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Impact perforation of thin stiffened CFRP panels
Composite Structures 48 (2000) 95±98
Impact perforation of thin stiened CFRP panels M.S. Found *, I.C. Howard, A.P. Paran SIRIUS, Department of Mechanical Engineering, University of Sheeld, Mappin Street, Sheeld S1 3JD, UK
Abstract Static indentation and dropweight impact tests have been performed on thin CFRP panels stiened with three T-blade stieners and comparisons made with similar tests on plain panels. The impact perforation threshold energy may be estimated from the work done by the indentor during a static indentation test for both plain and stiened panels. For impacts conducted in-line with the stiener the perforation energy is signi®cantly increased and the maximum damage levels occur at higher energies than for other panel loading conditions. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Static indentation; Impact; Perforation; Stiened panels
1. Introduction Due to the lack of adequate predictive methods for determining the damage tolerance of composite structures many present designs tend to be conservative such that the potential weight-saving is not achieved. Improvements in resistance to impact damage may be obtained with the use of a thin membrane to absorb the energy with structural stiness being provided by other means. Previously this has often involved the use of a honeycomb core with CFRP skins to provide a sandwich section. However in service such structures, if damaged, prove dicult to repair. Therefore the use of local stieners has been recognised as an alternative means of providing structural stiness for thin membranes [1±5]. Whilst there are no known references on the impact perforation of stiened panels attempts to model perforation of CFRP plain panels have been undertaken by a number of workers based on energy considerations. Cantwell and Morton [6] showed that elastic deformations, delaminations and shear-out were the main mechanisms, whilst Delfosse and Poursartip [7] identi®ed the proportions of matrix damage and ®bre damage absorbed at perforation. In a previous paper on stiened panels [1] we showed that for panels subjected to impact energies just sucient to produce damage when loaded between stieners that backface cracking and delamination area appeared *
Corresponding author. E-mail address: m.s.found@sheeld.ac.uk (M.S. Found)
to increase approximately linearly with small increases in incident energy. Here the work is extended on similar panels, but of constant stiener spacing, up to penetration of the panel by the indentor. Static indentation and dropweight impact tests are reported for panels loaded both between and in-line with the stieners and comparisons made with plain panels of similar lay-up. The paper suggests a means of predicting the impact perforation threshold energy based on the static perforation energy associated with a static indentation test.
2. Experimental CFRP panels nominally 350 ´ 350 mm, stiened with three parallel T-blades of 100 mm pitch were supplied by Hurel-Dubois UK. The blades measured 25 mm wide´ 12.5 mm deep and the webs were produced from two plies back-to-back with the ends bent at 90° to form a single thickness for the ¯ange. The panels were laid up in three plies as (0/90, 45, 0/90) and 0/90 stieners added, each panel being moulded in one shot by Hurel-Dubois at a nominal 58% ®bre volume fraction (see Fig. 1). Plain panels of the same three-ply lay-up were also supplied for comparison. The material was a ®ve-harness satin weave carbon ®bre preimpregnated with an epoxy resin designated 914C-713-40 and supplied by Hexcel Composites. An instrumented dropweight impact rig, described in Refs. [8,9], was used for both static indentation tests and low velocity impact tests. The panels were clamped to the same ring pressure using two annular rings ranging
0263-8223/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 8 2 2 3 ( 9 9 ) 0 0 0 7 9 - 3
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M.S. Found et al. / Composite Structures 48 (2000) 95±98
Fig. 1. Stiened panel geometry.
from 100 to 300 mm internal diameter, the lower ring having slots to accept the webs of the stieners. Dropweight impact tests were conducted from a height of 0.5 m to produce an impact velocity of the impactor of about 3 m/s. The impact forces and displacements were obtained from data that was processed through a digital low-pass ®lter set at a cut-o frequency of 3.5 kHz [9]. Static indentation and impact tests were performed on the stiened panels at three dierent locations namely; in the bay between the stieners, at the toe of stieners and directly in-line with stieners. Damage was assessed using x-radiography and microscopy techniques in order to determine the principal failure mechanisms of backface cracking, delamination and permanent indentation of the frontface.
