Structural Enhancements for Increased Energy Absorption in Composite Sandwich Structures SCOTT E. STAPLETON AND DANIEL O. ADAMS ∗ Department of Mechanical Engineering University of Utah, Salt Lake City, UT 84112, USA ABSTRACT: Facesheet debonding and sandwich buckling failure modes have often been the cause of low energy absorption in sandwich panels under edgewise compression loading. In this study, structural enhancements, including bevels, stitching, and vertical webbing, combined with fixture crush initiators were applied to four sandwich configurations in order to avoid debonding/buckling and induce high energy absorbing failures such as crushing. The sandwich configurations consisted of woven or random mat carbon fiber facesheets with foam or balsa wood cores. Dynamic edgewise compression test were used to evaluate the effectiveness of structural enhancements and crush initiators. Energy absorption and peak load were calculated and the failure progression monitored. Webbing and stitching were found to be the most effective enhancements, with energy absorption gains of up to 637%. KEY WORDS: sandwich, energy absorption, structural enhancement, compression, impact
INTRODUCTION
T
he use of carbon fiber-reinforced composite sandwich structures has received increased interest because of their high stiffness-to-weight ratios. For automotive applications, crashworthiness is also a major design consideration and has been the focus of much research in recent years [1]. Energy absorption in carbon fiber-reinforced composites is maximized by triggering a progressive series of localized fractures [2]. Such failures are difficult to achieve in sandwich structures because reduced-stiffness cores often inadequately support the facesheets during loading, causing buckling-type failures and facesheet debonding. Such failures result in little post-failure load carrying capacity, and therefore low energy absorption [3,4]. In the past, structural enhancements have been utilized in sandwich structures in an attempt to avoid global facesheet debonding. Raju and Tomblin [5] introduced through-the-thickness transverse stitching to foam-cored sandwiches with fiberglass facesheets to resist global buckling and debonding of the facesheets. In this case, stitching reduced the failure from facesheet global buckling to local bucking between the stitches, resulting in a progressive, accordion-like failure. In another study, through-the-thickness carbon fiber reinforcement was utilized to stiffen the core [6]. This reinforcement was independent of the facesheets, and did not provide any resistance to facesheet/core debonding. Although the intent was to improve sandwich resistance to damage from transverse loading, similar constructions could be used to improve sandwich
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Author to whom correspondence should be addressed. E-mail:
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performance and energy absorption under edgewise compression loading by averting bucklingtype failures. It has also been shown that even when buckling is avoided and the sandwich experiences a facesheet failure, the location of the initial failure is critical to energy absorption [4]. When the facesheets fail in the middle of the specimen or at different locations along the facesheets, the specimen often loses alignment with the load and absorbs little energy after the initial failure. However, facesheet failures occurring at the end of the specimen most often result in the specimen reloading and crushing in a progressive manner. Therefore, when maximizing energy absorption in sandwich structures, it is not only important to control the initial failure mode, but also the initial failure location. Through the introduction of specimen end-beveling and crush initiators, progressive crushing has been achieved in self-stabilizing structures such as tubes [7]. Crush initiators adapted to flat sandwich coupons have been shown to trigger similar failure progressions, but the success of initiators has been limited to sandwich configurations with stiff cores and relatively low-strength facesheets [8]. When these conditions were not met, the initiators did not trigger local crushing in the facesheets but caused them to deflect laterally, resulting in bending and facesheet/core debonding. The present study utilizes structural enhancements and crush initiators to optimize the energy absorption capabilities of four sandwich configurations consisting of woven or random mat carbon/epoxy facesheets with balsa wood or foam cores. These sandwich configurations were selected following a comprehensive material selection study [9,10]. The structural enhancements were designed to improve failure progression by either producing the desired initial failure mode, controlling the initial failure location, or both. Dynamic edgewise compression tests were performed on structurally enhanced sandwich specimens with and without crush initiators to identify the best-performing combination for each sandwich configuration. Once the best performing combination was identified through an initial screening process, more extensive testing was performed to quantify the improvement in energy absorption gained through the use of structural enhancements and initiators. Similar structural enhancements could be utilized in applications to improve the energy absorption of sandwich structures. METHOD Materials The facesheet and core materials used in the four sandwich configurations investigated are summarized below. These four sandwich configurations are identified by their facesheet/core materials and are referred to as: Woven Carbon/Balsa, Woven Carbon/Polyurethane, P4/Balsa, and P4/Polyurethane. All four sandwich configurations had facesheet thicknesses of approximately 1.4 mm, and core thicknesses of approximately 12.7 mm. Facesheet Materials: • Fiber: o Woven Carbon: T300B 3K woven carbon fabric ([(0/90)/(±45)]3T) [11] o P4: Random carbon mat [12] • Matrix:
