1-s2.0-0010436185902794-main

Instrumented drop weight impact testing of cross-ply and fabric composites J.D. WINKEL * and D.F. ADAMS **

(* EDO Corporation/** University of Wyoming. USA)

A state-of-the-art instrumented drop weight impact test system developed at the University of Wyoming was used to investigate the impact performance of thin, simply-supported composite laminates. System calibration, data acquisition and data reduction techniques developed for this impact test system, which makes use of a piezoelectric force transducer, are presented, along with insights into system resonance characteristics. Composite material plates were tested to identify performance differences between cross-ply and fabric material forms. The six composite material systems investigated included cross-ply and fabric laminates of Hercules AS4 graphite, DuPont Kevlar 49 and Owens-Coming E-glass fibres impregnated with a Hercules 3501-6 epoxy resin. Test results are presented along with results of a literature review in this area.

Key words: composite materials; laminates; instrumented impact drop weight impact E-glass fibres; graphite fibres; Kev/ar fibres; fabric; epoxy resins

During the past two decades, composite materials have been used in an increasing number of hardware applications that are susceptible to impact damage. The realization that some of the current advanced composite materials, particularly those incorporating graphite fibres, are far more susceptible to low-velocity impact damage than glass fibre composites has spurred the desire for a better understanding of this critical performance parameter. Future applications of advanced composites in components that will be susceptible to impact damage, such as aircraft wing primary structures, ensure that interest in the impact properties of composites will remain high for some time to come.

Most of the prior effort aimed at understanding composite impact damage mechanisms has centred around the use of Charpy and lzod tests, and particularly instrumented versions of these tests. 1 Although widely used, these tests are not necessarily suited to an understanding of composite material response since the test geometry often does not represent the end-use application of the composite. 2 Instrumented drop weight impact test (DWIT) systems have the advantage of more closely approximating the impacted plate configuration typical of a variety of composite material applications.

The present research had three objectives. First, an instrumented DWIT system was designed and put into

operation at the University of Wyomin~ Second, this instrumented DWIT system was used to explore impact response differences between balanced-weave fabric laminates and equivalent cross-ply tape laminates. Finally, a comprehensive literature survey of ballistic and instrumented DWIT impact results for fabric laminates was performed. Only the first two objectives are addressed here, additional details being given in Reference 3.

Many advantages have been cited in the literature (see, lot example, Reterences 4-7) in support of both fabric and cross-ply laminates. However, relatively little attention has been focussed on a comparison of these two material forms with respect to their impact properties. Consequently, the present study was initiated to address this lack of data. As noted in Reference 1, much of the prior impact testing of composites has been performed using hybrids. The present results are based only on non-hybridized laminates, in the hope of providing a simpler view of impact phenomena.

SYSTEM DESIGN AND CALIBRATION

A variety of possibilities can be explored when designing an instrumented drop weight impact" tester. Drop towers such as a guide-rod configurations and

0010-4361/85/040268-11 $03.00 1985 Butterworth Et Co (Publishers) Ltd

268 COMPOSITES. VOLUME 16 . NO 4 . OCTOBER 1985

Aluminium F ~ Pulley assembly

U| S.~r~ mechanism ~, , , steel I

. . . . I Guide ro Nicolet Explorer III

storage o~illo~cope

with floppy disc

j ? ;:2 ,.,..o.., Slotted Model 208A05 PCB force transducer

Specimen

I I Anvil infrared I -~- - - -= Motor transistor

! Model 464A PCB [

charge amp I

Fig. 1 University of Wyoming drop weight impact test (DWIT) tower

Fig. 2 University of Wyoming instrumented drop weight impact system

Gardner-style drop tubes 8,9 may be used. Instru- mentation may vary from impact tups fitted with strain gauge bridges to accelerometers attached to the cross- head at a location removed from the tup. However, any system will typically consist of three basic components:

1) a drop tower that guides a falling weight towards the specimen;

2) hardware and software dedicated to data acquisition and reduction; and

3) a force transducer.

Drop tower The drop tower in the University of Wyoming system is generally patterned after commercially available, guide- rod style drop towers, for two reasons. First, much of the reported literature available for comparison with the present results is based on testing conducted using drop towers manufactured by Effects Technology, Inc (see, for example, References 8, 10 and 11). Second, the Mechanical Engineering Department at the University of Wyoming already owned a Barry Controls model VP-35 Shock Machine which could be modified to be roughly equivalent to an Effects Technology, Inc model 8000 system. A drawing of the modified tower is presented in Fig 1. The overall test system is shown in Fig 2.

Modifications to the model VP-35 Shock Machine consisted of replacing the guide-rods to extend the drop height, adding a vertical support and base plate to increase stability and rigidity of the drop tower and minor modifications to the pulley and cross-head assemblies. The addition of the base plate to the original model VP-35 base resulted in a base weight of 320 kg The rated maximum cross-head weight for the system is approximately 32 kg~ In the present study, use was made of a 16 kg cross-head.

The University of Wyoming's instrumented DW|T system was designed to incorporate various tup and anvil configurations. This flexibility allows the drop

Plan view

/

I 1---

~ radius chamfer typical

Anvil

Mountin 9 plate

Side view

Fig. 3 Anvil configuration, University of Wyoming DWlT system

COMPOSITES. OCTOBER 1985 269

tower to be used for non-standard tests, plus Charpy, Izod and three-point bend tests in addition to normal plate impact tests. The only constraints imposed on test type by the drop tower are that the anvil and tup fit within the 250 250 mm space available between the guide rods Since the thrust of the present work was to investigate the normal impact behaviour of plates, a 152 152 105 mm anvil specimen support was built This fixture, providing a 127 127 mm simply- supported edge condition for the specimen plates, is shown in Fig 3.

