💥Failure Friday
Friday, August 14, 2026
Three jet airliners came apart in the sky between 1953 and 1954. The British investigation that followed rewrote how the whole industry thinks about fatigue. Every Boeing and Airbus you've ever flown in carries the lesson.
Term 1: Brittle Fracture
MaterialsSudden failure with little to no plastic deformation. The crack propagates faster than the material can yield to relieve the stress at the tip. Glass does this. So does high-strength steel below its ductile-to-brittle transition temperature. Once a brittle crack starts moving, it keeps moving until something stops it — and very little stops it.
Why it matters: Pressure vessel codes (ASME Section VIII) require Charpy impact testing at design temperature precisely because a brittle-fracture event in a vessel is catastrophic. The Liberty ship hull cracks of WWII drove the original specifications.
Term 2: Ductile Fracture
MaterialsFailure preceded by significant plastic deformation. You can see it coming if you're looking — the part necks down, the surface gets a fibrous gray appearance, and the final break often shows a cup-and-cone shape from a tensile specimen. Aluminum at room temperature does this. So does mild steel above its transition temperature.
Why it matters: Ductile failure is the failure mode you want, because it gives warning. The whole point of choosing 6061-T6 over a higher-strength but brittler aluminum alloy is often the failure mode trade. Same logic on stainless versus high-strength carbon steel in cold service.
Term 3: Charpy Impact Energy
MaterialsA standardized notched bar is struck by a swinging hammer; the energy absorbed during fracture is the Charpy V-notch value. Plot CVN versus temperature and you get the ductile-to-brittle transition curve. Most ferritic steels go from 100+ J ductile down to 5–10 J brittle over a fairly narrow temperature range — and that range often includes the operating temperature of something important.
Why it matters: Pipeline steels, pressure vessels, ship plate, and missile body materials all carry Charpy specs. Material certs are checked against them on every receiving inspection. The number you don't want to see on an MTR for cold-service equipment is a low Charpy value at design minimum metal temperature.
The longer read
The longer read
The de Havilland Comet was the world's first commercial jet airliner. It entered service in May 1952 with BOAC — a sleek, square-windowed, four-engine aircraft that cruised higher and faster than anything else in passenger service. For about a year and a half, British aviation owned the future.
Then the airplanes started coming apart.
On 2 May 1953, Comet G-ALYV broke up in a thunderstorm out of Calcutta. The investigation, hampered by limited wreckage recovery, attributed the loss to weather and pilot input. Service continued.
On 10 January 1954, Comet G-ALYP exploded in mid-air over the Mediterranean at 27,000 feet on a routine flight from Rome to London. Thirty-five souls. The British government grounded the fleet. The wreckage was painstakingly recovered from the seabed by the Royal Navy and reassembled in a hangar at Farnborough — the early ancestor of every aviation accident reconstruction since.
Comet service resumed in March 1954 after modifications. Two weeks later, on 8 April, G-ALYY broke up over the Mediterranean at 35,000 feet on a flight from Rome to Cairo. Twenty-one souls. The Comet was grounded for good.
The Royal Aircraft Establishment then did something the airline industry had never seen at scale. They put an entire Comet fuselage into a water tank, pressurized and depressurized it on a cyclic schedule that simulated flight loads, and ran it until failure. The tank arrived at the answer in months.
The fuselage failed from fatigue cracks initiating at the corners of the square ADF antenna windows. Each pressurization cycle drove the crack a little further through the skin. The skin gauge was thin. The corners had sharp radii. The fatigue life at the stress concentration was a small fraction of the calculated value because the calculation had used average skin stress, not the peak stress at the window corner. The crack reached critical length and the fuselage unzipped.
Every commercial aircraft that flies today carries the answer to that problem in its window geometry. The ovaled corners of the cabin windows on a 737, an A320, an 787, a 777 — those are not cosmetic. They are the de Havilland Comet lesson, made airworthy. Stress concentration at a sharp corner times pressurization cycle count equals fatigue failure. Round the corner. Spread the stress. Live.
The other lesson is methodological. The Farnborough water tank test became the template for full-scale fatigue testing of airframes — every commercial transport type certificate since has required a representative airframe to be cycled through its design service life inside a test rig, with crack inspections at defined intervals. The Comet investigation also pushed forward the science of linear elastic fracture mechanics — Griffith's 1920s crack theory finally got the engineering machinery it needed to become a routine design tool.
The Huntsville connection: every pressurized structure on every NASA program since Apollo has had its fatigue life qualified against the modern descendant of that Farnborough water-tank protocol. Crew modules. Habitat pressure vessels. Propellant tanks. Cycle the article, inspect for cracks, retire it before it gets to half its life. The discipline came from a wet hangar in southern England in 1954, and the cost of arriving at it was sixty-five passengers and crew.
When you next sit in an airliner and look at the window, look at the corner. That radius is engineering grief made geometry. We round the corners now because we used to lose airplanes that didn't.
📍 Huntsville Pulse
- •ASM International Huntsville is hosting a failure analysis case study night next month — invited speaker from MSFC NDE.
- •The Comet investigation report is online at the UK National Archives. It's still one of the cleanest aviation accident reports ever written; engineers new to fatigue should read the executive summary at minimum.
- •Friday at Three Caves Loop is one of the better short hikes for processing a hard week — about 1.5 miles, mostly level, with a quiet cave overlook near the midpoint.
- •Failure Friday pairing: Blue Oak BBQ post-shift, the brisket sandwich, and don't try to read the report at the bar — you'll get into an argument with someone who lived through the Challenger investigation.
“Q: How does a metallurgist describe heartbreak? A: A region of low fracture toughness that propagated catastrophically once a stress concentration was identified.”