o EPON 862 epoxy resin and EPON 9553 hardener [13] Core: • Polyurethane foam (FR-6710 General Plastics [14]) 160 kg/m3 • Balsa wood (S67 Alcan Baltek [15]) 112 kg/m3 All sandwich configurations were manufactured at the University of Utah. The two woven carbon sandwich configurations were fabricated using a single-step Vacuum Assisted Resin Transfer Molding (VARTM) process. The sandwich was infiltrated with resin while under vacuum, followed by oven curing for two hours at 50ºC. A two-step VARTM method was used to fabricate the P4 facesheet configurations. The sandwich was placed between two flat acrylic plates, which served as molds. Acrylic was selected for the mold material because resin infiltration could be visually monitored. After wet-out of the facesheets, additional pressure was applied to the panel to reduce the facesheet thickness to equal that of the woven carbon facesheets by placing the vacuum-bagged assembly into a heated press with a pressure of 1.0 MPa. Pressure was applied for two hours while the temperature of the platens was held at 50ºC. The vacuum was maintained on the bagged assembly throughout the cure process. At the completion of baseline (without structural enhancements) sandwich panel manufacturing, sandwich thickness measurements taken from each sandwich panel configuration were used to assess the degree of variability in the manufacturing process. For the Woven Carbon/Balsa, Woven Carbon/Polyurethane, and P4/Balsa configurations thickness measurements produced a coefficient of variation of less than 2.0%. A slightly higher thickness variation was recorded for the P4/Polyurethane configuration, which produced a coefficient of variation of 3.6%. Test Method Since energy absorption under dynamic loading was the primary focus of this investigation, dynamic drop-weight edgewise compression testing was performed to assess the performance of the four sandwich configurations with and without the crush initiators under in-plane compression loading. The test configuration used for dynamic edgewise compression testing is shown in Figure 1. The test fixture used to support the specimen during crushing (Figure 1) was constructed of low carbon steel and patterned after that used for quasi-static edgewise compression testing of sandwich composites at the University of Utah [10] and specified in ASTM C 364 [16]. The 76 mm wide by 127 mm tall specimen was placed into the stationary bottom fixture half. The specimen was held in place by means of a 6 mm tall clamped section on the bottom. The upper fixture half was attached to the crosshead and dropped onto the specimen. This upper fixture half restricted specimen movement by means of adjustable supports constructed with a 45° angled surface. This upper support was designed to keep the specimen aligned in the fixture during crushing and to serve as an optional crush initiator. Hardened tool steel contact plates were pressed into the upper and lower specimen contact areas of the fixture to prevent damage due to localized loading. The 40.6 kg impacting mass used for all drop-weight impact testing consisted of the steel crosshead and the upper half of the test fixture as shown in Figure 1. Vertical alignment of the crosshead was maintained using vertical guide rods and linear bearings. Damping springs and metal stoppers were used to keep the upper fixture from colliding with the bottom fixture during impact loading. The required drop height to produce at least 50.8 mm of crush displacement for
each sandwich configuration was determined based on the amount of energy absorbed in prior quasi-static edgewise compression testing [10]. The drop heights ranged from 0.92 to 3.35 meters, producing initial impactor velocities between 4.3 and 8.1 m/s. A Kistler 9372A quartz force transducer was affixed between the lower fixture and the tower base to provide a full force versus time history of the impact event. Numerical integration techniques were implemented to obtain velocity and displacement responses. A manual switch located on the crosshead and the top of the damping spring was used to signal the end of the crush section so that the springs and stoppers were not included in the impact event. A high speed FASTCAM-APX RS2 camera was used to capture the impact event. Based on high-speed video images of the failure sequence and examination of the failed specimens, the apparent initial failure mode and failure progression was determined. Failure modes were classified in a manner consistent with that outlined in Composite Materials Handbook 17 (CMH-17-6 [18], formerly MIL-HDBK-23A [19]) and observed in past research of composite sandwich structures under edgewise compression loading [20-24]. Four general failure mode classifications were considered relevant for edgewise compression loading: general buckling, local buckling, facesheet and core failure, and facesheet/core debonding.
