Dynamic response and spalling of cemented tailings backfill under cyclic impact

Investigation of the damage characteristics of rock material under the combined effect of cyclic load and impact load is extremely crucial for many mining engineering applications. Based on energy dissipation theory, we considered factors such as circulation times, the cyclic stress of a uniaxial cyclic load test, and the impact pressure (strain rate) of a split Hopkinson pressure bar (SHPB) test, studying the damage mechanism of red sandstone under the combined action of a uniaxial cyclic and impact loads. We found that when the load stress is 60%, 70%, and 80% of the uniaxial compressive strength (UCS) of red sandstone, the stress can still promote the development of microcracks and the generation of new cracks in the rock, increasing the inner damage of the rock so that it reduces the rock strength. As the cyclic time increases, the energy dissipation ratio presented a trend of decreasing dramatically and then maintaining basically no change, and the peak strain and the damage variable of the rock both increased first and then tended to remain relatively constant. The damage variable of the rock specimen under the combined action of the uniaxial cyclic loading and impact load had a significant corresponding relationship with the initial damage. As the rock specimen initial damage increases, the damage variable and the peak stress of the rock specimen both decreases almost linearly with initial damage. Meanwhile, the average particle diameter of the rock specimen after breakage gradually increased, showing a positively correlated tendency. The ratio between the fragment quality of the bigger particle diameter in the total rock specimen quality gradually increased. Under the conditions of the same initial damage, a higher impact pressure resulted in smaller rock fragment average particle diameters.

A series of impact compression tests were conducted to study the breakage characteristics of magnetite, as well as the impact pressure on its strain rate and dynamic compressive strength. The dynamic mechanical properties and fragmentation size distribution of magnetite under diverse impact loads and cyclic impact were investigated, with fractal theory as a basis and split Hopkinson pressure bar (SHPB). Breakage methods were also employed to analyze the fracture morphology of magnetite. According to the result, the fractal dimension can reflect the distribution of fragments in various sizes. If the strain rate increases, the fractal dimension will be larger, the fragment size will be finer, and the fragmentation degree will be more influential. A micro-analysis of SEM images demonstrates that the fracture morphology is determined by mineral properties. Under low load cyclic impact, intergranular fracture is the main fractography. Besides, the intergranular fracture will be changed to a transgranular one as the impact load increases.

Experimental evaluation of quasi-static and dynamic compressive properties of ambient-cured high-strength plain and fiber reinforced geopolymer composites

Recently, research on the impact resistance of transparent hybrid glass-polymer systems (layered and particle-matrix systems) has been conducted at the Netherlands Organisation for Applied Scientific Research (TNO), showing the potential of these materials for the application in protection concepts. A soft, transparent polyurethane elastomeric polymer, Clear Flex 75 (CF 75 in short) was selected as matrix material. Knowledge about the impact resistance of particle(s)-polymer systems is also relevant for propellants, consisting of a polymer binder and solid energetic particles. The impact-induced damage will change the combustion properties and the performances of the propellant. Against this background, a combined research programme of experimental tests and computational modelling was defined on the mechanical response of hybrid particle(s)-polymer systems under dynamic loading. In this programme, CF 75 polymer is the matrix material, which is embedded with single and multiple particles. In the computational part, multi-scale modelling techniques for dynamic conditions are developed. In the experimental part, a step-wise approach with increasing complexity is conducted for studying the dynamic response in compression and tension of (i) a monolithic polymer matrix, (ii) a single-particle-polymer system and (iii) a multiple-particles-polymer system. The used apparatus is a small scale split Hopkinson bar (SHB) device developed at TU Delft, which can conduct the dynamic compression (SHPB) and tension (SHTB) tests at various strain rates. The experimental results are important for (i) the evaluation of a transparent impact-resistant device of a soft polymer material backed by rigid glasses, (ii) the development of a particle(s)-polymer system for propellants and (iii) the calibration of computational models for designing a rigid-particle-modified composite material system for protective applications. Firstly, the experimental setups were built for high-strain-rate tension (SHTB) and compression (SHPB) tests. These two setups were carefully evaluated and proved to be reliable for dynamic tests on a soft polymer material at different strain rate levels. The details on the technology for casting the specimens of the monolithic CF 75 polymer material, the single-glass-polymer system and the multiple-particles-polymer system were given, as well as the basic physical properties of CF 75 polymer material. Afterwards, the dynamic tension and compression experiments were conducted at various strain rate levels. During these tests, a high-speed camera was employed to record the dynamic loading process and a scanning electron microscopy (SEM) was used to carry out the post-test analysis. From the dynamic tension tests, the damage, fracture and toughness mechanisms of the monolithic CF 75 polymer material were revealed. The rate dependency of the mechanical properties and dynamic deformation behaviour were characterized. Then, the bonding of the glass-polymer interface was also characterized by dynamic tension loading on a single-glass-polymer system of the CF 75 polymer matrix with an embedded single 3mm-diameter glass particle. Debonding, cavitation and necking were illustrated, as well as the effects on the stress-strain response at various strain rates. Dynamic compression tests were carried out to characterize the impact-resistant performance of the monolithic CF 75 polymer material. They revealed the deformation behaviour, damage mechanisms, fracture characteristics, rate dependency of mechanical properties and temperature rise. Then, the single-glass-polymer system was tested by dynamic compression tests. The results clarify the particle effect on the dynamic compressive response. The debonding-induced decrease of yield stress and the contribution of glass particle failure to fracture energy are quantitatively characterized at various strain rates. Finally, the impact-resistant performance of the multiple-particles-polymer systems of CF 75 polymer matrix with embedded 25wt.% and 50wt.% 0.5mm-diameter PMMA particles was characterized by dynamic compression tests. For the multiple-particles-polymer systems, an improved static stiffness is explained. Multiple cracking from the particle-polymer interface is observed, which induces the decrease of yield stress. A high particle density results in the decrease of maximum stress and strain energy. However, crushing of the particles causes a faster increase of maximum stress and strain energy with increasing strain rate (indicated by a higher strain rate sensitivity index). In this thesis, a systematical investigation about the dynamic mechanical response of soft polymers was carried out. It includes the monolithic polymer material (as matrix material in the particle(s)-polymer system), the single-glass-polymer system (for studying the interfacial bonding and single particle effect), and the multiple-particles-polymer system (as a representative of the real particle(s)-polymer system for impact-resistant applications).

