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The effect of specimen length on the static behaviour of filament-wound pipes
CompositeStructures29 (1994) 323-328 O 1994 Elsevier Science Limited Printed in Great Britain. All fights reserved 0263-8223/94/$7.00 ELSEVIER
The effect of specimen length on the static behaviour of filament-wound pipes K. L. Alderson & G. M. Simpson Department of Materials Science and Engineering, University of Liverpool, PO Box 147, Liverpool, UK, L69 3BX
The static failure process and residual mechanical properties of complete sections of open ended E-glass fibre/epoxy resin filament-wound pipes supported in half-circumference end-cradles were evaluated as a function of pipe length. The length of specimens under test varied from 500 to 150 mm with the aim of investigatingif small pipes could be tested to predict accurately the behaviour of much larger pipes. It was found that this is indeed possible if the region of interest is the first failure of a characteristic two-part failure process, which reveals the onset of delamination initiation. However, beyond this first failure point, it appears that although trends in behaviour can be predicted, absolute values cannot. This would appear to indicate that some form of scaling could be employed in tandem with experimental investigations to predict the behaviour of large pipes from much smaller pipes. Smaller pipes should be easier to handle in the laboratory situation and would represent a saving as far as materials and machinery are concerned. which are summarised here for completeness. Two support conditions were employed in the test programme, both of which were likely to be encountered during the service life of the pipes -these being floor supported along the pipe's length and supported in specially designed half-circumference end-cradles. Good correlations 1,2 were found between static and low velocity impact behaviour both in initial and residual property tests (i.e. those performed after the original impact or static test had been carried out), particularly for the cradled support condition, which is illustrated in Fig. 1. It should be noted that these tests were carried out with open ended pipes. This is applicable when considering possible damage during, say, pipe installation. However, in service, the pipes would normally be filled with fluid, which would have a considerable effect on the properties and damage caused by impact. Static tests were then employed to study the failure process of the open ended pipes and these revealed that for both specimen geometries tested, the filament-wound GRP pipes undergo a twopart failure process, illustrated by the load/displacement plot shown in Fig. 2. The first failure, at low loads, is of extreme importance 3 since work with electronic resistance strain gauges and extensive sectioning and microscopy has revealed that this is caused by the yielding of the resin matrix,
1 INTRODUCTION In a recent series of papers, aspects of the transverse low velocity impact behaviour of glass fibre reinforced plastic (GRP) filament-wound pipes were examined, including experimental studies of equivalence between low velocity impact and static behaviour ~-3 and the viability of performing finite element analysis 4 and simple mass-spring modelling 5 to predict specimen response. The low velocity impact response of composite materials is of particular interest since it can arise from situations such as the accidental dropping of hand tools on to the pipes during installation or servicing, resulting in what is known as barely visible impact damage. This consists of areas of local indentation and delamination which, as well as being difficult to detect, can have a dramatic effect on the properties of the composite under test, reducing its effectiveness as a structural material by up to 50%. 6 Clearly, this is undesirable and it is of great importance to be able to assess such damage and its effects on the properties of the composite materials under consideration, which in this case are GRP filament-wound pipes. The earlier work mentioned above has yielded some important findings concerning the nature and extent of the impact damage at low (i.e. up to 10 m/s) velocities on GRP filament-wound pipes, 323
324
K. L. Alderson, G. M. Simpson
II Fig. 1. The cradled support condition used in the experimental test programmes.
21"00
LOAD (kN) 7'00
DEFLECTION (mm) Fig. 2. A typical static load/displacement plot for cradled pipes of length 500 mm. which acts as a precursor to delamination initiation in many layers and has a marked effect on the pipe's residual properties. This appears to be a very localised effect, and is followed by the preferential growth of a favoured delamination, leading to a second major failure. All tests were carried out on 500 m m pipe lengths (cut from 5000 m m lengths manufactured by Wavin Repox B.V. 7) to avoid problems with end effects. However, due to the extremely localised nature of the first failure, it was decided to investigate the effects of varying the specimen length to see if tests could be performed on specimens of shorter lengths than 500 mm with the same results. This testing of pipes of reduced length would be of considerable benefit as it would reduce the costs of both the materials involved and the machinery required for testing. This area of impact testing of composite materials is currently under a great deal of investigation, with groups, for example, examining both the theoretical 8 and experimental 9-~ effects of using smaller laminates and cylinders to model the performance of larger components. These studies have tended to show that low velocity impact response is very dependent on the specimen size even at the first damage threshold, but have stated that at high velocities, where the damage is very localised, specimen size is of less importance? This paper examines the effect of varying the specimen length of GRP filament-wound pipes from 150 to 500 m m for the cradled support condition in simple static tests (found previously to be a close approximation to low velocity impact
behaviour) and then investigates the variation of residual properties with specimen length to see if the entire programme for testing previously carried out on 500 m m lengths of pipes can be performed on pipes of shorter lengths. These tests would not only be less expensive but would be easier to carry out than tests on larger specimens.
