Experimental Study on Mechanical Properties of 3D-Printed Specimens of Iron Oxide, Quartz, and Bedded Composites Under Uniaxial Compression and Indirect Tensile Strength

Abstract

The heterogeneity of natural rocks produces increased variations in the results of geomechanical and metallurgical tests making the repeatability of experimental work questionable. Fabricated test specimens have, therefore, become more attractive for fundamental studies. In this study, quasi-identical 3D-printed (3DP) specimens with 10 and 16 mm diameters were fabricated and tested to study material strength and understand the breakage characteristics at a scale more suitable for comminution. Cylinder specimens composed of quartz (named Si) and iron oxide (named Fe), with sorted grains of ∼100–150 μm in the form of homogeneous specimens (3DP-Si and 3DP-Fe) and heterogeneous specimens (bedded) (3DP-SiFeSi, 3DP-SiFe, and 3DP-FeSiFe) were tested. This article presents the results for experimental Unconfined Compressive Strength (UCS) and Brazilian Tensile Strength (BTS) tests. The elastic property was obtained from the UCS tests, while tensile strength was obtained from BTS tests. The strength of 3DP specimens of similar diameter decreases following the types: 3DP-Si (most competent), 3DP-Si-Fe, 3DP-SiFeSi, 3DP-FeSiFe, and 3DP-Fe (less competent). The results show that heterogeneous 3DP specimens were influenced by bedding angle, thickness, and mineral group composition. It also seems that the sequence of mineral composition and the number of beds play a role, rather than the overall grain percentage area for each cylinder, in influencing the strength and variability of fragments. Finally, the brittleness indices for 3DP specimens were calculated as a function of UCS and BTS.

Introduction

Mineral extraction involves a number of sophisticated processes, including blasting, comminution, and flotation.¹ The mechanical environment inside each of these processes is significantly complex and cannot be disregarded. However, to fully understand the interaction between stressing conditions and the structural features of rocks, tests must concentrate on these rock features/properties. Traditional metallurgical tests and modeling are limited in addressing this matter. Typically, optimal results treat the processed rock as a bulk material rather than by having a clear understanding of the properties and breakage characteristics of individual rock particles of various sizes (millimeter to micron), mineralogy, and texture, which influence the processing plant response.

Indeed, the high heterogeneity of natural rocks with varied shape and size makes the repeatability of experimental work even more challenging in mineral processing. Consideration of traditional guidelines for international standards of rock mechanics shows that its sampling and testing requirements are tedious and time-consuming.^2,3 In addition, no consideration exists to scale down properties from typical rock mechanics tests for metallurgists, the scale of interest is smaller. Therefore, being able to fabricate and test synthetic rock either isolating or controlling the effect of heterogeneity and having the benefit of infinite cloned specimens of varied smaller sizes is important for testing validation and attractive from a research perspective.

This study investigates the effect of size, mineralogy, and texture on the mechanical properties of fabricated rock particles using 3D-printed (3DP) specimens. It builds on previous studies using single mineralogy—quartz (fabricated sandstone) specimens of varied sizes and shapes.⁴ However, for the first time, 3DP specimens are fabricated with two different mineral grains to emulate contrasting mechanical characteristics and increase the degree of anisotropy, which is of interest across many disciplines in mining and metallurgy. The addictive manufacturing for printing rocks will help researchers on these challenges of repeatability, scale, mineral grain interactions, and fragmentation occurring in quasi-identical specimens.

Furthermore, the outcomes of this study can assist in establishing a practical approach to implement Discrete Element Modeling such as ESyS-particle⁵ to simulate the response of rock particles to different breakage mechanisms,⁶ which can then be utilized to improve comminution models and devices (e.g., crusher, rolls, and ball mills). Following King,⁷ an understanding of the breakage process should be used to design new comminution machines, leading to an improved comminution process.

This study investigates the influence of strength on homogeneous and heterogeneous 3DP cylinder specimens composed of two mineral groups, simulating different laminated strata, mechanical properties, and fracture characteristics under unconfining stress. The Unconfined Compressive Strength (UCS) and Brazilian Tensile Strength (BTS) tests were carried out on two and five different 3DP specimens, respectively. Brittleness indices for the 3DP specimens tested were then calculated using UCS and BTS. The experimental work performed shows the stress-strain relationship, deformation characteristics, strength characteristics, failure modes, and fragmentation of 3DP specimens. The results will help better understand and predict small particles’ behavior in breakage of contrasting minerals (i.e., mineralogy and texture controlling the particle's strength and influencing fragmentation), a pathway to improve breakage and liberation during mineral processing.