Deep mining coal body fissure development, and affected by high geostress and weak impact disturbance, deep mine dynamic disasters are frequent. To investigate the dynamic response behavior of pressurized coal under weak impacts under impact load, Separate Hopkinson Pressure Bar (SHPB) was used to analyze the dynamic mechanical parameters changes and damage evolution of pressurized coal under different number of weak impacts (0.2 Mpa) and crushing impacts (0.8, 1.0, 1.2, and 1.4 Mpa), and to construct visco-elastic damage constitutive model to describe the dynamic strain-strain curve. The study shows that: (1) Under the weak impact load and the same crushing impact load, the dynamic compressive strength and modulus of elasticity of the pressurized coal show a tendency to increase and then decrease with the increase of the number of weak impacts, and the dissipation rate changes in the opposite trend, and the specimens are first compacted and then damaged; (2) The degree of crushing is intensified with the increase of the crushing impact pressure, and the degree of crushing decreases due to the first three weak impacts; (3) The Z-W-T model is improved to construct a viscoelastic constitutive model to describe the dynamic mechanical behavior of pressurized coal under cyclic weak impact loading (R2 > 0.92).

Surface modification is one of the treatment methods that can be implemented to improve the strain rate sensitivity of composite materials. In this study, both untreated and treated polypropylene/muscovite layered silicate composites were tested under static and dynamic loading up to 1100 s-1 using the universal testing machine and the split Hopkinson pressure bar apparatus, respectively. Muscovite particles were treated with lithium nitrate and cetyltrimethylammonium bromide as a surfactant through ion exchange treatment. Results show that the treated polypropylene/muscovite specimens with fine state of dispersion level shows better rate of sensitivity as compared to untreated polypropylene/muscovite specimens under a wide range of strain rate investigated. Apart from that, the rate of sensitivity of both tested polypropylene/muscovite layered silicate composites also show great dependency on the strain rate sensitivity was steadily increased with increasing strain rate. Unfortunately, the thermal activation values show contrary trend. Key words: Ion exchange treatment; Strain rate sensitivity; Muscovite particles; Split Hopkinson pressure bar apparatus; Strain rates

This paper reports transverse impact behaviors of 3D braided composites T-beam at elevated temperatures. A modified split Hopkinson pressure bar apparatus combined with a self-designed heating device was used. The impact load–displacement curves and impact damage morphologies have been obtained. The impact peak load and total energy absorption of the T-beam under impact loading have been calculated from the curves. It was found that both the elevated temperature and transverse impact velocity influence the transverse impact responses of the T-beam. The responses were more sensitive to the transverse impact velocity than to the elevated temperatures. The results also showed that the failure mechanism changes from brittle failure to ductile failure as the temperature increases. The matrix crack and fragmentation are often occurred in front surface of the T-beam. The fiber breakage in the rib position and the flange were the failure modes in the rear surface.

