Normalizing impact energy by face sheet thickness for composite sandwich structure compression after impact testing

Abstract

The amount of impact energy used to damage a composite laminate face sheet of a sandwich structure is a critical parameter when assessing residual compression strength. The compression after impact strength of impacted laminates used as face sheets on honeycomb core sandwich structure is dependent upon how thick the face sheet laminate is and this has traditionally been accounted for by normalizing (dividing) the impact energy by the laminate’s thickness. However when comparing compression after impact strength values for a given lay-up sequence and fiber/resin system, dividing the impact energy by the specimen thickness has been noted by the author to give higher compression after impact strength values for thicker face sheet laminates. A study was thus undertaken to assess the comparability of compression after impact strength data of sandwich structure by normalizing the impact energy by the face sheet thickness raised to a power to account for the higher strength of thicker laminates. Two data sets generated in this study were analyzed by dividing the impact energy by the face sheet thickness to the 1.0, 1.5, 2.0 and 2.5 powers. Results show that raising the face sheet thickness to a power of approximately 2.5 and dividing the impact energy by this quantity yields more comparable compression after impact strength data for comparing 8- and 16-ply face sheet laminates. For comparison of 24-ply face sheet laminates to 8- or 16-ply face sheet laminates, a value closer to 2 was found to give more comparable compression after impact strength data.

Keywords

Impact damage tolerance compression after impact composite laminate honeycomb

Introduction

By using carbon/epoxy face sheets with honeycomb core, a lightweight sandwich structure that is strong and stiff can be achieved. If the sandwich structure is to experience compressive loads during use, then the compression after impact (CAI) strength of the sandwich structure may be a critical design driver. Although the face sheets on honeycomb sandwich structures are typically considered “thin,” the thickness of the laminate that comprises the face sheet for different sandwich structures can vary. If a comparison of CAI strength of various thicknesses of face sheets of a sandwich structure are to be made, the face sheet thickness must be taken into account as a thicker laminate will obviously have a higher CAI strength than thinner laminates for a given impact energy. A direct comparison of CAI strength values of sandwich structure with different thickness face sheets was desired for launch vehicle interstage and fairing sandwich structure in an effort to reduce the amount of testing needed for the ARES I crew launch vehicle program. This was the motivation behind this study.

In damage tolerance studies involving composite laminates (not necessarily used as face sheets in sandwich structure), it has not been uncommon to divide the impact energy by the laminate thickness to “normalize” data [1 –15]. While simplistic, this normalization of the impact energy data will certainly result in more comparable damage tolerance data across different laminate thicknesses. A simple linear correlation of impact energy with laminate thickness may cause damage tolerance data to be qualitatively more comparable, but a physical basis for doing so is not readily apparent. In a test for damage resistance (perforation) of plastic film (ASTM D-1709-09) [16] it is cautioned that: “The impact resistance of plastic film while partly dependent on thickness has no simple correlation with sample thickness. Hence, impact values cannot be normalized over a range of thickness without producing misleading data as to the actual impact resistance of the material. Data from these test methods are comparable only for specimens that vary by no more than ±25% from the nominal or average thickness of the specimens tested.”

Guynn and O’Brien [2] observed that with respect to CAI strength, thicker laminates failed at a higher compressive strength than thinner laminates for a given amount of impact energy per unit thickness of laminate. This was partially attributed to the thin laminate having different damage morphology. This result, along with the author’s experience that CAI data tends to be higher for thicker face sheets on sandwich structure even when the impact energy is normalized by the face sheet thickness was the inspiration for this study. In this paper, the results of a systematic experimental study was performed to support or refute the premise that dividing the impact energy by the face sheet thickness to a power other than 1.0 will give more uniform results for CAI strength data of composite sandwich structure of similar face sheet lay-up but different thicknesses.

It should be noted that detailed information about failure mechanisms both due to the impact event and during compression to failure testing are not presented in this study and only impact energies and associated CAI strengths of the sandwich structure are presented since these are the values of interest for the premise of this paper.

