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    Experimental and statistical analysis of low velocity impact response of filament wound composite pipes
    (Elsevier Sci Ltd, 2018) Gemi, Lokman; Kayrici, Mehmet; Uludag, Muhammet; Gemi, Dilek Soylu; Sahin, Omer Sinan
    Nowadays, filament wound composite pipes (GRP) are used as a structural element in many applications such as natural gas and oil transmission lines, and portable bridge constructions for military purposes. GRP pipes can expose to impact loading from various causes. This loading can cause an invisible level of damage. Thus, the detection and evaluation of such damages are of great importance. In this study, the low velocity impact response of (+/- 55 degrees)(3) filament wound E-glass/epoxy composite pipes has been studied. The pipes have been subjected to drop weight impact loading with various impact energies. The force-time and force-displacement relations have been examined. The impact damage formation was also evaluated. It is concluded that the damage development in the pipes is controlled by displacement trough radial direction. The obtained results were evaluated statistically by means of Weibull approach. Microscopy analysis of impacted region revealed that debonding, radial cracks, transfer cracks and delamination damage modes are the main observed damage modes.
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    Experimental investigation of axial compression behavior after low velocity impact of glass fiber reinforced filament wound pipes with different diameter
    (Elsevier Sci Ltd, 2022) Gemi, Dilek Soylu; Sahin, Omer Sinan; Gemi, Lokman
    The GRP (Glass Reinforced Polymer) composite pipes produced by filament winding (FW) technique are used in many applications such as the transmission of pressurized gases, liquids, oil, and natural gas transmission. These composite pipes may be subjected to low velocity impacts during production and/or service for various reasons. The impact on the composite pipes may lead to invisible damages such as matrix crack, fiber damage, and inter layer separation. In this study, pipes with three different diameters (empty set54, empty set72, and empty set96 mm) were produced to investigate the damage caused by low velocity impact and to determine the effect of these damages on the strength losses of the composite pipe. The pipes were subjected to low velocity impact test according to ASTM D 7136 with 1.5, 2, 2.5, and 3 m/s impact velocities. In order to examine the effects of impact damages on the strength of pipes, Compression after impact (CAI) tests were performed on pre-damaged pipes according to ASTM D 7137 and Force-Displacement and Stress-Strain relations were obtained. The results of impacted samples were compared with the behavior of undamaged samples. After the experiments, macro/micro damage analyses of GRP pipes were performed by optical microscope and SEM imaging.
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    Experimental investigation of the effect of diameter upon low velocity impact response of glass fiber reinforced composite pipes
    (Elsevier Sci Ltd, 2021) Gemi, Dilek Soylu; Sahin, Omer Sinan; Gemi, Lokman
    The GRP (Glass Reinforced Polymer) composite pipes produced by filament winding technique are used in many fields such as the transmission of pressurized chemical liquids, industrial liquids, oil and natural gas transmission and construction materials. GRP composite pipes may be subjected to low velocity impacts for various reasons both during installation and service. The impact on the GRP composite pipe wall may lead to some nonvisible damages such as matrix crack, fiber damage, delamination and inter-layer separation. Composite pipes suffered strength loss due to these damages caused by impact cannot support the expected strength values during service. Pipes with three different diameters (054, 072 and 096 mm) were produced to investigate the damage caused by low velocity impact and to determine the effect of these damages on the strength losses of the composite pipe. In order to investigate the influence of low velocity impact damage, the produced pipes were subjected to impacts at the velocity of 1.5, 2, 2.5 and 3 m/s according to ASTM D 7136 and preliminary damage was formed. During low velocity impact tests, Force-Time, Force-Displacement and Energy-Time graphs were obtained and dynamic behavior of the pipes were examined. With the increase in diameter; it has been observed that the effect of low velocity impact is reduced and the damage after impact transformed into delamination damage rather than multiple damage.
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    An experimental study on the effects of various drill types on drilling performance of GFRP composite pipes and damage formation
    (Elsevier Sci Ltd, 2019) Gemi, Lokman; Morkavuk, Sezer; Koklu, Ugur; Gemi, Dilek Soylu
    Drilling is the most commonly used machining operation among others for assembly applications. The necessity of this operation has come to the forefront in composite materials that are becoming widespread nowadays. In this study, drilling performance of glass fiber reinforced plastic (GFRP) composite pipes used in many sectors such as natural gas transmission lines, pressurized sewer lines, industrial waste transmission, defense industry, and construction industry was investigated. The GFRP pipe was produced by filament winding method with a winding angle of +/-55 degrees. Different drill types (conventional twist drill, brad and spur drill, and brad center drill) with a drill diameter of 4 mm were utilized for drilling the pipe and the effect of drill type on the drilling performance was investigated. Drilling tests were performed at a constant 5000 rpm spindle speed and six different feed rate parameters (25, 75, 125, 175, 225, and 275 mm/min). Thrust forces generated during drilling were recorded and after the drilling operations, hole exit surface damage, and borehole surface damage were examined by a digital microscope and scanning electron microscope (SEM). Results showed that the brad center drill produced lower thrust forces while the twist drill generated higher thrust forces. The severity of damages could vary depending on the tool geometry and feed rate. Especially, at lower feed rates, conventional twist drill results with increased delamination and uncut fibers as compared with other drills. The brad center drill presents better performance since it generates fewer damages. Also, it is observed that these damages formed in the winding angle direction (orientation).