Clamps
Force Transducer
Specimen
Fixture
Figure 1. Edgewise compression drop-weight impact test configuration.
Failure progression, peak load, energy absorption, and weight normalized energy absorption were determined for each specimen tested. Peak load was defined as the highest load held by the specimen. Energy absorption was found by calculating the area below the force vs. displacement curve over a 50.8 mm crush length (Figure 2). In some cases, the load reached negative values after the initial peak. It appears that this was an elastic response from the testing system due to sudden unloading (specimen failing catastrophically) and was subtracted from the total during energy absorption calculations. Because weight savings is a prime concern for carbon fiber sandwich structures, the energy absorbed was normalized with areal weight. The areal weight, or weight per unit surface area, is defined as
AW =
w bh
(1)
where AW = areal weight (kg/m2) w = specimen weight (kg) b = specimen width (m) h = specimen height (m)
40
Force (kN)
Peak Force
20
Energy Absorbed 0 0
25
50
Displacement (mm) Figure 2. Typical force vs. displacement plot illustrating the peak force and energy absorbed.
Structural Enhancements Structural enhancements were developed for each of the four sandwich configurations in an effort to produce the desired initial failure mode at the desired locations in the sandwich specimen. These structural enhancements consisted of end beveling, bidirectional stitching, and through-the-thickness core webbing. The first type of structural enhancement, end beveling, was intended to produce a stress concentration and initiate failure at the ends of the specimen. For the sandwich configurations with woven carbon facesheets, a 1.5 mm deep, 45° angled bevel was cut out of the core before resin infiltration, causing the facesheets to angle inward on the top and bottom of the specimen (Figure 3). Initial testing of the two sandwich configurations with P4/epoxy facesheets revealed that these angled bevels did not produce failure at the specimen ends. Thus, square-shaped bevels were pressed into the sandwich configurations with P4 facesheets (Figure 3). These square bevels went deeper into the core (3.2 mm) than the angled bevel, creating a larger stress concentration.
Angled Bevel
Square Bevel
Figure 3. Angled bevel for woven carbon configurations, and square bevel for P4 configurations.
The two remaining types of structural enhancements, bidirectional stitching and through-thethickness core webbing, were both developed to resist facesheet debonding from the core. The bidirectional stitching consisted of through-the-thickness Kevlar stitch rows with a 12.7 mm spacing. These stitch rows were inserted in both the longitudinal and transverse directions of the sandwich panels prior to resin infiltration (Figure 4). Two directions of stitching were used to further strengthen the facesheet/core interface and make this structural enhancement bidirectional. Additionally, through-the-thickness core webbing was developed as the final structural enhancement. For both the woven carbon and P4 facesheets, the core web was formed by passing selected layers of the facesheet through-the-thickness of the sandwich and integrating them into the facesheet on the other surface of the panel. The webbing configuration used with the woven carbon facesheets is shown in Figure 5. A single, vertical web was added at the center of the specimen. This web was constructed of two layers of woven carbon fabric from each side of the specimen, thus producing a four-layer through-the-thickness web. The webbing configuration used with the P4 facesheets is shown in Figure 6. Since the P4 facesheets consisted of a single preform layer, a different webbing design was required for this material. Two vertical webs were produced in each specimen; each placed approximately 1/8th of the specimen width inward from the unloaded edges. Each web, produced from the single P4 layer, extended from one facesheet to the other, forming a “Z-shaped” pattern as shown in Figure 6. This webbing design resulted in single-layer facesheet thicknesses in the central region of the specimen, with a double-layer thickness facesheet and a zero-layer thickness facesheet on the outer regions as shown. Initially, the P4 webbing produced gaps between the core sections which filled with resin during infiltration and increased the areal weight. A method was developed to clamp the core pieces together and close the gap for the P4/Polyurethane configuration, but the balsa core pieces broke when clamped. Therefore, the P4/Polyurethane webbing specimens resembled the “ideal” form in Figure 6, while the P4/Balsa specimens resembled the “non-ideal” form.
