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Fracture strength of E-glass fibre strands using acoustic emission
NDT&E International, Vol. 30, No. 6, pp. 383-388, 1997 PII: S0963-8695(97)00009-1
ELSEVIER
© 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0963-8695/97 $17.00+0.00
Fracture strength of E-glass f!bre strands using acoustic emission Sha Jihan a'*, A. M. Siddiqui b, M. A. S. Sweet b aSchool of Mechanical and Offshore Engineering, bSchooI of Applied Sciences, The Robert Gordon University, Aberdeen, UK Received 4 July 1996; revised 25 November 1996;accepted 27 November 1996
A method based on acoustic emission (AE) generated by fibre fractures during tensile test has been developed for the determination of the strength distribution of E-glass fibre strands containing as many as 4000 fibres (filaments). This technique is novel in that it allows fast data acquisition and provides accurate data for the load, location and time of fracture of each filament. A range of statistical data including the fracture strength and AE signal parameters can be extracted to characterise the strand of fibres under investigation. Software has been developed for the evaluation of the large number of collected data and for the graphical presentation of the AE signal parameters. The final test package thus allows automatic testing and provides the analyst with an overall picture of the real strength distribution of the fibres in a strand. The test and analysis for 4000 fibres is completed within 75 min. Complete strand tests of 4000 filaments using this method have revealed greater detail of the characteristic flaw distribution in a strand that was not possible with tests conducted on single filaments or bundles containing small numbers of filaments (see Ref. [1]). The new study has provided concrete evidence of the existence of a bimodal flaw population rather than the unimodal population that has been previously reported. © 1997 Elsevier Science Ltd. All rights reserved. Keywords: fracture strength, E-glass fibre strands, acoustic emission
Introduction
are prone to cause damage to the fibres during handling thus affecting the results of fibre strength. Two effects are responsible. Firstly most fibres are weakened (shifting the strength distribution), secondly the weakest fibres tend to break and are discarded and replaced by other stronger fibres (skewing the distribution). The strength distribution measured is thus changed.
E-glass fibres are widely used in the production of fibre reinforced composites. There has been a significant growth in the use of fibre-reinforced materials to meet the increasing demand for lightweight, high strength/stiffness and corrosion-resistant materials in domestic appliances, sports goods, automotive, offshore structures and the aircraft industries.
The application of the AE technique to study the effects of sample size (number of filaments) and of sample length of a commercial roll of E-glass fibres has shown significant variation in the bundle strength and Weibull parameters I6]. Samples tested across the width of the strand have also shown significant variations in these parameters. These results indicate that it is essential to test a complete strand of filaments for an accurate evaluation of fibre strength distribution. This information is vitally important for both the manufacturers and the users of E-glass fibres to establish optimum manufacturing conditions.
Some common methods of strength measurement of fibres include the single-filament test E2J, fractography t3], load drop [4] and strain measurement t51. These investigations were limited to single filaments and/or bundle of small numbers of filaments taken from a strand establishing a unimodal flaw distribution. The sampling method employed
* A u t h o r to w h o m all c o r r e s p o n d e n c e should be addressed.
383
S. Jihan et al. In the present investigation, an AE technique has been developed to test a complete strand of fibres during tensile loading. When a fibre under stress fractures, a fraction of the elastic energy released is used at the source in creating the fracture and the remainder is radiated in the form of stress waves. Figure 1 shows a typical AE signal waveform captured during a fibre breakage. Each fibre break in a strand is uniquely detected by recording the AE during fracture. The AE parameters are measured with reference to a preset threshold. The parameters can be quantified to characterise the AE source (fibre fracture).
where N(cr) is the cumulative number of flaws of strength less than cr per unit length. Weibull assumed an empirical form for the unknown fraction N(o) given by: N(a) = [(cr -
Oz)/Go]m, f o r
a > az
N ( a ) = O, f o r a < a z
(3)
where o z is the stress at which there is zero probability of failure, m and o0 are Weibull modulus (or shape parameter) and scale parameter respectively.
Weibull parameter
o z is taken as zero since it does not alter the shape of the curve representing the data.
One common method of characterising the strength of fibres is using the two-parameter distribution given by Weibull [71. Weibull statistics have been applied to study flaw distributions in bundles of glass fibres under tensile loadings tsl. The assumptions generally used in the Weibull analysis are:
Thus, Equations (1)-(3) yield the Weibull two-parameter distribution: P F = 1 - exp[
(1) The material is supposed statistically homogeneous throughout its length, i.e. the probability to find a critical flaw in a given volume element is the same as for the overall volume.
-
((7/00) m]
(4)
Rearranging: Ln[Ln(1/(1 - PF))] = m L n ( a ) - m L n ( a o )
(2) The stress and strain relationship for a single fibre follows Hooke's law up to fracture.
(5)
In a strand test, when fibres break at a certain load (F), the stress (a) in the remaining fibres Ns (fibres surviving the applied load) is given by:
(3) The applied load during a test is uniformally distributed among the surviving fibres at any instant of time.
a = F/(NsAs)
(4) Interaction between fibres such as friction and twisting are neglected.
