Test Methods for Fiber Reinforced Polymer (FRP) Composites John J. “Jack” Lesko Department of Engineering Science & Mechanics jlesko@vt.edu (540) 231-5259 Introduction to Polymeric Adhesives and Composites Short Course Copyright, 2004, J J Lesko, ESM, Virginia Tech, Blacksburg, Virginia. All rights reserved.

Partial List of Standardization Groups USA American Society for Testing and Materials (ASTM) MIL-HDBK-17 Committee (http://www.mil17.org/) Suppliers of Advanced Composite Materials Association (SACMA) Europe Deutsches Institut Fur Normung (DIN) Association Francaise de Normalization (AFNOR) British Standards Institute (BSI) East Japanese Industrial Standards International International Organization for Standardization (ISO)

ASTM Standard Test Methods* Definitions D3878--Definitions of Terms Relating to High-Modulus Reinforcing Fibers and Their Composites Fiber/Matrix Prepreg C613--Test Method for Resin Content of Carbon and Graphite Prepregs by Solvent Extraction D3379--Test Method for Tensile Strength and Young’s Modulus for High Modulus Single-Filament Materials D3529--Test Method for Resin Solids Content of Carbon Fiber-Epoxy Prepreg D3530--Test Method for Volatiles Content of Carbon Fiber-Epoxy Prepreg D3531--Test Method for Resin Flow of Carbon Fiber-Epoxy Prepreg D3532--Test Method for Gel Time of Carbon Fiber-Epoxy Prepreg D3544--Guide for Reporting Test Methods and Results on High Modulus Fibers D3800--Test Method for Density of High-Modulus Fibers D4102--Test Method for Thermal Oxidative Resistance of Carbon Fibers * Found in Vol. 15.03 of ASTM Annual Book of Standards

Key to Successful FRP Testing Stress Concentrations Alignment Uniformity of Stress

Damage & Strength of Composites

Composite Damage Modes Matrix Cracking & Delamination Tensile Failure Compression Failure

Tensile Strength

The tensile strength of a composite is controlled by the interface/phase, influencing the local stress concentrations and the size of the “ineffective length - d”.... f  f   f 1 2 3 4 5 6 7 8

Tensile Stress Concentration at a Fiber Break

Tensile Strength Models A very crude approximation of tensile strength from the Rule of Mixtures More sophisticated models include Batdorf, S. B. “Tensile strength of unidirectional reinforced composites--I,” Journal of Reinforced Plastics and Composites, Volume 1 (1982), pp.153-176. Gao, Z. and Reifsnider, K. L. “Micromechanics of tensile strength in composite systems,” Composite Materials: Fatigue and Fracture, Fourth Volume, ASTM STP 1156, W. W. Stinchcomb and N. E. Ashbaugh, Eds., ASTM, Philadelphia, (1993), pp. 453-470. Reifsnider, K., Iyengar, N., Case, S. and Xu, Y. “Kinetic Methods for Durability and Damage Tolerance Design of Composite Components,” Keynote Address, Conference on Composite Materials, Japan Society for Mechanical Engineers, June 26, 1995, Tokyo.

Stresses Around Filler Particles Monette, et al, J. Appl. Physics, 75 (3), 1994, 1442-1455.

Pultrusion Fabrication Flaw Microcrack -1.2mm long by .25mm wide 90º Tow 0º Tow “As received” pultruded cross ply laminate (E-glass/Derakane 441-400)

Transverse Strength Models Gibson, R. F. Principles of Composite Material Mechanics, McGraw Hill, New York (1994)

0° and Laminate Tension Testing of Composites Concerns in the Assessment of Modulus and Strength Uniformity of stress state Failure in the gage section (common problem between test specimens) Failure modes Material misalignment (1° misalignment can yield a 30% strength reduction) Specimens with cross reinforcement Gripping Transition region s concentration (common problem in all specimens) Tab geometry Grip region geometry Grip pressure

0° and Laminate Tension Testing of Composites Specimen Types Used in Tensile Testing Straight-Sided Coupon--MRG Preferred With and without tabs ASTM D638 Type I “Dogbone” Specimen Linear Tapered “Bowtie” Specimen 30% lower 0° strength compared to straight-sided specimen 10% lower 0° strength compared to dogbone specimen Woven cross-ply strengths  dogbone or tabbed specimen Streamline Specimen Comparable to straight-sided for 0°

Straight-Sided Specimen Advantages: No specimen tapering required; better results with cross-reinforced materials Disadvantages: Tabbing required; tab s-concentration; tight tolerances in thickness

Typical Failure Modes in Straight-Sided Coupons (Acceptable & common in unidirectional specimens) (Acceptable & common in 90° or 90° dominated layups) (May be found in crossply layups; unacceptable) (Unacceptable)

Typical Tab Failures in Straight-Sided Coupons

ASTM D 638 Type I “Dogbone” Specimen Advantages: No tabbing required; load introduction less of an issue Disadvantages: Careful specimen machining required; not suitable for unidirectional material

Streamline Specimen Advantages: No tabbing required; load introduction less of an issue; comparable to straight-sided Disadvantages: Careful specimen machining required; not suitable for unidirectional material; large specimen (12” [0°/90°]s; 24” [0°]) in order to keep the shear stresses low at the transition region

Linear-Taped “Bowtie” Specimen Advantages: No tabbing required; load introduction less of an issue Disadvantages: Careful specimen machining required; not suitable for unidirectional material; large specimen

Effect of Misalignment in Unidirectional Specimens

Compression Strength

Compression L = 41.8 mm

Compression Strength An approximation of crushing strength from the Rule of Mixtures Crushing Compression Strength Buckling Slenderness ratio (r/L)

Compression Strength Xu, Y. and Reifsnider, K. L. “Micromechanical modeling of composite compressive strength,” Journal of Composite Materials, Vol. 27 (6), (1993), pp. 572-588.

