Wire Testing

Part of Wire Drawing

Testing wire for strength, conductivity, and flexibility to ensure fitness for purpose.

Why This Matters

A coil of wire looks the same whether it will hold a bridge cable together for decades or snap the first time it bears a load. Internal flaws — micro-cracks from over-reduction, embrittlement from improper annealing, contamination from mixed scrap — are invisible to the naked eye. The only way to know if your wire will perform is to test it systematically before it goes into service.

In the old world, wire testing was performed by quality control labs with calibrated instruments. In a rebuilding scenario, you need to achieve the same confidence using simple, repeatable tests that any trained workshop member can perform. The tests described here require only basic tools — a vise, weights, a battery, and careful observation — but they reliably distinguish good wire from wire that will fail.

Testing is also how you improve your process. When a batch of wire fails the bend test, you investigate: was the annealing temperature too low? Was the reduction per pass too aggressive? Was the rod stock contaminated? Each failure, caught in testing rather than in the field, teaches you something that makes the next batch better.

Tensile Strength Testing

The Break Test

The most fundamental wire test: how much force does it take to break?

Setup:

1. Cut a test sample exactly 30 cm long
2. Clamp one end firmly in a bench vise with smooth jaws (serrated jaws create stress points that cause premature failure)
3. Attach a hook or loop to the free end for hanging weights
4. Prepare a collection of known weights (stones weighed on a balance, containers of water, metal pieces)

Procedure:

1. Add weight in small increments (100-200 g steps for thin wire, 500 g-1 kg for thick wire)
2. Wait 10 seconds after each addition for the wire to stabilize
3. Watch for necking — a visible thinning of the wire at one point
4. Continue adding weight until the wire breaks
5. Record the total weight at failure

Calculating tensile strength:

Tensile strength (MPa) = Breaking force (N) / Cross-sectional area (mm2)

Where: Force in Newtons = weight in kg x 9.81

Metal	Expected Tensile Strength
Annealed copper	200-250 MPa
Hard-drawn copper	350-400 MPa
Annealed mild steel	350-450 MPa
Hard-drawn steel	500-800 MPa
Annealed iron	250-350 MPa
Annealed brass	300-400 MPa
Aluminum (soft)	70-100 MPa

Test Multiple Samples

Never rely on a single test. Cut three samples from different points along the coil — beginning, middle, and end. If results vary by more than 15%, the wire has inconsistent quality and the entire coil should be re-evaluated.

Interpreting Break Patterns

How the wire breaks tells you as much as when it breaks:

Break Pattern	Indicates	Action
Cup-and-cone (necked)	Good ductility, proper annealing	Wire is healthy
Flat, clean break	Brittle, over-hardened	Re-anneal and retest
Jagged, irregular break	Internal inclusions or voids	Scrap or re-melt
Break at vise jaw	Stress concentration from clamping	Retest with padded jaws
Spiral or angled break	Torsional stress during drawing	Check draw alignment

Bend Testing (Ductility)

Standard Bend Test

Ductility — the ability to deform without breaking — is critical for any wire that will be bent, coiled, twisted, or formed in service.

Procedure:

1. Cut a 20 cm sample
2. Clamp one end vertically in a vise
3. Bend the wire 90 degrees around a mandrel whose diameter is twice the wire diameter
4. Bend it back to straight
5. Bend 90 degrees in the opposite direction
6. Count each 90-degree bend as one cycle
7. Continue until the wire breaks at the bend point

Expected results:

Metal/Condition	Minimum Acceptable Bends
Annealed copper	8-15
Hard-drawn copper	3-5
Annealed mild steel	4-8
Hard-drawn steel	1-3
Annealed brass	5-10
Annealed iron	4-6

What Poor Ductility Means

Wire that fails the bend test after only 1-2 bends is dangerously brittle. Possible causes:

1. Insufficient annealing: The wire was not heated enough or cooled too quickly (for steel)
2. Over-reduction: Too much area reduction per pass without intermediate annealing
3. Contamination: Impurities in the metal (sulfur and phosphorus make steel brittle; lead in copper causes hot shortness)
4. Hydrogen embrittlement: Can occur in steel wire that was acid-pickled without proper baking afterward

Remedy: Re-anneal the wire at proper temperature and retest. If it is still brittle after annealing, the problem is metallurgical and the wire should be scrapped.

Conductivity Testing

Why Test Conductivity

For electrical wire, conductivity is as important as strength. A wire can be perfectly strong but useless as a conductor if it contains the wrong alloy elements or has internal discontinuities.

