Hole Wall Research Articles

Machining responses of high-strength carbon/epoxy composites using diamond-coated brad spur drills

ABSTRACT Drilling of high-strength carbon/epoxy composites has been a critical procedure in the industry due to the need to create holes for mechanical assemblies. However, the inherent anisotropy and heterogeneity of the fibrous composites complicate the material removal process and induce tremendous challenges for the manufacturing sectors. The present paper addresses the machining issues of T700 carbon fibers reinforced epoxy laminates by using the diamond-coated brad spur drills. The cutting tools are featured by three protruding edge tips devoted to the minimization of drilling thrust forces for the composite laminates. The machining experiments were conducted under varying process conditions to investigate the parametric impacts on the machining responses. The cutting-induced delamination was studied using a novel quantification method. The work novelty is focused on revealing the machining characteristics of high-strength fibrous composites and evaluating the functionality of the specialized tools for the composite drilling. The process parameters greatly affect the drilling forces/temperatures and delamination factors. Delamination is shown to occur inside the inner plies of the composite and propagate rapidly toward the hole exit side. Moreover, surface cavities and fiber fractures dominate the failure of cut composite hole walls.

Experimental study on double-headed screw joints and their application in bamboo shear walls

Due to the weak withdrawal capacities of conventional nail joints, using double-headed screw joints as reliable connections in bamboo structures is investigated for the first time. A two-step test program is presented in this paper. In the first step, a double shear test is carried out to investigate the influences of the end distance and bamboo grain direction on the performance of double-headed screw joints. The test shows that there are four main failure modes of double-headed screw joints: double-headed screw shear failure, bearing failure of the hole wall, tensile failure of the bamboo cover panel and shear failure of the cover panel end. In the second step of test, the proposed double-headed screw joints are applied to three single-layer single-span bamboo shear walls, and low-cycle reversed loading tests are applied to the walls with double-headed screw spacings of 50 mm, 100 mm and 150 mm. The failure mode, hysteretic behaviour and energy dissipation performance of the shear walls are discussed. Test results show that the two main failure modes of the bamboo shear walls are the tensile failure of the edge of the wall and shear failure of the double-headed screws. Among the different spacings, the bearing capacity and effective stiffness of the wall with a double-headed screw spacing of 50 mm are the largest, the ductility and energy dissipation capacity of the bamboo shear wall with a double-headed screw spacing of 100 mm are the largest, and the bearing capacity and ductility of the bamboo shear wall with a double-headed screw spacing of 150 mm are the worst.

Safety Analysis of Lining Structure Influenced by Blasting of Tunnel with Extralarge Section and Small Space

In order to ensure the safety of the Daling tunnel with extralarge section and small space, the three-dimensional model of blasting dynamic simulation had been established and verified. Then, the model was used to analyze the influence of the surrounding rock characteristics and blasting design parameters on the blasting vibration of the first hole, and the analysis of the sensitivity of each factor was carried out. The results showed that blasting of the second hole had a serious impact on the safety of the first hole lining structure. Based on the safety threshold and analysis of sensitivity, the explosive velocity, charge density, and digging length were selected as the key parameters affecting the safety of the tunnel structure. Meanwhile, the corresponding engineering measure was taken based on the results of sensitivity and the actual situation on site, and, after that, the maximum PPV of the right wall of the first hole had been reduced to 9.3 cm/s, which effectively guaranteed the safe construction of the tunnel.

Numerical and experiment investigation on joining process and failure behaviors of CFRP/Al electromagnetic riveted joint

In this study, electromagnetic riveting (EMR) was proposed to connect CFRP/Al structures due to its little damage on composite. The joining process and failure behaviors of CFRP/Al joints were studied. Specifically, the finite element model was established to analyze the joining process. The riveted joints were tested under various loading velocities to obtain the failure behaviors. The simulation results showed that hole wall of the CFRP and Al sheets had uniform residual stresses, and no damage of the CFRP sheet was observed around the hole after EMR, indicating that the joint had superior performance. The tensile test results showed that the loading speed had a great influence on the performance of the joint. The maximum shear force at lower loading speeds was less than that at higher loading speeds, while the trend of energy absorption was opposite. The main failure position was in CFRP with a mix mode of bearing and shear out. Based on the failure analysis, a simplified analytical model for the maximum shear load was presented.

