INTERIAMINAR SHEAR STRENGTH

OF GLASS-FIBER-REINFORCED

PLASTIC LAMINATES

Ne.1848

September 1955

INFORMATION REVIEWEDAND REAFIIRMED

/960

This Veport is One of a Seriesissued hi Cooperation with theANC-17 PANEL ON MASTICS FOR AIRCRAFTof the Departments of theAIR FORCE, NAVY, AND COMMERCE

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UNITED STATES DEPARTMENT OF AGRICULTUREFOREST PRODUCTS LABORATORY

FOREST SERVICEMADISON 5, WISCONSIN

In Cooperation with the University of Wisconsin

INTERLAMINAR SHEAR STRENGTH OF GLASS-

FIBER-REINFORCED PLASTIC LAMINATES1—

By FRED WERREN, Engineerand

B. G. HEEBINK, Engineer

Forest Products Laboratory,? Forest ServiceU. S. Department of Agriculture

Summary

This report presents the results of tests to determine the interlaminarshear strength of glass-fiber-reinforced plastic laminates. Eleven differ-ent types of laminates were tested to investigate the effect of (1) typeof resin, (2) type of reinforcement, and (3) resin content on the inter-laminar shear strength of laminates. About 165 shear tests were made afternormal and wet conditioning and with loads applied at angles of 0° and 45°.

There was considerable variation in the test results, but the epoxidelaminate made with Epon 1001 resin was consistently highest in inter-laminar shear strength. Polyester laminates of 181 and 112 fabrics hadhigher shear strength than did similar laminates with reinforcements of143 glass fabric or glass mats. A decrease in resin content of polyesterlaminates of 181 fabric appeared to result in lower shear strength at the0° angle of load but higher shear strength at the 45° angle. In general,specimens tested after wet conditioning had somewhat lower shear strengththan those tested after normal conditioning.

Introduction

In certain structural applications of glass-fiber-reinforced plasticlaminates, there has been some evidence of interlaminar shear failure ofthe laminates. Interlaminar shear strength may be affected by variousfactors, including kind or type of resin, type of reinforcement, and per-centage of resin. The designer must have data available on these strength

-This progress report is one of a series prepared and distributed by theForest Products Laboratory under U. S. Navy, Bureau of AeronauticsOrder NAer 01610 and U. S. Air Force Order DO (33-616)53-20. Resultshere reported are preliminary and may be revised as additional databecome available.

4Iaintained at Madison, Wis., in cooperation with the University ofWisconsin.

Rept. No. 1848

Agriculture-Madison

values so that suitable laminates may be used in those structural applica-tions where interlaminar shear strength is of importance.

At the time this study was initiated, few data were available on the in-terlaminar shear strength of laminates. It was the purpose of this pro-gram to determine the interlaminar shear strength of (1) laminates madefrom a single fabric and four different types of resins, (2) laminatesmade from a single resin and five different types of glass-fiber reinforce-ments, and (3) laminates made from one combination of glass fabric andresin but having different resin contents. The study was undertaken atthe Forest Products Laboratory at the request of and in cooperation withthe ANC-17 Panel on Plastics for Aircraft.

The interlaminar shear, tensile, and compressive strengths of eleven dif-ferent laminates are presented in this report. Some of the materials testedwere not developed or intended by the manufacturer for the conditions towhich they have been subjected. Any failure or poor performance of amaterial is therefore not necessarily indicative of the utility of thematerial under other conditions or for all possible applications.

Description of Material

All polyester laminates and the silicone laminate were fabricated at theForest Products Laboratory. The phenolic laminate was fabricated by theresin manufacturer. The epoxide laminates had been forwarded to theLaboratory earlier by the Wright Air Development Center, and the inter-laminar shear properties, which have been previously reported along withother datarl are included in this report.

Prior to fabrication of laminates at the Forest Products Laboratory, checktests were made of materials to insure conformance with applicable specif-ications. The polyester resin was shown to meet the strength requirementsof Specification MIL-R-7575A, and the fabrics finished with Volan A werecapable of producing laminates meeting the strength requirements ofSpecification MIL-P-8013A. The phenolic laminate, as fabricated by theresin manufacturer, was requested to be "typical of a laminate of thistype, and of materials that conformed with applicable military specifica-tions."

A brief summation of the fabrication methods used with each laminate isgiven in table 1. Additional details of fabrication for the differenttypes of resin follow.

-Supplement to Mechanical Properties of Plastic Laminates. ForestProducts Laboratory Report No. 1820-3, 1955.

