Measured Hardness Values Research Articles

Correlation of hardness of aluminum composites obtained by stir casting technology and the size and weight fraction of reinforcing Al2O3 particles

In this work, the stir casting method was applied to obtain composites based on the alloy AN EW 6061 used as a metal base, and Al2O3 particles as a reinforcement. Composites play a significant role as engineering materials. Therefore, it is necessary to study, in detail, the production methods and the factors that affect their mechanical properties. For this purpose, we have carried out a planned experiment wi ASM International th the aim to use regression analysis to predict the influence of particle size and mass fraction on hardness of the obtained composites. The full factorial experimental design with two factors was used, which was analyzed at three levels. Hardness was observed as a system response, while particle size and mass fraction were set as influencing factors. Influencing factors were observed at three levels: 50, 80 and 110 ?m for the particle size and 2, 5 and 8 mass%. Measured hardness values of the composites ranged from 72 HV10 to 80 HV10. Based on the probability values (p<0.05), it was determined which factors are important for the system response. Statistical analysis has shown that linear terms of the influence factors (size and mass fraction of reinforcement particles) and the square term of the mass fraction have statistical significance on the hardness change. The square term of the particle size and the interaction term of the influencing parameters do not have a statistically significant contribution in predicting the hardness value. Thus, a second-order polynomial model was obtained by the regression analysis. Influence of input factors on the system response and the adequacy of the obtained mathematical model were determined by using the Analysis of Variance (ANOVA). Based on the statistical data analysis, it was established that, the particle mass fraction has a greater influence on hardness of the obtained composite in relation to the particle size. By comparing the experimental and predicted values, a high degree of agreement was achieved so that the chosen model of the factorial experiment was adequate (R2=0.989). It can be also concluded that the developed regression model can be applied to predict hardness of the aluminum composite reinforced by Al2O3 particles in the chosen variation interval of particle size and mass fraction.

Correlation between microhardness and wear resistance of dental alloys against monolithic zirconia.

PURPOSEThe aim of this study is to compare the hardness according to the conditions of metal alloys. Moreover, the correlation between the cast crown hardness before and after wear testing and the degree of wear for each dental alloy was assessed.MATERIALS AND METHODSCast crowns of three metal alloys (Co-Cr, gold, and Ni-Cr alloys) opposing smooth-surface monolithic zirconia were used. The Vickers microhardness of the ingot (which did not undergo wear testing) and the cast crown before and after wear testing were measured for each alloy. Two-way ANOVA and Scheffé tests were used to compare the measured hardness values. Moreover, the Pearson correlation coefficient was used to evaluate the relationship between the surface hardness and the wear of the cast crown (α=.05).RESULTSThere was no significant difference in the hardness before and after wear testing for the gold alloy (P>.05); however, the hardness of the worn surface of the cast crown increased compared to that of the cast crown before the wear tests of Ni-Cr and Co-Cr alloys (P<.05). Furthermore, there was no correlation between the wear and hardness of the cast crown before and after wear testing for all three metal alloys (P>.05).CONCLUSIONThere was a significant difference in hardness between dental alloys under the same conditions. No correlation existed between the surface hardness of the cast crown before and after wear testing and the wear of the cast crown.

Producing non-dendritic A356 and A380 feedstocks: Evaluation of the effects of cooling slope casting parameters

Abstract In this study, low superheat casting and cooling slope casting processes were carried out with A356 and A380 aluminum alloys in order to obtain feedstocks with near globular microstructures. Castings were conducted at 20 °C above of the liquidus temperature for both processes and molten alloys were cast through a copper cooling slope with two different lengths (350 mm and 650 mm) and two different tilt angles (30° and 60°). In order to evaluate the significance of boron nitride coating on cooling slope, castings in short slope were carried out under both coated and uncoated conditions. Microstructural examinations and hardness measurements were carried out. According to obtained results, cooling slope casting caused superior microstructural properties as compared to low superheat casting, especially for A356 alloy. 30° tilt angle was found more efficient in order to obtain a more globular microstructure for A356, while, on the other hand, the tilt angle of 60° was detected more favorable in that manner for A380. Obtained grain size measurements were slightly improved with the employment of short slope and coating was found beneficial especially for A356 alloy. The measured hardness values did not display any significant difference in the same alloy type except in the low superheat cast A356 specimen, which was obtained with coarser microstructure.

