Composite Material Testing v3.pdf - 第2页

Composit es and S truc tur es T es ting Applica tion Not e She ar T es ting Inst ead of manuf acturing a t esting c omponent that c an be pulled in a uniaxial tes ting fr ame to inves tiga te shear properties, the shear …

Composites and Structures Testing

Application Note

Introduction

Composites are increasingly used across a wide range

of industries to create lighter structures than their metal

equivalents. While steel has a density of 8.1 g/cm

, carbon

fibre only has a density of 1.6 g/cm

and potentially a higher

yield strength for the same cross section.

The main challenge with most fibre reinforced plastic

composites is that the strength of the structure is typically

not equal in all directions and depend on the direction of

the fibres running through the structure. Sheets of fibres

in one direction may be stacked at various angles, or fibre

tows are woven together to give a specific load bearing

structure or ‘architecture’. This allows the composite to

be stronger in certain directions, maximizing the weight

advantage, but presenting major challenges when testing

the structures to prove that they can stand their

design loads.

The common industry approach to composites is to use

uniaxial tensile testing machines, which were designed

for testing metals. While these machines are commonly

available, the testing that is carried out is limited to

uniaxial loading on an anisotropic material. The stresses on

composite structures are oen peeling or shearing loads,

however conventional testing machines were never built

with this kind of testing in mind. If the structures are tested

in a way that suits the testing machines, then the loads put

on the test samples may be very dierent than the loads

it will see while in service, so the design may be an over or

under estimate. This can lead to heavier structures where

weight reduction is paramount, or a structure that fails in

service due to poor understanding of the performance.

In this application note we explore the benefits of

testing composites and structures using a dedicated

materials tester.

Composites and Structures Testing

Application Note

Shear Testing

Instead of manufacturing a testing component that can

be pulled in a uniaxial testing frame to investigate shear

properties, the shear strength can be determined directly

from a component using shear testing. The component

is clamped in a workholder mounted on an XY stage and

sheared against a tool that measures the load. All of

the movement is measured by high precision encoders

within the stage, and further measurements of strain can

be carried out using independent strain gauges that can

interface directly with the materials tester.

Peel Testing

Many components can only be peel tested by

manufacturing materials with two flexible sides that can be

peeled at 90

, or one very flexible part that is bent around

180

and pulled away from the component. These tests are

diicult to set up and were only developed to make use of

uniaxial machines.

The materials tester can clamp very small features with

its pneumatic jaws and peel material away from the

component at a constant angle of 90

by moving the stage

in tandem with the z-axis. This provides a true 90

peel test.

Epoxy resins are aected by moisture, which penetrates into

the matrix from the surface. The ability to test fibre-matrix

interactions at a microscale allows for an investigation into

the depth that the composite has been aected, instead of

only testing the bulk properties.

Right: A high magnification

image of the same shear test,

using the onboard trinocular

camera.

Above: Shear testing of fibres from

a golf club sha to determine the

matrix-fibre interaction.

Le: A composite structure

is held rigidly in place using

high force pneumatic grips

and fibres are peeled from

the surface using precision

tweezer grips.

Bend Testing

While bend testing is more commonly available on

conventional machines, the materials tester can interface

with acoustic emission instruments, to allow cracking prior

to component failure to be detected. This gives a valuable

additional insight.

DAGE Paragon

Materials soware automatically

co-ordinates the materials tester and acoustic emissions

instrument by sending a trigger signal as soon as a user

defined pre-load is achieved.

As well as conventional bending, DAGE Prospector

can be

fitted with micro hooks that can be used to li fibre tows so

that the stiness can be measured.

Composites and Structures Testing

Application Note

Liing a 100 micron wide strip of carbon fibre from a

damaged component using a micro hook to investigate

local materials properties.

0.57 0.58 0.59 0.6 0.61

Axis Displacement (mm)

Force (gf)

1.5

1.45

1.4

1.35

1.3

1.25

1.2

1.15

1.1

1.05