It should be noted that cross-head friction was not negligible despite the fact the two of the model VP-35 cross-head bushings floated on 'O-ring' seals. Both bushing rod clearances and rod perpendicularity were unsuccessfully investigated in an effort to further reduce cross-head friction. The velocimeter instrumentation, however, allowed this friction to be taken into account when the cross-head velocity at impact was calculated. Consequently, the presence of friction did not affect the validity of test results. The most probable cause of this friction (binding due to rotation of the cross-head about a perpendicular to the guide- rod length) could be eliminated in future cross-head designs by the use of kinematic design principles. For example, reducing the number of contact points between the guide rods an~l the cross-head and the use of roller bearings would almost certainly improve the friction performance of the DWIT system.

Data acquisition and reduction

A Nicolet Explorer III model 206-2 digital oscilloscope was used to record and store the data output by the velocimeter and the piezoelectric force transducer. Data were then reduced using a Hewlett-Packard 21MX E-series minicomputer to preprocess the data and create a data file. This data file was then transferred to a Tektronix 4027 Interactive Terminal which was linked to the University of Wyoming's CDC Cyber 760 computer. Data were then reduced to provide the maximum impact load (Pm) and the corresponding energy (Em), as well as the total energy absorbed by the specimen during impact (ET). In addition, Fourier filtering techniques could be imple- mented to smooth output data curves These values, along with the complete force vs time curve, were then plotted using a Textronix 4662 Interactive Digital Plotter.

The Nicolet Explorer III model 206-2 digital oscilloscope is capable of storing up to 4096 data points per test in digital form on a small magnetic disc. The scope was triggered by the velocimeter, which consisted of a slotted steel flag of known dimensions that cut an infrared transistor beam. The slotted flag provided data from which pre- and post-impact velocities could be reduced. The velocimeter was equivalent to commercially available units that typically have an accuracy of 1% or better, ~2 with the exception that the velocimeter flag used here allowed determination of post-impact cross-head speeds. Since the velocimeter data were necessary for force/time data reduction, one channel of the Nicolet was dedicated to velocimeter output The other channel was dedicated to the force transducer output When operated in this mode, the Nicolet stored 2048 data points for each channel, at typical time intervals of 20 to 200 ms.

Stored data were reduced according to the following procedure. First, instantaneous cross-head velocities were calculated using the relation:

v(i) = v o -Av (1)

where

v(i) ---- cross-head velocity at time i;

v o = cross-head velocity at impact;

Av = F(i) +F(i+I) At 2 m'

m = cross-head mass;

F(z) = force at time i; and

At = time interval between data points.

Second, the instantaneous velocity was used to calculate the energy absorbed by the specimen up to time i using the relation:

E(/) ~- E( i -1) + F( i ) +F(i+I) 2 " vi" At (2)

Third, the maximum impact load sustained by the specimen was computed using a sorting routine in the data reduction program. Finally, the reduced data were plotted.

Force transducer instrumentation

The basic geometry of the impact tup was selected to be a 12.7 mm diameter steel rod with a hemispherical penetrating end. This geometry was chosen due to its earlier use by Wardle and Tokarsky? Takeda et aP 3 concluded from ballistic data that a hemispherical shape produced local crushing of the topmost lamina rather than dissipating the impact energy over a larger area as was characteristic with a blunt-nosed impacter. The material system they used was cross-plied Scotchply 1003 glass/epoxy. Although this work is arguably irrelevant in the non-ballistic context of the present study, it does suggest that impact data generated with a certain tup geometry should be used cautiously in applications involving different impact geometries.

Various options were considered in developing sufficiently sensitiwt tup instrumentation. This review resulted in the selection of a piezoelectric force transducer, mounted between the tup and the cross- head. This option was selected for the following reasons:

1) commercially available piezoelectric force transducers offered the desired sensitivity;

2) piezoelectric force transducers offered off-the-shelf reliability;

3) similar instrumentation (piezoelectric accelerometers) had been successfully used by Zoller t4 and Stellbrink and Aoki; 15 and finally

4) commercially available force transducers permit a convenient tup construction in which the gauge is connected in series with the tup and the tup base- plate, which is in turn attached to the cross-head. This configuration keeps the transducer close to the point of impact

270 COMPOSITES. OCTOBER 1985

Fig. 4 Photograph of tup assembly (anvil and velocimeter are also shown)

The specific force transducer selected was a PCB Piezotronics, Inc model 208A05 unit with a rated nominal load ~ of 22 kN. The assembled unit is shown in Fig, 4_

System calibration

The University of Wyoming's DWlT system was carefully calibrated in both static and dynamic modes. Since impact energy is calculated using Equation (2), the accuracy of the test results depends on valid measurements of instantaneous force (F'O, incremental time (At), and the cross-head mass. These quantities are needed to determine the initial impact velocity, vi, indicated in Equation (2). Consequently, system calibration effects centred on these quantities.

The weight of the cross-head was precisely measured using a calibrated Ohaus 20 kg balance and was determined to be 15.88 +__ 0.0014 kg. The accuracy of velocities calculated from the velocimeter output trace was highly dependent upon the precise measurement of the slot and space dimensions of the velocimeter flag, The velocimeter flag was precision-milled and measured to an accuracy of_--t-0.025 mm. The velocimeter was calibrated by comparing velocities calculated from the velocimeter data with three different known cross-head displacement rates in an MTS model 810 servo-hydraulic test frame. The maximum mean error between the DWIT velocimeter

and the MTS test frame data was 1.08%. Although the calibration was limited by the range of cross-head displacement rates available on the MTS, there was excellent agreement between the measured velocimeter and MTS velocities.