Background

The measurement of mechanical strength, toughness, and brittleness is important in determining the particles’ load-bearing capacity, deformation, fracturing and crushing. The UCS and the BTS are basic rock mechanics properties required for numerical modeling. UCS is a measurement of the rock's compressibility, and BTS is an indication of the cohesion of the bonds between material grains. According to Hucka and Das,⁸ a large difference exists between UCS and BTS in brittle rocks. Brittle rocks can present pre-existing fractures and/or cumulative damage, which facilitate fracture in tensile and shear modes, while ductile rocks present plastic deformation characteristics that inhibit fracture propagation. The benefits of predicting the brittleness-related material properties can affect geotechnical as well as metallurgical engineering practices such as cuttability and drillability of rocks,⁹ fracture toughness,¹⁰ rock burst and underground stability,¹¹ wellbore stability, and hydraulic fracture.¹²

While rock mechanical properties tend to be used to describe rock mass behavior, comminution deals with fragmentation (size reduction) of varied blocks/particles in mineral processing plants, which have limitations and challenges in testing. Also, it is not proved that relationships established to scale to rock mass holds when scaling down. In mineral processing, several mineral compositions at varied proportions are subject to similar stress concentration to active particle size reduction.

According to Kendall,¹³ different rock materials have specific size limits whereby cracks do not propagate, and the compression force corresponds to that required for gross yielding, meaning that there is a transition from brittle to ductile behavior as the specimen size decreases. In this context, it is recognized that there is a size limit to the efficient crushing of materials. It can also be seen that energy is wasted during crushing if no consideration is made to characterize rock/particle behavior, because smaller fragments might be deforming plastically rather than cracking.

In terms of material heterogeneity and anisotropy, natural rocks are composed of various mineral grains and composition formed through a geological process. The degree of heterogeneity depends on increase in depth of extraction, internal defects or natural fractures (porous, flaws), bedded zones or massive rocks (texture and fabric), lateral continuity and connectivity of the rocks following their mineralogical distribution and grade, preexisting geological structures, alteration, oxidation, in situ stress conditions/tectonic stress, and so on.^14,15 The anisotropy refers to specimens presenting a directionally dependent mechanical behavior. This feature can be attributed to a single plane of weakness on the intact core tested for strength.¹⁶ Overall, it is known that the direction of fractures, flaws, and heterogeneities into the specimen axis relative to the applied force orientation affect the plane of propagation and the crack orientation.^17–23

While geotechnical engineers and metallurgists face the opposite end goals of stability versus grindability, when studying the mechanical characteristics of rock, consideration of heterogeneity and anisotropic effects during sampling, testing, and data interpretation are equally important for both disciplines. However, metallurgical tests, such as the “standard” Julius Kruttschnitt Drop Weight Test (JKDWT), are not performed to capture heterogeneity and directionally dependent mechanical behavior. The comminution characterization approach using the “standard” breakage test breaks many particles of selected sizes with no concern for the varying angles between the applied force/energy (loading axis) and the orientation of the anisotropic features, particle shape or single-particle mass, nor secondary breakage of fragments.

The justification for this is that mineral processing devices are unable to apply these controls. Then, the rock breakage parameter, A × b index, is a single value to indicate amenability of the material to fragmentation²⁴ although no straight correlation to the rock mechanical strength can be made.²⁵ However, it has been agreed that a lack of understanding of this area, including orebody variability and characteristics, limits the chances for optimization in mineral processing.

In summary, the breakage of individual particles and variability of the rock are not considered when the degree of breakage is generalized (i.e., assumed to be similar) for a group of heterogeneous and anisotropic specimens. Breakage without an appropriate characterization is a missing opportunity to further understand the particle behavior. Consequently, the inability to track the variability of particles’ characteristics, breakage behavior, and other breakage mechanisms (e.g., attrition) result in unpredicted or uncontrolled variation in process performance. Also, because comminution is used to liberate the valuable mineral to a point where it can be separated from enough of the gangue to create a concentrate, the focus is given to the equipment's ability to grind fine, which might not be the best approach for all ore types.

In general, lack of knowledge on breakage and liberation of individual particles limits modeling capabilities and the prospect of improved recovery. Then a question to be investigated would be, for instance, to what extent a combination of mineralogy, shape and size of specimens’ matter to an entire processing plant. It means that the initial steps to integrate knowledge from different disciplines are growing.

Materials and Methods

Fabricated 3DP specimens

Much work has been dedicated to fabricated 3DP specimens using powder and mineral grains. The principle of 3D printing involves a controlled accumulation of sequential-layer material.²⁶ So, the fabricated specimens can be used to explore the material mechanical behavior,^4,27–33 indicating that absolute elastic properties might be comparable to natural rocks. 3DP specimens still present a degree of anisotropy, as demonstrated in the previous study.⁴ Also, it is anticipated that 3DP is advantageous to address the issue of coring procedures introducing fractures, uncontrolled heterogeneity, and grain size; bedding orientation; matrix content (chemical composition); and scale (specimen size).

This study made use of fabricated 3DP specimens to improve understanding of breakage and to clarify some aspects of fragmentation for comminution. For the first time, iron oxide (named Fe) mineral grains are introduced to the 3DP process to fabricate novel homogeneous (made of Fe) and bedded (made of quartz and iron oxide) specimens. For that, iron ore rocks were crushed and screened by size, then material retained at 150 μm sieve size were magnetically separated to obtain grains of Fe mineral group for the printer.