Producing non-dendritic A356 and A380 feedstocks: Evaluation of the effects of cooling slope casting parameters

Abstract In this study, low superheat casting and cooling slope casting processes were carried out with A356 and A380 aluminum alloys in order to obtain feedstocks with near globular microstructures. Castings were conducted at 20 °C above of the liquidus temperature for both processes and molten alloys were cast through a copper cooling slope with two different lengths (350 mm and 650 mm) and two different tilt angles (30° and 60°). In order to evaluate the significance of boron nitride coating on cooling slope, castings in short slope were carried out under both coated and uncoated conditions. Microstructural examinations and hardness measurements were carried out. According to obtained results, cooling slope casting caused superior microstructural properties as compared to low superheat casting, especially for A356 alloy. 30° tilt angle was found more efficient in order to obtain a more globular microstructure for A356, while, on the other hand, the tilt angle of 60° was detected more favorable in that manner for A380. Obtained grain size measurements were slightly improved with the employment of short slope and coating was found beneficial especially for A356 alloy. The measured hardness values did not display any significant difference in the same alloy type except in the low superheat cast A356 specimen, which was obtained with coarser microstructure.

MATERIALS SCIENCE RESEARCH DEFECT OCCURRENCE AND RUPTURE OF THE GAS PUMPING UNIT AXIAL COMPRESSOR BLADE

The article discusses the results of the study of the causes of destruction of the axial compressor blade and the causes of the defect of the working blade 10 stage GTU GPA type GTK-10-4 station No. 18 of the compressor station “Dzhangala”. The blade is made of X20Cr13 steel (AISI 420). The blades have an operating time of 23 284 hours after the last repair. Chemical analysis, fractographic analysis, and hardness measurements were performed. The probable cause of defect formation and destruction of the blade is established. The measured hardness values comply with the standard. А study of the chemical composition of metal blades is not revealed a deviation in the content of the main alloying elements. The results of fractographic analysis showed that the destruction caused the presence of a subsurface defect. The defect is an oxidizing shell of about 9×4 mm in size with propagation in operation to the outer surface. In operation, as a result of corrosion, static and dynamic stresses, a crack was formed in the area of the source with exit to the surface along the input edge of the blade. Non-destructive testing was performed on the remaining blades. No defects detected. On the basis of which it was decided to continue the operation of the working blades of the GTK-10-4 turbine, subject to periodic monitoring.

Design of a low density refractory high entropy alloy in non-equiatomic W–Mo–Cr–Ti–Al system

A new refractory high entropy alloy (RHEA) with non-equiatomic composition of W10Mo27Cr21Ti22Al20 was designed based on empirical rules and CALPHAD approach. Calculated results showed that W10Mo27Cr21Ti22Al20 RHEA had lower density and higher melting temperature than that of equiatomic alloy. The designed alloy was prepared by vacuum arc melting then the crystal structure, microstructure, chemical composition, density, hardness and compressive strength had been evaluated. The alloy exhibited a BCC solid solution as major phase with typical dendritic microstructures. The experimental results confirmed calculated results, suggesting a BCC solid solution phase. The density of the alloy was 7.48 g/cm3, which was approximately close to the calculated value, 7.61 g/cm3. However, the measured hardness value (511 kg/mm2) was significantly higher than the calculated value (125 kg/mm2), indicating the effect of solid solution strengthening. This alloy had compressive yield strengths of 1245 MPa at room temperature, 510 MPa at 600 °C, 201 MPa at 1000 °C and 105 MPa at 1200 °C. Compressive strain of the alloy was limited at room temperature, but it increased significantly to above 60% at 1000 °C and 1200 °C. Comparing with four conventional superalloys Inconel 718, Waspaloy, Hastaloy X and Haynes 230, the studied RHEA shows lower density and superior high-temperature yield strength.