3. Results and discussion Fig. 2 shows typical force±displacement plots for plain and stiened panels clamped at 100 mm diameter obtained from static indentation tests. Note that for panels of this size, indentation between the stieners is very similar to indentation on a plain panel since only a small length of the toes of adjacent stieners are encompassed within the test diameter. Hence, additional tests between stieners for this panel size were not deemed necessary. When the load is applied directly above the stiener the initial response is dominated by the stiener and the load±de¯ection response is essen-
Fig. 2. Static load±de¯ection response for 100 mm plain and stiened panels.
tially linear. The sudden large load reduction of the stiened panel is associated with a vertical crack in the web of the stiener immediately below the point of contact of the indentor. However, the load increases again as the rest of the panel responds at reduced stiness, but still greater than that of a plain panel, to produce a maximum force of more than twice that of a plain panel. The second small damage load identi®es onset of damage in the panel in the form of backface cracking and remains dominant up to about the average peak force when delaminations become dominant. At the start of the downside of the load±displacement plot ®bre fracture is signi®cant leading to perforation of the panel. Perforation of the stiened panel occurs by combination of the failure of the skin and failure of the fractured stiener by local buckling and or crushing. The fractured portion of the web appeared to obstruct the progress of the indentor and impeded the perforation of the panel. Fig. 3 shows the in¯uence of increasing the clamping ring and hence test panel size up to 300 mm such that testing at a position between the stieners is now dierent to the response of a plain panel. Whilst the peak force is similar for the plain panel and for testing between the stieners of a stiened panel a greater indentor displacement is required to perforate the plain panels. Hence the static perforation energy is higher for the plain panels due to their ability to store more energy. Comparing Figs. 2 and 3 shows that increasing the panel diameter makes the panels more compliant reducing the peak force on the stiener but increasing the displacement and perforation energy. Fig. 4 shows a plot of normalised peak force for indentation on the stiener for the 300 mm panel identi®ed in Fig. 3. Often the peak force is not clearly de®ned and therefore an average peak force is determined. This is obtained from the area under the force±displacement curve from the second damage load to the correspond-
Fig. 3. Static load±de¯ection response for 300 mm plain and stiened panels.
M.S. Found et al. / Composite Structures 48 (2000) 95±98
97
Fig. 4. Normalised static indentation plot for 300 mm stiened panel loaded in-line with stiener.
Fig. 5. Backface cracking of 300 mm plain and stiened panels.
ing position on the downside of the plot. The second damage load is identi®ed by the small change in the upside of the plot and is associated with damage in the panel which occurs at approximately 85% of the peak force. Note that the ®rst damage load is associated with cracking of the web of the stiener. Also shown in Fig. 4 is the normalised work done by the indentor during the test. The static perforation energy of the panel may be estimated in terms of maximum work done by the indentor to produce upperbound and lowerbound values. The former is obtained from the crossover of the two plots in Fig. 4 which occurs at approximately 82% of the maximum work done by the indentor to give an estimated static perforation energy of 5.58 J when the panel is tested in-line with the stiener. The lower value is determined from the work done by the indentor at the median displacement associated with the average peak force which occurs at approximately 60% of the maximum work done to produce an estimated static perforation energy of 4.12 J. The static perforation energy also appears to be related to the impact perforation threshold as shown in Table 1. The upperbound values of the static perforation energy appear to give a reasonable estimate of the impact perforation threshold energy. We have previously presented [8] energy maps as a means of identifying the dierent mechanisms that develop from initial damage to perforation for plain panels subjected to impact. In this paper Figs. 5±7 represent energy diagrams relating
to the principal failure mechanisms of stiened panels compared with plain panels of 300 mm diameter. Fig. 5 shows that whilst the maximum backface crack length is not signi®cantly dierent for each of the test conditions, that when tested in-line with a stiener the largest damage occurs at a much higher energy and that the perforation energy is also greater. The backface cracking presented in Fig. 5 refers to damage in the actual panel and is the sum of the maximum crack lengths in both the 0° and 90° directions. For a stiened panel subjected to impact on a stiener, there is additionally a vertical crack in the web of the stiener which occurs prior to
Fig. 6. Delamination of 300 mm plain and stiened panels.
Table 1 Comparison of lower and upperbound static perforation energies with impact perforation energies for 300 mm panels Panel type and indentor location
Plain panel
Stiened panel Between stieners
On stiener
Static perforation energy J
1.82 2.50 2.75
1.85 2.44 2.73
4.12 5.58 5.62
Impact perforation energy J
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M.S. Found et al. / Composite Structures 48 (2000) 95±98
eners is similar to that of plain panels. As expected, the perforation energy is signi®cantly increased when tests are conducted in-line with the stieners. The principal failure mechanisms of backface cracking, delamination and permanent indentation of the frontface have been identi®ed and represented in the form of energy diagrams. For panels loaded in-line with a stiener the maximum damage levels occur at higher energies than for the other panel conditions, however, the damage is preceded by vertical cracking of the web of the stiener.
