U.A. Khashaba

Delamination in drilling GFR-thermoset composites

Abstract Delamination is a major problem associated with drilling fiber-reinforced composite materials that, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration. Delamination-free in drilling different fiber reinforced thermoset composites is the main objective of the present paper. Therefore the influence of drilling and material variables on thrust force, torque and delamination of GFRP composites was investigated experimentally. Drilling variables are cutting speed and feed. Material variable include matrix type, filler and fiber shape. Drilling process was carried out on cross-winding/polyester, continuous-winding with filler/polyester, chopped/polyester, woven/polyester and woven/epoxy composites. A simple inexpensive accurate technique was developed to measure delamination size. The results show that the presence of sand filler in continuous-winding composites not only raised the values of cutting forces and push-out delamination but also increased their values with increasing cutting speed. In contrast, increasing the cutting speed in drilling cross-winding, woven and chopped composites reduces the push-out delamination as a result of decreasing the thrust force. The thrust forces in drilling continuous-winding composite are more than three orders of magnitude higher than those in the cross-winding composites. Chopped composites have lower push-out delamination than those made from woven fibers. For the same fiber shape, the peel-up and push-out delaminations of woven/epoxy composite are lower than that for woven/polyester composites. Delamination, chipping and spalling damage mechanisms were observed in drilling chopped and continuous-winding composites. In drilling woven composites the delamination was observed at different edge position angles due to the presence of the braids that made by the interlacing of two orthogonal directions of fibers tows (warp and fill). Delamination-free in drilling cross-winding composites was achieved using variable feed technique.

Factors affecting the machinability of GFR/epoxy composites

Abstract Drilling is an essential operation in the assembly of the structural frames of automobiles and aircrafts. The life of the joint can be critically affected by the quality of the drilled holes. The main objective of the present paper is to investigate the influence of some parameters on the thrust force, torque and surface roughness in drilling processes of fiber-reinforced composite materials. These parameters include cutting speed, feed, drill size and fiber volume fraction. The quasi-isotropic composite materials were manufactured from randomly oriented glass fiber-reinforced epoxy, with various values of fiber volume fractions ( V f ), using hand-lay-up technique. Two components drill dynamometer has been designed and manufactured to measure the thrust and torque during the drilling process. The dynamometer was connected with a data acquisition, which installed in a PC computer. This set-up enable to monitor and record the thrust force and torque with the aid of a computer program that designed using Lab View utilities. The results indicate that the start point of torque cycle is delayed by few seconds (depending on the value of feed) than the thrust force. This time is consumed to penetrate the specimen by chiseling edge. After the thrust force reached its maximum value it is gradually decreased during the full engagement of the drill and goes to zero when both the chisel edge and the cutting lips have exit of the laminate. In contrast the torque was gradually increased up to the end of the cycle and sudden jump to a value about 10 times the peak value. Cutting speed has insignificant effect on the thrust force and surface roughness of epoxy resin. For glass fiber-reinforced epoxy composites (GFREC) with V f =9.8–23.7% the thrust force and torque were decreased with increasing cutting speed. On contrast increasing feed, drill size and fiber volume fractions lead to increase the thrust force and torque. The drilled holes of GFREC with lower V f ratio at lower feed have greater roughness than that drilled at higher feed. Specimens with high V f ratio have a contrary behavior. Drill diameter combined with feed has a significant effect on surface roughness.

