The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics

The Effect of Glass-Resin Interface Strength on the Impact Strength of Fiber Reinforced Plastics*

PETER YEUNG and LAWRENCE J. BROUTMAN

Department of Metallurgical and Materials Engineering lllinois lnstitute of Technology

Chicago, lllinois 60616

The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotched specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength. The propagation energy and thus the total energy for polyester laminates dis- plays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and thus the total impact energy increases with increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.

INTRODUCTION e influence of silane coupling agents on the Th strengths of fabric laminates is well-documented

(1-4). Silane coupling agents a r e ambifunctional molecules with the unique ability to improve the bond between organic polymers and silicious mineral surfaces. Chemically, these are hybrid materials that possess the dual functionality of an organic reactive group at one end of the molecule and an inorganic methoxysilyl group on the other end. The inorganic group is capable of chemically bonding with the silicious surfaces and the organic group at the other end of the molecule may form chemical bonds with the resin.

In the case ofglass fabric laminates, the interface bond is extremely important because within each lamina or ply of a laminate there are orthogonally oriented fibers.

* Prewnted at t h r l977 Heiirloiced Yl.!stic\ Conf., Sriciet! of the Pl.i\tic\ Ii idu*tiir\ , Inc (Fell 1977)

Thus, the transverse strength (i.e., strength when measured perpendicular to the fiber direction) dependent only upon interface and resin strength, becomes very important. For example. when measuring the flexural strength of a balanced fabric laminate in one of the filler directions, 50 percent of the fibers will be at 90" to the stress direction. Failure will be initiated by cracking between these fibers. Not only will interface bonding influence this intralaminar strength, but, of course, the interlaminar shear strength and interlaminar tensile strength will be greatly influenced. Both of these properties are important in determiIiiiig failure modes resulting from a transverse impact force such a s occurs in the Charpy impact test. Both interlaminar tensile and shear stresses are created in such an impact.

The instrumented impact test provides much more information than can he obtained from the dial of an impact testing machine (5-8). The load and energy ah-

62 POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, V O ~ . 18, NO. 2

The Effect of Glass-Resin lnterface Strength

sorbed by the specimen can be determined during the entire duration of the impact. Thus, it is possible to determine the dynamic strength as well as to separate the total impact energy into the initiation and propagation energy.

The purpose of this study was to quantitatively define the relationship between interface strength and impact energy of a glass fabric laminate. I t is commonly thought that decreasing the interface strength can increase the amount of delamination thus allowing a laminate to absorb more energy. This principle is very important when using laminates to resist impacting forces such as pro- jectiles in armor systems where maximum energy absorption is required. Although the phenomenon is qual- itatively understood, it is difficult to find any quantita- tive verification. In this study, we have been able to quantitatively define how interface strength as determined by short beam shear strength influences energy absorption. It is shown that a critical interface strength exits and below this value impact failure is dominated by delamination. In this regime, impact energy can be maximized by reducing bond strength. However, if the bond strength is above the critical value, then fiber failure dominates the fracture and to maximize impact energy requires maximizing interface strength. Epoxy laminates appear always to be above this critical value of bond strength and thus to maximize their impact performance one must maximize interface strength. Poly- ester laminates can be above or below the critical value of bond strength and maximizing the impact performance can be achieved by reducing interface strength.

MATERIALS The matrix materials used in this study were an epoxy,

Epon 828*, and a polyester, Paraplex P-43**. These resins were selected because of their different properties, both physical and mechanical, and because they provided different interfacial bonding conditions with the glass reinforcing phase. The glass fiber reinforcement used was a woven glass fabric, style 181*** with Finish 112, and style 7581***.

In order to vary the nature of the resin-glass interface, different surface treatments were applied to the f'abric. The following coupling agents+ were investigated.

1. 2-6020: (Trimethyoxysilylpropyl) ethylenedi- amine

(CH30)3Si(CH2)3N HC H2CH2NH2 2. 2-6030: y-Methacrylosypropyltrimethoxysilane

0 II

H2C=C--COCH2CH2CH2Si(OCH3)3 I

C H3

3. 2-6032: N-P-(N-vinylbenzylaminoethyl) -7-aminopropyltrimethoxysilane hydrogenchlo- ride

on the Impact Strength of Fiber Reinforced Plastics

4. XF-1-3563: Polydimethylsiloxane, (a hydroxy

The first three surface treatments are intended to improve the adhesive bond strength of the glass-resin interface. 2-6020 is recommended for epoxy resins, 2-6030 and 2-6032 are recommended for polyester resins and the XF-1-3563 is a mold release agent intended to reduce adhesion.