2 EXPERIMENTAL M E T H O D S
All tests were carried out on filament-wound E-glass fibre/epoxy resin (Epikote 828) pipes with a winding angle of _+55 °. The pipes had a glass fibre volume fraction of 56 _+ 1% and an internal diameter of 150 mm. The wall thickness was 5.94 mm and it consisted of an outer gel coat of thickness 0-385 mm, an inner gel coat of thickness 0.533 m m and 18 layers of composite material, each of thickness 0.279 mm. In all cases, the pipes were supported in the specially designed haft-circumference end-cradles shown in Fig. 1 and tests were carried out on an Instron 1185 tensile testing machine set up in compression mode. A three-point bend configuration was employed with a strain rate of 5 mm/ min. The loading nose was a solid cylinder of length 150 mm and diameter 25 mm and was, as in previous work, applied at the midpoint transverse to the pipe's axis. The following investigations were carried out. 2.1 Effect of varying the specimen length on static behaviour
A 5000 mm pipe, as supplied, was sectioned to give complete cylinders for testing of length 500, 350, 250 and 150 mm. Static tests as described above were performed on several specimens of each designated length until the loading nose was about to penetrate the lower resin gel coat layer, a point previously defined as full specimen damage} The resulting load/displacement plots were analysed to obtain mechanical properties such as peak load and elastic energy. The area of damage for each of the specimens was measured using a simple backlighting technique, described elsewhere. ~ Basically, this consists of inserting a strong light source inside the damaged specimen and photographically recording the delamination patterns thus revealed. These can be subsequently analysed to give the total area of damage taking into account the delamination between all layers throughout the pipe's thickness.
Study on static behaviour of filament-wound pipes The damage sustained was further analysed by an extensive programme of sectioning of the damaged pipes for optical microscopy. This was performed on a Wild M8 optical microscope at low magnifications, i.e. x 6.
2.2 Effect of varying the specimen length on residual pipe properties A second test programme was then undertaken to study the effects of varying the specimen length on the residual pipe properties. A large amount of data has already been collected 2 for cradled specimens of length 500 mm and so it was decided to reduce this length to 150 mm and to repeat the tests on pipes of this length to assess the viability of using shorter specimen lengths to model residual behaviour. To these ends, partial and full specimen damage tests were conducted to the predetermined loads indicated in Fig. 2 on specimens of length 150 mm. Where required, several specimens were tested for each condition. After these initial tests had been carried out, mechanical properties were extracted from the load/deflection plots thus obtained, in particular the energy imparted to produce the amount of damage required, to be referred to as the incident energy. Then, the area of damage for all specimens was measured using the backlighting technique. After the damage had been assessed and measured for each tested specimen (a process normally taking one working week), the damaged specimens were then tested to full specimen damage on the Instron 1185. The same conditions as the initial tests were used, i.e. the pipes were cradled, the same loading nose was used and a strain rate of 5 mm/min was employed, with the test again taking place in three-point bend configuration. This yields the residual properties for the pipes and, in particular, values were obtained for residual peak load and residual deflection to failure from the residual load/deflection plots.
325
necessary to examine both failure points when analysing behaviour. Data extracted from the static load/displacement plots are given in Table 1 and plotted in Figs 3 and 4 for clarity. It can be seen from both Figs 3 and 4 that the first failure appears to occur at the same load and energy regardless of specimen length. This again emphasises the localised nature of this failure event. Also Table 1. Data extracted from the static load/displacement curves
Specimen Peak load at: Energy at length first failure (mm) 1st failure 2nd failure (J) (kN) (kN) 150 250 350 500
6"7__+0"8 7"3_+0"1 7"0+0"4 7-7-+0"1
21"0__+1"0 21"0_+0"3 19"4-+0"6 19'2_+0"1
14__+3 14_+1 13-+2 14_+1
Area of damage (m 2) 0"012_+0"001 0"012_+0-002 0"019__+0-002 0"050__+0"010
ENER6Y TO FIRST FAILURE (J) 201
I
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SPECIMEN LENGTH (ram) Fig. 3.
Graph of energy to first failure against specimen length.
LOAD AT FAILURE (kN) 30
0'
3 RESULTS
3.1 Effect of varying the specimen length on static behaviour The first thing to note when investigating the behaviour of the GRP filament-wound pipes tested here is that all, regardless of specimen length, underwent the same two-part failure process that is illustrated in Fig. 2. Thus, it was
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soo SPECIMEN LEN6TH (ram) Fig. 4. Graph of load at failure against specimen length. Load at first failure is shown as solid line and load at second failure as a dotted line.
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K. L. Alderson, G. M. Simpson
plotted in Fig. 4 is the peak load which each pipe attains before the second failure, which does show a behaviour trend linked to specimen length. This will be discussed below. Included in Table 1 are the areas of damage for each of the pipes measured using the backlighting technique after the specimens had been tested to full specimen damage. These reveal that as the pipe length is increased, so the delaminated area tends to be increased. This is as expected, as there is in the longer specimens obviously more physical length available for the delaminations to grow. It should, however, be noted that this increased delamination area as the specimen length increases also receives a contribution due to an increased number of delaminations growing during the test. Use of the backlighting technique in combination with microscopy revealed that only two favourable delaminations grow for the pipes of length 150 and 250 mm. For specimens of length 350 mm, four delaminations grow, with the third and fourth being most favourable, and for specimens of length 500 mm, five delaminations grow. Thus, the increased area of delamination is due to more as well as longer delaminations growing.