The effect of acid aging on the mechanical and tribological properties of coir–coconut husk-reinforced low-density polyethylene composites

The present study investigates the physical, thermo-mechanical and tribological properties of coir–coconut husk particulate-reinforced polymer composites subjected to a corrosive environment. The composites were prepared by the conventional facile hot compression molding method. The composite was immersed in a strongly acidic environment of pH 2.2 for a period of 3, 6, and 9 days. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis were used to elucidate the structure and morphology of the composites. The thermal analysis using differential scanning calorimetry, water absorption, hardness, coefficient of friction and wear rate was performed as per the ASTM standards to characterize the as-prepared and aged composites. The experimental test results revealed that with an increase in acid aging time, the acid aged samples lost surface matrix such that the fiber was seen on the surface. The effects of corrosion seemingly reduced the crystallinity of the acid aged samples allowing amorphous regions to be trapped within the crystals. Water absorption of the samples increased with aging time due to inherent voids in the specimens as weight gain values were 5.27, 16.80, 19.33 and 19.91%, respectively for control and acid aged samples. Hardness values initially decreased with immersion time and increased which was attributed to the crystallinity of the specimens and to some extent the elemental carbon present in the specimens before and after aging. The measured hardness values of the control and acid aged composites were 2.98, 7.27, 14.40 and 9.07 HV, respectively. From the thermal analysis, it was noticed that the glass transition temperature (Tg) of the polymer shifts to higher temperatures as the aging time in the acidic medium increased, which can be attributed to cross-linking of the polymer chains. The control specimen shows higher coefficient of friction (CoF) because they are more rigid than the acid aged samples, and hence under dry sliding can cause more friction leading to increased heat and CoF.

Processing, microstructure and mechanical characterization of a new magnesium based multicomponent alloy

In the present study, a new magnesium based multicomponent alloy consisting of five alloying elements was synthesized using the technique of Disintegrated Melt Deposition (DMD). Most of the conventional Mg based alloys have one dominant principle element with a small amount of other alloying elements. In this work, the alloy having a chemical composition of Mg70Al18Zn6Ca4Y2 (atomic pct.) was designed by incorporating multiple alloying elements (Al, Zn, Ca and Y) with high amount of alloying element concentration (2 to 18 atomic pct.) into principle element, Mg. Investigations were done on the microstructure and mechanical properties of the developed alloy. The formation and presence of phases and microstructural evolution of the alloy were examined using scanning electron microscopy, energy dispersive x-ray (EDX) analysis and x-ray diffraction (XRD) analysis. The mechanical properties of the developed alloy was determined by way of hardness tests and compression testing. Significantly high macrohardness (59 HRB) and microhardness (150 HV) was realized in Mg70Al18Zn6Ca4Y2 multicomponent alloy. The measured hardness values in this alloy was much higher when compared to those of commercial Mg alloys.

Lifetime extension prediction of the rejuvenated first stage gas turbine blades

ABSTRACTThis paper presents a novel method for the first stage gas turbine blades lifetime extension based on degraded GTD-111 substrates hardness evolution after the rejuvenation heat treatments (RHTs) process. Moreover, the RHT impact on directionally solidified (DS) GTD-111 nickel-based suparalloys homogenisation and blades hardness properties were revealed and discussed. The measured hardness values indicate that turbine blades hardness progress is closely related to the nucleation of topologically close packed (TCP) phases, irregular γ’ (Ni3, Al) precipitates growth, and Cr-rich M23C6 Coarsening at grain boundaries. Aspects like γ′/γ eutectic regions rafting, cooling holes oxidation attacks, and formation of cavities within γ-matrix were also identified and analysed.

Prediction of Jominy Hardness Curves Using Multiple Regression Analysis, and Effect of Alloying Elements on the Hardenability

The prediction of Jominy hardness curves and the effect of alloying elements on the hardenability of boron steels (19 different steels) are investigated using multiple regression analysis. To evaluate the hardenability of boron steels, Jominy end quenching tests are performed. Regardless of the alloy type, lath martensite structure is observed at the quenching end, and ferrite and pearlite structures are detected in the core. Some bainite microstructure also appears in areas where hardness is sharply reduced. Through multiple regression analysis method, the average multiplying factor (regression coefficient) for each alloying element is derived. As a result, B is found to be 6308.6, C is 71.5, Si is 59.4, Mn is 25.5, Ti is 13.8, and Cr is 24.5. The valid concentration ranges of the main alloying elements are 19 ppm < B < 28 ppm, 0.17 < C < 0.27 wt%, 0.19 < Si < 0.30 wt%, 0.75 < Mn < 1.15 wt%, 0.15 < Cr < 0.82 wt%, and 3 < N < 7 ppm. It is possible to predict changes of hardenability and hardness curves based on the above method. In the validation results of the multiple regression analysis, it is confirmed that the measured hardness values are within the error range of the predicted curves, regardless of alloy type.