Study on Microstructures of Complex Plant Polysaccharides Aerogels and Their Effects on Porosity

The microstructure of polysaccharide aerogels is a key factor affecting porosity, and directly determines the efficiency of its application as filter rod material. Scanning electron microscopy was used to observe the effects of different amounts of starch on the microstructure of the complex plant polysaccharide aerogels. The experimental results showed that the aerogel samples showed a complete and homogeneous three-dimensional network structure. With the increase of starch content, the pore size became smaller and the pore structure on the hole wall became smaller. After adding starch, the concentration of the system increased, that is, the solute in the unit volume increased, the moisture content decreased, and the air replaced the water in the gel after freeze drying, resulting in the increase of the density of the aerogel and the decrease of porosity. All aerogels showed a trend of increasing filtration efficiency with increasing particle size. The filtration performance of aerogels with different starch content was tested by using the comprehensive performance test bed of filter media. The test results showed that the capture of filter material was mainly through inertial collision. When the particle size was bigger, the greater the inertia, the greater the possibility of particle contamination being captured, so the higher the filtration efficiency.

Hole quality in longitudinal–torsional coupled ultrasonic vibration assisted drilling of carbon fiber reinforced plastics

Carbon fiber reinforced plastic (CFRP) composites are extremely attractive in the manufacturing of structural and functional components in the aircraft manufacturing field due to their outstanding properties, such as good fatigue resistance, high specific stiffness/strength, and good shock absorption. However, because of their inherent anisotropy, low interlamination strength, and abrasive characteristics, CFRP composites are considered difficult-to-cut materials and are prone to generating serious hole defects, such as delamination, tearing, and burrs. The advanced longitudinal–torsional coupled ultrasonic vibration assisted drilling (LTC-UAD) method has a potential application for drilling CFRP composites. At present, LTC-UAD is mainly adopted for drilling metal materials and rarely for CFRP. Therefore, this study analyzes the kinematic characteristics and the influence of feed rate on the drilling performance of LTC-UAD. Experimental results indicate that LTC-UAD can reduce the thrust force by 39% compared to conventional drilling. Furthermore, LTC-UAD can decrease the delamination and burr factors and improve the surface quality of the hole wall. Thus, LTC-UAD is an applicable process method for drilling components made with CFRP composites.

Numerical Simulation of Shallow Geothermal Field in Operating of a Ground Source Heat Pump System—A Case Study in Nan Cha Village, Ping Gu District, Beijing

The inefficient use of single energy and cold accumulation in the shallow geothermal field seriously affect the efficient operation of the ground source heat pump system (GSHPS). The operation of solar-assisted GSHPS can effectively solve the above problems. In this paper, a shallow geothermal utilization project in Nan cha Village, Ping Gu District of Beijing, is chosen as the study area. A three-dimensional numerical model of groundwater flow and heat transfer considering ambient temperature and backfill materials is established, and the level of model integration and validation are novel features of this paper. The thermal response test data in summer and winter conditions are used to validate the model. The results show that increasing hydraulic gradient has a positive impact on the heat exchange. The mixture of sand and barite powder is recognized as a more efficient and economical backfill material. The changes of thermal influence radius, heat balance, and shallow geothermal field are simulated and analyzed by three schemes. It is demonstrated that the thermal influence radius is 5 m, 3.9 m and 3.9 m for Scheme 1, Scheme 2 and Scheme 3, respectively. The ground temperature is always lower than the initial formation temperature in Scheme 1 and Scheme 2; however, under Scheme 3 it is higher than the initial values. The closer the hole wall is, the larger the difference between the initial formation temperature and the ground temperature, and vice versa. The thermal equilibrium of Scheme 1, Scheme 2 and Scheme 3 is −728 × 106 KJ, −269 × 106 KJ and +514 × 106 KJ. Through comprehensive analysis of the above three factors, Scheme 3 is regarded as the most reasonable scheme for a solar system to assist GSHPS.