Experiment

Material

CAI testing was performed on honeycomb sandwich structure with laminated face sheets of 8, 16 and 24 plies composed of IM7/MTM-45 carbon/epoxy. Table 1 gives the specific lay-ups and nominal thicknesses of the face sheet laminates manufactured. These face sheets were cured before bonding to the honeycomb core.

Table 1.

Lay-up of laminates used as face sheets in this study.

Material	Lay-up	Number of plies	Nominal laminate thickness (mm)
IM7/MTM-45	[+45, 0, −45, 90]_S	8	1.09
IM7/MTM-45	[+45, 0, −45, 90]_2S	16	2.18
IM7/MTM-45	[+45, 0, −45, 90]_3S	24	3.28

Laminates were manufactured by curing 61 cm square sections of prepreg in a heated platen press according to the manufacturer’s recommended cure cycle. After cure, these panels had one side prepared for bonding to aluminum honeycomb core since the four-point bend method was to be utilized to assess CAI strength [17]. The bottom (tensile) face sheet was identical to the top face sheet. The core density used for these specimens was 192 kg/m³ and the core thickness was 3.81 cm. The top and bottom face sheets were bonded to the aluminum honeycomb with FM-300 film adhesive. Once these bonds had cured, the panels were cut into 5.1 cm wide test specimens. A schematic of a specimen is shown in Figure 1.

Impact testing

Each sandwich specimen was impacted at its geometric center on the top (compression) face sheet. The impactor had a diameter of 6.4 mm and the specimen was placed on a solid steel plate during impact to give the highest rigidity, and thus most damage possible for a given impact energy [18,19]. This also ensured similar boundary conditions for all impacts. An instrumented drop weight impact apparatus was used to inflict damage to the specimens. The impact energy was measured by measuring the velocity of the impactor at the point of contact with the specimen since some of the initial potential energy of the drop weight was lost due to friction with the guide posts. A schematic of the impact apparatus used is given in Figure 2.

While not explicitly used in this particular part of the study, the extent of post-impact damage was assessed using infrared thermography (IRT), which can give an indication of the planar size of damage. Examples of the smallest and largest damage sizes measured are shown in Figure 3. Ideally, the specimen width would have been at least six times the damage diameter [1], however, since the CAI strength data in this study were to be directly compared to each other, any edge-damage interaction effects should be about the same for all specimens.

As mentioned previously, the four-point bend method [17] was utilized to generate compressive forces in the damaged face sheet. This methodology was chosen over end loading since an abundance of prepreg, film adhesive and honeycomb core was available while strain gages were not and the end loading method requires four strain gages per specimen [19].

The cross head rate used was 2.5 mm/min which caused the typical test to last approximately 1–2 min. All failures were similar in that fiber fracture occurred across the width of the upper face sheet through the point of impact.

From multiple tests for face sheet modulus (a strain gaged specimen taken to 30% of predicted ultimate failure load, removed from load frame and reloaded), the four-point bend testing methodology used for this study demonstrated repeatability within 2% assuming the modulus of the face sheet laminate did not change. The variability is most likely from imperfect specimen placement since the load cell used is rated for accuracy within 0.25% of the measured load. Other sources of variability may be present when calculating ultimate stress, however, the CAI strength results from this study were to be compared to each other and only the face sheet thickness and impact energy were varied for each specimen, thus any variability associated with calculation of stress from a breaking load should be fairly uniform across all of the specimens in this study. Thus caution should be used if comparing the CAI stress values from this study to those obtained from other studies.

Results

The results from the residual compression strength testing for the IM7/MTM-45 face sheet material are given in Table 2 (including plus or minus one standard deviation) and the results plotted in Figure 4. The average CAI strength values for each of the three thicknesses have a “best fit” power curve applied.

