Six Typical Fracture Surfaces of Bolts: Inferring Failure Causes from Fracture Morphology
Executive Summary
A broken bolt is itself a record. The location, morphology, color and texture direction of the fracture can point the cause to overloading, fatigue, hydrogen embrittlement, delayed fracture, stress corrosion or fretting fatigue. This article gives the morphological features, reading points and verification methods of six typical fractures, forming a reference system for inferring causes from morphology.
Table of Contents
The Fracture Is the Only Evidence That Does Not Lie
A batch of M10 grade 10.9 bolts began breaking one after another at the customer site. The field record read "broke after three days of installation; torques all passed." From that sentence alone, the possible causes—overload, hydrogen embrittlement, assembly misalignment, material defect—point in entirely different directions.
Put the broken part under a microscope and the key information appears immediately: the fracture has no necking, shows intergranular features, and is located at the first loaded thread tooth. The combination of these three features locks the direction onto hydrogen embrittlement. Fracture morphology is the only evidence that directly reflects the failure mechanism—appearance, dimensions and hardness data can only provide circumstantial evidence.
Three Basic Dimensions for Reading
Do three things first when looking at a fracture, in this order:
- Locate it: Where did it break? First loaded thread tooth, shank, head transition, thread run-out (last engaged tooth)—each position corresponds to different mechanical conditions.
- Characterize the morphology: Is there necking? Is the cross-section flat or rough? Are there fatigue marks, radial striations, or intergranular rock-candy features?
- Note color and deposits: Is the fracture fresh metal color, dark brown, or covered with corrosion products? Is there abrasive debris or secondary cracks?
Six Typical Fractures, Broken Down One by One
1. Ductile overload fracture
Morphology: fibrous (cup-cone) fracture with obvious necking and a shear lip at about 45° to the axis. Location: usually at the shank or the unengaged thread section (weakest cross-section). Cause: over-tightening, external load exceeding design, insufficient bolt strength, or excessive preload. Verification: check tightening tool torque settings and calibration records; re-test bolt hardness and tensile strength.
2. Fatigue fracture
Morphology: the fracture divides into a fatigue-initiation zone, a smooth propagation zone (clam-shell or beach markings) and a rough final-fracture zone. Location: the vast majority are at the root of the first loaded thread tooth, a few at the head transition or thread run-out. Cause: stress amplitude exceeding limits under alternating load, or impact cycles from joint-face opening. Verification: calculate stress amplitude as Φ × ΔFA / 2; check for joint-face opening marks; the larger the propagation zone, the lower the load and the longer the life.
3. Hydrogen-embrittlement delayed fracture
Morphology: flat, bright fracture with no necking; microscopically intergranular (rock-candy), sometimes with white bright spots. Location: often at the first loaded thread tooth or thread root, possibly at the head transition. Time signature: fractures within hours to days after assembly—this is the most valuable criterion. Cause: hydrogen introduced by electroplating pickling, insufficient baking, or baking done after passivation. Verification: run the pre-load test per GB/T 3098.17; check surface-treatment process and baking records (furnace within 4 h after plating, 190~230 °C, sufficient duration, before passivation).
4. Stress-corrosion fracture
Morphology: mostly intergranular or mixed fracture, covered with corrosion products, with multiple secondary cracks branching from the main fracture toward the side walls. Location: usually at stress-concentration areas or crevices where corrosive media accumulate. Cause: the three factors—tensile stress, specific corrosive medium and susceptible material—all present. Verification: analyze the composition of fracture corrosion products; confirm service-environment media (chloride, sulfide, ammonia, etc.); check whether protective coating has failed.
5. Fretting-fatigue fracture
Morphology: fatigue features plus obvious fretting wear marks near the fracture, reddish-brown iron-oxide debris on the surface, often with circumferential clusters of shallow cracks. Location: below the bearing surface, at the thread engagement, or near the hole wall. Cause: micrometer-scale relative motion of joint faces under alternating load; debris and microcracks become fatigue sources. Verification: inspect the depth of annular wear marks and debris distribution on the bearing surface; calculate joint-face pressure and relative slip.