Rept. No. 1848 -2-

Polyester Laminates--Selectron 5003 Resin

The polyester laminates, each 1/4 inch thick and about 18 inches square,were fabricated at the Forest Products Laboratory. The laminates werefabricated with resin from the same batch, and essentially the same fabri-cation procedures were used. The resin was catalyzed with 0.8 percent ofbenzoyl peroxide by weight. The lay-up of the glass-fiber reinforcementand the resin was made between cellophane-covered, 1/4-inch-thick aluminumcauls. Each panel was cured for 1 hour and 30 minutes in a press at atemperature that was gradually increased from 220° to 250° F.

All panels were parallel-laminated except panel No. 117, made of 143fabric, which was cross-laminated.

The three polyester laminates of 181 fabric were fabricated at differentpressures to obtain different resin contents, and were made from fabrictaken from the same roll. Sixty-nine sheets of fabric were cut and selectedat random for the three different panels. Panel 421 was cured at 14 poundsper square inch, panel 421A at 5 pounds per square inch, and panel 423 at50 pounds per square inch pressure. Thus, the panels were made fromrandomized fabric and the same batch of resin under identical fabricationconditions except for laminating pressure.

Silicone Laminate--Dow Corning DC 2104 Resin

The silicone laminate, nominally 1/4 by 18 inches square, was fabricatedat the Forest Products Laboratory. The panel was parallel-laminated of23 plies of heat-cleaned 181 fabric which had been impregnated with resinprior to receipt at the Laboratory.

The impregnated fabric was laid up between 1/16-inch cellophane-coveredaluminum cauls and placed in a press at a temperature of 340° F. and atlow pressure for 45 seconds. The press was then opened to "breathe" thepanel and closed to full pressure within 1 minute. The panel was curedfor 17 minutes at a pressure of 45 pounds per square inch and a tempera-ture of 340° F., cooled under pressure, and removed from the press. Thepanel was then post-cured in an oven for 16 hours at 90° C., 1 hour at125° C., 1 hour at 150° C., 1 hour at 175° C., 1 hour at 215° C., 140hours at 250° C., and cooled slowly in the oven.

Phenolic Laminate--Bakelite BVQ 11946 Resin

The phenolic laminate received from the manufacturer of the resin wasabout 1/4 by 14 by 16 inches. According to the manufacturer, the panelwas made up of 28 plies of 181 glass fabric finished with Volan A andparallel-laminated with BVQ 11946 resin. The manufacturer stated that,

Rept. No. 1848 -3-

after lay-up, the panel was cured in a press for 60 minutes at a tempera-ture of 325° F. and a pressure of 100 pounds per square inch, to 0.250-inchstops; the panel was then post-cured for 10 hours at 325° F.

Epoxide Laminates--Shell Epon 1001 and Epon 828 Resins

Two parallel-laminated panels were received at the Laboratory in November1953 from the Wright Air Development Center, Wright-Patterson Air-ForceBase, Ohio. A description of the laminates, as furnished by the supplier,follows:

"Panel No. 225 was a dry lay-up, nominally 3 feet square, consisting of12 plies of 181-Volan A glass fabric impregnated with Epon 1001 contain-ing 4 percent by weight dicyandiamide. The laminate was cured againststainless steel cauls with two sheets of 0.060 inch alpha-cellulose aspadding between each caul and the adjacent press platen. Contact pressurewas.maintained for 20 minutes, followed by 25 pounds per square inch for30 minutes. The temperature of the press was 345° F. The resin contentof this panel is 32 percent.

"Panel No. 226 was a wet lay-up, nominally 3 feet square, consisting of12 plies of 181-Volan A glass fabric impregnated with Epon 828 contain-ing 8 percent by weight of Curing Agent A. Cauls and padding were identi-cal to those described above. The laminate was cured for 30 minutes at25 pounds per square inch at 240° F. The resin content is 32 percent byweight.

"The Epon 1001 and Epon 828 resins and curing agents are products of theShell Chemical Company."

Values irom Cured Laminates

After fabrication and cure were completed, each laminate was trimmed,measured and weighed, and Barcol hardness readings were taken. Averagevalues of thickness, specific gravity, resin content, and Barcol hard-ness, 4 determined from Laboratory data, are presented in table 1.Since eq6h laminate was made by recommended and accepted fabricationprocedures, the panels are considered to be typical of their type. Ifdifferent fabrication methods had been employed, such as other laminat-ing pressures, the resulting laminates would have had different physicaland mechanical properties.

