Drilling of aerocomposites is very critical process in the production of aviation components. The defects in the drilled holes can lead to high stresses on the rivet and the component and thus, ...catastrophic failure of the structure. The economics of drilling-induced defects is significant, particularly considering the various stages related to the part as it reaches the final assembly line. In this study a novel, simple, inexpensive, and accurate AutoCAD Image Processing (ACIP) technique was developed to evaluate the damaged area in drilling FRP composites. Drilling operations are performed on woven GFRP composites without back-up plate. Scanning electron microscopic examination was conducted on representative specimens to clarify the different mechanisms of chip and burr formation with respect to fiber orientation and the instantaneous velocity vector of the cutting edge. The results revealed that the developed ACIP technique has an amazing accuracy without missing any details in the computed delamination and burr areas in comparison with the other methods. Because of the non-dimensional delamination parameters are not account the damaged area, in some cases they seems to be not suitable for representing the measured values, which considerably increased the importance of this study.
The recent climatic changes have resulted in a significant increase in temperatures over 50 °C during summer midday in certain countries. This has motivated the study of the low-velocity impact (LVI) ...response of angle-ply thin woven glass fiber-reinforced polymer composites under temperatures of 25 °C, 50 °C and 75 °C. The LVI performance of the laminates was assessed by eighteen parameters. A new inexpensive image processing method is developed to accurately measure the damage diameter, area, and perimeter length, which significantly aids in analyzing the damage mechanisms under various test conditions. The damage perimeter length is a new parameter that is used to interprets the change of material behaviors with temperature. The LVI results showed that the impact bending stiffness is increased with impact energy (Ei) due to increasing strain rates, which leads to decreasing the molecular mobility of polymer chains and hence, the materials tend to behave more brittle. Delamination threshold force increased with impact energy and vice versa with temperature. Soaking the specimens for 1 h at 50°C improved post curing and thus, enhanced the cross linking density, which significantly improved most of the impact properties. The penetration energy threshold of 25.5J (85 % of Ei) is observed at 30J-50 °C. Relaxation of the polymer chain within the glass transition region at 75°C leads to full fracture energy threshold at 30J, which correlated with open force-deformation curve, absorbed energy of 100 %, zero elastic energy, zero rebound energy, zero rebound velocity, and increase contact duration. The proposed multivariate regression models accurately predict the impact properties.
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•Eighteen impact parameters have been influenced by impact energy and temperatures.•Development new image processing method to evaluate damage area, size and perimeter.•Perimeter length used to interpret the change of material behavior with temperature.•A new concept for the ductility index was introduced through this study.
This study is motivated by the need to explore the low-velocity impact behavior of angle-ply thin woven GFRP composites with stacking sequence of 0/±45/90s, especially the lack of literature ...concerning damage analysis correlation with a comprehensive range of measurements. The damage diameter, area, and perimeter length are characterized in this study by a novel AutoCAD image processing technique. The test results revealed that specimens subjected to impact energies in the range of 2–30 J are free from edge delamination compared to those tested at 40–50 J. Decreasing the contact duration at impact energy of 60 J is due to perforation accompanied with damage propagation toward the specimen edges and thus, the contact force is sharply dropped from its maximum value of about 4.48kN to 1.3kN. Consequently, both rebound speed and coefficient of restitution dropped to zero. The direct measurement of the energy threshold is increased in the following order: energy threshold at Fmax (2.3–14.9 J), first penetration (load drop) after peak load (23.9–25.6 J), penetration energy threshold (37.9 J), perforation energy threshold (31.1 J), and perforation and crack propagation energies (43.3 J). The perimeter length represents a novel damage parameter, exhibiting a strong correlation with impact energy, as evidenced by a high coefficient of determination of 0.97.
•Falling weight impact tests are carried out on CFRE composites with impact energies ranging from 15 to 85J at room temperature, 50 °C and 75 °C.•Almost all impact energy is absorbed at room and 50 ...for impact energies higher than 35J. Penetration is observed at room and 50 for impact energies higher than 35J.•At 75°C, no penetration is observed though the highest reduction in the stiffness is recorded.•The threshold energy to initiate damage in the composite plates is measured/estimated using four methods and ranges between 2.8 and 9.6J.
Low-velocity impacts are unavoidable during the assembly stages and service life of aircrafts, marine and automobiles composite structures. This work aims to study the damage resistance of woven carbon fiber reinforced epoxy (CFRE) composite laminates subjected to a wide range of low-velocity impacts (15–85J). The composite laminates were fabricated using prepreg technique. The impact tests were carried out using falling weight impact tester at room temperature (RT), 50 °C and 75 °C. The selected test temperature of 50 °C represents the summer midday temperature of some countries, while the highest temperature value (75 °C) was used to study the possible effects of unexpected temperature rise. Compression-after-impact (CAI) were undertaken to measure their residual strength. Specimens that were tested at low impact energies (15 and 35J) as well as at higher temperature level (75 °C) are not penetrated by the impactor. Therefore, part of the kinetic energy is absorbed by the specimen in the form of damages. A second part is stored in the specimen as elastic energy, which is restituted back to the impactor for its rebound. Though no penetration is observed, the highest reduction in the stiffness and strength is observed at 75 °C. This is explained in terms of the softening and plastification of the epoxy matrix, the debonding at the interfacial fiber/matrix and the increase in the interfacial stress concentration of the re-solidified matrix. The threshold energy to initiate damage in the composite plates is measured/estimated using four methods and ranges between 2.8 and 9.6J.