6. Thread stripping
Morphology: the bolt is intact or only slightly deformed, while internal-thread teeth are sheared, turned out or peeled off, with tear-and-stretch marks on the flanks. Location: the first few teeth of the nut or internal-thread part. Cause: insufficient engagement length or low internal-thread material strength, making internal-thread shear strength lower than bolt tensile strength. Verification: calculate engagement length and the internal/external thread strength ratio; section the failed part metallographically to observe flank load distribution and the stripping initiation point.
Quick Reference Table of Fracture Features
| Fracture Type | Cross-Section Morphology | Necking | Fracture Location | Time Signature |
|---|---|---|---|---|
| Ductile overload | Fibrous, cup-cone, 45° shear lip | Obvious | Shank or unengaged section | At load application |
| Fatigue | Clam-shell propagation zone + rough final fracture | None | Root of first loaded thread tooth | Accumulated over service |
| Hydrogen embrittlement | Flat and bright, intergranular rock-candy | None | First loaded tooth / thread root | Hours to days after assembly |
| Stress corrosion | Intergranular or mixed, corrosion products, secondary cracks | None | Stress-concentration area or crevice | Long-term service (in corrosive media) |
| Fretting fatigue | Fatigue marks + wear marks + reddish-brown debris | None | Below bearing surface or engagement | Accumulated over service |
| Thread stripping | Bolt intact; internal threads sheared and turned out | — | First few internal-thread teeth | During tightening or service loading |
The Judgment Chain from Morphology to Cause
In practice you do not need to memorize all six categories at once; eliminate step by step along the following chain:
- Is there necking? Yes → ductile overload (check torque setting, load estimate, strength). No → next step.
- Are there fatigue marks on the fracture? Yes → fatigue (calculate stress amplitude and Φ), and check wear marks to judge whether fretting is superimposed. No → next step.
- Did it fracture hours to days after assembly? Yes → strongly suspect hydrogen embrittlement (check surface treatment and baking). No → next step.
- Are there corrosion products or secondary cracks on the fracture? Yes → stress corrosion (analyze medium and material). No → next step.
- Is the internal thread damaged while the bolt is intact? Yes → stripping (calculate engagement length and strength ratio). No → consider material defects, inclusions, abnormal heat treatment; metallographic and chemical analysis needed.
The value of the judgment chain is that it avoids preconceptions. The most common field misjudgments are "saw it break and called it overload," then upsizing; or "saw it fracture and blamed the material," then switching suppliers. Eliminating step by step keeps the direction from going wrong.
Supporting Verification Methods
| Suspected Cause | Verification Method | Key Criterion |
|---|---|---|
| Ductile overload | Recheck tightening torque and tool calibration; re-test hardness and tensile strength | Was torque over-set; does strength meet spec |
| Fatigue | Calculate stress amplitude and Φ; observe initiation site by SEM | Is stress amplitude over allowable; are there defects at the source |
| Hydrogen embrittlement | GB/T 3098.17 pre-load test; check baking records | Does the pre-load test fracture; do baking parameters meet spec |
| Stress corrosion | Compositional analysis of fracture products; service-medium investigation | Are chlorides, sulfides or other sensitive media present |
| Fretting fatigue | Observe bearing-surface wear marks; calculate joint-face pressure | Is there micrometer-scale relative motion |
| Stripping | Measure engagement length; section metallography | Engagement length and internal/external thread strength ratio |
| Material defect | Metallography, chemical composition, non-metallic inclusion rating | Are there inclusions, decarburization or abnormal microstructure |
Conclusion
The six typical fractures form the "morphology dictionary" of fastener failure: ductile overload shows necking, fatigue shows clam-shell markings, hydrogen embrittlement is flat and bright with delayed fracture, stress corrosion carries corrosion products and secondary cracks, fretting fatigue overlays wear marks, and stripping means the internal thread fails first.
The point of mastering this dictionary is that it turns failure analysis from "guessing" into "elimination." The broken bolt has already written the answer on the fracture; the remaining work is to read it out in order—and to protect the fracture so that no information is lost before it is read.
