The rotating cantilever fatigue test

The rotating cantilever fatigue test W. J. P U L L E N *

The use of composites, and fibre reinforced polymeric materials in general, as engineering materials, demands the provision of useful data for the design engineer. The life of such materials when subjected to cyclic stressing is a case in point. An attempt is made to demonstrate how meaningful fatigue data can be obtained by using a rotary cantilever method of test

The life in service of any material depends to a certain extent on its ability to withstand fracture when subjected to repeated stressing. Although the phenomenon of fracture under cyclic stressing or dynamic fatigue is well known to engineers in the metal industry, the increasing use of the new composite materials and fibre reinforced polymeric materials for stressed engineering parts means that the design engineer is urgently seeking data on the fatigue properties of such materials. The method used at this Establishment is shown schematically in Fig 1. The equipment permits tests to be carried out at high frequencies and thereby reduce the time required to obtain the relevant data. The stress system is that of a rotating cantilever in bend. Speeds from 13Hz to 133Hz are possible with a maximum stress of about 6 x 10~Nm-2 . In the series of tests reported here, the speed used was 83.3Hz (5000rpm). A typical series of S/N (alternating stress plotted against cycles to failure on a log scale) curves are given.

loss, results in an appreciable temperature rise of the specimen under test. This temperature rise is accelerated by two parameters, increasing stress and increasing frequency of stressing. With either parameter, an equilibrium temperature state can be reached. This phenomenon of temperature rise is illustrated in Fig 2 particularly with the S/N curve given for Perspex. Thermoset polymeric materials generally have a lower hysteresis loss than do the thermoplastics. Where the reinforcing fibre is a good heat conductor, as is the case with carbon fibre composites, then this temperature rise is small. In Fig 2 the difference in hysteresis losses for different polymer matrices is illustrated by the temperature rises given with the S/N curves for the glass/polyester and glass/polysulphone materials. For metal

i

I

Specimen

FUNDAMENTAL DIFFICULTIES WITH COMPOSITES HA VING A POL YMERIC M A T R I X The mechanical properties of polymeric materials are both time and temperature dependent. In the case of cyclic stressing, low heat conductivity coupled with hysteresis * Non-Metallic Materials Branch, Explosives Research and Development Establishment, Waltham Abbey, Essex

Stresses-up to 4 0 tons/~ in Speeds - up to BOOO r p rn

HG 1

Applied

Rotating cantilever fatigue machine

COMPOSITES June 1970

239

5.52

4.14

E

Z

3.45 Q 2.70 ~

2-07 I

I

~.

iOs

I

I

I

I

106 K~~ IOs Cycles to fracture

iOg

FIG 2 SIN curves showing temperature increases in the specimens (the ringed numbers are the temperature increases)

RESULTS A N D DISCUSSION

6'2 35

r bon~/epoxy

~25

~

~20

composites, eg an aluminium alloy reinforced with silicon carbide whiskers, this problem of temperature rise does not occur. In this work, an infra-red thermometer was used to measure the actual surface temperature of the specimen during the test. The basic philosophy behind the use of this method of fatigue testing, is the attempt to assess an endurance limit from the S/N curve produced, and to be able to do this within a reasonable time. This endurance limit will vary, depending on the rate of stress cycling and possibly on the nature of the environment under which the test is carried out. It will also be necessary to record the equilibrium temperature reached by the test specimen at this limit, when the test is carried out under normal ambient conditions. Provided all these factors are taken into account, it is considered that the information so obtained will be of use to the designer.

E

(:]lby+SiC

4.O.z Q

ASbe~t~s/¢po~

zs b I0 tn 5

~o~

CarbonI/epoxy/ / I

103

lo~

FIG 3

I

I

iOs IOs Cycles to fracture

I

iO7

I

lob I(~

SIN curves for some typical composites

Table 1

Fig 2 gives S/N curves for certain glass reinforced composites. The glass polyester material is a standard type of product in which the reinforcement is a random glass matt. The two curves are derived from tests carried out on specimens cut in two directions at right angles on the same laminate. The recorded temperatures given on the curve show clearly how rapidly the temperature of the specimen rises with increasing stress or increasing time at lower stress. As the fatigue endurance limit is reached so the temperature rise is much smaller.

Summary of results

Fatigue / Limit /

Composite

Approx fatigue limit ×107Nrn-2

Equilibrium temperature rise

Room temperature

°C

°C

Ultimate tensile strength x l 0 7 N r n-2

/ //

Ultimate tensile strength %

Type I carbon epoxy

20

5

25

59"3

34

Asbestos epoxy

24

5

25

48

50

AI alloy + SiC

25

Glass-polyester ( 1 )

3" 1

5

24

9"6

32

Glass-polyester (2)

3-1

5

24

6"9

45

Glass-polysulphone

2"6

9

22

11 "0

24

Perspex

2"5

25

25

8"9

28

240

COMPOSITESJune1970

The glass polysulphone material contains approximately 28 per cent by weight of short glass fibres incorporated into the thermoplastic resin. The resulting mixture can be readily injection-moulded into the required specimen shapes. The increase in specimen temperature throughout the S/N curve for this material is smaller and more consistent, indicating a lower hysteresis loss with this particular matrix. Fig 3 gives S/N curves for three typical composites. The metal composite consists of an aluminium alloy, heat treated, and reinforced with 18 per cent by volume of silicon carbide whiskers. The carbon composite is an epoxy resin matrix reinforced with a Type 1 continuous carbon fibre, the fibre content being 50 per cent by volume. Two results are also shown on Fig 3 for a similar material in which the reinforcement is 50 per cent by volume of Type 2 continuous carbon fibre. The asbestos/epoxy composite is a composite in which the reinforcement is an aligned but discontinuous chrysotile asbestos fibre. The composite contains 46 per cent by volume of fibre. (This particular asbestos composite is a material produced by a new process developed in ERDE.) Table 1 gives estimated fatigue limits obtained from the S/N curves shown in Figs 2 and 3, together with the equilibrium temperature rises at those endurance limits. The materials examined have a predicted

fatigue limit approximately 3 0 - 4 0 ultimate tensile strength.

per cent of their

CONCL USIONS

It is not suggested that the method is at this stage a foolproof approach to the problem of fatigue with respect to composites or reinforced polymeric materials, but that it does provide useful data. The specimen required is comparatively small being nominally some 5 6cm in length and 0-6-0"7cm in diameter. The actual shape of the specimen is shown in Fig 1. To obtain a S/N curve in a reasonable time, a minimum of eight specimens is required. The small specimen permits the evaluation under prescribed conditions of new types and formats of composites where only small experimental or pilot plant quantities are available. The effect of varying the speed, environment and temperature can all be studied. The effect of notches and other stress raisers on the fatigue endurance limit and the shape of the S/N curve can likewise be evaluated. For example, by fixing the stress and the speed of cycling, the effects of varying the fibre content or modifying the constitution of a particular matrix or matrices can readily be assessed.

Our new machine will sort out your fibres By using our fibre classifier it is now possible to accurately separate fibrous materials by length to your own requirements. A continuous flow process is achieved by the use of rotary screen units. Fibre lengths within the range of 2 cm to 0.003 cm can be classified easily, and it is possible to go outside these limits with slight modifications. The machine has been proven with such diverse materials as silicon carbide whiskers, asbestos and wood pulp. We have test facilities whereby we can classify your particular type of fibres.

Forfurtherdetailscontact Mr. A.C. Arno at

Glass Developments Ltd., Sudbourne Road, London, S.W.2. Telephone 01-274-4041.

COMPOSITES June 1970

241