Fatigue and Reliability Analysis of Unidirectional GFRP Composites under Rotating Bending Loads

Rotating bending fatigue tests have been conducted on unidirectional glass fiber reinforced polyester (GFRP) composites. Standard test specimens were manufactured in form of circular rods with various fiber volume fraction (V f) ratios. Failure modes of the composite rods have been examined using scanning electron microscope. The two-parameter Weibull distribution function was used to investigate the statistical analysis of the experimental fatigue life results. Safe design life based on time to first failure (TTFF) concept was calculated at high confidence level γ=0.99 and at two values of reliability, R=0.368 and 0.99. S–N diagrams of mean life, 50% survival life, lower bound life and safe design fatigue life have been constructed for GFRP rods with various values of V f ratios. These diagrams are of considerable value to the designer specially when the structure contains a critical component where any failure is catastrophic. No rigorous fatigue limit was observed within 107 cycles in these S–N diagrams. The fiber volume fractions have insignificant effect on the slope of the power function that fits the mean fatigue life. At the same number of cycles the stress amplitude required to fracture the specimens with V f=44.7% was increased by an order equal to 1.58 and1.25 than specimens with V f=15.8 and 31.8% respectively. The widest scatter was observed at the life range of 105 and 106 cycles for GFRP rode with different V f ratios. This tendency in the dispersion of fatigue life at varying stress levels is extremely important and deserves much attention for the design and application of GFRP composites.

Fracture Behavior of Woven Composites Containing Various Cracks Geometry

Recently, polymeric composite materials are widely used in automotive applications for reducing structural weight, saving fuel, and improving performance. Accidentalimpact damage is an important issue in the use of these composite materials as body components. The damages or cracks in the present work were represented by notched specimens. The test specimens were fabricated from polyester resin reinforced by woven E-glass fiber. A series of tests were conducted to measure the notched strength of composite laminates with various types of through-the-thickness cracks such as central normal notch (CNN), center inclined notch (CIN), single edge notch (SEN) and double edge notch (DEN). A Modified Point Stress Criterion (MPSC) was proposed for predicting the tensile strength of composite laminates with various aspect ratio (a/w), that is, notch length to specimen width ratio. The results show that the load–elongation diagrams of notched and unnotched glass fiber reinforced polyester (GFRP) specimens have a nonlinear behavior, with clear “Knee”, associated with audible noises. For the same notch length, the notched strength of GFRP specimens with symmetric cracks (CNN, CIN and DEN) is quite consistent. The lowest notched strength is for SEN specimens. An excellent agreement is found between the experimentalresults of centralnotched specimens (CNN and CIN) and the predicted values using the MPSC than the reviewed published theories. The critical stress intensity factor (K IC) was determined from the centralnotched specimens and used to predict the notched strength of edge notched specimens (SEN and DEN). The type of the central notch (inclined or normal) insignificantly affects the values of the stress intensity factor, K IC.

Drilling of polymer matrix composites: A review

This article addresses the effects of drilling parameters on the machinability parameters in drilling polymeric composite materials. Drilling parameters include: feed, speed, drill wear, drill geometry, drilling with special tools, and composite materials parameters. The machinability parameters include: thrust force, torque, residual strength, surface roughness, and mechanical and thermal damages of the composite materials. Delamination associated with drilling processes can often be the limiting factor for composite materials in structural applications. It results in reducing the structural strength, poor assembly tolerance and, hence, it has the potential for long-term performance deterioration. Therefore, special interest will be given for reporting factors affecting the delamination onset, delamination mechanisms, measurements, assessments, and delamination-free techniques in drilling fiber-reinforced composites.

Effect of mean stress on fatigue behaviour of GFRP pultruded rod composites

In weight-sensitive applications such as aircraft and space vehicles, high-performance composites (FRP) were used because of their high fatigue strength to weight ratio. To clarify the effect of mean stress on the behaviour of the S-N diagram of GFRP composites, constant deflection flexural fatigue tests were conducted on standard unidirectional glass fibre (ER 1150 F-183)-reinforced polyester (QL 8520 A) pultruded rods. The frequency of the testing machine was 25 Hz. The failure criterion was defined as when the residual stiffness of the test specimen reached 70% of the specimen stiffness at the start of the test. The results show that mean stress has a slight effect on the material constant (A; slope of the S-N power function). The relationship between the specific stiffness ratio EI(EI)o and the cycle ratio NNf was independent of the values of initial stress amplitude and mean stress. A master diagram for GFRP composite rods has been constructed for various cyclic lives. The area under the curves of the master diagram represents the safe design range, based on average values, of initial stress amplitude (plotted on the ordinate) and mean stress (plotted on the abscissa).