Finite Element Method Application to Determine Appropriate Splitting Parameters for Dimensional Stone Quarries

In dimension stone extraction, there have existed many block cutting techniques from intact rock or from a large block to smaller blocks such as disc sawing, chainsawing, diamond wire sawing and splitting techniques, one of which still has recently been used with the other modern methods is splitting techniques. The usage of the splitting methods has still based on the practical experience of mining workers on calculating the spacing of two holes in the co-planar line, resulting in increasing the roughness of the cutting face. The paper studies a relationship between beating force and two-hole spacing based on rock mass properties of dimension stone in order to find proper splitting parameters. The paper deploys the Finite Element Method (FEM) with computer simulation in 2D using Phase2 software. From input data including rock mechanics (unit density, Young’s modulus, Poisson’s rate), compressive pressure on the hole wall due to the force, the spacing, the simulation represents stress and displacement distributions along the two adjacent holes. A regression function for a relationship between the pressure and the spacing is established with the results of stress distribution along the two adjacent holes, compared to uniaxial tensile strength through running the software, which leads to making suggestions to appropriate splitting parameters.

Effect of an eccentric decoupled charge on rock mass blasting

Experimental and numerical analyses were conducted to investigate the effect of an eccentric loaded contour hole on a rock mass. In the concrete blocks used for the analyses, detonating cords were placed at the centre of the blast hole and eccentrically against the wall of the blast hole. PFC2D and AUTODYN were used for the numerical analyses, and the results of these software showed that an eccentric decoupled charge can result in the directional development of fractures, thereby enabling the control of cracks in the opposite direction. Even though both types of blasting have identical decoupling indexes, the crack and fracture patterns were affected by the location of the explosive, tamping, and other conditions. The results showed that an eccentric charge holder can be applicable to control the fracture direction and the damaged zone. For an eccentric charge, the initial crack was generated at 0.01 ms and expanded in the intended direction. For the eccentric charge, the maximum pressure at the area in contact with the blast hole wall exceeded that for the central decoupled charge by a factor of 5.5. Furthermore, the pressure in the intended direction was twice of that in the opposite direction.

Rock surface porosity measurement and pore distribution analysis based on hole wall images

This study proposes a method that uses image processing technology to identify, calculate, and analyze the pore structure in hole wall images. First, the R, B, and G components of a hole wall image are adjusted on the basis of an appropriate threshold to eliminate the effects of drilling fluid attached to the hole wall. Second, the hole wall image is converted into an S component image in the HSV color space to distinguish the pores and dark gray texture of the rock. Third, the best segmentation threshold of the S component image is obtained by the maximum interclass variance method, and the threshold is used to binarize the S component image. Finally, mathematical morphological operations are performed on the binary image to eliminate defects. On the basis of the identified pore structure in the hole wall image, the surface porosity of the rock can be calculated, and the pore distribution characteristics can be further analyzed. The results of this work provide ideas for measuring the porosity of deep rocks and analyzing their pore distribution.

Abrasive water jet piercing of inclined holes on ceramic coated nickel superalloy: A preliminary study

Micro inclined holes are widely used as cooling holes in the jet engine and gas turbine components for its protection from a high temperature gas stream. Usually, these components were made of thermal barrier coated (TBC) nickel superalloy as which often high corrosion and thermal fatigue resistance. In this work, the small diameter cooling holes with different inclination angles were fabricated on TBC nickel superalloy using the Abrasive Water Jet (AWJ) through a piercing mode. Different holes were fabricated through the employment of different settings of AWJ operational parameters namely jet impingement angle, stand-off distance and abrasive mass flow rate. The effect of piercing parameters on machining time, hole entry diameter and surface morphology were examined. From the experimental results, it is confirmed that the inclined hole surfaces do not contain any cracks and delamination. However, the rough morphology i.e wear tracks and microchips was seen in the hole wall surfaces. The preliminary results confirmed that the AWJ has capable for the production of cooling holes than other processing techniques based on lesser machining time and satisfactory hole quality. Yet, the further investigations would be required for comprehensive study of hole geometrical and surface features, and optimization of it.

All-optical modulator based on graphene-photonic crystal fiber

An all-optical modulator based on graphene-photonic crystal fiber (G-PCF) is demonstrated, where the G-PCF is fabricated by directly growing graphene onto inner hole walls of the PCF with atmospheric pressure chemical vapor deposition. A 1550 nm pump laser is used to induce all-optical intensity modulation of probe light. A G-PCF with a length of 2 cm achieves a significant modulation depth of 2.58 dB under a low pump power of 60 mW at wavelength of 895 nm. The flexible structure and all-optical control enable the G-PCF to have great potential in all-optical devices and photonics.