Micro hardness determination on a rough surface by using combined indentation and topography measurements

Micro hardness determination on rough surfaces is a topic of interest e.g. in industrial applications where the component surface is of functional relevance. In most approaches, hardness on a rough surface is determined by including profile roughness parameters like Ra (e.g., [–]) to adjust measured hardness values or to get the minimum indentation depth value where the influence of the surface topography is assumed to become negligibly small. In the present study, local surface topography data were used instead to enable precise micro hardness measurements on a sample with arbitrary surface topography. Samples made of 1.457 1 stainless steel with different surface states were face milled by using an end mill with different feed rates. Instrumented indentation tests were performed on these samples as well as on comparative samples with polished surfaces. From the resulting load-indentation depth (F-d)-curves the averaged indentation hardness was calculated for all surface states. A method was applied to manipulate and to average the F-d-curves to eliminate deviations, occurring at the beginning of the indentation. The indentation hardness was calculated from these modified F-d-curves and compared to the indentation hardness from the actually measured F-d-curves of the polished samples with feasible results. Using surface topography measurements is considered to enable deriving more accurate indentation hardness values directly and to put the investigations to another level. The surface topography of the samples was evaluated by confocal microscope measurements before and after the indentation tests. From the surface topography data at the location of indentation, four parameters were calculated: volume, projected contact area and depth of the indentation mark, as well as the curvature of the surface topography before indentation. These parameters were correlated with the hardness value from the respective indentation and compared to the indentation hardness of the polished sample. The results of the present study are the basis for combining optical imaging techniques like confocal microscopy or white light interferometry and indentation testing equipment to broaden the field of application.

Study of the mean size and fraction of the second-phase particles in a 13% chromium steel at high temperature

ABSTRACTThe mean size and fraction of the second-phase particles in a 13% chromium steel are investigated, while no plastic deformation was applied. The results of the measurement are compared with the modelling results from a physicallybased model. The heating sequence is performed on samples using a Gleeble thermo-mechanical simulator over the temperature range of 850–1200°C. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), the size distribution and composition of the carbides were evaluated, respectively. For obtaining particle size distribution (PSD), an image-processing software was employed to analyse the SEM images. Additionally, the relation between the 2D shape factor and size of the particles is also studied at different temperatures and most of the particles turned out to have a shape factor close to two. In order to measure the carbide weight fraction, electrochemical phase isolation was employed. The Ms and fraction of the martensite phase after quenching of samples are calculated and the results were comparable with the measured hardness values at corresponding temperatures. The measured hardness of the samples is found to comply very well with the measured mean size of the precipitates. The calculated mean size of the particles from the model shows very good agreement with both hardness value and experimentally measured mean size, while the calculated volume fraction from simulation follows a slightly different trend.

Precipitates evolution during tempering of 9CrMoCoB (CB2) ferritic heat-resistant steel

In this paper, the evolution of precipitate phases in 9CrMoCoB (CB2) steel after tempering at 650 °C for different times has been investigated using transmission electron microscopy. As the tempering time prolonged from 0 to 100 h, four types of secondary phases consisting of M3C, M2X, M7C3 and M23C6 were identified in the experimental steel. It was found that the needle-like M3C carbides could be precipitated during both the air cooling process after austenization and the early stage of tempering. After tempering for 15 to 120 min, fine M2X particles were observed. The precipitation and growth of M2X were derived from the partially consumption of M3C carbides. After tempering for 50 h, M2X disappeared, while the rod-like M7C3 carbides were observed to be precipitated independent of the early formed M3C precipitates. After tempering for 100 h, only M23C6 carbides persisted, of which the number density and size kept increasing throughout 30 min to 100 h owing to their high thermostability. In addition, the precipitation mechanisms of the four types of precipitates were explained, and the relationship between the measured hardness values and microstructure was discussed.