Rept. No. 1848 -4-

Testing

Polyester, Silicone, and Phenolic Laminates

Ten tension and ten compression specimens were cut parallel to the warpdirection of the top ply of each laminate, except that only five tensilespecimens were obtained from the phenolic laminate. Fourteen to sixteeninterlaminar shear specimens, at both 0° and 45° to the warp directionof the top ply, were also cut from each laminate.

Tensile specimens conformed with the requirements of the Type II specimenof Federal Specification L-P-406b.± They were loaded to failure in amechanical testing machine equipped with Templin tension grips. Load wasapplied at a head speed of 0.20 inch per minute.

Compression specimens were 1 inch wide, about 1 inch long, and the thick-ness of the laminate. The length of specimen was such as to provide aslenderness ratio between 11 and 15, as specified in Federal SpecificationL-P-406b. Bearing ends were carefully surfaced with a surface grinderprior to testing to insure flat and parallel loading ends. The specimenswere tested in a hydraulic testing machine at a head speed of 0.04 inchper minute. Failure was a combination of transverse shear and crushingof fibers.

Interlaminar shear specimens were of the general type used in testing theshear strength of glue joints in blocks of wood. Sketches of the testspecimen and of the test method are shown in figure 1. The bearing edgesof the specimens were carefully machined prior to testing to insure flatand parallel loading surfaces. The specimens were then mounted in theglue-line shear apparatus (fig. 2). Load was applied with a mechanicaltesting machine at a head speed of 0.01 inch per minute. In general,the failure was in delamination between two adjacent layers of reinforce-ment.

Tension, compression, and shear tests were made after (1) at least 2weeks' conditioning at a temperature of 73° F. and a relative humidityof 50 percent, and (2) 2 hours' immersion in boiling distilled water.Maximum loads were determined from each test.

4–Plastics, Organic: General Specifications, Test Methods. Sept. 1951.

Rept. No. 1848 -5-

Epoxide Laminates

Descriptions of the methods used to test specimens from the epoxidelaminates are given elsewhere,1 and details need not be repeated here.The interlaminar shear specimens were built up of three 1/8-inch pliesof laminate and, except for thickness, were the same as the specimens ofthe other laminates. Wet conditioning for the tensile and compressivespecimens was 30 days' immersion in water at room temperature instead ofthe 2-hour immersion in boiling water used with the other laminates. Theepoxide shear specimens, however, were subjected to 2 hours in boilingwater as were the shear specimens from the other laminates.

Presentation of Data

The polyester, silicone, and phenolic laminates described earlier werefabricated specifically for these tests, and the results given are based ontests of these materials. Results of tests of the epoxide laminates aretaken from an earlier report,I and are presented herein for comparativepurposes.

The results of tension and compression tests are given in table 2.Average strength values for both the dry and wet condition are given, aswell as the average percentage increase in weight of the compressionspecimens resulting from the 2-hour immersion in boiling water.

Table 3 presents the maximum, minimum, and average interlaminar shearstrength values, dry and wet, and at angles of 0° and 45° to the warpdirection, of five parallel laminates made of 181-Volan A glass fabricand different resins. Table 4 is a similar presentation except thatdata on five polyester laminates having different reinforcements aregiven. Data showing the effect of fabrication pressure and subsequentresin content on interlaminar shear strength of three polyester laminatesare given in table 5.

The average interlaminar shear strength of the different types oflaminates, as taken from tables 3, 4, and 5, are presented graphicallyin figure 3.

Discussion of Results

The purpose of obtaining the tensile and compressive strength valuesfor the polyester, silicone, and phenolic laminates (table 2) was toindicate the general quality of these laminates. In general, thetensile properties were reduced only slightly after the wet conditioningperiod, but there was an appreciable reduction in compressive strength.

Rept. No. 1848 -6-

The increase in weight due to boiling specimens was about two or threetimes greater for the mat-reinforced polyester laminates than for thefabric-reinforced polyester laminates, but the increase was still onlyabout two-thirds of 1 percent. The silicone laminate, however, increasedin weight about 7-1/2 percent during the period of immersion in boilingwater.

The 181, 112, and resin-bound mat polyester laminates all met the minimumtensile and compressive strength requirements of Specification MIL-P-8013A.Since the 143 laminate was cross-laminated, no check could be made forconformance with the specification. The laminate made with the mechanical-ly bonded mat failed to meet both tensile strength requirements, probablybecause of the high resin content.