References Fig. 7. Permanent indentation of 300 mm plain and stiened panels.
damage in the panel as earlier identi®ed for the static case. Figs. 6 and 7 for projected delamination area and permanent indentation of the frontface of the panels, respectively, show similar trends to those identi®ed in Fig. 5 for backface cracking. They con®rm that stiened panels subjected to impact between the stieners behave in a similar manner to that of plain panels. 4. Conclusions Static indentation and dropweight impact tests have been conducted on thin CFRP T±blade stiened panels and comparisons made with similar plain panels. The static perforation energy may be estimated from the work done by the indentor and the values used to estimate the impact perforation threshold energy. The response of stiened panels loaded between the sti-
[1] Found MS, Howard IC, Paran AP. Impact behaviour of stiened CFRP sections. Composite Structures 1997;39(3/4):229±35. [2] Wiggenraad JFM, Aoki R, Gadke M, Greenhalgh E, Hachenberg D, Wolf K, Bubl R. Damage propagation in composite structural elements ± analysis and experiments on structures. Composite Structures 1996;36:173±86. [3] Greenhalgh E, Bishop SM, Bray D, Hughes D, Lahi S, Millson B. Characterisation of impact damage in skin-stringer composite structures. Composite Structures 1996;36:187±207. [4] Gadke M, Geier B, Goetting HC, Klein H, Rohwer K, Zimmermann R. Damage in¯uence on the buckling load of CFRP stringerstiened panels. Composite Structures 1996;36:249±75. [5] Stevens KA, Ricci R, Davies GAO. Buckling and postbuckling of composite structures. Composite Structures 1995;26:189±99. [6] Cantwell WJ, Morton J. Impact perforation of carbon ®bre reinforced plastic. Composite Sci Technol 1990;38(2):119±41. [7] Delfosse D, Poursartip A. Energy-based approach to impact damage in CFRP aminates. Composites A 1997;28:647±55. [8] Found MS, Howard IC, Paran AP. Size eects in thin CFRP panels subjected to impact. Composite Structures 1997;38(1± 4):599±607. [9] Found MS, Howard IC, Paran AP. Interpretation of signals from dropweight impact tests. Composite Structures 1998;42:353±63.
Impact perforation of thin stiened CFRP panels M.S. Found *, I.C. Howard, A.P. Paran SIRIUS, Department of Mechanical Engineering, University of Sheeld, Mappin Street, Sheeld S1 3JD, UK
Abstract Static indentation and dropweight impact tests have been performed on thin CFRP panels stiened with three T-blade stieners and comparisons made with similar tests on plain panels. The impact perforation threshold energy may be estimated from the work done by the indentor during a static indentation test for both plain and stiened panels. For impacts conducted in-line with the stiener the perforation energy is signi®cantly increased and the maximum damage levels occur at higher energies than for other panel loading conditions. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Static indentation; Impact; Perforation; Stiened panels
1. Introduction Due to the lack of adequate predictive methods for determining the damage tolerance of composite structures many present designs tend to be conservative such that the potential weight-saving is not achieved. Improvements in resistance to impact damage may be obtained with the use of a thin membrane to absorb the energy with structural stiness being provided by other means. Previously this has often involved the use of a honeycomb core with CFRP skins to provide a sandwich section. However in service such structures, if damaged, prove dicult to repair. Therefore the use of local stieners has been recognised as an alternative means of providing structural stiness for thin membranes [1±5]. Whilst there are no known references on the impact perforation of stiened panels attempts to model perforation of CFRP plain panels have been undertaken by a number of workers based on energy considerations. Cantwell and Morton [6] showed that elastic deformations, delaminations and shear-out were the main mechanisms, whilst Delfosse and Poursartip [7] identi®ed the proportions of matrix damage and ®bre damage absorbed at perforation. In a previous paper on stiened panels [1] we showed that for panels subjected to impact energies just sucient to produce damage when loaded between stieners that backface cracking and delamination area appeared *
Corresponding author. E-mail address: m.s.found@sheeld.ac.uk (M.S. Found)
to increase approximately linearly with small increases in incident energy. Here the work is extended on similar panels, but of constant stiener spacing, up to penetration of the panel by the indentor. Static indentation and dropweight impact tests are reported for panels loaded both between and in-line with the stieners and comparisons made with plain panels of similar lay-up. The paper suggests a means of predicting the impact perforation threshold energy based on the static perforation energy associated with a static indentation test.
