10.4 Functionality, Force & Elastomer Hardness Testing

Functionality Testing: A Different Question

Every method covered so far asks whether a part matches its drawing. Functionality testing asks a different question: does the part do its job? A hose fitting can hold every dimension on the print and still leak. A crimped terminal can look perfect and pull off at half its required retention force. A spring can be the right free length and the wrong rate.

Because functional requirements are often the reason the dimensional requirements exist, functional tests frequently appear on inspection plans as the final gate before shipment, and they are the tests that a customer's incoming inspection is most likely to repeat.

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Tension and Pull Testing

Tension testing applies a pulling force along an axis. Section 10.1 covers the destructive tensile test to failure under ASTM E8. In functional inspection the more common variants are:

• Pull-off / retention testing. A crimped terminal, a bonded joint, a swaged fitting, or a molded insert is pulled to verify it holds a specified minimum force. Wire-crimp pull testing is a standard electrical-assembly inspection under specifications such as IPC/WHMA-A-620.
• Proof load testing. A force below the level that would damage the item is applied and held; the item is then inspected for permanent set or damage. Because the part survives and remains usable, proof testing is non-destructive. Lifting hardware, chain, slings, and threaded fasteners are commonly proof-loaded.
• Pull-to-failure testing. The specimen is pulled until it breaks, giving the actual breaking force and the failure mode. This is destructive and is applied to samples, never to product being shipped.
• Peel and adhesion testing. Tape, label, coating, or laminate adhesion measured as force per unit width at a specified peel angle.

The exam discriminator is proof versus destructive: a proof load verifies capability without consuming the part; a pull-to-failure test consumes it.

Failure mode matters as much as the number. A crimp that pulls apart at the barrel indicates a bad crimp; a wire that breaks outside the crimp at a higher force indicates the crimp is stronger than the wire, which is the desired outcome.

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Torque Testing

Torque is rotational force about an axis, and torque tests come in several distinct forms that candidates routinely confuse.

The residual torque audit is the workhorse of the inspection department because it does not disturb the joint: the auditor applies torque in the tightening direction and records the value at which the fastener first moves. Breakaway torque, applied in the loosening direction, destroys the original clamp load and requires re-torque.

Torque wrenches are calibrated under ISO 6789, typically at points spanning the range such as roughly 20%, 60%, and 100% of full scale. Section 3.4 covers the instrument families.

The concept every inspector must carry: torque is a proxy for clamp load, not a measurement of it. Most of the applied torque is consumed by friction under the head and in the threads, and lubrication, plating, surface finish, and reuse all change that friction. This is why critical joints specify torque-plus-angle or direct bolt-tension measurement rather than torque alone.

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Leak Testing

Leak testing verifies that a sealed or pressurized item contains its contents. The selection question on the exam is almost always about sensitivity: which method can detect the specified leak rate?

Leak rates are expressed in standard cubic centimeters per second (std cc/s) or in pressure-volume units such as mbar-L/s.

Two operational points generate exam questions. First, every leak test requires a stabilization or dwell time: pressurizing a part warms the gas inside it, and the pressure decay from that cooling is indistinguishable from a leak until the temperature settles. Second, a negative result is bounded by the method's sensitivity — "no bubbles observed" means the leak is smaller than roughly 1e-4 std cc/s, not that the part is hermetic. Leak test procedures therefore specify a calibrated reference leak used to verify the system can actually detect the rate it claims to detect.

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Compression Testing

Compression testing applies a squeezing force and measures the resulting deformation or the force required to reach a defined deflection.

• Spring rate and load at height. A compression spring is compressed to a specified working height and the force is recorded, verifying the spring rate rather than just the free length.
• Crush and buckling strength. Tubes, bushings, honeycomb cores, and containers are compressed to a defined load or to collapse.
• Compression set of elastomers. A seal is compressed a defined amount for a defined time at a defined temperature, then released; the percentage of thickness it fails to recover is its compression set. High compression set predicts seal leakage in service.
• Package compression. Loaded shipping containers are compressed to simulate stacking loads, commonly under ASTM D642 and the ISTA transit-test series.
• Indentation force deflection for foam cushioning.

