Experimental Investigation on Dynamic Performance of Micro-Pile with Predrilled Oversize Hole on Shaking Table Test

For the situation of lacking research on micro-pile with predrilled oversize hole, the key part of semi-integral abutment bridge, the micro-pile-soil interaction shaking table test is carried out by considering the reaming pore diameter, depth, packing and other parameters in the end of the micro-pile to obtain the acceleration, pile moment, displacement and pile-system response frequency and other basic dynamic response and dynamic interaction law. Results show that: 1) the change of predrilled-hole parameters has litter effect on the dynamic properties of soil outside oversize hole; 2) The change of predrilled-hole parameters can cause the change of structural frequency, so led to the change of inertia force of pile head; 3) Inertial interaction has an important influence on the response of the upside part of pile and little influence in the downside part (lower than 15D). These conclusions will provide reference for dynamic response of interaction between pile with predrilled oversize hole and soil and make contribution to the practical application and designing of micro-pile with predrilled oversize hole.

Numerical Study on the Entrance Effect of Penetration into Concrete Targets

Investigation on penetration into concrete targets is of great importance as concrete is widely used as the fundamental construction material. To achieve a more accurate prediction of penetration depths of concrete targets, a further study was conducted to explore the entrance effect by using AUTODYN hydrocode in this study. The numerical results on both deceleration-time history and depth of penetration of projectiles are in good agreement with experimental data, which demonstrate the feasibility of the numerical model in these conditions. A new target model was established with a predrilled hole around the symmetry axis to simulate the entrance effect of the crater phase on the penetration process. Compared with the regular target, the predrilled target enters the peak of acceleration earlier, leading to the reduction of the depth of penetration. In addition, simulation results indicated that nose shape significantly influenced crater region depth, while the depth was independent of the impact velocity and the target strength. Based on the simulation of entrance effect, a modified formula of penetration depth has been proposed and validated in terms of different nose shapes. The crater region depths obtained from the simulations can improve the accuracy of the predictions of the penetration depths for the penetration of concrete targets.

Cold Metal Transfer Plug Welding of Aluminum AA6061-T6-to-Bare Mild Steel

It was known that it is challenging to join lapped aluminum and bare steel with cold metal transfer (CMT) process because of the formation of significant brittle intermetallics. In this study, another attempt was made to join aluminum AA6061-T6 and bare mild steel with CMT plug welding. Welding tests were performed and the bonding mechanisms, fracture modes, and strengths of CMT plug welded joints were systematically characterized. It was found that it is feasible to join 1 mm thick bare mild steel-to-1 mm thick aluminum AA6061-T6. The material stacking sequence and the presence of a predrilled hole significantly affected the weldability of CMT plug welding bare mild steel-to-aluminum AA6061-T6. By positioning bare steel with a predrilled hole in the top aluminum AA6061-T6 and aligning a torch in the center of an 8 mm hole improved significantly the weld appearance and joint strength.

Cold Metal Transfer Spot Joining of AA6061-T6 to Galvanized DP590 Under Different Modes

In order to meet the upcoming regulations on greenhouse gas emissions, aluminum use in the automotive industry is increasing. However, this increase is now seen as part of a multimaterial strategy. Consequently, dissimilar material joints are a reality, which poses significant challenges to conventional fusion joining processes. To address this issue, cold metal transfer (CMT) spot welding process was developed in the current study to join aluminum alloy AA6061-T6 as the top sheet to hot dip galvanized (HDG) advanced high strength steel (AHSS) DP590 as the bottom sheet. Three different welding modes, i.e., direct welding (DW) mode, plug welding (PW) mode, and edge plug welding (EPW) mode were proposed and investigated. The DW mode, having no predrilled hole in the aluminum top sheet, required concentrated heat input to melt through the Al top sheet and resulted in a severe tearing fracture, shrinkage voids, and uneven intermetallic compounds (IMC) layer along the faying surface, leading to poor joint properties. Welding with the predrilled hole, PW mode, required significantly less heat input and led to greatly reduced, albeit uneven, IMC layer thickness. However, it was found that the EPW mode could homogenize the welding heat input into the hole and thus produce the most stable welding process and best joint quality. This led to joints having an excellent joint morphology characterized by the thinnest IMC layer and consequently, best mechanical performance among the three modes.

Measurement of the Internal Relative Humidity Distribution in Concrete

The internal relative humidity (RH) and humidity gradients in concrete at early ages have a significant influence upon the properties of concrete, where exists a great discrepancy among the test results under different methods. By comparing and analyzing the traditional measuring methods of the RH in concrete, a new measuring method will be developed in this study, which could measure the internal RH in cement-based materials accurately, conveniently and digitally. The changing laws of the internal RH in concrete at early ages were discussed. The results indicate that the measuring results under the new developed system was more accurate than that by the predrilled hole method, while the external environment does not have any effect on the measured results obtained by using the new measurement system. What’s more, the results achieved by the new developed system approached the real RH in concrete very quickly; the RH near the surface of the specimen decreased quickly, while the RH in the center of the specimen decreased slowly, the moisture contents unevenly distributed through the various height of the specimen. Hence, there exists an obvious humidity gradient in concrete.

On the Experimental Optimization of Tool Geometry for Uniform Pressure Distribution in Single Edge Gundrilling

An investigation into the effect of gundrill geometry on the coolant flow, in gundrilling, is carried out. This investigation deals mainly with the loss of coolant pressure occurring in a limited space between the flank of the gundrill and the bottom of the predrilled hole. This space is named as “bottom clearance.” The pressure loss in the bottom clearance is classified into, (a) pressure loss due to flow interaction with the bottom of the drilled hole (impact pressure loss), and (b) pressure loss due to hydraulic resistance of the annular groove connecting the bottom clearance and the chip removal passage. The study indicates that a significant part of the pressure loss occurs due to flow deflection at the bottom of the hole. The reduction of pressure loss can be achieved either by reducing the coolant velocity at the orifice exit, or, by increasing the coolant pressure in the bottom clearance. In this study, the shoulder dub-off angle of the gundrill is experimentally optimized to increase the coolant pressure in the bottom clearance, thereby achieving uniform coolant pressure distribution. This uniform pressure distribution resulted in increased gundrill life without compromising the quality of the machined hole.