The printing process was conducted at the University of Alberta in Canada. As in previous works using quartz (or silica sand, named Si), the printing process followed for all 3DP specimens used 20% binder saturation (8 vol. %) and post-curing at 80°C for 24 h.^30,34 For printing, the Si or Fe and the activator (p-toluene sulfonic acid) are mixed at low speed to coat the grains. Then, the acid-coated grains are added to a hopper of the M-Flex printer to be deposited into a vibrating spreader (recoater).

During the fabrication process, the print head dispenses binder liquid (furfuryl alcohol) on a bed of mineral grains (∼250 μm thick) while moving along the X and Y-axis, following the patter of the digital file preload onto the M-Flex computer.³⁴ Once printed, the job box is removed without disturbing the specimens and placed into a large oven set to 80°C. Afterward, the specimens are removed and cleaned with a wire brush to remove loose grains/particles.³⁴

The usage of different mineral groups to fabricate 3DP specimens is done to test the influence of two mineral compositions, Fe and Si, by playing with the bed sequence as well as thickness of printed beds. The fabrication of specimens of varied laminated strata allowed for different types of planar fabrics in the specimens. The term bed is used for layers of mineral grains distinctly different from overlying and underlying subsequent beds. In this context, the individual beds can be considered relatively homogeneous, while the effects of controlled heterogeneity and weak planes on deformation and strength characteristics should be attributed to the structure of the bed and sequence.

The 3DP bedded specimens are fabricated with a similar mineralogical composition to the nonbedded 3DP specimens. All 3DP specimens were printed in a vertical direction (i.e., layered horizontally while the cylinder is printed from base to top with a similar bedding angle). The 3DP specimens fabricated for this study, following their features, are named below:

• 3DP-Si: entire cylinder printed with layers of Si grains (similar to previous studies)^4,30,33;

• 3DP-Fe: entire cylinder printed with layers of Fe grains;

• 3DP-SiFe: cylinder with two beds, half specimen printed with layers of Si and other half printed with layers of Fe grains;

• 3DP-SiFeSi: cylinder with three beds, one-third printed with layers of Si, followed by one-third printed with layers of Fe, then another one-third printed with layers of Si grains;

• 3DP-FeSiFe: cylinder with three beds, one-third printed with layers of Fe, followed by one-third printed with layers of Si, then another one-third printed with layers of Fe grains.

3DP specimens with diameters of 16.2 ± 0.5 mm were tested for UCS and BTS. Also, smaller specimens with diameters of 10.4 ± 0.1 mm were tested for UCS. Following the orientation of the printed specimens, the strength measured in the UCS testing occurs when axial stress is applied perpendicular to bedding. While in the BTS testing the stress is applied parallel to the bedding direction. Figure 1 depicts different types of specimens tested in this study and the direction of applied stress.

FIG. 1.

3DP specimens, printed orientation, and testing. 3DP, 3D printed.

A summary of the specimens tested, which includes average density, mineral grain characteristics, the average diameter, average ratio (length-to-diameter ratio [L/D]) or (thickness-to-diameter ratio [t/D]), and the number of specimens tested, is shown in Table 1. The average density of specimens was measured with the helium pycnometer considering five measurements. Notice that reported average values with 95% confidence intervals are presented along this study.

Table 1.

Mineral Composition and Content of 3D Printed Tested for Unconfined Compressive Strength and Brazilian Tensile Strength

	Density (g/cm3)	Modal mineralogy weight (%)		UCS			BTS
		Quartz	FeOx Med-High	Specimens diameter	Avg. ratio (L/D)	Specimens tested	Specimens diameter	Avg. ratio (t/D)	Specimens tested
3DP-Si	2.6169 ± 0.0008	100	0	10.58 ± 0.07	2.25:1	6	16.61 ± 0.03	0.98:1	9
				16.55 ± 0.04
						6
3DP-Fe	4.3643 ± 0.0023	5	95	10.3 ± 0.07	2.29:1	4	16.27 ± 0.14	1.00:1	10
				15.93 ± 0.32		6
3DP-SiFe	3.1604 ± 0.0004	49	51	—	—	—	16.66 ± 0.04	1.07:1	10
3DP-SiFeSi	3.1242 ± 0.0011	—	—	—	—	—	16.64 ± 0.08	0.96:1	8
3DP-FeSiFe	3.6029 ± 0.0015	—	—	—	—	—	16.58 ± 0.04	1.07:1	13

3DP, 3D-printed; BTS, Brazilian Tensile Strength; Fe, iron oxide; L/D, length-to-diameter ratio; Si, quartz; t/D, thickness-to-diameter ratio; UCS, Unconfined Compressive Strength.