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Drop Testing

Drop testing evaluates whether a product or its packaging survives handling shocks.

• Packaged-product drop testing under ASTM D5276 releases a loaded shipping container in free fall onto a rigid surface. Drop height is selected from the package's gross weight — heavier packages are dropped from lower heights — and a standard sequence covers one corner, the three edges radiating from that corner, and all six faces. The pass criterion covers both package integrity and, more importantly, the condition of the product inside.
• Product drop and mechanical shock testing under standards such as IEC 60068-2-31 evaluates the device itself rather than its packaging.
• Related transit simulation includes vibration, incline impact, and compression, usually combined into an ISTA or ASTM series.

Drop testing is destructive in intent even when the specimen survives: a dropped unit is generally not returned to saleable stock.

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Durometer Hardness Testing of Elastomers and Plastics

Section 10.1 covered Rockwell, Brinell, Vickers, and microhardness for metals. Body of Knowledge topic III.D.4 also names the durometer, which is the standard method for rubber, elastomers, and plastics.

Principle

A durometer presses a spring-loaded indenter of specified geometry into the material and measures how far the indenter fails to penetrate. The scale runs 0 to 100, where 100 means no penetration (maximum hardness) and 0 means full penetration. Because the indenting force comes from a calibrated spring rather than a dead weight, the durometer is a portable, hand-held or stand-mounted instrument.

Scales (ASTM D2240)

ASTM D2240 defines twelve types; three dominate inspection work:

Type M is a micro-hardness durometer designed for small cross-sections such as O-rings, requiring far thinner specimens than the standard types.

Test Rules That Appear on Exams

• Specimen thickness must be at least 6 mm (0.25 in). Thinner material lets the indenter sense the anvil beneath, giving falsely high readings — exactly the anvil effect seen in Rockwell testing. Plying (stacking) thinner sheets to reach 6 mm is permitted.
• Readings must be taken at least 12 mm (0.5 in) from any edge, and separated from each other, because edges deform differently than the bulk material.
• Timing is part of the method. The reading is taken within 1 second of firm contact of the presser foot, unless the specification calls for a delayed reading at 15 seconds. Elastomers creep under the indenter, so a 15-second reading is always lower than a 1-second reading on the same material. A durometer result is meaningless without stating which timing was used.
• Curved surfaces read low or erratically. O-rings and tubing require a Type M durometer or a specified fixture.
• The scales overlap. When a Shore A reading exceeds 90, the material is too hard for reliable Type A measurement and Type D should be used; when a Shore D reading falls below 20, Type A is preferred. Reporting "Shore A 96" is a method error.
• Temperature matters. Elastomer hardness changes with temperature, so tests are run at a controlled standard laboratory temperature.

IRHD (International Rubber Hardness Degrees), defined in ISO 48, is the alternative method used widely outside North America. IRHD and Shore A numbers are similar in magnitude but are produced by different geometry and are not interchangeable by conversion.

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Method Selection Summary

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Common Exam Traps

• Proof load is non-destructive. Only tests carried to failure are destructive.
• Residual versus breakaway torque. Residual is applied in the tightening direction and preserves the joint; breakaway is applied in the loosening direction and destroys the original clamp load.
• Torque is not clamp load. Friction consumes most of the applied torque.
• A leak test result is bounded by method sensitivity. "No bubbles" is not "hermetic."
• Durometer timing must be stated. A 15-second reading is lower than a 1-second reading on the same specimen.
• Durometer thickness and edge distance. At least 6 mm thick and at least 12 mm from any edge; stacking to reach thickness is allowed.
• Shore A above 90 means switch to Shore D. Reporting a Shore A value in the 90s indicates the wrong scale was used.
• Durometer applies to polymers, not metals. If the stem describes hardened steel, the answer is Rockwell, Brinell, Vickers, or microhardness.