In Figure 4, low, medium and high impact severity levels are noted. While rather arbitrary, these levels may help give information about how impact severity affects the CAI strength to be compared. Each severity level chosen will require extrapolation of some of the data. Using specimens that were not impacted, the un-damaged compression strength of this laminated face sheet was measured as 690 MPa and using specimens with a 6.4 mm diameter hole, the open hole compression (OHC) strength measured as 270 MPa. These are the approximate upper and lower compression strength limits to curve extrapolation. Using the three damage severity levels shown in Figure 4 with the curve fit equations in Figure 4 yields the results presented in Table 3. Note that these are not directly measured values but rather values as inferred by the “best fit” power curves given in Figure 4.

Table 2.

Results from compression after impact (CAI) testing of sandwich specimens with face sheet laminates of IM7/MTM-45.

Number of plies	Thickness (mm)	Impact energy (J)	Number of specimens	CAI strength ± SD (MPa)
8	1.11	1.5	1	350
8	1.11	2.7	3	306 ± 29
8	1.11	3.8	4	290 ± 19
16	2.18	2.9	4	425 ± 17
16	2.18	5.3	3	344 ± 19
16	2.18	7.7	3	319 ± 18
24	3.28	4.5	4	477 ± 43
24	3.28	8.0	3	390 ± 7.3
24	3.28	11.2	3	354 ± 16

Table 3.

Difference in inferred compression after impact (CAI) strength values of un-normalized data using damage severity levels and curve fit equations in Figure 4. (CAI = f(IE)).

Impact severity level = IE	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply	24-Ply CAI strength (MPa)	% Increase over 8-ply	% Increase over 16-ply
Low = 2 J	315	414	31	549	74	33
Med = 3.5 J	289	354	22	456	58	29
High = 5.5 J	270	312	16	393	46	26

As expected, the sandwich structure with thicker face sheets have higher CAI strength for a given impact severity level. Of note is that as the impact severity level increases, the percent difference in CAI strength between the laminates of different thicknesses decreases. This makes physical sense as more severe impacts are closer to impactor penetration. Complete penetration is akin to an OHC test.

Rather than simply normalizing impact energy by specimen thickness (i.e. IE_N= IE/t where t= specimen thickness and IE_N= normalized impact energy), it was assumed that normalizing the impact energy by the specimen thickness raised to a power (denoted by m) such that IE_N= IE/t^m would give more comparable CAI strength results.

By dividing the impact energy by the face sheet thickness to a power of 1, 1.5, 2 or 2.5, the CAI strength data result as shown in Figure 5. In Figure 5, all of the data are pooled for all three face sheet thicknesses and a best fit power curve is applied. The equations and correlation coefficient (R values) for these curves are shown in the respective figures. The closer to 1.0 that R is the better the fit of the data. As with the unnormalized data, arbitrarily chosen low, medium and high impact severity levels are chosen for each of the four plots.

The thickness exponent m appears to fit the data for all three thicknesses across all impact severity levels best at a value of m= 2.0 (R= 0.98). Using m= 2.5 results in overcorrection of the CAI strength data for high impact severity levels (R= 0.95).

A best fit power law for each of the 3 thicknesses (rather than pooling) at each of the 4 values of m were applied; then the low, medium and high impact severity levels denoted in Figure 5 were used to find the inferred CAI strengths. The results are given in Tables 4–7.

Table 4.

Difference in inferred compression after impact (CAI) strength values in Figure 5 with m= 1 (CAI = f (IE/t^1.0)).

Impact severity level = IE/t	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply	24-Ply CAI strength (MPa)	% Increase over 8-ply	% Increase over 16-ply
Low = 1.4 J/mm	332	410	23	473	42	15
Med = 2.4 J/mm	306	353	15	396	29	12
High = 3.5 J/mm	289	318	10	349	21	10

Table 5.

Difference in inferred compression after impact (CAI) strength values in Figure 5 with m= 1.5 (CAI = f (IE/t^1.5)).

Impact severity level = IE/t^1.5	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply	24-Ply CAI strength (MPa)	% Increase over 8-ply	% Increase over 16-ply
Low = 0.75 J/(mm)^1.5	361	438	21	477	32	9
Med = 1.5 J/(mm)^1.5	326	361	11	380	17	5
High = 2.4 J/(mm)^1.5	303	316	4	325	7	3

Table 6.