To apply the surface treatments, 0.5 wt percent of 2-6020 and 2-6030 were first dissolved in a slightly acidic water solution, with a pH value of slightly higher than four. For the mold release compound, XF-1-3563, a toluene solution was used instead of water; and in the case of 2-6032, 0.2 percent by weight of the coupling agent was dissolved in water. The glass fabrics were then soaked in the solutions for approximately five h, and dried in air for a period between 24 to 28 h before being used to make a laminate.

The procedures as suggested by the manufacturers were followed in the preparation of the resins. The epoxy resin was cured using 14 pph of meta- phenylene-diamine. The polyester resin was cured with 1.5 pph of benzoyl peroxide which was dissolved in 10 pph of styrene. The epoxy resin laminates were fabri- cated using a vacuum bag technique. Twenty plies of fabric were used and the laminates were cured at 220°F for 2 h. A post cure at 300°F for 2 h was also used. A slightly different procedure was used to fabricate the polyester laminates. Prior to placing the wet lay-up in a vacuum bag, it was placed in a vacuum oven for 1 h. After much of the entrapped air was removed, the lay-up was transferred to a mold (preheated to 220°F) and excess resin was squeezed from the lay-up. The mold was then placed in a compression hydraulic press and cured at 220°F for 2 h at approximately 100 psi.

functional polydimet hylsiloxane).

EXPERIMENTAL METHODS AND PROCEDURES Flexural Tests

Three point hend tests were conducted on an Instron testing machine at a crosshead speed of 0.05 in./min. The specimens were 0.5 in. wide, 4 in. in length and the thickness varied between 0.20 and 0.25 in. Five specimens were tested to determine an average value.

Short Beam Shear Test The short beam shear test was conducted on an In-

stron testing machine by means of a three point bending fixture. Here, the span was kept small (approximately 1 in.) to achieve a span to thickness ratio of 5 as recommended by ASTM Test D-2344. The test was conducted at a crosshead speed of0.5 in./min. and at least five tests were conducted for each case.

It was observed that the maximum load did not always correspond to a shear failure at the midplane axis. In some cases, a tensile failure in the outer fiber preceded

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, Vol. 18, NO. 2 63

Peter Yeung and Lawrence J . Broutman

Instrumented Charpy Impact Test

Charpy impact tests were performed with unnotched specimens on an instrumented Tinius Olsen impact testing machine. The capacity of this machine was 264-ft-lb with a 16.8 ft/sec striking velocity of the hammer. (The striking tip is of radius 0.315 in. with a chordal distance of 0.157 t 0.002 in.) The distance between the specimen anvils, and therefore the span length, is fixed at 1.574 in. (40mm). The specimens were0.5in. wide, 2.5 in. long and the other dimension equivalent to the plate thickness. The latter would be the beam thickness in the impact test.

The impact machine is instrumented so that the dynamic load vs time can be recorded on an oscilloscope and this signal is also integrated to also record the energy vs time trace. A more complete description of this in- strumentation is presented in reference (7).

Figure I shows a schematic diagram of the load-time and energy-time response in a typical impact test. The load-time history can be divided into two distinct regions-a region of fracture initiation and a region of fracture propagation. i n the initiation region, as the load increases, elastic strain energy is accumulated in the specimen which it acquires on contact with the striking head of the pendulum. in this region, no gross failure takes place; but failure mechanisms on a microscale, for example, microbuckling of the fibers on the compression side, or debonding at the fiber-matrix interfi ‘ace are possible. Whcn acritical load is reached at the end ofthr: initiation phase, the composite specimen may fail either

ti TIME

by a tensile or a shear failure depending on the relative values of the tensile and interlaminar shear strengths. At this point, the fracture propagates either in a cata- strophic “brittle” manner or in a progressive manner continuing to absorb energy at smaller loads. The total impact energy, U t , as recorded on the impact machine or on the energy-time curve on the oscilloscope, is therefore, the sum of the initiation energy, Ui, and propagation energy, U p . Since a high strength brittle material, which has a large initiation energy but a small propagation energy, and a low strength ductile material which has a small initiation energy but a large propag a t’ ion energy, may have the same total impact energy, know- ing the value of U t alone is not sufficient to properly interpret the fracture behavior of the material. The values of total impact energy, U t , and initiation energy, U i , can be divided by the cross sectional area of each specimen in order to obtain their normalized values. Thus,

ut ut = - bh

ui u. - ’ bh

where b = width of specimen (in.), h = thickne.;s of specimen (in.) and ut and u, are in units of ft Ibiin.’.