(6)
and the number of surviving fibres N s = ( N r - N F ) where Nr = the original total number of fibres in the strand, NF = the number of fibres failed when the load (F) is reached and As = the cross-sectional area of a single fibre.
In the present investigation, it is assumed that the fibres in a strand are parallel, have the same length and are gripped at their ends so that they are equally loaded under a tensile load.
In the strand tests conducted in this work, the number of fibre breaks are detected by AE and the corresponding load (F) was measured.
The failure probability PF, that a fibre length L will fail below stress level a, is defined f91 as: (1)
P F = 1 - exp[ - N(a)]
(2)
The failure probability (PF) in a strand containing a total of N r fibres is
2
PF = NFIN~ 1
(7)
From the strand test data, a plot of Ln([Ln(1/(1 - PF))] against Ln(a) is anticipated to give a straight line with slope m.
Material and method
-I
Acoustic emission monitoring system -2-
An Acoustic Emission Testing (AET) 5500 system was used for monitoring energy released when E-glass fibres (mean diameter of 17 #m) fractures under tensile loading. Figure 2 illustrates a block diagram of the fundamental instrumentation required for the detection
I
0
320
640
T i m e (l~s)
Figure 1 A typical signal waveform captured during a fibre fractu re.
384
Generation of acoustic emissions by fibre fractures breaking even before reaching a quarter of the maximum load sustained by the bundle. Sample holders were thus designed and each sample of fibres for AE testing was prepared by gluing its ends to the sample holder. Fifty samples were prepared by cutting 80 mm lengths from the commercial roll of E-glass fibres (1,OCF R43A). Gloves were worn during the handling of the fibres to avoid any possible contamination. Each end of the sample was glued at a length of 15 mm to a sample holder using Araldite 2004 (high strength, two-pack epoxy paste obtained from Ciba Geigy Plastics). The fibres were carefully handled in order to ensure that they remain parallel during the gluing process. The specimens were cured in an oven at 40°C for 2 h. If required, a fast curing can be achieved within 15 min at a curing temperature of 80°C. With this improved preparation method, a test on a whole strand of 4000 filaments can be performed without fibre pull-outs. Printer
AE monitoring computer
Digital oscilloscope
The sensor, for detecting the energy released during fibre fractures, was secured on each of the aluminium sample holders using a constant force spring clamp to reduce interface losses. The sample holder also acts as an acoustic wave guide to transfer the energy from the glass material to the sensor. Aluminium was chosen as the material for the plates because it has similar acoustic characteristics to the glass material. This improves the transmission of signal energy reducing losses due to mode conversion and reverberation in the wave guides. The design also incorporates a circular cut groove in the sample holder to hold the sensor firmly in place. A couplant (silicone grease) was used in order to efficiently transmit the acoustic energy from the specimen under test to the sensors. This ensures that all AE events can be detected.
Figure 2 A schematic representation of the complete experimental layout for testing glass fibre strands using the AE technique.
and display of the acoustic emission signals generated by a strand of glass fibres during tensile loading. When a fibre breaks, the energy released propagate through the glass material and the acoustic wave guides (aluminium plates). The small displacements caused by the energy release from the fracture is detected by the sensors (S 1 and $2). AC375L piezoelectric sensors with a resonant frequency of 375 kHz, which are omnidirectional and differential, and with a sensitivity of better than - 7 0 decibels (dB) referred to 1 volt per microbar were employed.
Establishment of experimental parameters Normally one of the unknown parameters in testing a bundle containing a small number of fibres is the failure load of individual fibres B°]. In the previous work tl], the load at which each fibre breaks was obtained by noting the reading of the Instron chart-recorder at the point where a valid event was displayed on the AET terminal. This method was tedious and inaccurate. Furthermore, there was a tendency to miss records of the load readings when fibre breakages occur too quickly. An AE test on 200 fibres was conducted in 6 h with an operator required to note the load drops during fibre fractures. This method was intrinsically not suitable to carry out a test on a bundle containing a large number of fibres. In the present test, the output of the Instron load cell amplifier was used as a parametric input to the AET 5500. Thus, whenever a fibre breaks, the system records AE parameters and the corresponding parametric input value indicates the load at fracture. With this arrangement, an accurate method of determining the fibre fracture load was established.
The electrical signal from each sensor was pre-amplified using AET 140 dB pre-amplifier with a gain of 40 dB. A band pass filter between ranges 250 kHz to 500 kHz was used to eliminate undesirable low and high frequencies. The noise level of the pre--amplifier was less than 3 tzV. A threshold of 200 mV, at least 10 times the typical noise amplitude, was selected to ensure a safe margin to completely isolate the AE signals from the background noise signals. An Instron 1195 model loading frame was used in the investigation. It utilises high sensitive and accurate load weighing system employing strain gauge load cells for detecting the load applied to the specimen under test. For testing a strand of 4000 filaments, a load cell (type 2511-319) with a full scale load range of 2000N was employed.