Difference in inferred compression after impact (CAI) strength values in Figure 5 with m= 2. (CAI = f (IE/t^2.0)).

Impact severity level = IE/t²	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply	24-Ply CAI strength (MPa)	% Increase over 8-ply	% Increase over 16-ply
Low = 0.4 J/(mm)²	395	467	18	483	22	3
Med = 1.1 J/(mm)²	339	351	4	346	2	−1
High = 1.75 J/(mm)²	316	309	−2	297	−6	−4

Table 7.

Difference in inferred compression after impact (CAI) strength values in Figure 5 with m= 2.5. (CAI = f (IE/t^2.5)).

Impact severity level = IE/t^2.5	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply	24-Ply CAI strength (MPa)	% Increase over 8-ply	% Increase over 16-ply
Low = 0.25 J/(mm)^2.5	420	478	14	463	10	−3
Med = 0.75 J/(mm)^2.5	356	351	−1	322	−10	−8
High = 1.25 J/(mm)^2.5	330	304	−8	272	−18	−11

For each of the three pairs of face sheet laminate thicknesses (i.e. 16 ↔ 24, 8 ↔ 16 and 8 ↔ 24), the percent difference in inferred CAI strengths with respect to the four thickness exponents are summarized in Tables 8–10. If the medium impact severity level is used as a baseline, then the exponent that gives a percent difference closest to zero at the medium impact severity level should be chosen for normalization of face sheet thickness. For the data given, the impact energy data should be normalized (divided) by the face sheet thickness to the 2.0 power for comparing 16-ply and 24-ply data, to the 2.5 power for comparing 8-ply and 16-ply CAI data, and to the 2.0 power for comparing 8-ply and 24-ply data.

Table 8.

Percent difference in inferred compression after impact (CAI) strength values between 16- and 24-ply face sheet specimens for various IE/t^m values.

Comparison of percent difference in CAI strength for 16- and 24-ply face sheets (50% increase in thickness)
Impact severity level	Un-normalized	m= 1.0	m= 1.5	m= 2.0	m= 2.5
Low	33	15	9	3	−3
Med	29	12	5	−1	−8
High	26	10	3	−4	−11

Table 9.

Percent difference in inferred compression after impact (CAI) strength values between 8 and 16-ply face sheet specimens for various IE/t^m values.

Comparison of percent difference in CAI Strength for 8 and 16-ply face sheets (100% increase in thickness)
Impact severity level	Un-normalized	m= 1.0	m= 1.5	m= 2.0	m= 2.5
Low	31	23	21	18	14
Med	22	15	11	4	−1
High	16	10	+4	−2	−8

Table 10.

Percent difference in inferred compression after impact (CAI) strength values between 8- and 24-ply face sheet specimens for various IE/t^m values.

Comparison of percent difference in CAI Strength for 8- and 24-ply face sheets (200% increase in thickness)
Impact severity level	Un-normalized	m= 1.0	m= 1.5	m= 2.0	m= 2.5
Low	74	42	32	22	10
Med	58	29	17	2	−10
High	46	21	7	−6	−18

Figure 6 is a plot of the average of all the percent differences across all face sheet thicknesses and impact severity levels for any given value of m as a function of m (i.e. columns of Tables 8–10 averaged to one value). Note the large standard deviation values since data across all impact severity levels and thicknesses are being considered.

From the least squares linear fit of the data in Figure 6, the percent difference in CAI strengths is zero at m= 2.2 which is close to the value of m= 2.0 as determined by the visual examination of Figure 5. The CAI strength data plotted as a function of impact energy normalized by specimen thickness to the 2.2 power are given in Figure 7.

