Corrosion pits about 0.4 mm deep. The fatigue cracks that grew from them then turned through 90 degrees.
What the study did
An open-access paper published in Materials on 4 August 2026 looked at fatigue cracks that start from corrosion damage in AA 7085-T7452, a high-strength aluminium alloy. The specimens were machined from a die forging.
Two specimens were exposed for 14 days to a 5% sodium chloride salt fog at 35 °C to ASTM B117, which left pits of the order of 0.4 mm deep. They were then fatigue tested in laboratory air under a repeating block load spectrum with marker loads, at a peak stress of about 400 MPa.
What they found
The cracks started at the corrosion damage and grew normal to the surface, as expected. At about 2 mm deep they turned through roughly 90 degrees to the original fracture plane, giving a layered fracture. The authors describe this as a failure mechanism not previously reported.
The two specimens failed after 167,226 and 137,938 cycles. The authors estimate the stress intensity factor at the point where the cracks turned at about 22 and 20 MPa√m. Their earlier work measured the alloy's Mode I fracture toughness at 65 MPa√m.
Why it matters
Critical crack lengths for airworthiness assessments are normally calculated from Mode I fracture toughness measured to ASTM E399. The authors argue that, in these specimens, failure was governed by the lower value at which the crack turned rather than by the Mode I toughness. If that holds more widely, a critical crack length calculated in the usual way may not be conservative for this alloy in these conditions.
The authors are explicit that the work rests on two specimens and five cracks, that the mechanism is a hypothesis, and that the tests should be repeated.
What it means in practice
It was the fracture surface, not the crack length, that revealed the change. When a failed part shows a fracture surface that does not fit the expected pattern, that is evidence to be explained, not noise to be ignored.
What an engineer should check
- Where corrosion damage is accepted on fatigue-loaded aluminium parts, what fracture toughness value underpins the allowable crack size?
- Are failed or cracked parts examined fractographically, or only measured?
Source. Peng D., Ang A.S.M., Phan N., Brindza M.R., Main B., Jones R., "Unusual Failures Associated with Cracks That Nucleated from Corrosion Damage in Fatigue Tests on AA 7085-T7452 Specimens", Materials 19(15):3301 (2026), doi:10.3390/ma19153301, CC BY. Read the paper. This article summarises the source in our own words; see the original for full details. No figures from the paper are reproduced. It is general technical information, not advice on a specific component.
Related reading
- Fatigue or overload? Reading the fracture
- Why you should not clean a fracture surface
- Materials characterisation for non-specialists
- MTIS characterisation and testing services
- Follow MTIS on LinkedIn for new failure reviews and industry briefings.
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