1,720,973 research outputs found
The effect of nanofibrous membrane thickness on fracture behaviour of modified composite laminates – A numerical and experimental study
No full textDelamination is the most frequent failure mode in laminated composite materials and it may cause catastrophic failure in critical engineering structures. One of the most popular ways to prevent this failure is toughening the matrix. Recently, it has been proved that polyvinylidene fluoride (PVDF) nanofibers have the potential to increase the fracture toughness of epoxy-based composites. Therefore, in this study, the influence of PVDF-membrane thickness on mode-I fracture toughness of interleaved CFRP laminates is considered using experimental and numerical methods. For experimental investigation, the fracture behavior of interleaved laminates has been determined by Double Cantilever Beam (DCB) tests using two different thicknesses of membrane. On the other hand, finite element method (cohesive elements) is used for numerical considerations of fracture behavior during mode-I loading. The results show that thin and thick nanofibrous mat leads to 42% and 98% enhancement in mode-I fracture toughness, respectively. It is also shown that bi-linear traction-separation law is a suitable method to model PVDF-modified laminate under mode-I loading. By finding the three cohesive parameters K°, GI, σmax, and repeating the study for mode-II, it is possible to predict the behavior of nanomodified laminates under other loading conditions
The Effect of Pre-stress on Impact Response of Concave and Convex Composite Laminates
No full textAbstractCurved composite laminates are used in many applications while in the majority of the studies present in literature, flat laminates are investigated. Therefore in this paper, the influence of the curvature type (convex or concave) and preloading on impact response of curved laminates is considered. For this aim, a specific fixture and specimen were designed. The impact tests were performed at four different impact energies with the specimens fixed only in their straight sides. The results show that the effect of preloading on damaged area of concave laminates is lower than for convex ones. For considering the damage mechanism in different situations some optical pictures are also presented
The influence of pre-stress and curvature on impact response of curved composite laminates
No full textIn this paper the effect of pre-stress on impact response of composite laminates is considered. For this aim three various specimens were provided and impacted under 4 different impact energies: 1-A curved laminates without pre-stress (the initial curvature of A). 2-A pre-stressed curved laminate in which the initial curvature of A was changed to curvature of B by pre-stress. 3-A curved laminate without pre-stress and the initial curvature of B. In this way the effect of pre-stress and curvature were considered. The results showed that by applying pre-stress which leads to increasing the curvature (second group) the maximum force and damaged area increased significantly. Comparing the impact response of second and third specimens showed that in the same curvature the pre-stress caused less damaged area which is very important result for designing the structures containing curved laminate
Critical thrust force and feed rate determination in drilling of GFRP laminate with backup plate
No full textUsing backup plate is one of the most commonly used methods to decrease drilling-induced delamination of composite laminates. It has been shown that, the size of the delamination zone is related to the vertical element of cutting force named as thrust force. Also, direct control of thrust force is not a routine task, because, it depends on both drilling parameters and mechanical properties of the composite laminate. In this research, critical feed rate and thrust force are predicted analytically for delamination initiation in drilling of composite laminates with backup plate. Three common theories, linear elastic fracture mechanics, classical laminated plate and mechanics of oblique cutting, are used to model the problem. Based on the proposed analytical model, the effect of drill radius, chisel edge size, and backup plate size on the critical thrust force and feed rate are investigated. Experimental tests were carried out to prove analytical model
The effect of PVDF nanofibers on mode-I fracture toughness of composite materials
No full textIn this study, the fracture behavior of carbon/epoxy laminates interleaved by polyvinylidene fluoride (PVDF) nanofibers is investigated. For this aim, a mode-I fracture test is conducted on virgin and modified laminates. Unlike the results of other studies, it is shown that PVDF nanofibers can increase mode-I fracture toughness (GI) noticeably in a specific situation. The results show that GI is enhanced about 43% and 36% in initiation and propagation stages of the fracture, respectively, using PVDF nanofibers. The morphology of the fractured surface is also presented for investigating the mechanism of toughening
Improvement the impact damage resistance of composite materials by interleaving polycaprolactone nanofibers