Design of sample holders and specimen preparation
To monitor the effect of loading speed on bundle strength in a strand, tensile tests were conducted on three separate strands at crosshead speeds of 5, 0.5 and 0.05 mm min -1. The number of valid events (fibre breaks) obtained was 1325, 1997 and 3983 for crosshead speeds of 5, 0.5 and
The direct mounting of a bundle of fibres on to the grips of the Instron tensile testing machine was not practical as the fibres were severely damaged near the grips during loading,
385
S. Jihan et al. 0.05 nun min -1, respectively. The lower number of valid events obtained under the higher strain rates indicate that simultaneous fibre fractures occurred and the system was unable to resolve AE signal of each individual fracture. At a speed of 0.5 mm min-1 the number of valid events (3983) obtained closely corresponds to the number of fibres (4000) in a strand. As such all tensile tests, unless specified, were conducted at a crosshead speed of 0.05 mm rain-1 at room temperature of approximately 20°C and relative humidity of 50%.
Characteristics of a strang using AE signal parameters For each strand test, various AE signal parameters such as analog parameter (load), events, event duration and peak amplitude were recorded. These results obtained
(a) REGIONS
For each AE event, the time difference (t2ND - t l S T ) between recording the event at the two sensors S 1 and $2 (see Figure 3a) was found. This time difference was used by the existing and in-house software to locate the exact position of fibre breakage. This technique also permits out of range events to be eliminated. Out of range events are events occurring outside the specimen gauge length caused by such activities as noise in the grips, the breakage of fibres in the adhesive, cracking of the adhesive and sensor slippage if any. With this facility only events occurring within the fibre gauge length (region 3) as shown in Figure 3a were accepted as valid events (fibre breakages).
First ] q
Senso, I I S2 I 5 hit I_,1 J ..... m
tlSTI I
i
gauge length
I
t2ND| I I I
V Second Sensor hit
front view
and
t.... Fracture
! I I I
Typical results for source location of fibre breakages are illustrated in Figure 3b. Figure 3c shows the statistics of the events in the five regions between the two sensors where region 3 is the fibre gauge length.
Results
]. . . . .
discussion
2
Lj~
. . . . . 1
side view
(b) 500
Strength distribution in a strand Each strand test containing up to 4000 filaments was completed successfully within 1 h. The AE data was postprocessed and using the developed software, the results were evaluated in 15 min to characterise the strength of the tested strand. Fifty strands were tested, the results presented below and in the next section are from a typical strand.
g> 250
0
The relationship between the bundle load and the number of fibre fractures obtained for the test is shown in Figure 4a. The maximum bundle load (1000 _+ 50 N) can be estimated directly from the experimental data. In order to obtain the Weibull parameters, a plot of L n [ L n ( l / ( 1 - PF))] against L n ( t r ) was made. It is clear that a single straight line cannot be drawn through the resulting data points in Figure 4b and that the situation was accurately represented by two straight lines. Physically this means that the flaw distribution was bimodal in form rather than the unimodal distribution previously reported by other workers [1,11]. The failure probability is more accurately represented in terms of two sets of statistical parameters, each of which corresponds to one of the individual flaw populations. In Figure 4b, the flaw distribution in the tested strand was found to provide an upper and a lower limit of strength. Two moduli, mL = 1.5 - 0.5 for the lower strength population and m y = 6.5 - 0.5 relating to the upper strength population, were determined.
.
i
!
I
I I I I
I I I
0 4 8 1320 27 34 41 48 55 62 69 76 8 3 9 0 9 7 1 0 5 1 1 4 123132141150 Location
(c) 5000 -
o~ 2 5 0 0 tll
12345
Region Figure 3 (a) Events discrimination by regions. (b) Distribution of events by location (mm) for AE strand test. Number of intervals, 150;Interval size, 1 mm. (c) Total events by region for AE strandtest.
386
Generation of acoustic emissions by fibre fractures (a)
(a)
1200
•
6000°
1000
r,
.
5200
.
.
.
.
.
.
.
.
.
.
.
'200
........
800
600 "~ 4oo
i
2OO 500
1000 1500 2000
2500
-
3000
0
3 5 0 0 400(
-
00:00:00
NF
Time
=Y.__u
, oo mu
(b) 4 2
g'0
I
I
I
I
I
i
i
1
2
3
4
5
6
7 # 8
(b)
xU
500 -
I
I
9
I0
---2 ~
~-4 .2
-"
00:42:00
m
250
L = YL xL
--8 -10 m L --
lower modulus
m U -- upper moduhts
/ /
Figure 4 (a) The relationship between (NF) of a strand. (b) The VVeibull plot of
iiii
Xu
i i i i i i i ~
1
3 6 9 1 3 1 8 2 3 2 8 33 38 43 48 53 58 63 68 73 78 83 8893 98 PA
load and fibre fractures a strand.
(c) I00~ 9O 80
from a typical strand test of 4000 filaments are shown in Figure 5a-c.