Table 2. Residual property data for the 150 mm long pipes
Specimen Initial Residual Residual Incident Area of no. load peak load deflection energy damage (kN) (kN) (mm) (J) (m2) R1 R2 R3 R4
7.0 10"0 13.0 16.4
21"3 20-8 20-0 18.0
23"0 23"0 25"5 27-5
16-49 62.12 109.35 156.70
0.0005 0.0021 0"0083 0"0083
TOTAL DELAMINATED AREA (m2) 0.08 1
x" 3<" 0.0if
if/ // soo
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INCIDENT ENERGY (J) Fig. 5. Graph of total delaminated area against incident energy for pipes of length 150 mm (solid line) and 500 mm (dotted line). RESIDUAL PEAK LOAD 30(kN)
3.2 Effect of varying the specimen length on the residual pipe properties
One of the main aims of this work was to see if not only the mechanical properties of the pipes could be obtained from small samples but also the residual properties. The numerical data obtained from the residual load/displacement tests on pipes of length 150 m m are given in Table 2, namely the residual peak load and the residual deflection to failure. These two properties were selected because, from previous work 2 in the cradled support condition, these had the closest correlations between static and low velocity impact behaviour. Also included in Table 2 are the two damage parameters, these being the incident energy and the delaminated area imparted to the specimens during the initial loading measured using the backlighting technique. The relationship between the parameters is shown in Fig. 5 as a solid line. The dotted line in this figure represents the relationship between the two damage parameters for pipes of length 500 m m for comparison purposes. Figures 6 and 7 show the relationships between residual peak load and residual deflection, respectively, and delaminated area. Again, for
20
~x" ~ - .
10
O
0.08
DELAMINATION AREA (m2) Fig. 6. Graph of residual peak load against delamination area for pipes of length 150 mm (solid line) and 500 mm (dotted line). comparison purposes, the dotted line on each graph shows the behaviour of the specimens of length 500 mm. 4 DISCUSSION 4.1 Effect of varying the specimen length on static behaviour
The first thing to note from this particular group of tests is that it appears that the first failure seen
Study on static behaviour offilament-wound pipes
RESIDUAL DEFLECTION(ram) /
50"
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25 '1/
60u
6.08
DELANINATION AREA (m2) Fig. 7. Graph of residual deflection against delamination area for pipes of length 150 mm (solid line) and 500 mm (dotted line).
at low loads does not depend on the length of the specimen under test for the cradled support condition. This finding is underlined by Figs 3 and 4 and by Table 1 and agrees very well with the previous work. In this previous work, the effects of support condition (floor supported or cradled), test type (low velocity impact or static), 1 shape (1/ 16, 1/8, 1/4, 1/2 circumferential sections or complete pipes, all of length 500 mm) 12 and internal diameter (150 or 400 mm) 3 were examined and it was found that the first failure always occurred at the same load, dependent only on pipe wall thickness. Thus, the extremely localised nature of this first failure process, revealed by nondestructive and destructive testing techniques and by electronic resistance strain gauges, means that small specimens can be used to directly obtain mechanical properties at first failure of longer pipes. This is of considerable benefit due to increased ease of testing and reduction of cost as well as wastage. This first failure point marks a very important phase in the pipe's behaviour under three-point bend static and low velocity impact loading as it represents the yielding of the resin matrix immediately followed by the initiation of delamination throughout the pipe's wall thickness. Thus, if information is required on the load at which a pipe will undergo first failure, it can be concluded that a simple static test can be performed on a small section of it to give the absolute load necessary to initiate delamination. However, once the first failure point has occurred, the second part of the failure process does appear to show specimen length dependency. This is illustrated by Fig. 4, which shows
327
the variation with specimen length of both the load at first failure and the peak load attained before second failure occurs. Whilst first failure shows no variation at all with specimen length, it can clearly be seen that the peak load attained before second failure decreases as specimen length increases. This is in agreement with previous work by Qian and Swanson,/° who studied impact behaviour of carbon fibre/epoxy resin laminates in an attempt to use smaller specimens to model the response of larger components. They also found that smaller specimens were stronger than larger specimens and developed a set of scaling rules to allow behaviour of smaller samples to be used to predict the behaviour of larger samples. It would appear from this work that if the behaviour of large pipes beyond the point of delamination initiation is of interest, then it is not possible to perform tests on smaller pipes to obtain directly the mechanical properties, but it may be that by applying scaling rules such as those developed by Qian and Swanson, J° behaviour can be predicted by a combination of experimental and theoretical analysis.
