Mechanical Testing Protocol for Characterizing Composite Lamina Pre-Pregs

At a Glance

Metadata	Details
Publication Date	2017-01-01
Journal	DigitalCommons (California Polytechnic State University)
Authors	Kenneth Blain Ainslie, Joshua Gustav Brinkmann
Analysis	Full AI Review Included

Technical Analysis & Product Solutions: Precision Material Characterization in Aerospace Composites

Executive Summary

The analyzed research paper outlines a stringent mechanical testing protocol for characterizing thin Glass Fiber Reinforced Polymer (GFRP) phenolic pre-pregs used as facesheets in aerospace honeycomb sandwich panels. The findings underscore the critical need for extreme precision in sample preparation and metrology, highlighting areas where 6CCVD’s high-performance materials and fabrication techniques provide essential solutions for advanced engineering research.

Protocol Development: Established validated mechanical testing protocols (Tension/Shear: ASTM D3039/D3518; Compression: ASTM D6641) optimized for ultra-thin (1-14 ply) composite laminates.
Dimensional Integrity is Critical: Demonstrated that standard micrometry is insufficient due to surface roughness (up to 33 µm R_a), necessitating high-precision profilometry and diamond cutting to ensure accurate thickness measurement and flush sample ends.
Mechanical Performance: Confirmed that fiber orientation is a significant factor in mechanical performance, yielding higher ultimate strength in the warp direction (up to 517.7 MPa compressive strength).
Failure Mode Control: Developed precise dimensional requirements (minimum 0.04” thickness, specific slenderness ratios) to prevent unacceptable failure modes like Euler buckling in compression tests.
Tabbing Ineffectiveness: Found that applying tabs (aluminum or fiberglass) did not significantly alter tensile or shear properties, simplifying future protocol design and reducing labor/cost.
Bilinear Behavior: Identified a consistent bilinear stress-strain relationship in tension and compression, with the inflection point occurring between 0.4% and 1.0% strain.

Technical Specifications

Parameter	Value	Unit	Context
Material System	GFRP / Phenolic Resin	-	8-harness satin weave facesheet pre-preg
Tensile Strength (Warp, 0°)	391	MPa	Mean, 4-8 ply (Corrected Thickness)
Compressive Strength (Warp, 0°)	514	MPa	Mean, Excluding 2-ply buckled samples
Compressive Strength (Fill, 90°)	412	MPa	Mean, Excluding 2-ply buckled samples
Young’s Modulus (Tension)	21.7 - 28	GPa	Varies inversely with ply count (thinner = higher modulus)
Poisson’s Ratio (Tension, Avg)	0.195	-	Measured using biaxial strain gages
Shear Stress (Max)	130 - 210	MPa	Highly scattered, dependent on fiber alignment
Bilinear Strain Range	0.4 - 1.0	%	Inflection point in stress-strain curve
Surface Roughness (R_a)	up to 33	µm	Observed on 1-ply laminate (Figure 41, Table I)
Thickness Measurement Correction (1 Ply)	-21.3	%	Corrected micrometer reading based on profilometry
Compression Fixture Type	Combined Loading Compression (CLC)	-	ASTM D6641
Compression Bolt Torque	15	in-lb	Required torque for CLC fixture bolts
Compression Strain Rate	1.3	mm/min	ASTM 6641 prescribed rate
Tension Strain Rate	2.0	mm/min	ASTM D3039 prescribed rate

Key Methodologies

The experiment focused on mechanical testing (tension, shear, compression) of glass fiber phenolic laminates. Critical steps requiring high-precision tools or advanced metrology included:

Laminate Manufacture: Coupons were produced from rectangular sheets of B-stage pre-preg using a heated die press method and subsequently cured.
Initial Sample Cutting: Samples were water jet cut to nominal dimensions (e.g., 1” x 10” for tension/shear). This process resulted in beveling and non-orthogonal ends.
End Preparation Necessity: Due to unacceptable end-crushing failures in compression testing caused by slanted ends, excess tensile samples were precision cut using a diamond precision saw to achieve 90° flush surfaces.
Surface Metrology (Correction Factor): Sample thicknesses measured by standard micrometers (±0.00001”) were found unreliable. A profilometer (AmbiOS XP-1, 0.1 µm accuracy) was used to measure R_a and peak-to-valley displacement, generating thickness correction factors (Table I).
Microscopic Verification: Scanning Electron Microscopy (SEM) was performed on 1- and 2-ply samples (cut with a diamond embedded saw blade at 120 rpm) to confirm notable surface roughness and micrometer offset (up to 44 µm).
Strain Measurement: Uniaxial and biaxial strain gages (MicroMeasurements CEA-06 series) were applied. For thin compression samples (0.35” gage length), uniaxial gages were custom-fashioned by laser/precision cutting the vertical grid patterns from biaxial gages.
Data Processing: Stress-strain curves were analyzed using MatLab to determine Young’s modulus, Poisson’s ratio, and bilinear inflection points, utilizing the maximum second derivative method.