7O
~
Figure 5a shows a load history of the fibres in a strand. From calibration, a load cell :~ignal of 10 000 millivolts represents a load of 2000 N. The maximum bundle load sustained by a strand was 1000 ___ 50 N (5000 _ 200 mV). The nature of occurrence of the valid events (fibre breakages) up to the maximum load sustained by the strand was random whilst beyond the maximum load breakages occurred at much greater rate. A total of 750 - 50 fibre fractures was noted up to the peak load. A plot of the number of valid events against peak amplitude shows two distinct peak amplitude distributions (i) at a lower range between 53 dB and 73 dB and (ii) at an upper range between 73 dB and 95 dB as indicated in Figure 5b. This result extracted from statistical data is again indicative of a bimodal flaw distribution.
60
50 40 3O 20 10 0
-
0
0.5
,m
1.0
1.5
2.0
2.5
3.0
Stress
Figure 5 (a) Analog parameter (mV) vs time (rains) for AE strand test. Number of Intervals, 400; Interval size, 10s. {b) Distribution of valid events by peak amplitude (dB) for AE strand test. (c) Variation in the peak amplitude, PA {dB) distribution with stress (GPa).
dependence of strength on the condition of the glass surface [121. The general trend is towards lower strength with increasing surface damage. The flaws can be induced into the glass fibres during manufacturing processes and handling. It has been reported that fibres taken from the turn-around-points and fibres from the centre of the commercial roll possess different strength t13]. Hillig and Charles I14] observed the presence of intrinsic flaws in glass and suggested that the strength data can be accounted for if one assumes a bimodal distribution of flaw sizes. Hence in the present case, the two strength populations can be explained on the basis of flaw sizes, i.e. those with Griffith critical flaw size, caused by mechanical damage, fractured
Figure 5c shows two distributions of the peak amplitude of the signal with increasing stress, one in the range 0-1 GPa and the second in the range 1.53-3.0 GPa. It is apparent again from the plot that there are two populations of strength. A considerable number of weaker fibres were broken before the strand reached its maximum loading capacity. The population corresponding to a peak amplitude of 62 dB fractures at a stress below 1 GPa whilst the population corresponding to a amplitude of 83 dB fractures only above 1.5 GPa. One of the striking features of glass is the obvious
387
S. Jihan et aL
References
at low stress whilst in the stronger fibres flaws reached critical size at higher stresses and fractured.
1 Cowking, A., Attou, A., Siddiqui, A.M. and Sweet, M.A.S. Testing E-glass fibre bundles using acoustic emission. J. Mater Sci., 1991,26, 1301-1310. 2 ASTM 3379, 1982. 3 meecholsky, J.J., Rice, R.W. and Freiman, S.W. Prediction of fracture energy and flaw size in glass from measurements of mirror size. J. Am. Ceram Soc., 1974, 57, 440-445. 4 Fuwa, M., Bunsell, A.R. and Harris, B. An evaluation of acoustic emission technique supplied to carbon fibre composites. J. Appl.Phys., 1976, 9, 353-364. 5 Chi, Z., Chou, T.W. and Shen, G. Determination of single fibre strength distribution from fibre bundle testing. J. Mater. Sci., 1984, 19, 3319-3324. 6 Jihan, S., Siddiqui A.M. and Sweet, M.A.S. Effect of sample size and length on the strength of E-glass fibres using acoustic emission. In Proc. Int. Conf. on Electronic Measurements and Instrumentation, Tianjian University, China, 1992, pp. 210-215. 7 Weibull, W. A Statistical distribution function of wide applicability. J. Appl. Mech., 1951, 18, 293-296. 8 Scott, W.D. and Gaddipatl, A. Weibull parameters and the strength of long glass fibres. In Fracture Mechanics of Ceramics, R.C. Bradt, D.P.H. Hasselman and F.F. Lange, Eds., Plenum, New York, 1978, pp. 125-141. 9 Olshansky, R. and Murer, R.D. Tensile strength and fatigue of optical fibres. J. Appl. Phys., 1976, 47, 4497-4499. 10 Hamstad, M.A. and Moore, R.L., Acoustic Emission from single and multiple Kevlar 49 filament breaks. J. Comp. Mater., 1986, 20, 47-65. 11 Attou, A., Cowking, A., Siddiqui, A.M. and Sweet, M.A.S. Strength investigation of a commercial E-glass fibre roll using acoustic emission. Br. J. NDT, 1990, 32, 623-626. 12 Kurkjian, C.R. Strength and fatigue of oxide glasses. In Current trends in the Science and Technology of Glass, H. Jain, A.R. Cooper, K.J. Rat and D. Chakravorthy, Eds., World Scientific Co., Singapore, 1989, pp. 190-195. 13 Bailey, R., Bradsky, G.V. and Lehr, W. E-glass fibre strength: Their measurements and characteristics. ICI Report No. 11196, 1989, pp. 1-3. 14 Hillig, W.B. and Charles, R.J. In High Strength Materials, V.F. Zackay, Ed., John Wiley, New York, 1965, pp. 682-705. 15 Jihan, S. Ph.D. Thesis, Robert Gordon University, 1994.
