The T800 Carbon fiber reinforced polymer/plastic (CFRP) has been increasingly used for its considerable specific strength/modules to manufacture the primary load-carrying structures in aerospace ...industry. The abrasive carbon fibers can cause rapid tool wear in CFRP drilling, which deteriorate the quality of hole wall and result in unpredictable decrease of bearing capacity. In order to effectively reduce the tool wear, a cooling method namely external cooling lubrication (ECL) is applied in this study by using two different lubricants (Boelube 70104 and Castrol Syntilo 9828). The results show that Boelube 70104 lubricant exhibits the maximum flank wear (VB) reduction (34.5%) as compared to dry drilling after drilling 30 holes. And, the maximum CER reduction (57.4%) is obtained by using Castrol Syntilo 9828 lubricant. Different from previous researches, the maximum thrust force of drilling process using ECL presents obvious decrease as compared to dry drilling. And, the value of surface roughness (Ra) exhibits an obvious decrease when using Boelube 70104 lubricant. Two main different mechanisms for Ra decrease namely reducing surface cavity at FCA of 90° <
θ
< 180° and reducing saw-tooth surfaces at FCA of 0° <
θ
< 90° are separately observed for Boelube 70104 and Castrol Syntilo 9828 separately.
Application of carbon fiber reinforced plastic (CFRP) in aircraft load-carrying structure makes the component become thicker and damage tolerance get stricter. The heat accumulation highly increases ...the hole wall temperature along feeding direction during thicker CFRP component drilling due to the low thermal conductivity, which highly deteriorates the hole quality. Therefore, this paper proposed a hole wall temperature measurement experiment at various fiber cutting angles with different measuring distances to investigate the effects of heat accumulation on the cutting temperature and damages. Results show hole wall temperature in the vicinity of hole-exit increases by almost 20⁓40 °C and 30⁓65 °C separately for
S
=3000 rpm and
S
=5000 rpm. The fiber shearing and bending when against fiber cutting seriously increases temperature near the hole wall by increasing the fiber-rich regions and contact time between workpiece and cutting lip. Moreover, the maximum temperature distribution maps along feeding direction and radial direction were separately drawn for the first time to analyze the heat accumulation and overheating areas. The hole wall along the hole edge and feeding direction were detected, and severe matrix cracking, fiber bending, and fiber-matrix debonding occurred at the position where maximum temperature was located. In the future, the findings of borehole temperature characteristics can be used to refine the existing temperature model of CFRP drilling.
Carbon fiber-reinforced polymer and titanium alloy are widely employed in the aerospace industry. In the integrated drilling of CFRP/Ti stacks, the interface heat accumulation and severe tool wear ...are more serious than that in single CFRP drilling, which generated significant composite borehole damages. To address these complications, a fundamental study was conducted to explore the cutting mechanism of UD-CFRP using a controlled temperature and edge profile. Combined with the Digital Image Correlation technique, the loading states of carbon fiber and matrix were comprehensively investigated, and their effects on machined damages at four fiber cutting angles were analyzed. Results show that progressive wear of the cutting edge causes the initial fracture point of fiber to move up, leading subsequently to fiber bending or buckling. This further results in the separation induced by the compressive stress under the friction and ploughing functions of flank wear in the cutting process, affected by which the value of cutting force and shear strain increases along the worn edge profile. When the temperature exceeds the glass transition temperature (
T
g
= 110 °C), the low-rigidity fibers without support tend to overbend and fracture in the subsurface, ultimately resulting in matrix loosening and debonding damages at
Φ
= 90°~135° of the CFRP hole-wall at the interface region.
CFRP has been gradually used in the load-carrying structure with thicker component and stricter damage tolerance in aviation industry. However, the heat accumulation always occurs at hole exit-ply ...during thick CFRP drilling, resulting in temperature even higher than the glass transition temperature (Tg) of matrix, affected by which severe damages will occur based on an unknown cutting mechanism. Therefore, to investigate the new mechanism, this paper proposed a temperature controlled orthogonal cutting experiment by heating the cutting area to specified high temperature ranges (below and above Tg) before cutting. The results show that the resin matrix will be only softened by the high temperature below Tg and lose part of its strength, but the cutting mechanism is almost not changed. As for temperature above Tg, the resin matrix will be heated into molten state thereby losing the support function for machined fibers, which changes the cutting mechanism at 90° <θ < 180° into low efficiency cutting mode controlled by compressing stress. The fibers that lose support will be over bent and broken by flank face, and severe thermal mechanical damages including loose face, matrix cracking and fiber pull out can be captured at θ ~ 150° of hole wall near exit.