4.2 Effect of varying the specimen length on the residual pipe properties Firstly, it can be seen from Fig. 5 that for both sets of test data, i.e. for specimens of length 150 and 500 mm, a linear correlation exists between delamination area and incident energy. However, Fig. 5 immediately indicates that it is unlikely that it will be possible to use small specimens to directly determine the residual behaviour of larger pipes. If this were to be the case, then it would be expected that the data for the 150 mm pipes would directly superimpose on that for the pipes of length 500 mm and this is clearly not the case. However, it may be that similar trends in residual behaviour are observed for the two different pipe lengths, as there is a linear correlation between the damage parameters in both cases, again indicating that with a combination of experimental testing and theoretical modelling, tests could be performed on shorter pipes to predict the behaviour of longer pipes. This is borne out by observation of Figs 6 and 7. Figure 6 plots the residual peak load against delaminated area. Here, the residual peak load is seen to gradually decrease with increasing delamination area for the 500 mm length specimens. This is due to the initiation of delamination in many layers of the pipe at first failure. Beyond this
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K. L. Alderson, G. M. Simpson
point, the damage process continues with the growth of one favourable delamination, 3 which will reduce the amount of load a pipe may sustain as it grows. A similar trend is seen for the 150 m m length specimens, but here, the drop off in peak load is much more rapid. For the residual deflection, again the trend in behaviour is very similar for the two different length specimens, albeit on a smaller scale for the shorter pipes. For both pipe lengths, deflection is virtually constant until some critical value of delaminated area is reached. Beyond this, there is a sharp upturn in deflection needed to cause failure, which is thought to be due to the growth of one favourable delamination as the failure process continues. However, though the trend in behaviour is identical, Fig. 7 shows that absolute values of deflection and delamination area are very different. For example, the critical value at which the sharp upturn in deflection occurs after the plateau region is found at a delaminated area of 0.002 m 2 and a deflection of 23 mm for specimens of test length 150 mm. For a specimen of test length 500 mm, though, this occurs at a much bigger delaminated area of around 0"05 m 2 for a longer deflection of around 41 mm. Thus, again, some form of scaling procedure would be required if absolute values of residual properties were to be predicted. The overall comment for this section of the work carried out, then, shows that absolute values of residual mechanical properties cannot be predicted by only using tests carried out on shorter samples. However, it is possible to discover trends in residual behaviour, thus indicating that it is necessary to use scaling rules in combination with experimental testing to predict behaviour. This agrees with work reported by Cantwell and Morton, who say that care must be taken when using small samples to predict and characterise the impact response of larger samples, 9 at least for tests carried out beyond the initial first failure point, which is caused by delamination inititation.
5 CONCLUSIONS Regardless of support condition, test type, shape and now specimen length, GRP filament-wound pipes undergo a two-part failure process. The first failure occurs at the same load for the same energy even if the specimen length is considerably reduced for the pipe wall thickness (5.94 mm)
employed here. Therefore, this indicates that if this point, which marks the onset of delamination initiation, is of interest, it is possible to perform tests on very short specimens to gain exact information on the behaviour of much longer pipes. This will result in a considerable saving in material and machinery costs. However, for behaviour beyond this first failure, the length of the specimen becomes a more important parameter for both initial and residual mechanical properties. Then it is no longer possible to perform tests on small samples to gain exact information on behaviour of larger pipes although trends in residual pipe behaviour can be readily observed. This indicates the need for mathematical scaling to be employed in tandem with experimental testing to predict the behaviour of larger pipes from small pipes when tested to beyond delamination initiation.
REFERENCES 1. Alderson, K. L. & Evans, K. E., Low velocity transverse impact of filament-wound pipes: Part 1. Damage due to static and impact loads. Comp. Struct., 20 (1992) 37-45. 2. Evans, K. E. & Alderson, K. L., Low velocity transverse impact of filament-wound pipes: Part 2. Residual properties and correlations with impact damage. Comp. Struct., 20 (1992) 47-52. 3. Alderson, K. L. & Evans, K. E., Failure mechanisms during the transverse loading of filament-wound pipes under static and low velocity impact conditions. Composites, 23 (1992) 167-73. 4. Evans, K. E., Alderson, K. L. & Marks, P. R., Modelling of the transverse loading of filament wound pipes. Computers and Structures, 45 (1992) 1089-95. 5. Alderson, K. L. & Evans, K. E., Dynamic analysis of filament wound pipes undergoing low velocity transverse impact. Comp. Sci. &Tech., 45 (1992) 17-22. 6. Cantwell, W. J. & Morton, J., Detection of impact damage in CFRP laminates. Comp. Struct., 3 (1985) 241-57. 7. Wavin Repox B.V., JC Kellerlaan 3, PO Box 5, 7770 AA Hardenberg, The Netherlands. 8. Swanson, S. R., Mechanisms of transverse impact in fiber composite plates and cylinders. J. Reinforced Plastics and Composites, 12 (1993) 256-67. 9. Cantweli, W. J. & Morton, J., The impact response of composite materials -- A review. Composites, 22 ( 1991 ) 347-62. 10. Qian, Y. & Swanson, S. R., Experimental measurement of impact response in carbon/epoxy plates. AIAA Journal, 28 (1990) 1069-74. 11. Swanson, S. R., Scaling of impact damage in fiber reinforced composites from laboratory specimens to structures. Comp. Struct., 25 (1993) 249-55. 12. Ainsworth, K. L., Low velocity transverse impact damage of filament-wound E-glass/epoxy resin pipes. PhD thesis, University of Liverpool, 1990.