Apparently normalizing the impact energy by the face sheet thickness raised to a power of 2.2 as suggested by Figure 6 does not necessarily result in more comparable CAI strength results compared to the visually assessed result from Figure 5 of m= 2.0. Since the value of m differs between 2 and 2.5 depending on laminate thicknesses to be compared, perhaps the most fidelity that can be expected in m may be using m= 2 to give improved CAI strength comparability over m= 1 as is currently done.

Supplementary data

CAI testing was performed on sandwich structure with face sheets composed of 8 and 16 plies of IM7/8551-7 carbon/epoxy to supplement the data presented thus far. Specimen manufacture and testing was similar to that of the IM7/MTM-45 laminates presented earlier. Table 11 gives the results from the testing of the IM7/8551-7 laminates. These CAI strength results are plotted in Figure 8.

Table 11.

Results from compression after impact (CAI) testing of IM7/8551-7 face sheet sandwich specimens.

Number of plies	Thickness (mm)	Impact energy (J)	Number of specimens	CAI strength ± SD (MPa)
8	1.14	0.9	5	377 ± 22
8	1.14	1.5	6	344 ± 16
8	1.14	2.6	5	311 ± 8.0
8	1.14	3.8	5	294 ± 20
16	2.29	3.8	3	438 ± 24
16	2.29	6.2	7	372 ± 16
16	2.29	8.0	3	348 ± 15

Using the three damage severity levels shown in Figure 8 with the equations in Figure 8 yields the results presented in Table 12.

Table 12.

Difference in inferred compression after impact (CAI) strength values at three damage severity levels from Figure 8.

Impact severity level = IE	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply
2 J	328	535	63
4 J	292	431	48
6 J	272	380	40

Without normalization, the 16-ply laminate has between 40% and 63% more strength than the 8-ply laminate, depending on the impact severity level. As the impact severity level increases, the percent difference in CAI strength values between the two different thicknesses used is lessened as the impact damage approaches penetration, a result also found from testing of IM7/MTM-45 material.

If the impact energy is normalized by the specimen thickness raised to a power of 1, 1.5, 2 or 2.5, the data will result as shown in Figure 9. In Figure 9, all of the data are pooled and a best fit power curve is applied.

The thickness exponent m appears to fit the data for both thicknesses across all impact severity levels best at a value of m= 2.5, which is a higher value than that found for the IM7/MTM-45 face sheet laminates, although the same value of m (2.5) did give more comparable CAI strength data between 8- and 16-ply face sheet laminates for the IM7/MTM-45 face sheet material. A best fit power law for each of the two curves at each of the four values of m was applied; then arbitrarily selected low, medium and high impact severity levels (as noted in the figures) were used to infer CAI strengths. The results are given in Tables 13–16.

Table 13.

Difference in inferred compression after impact (CAI) strength values in Figure 9 with m= 1.0. (CAI = f (IE/t^1.0)).

Impact severity level = IE/t	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply
1 J/mm	361	512	42
2.25 J/mm	315	398	26
3.5 J/mm	292	347	19

Table 14.

Difference in inferred compression after impact (CAI) strength values in Figure 9 with m= 1.5. (CAI = f (IE/t^1.5)).

Impact severity level = IE/t^1.5	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply
1 J/(mm)^1.5	357	449	26
1.8 J/(mm)^1.5	323	374	16
2.75 J/(mm)^1.5	301	328	9

Table 15.

Difference in inferred compression after impact (CAI) strength values in Figure 9 with m= 2.0. (CAI = f (IE/t^2.0)).

Impact severity level = IE/t²	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply
0.7 J/(mm)²	375	441	18
1.5 J/(mm)²	329	348	6
2.25 J/(mm)²	308	307	0

Table 16.

Difference in inferred compression after impact (CAI) strength values in Figure 9 with m= 2.5. (CAI = f (IE/t^2.5)).

Impact severity level = IE/t^2.5	8-Ply CAI strength (MPa)	16-Ply CAI strength (MPa)	% Increase over 8-ply
0.5 J/(mm)^2.5	393	431	10
1.1 J/(mm)^2.5	343	338	−1
1.75 J/(mm)^2.5	317	293	−8

For the two laminate thicknesses, the percent difference in CAI strength with respect to the thickness exponent, m, are summarized in Table 17.