Fatigue tests were conducted on scarf adhesive joints (SAJs) with composite substrates at room temperature (RT) and 50 °C. The adhesive layer was modified with Al2O3-nanoparticles. Dynamic parameters ...such as damping factor, storage modulus, dissipated energy and potential energy (Up) were measured from the stress–strain hysteresis loops of the SAJs with the aim of knowing which ones are best for early prediction of joint collapse. Incorporation of Al2O3-nanoparticles into the adhesive layer improves the fatigue strength by about 4.8 and 24.5% at RT and 50 °C respectively. The glass transition temperature was improved by 8.3% and thus, at 50 °C the fatigue strength of the Al2O3-SAJs was decreased by 5.4% in return of 20.3% for the NE-SAJs. The variation of the Up with fatigue life is more pronounced compared to the other dynamic parameters and thus, can be modeled to monitor the integrity of the joint and preventing their catastrophic failure.
As the improved damping in fiber-reinforced composites can affect the other mechanical properties, therefore, the aim of this work is to investigate the effect of multiwall carbon nanotube (MWCNT) on ...the interfacial bond strength, flexural strength and stiffness, toughness and damping properties of hybridized glass-fiber reinforced epoxy (GFRE) composites. Nanophased epoxy resin was used to hybridize unidirectional and quasi-isotropic GFRE composites with 0/±45/90s and 90/±45/0s stacking sequences. Results from the interfacial characterizations of the hybridized composites showed improvement up to 30% compared to the control laminates. Hybridization of GFRE laminates with MWCNTs leads to decreasing the flexural and storage moduli, increasing flexural strength, toughness, natural frequencies and damping ratio. A high correlation coefficient of 0.9985 was obtained between static flexural and dynamic storage moduli. The highest flexural strength, flexural and storage moduli and natural frequency of quasi-isotropic laminate were observed for 0/±45/90s stacking sequence and vice versa for damping ratio.
In this study, the influence of cutting speed (16.3, 32.7 & 65.3m/min) and feed (0.025–0.1mm/rev) on cutting temperature, thrust force, torque, delamination factor, surface roughness and bearing ...strength in drilling thin woven glass-fiber reinforced epoxy (GFRE) composites was investigated, for the first time, grouped in one article. The results of drilling tests showed that the thrust force (Ft), torque, and delamination factor are increased with increasing feed and vice versa for the induced temperature. Drilling of GFRE composites at lower speed (16.3m/min) result in lower temperature rise, higher thrust force, higher torque and accordingly, higher delamination factor compared with that drilled at 65.3m/min. Drilling at temperature close to glass transition temperature is useful for reducing Ft, delamination and thus obtaining better bearing strength compared with drilling at lower temperature levels. The critical thrust force at the onset of push-out delamination was predicted for different uncut layers, in front of the drill chisel edge, using some analytical models. For the first time, in drilling 0°8 woven composites, the ABD stiffness matrices was effectively represented by 0°/90°4s cross-ply laminate with the advantages that the elastic properties were determined by the rule of mixtures (no need to perform mechanical tests).
Adhesive joints were fabricated by joining two woven carbon fiber composite adherends with scarf angle of 5° using Epocast adhesive system of 50-A1/946 modified with optimum weight percent of 1.5 wt% ...SiC-nanoparticles. The scarf adhesive joints (SAJs) were characterized through static tensile as well as tension-tension fatigue loading. The stress-number of cycles to failure curves are useful in determining the fatigue life, but they do not give any indication about the fatigue damage development during the joint's lifetime. In consideration of early detecting damage development in the SAJs, many dynamic parameters were estimated from the analysis of the stress-local strain cycles, with the aim to obtain more accurate predictions. These parameters include loss and storage moduli, potential and dissipated energies, and damping factor. It has been found that the tensile strength and fatigue limit of the SiC-SAJ were increased by 9.6% and 55% respectively with decreasing the bondline thickness from 0.25 mm to 0.17 mm. Incorporation of SiC-nanoparticles into the adhesive layer improves the tensile strength and the fatigue limit of the SAJs with bondline thickness of 0.17 mm by 26.1% and 22% respectively compared to the unmodified joint. At higher stress levels, the storage modulus and potential energy can be modeled for early prediction of the joint life and thereby, help in preventing their catastrophic failure. Zhang model showed good agreement with the measured potential energy compared to the storage modulus.
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A new approach was used in drilling woven glass fiber reinforced polymer (GFRP) composites to clamping the specimen by a controllable force at drill entry and exit sides. The influence of cutting and ...support conditions on thrust force, push-out and peel-up delaminations, and burr factor was investigated. A series of ASTM tests were implemented to determine tensile, in-plane shear, Mode I and Mode II fracture toughness properties, which were used to predict the critical thrust force (CTF) at delamination initiation. The delamination and burr areas were accurately measured via inexpensive, easy, and accurate image processing technique. Result from drilling processes revealed that the supported specimens are approximately free from burr accompanied with insignificant peeling within the drill (10 mm)/support hole (12 mm) tolerance. By using support plates, the thrust force was increased up to 8.3% compared to the unsupported conditions. On the other hand, the delaminations were decreased by about 65.5%. The delaminations were dominated by Mode I fracture toughness, which is lower than that of Mode II by 41%. The predicted CTF using concentrated load models are slightly lower than the measured values, which interpret the occurrence of delamination damage on the cutting zone.