Machinability and surface integrity of adhesively bonded Ti/CFRP/Ti hybrid composite laminates under dry and cryogenic conditions

The current work compares the machinability of conventional drilling of hybrid titanium/carbon fiber reinforced plastics/titanium ( Ti/CFRP/Ti ) stack laminates in a single shot under dry and cryogenic conditions. Damage at metal-composite interface (MCI), thrust force, torque, burr height, surface roughness, average hole size, circularity error, and perpendicularity error are considered as experimental outcomes to investigate the influence of cryogenic cooling on the machinability of Ti/CFRP/Ti stacks. The experimental outcomes indicate that the hole quality significantly improved under cryogenic condition compared to dry drilling for the range of drilling parameters chosen. However, cryogenic cutting yields an increased thrust force due to increase in the hardness of Ti sheet at low temperatures. Scanning acoustic microscopy (SAM) was capable of capturing damage at MCI. Scanning electron microscope (SEM) and micro-computed tomography (μ-CT) are also used to assess the drilling induced damages on hole wall surface under both (dry and cryogenic) conditions.

Hole integrity of carbon fibre reinforced epoxy composites using combined punching and drilling techniques

Drilling is the most common technique in the hole making of composite panels. Based on the previous study, punching was proposed to replace drilling. This study aims to investigate the effect of hole-making techniques, i.e. new combined technique and conventional drilling technique to the hole quality and performance by conducting experiment based on surface roughness and bearing strength, respectively. The surface roughness measurement was performed oversampling with 3.6-mm thickness at four quadrant points along the hole wall. A bearing test was conducted according to ASTM D5961 procedure-A double shear with a single-pin fastener. The failure modes resulting from an experiment are quantified and compared. It was found that there is a slight difference in the initial ply failure load (IPFL) between the conventional drilling and both combined technique for 0.65% and 7.90%, respectively. However, the difference is still low, i.e. less than 10%. The results confirmed that the use of the combined technique is almost similar to the conventional drilling alone in terms of bearing strength, failure mode, and surface roughness.

The transient intrusion effect of high energy fluid on the rock failure around the wellbore during the dynamic stimulation process

Dynamic wellbore stimulation technologies, such as high-energy gas fracturing, electrical pulse fracturing, hydraulic pulsation fracturing, are generally designed to multi-fracture the near-wellbore region through generating high energy pulse with intermediate or high loading rate. During the stimulating process, the dynamic strength and failure mode of saturated rocks around the wellbore can be largely affected by the transient intrusion of high energy fluid. Accordingly, an experimental method was developed to provide a method to apply the tailored pulse loading in the central borehole of the cylindrical rock samples. Three groups (Saturated + Center hole exposed (S-CHE), Saturated + Center hole wall isolated (S-CHI), Dry + Center hole wall isolated (D-CHI)) tests were carried out. The borehole fluid pressure-time curves and the failure modes of rock samples were tracked, analyzed and compared for all of the experiments. Experimental results demonstrate that: The relationship between rock dynamic strength and loading rate is monotonic direct proportionality. The dynamic strength of saturated rock shows a much stronger strain rate dependence than that of the dry rock. At the same loading rate, fluid penetration can largely weaken the dynamic strength of rock samples. The stress pulse generates a complex crushed zone around the wellbore, while the fluid penetration tends to produce radial multi-fractures. These phenomenons were explained using the micromechanics-based macroscopic damage theory, which is capable of obtaining the relationship between the fluid intrusion and the fracture deformation in the rock dynamic failure process.

Reducing the Collision Damage Done to the Tips of Steel Needles during Integrated Piercing by Using Shape Optimization with Feature Selection

Recent research has shown that, during integrated piercing, the yarn tension can displace the needles from the centers of the holes in the piercing template. To reduce the damage done to the needle tips when the needles and the hole walls collide when the needle displacement is too large, this paper proposes a method for optimizing the needle shape that satisfies the strength constraint while targeting minimum needle displacement. First, the optimal objective function and strength constraint conditions for the tip displacement of the integrated puncture needle are established, which are affected by many factors. Then, the feature selection method of machine learning is used to reduce the dimensionality of the objective function after data reduction, and the feasible region of key features is reduced to avoid falling into the local best point in the optimization iteration. Finally, machine vision is used to measure experimentally the tip displacement of a needle array. The results show that the average tip displacement is reduced by 18.16–31.42% and the collision risk is reduced from 43.14% to 5.54%. It shows that the optimization method of needle shape based on feature selection is effective for reducing needle tip impact damage during integrated piercing.