Table 17.

Percent difference in inferred compression after impact (CAI) strength values between 16 and 8-ply specimens for various IE/t^m values.

Comparison of percent difference in CAI Strength for 8 and 16-ply laminates (100% increase in thickness)
Impact severity level	Un-normalized	m= 1.0	m= 1.5	m= 2.0	m= 2.5
Low	+63	+42	+26	+18	+10
Med	+48	+26	+16	+6	−1
High	+40	+19	+9	0	−8

An examination of Table 17 shows that, if the medium impact severity level is used as a baseline, the exponent that gives the most comparable CAI strength data across all impact severity levels is m= 2.5.

Figure 10 is a plot of the average of all the percent differences in inferred CAI strengths across all three impact severity levels for any given value of m as a function of m (columns of Table 17 averaged to one value). From a least squares linear fit of the data in Figure 10, the percent difference in CAI strengths is zero at m= 2.3. This value is close to that found by a visual assessment of the plots in Figure 9 and the data in Table 17 which shows that m= 2.5 appears to give the most comparable CAI strength data across all impact severity levels.

The CAI strength data plotted as a function of impact energy normalized by the specimen thickness to the 2.3 power are given in Figure 11. As with the IM7/MTM-45 CAI strength data analyzed earlier, attempting to define m with a higher level of fidelity other than a simple visual examination, such as was done using Figure 9, does not necessarily result in more comparable CAI strength data.

Thus, it can be concluded that averaging the percent differences across all face sheet thicknesses and impact severity levels and inferring the value of m at which the average difference will be zero gives no better results than the visual assessments made by plotting CAI strength to impact energy divided by the specimen thickness raised to a power between 1 and 2.5 in increments of 0.5.

Conclusions

A definitive exponent that the face sheet thickness must be raised to in order to normalize impact energy to give comparable CAI strength results across all impact damage severities for composite sandwich structure probably cannot be achieved. An approximate exponent is the most that can be achieved in an attempt to compare CAI strength data across a range of impact damage severity levels.

As the impact severity level increases, the difference between the un-normalized CAI strengths becomes less, which makes physical sense as the sandwich specimens are essentially nearing the OHC strength of the laminated face sheet.

For the IM7/MTM-45 face sheet material CAI strength data examined in this paper, an exponent of m= 2 appears to give the best overall comparison of CAI strength values across the range of face sheet thicknesses and impact severity levels used. A value of m= 2.5 gave more comparable CAI strength data when comparing the two thinnest laminates tested (8 and 16 ply).

For comparing CAI strength of sandwich structure with 8-ply face sheets and 16-ply face sheets made of IM7/8551-7, an exponent of m= 2.5 appears to give the best overall fit.

The variability in the CAI strength data is mostly due to variability within the specimens rather than the test methodology.

It does appear that normalizing impact energy by face sheet thickness to a power greater than one gives more comparable CAI strength results within the parameters used in this study, however the value of this exponent cannot be achieved with enough fidelity to directly compare CAI strength results for launch vehicle interstage and fairing structure.

Footnotes

Funding

This work was funded by the National Aeronautics and Space Administration under the auspices of the Upper Stage Program Office at Marshall Space Flight Center (136905.08.05.12).

References

Husman GE, Whitney JM and Halpin J. Residual strength characterization of laminated composites subjected to impact loading. In: Foreign object impact damage to composites, ASTM (ed Greszczuk LB), 20 September 1973, paper no. STP 568, pp.92–113. Philadelphia, PA: American Society for Testing and Materials.

Guynn EG and O’Brien TK. The influence of lay-up and thickness on composite impact damage and compression strength. In: Proceedings AIAA/ASME/ASCE/AHS 26th structures, structural dynamics and materials conference, April 15--17 Orlando Florida, 1985; pp.187–196. Reston, VA: American Institute of Aeronautics and astronautics.

