A Closer Look at What Causes Fatigue & Microcracks
- Kirk Abolafia
- 23 hours ago
- 10 min read
A part can clear every dimensional check and every visual inspection on the shop floor and still be carrying a shortened fatigue life. The key issue: the defect isn't always visible; sometimes it's just a few microns deep below the surface, waiting for enough load cycles to matter. And when it matters, that can result in part failure on critical systems, from orthopedic fixtures to aerospace engines.
Working with a fatigue-sensitive feature in aerospace or medtech, or trying to figure out why a "good looking" part keeps failing under cyclic load? We want to learn more. Contact us at info@voxelinnovations.com
Key Takeaways
Fatigue is progressive damage from repeated loading...not from a single instance. A part can survive a thousand cycles but begin to fail by the millionth.
Microcracks often sit below the resolution of visual or dimensional inspection, which is why a part can pass acceptance yet still fail in service.
The manufacturing processes themselves can leave behind issues. Thermal processes like EDM and laser drilling can alter the surface microstructure in ways that create "crack-initiation sites", regardless of whether or not the part is up-to-spec.
PECM changes what's introduced at the surface, which matters specifically for the thermally-driven crack mechanisms discussed below (though fatigue life is never governed by process alone).
What Fatigue Actually Is
To start....what even is fatigue? Fatigue failure, by definition, is when a part or material fails under repeated loading well below a material's ultimate strength-- fatigue does not occur from a single load-bearing event. A bracket, a blade, an implant, or a fluid-delivery component can carry a load thousands or millions of times without visible distress, and then fail abruptly once a crack has grown to a critical size (think of a cardiovascular implant strained with each heartbeat, or a turbine component under stress as the blades rotate thousands of times a minute).
Fatigue is a sneaky problem because it generally gives little visual warning...before it doesn't. Unlike, say, an overload failure which may shows visible bending or necking as the material begins to yield, a fatigue fracture surface often shows almost no plastic deformation right up until final failure. Under a microscope, a fatigue fracture typically reveals "striations", fine parallel lines marking each individual load cycle's incremental crack growth, and sometimes alongside larger beach marks where crack growth paused. In fact, the transition from that smooth, striated propagation zone to a rougher, more granular final-fracture zone is usually considered some of the best evidence that a part failed from repeated fatigue rather than a single overload event.
One poignant case study of fatigue in action occurred in April 2018 aboard Southwest Airlines Flight 1380. A fan blade became separated in flight (after roughly 32,600 flight cycles) and the investigation concluded this was the result of a fatigue crack in the blade's dovetail root that had been growing, albeit undetected. Both visual and fluorescent-penetration inspections did not detect the microcracks, leading to a fragment of the blade to strike the engine cowling that lead to rapid cabin depressurization. One person was fatally ejected from the aircraft, and eight other passengers were injured.
Loosely speaking, the process of a part getting fatigued runs in three stages: initiation, propagation, and fracture, but amongst those three stages, initiation is where the manufacturing conversation matters most; a crack needs somewhere to start, after all.
Return to turbine blades and orthopedic implants: both are metal parts expected to survive an enormous number of load cycles in the field, often measured in the millions. High-cycle fatigue testing for titanium alloys used in orthopedic, spinal, and dental implants may be run to 10 million cycles or more before a part is considered qualified. (PMC). That scale of testing exists because a flaw too small to catch in standard inspection can still be large enough to start a crack after enough repetitions.
But those practices are still not perfect. A standard bench test usually applies the same load, over and over, at a steady rhythm, but real parts don't get that luxury. A turbine blade sees loads that constantly change in size and pattern as flight conditions shift, not a single repeated cycle. A cardiovascular implant sees mechanical stress from every heartbeat and body movement while also sitting in a corrosive, biological environment for years. None of that is fully captured by a single lab-qualified cycle count, which is one reason real-world fatigue life can diverge from what a bench test predicts.
As some evidence of this dichotomy: recent work on EDM-processed microholes ran ultra-high-cycle fatigue testing out to the 10-million to one-billion cycle range and found crack initiation tracing back to pre-existing microcracks, pores, and inclusions in the recast layer itself. (NCBI) In other words, a key observation from the research is that the number of cycles a part can survive is often decided before it ever gets loaded, by whatever flaw is already sitting at the surface.