•The thermo-mechanical relationship of tool-materials was decoupled around the tool wear mechanism.•The effect of drilling parameters on interface temperature and tool wear progression was ...quantified.•The cutting behavior at four FCAs under various cutting temperature and worn edges was revealed.•The subsurface damage formation and propagation mechanism of drilling process was analyzed.•The interface damages of thin-walled CFRP/Ti under high thermo-mechanical erosion were evaluated.
Due to the poor thermal conductivity and machinability of CFRP and Ti6Al4V, significant heat accumulation at the interface and rapid tool wear are considered the main factors restricting the hole quality. This study conducted a thin-walled CFRP/Ti stack drilling experiment to investigate the evolution of the thrust force, temperature and tool wear versus different drilled hole numbers and spindle speeds, affected by which the novel material removal behavior was revealed. Results showed that the thrust force and interface temperature increased with the hole number and spindle speed, causing a significant increase in the cutting edge rounding and flank face wear dominated by the many fiber bundles and metal debris. In turn, the various tool wear led radial interface temperature to alternately transfer between Φ = 45° and 135°. Under the cyclic thermo-mechanical function, the fibers underwent significant bending deformation as they lost the support of the degenerated matrix resin, forming severe subsurface damage when the interface temperature exceeded the matrix glass transition temperature (Tg = 110 °C). Combined with the comprehensive evaluation of exit damages, a relatively higher spindle speed was preferred for selecting the initial drilling of thin-walled CFRP/Ti stack and gradually reducing the spindle speed with increasing tool wear extent.
Carbon fiber reinforced polymers (CFRP) are widely used nowadays as primary and secondary bearing structures in the aviation industry. However, CFRP structures are threatened by unpredictable ...low-velocity impact events (caused by dropped tools, etc.) during the assembly process, which can cause impact damages. To prevent CFRP damage during the assembly process, a rubber layer can be placed on the surface of the CFRP to form a protective hybrid structure. The influence of rubber layer thickness on the protection effect and on the impact response of the hybrid structure were first investigated in this study. The experimental results indicate that the rubber layer significantly improved the CFRP’s impact resistance, with fewer damage modes and less damage area observed. Furthermore, the thicker the rubber layer places, the lower the CFRP delamination position occurs. The whole impact process of this hybrid structure was simulated using the mixed-mode damage criterion for CFRP laminate and the hyperelastic law for the rubber layer. The finite element model was refined with mesh size and contact behavior corresponding to the experiment condition, which showed good agreements with impact response and delamination area experiments. Some internal damage details and damage evolution were also discussed using the finite element model.
Delamination is the most critical damage in drilling the CFRP/Ti stacks under the impact of drilling parameters and tool structure, which makes the traditional theoretical or empirical models not ...have enough accuracy and be time-consuming due to the multi variables, while the machine learning model would suffer the unsuitable hyper-parameters and have a bad accuracy and generalization ability. This paper proposed an adaptive modelling approach to predict the delamination while drilling the CFRP/Ti stacks. This approach adapted the original arithmetic optimization algorithm (AOA) by adding a random disturbance phase to update the penalty coefficient C and the kernel coefficient γ of the support vector regression (SVR) automatically. In the meanwhile, the approach made use of the energy of the 5 stages in drilling the CFRP/Ti stacks and predicted the delamination damage both at the entrance and exit. The modified AOA optimized the training mean squared error(MSE) in predicting the entrance and exit delamination by 10.27 % and 33.63 %, while the accuracy of the proposed model can reach 96.7 % and 97.17 % respectively. The model got validated, and had a comprehensive ability containing the accuracy and generalization ability.