Investigation of the Mechanics, Composition, and Functional Behavior of Thick Tribofilms Formed from Silicon- and Oxygen-Containing Hydrogenated Amorphous Carbon

A custom-grown silicon and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) film is subjected to ball-on-flat tribometry under controlled sliding environments (ambient, dry air, and dry N2) at room temperature using a 52100 steel ball. The resulting friction coefficient is below 0.2 in ambient air and below 0.1 in dry N2. Tribofilms on the steel ball with thicknesses in excess of 500 nm are observed. The tribofilms are derived from the a-C:H:Si:O and grow on the steel ball, and display chemical and structural modifications relative to the original a-C:H:Si:O film. Sliding of the tribofilm-coated steel ball against bare silicon results in low friction, highlighting the inherent lubricity afforded by the tribofilm. Tribofilms grown through sliding against a-C:H:Si:O are characterized, post-sliding, with multiple spectroscopic and imaging techniques which collectively demonstrate that the composition and structure of the tribofilm is strongly dependent on the sliding environment. The unusually high tribofilm thickness allows for nanoindentation analysis, which demonstrates that the films are laterally heterogenous and softer than the original a-C:H:Si:O, with moduli and hardness values ranging over three orders of magnitude. Many regions of the tribofilms are extremely soft, with measured hardness values below 100 MPa and reduced Young’s moduli below 1 GPa, and also show a viscous mechanical response. Transmission electron microscopy and electron energy loss spectroscopy (TEM/EELS) characterization of the tribofilm demonstrates that the bulk structure is not graphitic, and indicates the tribofilms are enriched in C−H bonding. Additionally, there is a marked segregation within the tribofilm of Si/O and carbon. It is proposed that a primarily polymeric tribofilm structure can explain the observed mechanical properties.

Insights into intragranular precipitation and strengthening effect in Al2O3/SmAlO3 ceramic with eutectic composition

This work concerns the microstructure-mechanical properties relationship in Al2O3/SmAlO3 composites prepared by hot pressing at temperatures from 1450 °C to 1650 °C. Dense samples have been prepared using co-precipitated trigonal α-Al2O3 and orthorhombic SmAlO3 at the pseudo-binary eutectic composition. The overall microstructure coarsens with increasing processing temperature, but the nanoscale precipitates within α-Al2O3 and SmAlO3 grains contribute to coherency strains and distortions in Al2O3/SmAlO3 prepared at 1650 °C. The intragranular SmAlO3 precipitates in Al2O3 grains exhibit semi-coherent interface and an orientation relationship of the intragranular phase SmAlO3 and alumina matrix close to two different zone axes, namely [23¯1]SmAlO3 and [4¯401]Al2O3. Consequently, significant strengthening effect is found from the measured hardness values on the sample sintered at 1650 °C. The measured flexural strength and single edged notched beam fracture toughness of the Al2O3/SmAlO3 ceramic are comparable to traditional secondary phases reinforced alumina composites. The sample sintered at 1500 °C exhibits a flexural strength of 558 MPa and a fracture toughness of 5.59 MPa⋅m1/2.

Optical and Mechanical Characterization of Sputtered AlN/Ag Multilayer Films

This study explores the synthesis of ceramic/metal multilayers with a wide range of transmittance values while examining their resulting microstructure and mechanical behavior. AlN/Ag nanomultilayers are synthesized by reactive DC magnetron sputtering with either repeated bilayer thicknesses or layer thicknesses optimized for transmittance. Five structures are investigated, and all show agreement with predicted transmittance values. The measured hardness values range from 4.9 to 19.4 GPa, while the failure is brittle‐dominated fracture, independent of the Ag layer thickness. The optically optimized samples exhibit a fourfold increase in hardness and increase optical performance over the repeated bilayer samples, despite the similar deformation behavior.

Prediction of Hardness in Drawn Copper Wire by Effective Strain from Finite Element Simulation

The study developed a hardness-strain reference curve to be used with the finite element simulation for the prediction of the hardness in the drawn copper wire. The hardness values of the deformed copper specimens from tensile tests were analyzed to construct a relationship between hardness and strain. By using an industrial wire drawing machine, a copper wire was drawn by 5 passes to reduce its diameter from 8 to 4.64 mm. All drawing dies used the same configurations which include an area reduction ratio of 20 percent, an approach angle of 7°, and a bearing length of 0.5 times the feeding wire diameter. The finite element simulations of the wire drawing processes were also performed to predict the effective strains in the drawn copper wires. With the use of the developed hardness-strain curve, the hardness of the drawn wires can be estimated. The results show that the difference between the predicted and measured hardness values is about 10 percent lower in the early stage of the wire drawing process, and the difference increases with the number of passes to about 30 percent higher in the later stage of the process.