Compression Strength Ramberg-Osgood shear response Fleck, N. A. and Budiansky, B. “Compressive failure of fibre composites due to microbuckling,” IUTAM Symposium, Troy, New York, May 29-June 1, (1990), pp. 235-273.

Compression Testing of Composites Concerns in the Assessment of Modulus and Strength Uniformity of stress state End loaded Shear loaded Gage section dimensions Sandwich beam Gripping Stress concentration Tab geometry Tabbing material Alignment Buckling Failure modes Specimen machining tolerance Fixture characteristics

Compression Testing of Composites Classes of Test Methods Shear Loaded - Preferred Celanese & Wyoming modified Celanese IITRI (Illinois Institute of Technology Research Institute) & Wyoming modified IITRI End Loaded Boeing Compression ASTM D695 & Wyoming modified D695 Wyoming End Loaded Side Supported (ELSS) RAE (Royal Aircraft Establishment) Short Block Compression Sandwich Beam ASTM D3410, Method C--Flexure Axially Loaded Sandwich Column

IITRI - ASTM D3410 Advantages: Alignment; high data averages and low scatter; large specimens possible Disadvantages: Expense; specimen tabbing & machining critical; tab s-concentration

Celanese: ASTM D3410 Advantages: Alignment; high data averages and low scatter; long-standing test fixture Disadvantages: Specimen tabbing & machining critical; tab s-concentration; sensitive to fixture accuracy; expense (latter two concerns addressed in Wyoming-modified)

Boeing Modified ASTM D695 Advantages: Small, thin specimen; reduced material; highly supported against buckling Disadvantages: No s-e curve; untabbed for modulus; tabbed for strength; tab s-concentration

Wyoming End Loaded Side Supported (ELSS) Advantages: No tabbing required; simple fixture; inexpensive; simple alignment; some shear loading Disadvantages: End crushing for highly orthotropic specimens; support s-concentration; specimen tolerances critical

Sandwich Beam Flexure - ASTM D3410 (ASTM C 393) Advantages: Simple fixture; reliable results with proper specimen (core) design Disadvantages: Large specimens (materials expense); failure must occur in compressive face sheet

Axially Loaded Sandwich Column Advantages: Simple fixture; simple data analysis; standard compression fixture Disadvantages: Expense in fabricating sandwich panel; end crushing; end s-concentration

Other Compression Tests Block Compression Test Advantages: Simple untabbed specimen; simple fixture; inexpensive Disadvantages: Thick specimen required; end crushing; end s-concentration; misalignment sensitive RAE Compression Test Advantages: No tabbing required; simple fixture; inexpensive; shear and end loading Disadvantages: Not widely used; tolerance sensitive for thickness taper; misalignment upon debonding; specimen buckling

Shear Strength

Shear Strength Models Gibson, R. F. Principles of Composite Material Mechanics, McGraw Hill, New York (1994)

Shear Testing of Composites Concerns in the Assessment of Modulus and Strength In-plane: t12 Interlaminar: t13 Uniformity of Stress State Failure in the gage section (common problem between test specimens) Failure modes: buckling out of plane; scissoring Material alignment Uniform shear Load Introduction Transition region s concentration (common problem in all specimens) Loading arrangement and assessment of results Grip region geometry

Shear Testing of Composites In-plane: t12 Iosipescu ASTM D5379 (Preferred for shear strength) (45)ns Tension ASTM D3518 (Preferred for modulus) Off-axis Tension Rail Shear ASTM D4255 Torsion of bar (circular/rectangular) Torsion of a tube ASTM D5448 Interlaminar: t13 Short Beam Shear ASTM D2344 Iosipescu ASTM D5379 (experimental) bonded laminates

Shear Directions 3 S13 S12 S23 2 S12 S23 1 Material Coordinate System 1, 2, 3 S13

Iosipescu Shear Test ASTM D5379 Advantages: Excellent shear strength measurement; small specimen; 0°, 90°, [0°/90°]ns layups Disadvantages: Tight tolerances on specimen; alignment; twist failure; quality fixture required; expense

(45)ns Tension ASTM D3518 Advantages: Simple; uniform stress state; no fixture; damage growth representative of laminates Disadvantages: Tabbing; alignment; strength dependent on layup; scissoring; t12 and t13 failure; edge delamination; s-concentration due to tabs

Short Beam Shear ASTM D2344 Advantages: Simple test and fixture; small specimen Disadvantages: Load introduction; no strain measurement; no modulus measurement; improper assumption of parabolic stress distribution; mixed mode failure

Stress Distribution in a Short Beam Shear Specimen Elasticity Solution Beam Theory

Interlaminar Fracture

Double Cantilever Beam (DCB) Test Data – ASTM D5528 P D a1 a2 a3 an

DCB Data Reduction: Modified Beam Theory b=width y = 0.429941x + 0.001997 R 2 = 0.9997 0.01 0.02 0.03 0.04 0.05 0.06 0.07 -0.05 0.1 0.15 0.2 Crack Length, a [m] Cube Root of Compliance C 1/3 (J/m2)1/3 x 1 m a P D Find C: Plot C1/3 vs a Find fit:

DCB Data Reduction: Compliance Calibration Method b=width a P D log(C) Find C: Plot log(C) vs log(a) Find the slope m2 m2 1 log(a)

DCB Data Reduction: Compliance Calibration Method b=width a P D Find C: Plot a/h vs C1/3 Find the slope m3 m3 a/h 1 C1/3

Edge Notch Flexure (ENF) P 1/C PMax P95% Pnl 95% of 1/C Load, P a L L Mid-span Displacement, D b=width Of the uncracked region

QUESTIONS