Mechanical behavior and constitutive model of frozen soil subjected to cyclic impact loading

Experimental study on the dynamic behaviour of aluminium foam sandwich plates under single and repeated impacts at low temperature

This study investigates the mechanical properties and damage processes of cement-consolidated soils with Pisha sandstone geopolymer under impact loading using the Hopkinson lever impact test. The mechanical properties of cement-cured soils containing Pisha sandstone geopolymer were examined at various strain rates. The relationship between strain rate and strength of the geopolymer-cemented soil was established. As the strain rate increased, the coefficient of power increase for the Pisha sandstone geopolymer cement-cured soil initially rose before gradually stabilizing. The pore structure of the crushed specimens was analyzed using Mercury intrusion porosimetry. Based on the observed pore changes under impact loading, the pore intervals of the geopolymer-cemented soil were defined. A fitting model linking strain rate and porosity was developed. As strain rate increased, the porosity of the specimens first increased and then decreased, with larger internal pores gradually transforming into smaller ones. The highest porosity was observed at a strain rate of 64.67 s− 1. Crushing characteristics of the cement-cured soils under impact loading were determined through sieving statistics of the crushed particles. The average particle size of the fragments decreased as the strain rate increased. The fractal dimension initially decreased and then increased with the rise in strain rate, reaching its lowest value at a strain rate of 64.67 s− 1. Based on the dynamic mechanical properties, microscopic porosity, and fracture characteristics, the critical strain rate and damage form for cement-consolidated soils with Pisha sandstone geopolymer under impact loading were determined. This study offers valuable insights for the practical application of Pisha sandstone geopolymer cement-cured soils in engineering.

Spalling characteristics of high-temperature treated granitic rock at different strain rates

The strain rate-dependent behavior of a unidirectional non-crimp fabric (UD-NCF) carbon fiber/snap-cure epoxy composite loaded along the transverse direction under quasi-static and dynamic conditions was characterized. Transverse tension and compression tests at quasi-static and intermediate strain rates were performed using hydraulic testing machines, while a split Hopkinson pressure bar (SHPB) apparatus was used for transverse compression tests at high strain rates. A pulse shaper was used on the SHPB apparatus to ensure dynamic equilibrium was achieved and that the test specimens deformed homogenously with a nearly constant strain rate. The transverse tensile strength at a strain rate of 16 s−1 increased by 16% when compared to that at quasi-static strain rates, while distinct localized fracture surface morphology was observed for specimens tested at different strain rates. The transverse compressive yield stress and strength at a strain rate of 325 s−1 increased by 94% and 96%, respectively, when compared to those at quasi-static strain rates. The initial fracture plane orientation for the transverse compression tests was captured with high-speed cameras and found to increase with increasing strain rate. The study provides an important data set for the strain rate-dependent response of a UD-NCF composite material, while the qualitative fracture surface observations provide a deeper understanding of the failure characteristics.

This study systematically investigates the failure patterns, energy dissipation, and fracture behavior of rock specimens containing a vertical hole under impact loads. First, an improved damage calculation equation suitable for the analysis of rock specimens with a vertical hole is obtained based on the one‐dimensional stress wave theory and the interface continuity condition. After that, the Hopkinson pressure bar (SHPB) device was used to conduct cyclic impact tests with different impact pressures and impact modes (impact pressures with equal amplitude and unequal amplitude). The experimental results suggest that, under the equal‐amplitude high pressure and unequal‐amplitude pressure, the degree of damage of the rock significantly increased, the bearing capacity greatly reduced, and the rock gradually transitions from having good ductility to experiencing brittle failure. The cumulative specific energy absorption value gradually increases with the increase in the cyclic impact. Compared to that of the equal impact condition, the degree of damage to the rock is more severe for the case of equal‐amplitude high pressure and unequal impact, and the failure mode undergoes a transformation from transverse tensile failure to transverse tensile failure‐axial splitting failure combination and axial splitting failure. Through the analysis of rock energy changes and rock failure patterns during cyclic impact, it will be helpful to predict and control the fracture caused by local stress concentration during excavation, thus can reduce the cost of support and reinforcement in excavation and improve the stability of surrounding rocks.

The mechanical properties of a product made in Fused Deposition Modelling (FDM) rapid prototyping are strongly dependant on process parameters selected during part fabrication. Acrylonitrile butadiene styrene (ABS) is a common material used in FDM systems. The advantages of ABS include high strength and rigidity with toughness and these properties make it one of the most common thermoplastics used in engineering applications. This paper describes an experimental investigation of dynamic stressstrain response of ABS parts made by fused deposition modelling for three different part build orientations. Currently there is limited research available for this aspect of ABS material processed by FDM systems. The high strain rate compression tests were performed using a Split Hopkinson Pressure Bar apparatus to determine the dynamic stress-strain response and results were compared with quasi-static behaviour of the same specimens.

Ti-6Al-4V alloy is one of the most important engineering alloys, combining attractive properties with inherent workability. The aim of this study is to investigate the effect of strain rate on the compressive mechanical properties of Ti6Al4V alloy manufactured by a selective laser melting process. The mechanical tests were performed by means of a compression split Hopkinson pressure bar apparatus under high strain rate ranging from 1400 s-1 to 4500 s-1. The true stress-strain curves obtained from static and dynamic compressive tests show strain rate sensitivity from quasi-static (peak strength 1300MPa) to high strain rate (peak 1500 MPa). Within the high strain rate range tested, the strain rate sensitivity is not remarkable. The fractographic analysis shows a relatively smooth and smeared fractured surface along with a dimple like structure. The observation of elongated dimples confirms the operation of a dynamic shear failure mechanism for the additively manufactured Ti-6Al-4V parts.