2. Experimental CFRP panels nominally 350 ´ 350 mm, stiened with three parallel T-blades of 100 mm pitch were supplied by Hurel-Dubois UK. The blades measured 25 mm wide´ 12.5 mm deep and the webs were produced from two plies back-to-back with the ends bent at 90° to form a single thickness for the ¯ange. The panels were laid up in three plies as (0/90, 45, 0/90) and 0/90 stieners added, each panel being moulded in one shot by Hurel-Dubois at a nominal 58% ®bre volume fraction (see Fig. 1). Plain panels of the same three-ply lay-up were also supplied for comparison. The material was a ®ve-harness satin weave carbon ®bre preimpregnated with an epoxy resin designated 914C-713-40 and supplied by Hexcel Composites. An instrumented dropweight impact rig, described in Refs. [8,9], was used for both static indentation tests and low velocity impact tests. The panels were clamped to the same ring pressure using two annular rings ranging
0263-8223/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 8 2 2 3 ( 9 9 ) 0 0 0 7 9 - 3
96
M.S. Found et al. / Composite Structures 48 (2000) 95±98
Fig. 1. Stiened panel geometry.
from 100 to 300 mm internal diameter, the lower ring having slots to accept the webs of the stieners. Dropweight impact tests were conducted from a height of 0.5 m to produce an impact velocity of the impactor of about 3 m/s. The impact forces and displacements were obtained from data that was processed through a digital low-pass ®lter set at a cut-o frequency of 3.5 kHz [9]. Static indentation and impact tests were performed on the stiened panels at three dierent locations namely; in the bay between the stieners, at the toe of stieners and directly in-line with stieners. Damage was assessed using x-radiography and microscopy techniques in order to determine the principal failure mechanisms of backface cracking, delamination and permanent indentation of the frontface.
3. Results and discussion Fig. 2 shows typical force±displacement plots for plain and stiened panels clamped at 100 mm diameter obtained from static indentation tests. Note that for panels of this size, indentation between the stieners is very similar to indentation on a plain panel since only a small length of the toes of adjacent stieners are encompassed within the test diameter. Hence, additional tests between stieners for this panel size were not deemed necessary. When the load is applied directly above the stiener the initial response is dominated by the stiener and the load±de¯ection response is essen-
Fig. 2. Static load±de¯ection response for 100 mm plain and stiened panels.
tially linear. The sudden large load reduction of the stiened panel is associated with a vertical crack in the web of the stiener immediately below the point of contact of the indentor. However, the load increases again as the rest of the panel responds at reduced stiness, but still greater than that of a plain panel, to produce a maximum force of more than twice that of a plain panel. The second small damage load identi®es onset of damage in the panel in the form of backface cracking and remains dominant up to about the average peak force when delaminations become dominant. At the start of the downside of the load±displacement plot ®bre fracture is signi®cant leading to perforation of the panel. Perforation of the stiened panel occurs by combination of the failure of the skin and failure of the fractured stiener by local buckling and or crushing. The fractured portion of the web appeared to obstruct the progress of the indentor and impeded the perforation of the panel. Fig. 3 shows the in¯uence of increasing the clamping ring and hence test panel size up to 300 mm such that testing at a position between the stieners is now dierent to the response of a plain panel. Whilst the peak force is similar for the plain panel and for testing between the stieners of a stiened panel a greater indentor displacement is required to perforate the plain panels. Hence the static perforation energy is higher for the plain panels due to their ability to store more energy. Comparing Figs. 2 and 3 shows that increasing the panel diameter makes the panels more compliant reducing the peak force on the stiener but increasing the displacement and perforation energy. Fig. 4 shows a plot of normalised peak force for indentation on the stiener for the 300 mm panel identi®ed in Fig. 3. Often the peak force is not clearly de®ned and therefore an average peak force is determined. This is obtained from the area under the force±displacement curve from the second damage load to the correspond-
Fig. 3. Static load±de¯ection response for 300 mm plain and stiened panels.
M.S. Found et al. / Composite Structures 48 (2000) 95±98
97
Fig. 4. Normalised static indentation plot for 300 mm stiened panel loaded in-line with stiener.