The propagation energy per unit area, u,, 1s given by:

or u, = ut - ui

Another proposed characteristic of the material that can be obtained from these relationships is the Ductility index ( D I ) , which is a dimensionless parameter arid is defined as the ratio of propagation energy to the initiation energy (9). Thus,

(4 )

Double Cantilever Cleavage Tests

The interlaminar cleavage test was readily adapted from standard double beam cleavage tests. A crack was initiated between the central laminae of thc specimcn 1)y means of LaTeflon film. The film was placed at one end ofthe plate, between the tenth and eleventh ply, during the f:abrication of the laminate. Because of the inability of Tcflon to properly hond with the resin material, a discontinuity was initiated at that end when the plate was cnred. Cleavage specimens ww then cut from the plate with dimensions a s shown in F i g . 2 . Aluminum blocks were bonded to the pre-cracked uid of the specimen to provide the means for load application. Half-inch spacings were then carefully marked on the sides of the specimens in order to determine the position of the crack front with respect to the load-deflection curve during the test.

The specimen w a s pinloaded on an Insti-on tcsting machine at a constant rate of deflection of 0.0s in./min.,

64 POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, VOI. 18, NO. 2

The Effect of Glass-Resin lnterfuce Strength on the l m p a c t Strength of Fiber Reinforced Plustics

ALUMINUM BLOCKS

TINTERLAMINAR STARTER CRACK

! i

Fig. 2. Double cuntilever interlaminar cleavage specimen.

and the crack was slowly propagated in the opening mode down the length of the specimen. The load- deflection curve was continuously recorded on the In- stron chart, and the area under the load-deflection curve, which represented the energy, was recorded on an area integrator attached to the testing machine. The load initially increased with deflection until the crack began to propagate, and subsequently, the load would gradually decrease. A typical load-deflection curve from the cleavage test is shown in Fig . 3 .

In most cases, the crack front could be located by using a high-intensity spot light focused on the specimen. The distance traveled by the crack front was recorded at half-inch intervals and the corresponding read- ings on the load-deflection curve and the area integrator were recorded. Crack growth in the cleavage specimen is inherently stable: increasing deflection must be applied to sustain crack growth through the specimen. Typically, the crack extends itself by short jumps, followed by arrest and re-initiation. The fracture surface work can he obtained from the load-deflection curve by measuring the instantaneous crack lengths at several

A y=21w

w=crock width

points and calculating the corresponding energy released during the crack growth process between these points. Since the fracture work is found to be indepen- dent of the crack length, except at the extreme ends of the specimens, the data can be presented by averaging a number of values for the central portion of the specimen.

Referring to Fig. 3, the determination of the interlaminar fracture surface work is shown below: If

y = interlaminar fracture surface work -

1 = distance travelled by crack front (in.)

t ~ : = width of crack (in.)

3

Then

the total energy released = 2ylw

But the energy is also represented by A , the area prescribed in a loading-cracking-unloading cycle; therefore:

2ylw = A (7)

RESULTS AND DISCUSSION Mechanical Properties of Epoxy Resin Composites

The effects of fiber surface treatments on the properties of composite materials can be best illustrated in cases where the bonding between the matrix and the reinforcement phase is weak. In the case of epoxy resin laminates, the matrix (epoxy) is capable of establishing a good ljond with the untreated glass surface. Thus, the significance of the surface treatments is less obvious, and other variables such as void content of the plates and the thickness of the specimens will also play an important role in determining the strength properties of the materials.

Table I lists some of the properties of the laminates prepared. In the flexural tests, all specimens failed by tensile failure of the outer fibers. It is seen that the flexural strength ofthe laminates is only slightly affected hy the surface treatments applied to the glass fabric, with the 2-6020 treated fiber composite showing the highest strength and the mold release compound treated composite showing the lowest. The flexural modulus is a small strain property of the material and is not expected to differ much for the various surface treatments. The lower modulus for the 2-6020 treated laminate may be attributable to the higher void content and the lower fiber content that exist in the laminate itself.