Comparative Resistance Test

Without a precision ohmmeter, you can compare resistance between your wire and a known reference:

Setup:

1. Cut a 1-meter length of your test wire
2. Cut a 1-meter length of known-good copper wire of the same gauge
3. Connect each to an identical battery and lamp circuit
4. Use the same battery, same lamp, same connections for both tests

Evaluation:

Observation	Interpretation
Equal brightness	Conductivity matches reference — good
Slightly dimmer	10-20% higher resistance — acceptable for most uses
Noticeably dimmer	30%+ higher resistance — investigate cause
Much dimmer or no light	Wire may be wrong metal, heavily contaminated, or internally broken

The Heating Test

Run a known current through a 1-meter sample for 60 seconds and feel the wire temperature:

1. Connect the wire to a battery through a load that draws a moderate current
2. After 60 seconds, touch the wire (carefully) at several points along its length
3. Uniform warmth indicates consistent conductivity throughout
4. A hot spot indicates a point of higher resistance — likely a thin section, internal crack, or poor splice

Hot Spots Mean Danger

A wire that develops hot spots under load is a fire hazard. A hot spot means current is being forced through a constriction, heating that point far above the rest of the wire. Cut out the hot spot section and splice or re-draw.

Metal Identification by Resistance

If you are unsure what metal your wire is made of, relative resistance helps identify it:

Metal	Relative Resistivity (copper = 1.0)
Copper	1.0
Aluminum	1.6
Brass	3.7-4.5
Iron	5.8
Steel	7-13
Nichrome	65

If your “copper” wire shows resistance similar to brass, it is likely a copper alloy rather than pure copper. This matters for generator coils and transformer windings where pure copper’s low resistance is essential.

Torsion Testing

The Twist Test

Torsion testing reveals internal defects that other tests miss. It is especially important for spring wire and any wire that will be twisted in service (rope-making, cable-laying).

Procedure:

1. Cut a sample 20 cm long
2. Clamp one end in a vise
3. Grip the other end with pliers
4. Twist the wire by rotating the pliers, counting full rotations
5. Continue until the wire breaks
6. Record the number of twists

Expected results (for wire of approximately 2 mm diameter):

Metal	Minimum Twists Per 10 cm
Annealed copper	25-40
Annealed mild steel	15-25
Hard-drawn steel	8-15
Annealed brass	20-30

Interpreting Twist Failures

• Clean helical fracture: Normal failure mode — wire is sound
• Longitudinal splitting: Internal seam or fold from forging — rod stock defect
• Early failure with rough surface: Over-hardened, needs annealing
• Uneven twisting (tight in some spots, loose in others): Inconsistent diameter or uneven annealing

Corrosion Resistance Testing

Accelerated Corrosion Test

You cannot wait years to see if wire corrodes. An accelerated test gives a relative indication:

1. Cut three 10 cm samples of the wire to test
2. Submerge one in fresh water, one in salt water (1 tablespoon salt per cup), and leave one in open air
3. Check daily for 7 days
4. Record the onset and progression of corrosion

Expected results for unprotected wire:

Metal	Fresh Water	Salt Water	Air
Copper	Slight tarnish (7 days)	Green patina (3-5 days)	Slow tarnish (weeks)
Iron	Light rust (2-3 days)	Heavy rust (1 day)	Rust spots (3-5 days)
Steel	Light rust (2-3 days)	Heavy rust (1 day)	Rust spots (3-5 days)
Brass	Minimal (7 days)	Light tarnish (5-7 days)	Very slow tarnish
Galvanized steel	No rust (7 days)	Minor spots (5-7 days)	No rust (months)

Coating Effectiveness Test

If you have applied a protective coating (oil, wax, or galvanizing), test its effectiveness:

1. Coat a sample and an uncoated control piece
2. Subject both to the salt water test
3. Compare corrosion onset — the coating should delay corrosion by at least 5x
4. If the coating fails quickly, reformulate or apply more thickly

Creating a Test Record System

The Wire Test Log

Maintain a written log of all wire tests. Each entry should record:

1. Date of test
2. Wire batch identifier (drawer, date drawn, metal, gauge)
3. Tests performed and results (tensile, bend, conductivity, torsion)
4. Grade assigned (A/B/C/D — see Quality & Storage)
5. Disposition — accepted for stock, re-annealed, scrapped

Statistical Tracking

Over time, your test log reveals patterns:

• If tensile strength drops over a production run, your die is wearing
• If bend test results decline, your annealing practice needs adjustment
• If conductivity varies between batches, your scrap sorting is inconsistent
• If a particular drawer’s wire consistently tests lower, they need technique coaching

Testing Is Not Waste

Cutting test samples from every coil feels like wasting wire. It is not. A 30 cm test sample costs minutes. A failed installation costs days or weeks. Test every coil, every time.

Sample Retention

Keep a labeled test sample from every batch for at least 6 months. If wire fails in service, you can retest the retained sample to determine whether the batch was defective or the failure was caused by installation conditions. Store samples in a dry, labeled container organized by date.