The effect of specimen length on the static behaviour of filament-wound pipes K. L. Alderson & G. M. Simpson Department of Materials Science and Engineering, University of Liverpool, PO Box 147, Liverpool, UK, L69 3BX
The static failure process and residual mechanical properties of complete sections of open ended E-glass fibre/epoxy resin filament-wound pipes supported in half-circumference end-cradles were evaluated as a function of pipe length. The length of specimens under test varied from 500 to 150 mm with the aim of investigatingif small pipes could be tested to predict accurately the behaviour of much larger pipes. It was found that this is indeed possible if the region of interest is the first failure of a characteristic two-part failure process, which reveals the onset of delamination initiation. However, beyond this first failure point, it appears that although trends in behaviour can be predicted, absolute values cannot. This would appear to indicate that some form of scaling could be employed in tandem with experimental investigations to predict the behaviour of large pipes from much smaller pipes. Smaller pipes should be easier to handle in the laboratory situation and would represent a saving as far as materials and machinery are concerned. which are summarised here for completeness. Two support conditions were employed in the test programme, both of which were likely to be encountered during the service life of the pipes -these being floor supported along the pipe's length and supported in specially designed half-circumference end-cradles. Good correlations 1,2 were found between static and low velocity impact behaviour both in initial and residual property tests (i.e. those performed after the original impact or static test had been carried out), particularly for the cradled support condition, which is illustrated in Fig. 1. It should be noted that these tests were carried out with open ended pipes. This is applicable when considering possible damage during, say, pipe installation. However, in service, the pipes would normally be filled with fluid, which would have a considerable effect on the properties and damage caused by impact. Static tests were then employed to study the failure process of the open ended pipes and these revealed that for both specimen geometries tested, the filament-wound GRP pipes undergo a twopart failure process, illustrated by the load/displacement plot shown in Fig. 2. The first failure, at low loads, is of extreme importance 3 since work with electronic resistance strain gauges and extensive sectioning and microscopy has revealed that this is caused by the yielding of the resin matrix,
1 INTRODUCTION In a recent series of papers, aspects of the transverse low velocity impact behaviour of glass fibre reinforced plastic (GRP) filament-wound pipes were examined, including experimental studies of equivalence between low velocity impact and static behaviour ~-3 and the viability of performing finite element analysis 4 and simple mass-spring modelling 5 to predict specimen response. The low velocity impact response of composite materials is of particular interest since it can arise from situations such as the accidental dropping of hand tools on to the pipes during installation or servicing, resulting in what is known as barely visible impact damage. This consists of areas of local indentation and delamination which, as well as being difficult to detect, can have a dramatic effect on the properties of the composite under test, reducing its effectiveness as a structural material by up to 50%. 6 Clearly, this is undesirable and it is of great importance to be able to assess such damage and its effects on the properties of the composite materials under consideration, which in this case are GRP filament-wound pipes. The earlier work mentioned above has yielded some important findings concerning the nature and extent of the impact damage at low (i.e. up to 10 m/s) velocities on GRP filament-wound pipes, 323
324
K. L. Alderson, G. M. Simpson
II Fig. 1. The cradled support condition used in the experimental test programmes.
21"00
LOAD (kN) 7'00
DEFLECTION (mm) Fig. 2. A typical static load/displacement plot for cradled pipes of length 500 mm. which acts as a precursor to delamination initiation in many layers and has a marked effect on the pipe's residual properties. This appears to be a very localised effect, and is followed by the preferential growth of a favoured delamination, leading to a second major failure. All tests were carried out on 500 m m pipe lengths (cut from 5000 m m lengths manufactured by Wavin Repox B.V. 7) to avoid problems with end effects. However, due to the extremely localised nature of the first failure, it was decided to investigate the effects of varying the specimen length to see if tests could be performed on specimens of shorter lengths than 500 mm with the same results. This testing of pipes of reduced length would be of considerable benefit as it would reduce the costs of both the materials involved and the machinery required for testing. This area of impact testing of composite materials is currently under a great deal of investigation, with groups, for example, examining both the theoretical 8 and experimental 9-~ effects of using smaller laminates and cylinders to model the performance of larger components. These studies have tended to show that low velocity impact response is very dependent on the specimen size even at the first damage threshold, but have stated that at high velocities, where the damage is very localised, specimen size is of less importance? This paper examines the effect of varying the specimen length of GRP filament-wound pipes from 150 to 500 m m for the cradled support condition in simple static tests (found previously to be a close approximation to low velocity impact
behaviour) and then investigates the variation of residual properties with specimen length to see if the entire programme for testing previously carried out on 500 m m lengths of pipes can be performed on pipes of shorter lengths. These tests would not only be less expensive but would be easier to carry out than tests on larger specimens.
