation (ABI) and small punch test (SPT) methods have proved to be more promising. These test methods are basically non-destructive in nature and are proficient enough to extract the flow properties of the materials using small volume specimen. In this work, tensile properties of a pressure vessel steel (P12) have been estimated through ABI and SPT. The objective is to compare the capability of these test methods in determining the tensile properties. The influence of lubrication (between the indenter and the specimen) on the ABI response is also investigated. The ABI response is found to be similar and the effect on the tensile properties was under 2 %. The tensile properties estimated from ABI and SPT are found to be in good agreement with conventional tensile test results. Nevertheless, in case of SPT, the error in the estimation of yield strength and ultimate tensile strength using empirical correlations is significantly high. However, the use of analytical formulations to convert the SPT load–displacement response to stress–strain curve are found to be reliable, since the error in the estimated properties is considerably less. The ABI process is numerically simulated to study the stress–strain field beneath the indenter. The maximum strain occurs at the edge of the contact indicating the material displacement along the radial direction. The plastic zone beneath the indenter resemble hemispherical shape which is in agreement with the expanding cavity model. The nature of stress changes from compressive (right below the indentation axis) to tensile at the edge of contact. This indicates that radial cracks may initiate on the specimen and propagate outwards. The pile-up is significantly higher in the case of frictionless contact between the indenter and the specimen but found to converge for a value of around 0.2.

Evaluation of mechanical properties across micro alloyed HSLA steel weld joints using Automated Ball Indentation

The object of the present investigations is to evaluate the mechanical properties across the SMAW, SAW, FCAW and A-GTAW weld joints of micro alloyed HSLA steel. The correlation between microstructure, micro-hardness and tensile properties obtained using automated ball indent (ABI) has been undertaken. The variation of yield strength (YS), ultimate tensile strength (UTS) and strain hardening component (n) values across base metal, heat affected zone and weld metal were obtained from the ABI results and validated with the standard conventional tensile test results. The cross-weld joints were found to have minor increase in yield strength with corresponding decrease ductility as compared to the base steel. The high strength of weld metals was attributed to the presence of high percentage of bainitic and acicular ferritic microphases. The transformation of base metal to bainitic structure was found to be the significant factor contributing to the strength of HAZ in the weld joints. The tensile strength was observed to vary significantly across the weld joints. The strength values (YS and UTS) decreased systematically across the weld joint from weld metal to base metal. The strain hardening exponent was found to be comparable across the weld joints with marginal higher values for HAZ. The comparable values of strain hardening exponent for weld metal and base metal showed the balanced strength and ductility of the welds. The FCAW and SAW were found to be better welding techniques that provide sufficient strength and least variation in mechanical properties across the weld joints.

Comparison of Conventional Mechanical Testing with Innovative Techniques for Determination of Mechanical Properties of Nuclear Power Plant Components Materials

Within the nuclear power plant operational life management, components lifetime extension requires information on structural material degradation. Innovative testing methods of Small punch testing and Automated ball indentation test are based on the determination of material properties from sub-sized specimens. Present paper is focused on the employment of these techniques in the NPP irradiated materials testing and evaluation at the accredited hot cell testing laboratory of ÚJV Řež, a. s., Mechanical testing department. Comparison with the testing results from the conventional methods is depicted.

Evaluation of variation of tensile strength across 316LN stainless steel weld joint using automated ball indentation technique

Tensile strength variation across 316LN stainless steel fusion welded joint comprising of base metal, deposited weld metal and heat affected zone (HAZ) has been evaluated by Automated Ball Indentation (ABI) technique. Automated Ball Indentation tests were conducted on the various zones of the steel weld joint at 300, 523 and 923 K. The flow curves obtained from ABI results were consistent with corresponding conventional uniaxial tensile test results. The HAZ exhibited higher tensile strength than the other regions of the steel weld joint at all investigated temperatures. The ratio of ultimate tensile strength to yield stress (YS), which represents the work hardening behaviour, increased with an increase in temperature for the base metal and HAZ; whereas it remained nearly the same for the weld metal.