Molecular dynamics and experimental studies of nanoindentation on nanoporous silica aerogels

The nanomechanics during the indentation test on low-density nanoporous silica aerogels remains one of the least understood and explored areas of mechanics. In the present work, we performed nanoindentation using a spherical indenter on silica aerogels to investigate the mechanical properties, such as elastic modulus and hardness, and also, the deformation behaviour. Using all-atom simulations on large samples, the elastic modulus is computed from the elastic part of force–depth curves that can be fitted to the Hertz law, which shows that it increases with density. We proposed a novel approach to calculate the projected true contact area in nanoindentation and to estimate an accurate hardness of silica aerogel, which has a highly complex and randomly arranged network of atoms structure. The experimental studies of nanoindentation are performed on silica aerogel, which reveals that the measured elastic modulus is in good agreement with the simulations. However, the measured hardness values are nearly close to the projected contact area method. It suggests that in all-atom simulations the computed high hardness values using the proposed true area method are the actual local contact pressure. This new understanding may help to expand the use of computer simulations to explore the nanoindentation processes at the molecular level and to advance the macroscopic hardness calculation.

Fracture Toughness Evaluation and Plastic Behavior Law of a Single Crystal Silicon Carbide by Nanoindentation

Nanoindentation-based fracture toughness measurements of ceramic materials like silicon carbide (SiC) with pyramidal indenters are of significant interest in materials research. A majority of currently used fracture toughness models have been developed for Vickers indenters and are limited to specific crack geometries. The validity of the indentation-cracking method for the fracture toughness measurement of single crystal SiC, the elastic-plastic anisotropy and orientation dependence around the c-axis when indented in the &lt;0001&gt; direction is examined using nanoindentation with different pyramidal indenters. The residual impressions are analyzed using scanning electron microscopy to measure the crack lengths and the validity of existing fracture toughness measurement methods and equations is analyzed. A combination of nanoindentation with different pyramidal indenters to produce a wide range of effective strains and finite element simulation is used to extract flow properties of single crystal SiC in the &lt;0001&gt; direction. It is observed that there is no orientation dependence around the c-axis when SiC-6H is indented in the &lt;0001&gt; direction with a Berkovich indenter, i.e., it is transversely isotropic. It is also found that for a Berkovich indenter, the Jang and Pharr model, which is based on the Lawn model for cone/halfpenny cracks, gives approximately constant values at low loads (&lt;1 N), while at higher loads (&gt;1 N), the Laugier model gives constant fracture toughness values. Finite element analysis using equivalent cones is used along with measured hardness values to estimate the yield strength, the work hardening exponents and the stress–strain curve for single crystal SiC-6H in the &lt;0001&gt; direction.

Comparison and combination of two NDT methods with implications for compressive strength evaluation of selected masonry and building stones

The classic Schmidt hammer (SH) and the Equotip hardness tester (EHT) are the widely used nondestructive (NDT) instruments for measuring surface dynamic hardness (rebound hardness) of materials. In previous studies, both instruments have been used individually to establish empirical models for uniaxial compressive strength (UCS) evaluation of rock materials. However, no specific research seems to have been undertaken to investigate their relative UCS prediction effectiveness when performed on the same rock samples. In this study, we evaluated the relative UCS prediction performance of these two NDT instruments by performing laboratory tests on core specimens of some selected rock materials used as masonry and building stones. The correlations of Schmidt hammer rebound hardness (SHR) and Equotip hardness (HLB) to UCS were determined by using regression analyses and test statistics. Also, the variability of each test method was evaluated by employing variance analyses. The results indicated that the established UCS prediction models for both NDT methods were significant in the statistical sense (p value <0.05), with the EHT method showing somewhat better prediction accuracy (R2 = 0.87; accuracy ratio = 0.60) compared to the SH method (R2 = 0.84; accuracy ratio = 0.78). Data scatter graphs showed increasing standard deviation in UCS for increasing rebound indices. An average conversion factor (k) of 15.7 was derived in the zero-intercept regression model HLB = k ⋅ SHR, which may be used in practice as a guide to translate the measured hardness values from one instrument to another. For improved UCS prediction accuracy, an attempt was made to evaluate the potential use of a newly proposed combined NDT method involving the simultaneous use of SH and EHT test results. With this kind of approach, a relatively better prediction accuracy was achieved (R2 = 0.89; accuracy ratio = 0.52) compared to the use of the SH and EHT methods individually.