Carbon Fibre Composite

Strength of Laminated Fibrous Composites Lab

• Summary

This report compares the results of theoretical calculations undertaken to predict the tensile strength of carbon fibre reinforced composites against experimental results. Two main methods for the calculation of tensile strength are used; the rule of mixtures and the ten percent rule. These standard theories appear to be valid for the majority of cases although inconsistencies are observed for the 90°, transversely loaded, isostress condition.

• Aims

• To become familiar with composite materials, their structure and quality

• To understand how carbon fibre reinforced polymer, CFRP, materials are tensile tested

• Objectives

• To examine samples of composites and their precursors

• Examine typical microstructure

• Tensile test different samples of CFRP composites

• Introduction

This work looks at the tensile strength of different structural make-ups of carbon fibre reinforced polymers. Carbon fibres have greatest strength when they are loaded in the isostrain (0°) condition as illustrated in Figure 4.1. In the isostrain condition, the majority of the applied load is transferred to the long fibre strands whereas in the isostress condition, the load is acting perpendicular to the fibre length and yields a far lower maximum strength. When there are multiple layers of fibres in the composite, orientated in different directions, otherwise known as a mixed ply lay-up, the maximum tensile strength varies with respect to the fibre angle.

Figure 4.1 – Composite Loading

Four different composite lay-up structures are looked at in this report. Each lay-up consists of a total of 16 layers which are symmetrical about the neutral axis. The layups of the composites are:

Where subscripts:

2, 4, 8 = multiple of layers noted in the brackets

S= refers to symmetry about the neutral axis

A – [ 0° ]_8S

B – [ 90° ]_8S

C – [ +45°,-45° ]_4S

D – [0°/+45°/-45°/90°]_2S

The test specimens consist of a section of CFRP with metal plates attached with epoxy resin at either end as shown later on in Figure 5.1. These metal plates are purely for clamping of the CFFP section into the in the tensile testing rig as shown in Figure 5.2.

• Experimental Method

The dimensions of the test specimens are noted for each piece prior to any testing and will be used for fair comparison in the theoretical calculation methods. Dimensions for each specimen, A-D are shown below where x and y relate to the dimensions shown in Figure 5.1.