The shear strength properties are also reported in Table 1 . For the epoxy resin laminates all specimens failed by a shear mechanism. A comparison between the shear strength values shows that the 2-6020 treated specimens have the highest shear strength and the mold release compound treated specimens have the lowest,

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, Vol. 18, No. 2 65

Peter Yeung and Lawrence]. Broutman

Table 1. Mechanical Properties of Epoxy-Glass Laminates'

Short beam shear

treatment wlo vol % ratio ksi x 1 O6 psi ratio psi

Fiber Void Span to Flexural Flexural Span to Apparent Surface content, content, depth strength, modulus, depth shear strength,

No surface 61 0.08 18 64.3 2.9 5 treatment Coupling agent 56 1.08 16 68.4 2.6 4.5 6350

Mold release 61 0.38 18 59.5 3.0 5 5530 compound

(Z- 6020)

(XF-1-3563)

* Fiber material: E-glass, fabric style 7581, 20 plies; matrix material: Epon 828 + meta-phenylenediamine.

with the non-treated specimens having strength values in between the two. The differences observed are minimal.

In the cleavage test for determining interlaminar fracture surface work, the crack propagated primarily along the initial interface in most cases, though the crack follows a tortuous path resulting from the fabric geometry. Table 2 shows the interlaminar fracture surface work for the laminates tested, with the 2-6020 treated composite having the highest value for the fracture surface work. It should be noted, however, that in the cases of non-treated and mold release treated laminates, separation of an adjacent ply also occurred for most of the test duration, thus creating a double- delamination situation. Both resulting cracks then propagate down the length of the specimens as the deflection increases. To account for this phenomenon, it has been assumed that the area of surface created in the interface of the adjacent ply is exactly equal to the area of travel by the main crack, thus the values of the interlaminar fracture surface work are reduced by 50 percent. This as- sumption can only be accepted with reservation since two distinctly separated interfaces did not occur and the values of fracture surface work for these laminates can not accurately be determined. The interlaminar fracture surface work values are at least two times greater than for the unfilled epoxy, the difference resulting from the increased crack area, constraint of the resin between the glass and mixed interface-resin failure path.

The impact properties of the epoxy laminates are listed in Table 3 . In order to obtain a direct correlation between impact and conventional static properties,

Table 2. lnterlaminar Fracture Surface Work of Epoxy-Glass Laminates

Surface treatment

No surface treatment Coupling agent

Mold release compound

(2-6020)

(XF-1-3563)

lnterlaminar fracture Fiber Void surface work, y'

WIO vol O/O R-lblin:L erglcm' content, content, 105,

3.30** ave 0.16" 0'08 range 0.14-0.19 61

4.53 ave 0.22 range 0.1 7-0.30 56

3.35" o,38 ave 0.16" range 0.1 3-0.20 61

' The fracture surface work for the unfilled epoxy resin is 0.083 fl Ib/in.* (ref. 10). * * Assuming double delamination. The actual measured values are 0.32, but they have been divided by two in order to account for the additional ply separation.

Table 3. Impact Properties of Epoxy-Glass Laminates

Fiber Void Dynamic Duc- Surface content, content, ut ui u, strength tility

treatment w/o vol % ft-lb/in.2 U, psi index*

No surface 61 0.08 137 46 91 80200 2.0 treatment Coupling 56 1.08 132 43 89 80900 2.1 agent

Mold release 61 0.38 130 45 85 84100 1.9 compound

(2-6020)

(XF-1-3563)

*Ductility index =". u, '

3-point static bend tests were also performed, using a span length equal to that ofthe impact test (40 mm) and a crosshead speed of0.02 in./min. The results of the static bend tests are listed in Table 4 . The effect of this reduced span-to-depth ratio is to slightly increase the calculated flexural strength and reduce the calculated modulus (see Table 1 ) . Also, because of the reduced span-to-depth ratio, the failure mode for some of the flexural tests appeared to be a shear mode. The energy values have been normalized by dividing by the cross- sectional area of. each specimen, thus removing the influence of small variations in cross-sectional dimensions from specimen to specimen. This is not to imply, however, that the results presented here are representa- tive of those that would be obtained using a specimen of different dimension, since size effects may be pro- nounced in composite materials.

