What is Metallic Foil Tension Testing?
Metallic foil tension testing, as defined by ASTM E345, is a specialized mechanical evaluation method designed to determine the strength and ductility of metallic materials in very thin gauges (less than 0.006 in. or 0.150 mm). Unlike standard tensile tests for bulk materials (ASTM E8), this standard addresses the unique challenges of handling, gripping, and measuring extremely thin specimens that are prone to tearing, buckling, and edge defects. It provides specific procedures for specimen preparation (machining vs. shearing), thickness determination (weighing vs. micrometry), and gripping techniques to ensure valid data for quality control, alloy development, and design applications involving foils used in electronics, packaging, and aerospace.
What is a typical Metallic Foil Tension Test Result?
The results provide key mechanical properties adapted for thin-gauge materials:
① Tensile Strength: The maximum stress the foil can withstand, calculated by dividing the maximum load by the original cross-sectional area. Due to the thinness, accurate area determination is critical.
② Yield Strength: Determined by the offset method (typically 0.2%) or the extension-under-load method. The standard specifies strict strain rates (0.002 to 0.010 in./in.·min) during yield determination to ensure accuracy.
③ Elongation: A measure of ductility. For Type A specimens, it is measured over a 2-in. gage length marked on the specimen. For Type B specimens, it can be derived from grip separation or chart recordings, acknowledging that elongation values become less reliable as thickness decreases.
④ Thickness Accuracy: A critical derived parameter. For soft foils or those ≤0.002 in., thickness is often determined by weighing (per ASTM E252) rather than direct measurement to achieve ±2% accuracy, avoiding errors from micrometer pressure.
⑤ Specimen Integrity: Qualitative assessment of edge quality. Specimens with visible scratches, creases, or edge discontinuities (checked at 20x magnification) are rejected, as these defects disproportionately affect foil strength.
⑥ Fracture Location: Validity check ensuring fracture occurs within the gage length (and specifically within the middle half for Type A elongation tests); otherwise, the test is discarded.
Discovery and Evolution of Foil Testing
As the use of metallic foils expanded in the mid-20th century for capacitors, shielding, and flexible circuits, standard tensile methods proved inadequate. Conventional grips crushed thin foils, and micrometers distorted them, leading to erroneous data. ASTM E345 was developed to codify best practices for these delicate materials, introducing the “weighing method” for thickness and specific specimen geometries (Type A and Type B) to mitigate buckling and edge effects. The 1993 version (reapproved 2002) refined specimen preparation techniques, distinguishing between machined edges for harder materials and sheared edges for softer ones, and established clear criteria for rejecting defective specimens.
ASTM E345-93 distinguishes itself from general tensile standards by:
Thickness Measurement: Prioritizing the gravimetric (weighing) method for thin/soft foils to avoid compression errors inherent in micrometers.
Specimen Preparation: Allowing sheared edges (Type B) for soft materials while mandating machined and polished edges for others, with strict visual inspection criteria.
Gripping Protocols: Specifying smooth-faced grips and buffer materials for ultra-thin gauges to prevent grip-induced fractures.
Buckling Prevention: Recommending specific fillet radii and grip widths for Type A specimens to avoid lateral instability during loading.
The key contents covered by the standard include:
Scope: Applicable to metallic foils <0.006 in. thick.
Apparatus: Requirements for testing machines (ASTM E4), wedge vs. smooth grips, and thickness measuring devices.
Specimens: Detailed dimensions for Type A (machined) and Type B (sheared/parallel) specimens.
Procedures: Step-by-step guides for specimen preparation (milling, shearing, polishing), thickness calculation, and testing speeds.
Calculations: Methods for determining yield strength (offset/extension), tensile strength, and elongation, including rounding rules.
Replacement Criteria: Specific conditions under which a test result can be discarded (e.g., fracture outside gage length, equipment malfunction).
Reporting: Mandatory data points including alloy, temper, specimen type, and test results.
Appendix: Density values for common aluminum and copper alloys to facilitate thickness calculations.
Referenced Standards
ASTM E4: Practices for Force Verification of Testing Machines.
ASTM E6: Terminology Relating to Methods of Mechanical Testing.
ASTM E8/E8M: Test Methods for Tension Testing of Metallic Materials.
ASTM E29: Practice for Using Significant Digits in Test Data.
ASTM E252: Test Method for Thickness of Thin Foil and Film by Weighing.
ASTM E796: Test Method for Ductility Testing of Metallic Foil (alternative for ductility).
ASTM B193: Test Method for Resistivity of Electrical Conductor Materials (for density references).
ISO 15578: Metallic materials — Sheet and strip — Determination of plastic strain ratio r (often relevant for foil formability).
ASTM E796: Specifically addresses ductility testing for foils where tension elongation is unreliable.
IPC TM-650: Test methods for printed board materials (often references foil tensile properties).
JIS Z 2241: Japanese standard for tensile testing of metallic materials (includes thin sheet provisions).
Environment:
Temperature: Room temperature (standard laboratory conditions).
Atmosphere: Ambient air. No special environmental controls are specified unless required by material specifications.
Test Procedure:
- Specimen Selection: Choose Type A (machined) or Type B (sheared) based on material specification and hardness.
- Preparation:
- Inspection: Examine all specimens at 20x magnification. Reject any with scratches, creases, or rough edges.
- Thickness Measurement:
- Width Measurement: Measure width to the nearest 0.001 in.
- Grip Setup:
- Alignment: Ensure specimen axis coincides with the machine center line to avoid bending.
- Testing:
- Data Recording: Record maximum load, yield point/strength, and elongation (via gage marks or grip separation).
- Post-Test: Check fracture location. If outside the valid zone, discard and retest.
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