Electrical components such as the Nicolet Explorer III oscilloscope and the PCB 464A amplifier played a role in determining F i, v i and At Consequently, these two components were calibrated using known outputs of a Hewlett-Packard model 6204B power supply and a Hewlett-Packard model 202C low-frequency oscillator.

Although the model 208A05 piezoelectric force transducer was designed for impact applications, the literature review revealed that it had not been used in impact of composite materials prior to the present application, Early momentum balance (dynamic) calibrations run with the model 208A05 on an "inelastic' medium provided unexpected results and further impetus for a thorough calibration of the system. Static calibration data were provided with the gauge by PCB Piezotronics, Inc. This calibration was verified by compressively loading the entire tup assembly in an Instron model 1125 test frame.

Dynamic calibration was performed with the gauge mounted both in the drop weight system and isolated from it. The literature reviewed did not report results of dynamic calibration of instrumented drop weight impact systems. Consequently, efforts to dynamically calibrate the University of Wyoming's DWIT system were deemed to be useful in demonstrating the general validity of instrumented impact testing. Initial dynamic calibration efforts centred on confirming the conservation of momentum when the tup impacted an assumed inelastic medium (clay). Forces generated by the transducer for this test configuration were apparently too high, and momentum appeared not to be conserved. Similar force discrepancies were obtained in low-energy tests of eight-ply, quasi- isotropic and cross-plied graphite/epoxy plates.

The above tests were repeated with a variety of other gauges in an effort to determine whether momentum discrepancies resulted from a bad gauge. From these tests it was concluded that the calibration procedure was fundamentally flawed for two reasons. First the assumption of an inelastic medium was not valid. Second, the additional mass of clay adhering to the tup needed to be taken into account for momentum to be conserved during rebounc[

Some additional work was then done to isolate the cause of the low-energy graphite/epoxy plate results. This work consisted of subjecting plates of various materials to very low impact energies. Impact energies were low enough so that the plates were undamaged, and simply caused the cross-head to rebound. This rebound phenomenon was assumed to be a perfectly elastic collision, ie the cross-head velocity was expected to go to zero at the location of maximum force, and the maximum impact energy was expected to correspond to the potential energy of the cross-head. Experimental drops verified both of these expectations. Therefore, it was concluded that the instrumentation was not responsible for the noticed force discrepancies. These discrepancies appeared, instead, to be due to an undetermined plate/tup interaction phenomenon. It further appeared that discrepancies were present only when the impact energy required for plate penetration


became a very significant portion (90% or more) of the cross-head potential energy. Such a test condition was not representative of the present testing; therefore no further efforts were made to isolate the observed phenomenon.

The University of Wyoming DWIT system was dynamically calibrated by abandoning the 'inelastic' test and instead correlating the loss of cross-head kinetic energy with energy absorbed by plate specimens. This calibration method was verified using thin Plexiglass plates, and became a standard feature of subsequent testing

SPECIMEN PREPARATION All specimens were prepared from prepregs produced by the present authors, in order to remove material processing as a variable which might effect the final results. Balanced, plain-weave fabric laminates and equivalent laminates made from cross-plied, unidirectional tapes of Kevlar 49/3501-6 epoxy, AS4 graphite/3501-6 epoxy and E-glass/3501-6 epoxy were chosen for the current testing~ Both filament tow and fabric material forms were impregnated with Hercules

Table 1. Average plate thicknesses and fibre volumes

Material Lay-up Average Average thickness* fibre (ram) volume**

(90)

AS4 graphite Cross-ply [0/9015s 3.00 (0.05) 42.8 (0.02) Fabric [0/9011o 2.16 (0.02) 49.7 (0.02)

Kevlar 49 Cross-ply [0/9015s 2.72 (0.03) 42.9 (0.02) Fabric [0/9011o 2.34 (0.02) 46.2 (0.01)

E-glass Cross-ply [0/9013s 2.54 (0.06) 45.0 (0.04) Fabric [0/90110 1.85 (0.03) 36.6 (0.07)

Coefficients of variation are given in parentheses *Plate thickness is an average of 16 measurements (four per specimen) **Fibre volume is an average of three measurements, per ASTM D3171-76

3501-6 epoxy resin at the University of Wyoming Unidirectional fibres were wet filament wound on a drum winder which had been previously developed at Wyoming Fabric material was impregnated using a cold slurry process which was developed as part of the present effort

Plates were laid up by hand and cured using the prepreg described above. All plates were carefully compacted during lay-up and were then cured in a blanket press This press consists of a pressurized silicone rubber blanket in a heated cavity which applies the desired level of pressure and heat to the laminate. Steel caul plates were used when curing the laminates in the blanket press, to ensure even compaction.

All laminated plates were approximately 380 380 mm in size. These large plates were C-scanned and then cut into four 152 mm square impact specimens, three tensile specimens and three losipescu shear specimens. All plates were nominally 2.5 mm thick Actual plate thicknesses and fibre volumes are presented in Table 1. Impact specimens were cut using an abrasive cutting wheel (LECO type 811-035) mounted on a radial arm saw. Tensile and shear specimens were cut using the same type of abrasive cutting wheel mounted on a surface grinder. The grinder provided a firm surface on which to cut the specimens, and thereby minimized edge delaminations. Based on C-scans of scrap E-glass, Kevlar and graphite composite specimens (which were used to verify test procedures prior to performing the present testing), impact specimens were expected to exhibit little sensitivity to the quality of specimen edge preparation. Consequently, the radial arm saw provided the most efficient means to cut impact specimens.

C-scans were used as the basis for quantitative comparisons of impact damage area in the fabric and tape specimens. All scans were performed with a Sonic Instruments Mark IV Ultrasonic Flaw Detector.