Transverse stitches
Longitudinal stitches Figure 4. Bidirectional stitching specimen.
4 layers
2 layers
Top View
Figure 5. Woven Carbon webbing structural enhancement.
Ideal
Non-Ideal
Resin Channels
Figure 6. P4 webbing structural enhancement with ideal and non-ideal construction.
Based on initial testing, a modified version of the woven carbon webbing design was developed to force facesheet failure to occur at the ends of the specimen. As shown in Figure 7, the facesheet thicknesses tapered from six to four plies at a distance of 6.4 mm from the specimen ends. Additionally, the through-the-thickness webbing was terminated at this distance such that there was no web reinforcement at the specimen ends. These modifications were made in an attempt to initiate a progressive end failure before loading the webbed section of the specimen.
4 layers, no webbing
6 layers, webbing
Side View
Figure 7. Woven Carbon modified webbing structural enhancement.
Crush Initiators In addition to incorporating structural enhancements into the sandwich specimens, crush initiators were incorporated into the test fixture in an effort to trigger high-energy absorbing failures. The two initiators used are shown in Figure 8. Stapleton and Adams [8] evaluated the effects of these two initiators for the four baseline sandwich configurations (i.e. without structural enhancements). The internal plug initiator was placed between the angled clamps in the upper test fixture half as shown in Figure 8a. This initiator consisted of a tapered steel plug which contacted the core first, and subsequently contacted the facesheets at a 45º angle. This initiator was designed to cause local crushing in the facesheets and force debris away from the specimen. The external wedge initiator (Figure 8b) was a built-in feature of the impact test fixture. The external wedge was activated by moving the angled clamps in the upper fixture half inward, the 45˚ angle of the clamp surface producing an external wedge. Under compressive loading, this external wedge also served to guide the specimen back to the center during crushing. This initiator was designed to crush the facesheets locally, and force the debris inward. In this study, specimens with structural enhancements were tested with and without an initiator to find the highest energy absorbing enhancement/initiator combination. Based on results obtained previously without the use of structural enhancements [8], the internal plug initiator was used with the polyurethane core configurations whereas the external wedge initiator was used with the balsa core configurations.
(a.)
(b.)
Figure 8. Crush initiators used to enhance energy absorption: (a.) Internal plug initiator, (b.) External wedge initiator.
RESULTS AND DISCUSSION Initial Screening Testing Dynamic edgewise compression tests were performed on all four sandwich configurations with and without structural enhancements and crush initiators in order to determine the effectiveness of structural enhancements and to identify the highest energy absorbing enhancement/initiator combinations. For each sandwich configuration, three “baseline” specimens, without structural enhancements or initiators, were tested. Additionally, three specimens from each sandwich configuration were tested with each structural enhancement. For each structural enhancement, the first specimen was tested without an initiator, and the second with an initiator. The condition producing the highest energy absorption was used for the third specimen. As described in the previous section, the internal plug initiator was used for the polyurethane core configurations whereas the external wedge initiator was used for the balsa core configurations. The purpose of this initial screening testing was not to gather statistically significant data, but rather to identify the highest energy absorbing initiator/enhancement combination based on a small sampling of many test configurations. The small amount of samples per configuration facilitated the testing of four structural enhancements with and without initiators for four sandwich material systems. All together, this initial screening round represented 69 dynamic edgewise compression tests. A summary of the results of the initial screening of dynamic edgewise compression testing is presented in Table 1. Results are presented as average energy absorbed along with weight normalized energy absorption. The weight-normalized metric is included because weight savings is a primary concern when dealing with composite materials. Therefore, the decision as to highest energy absorbing initiator/enhancement combination was chosen based on weight normalized energy absorption. The following sections contain a general discussion of the baseline failure modes, followed by a discussion of the general effects of the initiators and structural enhancements.