Notched and Pin Bearing Strengths of GFRP Composite Laminates

An experimental study was conducted to determine the notched and pin bearing strengths of randomly oriented GFRP composites having various values of fiber volume fractions (V f ). To achieve this, the effect of varying hole size (D = 3 to 12 mm) on the tensile strength of GFRP specimens, with constant width (W), has been investigated. A series of pin bearing tests has been carried out to examine the effect of W/D ratio on the bearing strength of the composite laminates. The failure sequence and the fractured surfaces of bearing specimens were examined using the scanning electron microscope. The results show that, fiber volume fraction has a significant effect on load-pin bearing displacement behavior. The value of W/D must be greater than 5 for the development of full bearing strength of the composite laminates. The stress intensity factor (K lc ) was accurately calculated for GFRP composites with various values of V f , in the range of hole sizes investigated.

Effect of fibre volume fraction on the fatigue behaviour of GRP pultruded rod composites

Abstract The aim of this paper is to study the fatigue behaviour of GRP rods manufactured by a modified pultrusion technique. Constant-deflection flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R = −1. The frequency of the testing machine was 25 Hz. Failure was defined as a 30% drop in the specimen stiffness (EI). The relationship between the endurance limit and the static flexural strength and fibre volume fraction, Vf, was determined, and the influence of Vf on the induced surface temperature, S-N relationships and failure modes was investigated. The results show that, in the range of Vf values investigated, as Vf increased fatigue strength increased. The relationship between the residual stiffness ratio ( EI (EI) 0 ) and the normalized fatigue life ( N N f ) was independent of stress amplitude (Sa) and Vf. This result was supported by using the residual stiffness ratio as a failure criterion. The predicted values of endurance limit by using the proposed equation gave excellent agreement with the experimental data.

Experimental and numerical analysis of pinned-joints composite laminates: Effects of stacking sequences

The present study investigates the effect of stacking sequence on the failure loads (strength) and modes of pinned-joints glass-fiber reinforced epoxy composite laminates. Specimens with [0/90]2S, [15/−75]2S, [30/−60]2S and [45/−45]2S stacking sequences were investigated both experimentally and numerically. A series of ASTM tests were performed on unidirectional [8]0 glass-fiber reinforced epoxy composite laminate to determine the properties of the single lamina that was needed for the finite element analysis. A 3D progressive damage model was built with the aid of ABAQUS software, failure criteria and property degradation rules to simulate the problem. The results showed that the [0/90]2S laminate has the highest ultimate strength. The minimum bearing and ultimate strength was observed for [30/−60]2S laminate. Loading the specimens up to the ultimate value lead to shear-out failure mode for [0/90]2S, [15/−75]2S and [30/−60]2S stacking sequences, while specimens with [45/−45]2S stacking sequence are characterized by bearing failure mode. The experimental and numerical results agree well with a maximum Euclidean error norm of 8.57%.

Drilling analysis of woven glass fiber-reinforced/epoxy composites

This article deals with the effect of drilling parameters (feed, speed, and drill pre-wear) on the machinability parameters (thrust force, torque, peel-up and push-out delaminations, surface roughness, and bearing strength) in drilling woven glass fiber-reinforced. The results show that at high feeds (0.45 mm/rev), the drill point acts as a punch that pierces the laminate with approximately constant push-out delamination size irrespective to the value of the thrust force. Surface roughness increases with the increase of drill pre-wear due to the generated heat that assisted by the low thermal properties of polymeric composites. Drilling at high feeds reduces the stiffness of the specimens and its ultimate bearing loads. Artificial neural network and multivariable regression models were developed for predicting the bearing strength of drilled holes.