Fig. 5. Backface cracking of 300 mm plain and stiened panels.
ing position on the downside of the plot. The second damage load is identi®ed by the small change in the upside of the plot and is associated with damage in the panel which occurs at approximately 85% of the peak force. Note that the ®rst damage load is associated with cracking of the web of the stiener. Also shown in Fig. 4 is the normalised work done by the indentor during the test. The static perforation energy of the panel may be estimated in terms of maximum work done by the indentor to produce upperbound and lowerbound values. The former is obtained from the crossover of the two plots in Fig. 4 which occurs at approximately 82% of the maximum work done by the indentor to give an estimated static perforation energy of 5.58 J when the panel is tested in-line with the stiener. The lower value is determined from the work done by the indentor at the median displacement associated with the average peak force which occurs at approximately 60% of the maximum work done to produce an estimated static perforation energy of 4.12 J. The static perforation energy also appears to be related to the impact perforation threshold as shown in Table 1. The upperbound values of the static perforation energy appear to give a reasonable estimate of the impact perforation threshold energy. We have previously presented [8] energy maps as a means of identifying the dierent mechanisms that develop from initial damage to perforation for plain panels subjected to impact. In this paper Figs. 5±7 represent energy diagrams relating
to the principal failure mechanisms of stiened panels compared with plain panels of 300 mm diameter. Fig. 5 shows that whilst the maximum backface crack length is not signi®cantly dierent for each of the test conditions, that when tested in-line with a stiener the largest damage occurs at a much higher energy and that the perforation energy is also greater. The backface cracking presented in Fig. 5 refers to damage in the actual panel and is the sum of the maximum crack lengths in both the 0° and 90° directions. For a stiened panel subjected to impact on a stiener, there is additionally a vertical crack in the web of the stiener which occurs prior to
Fig. 6. Delamination of 300 mm plain and stiened panels.
Table 1 Comparison of lower and upperbound static perforation energies with impact perforation energies for 300 mm panels Panel type and indentor location
Plain panel
Stiened panel Between stieners
On stiener
Static perforation energy J
1.82 2.50 2.75
1.85 2.44 2.73
4.12 5.58 5.62
Impact perforation energy J
98
M.S. Found et al. / Composite Structures 48 (2000) 95±98
eners is similar to that of plain panels. As expected, the perforation energy is signi®cantly increased when tests are conducted in-line with the stieners. The principal failure mechanisms of backface cracking, delamination and permanent indentation of the frontface have been identi®ed and represented in the form of energy diagrams. For panels loaded in-line with a stiener the maximum damage levels occur at higher energies than for the other panel conditions, however, the damage is preceded by vertical cracking of the web of the stiener.
References Fig. 7. Permanent indentation of 300 mm plain and stiened panels.
damage in the panel as earlier identi®ed for the static case. Figs. 6 and 7 for projected delamination area and permanent indentation of the frontface of the panels, respectively, show similar trends to those identi®ed in Fig. 5 for backface cracking. They con®rm that stiened panels subjected to impact between the stieners behave in a similar manner to that of plain panels. 4. Conclusions Static indentation and dropweight impact tests have been conducted on thin CFRP T±blade stiened panels and comparisons made with similar plain panels. The static perforation energy may be estimated from the work done by the indentor and the values used to estimate the impact perforation threshold energy. The response of stiened panels loaded between the sti-
[1] Found MS, Howard IC, Paran AP. Impact behaviour of stiened CFRP sections. Composite Structures 1997;39(3/4):229±35. [2] Wiggenraad JFM, Aoki R, Gadke M, Greenhalgh E, Hachenberg D, Wolf K, Bubl R. Damage propagation in composite structural elements ± analysis and experiments on structures. Composite Structures 1996;36:173±86. [3] Greenhalgh E, Bishop SM, Bray D, Hughes D, Lahi S, Millson B. Characterisation of impact damage in skin-stringer composite structures. Composite Structures 1996;36:187±207. [4] Gadke M, Geier B, Goetting HC, Klein H, Rohwer K, Zimmermann R. Damage in¯uence on the buckling load of CFRP stringerstiened panels. Composite Structures 1996;36:249±75. [5] Stevens KA, Ricci R, Davies GAO. Buckling and postbuckling of composite structures. Composite Structures 1995;26:189±99. [6] Cantwell WJ, Morton J. Impact perforation of carbon ®bre reinforced plastic. Composite Sci Technol 1990;38(2):119±41. [7] Delfosse D, Poursartip A. Energy-based approach to impact damage in CFRP aminates. Composites A 1997;28:647±55. [8] Found MS, Howard IC, Paran AP. Size eects in thin CFRP panels subjected to impact. Composite Structures 1997;38(1± 4):599±607. [9] Found MS, Howard IC, Paran AP. Interpretation of signals from dropweight impact tests. Composite Structures 1998;42:353±63.