The modal mineral composition was verified using a scanning electron microscope (SEM) based image analysis. The specimen selected for the Mineral Liberation Analyzer (MLA) was the 3DP-SiFe (16.7 mm diameter and 15.5 mm height) (Fig. 2). The sample block containing a cylinder, sectioned perpendicular to the specimen diameter, was mounted in an epoxy resin, polished and carbon-coated before measurement to obtain an electrically conducting surface. The sample was analyzed with a grain X-ray mapping to produce a back scattered electron image. The MLA image shows how the printed process ended up for the 3DP-SiFe specimen (one bed of Si and one bed of Fe).

FIG. 2.

Image from MLA (SEM-based classified image) for 3DP-SiFe specimen. MLA, Mineral Liberation Analyzer; SEM, scanning electron microscope.

Zooming in on the two beds’ contact shows the Si layers (left) of quite uniform sub-rounded, medium-grained material where 78% of the grains range from 106 to 180 μm (D50 is equal to 150 μm), evenly distributed. In contrast, the Fe layers (right) shows that the majority of the layers comprised Fe grains (classified as medium to high iron content) of irregular shape, fine-to-medium grained, in addition to some quartz grains of both an irregular and subrounded shape. The grain size distribution for Fe is more widespread, with 78% of the grains ranging from 53 to 150 μm (D50 is equal to 100 μm).

Experimental apparatus and procedure

The fabrication and testing of the specimens follow the recommendations and guidelines of the American Society for Testing and Materials (ASTM),³⁵ and the International Society for Rock Mechanics (ISRM),^36–38 and suggested method of the ISRM,³⁹ except that smaller cylinder specimens rather than standard size cores, are used. The specimen L/D was 2.27 ± 0.03 for the UCS, and the t/D was 1.01 ± 0.01 for the BTS. It is worth noting that the end surfaces (top and base) of 3DP-Fe specimens fabricated for the UCS test were friable (lost grains); therefore, contact surfaces were not perfectly flat and smooth (according to the guidelines). Such a condition might influence the results presented for 3DP-Fe specimens tested for UCS. The 3DP-Fe specimens fabricated for BTS tests were more consistent in shape with only a couple of specimens showing grain loss.

UCS testing

The experimental work was carried out on a servo-controlled INSTRON 5584 load frame with an advanced video extensometer (AVE) system to ensure accurate control and measurement of 3DP specimens tested for UCS. This testing apparatus allows for measurements to be recorded digitally to obtain stress-strain data, while the AVE records the specimens’ axial strain during the entire test. The strain using the AVE is estimated based on tracking the proximity of two dot points painted on each of the 3DP specimens (i.e., the relative difference in axial displacements from pixels) and detected by an advanced digital image correlation algorithm. The tangent Young's modulus was calculated following ASTM, passing the initial portion of the stress-strain curve, where the middle (50%, average slope) of the linear prepeak portion of the stress-strain curve and the end of the stress-strain curve immediately before maximum stress were taken.

Fabricated 3DP-Si and 3DP-Fe specimens were tested at room temperature (22–23°C) using ± 10 kN load cell (maximum force capability) with the ability to accurate measure forces as low as 1/500th of the load cell capacity to an accuracy of 0.5% of reading. The specimens were compressed at a rate of ±0.2 mm/min until failure. The specimens underwent an initial load of ±0.05 kN for 3DP-Si and ±0.01 kN for 3DP-Fe to establish close contact between the specimen and INSTRON frame platens before loading. The maximum load at failure was applied to calculate the UCS of the specimens.

BTS testing

The BTS test is a well-known indirect method using a circular disk load under diametral compression. An INSTRON 4505 load system was used for compression during BTS testing to provide high control accuracy and data collection. Measurements were recorded digitally to obtain stress-strain data.

Fabricated 3DP specimens were tested at room temperature (22–23°C) using ±1 kN for 3DP-Fe specimens and ±5 kN load cell for all other specimens. These load cells also provide a wide range allowing accurate measure force to make downs to 1/500th of the load cell capacity to an accuracy of 0.5% of reading. The specimens were compressed at a loading rate of ±1.2 mm/min for 3DP-Fe, ±2.5 mm/min for 3DP-Si, 3DP-SiFe, and 3DP-SiFeSi, and ±1.8 mm/min for 3DP-FeSiFe, until failure occurred within the time range proposed at the ISRM, suggested method.³⁹

Since 3DP specimens were printed with a diameter of 16.55 ± 0.05 mm, a mini-Brazilian jaw (curve loading ring) with two symmetrical peripheral arcs was built to accommodate the 3DP specimens (Fig. 3). When performing the BTS test, none of the specimens was preloaded; however, close contact between the mini-Brazilian jaw and the INSTRON frame platens was verified before testing.

FIG. 3.

Standard Brazilian jaw and mini-Brazilian jaw used for BTS test. BTS, Brazilian Tensile Strength.