Table 1. Summary of results of the first round of dynamic compression tests. Structural enhancement None (Baseline) End bevel Bidirectional stitching Webbing (Woven or P4) Modified webbing
Woven Carbon/Balsa Initiator? no yes no yes no yes no yes no yes
EA 384.3 328.7 137.7 150.5 532.1 518.1 1158.5 756.0 1107.5 753.5
NEA 65.5 55.7 22.2 24.6 78.4 76.8 181.6 124.2 190.0 128.2
Woven Carbon/ Polyurethane EA 87.4 94.9 55.2 53.4 317.9 359.3 306.0 273.5 332.2 258.1
NEA 14.7 15.9 9.1 8.8 51.5 47.8 48.6 44.3 51.4 39.5
P4/Balsa EA 470.9 927.5 364.0 791.0 233.6 487.2 859.5 1166.2 ---
NEA 74.8 140.5 57.6 114.2 30.1 66.7 103.9 135.0 ---
EA= Energy absorption (N-m), NEA=Weight normalized energy absorption (N-m/kg/m2)
P4/ Polyurethane EA 109.8 112.8 68.8 54.0 274.8 482.4 494.7 681.2 ---
NEA 18.4 19.0 11.7 9.0 37.3 66.5 65.2 95.0 ---
The baseline specimens for all configurations failed by either buckling or facesheet compression failure as illustrated in Figure 9. The Woven Carbon/Polyurethane configuration baseline specimens all failed by facesheet buckling and facesheet debonding (Figure 9b). The P4/Polyurethane configuration appeared to also fail by bucking, accompanied by a facesheet failure (Figure 9d.). One of the facesheets usually failed, and the other one buckled and debonded from the core. This type of failure absorbed a comparable amount of energy to a buckling failure. The Woven Carbon/Balsa and P4/Balsa configurations both experienced facesheet failures (Figure 9a, c.). While the baseline specimens of these two sandwich configurations absorbed more energy than the polyurethane core configurations, there was high variation between specimens. The energy absorption was dependant upon initial failure location. Failures near the end of the specimen resulted in progressive crushing and high energy absorption (Figure 9c.), while failures in the middle caused more catastrophic failures (Figure 9a).
(a.)
(c.)
(b.)
(d.)
Figure 9. Typical failure progressions for baseline specimens for all four sandwich configurations: (a.) Woven Carbon/Balsa, (b.) Woven Carbon/Polyurethane, (c.) P4/Balsa, (d.) P4/Polyurethane.
In general, the effectiveness of the initiators was highly dependant on the facesheet material. For the woven carbon facesheet configurations, the use of a crush initiator resulted in a decreased improvement in energy absorption in almost all cases. This is because the facesheets were strong, and the angled surface of the crush initiators caused the facesheets to deflect laterally rather than crush locally (Figure 10a.). This lateral deflection usually resulted in the facesheet debonding from the core, causing decreased energy absorption. The P4 facesheet configurations, on the other hand, benefited from the crush initiators in almost every case. The P4 facesheets were not as strong as the woven carbon facesheet, so they often crushed locally at the ends when contacted by the angled surface of the initiator (Figure 10b.). This local crushing triggered a more progressive failure and usually resulted in increased energy absorption. For the second round of testing, no initiator was used for the woven carbon facesheet configurations, while the internal plug initiator was applied to the P4/Polyurethane configuration and the external wedge initiator was applied to the P4/Balsa configuration.
(a.)
(b.)