No full textIn this study, the impact response of glass/epoxy laminates interleaved by Polycaprolactone (PCL) nanofibers is considered. PCL is a thermoplastic polymer, which is a good choice for toughening epoxy-based composite. The impact tests were conducted on curved laminates and under 24 and 36J. The results showed that the effect of interleaving on impact parameters such as maximum load is negligible, but on the other hand could decrease damaged area significantly. By inserting 30μm of PCL nanofibers between each layer of laminate the damaged area decreased about 27%
Influence of electrospun nanofibers on the interlaminar properties of unidirectional epoxy resin/glass fiber composite laminates
No full textNylon 6,6 nanofibers were interleaved in the mid-plane of glass fiber/epoxy matrix composite laminates for Mode I and II fracture mechanic tests. The present study investigates the effect of the nanofibers on the laminates' mechanical response. Results showed that Nylon 6,6 nanofibers improved specimen's fracture mechanic behavior: the initial energy release rates GIC and GIIC increased 62% and 109%, respectively, when nanofibrous interlayer was used. Scanning electron microscope micrographs showed that nanofiber bridging mechanism enhances performances of the nanomodified specimens, still able to link the layers when the matrix is broken
Comparing various toughening mechanisms occurred in nanomodified laminates under impact loading
No full textDelamination is the most frequent failure mode in thermoset-based composite laminates which can be easily occurred under an impact loading. Applying nanofibers between composite layers is one of the attractive methods to decrease the influence of this phenomenon (delamination) on the operation of an engineering structure. In this study, various nanofibers were utilized to consider their effects on impact response of glass/epoxy laminates. The nanofibers were made of PA66, PCL, and their mixture (PA66/PCL) which their toughening mechanisms are different. The results showed that PA66 and PA66/PCL had the best effectiveness on damaged area caused by the impactor (reduction of 60%). According to these results and some other evidences, it is concluded that bridging between composite layers which was established by PA66 nanofibers is the best mechanism for toughening laminates. On the other hand, depending on the curing cycle, PCL can toughen the composite laminates with two various mechanisms: 1- bridging between the layers 2- phase separation. The evidences show that the first mechanism is much more effective than the second one
An Investigation on the Fatigue based Delamination of Woven Carbon-epoxy Composite Laminates Reinforced with Polyamide Nanofibers
Full text linkAbstractDelamination is the most frequent failure mode in laminated composite materials and it may cause catastrophic failure in critical engineering structures. One of the ways to prevent this failure is to toughen the crack against initiation and propagation. There is lack of studies in literature for toughening the delamination using nanofibers, especially in the case of fatigue behavior. Therefore, the present work aims to investigate effect of interleaved nanofiber mat on fatigue interlaminar properties of mode I delamination in carbon-epoxy composite laminates. To reach this aim, the electrospun polyamide nanofiber fabrics were put in the mid-plane of woven carbon/epoxy laminates. Then the fatigue Double Cantilever Beam (DCB) tests were performed on both virgin and nanomodified specimens, based on ASTM D6115. The experimental results show that the interlaminar fracture toughness and delamination onset fatigue life of the specimens can be substantially improved by addition of the Polyamide nanofiber interlayer. As a matter of fact, there is negligible increase in the thickness of the specimens, less than 1%, but there is noticeable out-plane mechanical properties increase of the modified specimens. The increases in the delamination toughness are up to 150%, at the static tests, and around 100% at the high cycle fatigue loading conditions. Crack paths were investigated by micrograph analysis and different behaviors in the virgin and nano-modified ones were observed and related to the different mechanical results
Shear mode of fracture in composite laminates toughened by polyvinylidene fluoride nanofibers
No full textEpoxy is one of the most commonly used resins in the composite industry, but its inherent brittleness can limit its application. The insertion of thermoplastic polymers in the form of nanofibers between composite layers is a recently introduced method for increasing the fracture toughness of these materials. In this study, electrospun polyvinylidene fluoride (PVDF) nanofibers were utilized to toughen carbon/epoxy laminates under mode-II loading. The results showed that the thickness of the nanomat is an important factor that determines the response of the modified laminates: double thickness leads to about double fracture toughness. The pictures obtained by optical microscope and SEM proved that the PVDF nanofibers melted and mixed with epoxy during the curing process and the toughening mechanism due to the plastic zone formation at the crack front. In the second part of the study, the ABAQUS software was used to find out cohesive parameters of the reference and nanomodified laminates and to consider the effect of the nanomat thickness on them
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