Prevention Of Spalling Research Articles

Influence of sidewall size on spalling in deep D-shaped tunnels: An experimental simulation

A series of true triaxial compression tests was performed herein on cubic red sandstone samples containing a D-shaped hole with three sidewall sizes to study the influence of sidewall size on spalling in D-shaped tunnels. The initial horizontal stress of the tests refers to the in-situ stress at 500 m depth. The hole axis was arranged along the direction of the maximum horizontal principal stress. The vertical stress was increased to simulate the increasing surrounding rock stress after tunnel excavation. During the test, a self-made monitoring device was used to monitor and record the failure process of the tunnel models in real time. Furthermore, the spalling process of the D-shaped tunnels with different sidewall sizes was reproduced indoors. The failure process and the stress and fracture characteristics of the tunnel models with three different sidewall sizes were then analyzed and summarized. The effect of the sidewall size on spalling in D-shaped tunnels will be discussed in this paper. The results show that the failure of the tunnel models with different sidewall sizes first occurred on the sidewall near the corner then gradually developed upward and axial along the sidewall with the increase of the vertical stress. The width and the depth of the failure zone gradually increased, and a V-shaped notch was finally formed on the sidewall. No failure occurred on the roof and the floor of the tunnel models. The dominant failure modes were tensile failure. The failure forms were mainly spalling occasionally accompanied by slab ejection. The slabs were shaped in thin plates. The vertical stress that caused the initial failure of the tunnel models gradually decreased with the increasing sidewall size. Furthermore, the ability of the surrounding rock to store elastic strain energy decreased. The tensile characteristics became more prominent, and the size of the plate-shaped slabs increased. In the same stress environment, the greater the sidewall size, the larger the failure width and depth on the sidewall, and the more visible the V-shaped failure zone. The stability of deep D-shaped tunnels can be improved, and spalling can be mitigated by reducing the sidewall size. However, the risk of a rockburst will increase. This finding has an important engineering significance for the optimization design of a tunneling cross-section and the prevention of spalling and rockburst in D-shaped tunnels.

Spalling Resistance of Fiber-Reinforced Ultra-High-Strength Concrete Subjected to the ISO-834 Standard Fire Curve: Effects of Thermal Strain and Water Vapor Pressure.

The prevention and mitigation of spalling in high-strength concrete (HSC) rely on mixing polypropylene (PP) as an additive reinforcement. The dense internal structures of ultra-high-strength concrete (UHSC) result in risks associated with a high thermal stress and high water vapor pressure. Herein, the effects of pore formation and thermal strain on spalling are examined by subjecting fiber-laden UHSC to conditions similar to those under which the ISO-834 standard fire curve was obtained. Evaluation of the initial melting properties of the fibers based on thermogravimetric analysis (TGA) and differential thermal analysis (DTA) demon strated that although nylon fibers exhibit a higher melting point than polypropylene and polyethylene fibers, weight loss occurs below 200 °C. Nylon fibers were effective at reducing spalling in UHSC compared to polypropylene and polyethylene fibers as they rapidly melt, leading to pore formation. We anticipate that these results will serve as references for future studies on the prevention of spalling in fiber-reinforced UHSC.

Experimental, numerical and analytical study on the shock wave propagation through impedance-graded multi-metallic systems

The behaviour of multi-material systems subjected to dynamic loads is currently a topic of interest. This paper aims to study the performance of an impedance-graded multi-metallic (IGMM) system in mitigating the effects of stress waves generated during a high-velocity impact event. The IGMM system was designed by placing different metals in their reducing order of impedance. The experimental work was carried out using a single-stage light gas gun with a steel flyer fired at a velocity of 350 m/s. Armour-grade steel, titanium and aluminium were chosen as the materials and were used to design monolithic, bi-metallic and tri-metallic impedance graded targets. The free surface velocity of the final material in the target was measured using PDV probes and the generated velocity profile was used to quantify the Hugoniot Elastic Limit and the magnitude of the transmitted shock waves. A two-dimensional axisymmetric numerical model was used to simulate this impact event, which was carried out using the non-linear finite element code LS-DYNA. An analytical model was developed based on the shock wave propagation theories, using Matlab, to quantify the magnitude of the stresses within the different materials in the target. The output from the numerical and analytical models were in good agreement with the experimental results. The main findings from this study highlighted the potential of the IGMM system in reducing transmitted compressive stresses as well as prevention of spalling during impact loading events.

Simulation test of spalling failure of surrounding rock in rectangular tunnels with different height-to-width ratios

Spalling (Slabbing) is a typical failure phenomenon, which often occurs after excavation of rectangular tunnel. It has a serious impact on the construction, support and stability of the tunnel. To investigate the characteristics and mechanism of spalling-failure in rectangular tunnel, a series of tests were carried out on granite rectangular tunnel with different height-to-width ratios (H/Ws) (1/2, 3/4, 1/1, 4/3 and 2/1) by using true triaxial system. The whole process of spalling was monitored and recorded by wireless micro camera. According to the test results, the failure mode and failure characteristics of the sidewall spalling were summarized, and the influence of the H/Ws of the rectangular hole on the failure characteristics and stability of the tunnel were discussed in detail. The test results indicate that when the vertical principal stress is the maximum principal stress, the sidewalls of the specimens experience spalling failure. When the spalling occurs, the crack occurs at the shoulder corner of the sidewall and propagates along the axis, resulting in the spalling damage gradually developing from the local area to the whole sidewall along the axis, and forming the thin rock slab parallel to the sidewall. In addition, rectangular tunnels with different H/Ws clearly influence spalling failure. When the height was the same, the degree of spalling damage decreases with the decrease of its width. On the contrary, when the width was the same, the degree of spalling damage does not decrease with the increase in the height. When the height was equal to the width, the damage degree of the tunnel spalling is the largest, which seriously affects the safety of the tunnel. In the case of the same cross-section area of the tunnel, the stability of the tunnel can be improved by selecting the layout mode with the height greater than the width. It is necessary to strengthen the monitoring of the cracks at the shoulder corner of the tunnel, which can reduce the safety risk of the tunnel. These conclusions are helpful to the design of the tunnel, the prevention of spalling and the improvement of the stability of the tunnel.

Prevention of spalling by the self-formed reaction barrier layer on controlled collapse chip connections under bump metallization

The spalling phenomenon on under bump metallization (UBM) is one of the current urgent reliability issues for the Pb-free solder implementation in flip chip technology. In this paper, we report that spalling of Ni thin UBM can be prevented during the soldering reaction, if a Cu reservoir is introduced into the structure of controlled collapse chip connections (C4) solder joints. Once molten Sn-3.5Ag solder was saturated with Cu atoms, Cu precipitated out as a layer of Cu-Sn compound on Ni thin UBM. The Cu-Sn compound layer served as a reaction barrier to retard the consumption of Ni thin UBM. So, spalling was retarded. After prolonged reflowing, Ni thin UBM was converted to ternary Cu-Sn-Ni compounds. Unlike interfaces of the Ni-Sn compound/Cr, the interface of the Cu-Sn-Ni compound/Cr was very stable and no spalling was found.