Figure 10. Typical failure progressions for sandwiches using crush initiators: (a.) Woven Carbon facesheet sandwich, (b.) P4 facesheet sandwich.
The end bevel structural enhancement did not induce end crushing as intended and had a detrimental effect on energy absorption for all but one configuration. Rather than the bevels breaking in and triggering facesheet end curling, the notches induced bending and forced the facesheets away from the core for every configuration (Figure 11). However, when applied to the P4/Balsa configuration with the initiator, the initiator still caused crushing and improved energy absorption. However, it is believed that the improvement was due to the initiator, not the end bevel. In general, this structural enhancement did not improve energy absorption for the four sandwich configurations.
Figure 11. Typical failure progression for sandwiches with the end bevel structural enhancement.
The bidirectional stitching enhancement was successful at increasing the energy absorption for every sandwich configuration except the P4/Balsa configuration. The stitches introduced a stress concentration in the middle of the P4/Balsa specimens rather than the ends, causing facesheet failures in the middle and subsequent specimen misalignment. The stitching increased energy absorption in the other three configurations by improving post-buckling energy absorption. Specimens from these configurations buckled, and the stitching held the facesheets to the core, allowing the specimen to realign and reload. This enhancement was the most successful structural enhancement for the Woven Carbon/Polyurethane configuration. The webbing enhancement was very effective at increasing the energy absorption for all four sandwich configurations. For the polyurethane core configurations, it changed the initial failure mode from global buckling to facesheet compression failure by stiffening the core. However, although the specimens failed in a progressive manner, it appeared that sections of facesheet still buckled from the core, causing large debris and less energy absorption. The two balsa core configurations crushed progressively the entire impact event. This structural enhancement resulted in the highest increase in energy absorption for the P4/Balsa and P4/Polyurethane configurations when applied in conjunction with crush initiators. Applied only to the woven carbon facesheet configurations, the modified webbing structural enhancement improved the energy absorption in both cases. The ply drops on the top and bottom of the specimens caused them to crush at the top and bottom before displaying failures similar to those of the webbing specimens. This enhancement was the most effective enhancement for the Woven Carbon/Balsa configuration, and almost the most effective for the Woven Carbon/Polyurethane configuration. The initial screening testing showed that the bidirectional stitching, webbing, and modified webbing structural enhancements were generally advantageous to the energy absorption of specimens from all configurations with some exceptions. The P4 facesheet configurations benefited from the initiators, while the woven carbon facesheet configurations did not. The end bevel was detrimental to the energy absorption of all four sandwich configurations.
Follow-on Testing with Most Effective Enhancement/Initiator Combination The initial screening testing identified the most beneficial combination of structural enhancement/initiator condition for each of the four configurations. In order to better quantify the energy absorption improvement, a total of five dynamic edgewise compression tests were performed for each configuration baseline and combination of structural enhancement/initiator condition. A summary of the four configurations and the enhancement/initiator combination tested in the follow-on testing is found in Table 2. The results of the follow-on testing are contained in Table 3, including, average values and standard deviation of peak load, energy absorption, and weight normalized energy absorption. Table 2. Highest energy absorbing case for each sandwich configuration. Highest energy absorbing combination Configuration Woven Carbon/Balsa Woven Carbon/Polyurethane P4/Balsa P4/Polyurethane
Structural enhancement
Initiator
Modified webbing Bidirectional stitching P4 webbing P4 webbing
No initiator No initiator External wedge initiator Internal plug initiator
Table 3. Results of the highest energy absorbing initiator/enhancement combination for each sandwich configuration.
Ave SD Ave SD Ave SD Ave SD Ave SD Ave SD Ave
Peak load (kN) 68.9 10.0 57.6 8.0 56.5 8.4 51.9 6.7 34.3 5.0 64.4 6.2 27.0
Energy absorption (N-m) 431.3 88.8 980.7 112.4 90.3 14.5 327.8 63.3 310.4 146.0 1013.5 173.2 92.4
Weight normalized energy absorption (N-m/kg/m2) 69.9 14.6 163.1 25.1 14.2 2.2 50.8 10.3 47.3 21.5 114.4 19.2 15.0
SD
3.5
31.5
5.2
Ave
26.6
713.9
126.1
SD
2.0
111.9
46.6
Configuration Woven Carbon/ Balsa
Baseline
Woven Carbon/ Polyurethane
Baseline
Best enhancement/initiator combination
Best enhancement/initiator combination Baseline
P4/Balsa
Best enhancement/initiator combination Baseline
P4/ Polyurethane
Best enhancement/initiator combination
The difference in peak load between the baseline and best enhancement/initiator combination for all four sandwich configurations in the follow-on testing is shown graphically in Figure 12. The thick bars represent the average peak load values of five specimens, and the error bars represent one standard deviation above and below the average. The two woven carbon facesheet configurations had structural enhancements which reduced the peak force, because the ply-drop and the stitches reduced the strength of the facesheets. The webbing enhancement combined with the initiator increased the peak load for the P4/Balsa configuration, while the webbing and initiator had little change on the peak force for the P4/Polyurethane configuration.
80
Baseline Best Enhancement/Initiator Combination
70
Peak Load (kN)
60 50 40 30 20 10 0
Woven Carbon/ Balsa
Woven Carbon/ Polyurethane
P4/ Balsa
P4/ Polyurethane
Figure 12. Peak load for all four sandwich configurations.
The difference in energy absorption between the baseline and best enhancement/initiator combination is illustrated in Figure 13. The thick bars represent the average energy absorption values of five specimens, and the error bars represent one standard deviation above and below the average.
1200
Baseline Best Enhancement/Initiator Combination
Energy Absorbed (N-m)
1000
800
600
400
200
0
Woven Woven Carbon/ Carbon/ Balsa Polyurethane
P4/ Balsa
P4/ Polyurethane
Figure 13. Dynamic energy absorption for the baseline and best enhancement/initiator combination for all four sandwich configurations.
The Woven Carbon/ Balsa configuration had the highest average energy absorption of all of the baselines and structural enhancement/initiator combinations. Although the baseline specimens often crushed at the ends (Figure 14a), this was often followed by buckling-type failures which rendered the specimen incapable of holding much load as can be seen in the force versus displacement graph in Figure 15. The modified webbing enhancement increased the average energy absorption by 156% by causing the facesheets to crush on the top bottom followed by crushing from the top (Figure 14b). This crushing allowed the specimen to hold more load after initial failure, and greatly increased the energy absorption.
(b.)
(a.)
Figure 14. Typical failure progressions for Woven Carbon/Balsa specimens: (a.) Baseline specimen, (b.) Modified webbing specimen.
Baseline Specimen Modified Webbing Specimen
70
Force (kN)
Facesheet Curl
50
Buckle Bottom Facesheet Curl/Crush
30
10
-10 0
10
20
30
40
50
Displacement (mm) Figure 15. Force vs. displacement plots for Woven Carbon/Balsa specimens.
The bidirectional stitching enhancement applied to the Woven Carbon/Polyurethane configuration had the lowest energy absorption of all the sandwich configurations, even though it improved the energy absorption of the baseline by 297%. The baseline specimens displayed a buckling-type failure, which resulted in low post-buckling energy absorption (Figure 16a and Figure 17). The stitching did not prevent specimens from buckling, but only improved the postbuckling energy absorption by causing the specimens to reload (Figure 16b).
(a.)
(b.)
Figure 16. Typical failure progressions for Woven Carbon/Polyurethane specimens: (a.) Baseline specimen, (b.) Bidirectional stitching specimen
80
Baseline Specimen Bidirectional Stitching Specimen
Force (kN)
60
Global Buckling
40
Reloading
20 0 -20 0
10
20
30
40
50
Displacement (mm) Figure 16. Force vs. displacement plots for Woven Carbon/Polyurethane specimens.