Conclusions It has been established that AE can be used as a suitable means of determining fracture strength distribution of E-glass fibres in a strand under tension. The work has shown that for reliable results, tests across a strand (typically 4000) are essential. The established AE technique has the potential for testing even larger number of filaments ( > 4000) if required). AE events due to fibre fractures are characterised by high peak amplitudes and were easily distinguished from out of range events. Strength analyses of single filaments or small numbers of filaments in selected bundles have normally been represented by a unimodal flaw distribution. The present investigations have shown this to be inadequate. Testing fibres in a strand revealed a bimodal nature. Analysis of AE signal parameter data has further substantiated the existence of a bimodal flaw population. The AE strand testing technique developed in this work has the potential to monitor the effect of manufacturing (e.g. production of long fibre reinforced composites using the pultrusion process) variables such as temperature, friction, environment and any combination of these factors applied to a strand I~5].
Acknowledgements Mr S. Jones, LNP Engineering Plastics and Dr R. Bailey, ICI, deserve special mention for the discussions concerning the developed AE technique and for the supply of the materials.
388
ELSEVIER
© 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0963-8695/97 $17.00+0.00
Fracture strength of E-glass f!bre strands using acoustic emission Sha Jihan a'*, A. M. Siddiqui b, M. A. S. Sweet b aSchool of Mechanical and Offshore Engineering, bSchooI of Applied Sciences, The Robert Gordon University, Aberdeen, UK Received 4 July 1996; revised 25 November 1996;accepted 27 November 1996
A method based on acoustic emission (AE) generated by fibre fractures during tensile test has been developed for the determination of the strength distribution of E-glass fibre strands containing as many as 4000 fibres (filaments). This technique is novel in that it allows fast data acquisition and provides accurate data for the load, location and time of fracture of each filament. A range of statistical data including the fracture strength and AE signal parameters can be extracted to characterise the strand of fibres under investigation. Software has been developed for the evaluation of the large number of collected data and for the graphical presentation of the AE signal parameters. The final test package thus allows automatic testing and provides the analyst with an overall picture of the real strength distribution of the fibres in a strand. The test and analysis for 4000 fibres is completed within 75 min. Complete strand tests of 4000 filaments using this method have revealed greater detail of the characteristic flaw distribution in a strand that was not possible with tests conducted on single filaments or bundles containing small numbers of filaments (see Ref. [1]). The new study has provided concrete evidence of the existence of a bimodal flaw population rather than the unimodal population that has been previously reported. © 1997 Elsevier Science Ltd. All rights reserved. Keywords: fracture strength, E-glass fibre strands, acoustic emission
Introduction
are prone to cause damage to the fibres during handling thus affecting the results of fibre strength. Two effects are responsible. Firstly most fibres are weakened (shifting the strength distribution), secondly the weakest fibres tend to break and are discarded and replaced by other stronger fibres (skewing the distribution). The strength distribution measured is thus changed.
E-glass fibres are widely used in the production of fibre reinforced composites. There has been a significant growth in the use of fibre-reinforced materials to meet the increasing demand for lightweight, high strength/stiffness and corrosion-resistant materials in domestic appliances, sports goods, automotive, offshore structures and the aircraft industries.
The application of the AE technique to study the effects of sample size (number of filaments) and of sample length of a commercial roll of E-glass fibres has shown significant variation in the bundle strength and Weibull parameters I6]. Samples tested across the width of the strand have also shown significant variations in these parameters. These results indicate that it is essential to test a complete strand of filaments for an accurate evaluation of fibre strength distribution. This information is vitally important for both the manufacturers and the users of E-glass fibres to establish optimum manufacturing conditions.
Some common methods of strength measurement of fibres include the single-filament test E2J, fractography t3], load drop [4] and strain measurement t51. These investigations were limited to single filaments and/or bundle of small numbers of filaments taken from a strand establishing a unimodal flaw distribution. The sampling method employed
* A u t h o r to w h o m all c o r r e s p o n d e n c e should be addressed.
383
S. Jihan et al. In the present investigation, an AE technique has been developed to test a complete strand of fibres during tensile loading. When a fibre under stress fractures, a fraction of the elastic energy released is used at the source in creating the fracture and the remainder is radiated in the form of stress waves. Figure 1 shows a typical AE signal waveform captured during a fibre breakage. Each fibre break in a strand is uniquely detected by recording the AE during fracture. The AE parameters are measured with reference to a preset threshold. The parameters can be quantified to characterise the AE source (fibre fracture).
where N(cr) is the cumulative number of flaws of strength less than cr per unit length. Weibull assumed an empirical form for the unknown fraction N(o) given by: N(a) = [(cr -
Oz)/Go]m, f o r
a > az
N ( a ) = O, f o r a < a z
(3)
where o z is the stress at which there is zero probability of failure, m and o0 are Weibull modulus (or shape parameter) and scale parameter respectively.
Weibull parameter
o z is taken as zero since it does not alter the shape of the curve representing the data.
One common method of characterising the strength of fibres is using the two-parameter distribution given by Weibull [71. Weibull statistics have been applied to study flaw distributions in bundles of glass fibres under tensile loadings tsl. The assumptions generally used in the Weibull analysis are:
Thus, Equations (1)-(3) yield the Weibull two-parameter distribution: P F = 1 - exp[
(1) The material is supposed statistically homogeneous throughout its length, i.e. the probability to find a critical flaw in a given volume element is the same as for the overall volume.
-
((7/00) m]
(4)
Rearranging: Ln[Ln(1/(1 - PF))] = m L n ( a ) - m L n ( a o )
(2) The stress and strain relationship for a single fibre follows Hooke's law up to fracture.
(5)
In a strand test, when fibres break at a certain load (F), the stress (a) in the remaining fibres Ns (fibres surviving the applied load) is given by:
(3) The applied load during a test is uniformally distributed among the surviving fibres at any instant of time.
a = F/(NsAs)
(4) Interaction between fibres such as friction and twisting are neglected.
(6)
and the number of surviving fibres N s = ( N r - N F ) where Nr = the original total number of fibres in the strand, NF = the number of fibres failed when the load (F) is reached and As = the cross-sectional area of a single fibre.
In the present investigation, it is assumed that the fibres in a strand are parallel, have the same length and are gripped at their ends so that they are equally loaded under a tensile load.
In the strand tests conducted in this work, the number of fibre breaks are detected by AE and the corresponding load (F) was measured.
The failure probability PF, that a fibre length L will fail below stress level a, is defined f91 as: (1)
P F = 1 - exp[ - N(a)]
(2)
The failure probability (PF) in a strand containing a total of N r fibres is
2
PF = NFIN~ 1
(7)
From the strand test data, a plot of Ln([Ln(1/(1 - PF))] against Ln(a) is anticipated to give a straight line with slope m.
Material and method
-I
Acoustic emission monitoring system -2-
An Acoustic Emission Testing (AET) 5500 system was used for monitoring energy released when E-glass fibres (mean diameter of 17 #m) fractures under tensile loading. Figure 2 illustrates a block diagram of the fundamental instrumentation required for the detection
I
0
320
640
T i m e (l~s)
Figure 1 A typical signal waveform captured during a fibre fractu re.
384
Generation of acoustic emissions by fibre fractures breaking even before reaching a quarter of the maximum load sustained by the bundle. Sample holders were thus designed and each sample of fibres for AE testing was prepared by gluing its ends to the sample holder. Fifty samples were prepared by cutting 80 mm lengths from the commercial roll of E-glass fibres (1,OCF R43A). Gloves were worn during the handling of the fibres to avoid any possible contamination. Each end of the sample was glued at a length of 15 mm to a sample holder using Araldite 2004 (high strength, two-pack epoxy paste obtained from Ciba Geigy Plastics). The fibres were carefully handled in order to ensure that they remain parallel during the gluing process. The specimens were cured in an oven at 40°C for 2 h. If required, a fast curing can be achieved within 15 min at a curing temperature of 80°C. With this improved preparation method, a test on a whole strand of 4000 filaments can be performed without fibre pull-outs. Printer
AE monitoring computer
Digital oscilloscope
The sensor, for detecting the energy released during fibre fractures, was secured on each of the aluminium sample holders using a constant force spring clamp to reduce interface losses. The sample holder also acts as an acoustic wave guide to transfer the energy from the glass material to the sensor. Aluminium was chosen as the material for the plates because it has similar acoustic characteristics to the glass material. This improves the transmission of signal energy reducing losses due to mode conversion and reverberation in the wave guides. The design also incorporates a circular cut groove in the sample holder to hold the sensor firmly in place. A couplant (silicone grease) was used in order to efficiently transmit the acoustic energy from the specimen under test to the sensors. This ensures that all AE events can be detected.
Figure 2 A schematic representation of the complete experimental layout for testing glass fibre strands using the AE technique.
and display of the acoustic emission signals generated by a strand of glass fibres during tensile loading. When a fibre breaks, the energy released propagate through the glass material and the acoustic wave guides (aluminium plates). The small displacements caused by the energy release from the fracture is detected by the sensors (S 1 and $2). AC375L piezoelectric sensors with a resonant frequency of 375 kHz, which are omnidirectional and differential, and with a sensitivity of better than - 7 0 decibels (dB) referred to 1 volt per microbar were employed.
Establishment of experimental parameters Normally one of the unknown parameters in testing a bundle containing a small number of fibres is the failure load of individual fibres B°]. In the previous work tl], the load at which each fibre breaks was obtained by noting the reading of the Instron chart-recorder at the point where a valid event was displayed on the AET terminal. This method was tedious and inaccurate. Furthermore, there was a tendency to miss records of the load readings when fibre breakages occur too quickly. An AE test on 200 fibres was conducted in 6 h with an operator required to note the load drops during fibre fractures. This method was intrinsically not suitable to carry out a test on a bundle containing a large number of fibres. In the present test, the output of the Instron load cell amplifier was used as a parametric input to the AET 5500. Thus, whenever a fibre breaks, the system records AE parameters and the corresponding parametric input value indicates the load at fracture. With this arrangement, an accurate method of determining the fibre fracture load was established.