Results and Discussion

UCS results

The UCS was carried out on intact 3DP-Si and 3DP-Fe specimens. Figure 4a shows results from the 3DP-Si specimens, the main crack tends to propagate in a combination of longitudinal splitting with a typical “end cone” (Y-shaped shear failure) where two shear planes occurred along the loading direction.^34,40 It is noted that the fracture on 3DP-Si specimens tend to propagate through the cylinder and preventing sudden structural failure. Figure 4b shows that no axial fracture propagation was observed for 3DP-Fe specimens. Instead, a more rounded “end cone” (similar to U-shape) formed on either the top or the base of the specimens. The applied pressure has different effects on 3DP specimens of different mineral composition. Consequently, a change in grain morphology influences the fracture characteristics of 3DP specimens.

FIG. 4.

Failure modes observed on (a) 3DP-Si and (b) 3DP-Fe specimens (≈10.5 mm diameter and ≈24 mm length) under uniaxial compression.

Figure 5 shows the stress-strain curves obtained for 3DP specimens of different mineral composition, 3DP-Si in Figure 5a, b, and 3DP-Fe in Figure 5c, d. The specimen numbers shown in the graphs (Fig. 5) can be compared to the fracture mode pattern in Figure 6. Note that consistent failure modes were observed across the specimens tested for particular mineral composition. The constitutive model for 3DP-Si specimens seem to be of the elastic-softening behavior, while for the 3DP-Fe specimens, a strain-softening behavior is more evident.

FIG. 5.

Typical stress-strain curve of UCS test (a) 3DP-Si (ø10.58 ± 0.07 mm), (b) 3DP-Si (ø16.55 ± 0.04 mm), (c) 3DP-Fe (ø10.3 ± 0.07 mm), and (d) 3DP-Fe (ø15.93 ± 0.32 mm) specimens. UCS, Unconfined Compressive Strength.

FIG. 6.

Fracture pattern of UCS test (a) 3DP-Si (ø10.58 ± 0.07 mm), (b) 3DP-Si (ø16.55 ± 0.04 mm), (c) 3DP-Fe (ø10.3 ± 0.07 mm), and (d) 3DP-Fe (ø15.93 ± 0.32 mm) specimens.

The stress-strain curves for 3DP-Si show higher repeatability for the 16.55 ± 0.04 mm diameter specimens than the 10.58 ± 0.07 mm diameter specimens (Fig. 5a, b). The graphs of 3DP-Fe specimens show variation in specimens’ response on the stress-strain curves (Fig. 5c, d). There is an increase in the heterogeneity and anisotropy of the 3DP-Fe for both the 15.93 ± 0.32 mm and the 10.3 ± 0.07 mm diameter specimens. From the peak of the curves, the UCS for 3DP-Si is 16.8 ± 0.6 MPa and 17.8 ± 0.4 MPa for 10.58 ± 0.07 mm and 16.55 ± 0.04 mm diameter specimens, respectively. However, the UCS for 3DP-Fe is 0.30 ± 0.06 MPa and 1.18 ± 0.13 MPa for 10.3 ± 0.07 mm and 15.93 ± 0.32 mm diameter specimens, respectively.

The 3DP-Si specimens are more competent than the 3DP-Fe specimens. Because the matrix (binder) used in the printing process is the same and the identical printing process is followed for the production of all the specimens, it is concluded that the uniaxial compressive strength has been influenced by grain mineral composition, grain shape, and its mechanical properties.

3DP-Si specimen in comparison with previous dataset

Quasi-identical 3DP specimens are being utilized for the benefit of reproducibility and repeatability of testing works. Because studies were previously conducted^4,30,34 for 3DP-Si specimens, a synthetic sandstone rock, a comparison for the UCS property is made. Unfortunately, the results shows that the current work for UCS using 3DP-Si specimens did not corroborate previous dataset performed on similar specimens (same dimensions and printing direction), which was unexpected. Notice that⁴¹ the observed aging of the 3DP material, that is, the strength builds up over time, can be related to the kinetic of the polymerization. The kinetic of acid catalyzed Furfuryl alcohol polymerization in isothermal curing conditions follows a linear behavior until reaching a plateau.⁴²

It is observed that, the timeline between fabrication and UCS tests for 3DP specimens is recorded 10 weeks for the current work and from 11 to 12 weeks for the previous studies. Previous testing was completed using the INSTRON 4505, set up with ±10 kN load cell. Those specimens were preloaded to 0.2 kN axial load, compressed at a rate of 0.25 mm/min, and extensometers were used.⁴

Figure 7a illustrates the variation of UCS for 3DP-Si specimens of 16 mm average diameter of the tested specimens by Barbosa et al.⁴ and the current work. Figure 7b shows a direct relationship between UCS and the specimen diameter for the previous dataset on 3DP-Si specimen that is, the trend of increasing UCS as the specimen diameter increases (the dataset was extrapolated to the UCS on 25 mm diameter). Although data for the current study (two lower points) is not conclusive, due to a limited number of sizes (diameter tested), it is clearly offset from the expected trend (Fig. 7b).

FIG. 7.

UCS test results for 3DP-Si specimens compared data from Barbosa et al.⁴ and current work (a) 16 mm average diameter (b) different diameter.

Table 2 summarizes some mechanical properties such as UCS, maximum axial strain (ɛA), and Young's modulus (E) of 3DP-Si and 3DP-Fe specimens for the current work and 3DP-Si specimens for Barbosa et al.⁴ The 3DP-Si specimens tested by Barbosa et al.⁴ present high Young's modulus values. Therefore, specimens were stiffer (more rigid) and more competent in terms of strength (higher) than 3DP-Si specimens tested in the current work. The stress-strain curve of UCS tests highlights the differences.

Table 2.

Summary of Mechanical Properties of the 3D-Printed Specimens from Barbosa et al.⁴ and Current Work

UCS	Barbosa et al.4		Current work
Specimen	3DP-Si		3DP-Si		3DP-Fe
Diameter (mm)	10.09 ± 0.02	16.52 ± 0.04	10.58 ± 0.07	16.55 ± 0.04	10.3 ± 0.07	15.93 ± 0.32
UCS (MPa)	25.0 ± 0.7	32.7 ± 0.9	16.8 ± 0.6	17.8 ± 0.4	0.3 ± 0.06	1.18 ± 0.13
ɛA (%)	2.0 ± 0.2	1.8 ± 0.2	1.73 ± 0.07	1.66 ± 0.05	0.37 ± 0.05	0.56 ± 0.09
Young's modulus (GPa)	1.8 ± 0.2	1.9 ± 0.1	1.40 ± 0.09	1.62 ± 0.09	0.06 ± 0.01	0.12 ± 0.03

It is disappointed that the current data for the 3DP-Si dataset show lower strength and a less sensitive response to specimen size. It is known that the strength of the bulk material also depends on curing process and sand.^30,43 However, the quartz supplier, printing technique, printer machine, and binder composition using Si were unchanged. The literature⁴⁴ highlights the need for more published data regarding the capability and characteristics of the printing process.

According to Coniglio et al.,⁴⁵ the influence of the 3D printing process parameters (average grain size, activator content, inhibitor content, print head voltage, layer thickness, heating temperature, print resolution, and recoater speed) can influence the specimen strength and permeability. The binder quantity only affects the specimen strength but increasing the recoater speed leads to both greater permeability and reduced strength due to the reduced sand compaction.⁴⁵ Following that, the quantification of the printing parameters used during the printing processes is presented below (Table 3):

Table 3.

Printer Parameters

	Print Job 1 and Job 2 Barbosa et al.4 and current work
Print speed (mm/s)	200
Frequency (Hz)	3500
dX resolution (μm)	Y-drop spacing is 56 μm, X-drop spacing is fixed at 63.5 μm
Pulse voltage (V)	NA
Droplet mass (pL)	80
Binder mass ratio (mbinder/mtotal) (%)	NA
Binder volumetric ratio (Vbinder/Vtotal) (%)	8 vol. %
Recoating speed (m/s)	150

NA, not applicable.

The studies conducted by Del Giudice and Vassiliou⁴¹ concluded that the mechanical properties of fabricated 3DP specimen depend on the age of the binder at the time of the print. Also, the room's relative humidity (RH) plays a role for the matrix (binder) to present flaws/voids, thus reducing the strength of 3DP-Si specimens.^30,34,46,47 According to Ardila-Angulo⁴⁸ humidity changes can be detrimental to the mechanical strength of 3DP specimens fabricated with furfuryl alcohol. The mechanism of mechanical loss due to water is unclear, as the furfuryl alcohol is cross-linked on addition of the acid catalyst and should be resistant to dissolution.

However, water has been shown to alter the adhesion properties of furfuryl alcohol,⁴⁹ which will attribute to a lower mechanical strength. It was the rainy season when 3DP specimens were printed in Canada, and the RH approached 50–60% RH in the laboratory rather than the ∼25–30% RH recorded for previous specimens. Therefore, it is concluded that the age of the binder and/or an external condition influenced the strength of specimens tested in the present work. While further investigation and/or better controls are required to guarantee the quality of 3DP specimens for future studies.

Specimens’ characteristics

Figure 8 shows the SEM image on grain scale observations similar to those of the 3DP specimens tested for impact (nor UCS neither BTS). Nevertheless, dashed lines highlight observations of flaws/voids randomly distributed inside 3DP-Si and 3DP-Fe specimens (Fig. 8). This information clarifies the fact that specimens fabricated for the current study cannot be compared to previously printed sets, but observations are still valid when comparing specimens of same printing conditions.

FIG. 8.

SEM image (a) 3DP-Si and (b) 3DP-Fe specimens.

Figure 8 also show that grain morphology, and binder gathering at the binder neck can influence the strength and fracture characteristics. Figure 8a shows that with a 3DP-Si specimen, the fracture propagation is exclusively through the matrix, that is, around grain boundaries at the neck created between grains, identified by the arrows in the picture. In the case of 3DP-Fe specimens (Fig. 8b), it is more difficult to observe the binder neck created between grains. The roughness and angularity of Fe grains appear to allow for the accumulation of the binder on the grain surface rather than well-formed binder necks. The grain-to-grain binder of 3DP-Fe specimens is reduced, which might facilitate breakage between grains without crack propagation, as observed in Figure 6.

Following on that, samples tested for MLA allow an observation of the particle size distribution (PSD) of Si and Fe when 3DP-Fe (Fig. 9a) is compared against 3DP-SiFe (Fig. 9b). The 3DP-Fe specimen comprises nearly the same PSD for both minerals, except for some minimum variation on finer grains (below 90 μm) (Fig. 9a). Figure 9b shows self-similar PSD for grains of Fe, compared to 3DP-Fe, while Si grains are larger sizes for 3DP-SiFe specimens. Because smaller grains result on large surface area, it is anticipated that less amount of binder per particle is available, contributing to reduce the grain-to-grain binder of specimens containing Fe.

FIG. 9.

Grain size distribution for (a) 3DP-Fe and (b) 3DP-SiFe specimens.

A possible chemical reaction between the grains of Fe and the binder resulting in weathering, alteration, and grain breakdown is not excluded. It is, therefore, important to consider the chemical interaction between binder and grains when selecting minerals for the 3D printing. Although based on a single observation, Figure 8b shows that the 3DP-Fe specimen is likely to contain more quantity and/or large flaws/voids spread inside the specimen and less effective binder structure to “cement” grains together.

In this context, the presence of flaws and a choice of mineral composition can affect the deformation and strength characteristics of 3DP specimens. In terms of grain hardness, the Fe mineral composition is softer than Si grain mineral composition. The Mohs scale hardness of iron is 4 and magnetite is 6, while the Mohs scale for quartz is 7.⁵⁰ Therefore, consideration of compressive strength and hardness shows that Fe grains represent the weaker and softer component compared with Si, which is a harder and stronger component.

BTS results

The BTS was carried out on five 3DP specimens. The homogeneous 3DP-Si and 3DP-Fe specimens along with the heterogeneous 3DP-SiFeSi, 3DP-FeSi, and 3DP-FeSiFe specimens, all with 16.55 ± 0.07 mm diameter, are tested in this study. The stress-strain conditions and failure mechanisms are investigated to understand the influence of different minerals and bed arrangements of 3DP specimens on tensile strength. Since 3DP specimens were printed using the same binder, this test gives more insight into the cohesion of the bonds between material grains influenced by internal flaws and the binder neck.

Fractures, flaws or heterogeneities are known to be inherent in the natural rock mass and are important zones for the subsequent crack initiation and propagation under compressed conditions.^14,51 With certain rocks, mineral grain boundaries act as Griffith cracks⁵² or contain microcracks (microplane of weakness); therefore, the first detectable fracture might initiate at these grain boundaries.^53,54 But, the simple theory for the BTS test suggests that fracture is characterized by a central tensile crack, that is, the crack initiated at the disk center during loading, which then propagates along the loading diameter. The crack splits the specimens into two halves when a typical BTS test is performed.^39,55,56 However, Dan et al.,⁵⁷ showed that the tensile crack could initiate in regions near the load jaw arc or especially along weak planes in transverse anisotropic specimens.

Figure 10 shows the calculated average values and standard deviation for the tensile strength observed using the volume of Si and Fe mineral grains for 3DP specimens. As expected, the tensile strength results show 3DP-Si specimens are the most competent specimens with 5.58 ± 0.28 MPa, and 3DP-Fe specimens are the least competent specimens with 0.59 ± 0.04 MPa. The tensile strength of 3DP specimens with bed composition are 3.07 ± 0.13 MPa for 3DP-SiFe; 2.32 ± 0.26 MPa for 3DP-SiFeSi, and 1.6 ± 0.14 MPa for 3DP-FeSiFe. The 3DP specimen does not present significant differences for tensile strength for specimens of the same type. But a straight correlation on the tensile strength cannot be recognized for 3DP-SiFeSi and 3DP-FeSiFe specimens when compared to 3DP-Si, 3DP-Fe, and 3DP-SiFe specimens.

FIG. 10.

BTS test results for 3DP specimens of 16.55 ± 0.07 mm diameter.

The homogeneous specimens (i.e., 3DP-Si and 3DP-Fe) tended to present both a central and a noncentral fracture pattern initiated within the load jaw arc. Thus, showing that the tensile failure occurs somewhere along the loaded contact area but not necessarily from a central crack, propagating perpendicular through the specimen's diameter. Figure 11 shows the failure pattern after the peak strength had been reached for homogeneous specimens tested for the BTS.

FIG. 11.

Failure pattern mechanisms in the BTS test using (a) 3DP-Si and (b) 3DP-Fe specimen.

The heterogeneous specimens (i.e., bedded 3DP-FeSi, 3DP-SiFeSi, and 3DP-FeSiFe) tended to fracture along bedding planes (parallel to the diameter forming disks between the different mineral compositions), while breakage near the load jaw arc (perpendicular to the diameter) can also occur. This composite failure of bedded 3DP specimens shows that the tensile crack tends to develop through the weak planes (i.e., close to the bedding contact) and it might also move near the load jaw arc (again not necessarily a central crack). This composite breakage behavior tends to generate more fragments compared with homogeneous specimens. Overall, the fracture pattern of bedded 3DP specimens is influenced by the orientation of the weak planes as well as the thickness (or number) of the beds.

Brittleness results

While several empirical methods have been proposed to quantify rock brittleness, the authors take advantage of the brittleness coefficients based on UCS and BTS.^8,58–60 Equations 1–3 were used to calculate the brittleness indices of 3DP specimens using the three approaches.

In Eqs. (1–3), $B{I}_1$, $B{I}_2$, and $B{I}_3$ are brittleness indices. The UCS is the uniaxial compressive strength value in MPa, and BTS is the indirect BTS value in MPa.

It is known that brittle and ductile behaviors are strongly related to mineral composition. This experimental work showed that homogeneous 3DP specimens printed with Si are brittle while those printed with Fe behave as ductile materials. Table 4 lists the mechanical properties and average values for UCS and BTS for the 16 mm diameter 3DP specimens. Notice that the estimations of UCS values for 3DP-SiFe, 3DP-SiFeSi, and 3DP-FeSiFe are based on parity plots of measured and fitted strength values, which shows good agreement with the data in Figure 12. Then the calculation of the brittleness indices is complete for all the 3DP specimens (Table 4).

Table 4.

Brittleness Indices of 3D-Printed Specimens

	UCS (MPa)	BTS (MPa)	BI1 (MPa)	BI2 (MPa)	BI3 (MPa)
3DP-Si	17.8 ± 0.4	5.58 ± 0.28	3.19	0.52	7.05
3DP-SiFe	(9.5)a	3.07 ± 0.13	3.09	0.51	3.82
3DP-SiFeSi	(7)a	2.32 ± 0.26	3.02	0.50	2.85
3DP-FeSiFe	(4.6)a	1.6 ± 0.14	2.88	0.48	1.92
3DP-Fe	1.18 ± 0.13	0.59 ± 0.04	2.0	0.33	0.59

^a Estimated value from the parity plots of measured and fitted strength values.

FIG. 12.

Parity plots between measured and fitted strength values for 3DP specimens.

The brittleness of 3DP-Si is much higher than that of 3DP-SiFe, 3DP-SiFeSi, and 3DP-FeSiFe based on $B{I}_3$. However, all specimens are almost comparable based on $B{I}_1$ and $B{I}_2$ results, there is no evidence of a higher ratio of compressive strength to tensile strength except for 3DP-Fe. A better prediction of brittleness is seen to be obtained using the Altindag and Guney,⁵⁹ Altindag⁶⁰ approach ($B{I}_3$), which quantifies the brittleness as a function of the area under the curve in the UCS and the BTS. Results show that the brittleness indices of the 3DP increases with increasing strength, which is attributed to the amount of brittle mineral (quartz) in the specimens tested.

Conclusions

3DP specimens with two different mineral grains were fabricated to emulate contrasting mechanical characteristics and to increase the degree of anisotropy, which is of interest across many disciplines in mining and metallurgy. The deformation and strength characteristics of 3DP specimens were studied to measure the mechanical properties and further understand the fracture mechanism and responses. This study is a pathway to improve knowledge of comminution/grindability using printed specimens of known mineralogical composition and texture. Experimental work for UCS and indirect BTS was carried out and the following conclusions can be drawn:

• The Fe grains are the component within the 3DP specimens deforming plastically rather than cracking. The Fe composition produces weak beds, which increases the heterogeneity and anisotropy of the 3DP specimens. It is suggested that the grain composition and physical characteristics (smooth, rough, spherical, irregular, size distribution) and the binder neck (grain-to-grain binder conditions) on fabricated 3DP specimens have influenced the material structural failure.

• The tensile strength of 3DP specimens is conditioned by the mineralogy or lithology for specimens. Also, the mineral composition, bed sequence, and number of beds play an important role in the failure of heterogeneous 3DP specimens observed on the BTS tests. While the orientation of the specimens in relationship to the loading direction is less important for nearly isotropic materials, it is anticipated that the orientation of the specimens with varied bed planes influences the fragmentation response and the measured peak load.

• Quasi-identical homogeneous 3DP specimens tend to split, while the variability on fragmentation tends to increase for heterogeneous 3DP specimens.

• Better brittleness index prediction of 3DP specimens of varied mineralogical composition is obtained using the Altindag approach.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The authors acknowledge the University of Queensland's financial support for the UQ Early Career Research grant no. 2019003181.