Effect of tool wear on drilling unidirectional CFRP laminates in different fiber cutting angles

Drilling performance of carbon fiber reinforced polymer (CFRP) aggravates significantly with serious tool wear caused by highly abrasive fibers. Because of the anisotropy of CFRP, the angle between fiber orientation and cutting direction, namely, fiber cutting angle, also plays a crucial part in the drilling performance. To enhance drilling performance, this paper initially conducts a comprehensive experimental study on the effects of tool wear on thrust force, exit temperature, and hole wall defect in different fiber cutting angles in the drilling of unidirectional CFRP. The results demonstrate that the profile of worn cutting edge is an approximate elliptical arc and the half axis lengths of the elliptical arc are utilized to evaluate cutting edge wear. Tool wear on the cutting edge, flank face, and rake face all deteriorates with the number of drilled holes. Tool wear has a more significant influence on thrust force in the fiber cutting angle region of 0~90° than in the fiber cutting angle region of 90~180°. With increasing tool wear, the exit temperature distribution can still be approximate to an ellipse. The variation of exit temperature caused by tool wear in the major axis direction is still no less than that in other directions. In addition, on hole wall, tool wear causes severe crack defects in the fiber cutting angle region of 0~90° and cavity defects in the fiber cutting angle region of 90~180°.

A comparison of different heat flux density distribution models to predict the temperature in the drilling process

This work aims to improve heat flux distribution models to predict the temperature during drilling. The workpiece temperature is simulated by applying heat flux load only on the hole wall surface. Finite element method is used to simulate the temperature over process time. An iterative identification algorithm using particle swarm optimization (PSO) enables to identify the heat flux distribution parameters through the best fit of the experimental and calculated temperatures. Two heat flux density distributions presented in the literature, namely linear and polynomial, are investigated. A concentrated heat flux distribution is calculated based on measured torque and serves for a comparison purpose. Additionally, the polynomial approach is expanded by adding a concentrated heat flux in order to compensate the lack of hole bottom surface. The best results in terms of residual temperature are obtained for the hybrid heat flow distribution.

Friction-Stir Spot Mechanical Joining between Hot-Dip 55% Aluminum–Zinc Alloy-Coated Steel Sheet and A5083 Aluminum Alloy Plate Using Conventional Punching

In this study, we fabricated a mechanical joining with spot friction-stirring between an aluminum alloy plate and an aluminum–zinc alloy-coated steel sheet; this was achieved by utilizing a conventional press-punching preprocess. In the preprocessing, a hot-dip 55% aluminum–zinc alloy-coated steel sheet was punched using a press and an inclined surface was generated at the wall of a punched hole due to shear droop. Subsequently, an A5083P-O aluminum alloy plate was positioned against it, and friction stirring was conducted on its rear surface. The hole was filled with the aluminum alloy to generate a mechanical interlock at the tilt of the wall. Further, we evaluated its cross tensile strength (CTS) and tensile shear strength (TSS) and obtained an average CTS and TSS of 368 kN and 1470 kN, respectively.

Improvement of machining accuracy in EDCM by enhanced electrochemical reaming based on a non-metallic backing layer

Electrochemical Discharge Machining (ECDM) is potentially applicable for the fabrication of film-cooling holes. However, It is extremely difficult for the holes to achieve higher precision and machining quality owing to the working liquid diminish in the lateral machining gap. In this study, a non-metallic backing layer was proposed to overcome the diminish of working liquid, and the electrochemical reaming, as a post-processing method for ECDM, was used to further improve the machining accuracy and quality of the holes. First, the three-dimensional morphology of the melted pit of a paraffin backing layer was scanned to obtain the geometric parameters. Then, simulation analysis and experimental verification of auxiliary flushing by using the non-metallic backing layer were performed. The machining performance of the holes machined with electrochemical reaming based on non-metallic backing layer was confirmed by the observations of the surface topography of the hole wall and orifice, measurement of the orifice precision, and analysis of the element composition on the surface of the orifice wall. Finally, an optimum combination of machining parameters for electrochemical reaming is obtained through a process parameter optimization experiment.