At the present time, there is no accepted standard method for making inter-laminar shear tests. Various methods are used and others have been sug-gested, including tensile-, compression-, and flexural-type specimens.Suggested methods which result in a reasonably uniform shear stress dis-tribution over the shear area require a rather complex and expensive testspecimen and procedure.

Experience in determining the shear strength of wood and the shear strengthof a glue bond between blocks of wood led to the selection of the type ofspecimen used in this study. This type of block-shear test obviously doesnot apply pure shear stress to the specimens. It has the further dis-advantage of requiring a rather bulky apparatus (fig. 2). When used withwood specimens which are carefully prepared and tested, however, it hasdemonstrated reproducibility and consistency of results. If shear speci-mens of laminates are also carefully prepared and tested, it is believedthat reasonable shear values may be expected. Although these values maynot be the true interlaminar shear strength of the material, tests ofdifferent types of laminates, such as have been evaluated in this study,may be expected to show the relative shear strength of the differentmaterials.

Of the 181 laminates made with different types of resins, the siliconelaminate had much lower interlaminar shear strength than did the otherlaminates. The epoxide laminate made with Epon 1001 resin had a some-what higher shear strength than did laminates made with other resins.

The interlaminar shear tests of the polyester laminates with differenttypes of glass-fiber reinforcement indicated that the 181 and 112laminates had the highest and about the same shear strength, while the143 and mat-reinforced laminates were slightly lower in strength.

Within the range of resin contents tested for the 181-polyester laminates,dry shear strength at 0° appeared to remain about the same. At thelowest resin content, however, the wet shear strength drcpped appreciably.

Rept. No. 1848 -7-

It seems reasonable to assume that further reductions in resin contentmight further reduce the shear properties at 0° loading. Shear strengthat 45°, however, generally increased with decreased resin content.

The shear strength of the 181 laminates was, in general, higher at the45° direction of loading than at 0° loading. This may possibly be dueto the "nesting" of the threads in adjacent laminations, which offeredlateral resistance to 'shear movement. In the polyester laminates, thisdifference became more pronounced as laminating pressure was increasedand resin content decreased, which, within limits, is to be expected. The181-phenolic laminate also showed an appreciable increase in shear strengthat 45° over that at 0°, but in the epoxide (Epon 1001) and silicone lamin-ates the difference in shear strength at the two angles of loading wassmaller. An appreciable directional increase at 45° was found also withthe 143 laminate, but the increase at 45° was less for the 112 laminate.Since the 112 fabric is much thinner than the 181 and 143 fabrics, "nest-ing" effects would be expected to be less pronounced.

There were no definite directional effects in the mat laminates. Iffiber distribution and orientation is uniform within a mat, propertieswould be expected to be the same in all directions in the plane of thelaminate.

Wet conditioning, in general, appeared to result in a slight reductionin shear strength of the laminates, but results were somewhat erratic.Wet strengths, on the average, were about 7 percent lower than the drystrengths for the polyester laminates, the greatest reduction occurringin the mechanically bonded mat (tables 4 and 5). Wet conditioning hadthe least effect on the epoxide laminates, with only a minor drop instrength for the Epon 1001 laminate and increased wet strength for theEpon 828 laminate.

Summary of Results

Interlaminar shear tests were made of 11 different glass-fiber-reinforcedplastic laminates. The tests were made to determine the effect of (1)type of resin, (2) type of reinforcement, and (3) resin content on theshear strength of the laminates. Data were obtained also to show thevariation of shear strength with direction of loading and conditioningof specimens. Briefly, the results may be summarized as follows:

Rept. No. 1848 -8-

(1) The highest shear strength was obtained with the epoxide laminatemade with Epon 1001 resin, and the lowest shear strength wasobtained with the silicone laminate.

(2) Of the different types of reinforcement, the 181 and 112 laminateshad generally higher shear strength than did the 143 and matlaminates.

(3) Dry shear strength at 0° remained about the same within the rangeof resin contents of the 181 polyester laminates. Wet shearstrength at 0° dropped appreciably at the lower resin content.At 45°, however, shear strength generally increased with a decreasein resin content.

(4) In most cases, the interlaminar shear strength at 45° was higherthan at 0°.

(5) Wet conditioning generally resulted in a slight reduction in inter-laminar shear strength, the greatest change occurring in the poly-ester laminate and the least in the epoxide laminates.

Rept. No. 1848 -9- 1. -18

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