The electrical signal from each sensor was pre-amplified using AET 140 dB pre-amplifier with a gain of 40 dB. A band pass filter between ranges 250 kHz to 500 kHz was used to eliminate undesirable low and high frequencies. The noise level of the pre--amplifier was less than 3 tzV. A threshold of 200 mV, at least 10 times the typical noise amplitude, was selected to ensure a safe margin to completely isolate the AE signals from the background noise signals. An Instron 1195 model loading frame was used in the investigation. It utilises high sensitive and accurate load weighing system employing strain gauge load cells for detecting the load applied to the specimen under test. For testing a strand of 4000 filaments, a load cell (type 2511-319) with a full scale load range of 2000N was employed.
Design of sample holders and specimen preparation
To monitor the effect of loading speed on bundle strength in a strand, tensile tests were conducted on three separate strands at crosshead speeds of 5, 0.5 and 0.05 mm min -1. The number of valid events (fibre breaks) obtained was 1325, 1997 and 3983 for crosshead speeds of 5, 0.5 and
The direct mounting of a bundle of fibres on to the grips of the Instron tensile testing machine was not practical as the fibres were severely damaged near the grips during loading,
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S. Jihan et al. 0.05 nun min -1, respectively. The lower number of valid events obtained under the higher strain rates indicate that simultaneous fibre fractures occurred and the system was unable to resolve AE signal of each individual fracture. At a speed of 0.5 mm min-1 the number of valid events (3983) obtained closely corresponds to the number of fibres (4000) in a strand. As such all tensile tests, unless specified, were conducted at a crosshead speed of 0.05 mm rain-1 at room temperature of approximately 20°C and relative humidity of 50%.
Characteristics of a strang using AE signal parameters For each strand test, various AE signal parameters such as analog parameter (load), events, event duration and peak amplitude were recorded. These results obtained
(a) REGIONS
For each AE event, the time difference (t2ND - t l S T ) between recording the event at the two sensors S 1 and $2 (see Figure 3a) was found. This time difference was used by the existing and in-house software to locate the exact position of fibre breakage. This technique also permits out of range events to be eliminated. Out of range events are events occurring outside the specimen gauge length caused by such activities as noise in the grips, the breakage of fibres in the adhesive, cracking of the adhesive and sensor slippage if any. With this facility only events occurring within the fibre gauge length (region 3) as shown in Figure 3a were accepted as valid events (fibre breakages).
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Results
]. . . . .
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(b) 500
Strength distribution in a strand Each strand test containing up to 4000 filaments was completed successfully within 1 h. The AE data was postprocessed and using the developed software, the results were evaluated in 15 min to characterise the strength of the tested strand. Fifty strands were tested, the results presented below and in the next section are from a typical strand.
g> 250
0
The relationship between the bundle load and the number of fibre fractures obtained for the test is shown in Figure 4a. The maximum bundle load (1000 _+ 50 N) can be estimated directly from the experimental data. In order to obtain the Weibull parameters, a plot of L n [ L n ( l / ( 1 - PF))] against L n ( t r ) was made. It is clear that a single straight line cannot be drawn through the resulting data points in Figure 4b and that the situation was accurately represented by two straight lines. Physically this means that the flaw distribution was bimodal in form rather than the unimodal distribution previously reported by other workers [1,11]. The failure probability is more accurately represented in terms of two sets of statistical parameters, each of which corresponds to one of the individual flaw populations. In Figure 4b, the flaw distribution in the tested strand was found to provide an upper and a lower limit of strength. Two moduli, mL = 1.5 - 0.5 for the lower strength population and m y = 6.5 - 0.5 relating to the upper strength population, were determined.
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Region Figure 3 (a) Events discrimination by regions. (b) Distribution of events by location (mm) for AE strand test. Number of intervals, 150;Interval size, 1 mm. (c) Total events by region for AE strandtest.
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from a typical strand test of 4000 filaments are shown in Figure 5a-c.
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Figure 5a shows a load history of the fibres in a strand. From calibration, a load cell :~ignal of 10 000 millivolts represents a load of 2000 N. The maximum bundle load sustained by a strand was 1000 ___ 50 N (5000 _ 200 mV). The nature of occurrence of the valid events (fibre breakages) up to the maximum load sustained by the strand was random whilst beyond the maximum load breakages occurred at much greater rate. A total of 750 - 50 fibre fractures was noted up to the peak load. A plot of the number of valid events against peak amplitude shows two distinct peak amplitude distributions (i) at a lower range between 53 dB and 73 dB and (ii) at an upper range between 73 dB and 95 dB as indicated in Figure 5b. This result extracted from statistical data is again indicative of a bimodal flaw distribution.