Table 3 indicates that the normalized initiation, propagation and total impact energies, as well as the dynamic strength of epoxy laminates are quite similar, regardless of the surface treatments applied to the glass fabric. A closer examination of the failed specimens shows that two modes of failure are dominant in impact fracture of reinforced laminates, namely delamination and fiber breakage. Figures 4-6 show the load and energy history, as well as the failed specimen for some of the impact tests. It can be seen that the impact load rises to a maximum and then decreases; but in some other cases, other maxima also exist. This indicates an ability for the material to carry reasonable loads even after initial failure has occurred. By examining the failed specimens, it can he noticed that for a well-bonded composite, such as the 2-6020 treated laminate, the degree of delamination is considerably less, when compared with other non-

66 POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, Vol. 18, No. 2

The Effect of Glass-Resin lnterface Strength on the lmpact Strength of Fiber Reinforced Plastics

Table 4. Slow Bend Properties of Epoxy-Glass Laminates Using a Span Length of 40 mm

Flexural Flexural Normalized Surface Fiber content, Void content, strength, modulus, Static energy, static energy,

treatment WIO vol Yo ksi x106 psi in.-lb R-lblin.2

No surface 61 0.08 68.4 2.35 44.6 36.2 treatment Coupling agent 56

Mold release 61 (2-6020)

1.08 70.6 1.88 40.8 29.7

0.38 65.6 2.41 45.0 36.7 compound (XF-1-3563)

treated laminates. The bulk of the impact energy has been dissipated through the breakage of the fibers during the initial load drop. Subsequent failure involves limited delamination of an individual ply as shown in Fig. 4 . In the case of the non-treated or mold release treated specimen, a few additional delaminations occur (Figs. 5 and 6 ) but on the average only minimal energy differences exist for the three types of laminates.

The static flexural energies of the laminates tested are considerably lower than the dynamic values. This may result from the lower strengths of the specimens tested at low strain rates and it was also observed that less delamination occurred in these specimens.

Mechanical Properties of Polyester Resin Composites Polyester resin is known to have a lower bond

strength to E-glass than the epoxy resin. Thus, by con- ducting experiments with fiberglass laminates using the polyester as the matrix material, it was expected that the effect of glass surface treatment on the properties of the laminate will lie more evident.

Table 5 lists the flexural properties of the fiberglass- polyester laminates. It should be noted that two types of fiabrics were used: style 181 with a 112 finish and style 7581. The 112 finish is a heat-cleaning process that is carried out by the supplier to burn off excess sizing

compound that may be present. The two types offabrics are extremely similar and results for the style 7581 laminates are included in order to compare the properties of epoxy and polyester composites. The effects of the surface treatments are obvious, the 2-6030 treated specimen showing the highest strength of approximately 70 ksi. The application of a coupling agent improves the flexural strength of the laminate by over 30 percent. The mold release treatment weakens the bond at the interface and decreases the strength relative to the 112 finish by over 40 percent. The manner in which the flexural specimens fail is ofparticular interest (Fig. 7 ) . In the case of a well-bonded specimen, such as the 2-6030 treated or the 2-6032 treated laminates, the initial failure occurs on the outermost ply which is subjected to the highest strain. After the outer fibers have broken in tension, and upon further deflection, the newly-created cracks will travel along the interface between the outermost ply and the adjacent ply in a delamination mode towards the ends of the specimen. The cracks will con- tinue to propagate until the strain is such that the second outermost ply begins to fail in tension. Delamination again occurs, and this phenomenon continues until the specimen can no longer carry any load. In the non-

TIME (0.5 msec/div )

TIME (0.5 mwc/div 1

F i g . 4 . Locitl ( i i i d energ!/ curccs f o r 2-6020 trented gl(~s.r.-epox!/ composite (1nd failed i i i ip /~c t .specivieri.

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, Vol. 18, No. 2

Fig . .5. Looil i111i1 energ!/ curue,s for non-treated glass-epoxy ('0 rti posi te ti 11 (1 f i i i led irn p e t specimen .

67

Peter Yeung and Lawrence J . Broutrnan

112 FINISH

2 - 6 0 3 0

TIME ( 0.5 msecldiv )

Fig. 6. Loud und energy curves of mold releuse (.YF-l-3563) treated glass-epoxy composite arid fuiletl impuct specimen.

treated or the mold release treated specimens, little delamination can be observed and tensile fiailure under bending is produced.

Improvements in the shear strength of fiberglass- polyester laminates for surface treated glass fabrics can also be seen in Table 5. A better than 60 percent im- provement in the shear strength can be achieved in the case of the 2-6030 treated specimens. The 40 percent

Fag 7 Comparison of modep of fualure zn bendaiig f o r 2-6030 (well-honded) arid non-treutecl polyester-glnss laminutes

decrease in the shear strength for the mold release treated specimens is about the same as in the flexural strength.