TEST RESULTS

Static material properties

Tensile and shear static tests were first performed using material cut from the same laminated plates as the impact specimens. Results of these tests are presented, along with predicted values, in Tables 2 and 3.

Table 2. Tensile properties of the various materials tasted

Composite Number Measured Reported Number Measured Reported material of modulus modulus of strength strength system specimens (GPa) (GPa) specimens (MPa) (MPa)

[ Reference] [ Reference]

AS4 graphite/epoxy Cross-ply 3 52.6 (0.056) 51.2 [18] 3 Fabric 3 65.2 (0.018) 59.4 [18] 3

Kevlar 49/epoxy Cross-ply 1 26.1 (--) 30.1 [19] 2 Fabric 3 29.5 (0.054) 32.2 [19] 3

E-glass/epoxy Cross-ply 3 22.2 (0.039) 19.6 [20] 3 Fabric 3 20.8 (0.018) 16.9 [20] 3

535 (0.072) 738 [18] 697 (0.056) 855 [18]

470 (0.117) 496 [19] 437 (0.117) 531 [19]

340 (0.383) 490 [20] 157 (0.082) 421 [20]

Coefficients of variation are given in parentheses

272 COMPOSITES. OCTOBER 1 985

Table 3. Iosipescu shear properties of the various materials tested

Composite Number Measured Reported material of modulus modulus system specimens (GPa) (GPa)

[Reference]

Number Measured Reported of strength strength specimens (MPa) (MPa)

[Reference]

AS4 graphite/epoxy Cross-ply 2 3.31 (0.073) * Fabric 3 3.86 (0.105) *

Kevlar 49/epoxy Cross-ply 2 1.79 (0.347) 1.52 [19] Fabric 3 1.59 (0.102) 1.52 [19]

E-glass/epoxy Cross-ply 3 4.41 (0.177) * Fabric 2 4.28 (0.057) *

2 157 (0.18) * 3 150 (0.03) *

2 95 (0.15) 60 [19] 3 99 (0.17) 60 [19]

2 108 (0.07) * 2 124 (0.04) *

Coefficients of variation are given in parentheses "Values not available

Tensile tests were performed according to ASTM Standard D3039-76, except that recommended width- to-length ratios could not be maintained due to the limited amount of material available for specimens. The recommended width-to-length ratio given in ASTM D3039-76 is 1:5; actual specimens ranged from approximately 1:2.5 to the full 1:5 ratio. Some of the discrepancy between strength predictions and test data can probably be attributed to stress concentration effects at the grips induced in specimens having shorter lengths than those specified by ASTM D3039-76.

Shear tests were performed using the Iosipescu shear test methodY ~-23 The test method was limited by the fact that the strain gauges used to measure shear strain on the Iosipescu specimen had a linearity limit of 3% strain. Thus, the stress/strain curves obtained probably reflected both material and strain gauge non-linearities above 3% strain. Shear strength values obtained (see Table 3) were higher than those listed in the literature for unidirectional material. Although little shear strength difference was expected between unidirectional and cross-plied laminates, this result is likely to be valid. One possible explanation for the higher shear strengths exhibited by the cross-plied composites is that perpendicularly oriented fibres tend to blunt

cracks that attempt to propagate between plies. Since shear failure modes are currently not well understood, future work in this area would be quite worthwhile.

Both tensile and shear properties obtained experimentally were compared with values available in the literature. In those cases where adequate data were not found, the fibre supplier data sheet values were used, along with a rule-of-mixtures estimation, to generate values for comparison. Matrix property values used in the rule-of-mixtures estimations were previously measured at the University of Wyoming ~7

Static test results indicated that the materials used in the impact test specimens were of sufficient quality to remove material processing as a variable that would affect the impact results.

Impact test results

Impact tests were carried out on 24 specimens, four each for the cross-ply and fabric forms of the three selected material systems. Specimens were chosen to be 152 mm square plates approximately 2.5 mm thick~ so that the results could be compared with those obtained by other investigators (see below). Specimen and support (anvil) geometries resulted in a simply-

Table 4. Impact properties of the various materials tested

Composite Number Impact Specimen Peak force, Energy to Total energy, Energy loss, material of velocity thickness Pm peak, E m E T AKE system specimens (ms -1) (mm) (kN) (J) (J) (J)

ASS graphite/epoxy l O-ply cross-ply 3 3.725 (0.02) 2.997 (0.02) 3.22 (0.04) 8.76 (0.18) 21.65 (0.07) 20.65 (0.06) lO-ply fabric 4 3.645 (0.03) 2.159 (0.01) 2.60 (0.01) 8.51 (0.13) 15.55 (0.05) 15.00 (0.07)

Kevlar 49/epoxy lO-ply cross-ply 4 3.636 (0.03) 2.718 (0.01) 3.15 (0.10) 14.57 (0.12) 21.76 (0.02) 21.20 (0.02) lO-plyfabric 4 3.725 (0.02) 2.337 (0.01) 2.14 (0.02) 10.26 (0.05) 17.92 (0.04) 17.71 (0.04)

E-glass/epoxy 6-ply cross-ply 4 3.609 (0.03) 2.540 (0.03) 7.19 (0.08) 35.43 (0.12) 46.73 (0.01) 47.70 (0.02) lO-plycross-ply 2 3.243 (0.20) 3.785 (0.01) 10.97 (0.10) 56.35 (0.16) 81.77 (0.10) * l O-ply fabric 4 3.71 2 (0.02) 1.854 (0.01) 1.83 (0.02) 7.21 (0.08) 13.33 (0.02) 11.88 (0.07)

Coefficients of variation are given in parentheses

*These data were obtained before minor modifications to the velocimeter were made which permitted post-impact velocities to be measured with a high degree of accuracy


supported area that was 127 mm square.