Where Do Microcracks Come From?
A microcrack is exactly what it sounds like: a crack small enough that it doesn't show up under a visual check or a basic dimensional inspection, but large enough to act as a stress riser once cyclic load is applied.
In general, three manufacturing pathways tend to create microcracks:
Thermal recast from EDM. During electrical discharge machining, localized melting and rapid re-solidification leaves behind what's called a recast layer with different mechanical properties than the base material. Recent research on high-speed EDM microholes found the recast layer showed a "measurable increase in hardness and a decrease in elastic modulus compared to the substrate, and that fatigue cracks originated from microcracks, pores, and inclusions within that layer or at its interface with the base material". (Materials, via NCBI)
[We have written extensively about recast layers and heat-affected-zones and their other detrimental affects to critical components, consider reading those deep-dives]
Heat-affected zones (HAZ) from laser processing. Laser drilling and cutting introduce a heat-affected zone alongside their own recast layer, which are areas that are molecularly altered from the base material (like recast layers) but are often invisible (like microcracks). In nickel-based superalloys used for turbine blade cooling holes, that heat-affected-zone has been shown to cause a measurable reduction in corrosion fatigue life at elevated temperature.
Mechanical residual stress from cutting or grinding. Even without heat, a tool pressing into material can leave compressive or tensile residual stress at the surface, depending on the operation and tool condition. Tensile residual stress is the less forgiving of the two, since it works in the same direction as the applied cyclic load rather than against it. This is albeit less common, but still worth mentioning.
How big are these microcracks? EDM recast layers typically run anywhere from about 3 to 70 microns thick depending on discharge energy and pulse duration, with micro-EDM on Inconel 718 producing recast in the 5-9 micron range. (PMC) Laser-drilled superalloys tend to run heavier: recast layers around 5-50 microns and a heat-affected zone that can extend roughly 250 microns deep in materials like Inconel 625. (MDPI) The microcracks themselves form within that recast layer or right at its interface with the base material, rarely deep in the bulk part, which is exactly why they're easy to miss: they're sitting in a thin surface band that standard inspection isn't built to see.
Why Aerospace and Medtech Are Especially Exposed
Both industries rely on tough, complex part geometries with relatively low margins: thin walls, dense microhole arrays, and features with high stress concentration...all by design. A turbine blade's film-cooling holes and an orthopedic implant's fixation features both sit in regions where the part is already working close to its structural limit; there isn't much extra material available to absorb a crack-initiation site that a bulkier part could tolerate without consequence.
Furthermore, as evidenced above, hidden metal fatigue can sometimes result in catastrophic consequences aboard aircraft.
Another reason these industries are especially sensitive to fatigue is that inspection (already challenging!) gets even harder: a microcrack inside a cooling hole or a cannulated screw isn't something a visual check or a coordinate-measuring machine will catch, be it SEM or metallographic cross-sectioning.
As a quick aside: SEM, or scanning electron microscopy, images the surface at magnifications far beyond an optical microscope, high enough to resolve a crack that's only a few microns wide. Metallographic cross-sectioning, on the other hand, goes a step further: the part is cut open, the cut face is mounted, polished, and chemically etched, and the exposed cross-section is examined directly under a microscope to see the recast layer, HAZ, or subsurface cracking in profile. Both methods require sacrificing the part to be inspected, which is why they're run on sample parts pulled from a batch rather than on 100% of production.
Ultimately, that gap between what's easy to inspect and what actually predicts fatigue life is a large part of why this problem tends to surface after shipment rather than during acceptance.
How PECM Changes The Concept of Fatigue
The straightforward answer: PECM helps prevent fatigue from setting in from the way it inherently machines material in the first place.
PECM removes material through anodic dissolution rather than thermal melting or mechanical shear: it doesn't introduce a recast layer or heat-affected zone in the first place.
To be clear: that's not a claim that PECM-made parts can't fatigue; every part fatigues eventually under enough load. Rather, the specific crack-initiation pathway created by recast/HAZ microcracking, discussed above, isn't part of the PECM process.
