Breaking Force Failure Analysis: Root Cause Investigation of Tensile Break Failures

Breaking force failure analysis is a systematic laboratory investigation that identifies why a wire, terminal, joint, or material fractures below its specified tensile load. By combining standardized force testing with fractography and material characterization, this service helps manufacturers pinpoint root causes such as material defects, process errors, and design weaknesses. This article explains the testing scope, methods, applicable standards, and typical applications of professional breakage force failure analysis.

Technical Overview

Breaking force, also known as tensile breaking force or pull-off force, is the maximum tensile load a specimen can withstand before fracture. When a product fails below its specified breaking force—whether during production pull testing, incoming inspection, or field use—a structured failure analysis is required to determine the root cause. Breaking force failure analysis integrates mechanical testing, fracture surface examination, material verification, and process review to answer three key questions: how strong is the failed part compared to specification, where and how did it fracture, and what material or process condition caused the weakness.

Typical failure modes investigated include conductor strand breakage, crimp terminal pull-out, brittle intermetallic fracture at welded or soldered joints, necking failure of plastic strapping, delamination at adhesive bonds, and low seal strength in flexible packaging. Results are documented in a formal failure analysis report that supports engineering corrective action, supplier claims, and regulatory submissions.

Testing Samples

Wires and Cable Conductors

Bare and insulated copper, tinned copper, copper-clad aluminum, and aluminum alloy conductors, including stranded, solid, and tinsel constructions. Common failures include broken strands, conductor necking, and hydrogen embrittlement of high-strength alloys.

Crimped Terminals and Connectors

Closed-barrel and open-barrel crimp terminals, splices, and coaxial contacts that fail the crimp pull-off force requirement. Failure mechanisms include under-crimping, over-crimping, cracked bell mouths, wrong conductor-to-terminal matching, and contaminated strands.

Welded, Soldered, and Ultrasonic Joints

Resistance-welded wire bundles, ultrasonic splices, soldered connections, and laser-welded battery interconnects. Analysis focuses on intermetallic brittleness, cold welds, voids, and insufficient weld nugget formation.

Strapping, Fasteners, and Ties

Plastic cable ties, steel strapping, lashing wire, and hook-and-loop restraints that snap under installation or service loads, often due to notch sensitivity, moisture absorption, or degraded polymer.

Flexible Packaging and Seals

Heat-seal and peel-seal areas of pouches, lids, and films that exhibit low seal strength or sealant fracture, evaluated through seal strength and hot tack testing.

Textiles, Yarns, and Webbing

Sewing threads, industrial yarns, sling webbing, and straps tested for breaking force and knot breaking force in accordance with textile testing standards.

Testing Items

• Breaking force and tensile strength of conductors and metal wires
• Elongation at break and reduction of area at fracture
• Crimp terminal pull-off force and push-out force
• Peel and pull strength of welded and soldered joints
• Seal strength of flexible packaging seals
• Knot breaking force and loop breaking force of yarns and threads
• Breaking force retention after thermal aging, humidity exposure, or salt spray
• Fracture surface morphology and failure mode classification
• Cross-sectional analysis of crimp or weld zones
• Hardness and microstructure verification of base materials
• Chemical composition verification by spectroscopic methods
• Surface contamination and corrosion product identification

Testing Methods

Mechanical force measurement is performed on calibrated universal testing machines or dedicated pull testers equipped with appropriate grips, pneumatic clamps, or terminal holders, with load cells selected to keep the measured force within the calibrated range. The applied test method depends on the product category:

• Metallic conductors: tensile testing per ISO 6892-1, ASTM E8/E8M, or GB/T 228.1, with fracture position and mode recorded.
• Crimped connections: pull-out force testing per IEC 60352-2, UL 486A-486B, or equivalent national standards, using specified pull rates and gripping methods.
• Plastic strapping and ties: tensile properties per ASTM D638 or ISO 527, with break force and break elongation reported.
• Flexible packaging seals: seal strength per ASTM F88 and burst strength per ASTM F1140 or ISO 11607 series requirements.
• Yarns and threads: breaking force and elongation per ISO 2062, ASTM D2256, or corresponding GB and JIS standards.
• Battery and busbar welds: pull and shear testing per IEC 62133-related component evaluations and customer specifications.

Following the force test, root cause analysis typically combines stereomicroscopy and scanning electron microscopy to classify the fracture as ductile, brittle, fatigue, or intergranular, supported by energy-dispersive spectroscopy for contamination or oxidation, metallographic cross-sectioning for crimp or weld quality, microhardness testing, and hardness/composition checks against material certificates. The final report correlates measured force values, fracture morphology, and material data to identify the dominant failure mechanism and provide corrective recommendations.

Applications

• Automotive wiring harness manufacturing, for crimp force qualification and production line break failures
• Consumer electronics and appliances, for connector pull-off failures and flex cable fracture
• Energy storage and battery systems, for tab weld and busbar joint integrity investigations
• Aerospace and rail transit, for control cable, lashing, and structural wire break analysis
• Medical device packaging, for seal strength failures in sterile barrier systems
• Logistics and steel industry, for strapping breakage during bundling and transport
• Textile and lifting equipment industries, for webbing, sling, and thread break investigations
• Supplier quality disputes, customer complaints, and incoming material rejections

FAQ

What does a typical breaking force failure analysis report include?

The report includes specimen description, test conditions and standards applied, measured force values with comparison to specification, fracture surface photographs and microscopy images, failure mode classification, identified root cause, and engineering recommendations for corrective action.

How many samples are needed for a meaningful analysis?

For statistical comparison, a minimum of five to ten failed samples plus several known-good reference samples is recommended. If availability is limited, a single failed part can still be analyzed, though conclusions on process variation will carry lower confidence.

Why would a properly crimped terminal still fail the pull-off force test?

Common causes include conductor strand damage or contamination before crimping, incorrect crimp height and tooling wear, mismatched terminal and conductor sizes, work-hardened or impure conductor material, and stress concentration at the insulation crimp. Cross-section analysis and fractography distinguish among these mechanisms.

Can analysis determine whether the failure was caused by material or by process?

Yes. Composition, hardness, and microstructure verification isolate material-related causes, while crimp dimension checks, cross-section wire fill assessment, and fracture location mapping isolate process-related causes. Many failures involve both, which is why a combined approach is used.

What is the typical turnaround time?

Standard failure analysis projects are generally completed within five to ten working days, depending on sample quantity and the depth of microscopy or material characterization required. Urgent screening can often be delivered faster by accredited third-party laboratories.

Which acceptance criteria are applied?

Acceptance values are taken from the applicable product standard, such as minimum pull-out forces in IEC 60352-2 or UL 486A-486B, conductor tensile limits in ISO 6892-1 based material specifications, or customer drawings and engineering specifications when no public standard exists.