The interlaminar fracture surface work obtained by the cleavage technique for the fiberglass-polyester laminates is shown in Table 6, and little or no improve- ment can be seen for the 2-6032 treated specimens. In fact, a higher value for yis observed for the mold release treated specimens. This may result from additional delamination comparable to the effect with the epoxy resin laminates.

The impact data and the results from static bend tests using a span length equal to that used in the impact test are listed in Tables 7 and 8 . In comparing the results from Table 8 to those of Table 5 , it should be noted that the void contents are different and thus the effect of the span-to-depth ratio cannot be clearly defined.

Table 5. Mechanical Properties of Polyester-Glass Laminates*

Void Flexural Flexural Short beam Fabric Fabric content, strength, modulus shear strength style finish Surface treatment vol Yo ksi x1Oe psi SH, psi

7581 - None 0.88 40.4 2.52 3530 181 112 None 0.52 53.4 2.89 4760 181 112 Coupling agent 0.16 70.1 2.99 7650

181 112 Coupling agent 0.31 67.5 2.90 6630

181 112 Mold release 0.76 30.3 2.58 2680

(2-6030)

(2-6032)

compound (XF-1-3563)

* Matrix material: Polyester, Paraplex P-43 + 10 w/o styrene + 1.5 wio benzoyl peroxide; glass fiber content: 56-58 wio. Span to depth ratio for flexural tests was 17 and for short beam shear tests was 4.3.

Table 6. lnterlaminar Fracture Surface Work of Polyester-Glass Laminates'

lnterlaminar fracture surface work, y**

Surface treatment Fiber Content Void content ft-lblin? x 10" erglcm'

No surface treatment Coupling agent (2-6032)

58

58

0.20 ave 0.18 range 0.14-0.22 3.70

3.74 ave 0.19 range 0.13-0.20 0.1 7

4.41 ave 0.21 range 0.18-0.25 58 0.98 Mold release

compound (XF-1-3563)

* Fiber material: E-glass style 181 finish 112 20 piles. matrix material: polyester resin, Paraplex P-43 + 10 wio styrene + 1.5 wlo benzoyl peroxide. ** The fracture surface wbrk of the hnfilled poiyester re'sin is 0.019 R Ib/in.2 (ref. 10).

68 POLYMER ENGlNEERlNG AND SCIENCE, MID-FEBRUARY, 1978, Vol. 18, No. 2

The Effect of Glass-Resin Interface Strength on the lrnpact Strength of Fiber Reinforced Plastics

Table 7. lrnpact Properties of Polyester-Glass Laminates

Fabric Fabric content, content, ut U, U P strength Ductility style finish Surface treatment wlo vol Yo ft-lbflin. urn, Psi index

7581 - None 57 0.81 69 10 59 41 200 5.8 181 112 None 58 0.52 59 18 41 601 00 2.4 181 112 Coupling agent 56 0.16 86 22 65 71 100 3.0

181 112 Coupling agent 58 0.31 87 20 67 65300 3.0

181 112 mold release 56 0.76 125 8 117 35900 14.5

Fiber Void Dynamic

(2-6030)

(2-6032)

compound (X F-1-3563)

Table 8. Static Bend Properties of Polyester Glass Laminates Using a Span Length of 40 mm

Flexural Flexural Normalized Surface Fiber content, Void content, strength, modulus, Static energy, static energy,

treatment wlo vol Yo ksi x106 psi in.-lb ft-lblin? ~~ ~~

No surface 58 treatment Coupling agent 57 (2-6030) Coupling agent 57 (2-6032) Mold release 57 compound (XF-1-3563)

0.70

0.80

0.78

0.74

59.5 2.24 38.2 28.5

56.9 2.1 1 63.2 46.3

62.2 2.19 49.8 36.6

29.9 1.92 53.3 39.6

Since the total impact energy of a composite is the sum of the initiation and propagation energies, the fac- tors governing the individual components will indeed have a profound effect on the total impact energy. Two composite laniiilates may have an identical total energy with one having a high initiation energy and the other a high propagation energy. By examining the modes of failure from the fractured specimens and correlating these with the data obtained from the load and energy vs time curves, it has been found that the interfacial bonding of a laminate bears an important role in determining the modes of failure and thus the impact energies of the system.