2 EXPERIMENTAL M E T H O D S
All tests were carried out on filament-wound E-glass fibre/epoxy resin (Epikote 828) pipes with a winding angle of _+55 °. The pipes had a glass fibre volume fraction of 56 _+ 1% and an internal diameter of 150 mm. The wall thickness was 5.94 mm and it consisted of an outer gel coat of thickness 0-385 mm, an inner gel coat of thickness 0.533 m m and 18 layers of composite material, each of thickness 0.279 mm. In all cases, the pipes were supported in the specially designed haft-circumference end-cradles shown in Fig. 1 and tests were carried out on an Instron 1185 tensile testing machine set up in compression mode. A three-point bend configuration was employed with a strain rate of 5 mm/ min. The loading nose was a solid cylinder of length 150 mm and diameter 25 mm and was, as in previous work, applied at the midpoint transverse to the pipe's axis. The following investigations were carried out. 2.1 Effect of varying the specimen length on static behaviour
A 5000 mm pipe, as supplied, was sectioned to give complete cylinders for testing of length 500, 350, 250 and 150 mm. Static tests as described above were performed on several specimens of each designated length until the loading nose was about to penetrate the lower resin gel coat layer, a point previously defined as full specimen damage} The resulting load/displacement plots were analysed to obtain mechanical properties such as peak load and elastic energy. The area of damage for each of the specimens was measured using a simple backlighting technique, described elsewhere. ~ Basically, this consists of inserting a strong light source inside the damaged specimen and photographically recording the delamination patterns thus revealed. These can be subsequently analysed to give the total area of damage taking into account the delamination between all layers throughout the pipe's thickness.
Study on static behaviour of filament-wound pipes The damage sustained was further analysed by an extensive programme of sectioning of the damaged pipes for optical microscopy. This was performed on a Wild M8 optical microscope at low magnifications, i.e. x 6.
2.2 Effect of varying the specimen length on residual pipe properties A second test programme was then undertaken to study the effects of varying the specimen length on the residual pipe properties. A large amount of data has already been collected 2 for cradled specimens of length 500 mm and so it was decided to reduce this length to 150 mm and to repeat the tests on pipes of this length to assess the viability of using shorter specimen lengths to model residual behaviour. To these ends, partial and full specimen damage tests were conducted to the predetermined loads indicated in Fig. 2 on specimens of length 150 mm. Where required, several specimens were tested for each condition. After these initial tests had been carried out, mechanical properties were extracted from the load/deflection plots thus obtained, in particular the energy imparted to produce the amount of damage required, to be referred to as the incident energy. Then, the area of damage for all specimens was measured using the backlighting technique. After the damage had been assessed and measured for each tested specimen (a process normally taking one working week), the damaged specimens were then tested to full specimen damage on the Instron 1185. The same conditions as the initial tests were used, i.e. the pipes were cradled, the same loading nose was used and a strain rate of 5 mm/min was employed, with the test again taking place in three-point bend configuration. This yields the residual properties for the pipes and, in particular, values were obtained for residual peak load and residual deflection to failure from the residual load/deflection plots.
325
necessary to examine both failure points when analysing behaviour. Data extracted from the static load/displacement plots are given in Table 1 and plotted in Figs 3 and 4 for clarity. It can be seen from both Figs 3 and 4 that the first failure appears to occur at the same load and energy regardless of specimen length. This again emphasises the localised nature of this failure event. Also Table 1. Data extracted from the static load/displacement curves
Specimen Peak load at: Energy at length first failure (mm) 1st failure 2nd failure (J) (kN) (kN) 150 250 350 500
6"7__+0"8 7"3_+0"1 7"0+0"4 7-7-+0"1
21"0__+1"0 21"0_+0"3 19"4-+0"6 19'2_+0"1
14__+3 14_+1 13-+2 14_+1
Area of damage (m 2) 0"012_+0"001 0"012_+0-002 0"019__+0-002 0"050__+0"010
ENER6Y TO FIRST FAILURE (J) 201
I
10
1
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'
t
÷
2so
Zoo
SPECIMEN LENGTH (ram) Fig. 3.
Graph of energy to first failure against specimen length.
LOAD AT FAILURE (kN) 30
0'
3 RESULTS
3.1 Effect of varying the specimen length on static behaviour The first thing to note when investigating the behaviour of the GRP filament-wound pipes tested here is that all, regardless of specimen length, underwent the same two-part failure process that is illustrated in Fig. 2. Thus, it was
10 }
'
!
|
soo SPECIMEN LEN6TH (ram) Fig. 4. Graph of load at failure against specimen length. Load at first failure is shown as solid line and load at second failure as a dotted line.
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K. L. Alderson, G. M. Simpson
plotted in Fig. 4 is the peak load which each pipe attains before the second failure, which does show a behaviour trend linked to specimen length. This will be discussed below. Included in Table 1 are the areas of damage for each of the pipes measured using the backlighting technique after the specimens had been tested to full specimen damage. These reveal that as the pipe length is increased, so the delaminated area tends to be increased. This is as expected, as there is in the longer specimens obviously more physical length available for the delaminations to grow. It should, however, be noted that this increased delamination area as the specimen length increases also receives a contribution due to an increased number of delaminations growing during the test. Use of the backlighting technique in combination with microscopy revealed that only two favourable delaminations grow for the pipes of length 150 and 250 mm. For specimens of length 350 mm, four delaminations grow, with the third and fourth being most favourable, and for specimens of length 500 mm, five delaminations grow. Thus, the increased area of delamination is due to more as well as longer delaminations growing.