Due to lack of effective backup when drilling aircraft thin-walled CFRP structure, serious dynamic structural deformation including vertical and in-plane deformation usually occur, which extremely ...changes the cutting behavior and materials removed mechanism, causing severe entrance and exit damages. Therefore, this paper proposed a dynamic deformation measurement experiment in vertical and horizontal direction at various processing parameters to analyze their effects on cutting mechanism and damages distribution. Results show the vertical and in-plane deformation occurrence are highly dependent on the actual positions of tool-CFRP system. Namely elastic deformation, local deformation and drawback reflection appear many a time in different stages, causing obvious sudden change of thrust force and different initiation and propagation characteristics of damages. With the feed rate increasing and workpiece thickness decreasing, the maximum displacements of every stages all increase and some thinner workpiece displacement exceed several times the plate thickness. The larger in-plane deformation will nonuniformly distribute along borehole exit due to the different cutting mechanism and strength of different fiber cutting angles. The larger elastic deformation and drawback action in vertical direction will change the out-ply material remove mechanism into mainly puncture rather than only cutting. Under the joint action of deformation in two directions, the loose surface damages composed of delamination and uncut fiber will gather heavily at exit.
•The vertical and in-plane deformation were characterized in divided stages by DIC.•The influence of drilling parameters on deformation and drawback was investigated.•The cutting behavior at various FCA under large drawback action was revealed.•The damage formation and propagation mechanism of drilling process was analyzed.•The in-plane damages in entrance and exit regions after deformation were evaluated.
•Hole-location error leads to deformation of composite joints which distributed symmetrically along direction of the error.•Forced connection causes non-uniform deformation of exit and damage on ...surface and hole of joints.•Average deformation can be reduced by more than 70% after changing installation sequence form middle to both sides.
In this paper, deformation of composite multi-bolted joints considering hole-location error and installation sequence was investigated. Experiments were conduct to examine deformation and damage of joints with different hole-location error and installation sequence. Deformation of joints was measured by laser measurement system. Bolt-composite interfaces were examined microscopically to reveal damage caused by forced installation. Results showed that hole-location error caused vertical deformation of composite joint. Non-uniform extrusion of up and down hole was caused by forced installation. Besides, deformation mode of hole exit was dominated by deformation asymmetry. Damage distribution was influenced by radial extrusion direction of up and down holes. Thus, damage was distributed no-uniformly along depth of holes. During forced installation, deformation accumulation of multi-bolted joints was generating. Different installation sequences caused various deformation distribution of composite joints. Average deformation were reduced by more than 70% after changing the installation sequence. Conclusions of this paper are of significance for developing deformation model and deformation reduction of composite multi-bolted joints assembly.
Limited information is available regarding spatiotemporal variations of particles with median aerodynamic diameter < 2.5 μm (PM2.5) at high resolutions, and their relationships with meteorological ...factors in Beijing, China. This study aimed to detect spatiotemporal change patterns of PM2.5 from August 2013 to July 2014 in Beijing, and to assess the relationship between PM2.5 and meteorological factors.
Daily and hourly PM2.5 data from the Beijing Environmental Protection Bureau (BJEPB) were analyzed separately. Ordinary kriging (OK) interpolation, time-series graphs, Spearman correlation coefficient and coefficient of divergence (COD) were used to describe the spatiotemporal variations of PM2.5. The Kruskal-Wallis H test, Bonferroni correction, and Mann-Whitney U test were used to assess differences in PM2.5 levels associated with spatial and temporal factors including season, region, daytime and day of week. Relationships between daily PM2.5 and meteorological variables were analyzed using the generalized additive mixed model (GAMM).
Annual mean and median of PM2.5 concentrations were 88.07 μg/m3 and 71.00 μg/m3, respectively, from August 2013 to July 2014. PM2.5 concentration was significantly higher in winter (P < 0.0083) and in the southern part of the city (P < 0.0167). Day to day variation of PM2.5 showed a long-term trend of fluctuations, with 2-6 peaks each month. PM2.5 concentration was significantly higher in the night than day (P < 0.0167). Meteorological factors were associated with daily PM2.5 concentration using the GAMM model (R2 = 0.59, AIC = 7373.84).
PM2.5 pollution in Beijing shows strong spatiotemporal variations. Meteorological factors influence the PM2.5 concentration with certain patterns. Generally, prior day wind speed, sunlight hours and precipitation are negatively correlated with PM2.5, whereas relative humidity and air pressure three days earlier are positively correlated with PM2.5.
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