Avva VS, Vala JR and Jeyaseelan M. Effect of impact and fatigue loads on the strength of graphite/epoxy composites. In: Composite materials testing and design (seventh conference), ASTM (ed Whitney JM), 2–4 April 1984, paper no. STP 893, pp.196–206. Philadelphia, PA: American Society for Testing and Materials.

Cano RJ and Dow MB. Properties of five toughened matrix composite materials. NASA-TP-3254, 1992.

Strait

Karasek

Amateau

. Effects of stacking sequence on the impact resistance of carbon fiber reinforced thermoplastic toughened epoxy laminates. J Compos Mater 1992; 26(12): 1725–1740.

Bannister M, Callus P and Herszberg I. Compression after impact behavior of multilayer woven glass/vinyl ester composites. In: Proceedings 21st international council of the aeronautical sciences (ed Marec JP), Melbourne, Australia, 13–18 September 1998, pp.5.9.1. (Pages are numbered by Day, Session # and Paper#) Bonn, Germany: International Council of the Aeronautical Sciences.

Ishikawa

Sugimoto

Matsushima

. Some experimental findings in compression-after-impact (CAI) tests of CF/PEEK (APC-2) and conventional CF/Epoxy flat plates. Compos Sci Technol 1995; 55: 349–363.

Tai

Yip

Lin

. Effects of low-energy impact on the fatigue behavior of carbon/epoxy composites. Compos Sci Technol 1997; 58: 1–8.

Duarte

Herszberg

Paton

. Impact resistance and tolerance of interleaved tape laminates. Compos Struct 1999; 47: 753–758.

10.

Khondker

Leong

Herszberg

. Impact and compression-after-impact performance of weft knitted glass textile composites. Compos Part A 2005; 36: 638–648.

11.

Mourtis

. Comment on the impact damage tolerance of stitched composites. J Mater Sci Lett 2003; 22: 519–521.

12.

Byun

J-H

Song

S-W

Lee

C-H

. Impact properties of laminated composites with stitching fibers. Compos Struct 2006; 76: 21–27.

13.

Aoki Y, Kondo H and Hatta H. Effect of specimen thickness on impact-induced delamination and CAI behavior. In: Proceedings 39th international SAMPE technical conference (eds Klosterman DA, Strong KL and Beckwith SW), Cincinnati, OH, 29 October–1 November 2007. Covina, CA: Society for the Advancement of Materials and Processes Engineering.

14.

Uda N and Ono K. Compression fatigue failure of CFRP laminates with impact damage. In: Proceedings 16th international conference on composite materials (eds Kageyama K, Ishikawa T, Takeda N, Hojo M, Sugimoto S and Ogasawara T), Kyoto, Japan, 8–13 July 2007. Kyoto, Japan: Japan Society for Composite Materials.

15.

Hoshi H, Nakano K and Iwahori Y. Study on repair of CFRP laminates for aircraft structures. In: Proceedings 16th international conference on composite materials (eds Kageyama K, Ishikawa T, Takeda N, Hojo M, Sugimoto S and Ogasawara T), Kyoto, Japan, 8–13 July 2007. Kyoto, Japan: Japan Society for Composite Materials.

16.

ASTM Standard D 1709-09. Standard Test Methods for Impact Resistance of Plastic Film by the Free Falling dart method, 2009.

17.

Nettles AT, Jackson JR and Gates TS. Compression after impact testing of sandwich structures using a four point bend test. In: Proceedings, SAMPE fall technical conference and exhibition, Memphis, TN, 8--11 September 2008.

18.

Tomblin JS, Ng YC and Raju KS. Material qualification and equivalency for polymer matrix composite material systems, Final Contract Report DOT/FAA/AR-00/47, 2001.

19.

Nettles AT and Jackson JR. Developing a material strength design value based on compression after impact (CAI) damage for the ARES I composite interstage. AL, USA: NASA/TP-2009-215634, Marshall Space Flight Center, 2009.