Mechanical Behavior of Nylon 66 and Glass Fiber Reinforced Nylon 66 - Lab Report Example

Mechanical Properties of Glass Fiber Reinforce Nylon 66 Composite Objective: The experiment is aimed at making a comparative study of mechanical behavior of Nylon 66 and Glass Fiber Reinforced Nylon 66 by carrying out carrying out tensile testing. Theory: Polymeric materials have low stiffness and are ductile. Ceramic materials on the other hand are stiff and strong, but at the same time are catastrophically brittle. Composite materials have been developed to exploit the best of both i.e. ductility of polymers with stiffness and strength of ceramic materials. Therefore certain plastics with high ductility are being reinforced with high strength glass and aramid fibers to produce composites with high strength and high ductility (Hertzberg R. W. 1996). The properties of a composite material depends on shape, size, volume fraction, distribution, alignment etc. of the reinforcing material and is some sort of weighted average of the properties of the matrix and the reinforcing constituent. For a fiber reinforced composite material aligned parallel to the loading direction, the strength and stiffness are weighted average of the strength and ductility respectively of the matrix and the reinforcing constituent and the weighted average is taken over the volume fraction of the individual phases. The stiffness and the strength of the composite material are given by the following expression: (1) Where, EC = Elastic Modulus of the Composite Em = Elastic Modulus of the Matrix Ef = Elastic Modulus of the Reinforcing Fiber Vm = Volume Fraction of the Matrix Vf = 1 - Vm = Volume Fraction of the Reinforcing Fiber And (2) Where, = Tensile Strength of the Composite = Yield Strength of the Matrix = Fracture Strength of the Reinforcing Fiber Experimental Procedure: Tensile test of Nylon 66 and Glass Fiber Reinforced Nylon 66 Composite was performed using on rectangular specimen on Instron Machine and load – extension plot was obtained. From this plot the load at the yield point, the maximum load and the extension before fracture was measured for calculating useful properties like yield strength, ultimate tensile strength and ductility of the material. Dimensions of the tensile specimen were measured using a micrometer and the same are presented in Table 1, below. Table 1: Important Dimensions of Tensile Specimen Material Dimensions Nylon 66 Glass Fiber Reinforced Nylon 66 Composite Gage Length (mm) 77 77 Width (mm) 9.80 3.75 Thickness (mm) 10.00 3.79 Cross-section of the Glass Fiber Reinforced Nylon 66 Composite was examined at high magnification. Volume fraction of the Reinforcing Glass Ceramic was calculated using “Point Count Method”. Results: The load – extension plot for Nylon 66 and Glass – Fiber Reinforced Nylon 66 composite is presented in Fig. 1 and Fig. 2 respectively. Fig. 1: Load – Extension Plot for Nylon 66 Fig. 2: Load – Extension Plot for Glass Fiber Reinforced Nylon 66 Composite The important parameters derived from these plots and correspondingly important tensile properties of these materials are presented in Table 2, below. Material Dimensions Nylon 66 Glass Fiber Reinforced Nylon 66 Composite Yield Force (N) 2276 5693 Yield Strength (MPa) 61.9 150.2 Maximum Force (N) 3308 5693 Tensile Strength (MPa) 90 150.2 Extension (mm) 324 7.9 %Elongation 420.6 10.3 Calculation of Elastic Modulus from the load extension plot was done by taking slope of the initial linear portion. The values of the Elastic Modulus of the Nylon 66 (Em) and Glass Fiber Reinforced Nylon 66 Composite (EC) are the following: For Nylon 66: Linear Portion of the load – extension plot was taken upto 1600 N Load. For this load Stress  = 1600/(3.75 * 9.8) MPa = 43.54 MPa Extension = (1.3/13.3)*40 mm = 3.91 mm Strain = Extension / Gage Length = 3.91 / 77 = 0.051 Therefore, Em = (Stress / Strain)elastic region = 863.7 MPa For Glass Fiber Reinforce Nylon 66 Composite: Linear Portion of the load – extension plot was taken upto 1500 N Load. For this load Stress  = 1500/(3.79 * 10.00) MPa = 39.58 MPa Extension = (21 / 24)*1.6 mm = 1.4 mm Strain = Extension / Gage Length = 1.4 / 77 = 0.018 Therefore, Em = (Stress / Strain)elastic region = 2198.8 MPa From Fig. 1 it can be seen that Nylon 66 is very ductile material. A gage length of 77 mm has got extended by ~ 325 mm before fracture. This is a fantastic value for ductility by any standard. However, its stiffness and yield strength are much lower at 863 MPa and 62 MPa respectively. Owing to reinforcement by glass fibers there has been significant improvement in the stiffness and yield strength which has gone to 2200 MPa and 150 MPa respectively with a decent amount of tensile ductility which is ~ 10%. The volume fraction of the reinforcing glass fiber was calculated by point count method on the magnified cross-section of the glass fiber reinforced composite (Fig. 3). For this purpose a grid of lines was drawn with spacing of 5 mm and then the number of nodes being cut by the fiber was counted which was then divided by the total number of nodes in the grid. Volume fraction of the reinforcing fiber Vf = 0.30 Therefore, volume fraction of the matrix Vm = 1 – Vf = 0.70 Fig. 3: Cross-section of Glass Fiber Reinforced Glass Ceramic Using rule of mixtures (1); Elastic Modulus of the Glass Fiber MPa Similarly, using equation (2); Fracture Strength of the Glass Fiber MPa The values of the elastic modulus reported in the literature for nylon 66 matrix (Em), and reinforcing glass fiber (Ef) given in the literature are the following. Elastic Modulus of Nylon 66 Em = 2.05 GPa (Mean of the range 1.2 – 2.9 GPa) (Hertzberg R. W. 1996) Elastic Modulus of Glass Fiber Ef = 72 GPa (Ramchandran P. 2009) It is clearly that the experimentally measured values of the elastic modulus for the matrix and the fiber from the tensile test are way below what is reported in the literature. This is because the values of elastic modulus were measured using the strain as indicated by the movement of the cross-head of the tensile testing machine. This value is not a correct measure of the actual elastic strain in fact it is higher