All specimens were impacted by a 12.7 mm diameter tup mounted on a failing cross-head that weighed 15.88 kg. The nominal impact velocity was 3.66 ms -l, corresponding to a drop height of approximately 876 mm. The cross-head was manually released after the PCB model 464A amplifier and Nicolet oscilloscope settings were verified. Typical settings consisted of running the amplifier in the short time- constant mode with a full-range output of 8.90 kN. The oscilloscope was typically set to sample data at 20 ms intervals, with a full scale range of__+10 V. The guide rods were liberally lubricated to minimize friction and thus reduce scatter in the resulting impact velocities.

Impact velocity, average specimen thickness, peak impact force (Pm) and corresponding impact energy (E,n), and total impact energy (ET) is given in Table 4 for each specimen. Cross-head kinetic energy losses are also presented. Comparison of these data with total impact energy data supports the previously stated conclusion that the University of Wyoming DWlT system exhibits good dynamic calibration.

Instrumented impact testing represents a powerful tool for understanding materials behaviour during impact, and can be correlated with more conventional concepts of impact performance. This is illustrated particularly well when Fig: 5 is contrasted with Figs 6-11. Fig. 5 shows the brittle behaviour exhibited by one of the Plexiglass calibration specimens; Figs 6-11 illustrate considerably tougher impact behaviour and are representative of the six composite material configurations tested. Additional plots, along with photographs of failed specimens, are included in Reference 3. The results indicate that each of the composite materials tested continued to carry some load beyond the maximum. Brittle materials such as the Plexiglass achieve a maximum impact loading beyond which the force rapidly drops to zero. In Fig 5, the sudden drop in the force/time curve corresponded to shattering of the specimen.

"3 _3

|

o

= = = =

i I Prom 2.28 kN Era= = 5.05 J

Etm t 5.05 J

5.0

0 5 tO 15 Time (ms)

Fig. 5 Typical impact response of a 6.10 mm thick Plexiglass specimen

A

2.5 "' ~o . . J

7.5

Pro= = 3.32 kN Em I = 7.44 d Etot, I = 2102 J

5.0

_ J

2.5

0 , I . . . . 0 0 2.5 5.0 Z5

Time (ms)

Fig. 6 Representative impact response of an AS4/3501-6 [0/9015 s cross-ply laminate

30

"-3

20

U.I

I0

4 Pro== = 2.58 kN Em, x = 9.58 J

Eto,= = = 15.58 d 15

5

A

z

.. J

I

0 , I , i i i 0 0 2.5 5.0 7.5

Time (me)

Fig. 7 Representative impact response of an AS4/3501--6 [0/90110 fabric laminate

I0 ~-

b J

7.5

'max = 3.17 kN Emo = = 14.04 J

E~t,t = 22.40 d

5.0

25

LU

2,5

I I i I ~0 O0 2.5 5.0 7.

Time (ms)

Fig. 8 Representative impact response of a Kevlar 49/3501-6 [0/9015S cross-ply laminate

274 COMPOSITES . OCTOBER 1985

IOO

DISCUSSION It was observed in the present impact testing that results were dependent upon plate thickness. The force and energy data of Table 4 have consequently been normalized by dividing through by the appropriate plate thicknesses. The normalized values are given in Table 5. These normalized data will be used when discussing the present results.

The data of Figs 6-11 were processed to increase their usefulness and clarity. For example, the data were run through a Fourier transform filtering process to remove resonance oscillations from the data signal. Fig. 12 is a non-filtered data trace corresponding to F'ig. 11. Data were transformed with the goal of achieving as "true" a data signal as possible by limiting use of the filtering process. The filter threshold was set to smooth out only high frequency oscillations. Typical oscillation frequencies, such as those shown in Fig. 12, were of two types. A low frequency oscillation was present in all data; this is discussed in more detail below. A second, higher frequency oscillation with a range of

25

'max = 6.49 kN

Ema x = 90.50 J 50

Etot= I = 47.20 J 7.5

5 ~ ~ v

50

" 25

2.5

0 ' ' ' ~ 0 0 25 5.0 7.5

Time (ms)

Fig. 10 Representative impact response of an E-glass/3501-6 [0 /90]3 s cross-ply laminate

2.~

1.5

1.0

05

o 0 2.5

Time (ms)

'max = 1.82 kN

Ema x = 6.94 J

Etot= I = 13.46 J

I0

uJ

5

I , i i i

Fig.11 Representative impact response of an E-g lass/3501-6 [0 /90]10 fabric laminate

Table 5. Normal ized impact force and energy values*

Composite Normalized Normalized material system force, maximum energy

Pm/t (kN mm -1) Em/t (J mm -1)

AS4 graphite/epoxy 1 O-ply cross-ply 1.07 2.92 1 O-ply fabric 1.21 3.94

Kevlar 49/epoxy 1 O-ply cross-ply 1.16 5.36 1 O-ply fabric 0.91 4.39

E-glass/epoxy 6-ply cross-ply 2.83 13.95

10-ply cross-ply 2.90 14.89 1 O-ply fabric 0.99 3.89

*Data of Table 4 normalized by dividing by the corresponding plate thicknesses

2.0

1.5 z

1.0

0.5

i 2_5

Time (ms)

/~m== = 182 kN

Ema x : 6.58 J Etota I = 12.77 J

Pmax = ?-.16 kN

Ema x = 10.18 J

Etoro I = 16.83 d 20 3

z .-o

g ~ _.1

10

I

[ I I f I o 50 Time (ms)

Fig. 9 Representative impact response of a Kevlar 49 /3501-6 [0/9011o fabric laminate

I i i i 5.0

IO

Ld

5

1 Fig. 12 Non-filtered E-glass/3501-6 [0/9011 o fabric laminate impact data corresponding to Fig. 11


10 to 20 kHz was also present in all tests and was smoothed by means of the Fourier transform filtering process. The source of these high frequency oscillations was not clearly pinpointed. The electrical characteristics of the data acquisition hardware at rest were investigated but revealed no periodic frequency of the above magnitude; thus, electrical oscillation did not appear to be a realistic source. A more likely source may be related to the fact that the resonant frequency of the gauge and the observed high frequency oscillations have the same order of magnitude.

Each of Figs 6-12 exhibit a relatively low frequency oscillation during the initial portion of the impact event This oscillation has a typical frequency of ~1300-2500 Hz, and is similar to apparent signal oscillations in data traces reported in the literature. H,24 Since it has a periodic character, this signal oscillation does not affect impact energy test data. Because impact energy is an integral of the force/deflection data, the oscillatory nature of the signal will be removed when impact energies are calculated. Peak impact force similarly does not appear to be affected since the magnitude of these signal oscillations appears to be damped down to a negligible amount at the peak condition.

It was initially assumed that the observed 1300- 2500 Hz oscillation was due to vibration of the simply- supported specimen prior to damage inducement. However, the calculated 462 Hz natural frequency of a 5.94 mm thick Plexiglass plate that had been tested earlier was considerably lower than the observed frequency. This disagreement was further investigated by affixing a PCB model 308B10 accelerometer to the centre of the Plexiglass plate. The mass of the accelerometer was included in the above calculation of plate natural frequency. When the centre of the plate was tapped, a vibration frequency of 429 Hz resulted. This matched closely the predicted natural frequency of 462 Hz.

Subsequent investigation (dropping a weight onto the cross-head-tup assembly while it was resting on an unbroken specimen) revealed transducer output signal oscillations of 1670-2860 Hz. It was concluded that the observed low frequency oscillation resulted from resonance inherent in the conventional DWIT design. This resonance could possibly be avoided by removing the gauge/tup assembly from the cross-head and mounting it in such a way as to maximize its vibration isolation from the guide-rods. This would necessitate mounting the specimen on either the falling cross-head or directly above the tup and striking its free edges.

In addition to the above conclusions, a look at the data in Table 5 confirms the expectation that E-glass/epoxy should outperform Kevlar 49/epoxy which, in turn, should outperform the AS4 graphite/epoxy. The E-glass fabric/epoxy performance is regarded as anomalous due to its low fibre volume (see following discussion).

A review of representative pre- and post-impact C-scans, along with visual inspection of the failed specimens, led to the following conclusions, First, impact of the fabric specimens resulted in smaller damage areas than were present in the cross-ply specimens. Second, cross-ply specimens exhibited back-face fibre splitting with the exception of the Kevlar/epoxy specimens. Third, the Kevlar/epoxy cross- ply specimens exhibited only slightly larger damage areas than did the fabric specimens. Consequently, it is

felt that reduced damage area might be a justification for selecting fabric material forms in the case of E-glass and graphite, but not in the case of Kevlar 49.

The remaining discussion will focus on impact force as an indicator of specimen performance. Although impact energies can be expected to mirror the trends indicated by impact force data, they will not be used in the following discussion for two reasons. First, the use of impact energy obscures small magnitude or short duration differences in comparable data traces since it is an integrated quantity. Second, there is a tendency on the part of some designers to assume that comparable applied energies will yield comparable impact performance. By discussing energies rather than forces it is too easy to disregard factors (such as impact velocity) that play a definite role in impact performance of composites.

AS4 graphite~3501-6 epoxy

Little difference in performance was expected between the AS4/3501-6 cross-ply and fabric laminates as a result of the literature review? This conclusion was particularly supported by Miller et al.X~ Their work also suggested that incipient damage forces were slightly higher for fabric laminates, and that peak force varied linearly with thickness for thin specimens. The results of the present testing generally support this conclusion, and that there is little difference between peak impact force (Pm) performance of AS4/3501-06 fabric and cross- ply laminates when normalized with respect to thickness. Thus, the peak force data shown in Table 5 are directly comparable since they have been normalized with respect to thickness. After normalization, the fabric specimens exhibited peak forces about 11% higher than the cross-ply specimens.

Kevlar 49/3501-6 epoxy

Work by Miner et aP suggests that plain-weave Kevlar 49 fabric absorbs more energy prior to damage than eight-harness satin-weave material, due to the greater straightening effect possible in the plain-weave. Consequently, it was expected that Kevlar fabric specimens would provide better impact performance than the comparable cross-ply specimens. Just the opposite proved to be true. Comparisons of the peak force data of Table 5 indicate a force 27% higher lor the cross-ply laminate than for the fabric, after normalization with respect to thickness. A linear force- to-thickness relationship was again assumed. Since this result was unexpected, a closer look was taken at the other available data.

Further discussion 25 with one of the authors of Reference 10 revealed results similar to those reported above. Thickness and lay-up data made available by Wardle a5 allowed a more direct comparison of the quasi-isotropic fabric and cross-ply data presented in Reference 10, by suggesting the existence of a linear force-to-thickness relationship. A direct comparison of the reported fabric and cross-ply data remains difficult, however, due to the use of somewhat different resin systems and lay-ups between the fabric and tape laminates. While the comparisons of present results with those given by Wardld ,25 should not be viewed as conclusive, they do suggest that the present results are plausible. Possible explanations for the superior perlormance of Kevlar 49 cross-ply laminates will be discussed later.


E-glass/3501-6 epoxy No literature was available that suggested an expected behaviour for the E-glass fabric and cross-ply laminate specimens. The applicability of linear force-to- thickness scaling in the case of the graphite/epoxy and Kevlar/epoxy specimens suggested, however, that a similar relationship might exist for the E-glass/epoxy specimens. A limited amount of testing was performed that tended to confirm the existence of a linear relationship for the specimen thicknesses being investigated. Demonstra- tion of this relationship was particularly important since it allowed comparison of disparate cross-ply and fabric specimens. The loose weave of the E-glass cloth used created difficulties in matching both the fibre volume and thickness of the comparable cross-ply specimens. Laminate processing samples, for example, indicated that a fibre volume of 39% or more corresponded to a poor quality, porous laminate. When fibre volumes exceeded the above value, the laminate lacked enough resin to physically fill the spaces between fibres in the loosely woven fabric used in the present testing Consequently, the fibre volume mismatch between E-glass cross-ply and fabric specimens was greater than that for either the graphite/ epoxy or Kevlar/epoxy specimens. For the cross-ply data (see Table 4), the thickness relationship was linear, however. After scaling the cross-ply laminate specimen data to the corresponding fabric specimen thicknesses, average peak forces of 5.25 kN and 1.83 kN were noted for the cross-ply and fabric specimens, respectively. The fact that the fabric performance was lower than that of the cross-ply material was consistent with the observed failures. While all other through-penetrated specimens clung to the tup after impact, the E-glass fabric/epoxy composites specimens hung very loosely from it. Most displaced material also tended to move back into place when the other specimens were removed from the tup; this was not the case with the E-glass fabric composite.

Because of the differences in apparent failure modes noted above, it is surmised that the poor impact performance of the E-glass fabric laminate was an artefact of the loose weave of the fabric style used in these specimens. Tighter weaves with higher fibre volumes would likely provide better impact performance.

Correlations with theory The complex nature of composite material impact

response has limited the development of a generalized theoretical model. Much of the existing correlation work is empirical in nature. Although empirical approaches do provide general guidelines, it should be noted that results obtained may not be reliable, and there are bound to be many exceptions to the general approximations obtained.

In the case of normal plate impacts, Coppa et aF 6 suggest that, as a first approximation, plate impact energy is related to quasi-static flexural failure energy. For a beam-type structure, this suggests that the impact energy should be proportional to the quantity tTe (or o2/E), where cr and e are the maximum stress and strain of the beam at failure and E is the modulus. Wardle and Tokarsky ~ have expanded this approach and suggest that the impact energy should be proportional to to2/E, where t is the specimen thickness and and E are the specimen extensional (rather than flexural) strength and stiffness.

Although this is a simple approach, it does predict the general trends observed in the present testing Values of the parameter to~/E have been calculated for each material, using average specimen thickness values given in Table 1 and the strength and modulus values given in Table 2. These calculated energy parameter values are listed in Table 6. The energy parameter to~/E is regarded here as only a qualitative measure of data trends since it is derived from an approximate formulation that will be particularly dependent on specimen span-to-depth ratios, specimen edge conditions, etc. The data trends which do appear to be modelled by this simple energy parameter are:

1) similar maximum AS4/3501-6 cross-ply and fabric impact forces. Fabric impact data exhibited peak forces about 11% higher than cross-ply data. Use of the energy parameter toa/E suggests cross-ply laminate performance 1.6% higher than fabric performance: ie a negligible difference;

2) maximum Kevlar 49/3501-6 cross-ply laminate impact forces higher than fabric values. Experi- mentally measured maximum impact forces were 43% higher for the cross-ply than for the fabric laminates. A cross-ply laminate performance 52% higher than for the fabric laminate is indicated by the energy parameter to~/E;

3) the wide disparity between E-glass fabric and cross- ply laminate data. The energy parameter to~/E clearly models this disparity, and supports the

Table 6. Laminate energy parameter values 1 for the various materials tested

Composite Specimen Tensile Tensile Energy material thickness, strength, modulus, parameter, system t (mm) a(M Pa) E (GPa) t~/E (kPa- m)

AS4 graphite/epoxy Cross-ply 3.00 535 52.6 16.3 Fabric 2.16 697 65.2 16.1

Kevlar 49/epoxy Cross-ply 2.72 470 26.1 23.0 Fabric 2.34 437 29.5 15.1

E-glass/epoxy Cross-ply 2.54 340 22.2 13.2 Fabric 1.85 157 20.8 2.2

COMPOSITES . OCTOBER 1985 277

present experimental finding of poor fabric performance.

In summary, the simple energy parameter model appears to provide a good means of relating tensile coupon data to relative impact performance of thin composite plates. It is clear, however, that the insights gained by applying this model will be dependent on the quality of the strength and modulus data used.

CONCLUSIONS

The University of Wyoming's drop weight impact test (DWIT) system was used to provide new insights into low-leveL normal impacts of composite plates. The present effort provided a variety of insights into drop weight impact testing which may be useful to future investigators. Test results showed a strong dependence on specimen thickness, which agreed with results of previous work by other investigators? TM That is, the existence of a linear force-to-thickness relationship for thin graphite/epoxy and Kevlar/epoxy laminates was verified. This relationship allows comparisons of data from specimens of varying thickness.