6CCVD Solutions & Capabilities

This research highlights the absolute necessity of ultra-precise material processing and measurement capabilities—domains where 6CCVD’s specialization in MPCVD diamond technology provides a distinct competitive advantage for researchers and engineers.

Applicable Materials

While 6CCVD does not manufacture GFRP/Phenolic composites, our diamond materials are essential for creating the durable, precise tooling and metrology standards required for rigorous aerospace testing protocols.

Ultra-Polished PCD or SCD: Ideal for creating high-precision reference standards for profilometry and SEM calibration. The study found material roughness up to 33 µm R_a. 6CCVD offers Optical Grade SCD polished to R_a < 1 nm, which can serve as an absolute zero-reference for surface analysis, critical for correcting thickness measurements in thin laminates.
Custom SCD Components: High-stiffness Single Crystal Diamond can be used to engineer replacement wear components for the Combined Loading Compression (CLC) fixture (ASTM D6641). Diamond’s superior rigidity and low thermal expansion compared to steel blocks ensure minimal fixture compliance, improving measurement accuracy in high-stress compression tests.

Customization Potential

The paper clearly demonstrated that water jet cutting failed to provide the necessary sample integrity for acceptable compression testing, requiring subsequent precision cutting by a diamond saw. 6CCVD provides the expertise needed to solve these exact fabrication challenges globally.

Precision Diamond Cutting Services: 6CCVD routinely utilizes MPCVD diamond technology for ultra-precise laser and mechanical cutting. We can supply materials for, or directly fabricate, highly specific coupon geometries, ensuring 90° orthogonal edges (as required by the study, Figure 23) and superior edge quality compared to water jetting, eliminating catastrophic end-crushing failure modes.
Custom Dimensional Control: We offer plates and wafers up to 125 mm (PCD) and substrates up to 10 mm thick, capable of being manufactured to the tight tolerances required by demanding ASTM standards (e.g., D3039, D6641), including the revised compression dimensions of 1.1” x 5.35” x ≥ 0.04”.

Engineering Support

The challenges faced in this composite characterization (failure mode identification, metrology correction, strain gauge implementation, slenderness ratio modeling) mirror complexities found in characterizing advanced materials like diamond.

Metrology Expertise: 6CCVD’s in-house PhD engineering team provides consultation on optimizing metrology protocols for high-aspect ratio materials, particularly concerning surface roughness (R_a < 1 nm capability) and dimensional stability, ensuring that derived material properties (Modulus, Strength) are accurate and not skewed by measurement artifacts.
Failure Analysis & Protocol Design: We offer consultation services leveraging our deep expertise in mechanics of materials to assist researchers in designing effective testing fixtures and protocols for new structural materials, similar to the refinement required here for high-strength, thin composite laminates.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Zodiac Aerospace currently employs finite element analysis (FEA) computer models to predict the material behavior of its composite products. The objective of this project was to develop a testing protocol for obtaining detailed material property data to use in these FEA models. With accurate material data, FEA models can reduce the need for expensive physical testing and achieve timely troubleshooting when testing complex components. The specific material characterized in this project was an 8-satin weave fiberglass phenolic pre-preg used as the facesheet material in many of Zodiac’s sandwich panel composites. The developed testing protocol involved mechanical testing of lamina and laminate samples in tension, shear, and compression. Relationships between fiber orientation, sample tabbing, and sample thickness were examined using multiple full factorial experimental designs. Results indicated that nominal fiber orientation was significant in determining the mechanical properties of all samples, while tabbing was not significant for any sample. Sample thickness was less influential in determining tensile and shear properties but more influential in determining compressive properties. The resultant testing protocol therefore recommends that samples used to determine the mechanical properties of pre-preg materials be laminate samples manufactured without tabs, with minimized slenderness ratios, and with differing fiber orientations. Additional parameters of the protocol include surface roughness analysis via profilometry to determine accurate sample thicknesses, and end-cutting of samples via diamond saw to ensure flush sample surfaces. The finalized protocol is intended to be used in the characterization of current and future pre-preg materials produced by Zodiac.

Tech Support

Original Source

DOI: None