than the actual value because of the spring constant of the machine and therefore yields a much lower value for the elastic modulus. Therefore, the true measure of the elastic modulus can be done only by using a strain gage attached to the gage of the tensile sample or by some other method like measurement of the velocity of acoustic signal through the material. References: Hertzberg R. W. “Deformation and Fracture Behavior of Engineering Materials”, John Wiley and Sons Inc. New York, 1996 Ramchandran P. “Experimental and Numerical Modeling of Stresses In Non-Conventional Cross-Sectioned Composite Pipes” Retrieved from http://etd.lsu.edu/docs/available/etd-01072009-112827/unrestricted/ramachandranthesis.pdf on November 12, 2009. Answer of Supplementary Questions Q1) Elastic modulus cannot be measured with any accuracy directly from the stress-strain read outs because, elastic strain portion of this plot has contributions from elastic deformation of the machine itself which has finite stiffness or rigidity. Thus the elastic strain read from such chart corresponding to any stress in the elastic region is higher than the correct value of the elastic strain in the tensile specimen. This invariably leads to lower value of elastic modulus than the actual value. To get rid of this problem and therefore for measuring value of the elastic modulus in tensile testing what one can do is to use an strain gage to measure the strain in the gage section only. This will lead to accurate measurement of elastic modulus using a tensile test. Q2) a) Glass Fibers are most commonly used for reinforcement because (i) It has very high stiffness and therefore provides high stiffness to the composite. (ii) It has very high fracture strength and therefore provides high strength to the composite (iii) It is compatible with polymeric matrix and therefore, good bonding with the matrix is possible. (iv) It is economical and therefore, there are cost side incentives. Q2) b) Surface perfection of glass fiber is important because only then there can be good bonding between the matrix and the fiber and this is very essential for manufacturing a composite material with improved properties. Q2) c) A protective cover of resin is given to protect the surface of glass fibers. Q3) a) Four different sports implements made of or containing composite materials are the following: (i) Pole of pole vault (ii) Racket of Lawn Tennis (iii) Golf Stick (iv) Hockey Blade Q3) b) Pole of pole vault is made of carbon fiber- epoxy resin composite. It has three layers having different orientation of the reinforcing fiber – filament wound glass fiber core, woven carbon fiber – epoxy resin and unidirectional carbon fiber – epoxy resin from center towards the outer side as shown in the following figure (http://www.download.bham.ac.uk/eng/metallurgy/kukureka/presentation/PoleVaultforEngineers.pdf). Manufacturing of Pole Vault (http://www.download.bham.ac.uk/eng/metallurgy/kukureka/presentation/PoleVaultforEngineers.pdf) “The Composite tubing is produced by filament winding. This is the automated process of wrapping resin impregnated filaments (rovings or tows) in a geometric pattern over a rotating male mandrel. The component is then cured under high pressure and temperature.” The process is shown in the following figure: Elastic Toughening of Polymers Objective: The experiment is aimed at making a comparative study of mechanical behavior of polystyrene and elastomer toughened polystyrene by carrying out carrying out tensile testing. Theory: Many polymers are stiff and strong but highly brittle due to very rigid three dimensional network. Polystyrene is an example of such materials. However, for certain application a suitable combination of high stiffness and strength and toughness is required. To incorporate toughness into stiff and brittle materials fine globules of an elastomer is incorporated into the brittle polymer matrix. This results in significant improvement in the ductility of the resultant composte material with some loss in the strength. Experimental Procedure: Tensile test of Polystyrene and Elastomer toughened polystyrene composite was performed using on rectangular specimen on Instron Machine and load – extension plot was obtained. From this plot the load at the yield point, the maximum load and the extension before fracture was measured for calculating useful properties like yield strength, ultimate tensile strength and ductility of the material. Dimensions of the tensile specimen were measured using a micrometer and the same are presented in Table 1, below. Table 1: Important Dimensions of Tensile Specimen Material Dimensions Polystyrene Elastomer Toughened Polystyrene Composite Gage Length (mm) 76 70.5 Width (mm) 4.07 4.17 Thickness (mm) 10.30 10.10 Results: The load – extension plot for polystyrene and elastomer toughened polystyrene is presented in Fig. 4 and Fig. 5 respectively. Fig. 4: Load – Extension Plot for polystyrene Fig. 5: Load – Extension Plot for elastomer toughened polystyrene The important parameters derived from these plots and correspondingly important tensile properties of these materials are presented in Table 2, below. Material Dimensions Polystyrene Elastomer Toughened Polystyrene Composite Yield Force (N) 1536 960 Yield Strength (MPa) 36.64 22.79 Maximum Force (N) 1536 960 Tensile Strength (MPa) 36.64 22.79 Extension (mm) 2 206 %Elongation 2.63 291.8 From this table as well as from the load – extension plots in Fig. 4 and 5 it is very clear that polystyrene has higher strength but very low ductility at the same time. On the other hand elastomer toughened polystyrene composite has somewhat lower strength but significantly higher strength. Hence it can be concluded that elastomer toughening of polymers lead to significant improvement in the toughness of the material at the cost of a marginal decrease in the strength. It is important to some something on the strength if it can lead to increase in toughness. This is because loss of strength can be taken care of by design but loss of toughness is something that cannot be allowed as it will lead to catastrophic and unpredictable fracture behavior. Read More