For a well-bonded laminate, such as the 2-6030 or the 2-6032 treated specimens, the load can be transferred inore adequately from matrix to fibers, and the transverse strength is greater. Thus, the stress required to cause cracking between the transverse fibers will he greater. The initial impact energies are thus greater for the well-bonded systems. The good bonding at the interface tends to restrain dt.lamination due to the impacting load normal to the specimen surface and the specimen fails mainly by fiher breakage with a small degree of delamination (as shown in Fig. 8). The total impact energy of these composites is greater than for the composites with no surface treatment but not a s great as for the composites treated with a mold release*. This will be discussed in more detail later.

Careful examination of the non-treated or the mold release treated specimens shows that less filier breakage is involved in the failure ofthew specimens. Most ofthe energy is dissipated by delamination of the individual plies (Figs. 9 and 10). The stress normal to the laminate surface created by the impact and the weak transverse strength of these laminates induce delaminations be-

tween plies. It has also been noted that the indentation upon impact is deeper and local buckling of fibers, associated with delamination, increases the propagation energy of the mold release treated composite, thus giv- ing a very high total impact energy value for these specimens (Fig. 11 ). However, the strength of the laminate is greatly decreased by the mold release treatment, as can be observed in both the impact and the static bend tests. Because of the high propagation energy associated with the buckling and delamination, the ductility index

TIME ( 0.5 rnsec/div )

Fig. 8 . Loud c i n d energ!/ curz;es f o r 2-6032 treuted glass- pol!/cster coinpimite und fuiled inipuct specitneti.


Peter Yeung and Lawrence J. Broutman

- impact energy of the mold release treated laminate is 40 percent greater than for the silane coupled laminates.

As in the case of epoxy laminates, the energies absorbed in slow bend tests are much less than in impact (0

Fig. 11. Deep indentation und compressivejiber buckling in a mold releuse treated glass-polyester composite in impact.

I I I I I I \I: 3 4 5 6 7 8

TIME. (0.5 msec/div )

Analysis of Impact Behavior

Through the use of the instrumented impact test, analysis of failed specimens, and measurement of the laminate mechanical properties it is possible to establish correlations between impact strength and other mechanical properties and to assess the importance of material variables on impact strength.

The objective of this study was to relate the glass-resin interfacial strength to the transverse impact strength of a laminate as measured by an unnotched Charpy impact test. The apparent shear strength as determined from a short beam shear strength test is used as a measure ofthe interface strength. The results for the polyester and epoxy laminates are shown in Figs. 12 and 1.3 in which the values of ui, up and ut are plotted as a function of laminate shear strength. Also included in these figures are data points for specimens which had either been exposed to water at room temperature for 7 days or boiling water for 2 h. Although these experiments were

Fig. 9, Loud und energy curves f o r non-trented glass-polyester composite und fuiled impact specimen.

TIME ( 0.5 mrec/div )

150

hl c \

I c .c

.- 9

IOC

n - I) .- I)-

Fig. 10. Loud ui id energ!/ curces f o r mold re1eci.w treuted pdy- mtcr coinpo~site uiid fuilcd iifipuct .specinieii.

F 5(

2 r 0

of a mold release treated specimen is more than 4 times greater than that for a well-bonded laminate. The total

The Effect of G~as,~-Re.sin lnterface Strength on the lmpact Strength of Fiber Reinforced P1ustic.s

1501

EPOXY LAMINATES

WATER DRY BOILED

2 HRS. A u, A

up 0 0 ui 0

0

I l l , I , 5 6 7

APPARENT SHEAR STRENGTH CS,,), ksi F i g . 13. linpuct energ!! us short beuni slaeur strerigth f o r fiberglass-reinforeeci e p o q laminates.

not discussed earlier, the effect of environmental expo- sure was to reduce the shear strengths with modest effects on impact strength. These studies will bc discussed more complttely in a later pulilication. It is shown inFigs. 12 and13 that the points representing the environmentally exposed specimens can he plotted with thc points representing dry specimens and a single curve may fit both sets of points when plotting impact energy vs shear strength.

It can be seen that, ui, the initiation energy increases with increasing shear strength for both polyester and cpoxy laminates. As the shear strength increases. the flexural strength of these fiabric laminates also increases reflecting better interfacial bonding and greater values of transverse and undoul)tedly interlaminar tensile strengths. The initiation impact energies are much grmter for the c p x y laminates again reflecting their grcater flexural strengths.