Table 2. Residual property data for the 150 mm long pipes
Specimen Initial Residual Residual Incident Area of no. load peak load deflection energy damage (kN) (kN) (mm) (J) (m2) R1 R2 R3 R4
7.0 10"0 13.0 16.4
21"3 20-8 20-0 18.0
23"0 23"0 25"5 27-5
16-49 62.12 109.35 156.70
0.0005 0.0021 0"0083 0"0083
TOTAL DELAMINATED AREA (m2) 0.08 1
x" 3<" 0.0if
if/ // soo
iooo
INCIDENT ENERGY (J) Fig. 5. Graph of total delaminated area against incident energy for pipes of length 150 mm (solid line) and 500 mm (dotted line). RESIDUAL PEAK LOAD 30(kN)
3.2 Effect of varying the specimen length on the residual pipe properties
One of the main aims of this work was to see if not only the mechanical properties of the pipes could be obtained from small samples but also the residual properties. The numerical data obtained from the residual load/displacement tests on pipes of length 150 m m are given in Table 2, namely the residual peak load and the residual deflection to failure. These two properties were selected because, from previous work 2 in the cradled support condition, these had the closest correlations between static and low velocity impact behaviour. Also included in Table 2 are the two damage parameters, these being the incident energy and the delaminated area imparted to the specimens during the initial loading measured using the backlighting technique. The relationship between the parameters is shown in Fig. 5 as a solid line. The dotted line in this figure represents the relationship between the two damage parameters for pipes of length 500 m m for comparison purposes. Figures 6 and 7 show the relationships between residual peak load and residual deflection, respectively, and delaminated area. Again, for
20
~x" ~ - .
10
O
0.08
DELAMINATION AREA (m2) Fig. 6. Graph of residual peak load against delamination area for pipes of length 150 mm (solid line) and 500 mm (dotted line). comparison purposes, the dotted line on each graph shows the behaviour of the specimens of length 500 mm. 4 DISCUSSION 4.1 Effect of varying the specimen length on static behaviour
The first thing to note from this particular group of tests is that it appears that the first failure seen
Study on static behaviour offilament-wound pipes
RESIDUAL DEFLECTION(ram) /
50"
x
! !
25 '1/
60u
6.08
DELANINATION AREA (m2) Fig. 7. Graph of residual deflection against delamination area for pipes of length 150 mm (solid line) and 500 mm (dotted line).
at low loads does not depend on the length of the specimen under test for the cradled support condition. This finding is underlined by Figs 3 and 4 and by Table 1 and agrees very well with the previous work. In this previous work, the effects of support condition (floor supported or cradled), test type (low velocity impact or static), 1 shape (1/ 16, 1/8, 1/4, 1/2 circumferential sections or complete pipes, all of length 500 mm) 12 and internal diameter (150 or 400 mm) 3 were examined and it was found that the first failure always occurred at the same load, dependent only on pipe wall thickness. Thus, the extremely localised nature of this first failure process, revealed by nondestructive and destructive testing techniques and by electronic resistance strain gauges, means that small specimens can be used to directly obtain mechanical properties at first failure of longer pipes. This is of considerable benefit due to increased ease of testing and reduction of cost as well as wastage. This first failure point marks a very important phase in the pipe's behaviour under three-point bend static and low velocity impact loading as it represents the yielding of the resin matrix immediately followed by the initiation of delamination throughout the pipe's wall thickness. Thus, if information is required on the load at which a pipe will undergo first failure, it can be concluded that a simple static test can be performed on a small section of it to give the absolute load necessary to initiate delamination. However, once the first failure point has occurred, the second part of the failure process does appear to show specimen length dependency. This is illustrated by Fig. 4, which shows
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the variation with specimen length of both the load at first failure and the peak load attained before second failure occurs. Whilst first failure shows no variation at all with specimen length, it can clearly be seen that the peak load attained before second failure decreases as specimen length increases. This is in agreement with previous work by Qian and Swanson,/° who studied impact behaviour of carbon fibre/epoxy resin laminates in an attempt to use smaller specimens to model the response of larger components. They also found that smaller specimens were stronger than larger specimens and developed a set of scaling rules to allow behaviour of smaller samples to be used to predict the behaviour of larger samples. It would appear from this work that if the behaviour of large pipes beyond the point of delamination initiation is of interest, then it is not possible to perform tests on smaller pipes to obtain directly the mechanical properties, but it may be that by applying scaling rules such as those developed by Qian and Swanson, J° behaviour can be predicted by a combination of experimental and theoretical analysis.