A number of conclusions can be drawn from the present effort These include:

1) instrumented drop weight impact testing can provide useful insights into impact behaviour of composite materials. It is particularly useful in closely repli- cating impact conditions that are typical of actual applications;

2 piezoelectric force transducers provide a practical alternative to more conventional strain-gauge instrumentation;

3) instrumented drop weight impact test results are strongly dependent upon changes in specimen thickness. A test configuration such as that used in the present testing should not be regarded as a potential standard unless special concern is given to assuring repeatable specimen thicknesses;

4) results of the present testing generally confirmed previous observations of increasing impact performance when progressing from graphite/epoxy to Kevlar 49/epoxy to E-glass/epoxy composites. The present testing also demonstrated smaller post- impact damage areas for graphite and E-glass fabric laminates than for comparable cross-ply laminates. There were no significant damage area differences between Kevlar fabric and cross-ply laminate forms; and

5) the present results demonstrate little difference between the impact performances of AS4 graphite fabric and cross-ply forms, superior performance for the Kevlar 49 cross-ply form and inconclusive results for the E-glass fabric vs cross-ply forms.

REFERENCES

1 Adams, D.F. "Impact response of polymer-matrix composite materials" Composite Materials: Testing and Design (Fourth Conference). ASTM STP 617 (American Society for Testing and Materials, 1977) pp 409-426

2 Ireland, D.R. "Instrumented impact testing for evaluating end-use performance" Physical Testing of Plastics-Correlation with End-Use Performanc~ ASTM STP 736 (American Society for Testing and Materials, 1981) pp 45-58

3 Winkel, .I.D. and Adams, D.F. "Instrumented drop weight impact testing of composite materials" Report UWME-DR-

301-108-0 (Department of Mechanical Engineering. University of Wyoming, USA. December 1983)

4 "Weaving engineered fabrics" Textile Industries(February 1975) pp 88-89

5 Van Hammersfeld, J. "Extensive cost reduction studies -- composite empennage component -- L-1011 commercial airliner" SAMPE Journal (May/June 1978) pp 25-33

6 Wood, S.A. 'Structural composites find big new market: jumbo jetliners" Modern Plastics (November 1981) pp 57-59

7 LeBlanc, D.J. et al'Advanced composite cost estimating manual -- volume I" Technical Report AFFDLL TR-76-87 (Northrop Corporation, Aircraft Division, Hawthorne, CA. USA, August 1976)

8 "Advanced impact resistant multidimensional composites" 'Final Technical Report (submitted to Naval Air Systems Command by Fibre Materials, lnc under NASC contract no N00019-77-C-0430, January 1979)

9 Miner, LH., Wolffe, R.A. and Zweben, C.H, "Fatigue. creep, and impact resistance of aramid fibre reinforced composites" Composite Reliabili~ ASTM STSP 580 (American Society for Testing and Materials, 1975) pp 549-559

10 Wardle, M.W. and Tokarsky, E.W. "Drop weight impact testing of laminates reinforced with Kevlar aramid fibres, E-glass and graphite" Composites Technology Review 5 No I (Spring 1983) pp 4-10

11 Miller, A.G., Hertzberg, P.E. and Rantala, V.W. "Toughness testing of composite materials" SAMPE Quarterly (January 1981) pp 36-42

12 Telecommunication with John RuybalL Effects Technology, Inc, Santa Barbara. CA, USA` July 1983

13 Takeda, N., Sierakowski, R,L. and Malvern, L.E. "Studies of impacted glass fibre-reinforced composite laminates' SAMPE Quarterly (January 1981) pp 9-16

14 Zoller, P. "Instrumentation for impact testing of plastics" Polymer Testing 3 (1983) pp 197-208

15 Stellbrink, ICK. and Aoki, ll.M. 'Effect of defect on the behaviour of composites' Proc 4th Int Conf on Composite Materials Tokyo. Japan 1982 pp 853-860

16 Transducer instrumentation, Model 208 series force transducer" (PBC Piezotronics, lnc, Buflhlo, NY. USA)

17 Cairns, D.S. and Adams, D.F. "Moisture and thermal expansion properties of unidirectional composite materials and the epoxy matrix' J Reinforced Plastics and Compo.~ites 2 No 4 (October 1983) pp 239-255

18 Hercules Product Data Sheet No 843-1 (Hercules, Inc. Wilmington, DE, USA, August 1981)

19 Kevlar49 Data Manual(E.l. DuPont de Nemours and Company, Inc, Wilmington, DE, USA, March 1978)

20 "Comparative data E, S and $2 glass" (Owens-Corning Fiberglas Corp, Toledo, OH, USA` May 1975)

21 losipesett, N. "New accurate procedure for single shear testing of metals' JMater 2 No 3 (September 1967) pp 537- 666

22 Walrath, D.E. and Adams, D.F. 'The iosipescu shear test as applied to composite materials" Experimental Mechanics 23 No 1 (March 1983) pp 105-110

23 Adams, D.F. and Walrath, D.E. "losipescu shear properties of SMC composite materials' Composite Materials: Testing and Design (Sixth Conference). ASTM STP 787 (American Society for Testing and Materials, 1982) pp 19-33

24 Wardle, M.W. "Impact damage tolerance of composites reinforced with Kevlar aramid fibres' Proc 4th lnt Confon Composite Materials op cit pp 837-844

25 Personal communication with M.W. Wardle, E.I. DuPont de Nemours and Company, Inc, Wilmington, DE, USA. October 1983

26 Coppa, A.P., Zweben, C.H. and Mirandy, L. 'Flywheel containment technology assessment' Report No UCRL-15261 (Lawrence Livermore Laboratory, Livermore, CA. USA. July 1980)

AUTHORS

Mr Winkei is currently a Project Engineer at the Fibre Sciences Division, EDO Corporation, Salt Lake City. UT, USA. Professor Adams, to whom inquiries should be addressed, is with the Department of Mechanical Engineering, University of Wyoming, Box 3295, Laramie, WY 82071, USA.


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