In the case of polyester laminates the curves for propagation and total impact encrgy appear to have a minimum. Ahove a critical valuc of interlaminar shear strength the total impact energy increases with increasing shear strength. Below the critical value of shear strength. the impact energy increases with decreasing values of shear strength. As shown in Fig. 12, and from Fig.9. 8-10, delamination appears to Iw the dominant failure inode lielow the critical value. while fil)er failrirc, is the dominant mode almve a critical value of shear strength. Thus, in case of polyester laminates (me can optimize the total impact resistaiict. by reducing the

interfacial bonding. The mold release treated surface produced the greatest value of impact strength. It should be noted (Figs. 8-10) that the initiation of failure will require less energy for the mold release treated laminate and the large value of total impact energy is achieved during the delamination phase occurring after failure initiation. The specimen supports less load during propagation but absorbs more energy due to the large deflections which the specimen can sustain. In the case of epoxy laminates, the interface bonding is not reduced to a low enough value to induce severe delamination (Figs. 5 and 6 ) . Thus, as in the case of polyester laminates above a critical shear strength value, the impact energy increases with increasing shear strength. It is interesting to note that the maximum impact energy observed for a silane treated epoxy laminate is nearly identical to the impact energy for a mold release treated polyester laminate. The failure mode for the epoxy laminate is predominantly fiber failure whereas for the mold release treated polyester laminate it is delamination.

In the cases where delamination is the principal failure mode one must consider whether the absorption of energy is influenced by the delamination fracture work or b y the area of delamination. For example, by altering the interface strength through glass surface treatments, does the change in impact energy result from an altera- tion of the delamination work or from the amount of delamination? From our results one can conclude that the surface treatment did not have a significant effect on interlaminar fracture surface work (Table 6). Figure 14 also demonstrates that the fracture surface work is inde- pendent of interlaminar shear strength. Thus, it appears that the reduction of interface strength allows more extensive delamination to occur as opposed to changing the work of delamination.

ACKNOWLEDGMENTS This work was submitted as an M.S. thesis by Mr.

Peter Yeung in partial fulfillment of the requirements of the degree of Master of Science in Metallurgical and Materials Engineering at Illinois Institute of Technol- ogy. The authors gratefully acknowledge the support of the U.S. Air Force Office of Scientific Research, Grant No. AFOSR 72-2214F, and Mr. William Walker, our technical monitor.

0 DRY

0 WATER-SOAKED 7 DAYS

T 4 i


Peter Yeung nnd Luwrence]. Broutman

REFERENCES

1. L. J . Broutman, “Fiber Reinforced Plastics,” in “Modern Composite Materials,” Rroritrnaii and Krock, Atldison- Wesley Publishing Co., Re;tding, Mass. (1967).

2. E. Pluedderniann, Ed., “Interfaces in Polymer Matrix Composites,” Ch 6, Vol 6 i n “Composite Sllaterials,” Academic Press (1974).

3. P. J . Orenski, S. E. Berger, and hl. W. Ranney, “Silane Coupling Agents-Performance in Engineering Plastics,” 28th Ann. Tech. Conf. Proc., RPK Inst., SPI, Sec. 3-E (1973).

4. Sf. W. Ranney, S. W. Berger, and J . G . Marsden, “Silane Coupling Agents in Particulate Slineral-Filled Cornpos-

72

ites,” 27th Ann. Tech. Conf. Proc., RPiC Inht., SPI, Sec. 21-D (1972).

5. D. F. Adanis and J. L. Perry, Composites, 6, 1, (1975). 6 . D. F. Adams and J . L. Perry, Fiber Sci. Techttol., 8, 275. 7. L. J . Broutinan and P. K . Mallick, “hipact 13ehaviour of

Hybrid Composites,” 30th Ann . Tech. Conf. Proc., RPiC Inst., SPI, Sec. 18-F (1975).

8. D. F. Adams and A. K. Miller,Mat. Sci. Eiig., 19.245(1975). 9. P. Beaumont, P. Riewald, and C. Zweben, “ntlethods for

Irnprovirig the Impact Resistance of Conipositc Materials,” ASTSI STP 568. p. 134 (1974).

10. F. J . McGarry and J . F . Mandell, “Fracture Toughness of Fibrous Glass Reinforced Plastic Composites,” 27th Ann . Tech. Conf. Proc., RPiC Inst., SPI, Sec. 9-A (1972).

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY, 1978, V O ~ . 18, NO. 2

Documents

The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics