Experimental Study of Breakage of Particles under Compression

Granular materials are used widely and can be seen in natural and industrial applications such as sand bags or pharmaceutical pills. During their manufacturing, processing, transport and use, granular materials are subjected to various kinds of loadings. If the amplitude of the loading is above the strength threshold, particles constituting granular materials may fracture. It is very important to understand the failure of particles under these loading conditions to prevent or control their failure during all stages of their manufacturing and use. Better characterization of the fracture behavior of particles composed of different materials and sizes will allow more precise application and better maintenance of granular materials in commercial usage. The effects of size and material properties on the deformation and fracture behavior of granular particles are studied by investigating particles from three different size ranges for three different materials. The mechanical behavior is characterized by force-displacement and stress-strain plots under quasi-static compression (strain rate = 10-2s-1). Along with the deformation behavior, the strengths of particles are also recorded and Weibull distribution is fitted to the fracture stresses. It was observed that the smaller particles break at lower forces but actually withstand higher stress at fracture. The calculated Weibull moduli for different size range and materials show that the flaw population from the manufacturing process is different for different sizes and materials. This study shows that size and material properties alter the fracture stresses. Future experiment can be performed for the same particles under dynamic compression to better understand effects of strain rate on the fracture of particles

Fracture behavior of brittle microspheres using indentation and compressive loading techniques

The fracture behavior of small spheres of silica, glass, silicon, and yittria-stabilized zirconia (YSZ) has been studied with different loading techniques. Spherical particles come under impact loading both during materials processing, transportation and construction, and high strain rate loading of ballistics; the resulting integrity of the spherical particles plays a role in both the subsequent processing and the energy absorption capabilities of the material. Nanoindentation has been used to measure the hardness (H) and elastic modulus (E), and microindentation has been used to measure the fracture toughness (T) of the materials. High speed X-ray phase contrast imaging was used to examine the failure mechanism under dynamic compression for individual particles with diameters (d) that range between 500 and 2000 µm. Static testing of particulates using bulk indentation of granular solids also can lead to sample failure. A pulverization model has been developed to better understand the failure of the materials. The pulverization parameter is presented by P = Hd 0.5 /T 2. The preliminary results show that materials that exhibit pulverized failure under high strain rate compressive loading, such as silica and glass, have high P values. YSZ shows a single crack under compressive loading and has the smallest P value, whereas silicon exhibits substantial but still distinct cracking and has a medium P value. The effects of strain rates are also discussed in this presentation

Experimental assessment of fracture of individual sand particles at different loading rates

Failure mechanisms in individual sand particles under compressive loading at different loading rates were -investigated using X-ray imaging. High speed X-ray phase contrast imaging was utilized to study the damage mechanisms in dry and wet sand under dynamic compressive loading. A modified Kolsky bar setup was used to apply controlled dynamic compression on two contacting sand particles. One of the particles was observed to pulverize, whereas other particle remained intact for dry sand particles with average failure load of 34.344 N. In wet conditions, one of the particles was observed to break into large subparticles which pulverized upon further loading. Other particle was observed to stay intact. The failure load was observed to increase to 65.466 N for wet particles. 3D X-ray tomography was used to assess the failure of dry sand particles under static compressive loading. One particle broke into large subparticles which subsequently pulverized under static compressive loading. Even under static loading, second particle did not fail until first particle was completely pulverized. The pulverization load under static compressive loading was observed to be 42 N. The order of pulverization for the particles was observed to be random in all experiments

Bulk-Explosion-Induced Metal Spattering during Laser Processing

Spattering has been a problem in metal processing involving high-power lasers, like laser welding, machining, and recently, additive manufacturing. Limited by the capabilities of in situ diagnostic techniques, typically imaging with visible light or laboratory x-ray sources, a comprehensive understanding of the laser-spattering phenomenon, particularly the extremely fast spatters, has not been achieved yet. Here, using MHz single-pulse synchrotron-x-ray imaging, we probe the spattering behavior of Ti-6Al-4V with micrometer spatial resolution and subnanosecond temporal resolution. Combining direct experimental observations, quantitative image analysis, as well as numerical simulations, our study unravels a novel mechanism of laser spattering: The bulk explosion of a tonguelike protrusion forming on the front keyhole wall leads to the ligamentation of molten metal at the keyhole rims and the subsequent spattering. Our study confirms the critical role of melt and vapor flow in the laser-spattering process and opens a door to manufacturing spatter- and defect-free metal parts via precise control of keyhole dynamics

Revealing Particle-Scale Powder Spreading Dynamics in Powder-Bed-Based Additive Manufacturing Process by High-Speed X-Ray Imaging

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior

Pore Elimination Mechanisms during 3D Printing of Metals

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals

Ultrafast X-Ray Imaging of Laser-Metal Additive Manufacturing Processes

The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts

High Speed X-ray Phase Contrast Imaging of Energetic Composites under Dynamic Compression

Fracture of crystals and frictional heating are associated with the formation of “hot spots” (localized heating) in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and the fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and the fracture mechanisms in PBXs composed of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene binder doped with iron (III) oxide. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binder, even at ultrafast recording rates. Under dynamic compression, most of the cracking in the crystals was observed to be due to the tensile stress generated by the diametral compression applied from the contacts between the crystals. Tensile stress driven cracking was also observed for some of the crystals due to the transverse deformation of the binder and superior bonding between the crystal and the binder. The obtained results are vital to develop improved understanding and to validate the macroscopic and mesoscopic numerical models for energetic composites so that eventually hot spot formation can be predicted

Fracture of Spherical Particles under Compression

The fracture behaviors of individual and contacting particles are studied using experimental techniques. Six different materials were studied: soda lime glass, silicon dioxide, silicon, barium titanate glass,poly methyl methacrylate, and yttria stabilized zirconia. The fracture mechanisms were studied at quasi-static and dynamic loading rates. Single, two, and multiple contacting particle geometries were studied to investigate the effects of contact conditions on the fracture mechanisms. The experiments were performed for various diameter ranges to assess the effects of size on fracture behavior. The quasi-static fracture behavior of particles was studied using the single particle compression experiments, performed using a servo-hydraulic machine. Two deterministic models based on the tensile stresses at the contact and near the center of the particle and a statistical model based on the Weibull weakest link theory were proposed to identify the fracture modes of the particles. The models provided an upper and lower bounds for the fracture stresses, however, the variability in the experimental data prevented definitive identification of the fracture modes. ^ A modified Kolsky bar apparatus was synchronized with the high speed X-ray phase contrast imaging setup to record the in-situ fracture mechanisms of single, two, and multiple contacting particles under dynamic compression. No significant size effects on the fracture mechanisms were observed. For the single particle experiments, the crack initiation was observed to occur near the center of the particle for all materials. For two particle experiments, the crack initiation occurred near the particle-particle contact in form of angular Hertzian cracks for brittle materials. These angular cracks only separated fragments near the contact and did not ultimately fracture the particle. Angular cracks with resulting contact fragments were also observed in the multi-particle experiments for soda lime glass particles. The elastic-plastic particles did not show any contact cracks. For all particles, the cracks that caused the catastrophic fracture initiated under the particle-particle contact or near the center of the particle. For the single and two particle experiments, the cracks propagated toward the contact, thus forming meridional cracking pattern. For the multi-particle experiments, cracks spanned the contact points with one major meridional crack observed in all experiments. Regardless of the contact conditions, three major catastrophic fracture modes were observed for the particles depending on the material properties: (1) explosive fragmentation or pulverization (soda lime glass), (2) finite number of large cracks or major cracking (silicon, silicon dioxide, and barium titanate glass), and (3) single crack (poly methyl methacrylate, and yttria stabilized zirconia). ^ A new phenomenological model was then proposed to better describe the observed morphological fracture mechanisms under dynamic compression. This phenomenological model was represented by a pulverization parameter that was related to the hardness, elastic modulus, fracture toughness and the size of the particles assuming displacement driven loading conditions. High pulverization parameter values were associated with explosive fragmentation failure, low pulverization parameter values were associated with single cracking failure, and intermediate pulverization parameter values spanned a range of failure modes between explosive fragmentation and single cracking failure. ^ Scanning electron microscopy was then used to image the generated fragments from the dynamic fracture experiments. All particles showed some variant of the prominent brittle fracture features which included hackle lines and Wallaner lines. In some polycrystalline particles, inter-granular crack propagation was observed. The sharpness of the features were observed to be related to the magnitude of the pulverization parameter for the materials.