The P4 webbing with the external wedge initiator improved the energy absorption of the P4/Balsa configuration by 275%. The baseline specimens displayed facesheet compression failures, but if the specimen failed initially in the middle, the specimen would often bend before reloading causing large specimen displacement with little load bearing (Figure 17a and Figure 18). The webbing stiffened the core, causing the specimen to hold more load while the external wedge initiator force the specimen to consistently crush from the end in a progressive manner (Figure 17b). When comparing the weight normalized energy absorption to that of the other configurations, it seems logical to draw the conclusion that the P4/Balsa configuration with the
webbing enhancement did not perform as well as the Woven Carbon/Balsa and the P4/Polyurethane configurations. However, this is due to the weight normalization. The P4 webbing specimens with a balsa core had resin channels in between the core sections due to insufficient compression during curing. The extra resin made the specimens heavy without adding any significant strength. Although these specimens absorbed more energy than any other specimens, their heavy weight caused the weight normalized energy absorption to be relatively lower. If a new manufacturing method were developed to solve this problem, it can be concluded that the specimens would have weight normalized energy absorption values as high as or higher than the other configurations.
(a.)
(b.)
Figure 17. Typical failure progressions r for the P4/Balsa specimens: (a.) Baseline specimen, (b.) Webbing specimen with initiator.
60
Force (kN)
Baseline Webbing with Initiator
Buckling
50 40
Reloading
30 20 10 0 -10 0
10
20
30
40
50
Displacement (mm) Figure 18. Force vs. displacement plots for P4/Balsa specimens.
The P4 webbing enhancement increased the energy absorption of the P4/Polyurethane configuration by 637% because it changed the initial failure mode from global buckling (Figure 19a) to facesheet failure (Figure 19b). The webbing kept the facesheets from debonding from the core while also stiffening the core. This change in failure mode caused a huge increase in energy absorption because the specimen’s capacity to carry load after initial failure dramatically increased (Figure 20). It is worth noting that the increase in weight-normalized energy absorption over the baseline increased substantially from round 1 to round 2 testing. This is because new manufacturing methods were discovered to make the specimens more like the ideal form of Figure 6. Without heavy resin channels, the weight normalized energy absorption increased, causing the greatest increase in energy absorption from a structural enhancement/initiator combination.
(a.)
(b.)
Figure 19. Typical failure progressions for P4/Polyurethane specimens: (a.) Baseline specimen, (b.) Webbing specimen with initiator.
40 Buckling
Force (kN)
30
Baseline Specimen Webbing Specimen with Initiator Sectional Crushing
20 10 0 -10 0
10
20
30
40
50
Displacement (mm) Figure 20. Force vs. displacement plots for P4/Polyurethane specimens.
SUMMARY AND CONCLUSION Structural enhancements were applied to sandwich specimens, consisting of random mat (P4) or woven carbon facesheets and balsa or polyurethane cores, to induce high energy absorbing failures and increase the energy absorption of the four configurations. These structural enhancements had at least one of two purposes: to control the failure mode and location by introducing a weakness into the specimen, or to change the initial failure mode by stiffening the specimen. An initial screening round of dynamic edgewise compression testing was performed on baseline and structurally enhanced specimens of all four configurations to find the enhancements most beneficial to energy absorption. Initiators were also used in conjunction with structural enhancements to trigger energy absorbing failures. The top energy absorbing enhancement/initiator combinations were identified for each configuration. A follow-on round of dynamic edgewise compression testing was performed on five specimens of the top enhancement/initiator combinations to better quantify the increase in energy absorption over the baseline. Structural enhancements and crush initiators caused up to 637% energy absorption increase over baseline specimens. The webbing structural enhancement appeared to be the most successful in increasing energy absorption for the P4 facesheet configurations. The initiators had the most beneficial effect on these configurations, because local end fractures could be induced in the lower strength P4 facesheets. The modified webbing, with a facesheet ply-drop on both of the loaded ends, was the most successful structural enhancement for the Woven Carbon/Balsa configuration because it induced facesheet end curling in both the top and the bottom of the specimen followed by progressive crushing. None of the structural enhancements prevented buckling in the Woven Carbon/Polyurethane configuration, but the bidirectional stitching
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