4.2 Effect of varying the specimen length on the residual pipe properties Firstly, it can be seen from Fig. 5 that for both sets of test data, i.e. for specimens of length 150 and 500 mm, a linear correlation exists between delamination area and incident energy. However, Fig. 5 immediately indicates that it is unlikely that it will be possible to use small specimens to directly determine the residual behaviour of larger pipes. If this were to be the case, then it would be expected that the data for the 150 mm pipes would directly superimpose on that for the pipes of length 500 mm and this is clearly not the case. However, it may be that similar trends in residual behaviour are observed for the two different pipe lengths, as there is a linear correlation between the damage parameters in both cases, again indicating that with a combination of experimental testing and theoretical modelling, tests could be performed on shorter pipes to predict the behaviour of longer pipes. This is borne out by observation of Figs 6 and 7. Figure 6 plots the residual peak load against delaminated area. Here, the residual peak load is seen to gradually decrease with increasing delamination area for the 500 mm length specimens. This is due to the initiation of delamination in many layers of the pipe at first failure. Beyond this
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K. L. Alderson, G. M. Simpson
point, the damage process continues with the growth of one favourable delamination, 3 which will reduce the amount of load a pipe may sustain as it grows. A similar trend is seen for the 150 m m length specimens, but here, the drop off in peak load is much more rapid. For the residual deflection, again the trend in behaviour is very similar for the two different length specimens, albeit on a smaller scale for the shorter pipes. For both pipe lengths, deflection is virtually constant until some critical value of delaminated area is reached. Beyond this, there is a sharp upturn in deflection needed to cause failure, which is thought to be due to the growth of one favourable delamination as the failure process continues. However, though the trend in behaviour is identical, Fig. 7 shows that absolute values of deflection and delamination area are very different. For example, the critical value at which the sharp upturn in deflection occurs after the plateau region is found at a delaminated area of 0.002 m 2 and a deflection of 23 mm for specimens of test length 150 mm. For a specimen of test length 500 mm, though, this occurs at a much bigger delaminated area of around 0"05 m 2 for a longer deflection of around 41 mm. Thus, again, some form of scaling procedure would be required if absolute values of residual properties were to be predicted. The overall comment for this section of the work carried out, then, shows that absolute values of residual mechanical properties cannot be predicted by only using tests carried out on shorter samples. However, it is possible to discover trends in residual behaviour, thus indicating that it is necessary to use scaling rules in combination with experimental testing to predict behaviour. This agrees with work reported by Cantwell and Morton, who say that care must be taken when using small samples to predict and characterise the impact response of larger samples, 9 at least for tests carried out beyond the initial first failure point, which is caused by delamination inititation.
5 CONCLUSIONS Regardless of support condition, test type, shape and now specimen length, GRP filament-wound pipes undergo a two-part failure process. The first failure occurs at the same load for the same energy even if the specimen length is considerably reduced for the pipe wall thickness (5.94 mm)
employed here. Therefore, this indicates that if this point, which marks the onset of delamination initiation, is of interest, it is possible to perform tests on very short specimens to gain exact information on the behaviour of much longer pipes. This will result in a considerable saving in material and machinery costs. However, for behaviour beyond this first failure, the length of the specimen becomes a more important parameter for both initial and residual mechanical properties. Then it is no longer possible to perform tests on small samples to gain exact information on behaviour of larger pipes although trends in residual pipe behaviour can be readily observed. This indicates the need for mathematical scaling to be employed in tandem with experimental testing to predict the behaviour of larger pipes from small pipes when tested to beyond delamination initiation.
REFERENCES 1. Alderson, K. L. & Evans, K. E., Low velocity transverse impact of filament-wound pipes: Part 1. Damage due to static and impact loads. Comp. Struct., 20 (1992) 37-45. 2. Evans, K. E. & Alderson, K. L., Low velocity transverse impact of filament-wound pipes: Part 2. Residual properties and correlations with impact damage. Comp. Struct., 20 (1992) 47-52. 3. Alderson, K. L. & Evans, K. E., Failure mechanisms during the transverse loading of filament-wound pipes under static and low velocity impact conditions. Composites, 23 (1992) 167-73. 4. Evans, K. E., Alderson, K. L. & Marks, P. R., Modelling of the transverse loading of filament wound pipes. Computers and Structures, 45 (1992) 1089-95. 5. Alderson, K. L. & Evans, K. E., Dynamic analysis of filament wound pipes undergoing low velocity transverse impact. Comp. Sci. &Tech., 45 (1992) 17-22. 6. Cantwell, W. J. & Morton, J., Detection of impact damage in CFRP laminates. Comp. Struct., 3 (1985) 241-57. 7. Wavin Repox B.V., JC Kellerlaan 3, PO Box 5, 7770 AA Hardenberg, The Netherlands. 8. Swanson, S. R., Mechanisms of transverse impact in fiber composite plates and cylinders. J. Reinforced Plastics and Composites, 12 (1993) 256-67. 9. Cantweli, W. J. & Morton, J., The impact response of composite materials -- A review. Composites, 22 ( 1991 ) 347-62. 10. Qian, Y. & Swanson, S. R., Experimental measurement of impact response in carbon/epoxy plates. AIAA Journal, 28 (1990) 1069-74. 11. Swanson, S. R., Scaling of impact damage in fiber reinforced composites from laboratory specimens to structures. Comp. Struct., 25 (1993) 249-55. 12. Ainsworth, K. L., Low velocity transverse impact damage of filament-wound E-glass/epoxy resin pipes. PhD thesis, University of Liverpool, 1990.