The second, separate advantage is about geometry rather than surface integrity. PECM can hold tolerance on thin walls, internal channels, and complex features that are difficult or impossible to reach with an EDM electrode or a laser's line of sight, including geometries increasingly enabled by additive manufacturing. That matters for fatigue-critical design because it removes a common trade-off: a designer no longer has to simplify a fatigue-optimized geometry down to whatever a thermal process can reach, then accept the recast/HAZ penalty on top of it.
Process | What it leaves behind | Fatigue-relevant risk |
EDM | Recast/white layer, tensile residual stress | Recast-layer hardness and stiffness mismatch with base material, crack initiation at layer/substrate interface |
Laser drilling | Recast layer, heat-affected zone | HAZ microstructure alteration, recast-driven fatigue-life reduction reported at elevated temperature |
PECM | Minimal thermal or mechanical alteration to the surface | No cutting edge, no thermal melt-and-resolidify step, and minimal cathode wear across production runs |
These specifications are most meaningful in the context of a full process evaluation. Geometry, load spectrum, alloy, and surface finish all influence fatigue life alongside the manufacturing method. Voxel's vertically integrated PECM process connects those variables during feasibility review rather than treating surface integrity as an isolated claim.
What This Means for a Design Under Review
A feature that shows thermally-driven microcracking as a strong candidate for PECM still deserves a closer look before assuming it's a fit. Load spectrum, alloy, geometry, and the specific failure history of the part all shape whether recast- or HAZ-driven microcracking is actually the dominant risk, or whether something else in the design is the bigger driver of fatigue life. The reverse holds too: a part that looks like it's outside the obvious PECM window may still be worth a conversation if fatigue failures, unexplained rejections, or inconsistent test results keep showing up.
That's the kind of question a PECM engineer is built to help answer early, before a design commits to a manufacturing path that turns out to be the wrong one for a fatigue-critical feature.
Trying to figure out whether recast layer, HAZ, or residual stress is driving a fatigue problem on a conductive-metal part? Contact Voxel at info@voxelinnovations.com to discuss burr, HAZ, and surface-integrity risk with Mike or Kirk.
Mini-FAQ
What even is fatigue?
Fatigue is progressive damage from repeated loading, well below the load a part could survive in a single event. A part can carry a load thousands or millions of times without visible distress, then fail abruptly once a crack has grown large enough. It's a different failure mode than overload, which happens all at once and usually shows visible bending or deformation first.
How long does it take before parts tend to get fatigued?
It depends heavily on the part, the material, and how it's loaded, but it's often measured in the millions of cycles rather than hundreds or thousands. Titanium implants used in orthopedic and spinal applications, for example, are commonly qualified through testing to 10 million cycles or more. Real-world life can still diverge from that lab number once you factor in variable loading, combined mechanical and thermal stress, or a corrosive environment.
What's so problematic about microcracks?
Microcracks act as stress risers, giving a fatigue crack a starting point it wouldn't otherwise have. The bigger problem is that they're often invisible to standard inspection. A microcrack sitting a few microns below the surface won't show up on a visual check or a dimensional inspection, only SEM or metallographic cross-sectioning will catch it, and both are destructive, so they're run on sample parts rather than every unit that ships.
What environments, materials, or designs make a part more likely to experience fatigue quicker?
Thin walls, dense microhole arrays, and features with high stress concentration by design all leave less material margin to absorb a crack-initiation site. Parts exposed to variable-amplitude loading, combined thermal and mechanical cycling, or a corrosive or biological environment (like a cardiovascular implant under constant mechanical stress and body chemistry) tend to see fatigue life diverge further from a standard lab-qualified number.
How do conventional methods deal with fatigue?
Mostly by managing it rather than removing its source. EDM and laser drilling both leave a recast layer, and laser drilling adds a heat-affected zone on top of it. Downstream steps like polishing, etching, or shot peening can reduce that risk after the fact, but they add cost and don't change the fact that the thermal process created the risk in the first place.
How does PECM mitigate fatigue?
PECM removes material through anodic dissolution rather than thermal melting or mechanical shear, so it doesn't introduce a recast layer or heat-affected zone at all. That removes one specific crack-initiation pathway rather than addressing fatigue risk broadly. It's worth a closer look for a thermally-processed part showing fatigue problems, though geometry, load spectrum, and alloy still shape the overall picture, and a PECM engineer reviewing the specific application is the fastest way to know if it's the right fit.
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