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Figure 5 (a) Analog parameter (mV) vs time (rains) for AE strand test. Number of Intervals, 400; Interval size, 10s. {b) Distribution of valid events by peak amplitude (dB) for AE strand test. (c) Variation in the peak amplitude, PA {dB) distribution with stress (GPa).
dependence of strength on the condition of the glass surface [121. The general trend is towards lower strength with increasing surface damage. The flaws can be induced into the glass fibres during manufacturing processes and handling. It has been reported that fibres taken from the turn-around-points and fibres from the centre of the commercial roll possess different strength t13]. Hillig and Charles I14] observed the presence of intrinsic flaws in glass and suggested that the strength data can be accounted for if one assumes a bimodal distribution of flaw sizes. Hence in the present case, the two strength populations can be explained on the basis of flaw sizes, i.e. those with Griffith critical flaw size, caused by mechanical damage, fractured
Figure 5c shows two distributions of the peak amplitude of the signal with increasing stress, one in the range 0-1 GPa and the second in the range 1.53-3.0 GPa. It is apparent again from the plot that there are two populations of strength. A considerable number of weaker fibres were broken before the strand reached its maximum loading capacity. The population corresponding to a peak amplitude of 62 dB fractures at a stress below 1 GPa whilst the population corresponding to a amplitude of 83 dB fractures only above 1.5 GPa. One of the striking features of glass is the obvious
387
S. Jihan et aL
References
at low stress whilst in the stronger fibres flaws reached critical size at higher stresses and fractured.
1 Cowking, A., Attou, A., Siddiqui, A.M. and Sweet, M.A.S. Testing E-glass fibre bundles using acoustic emission. J. Mater Sci., 1991,26, 1301-1310. 2 ASTM 3379, 1982. 3 meecholsky, J.J., Rice, R.W. and Freiman, S.W. Prediction of fracture energy and flaw size in glass from measurements of mirror size. J. Am. Ceram Soc., 1974, 57, 440-445. 4 Fuwa, M., Bunsell, A.R. and Harris, B. An evaluation of acoustic emission technique supplied to carbon fibre composites. J. Appl.Phys., 1976, 9, 353-364. 5 Chi, Z., Chou, T.W. and Shen, G. Determination of single fibre strength distribution from fibre bundle testing. J. Mater. Sci., 1984, 19, 3319-3324. 6 Jihan, S., Siddiqui A.M. and Sweet, M.A.S. Effect of sample size and length on the strength of E-glass fibres using acoustic emission. In Proc. Int. Conf. on Electronic Measurements and Instrumentation, Tianjian University, China, 1992, pp. 210-215. 7 Weibull, W. A Statistical distribution function of wide applicability. J. Appl. Mech., 1951, 18, 293-296. 8 Scott, W.D. and Gaddipatl, A. Weibull parameters and the strength of long glass fibres. In Fracture Mechanics of Ceramics, R.C. Bradt, D.P.H. Hasselman and F.F. Lange, Eds., Plenum, New York, 1978, pp. 125-141. 9 Olshansky, R. and Murer, R.D. Tensile strength and fatigue of optical fibres. J. Appl. Phys., 1976, 47, 4497-4499. 10 Hamstad, M.A. and Moore, R.L., Acoustic Emission from single and multiple Kevlar 49 filament breaks. J. Comp. Mater., 1986, 20, 47-65. 11 Attou, A., Cowking, A., Siddiqui, A.M. and Sweet, M.A.S. Strength investigation of a commercial E-glass fibre roll using acoustic emission. Br. J. NDT, 1990, 32, 623-626. 12 Kurkjian, C.R. Strength and fatigue of oxide glasses. In Current trends in the Science and Technology of Glass, H. Jain, A.R. Cooper, K.J. Rat and D. Chakravorthy, Eds., World Scientific Co., Singapore, 1989, pp. 190-195. 13 Bailey, R., Bradsky, G.V. and Lehr, W. E-glass fibre strength: Their measurements and characteristics. ICI Report No. 11196, 1989, pp. 1-3. 14 Hillig, W.B. and Charles, R.J. In High Strength Materials, V.F. Zackay, Ed., John Wiley, New York, 1965, pp. 682-705. 15 Jihan, S. Ph.D. Thesis, Robert Gordon University, 1994.
Conclusions It has been established that AE can be used as a suitable means of determining fracture strength distribution of E-glass fibres in a strand under tension. The work has shown that for reliable results, tests across a strand (typically 4000) are essential. The established AE technique has the potential for testing even larger number of filaments ( > 4000) if required). AE events due to fibre fractures are characterised by high peak amplitudes and were easily distinguished from out of range events. Strength analyses of single filaments or small numbers of filaments in selected bundles have normally been represented by a unimodal flaw distribution. The present investigations have shown this to be inadequate. Testing fibres in a strand revealed a bimodal nature. Analysis of AE signal parameter data has further substantiated the existence of a bimodal flaw population. The AE strand testing technique developed in this work has the potential to monitor the effect of manufacturing (e.g. production of long fibre reinforced composites using the pultrusion process) variables such as temperature, friction, environment and any combination of these factors applied to a strand I~5].
Acknowledgements Mr S. Jones, LNP Engineering Plastics and Dr R. Bailey, ICI, deserve special mention for the discussions concerning the developed AE technique and for the supply of the materials.
388