{"id":23512,"date":"2025-12-18T18:33:31","date_gmt":"2025-12-19T02:33:31","guid":{"rendered":"https:\/\/shengenfab.com\/?p=23512"},"modified":"2026-08-24T19:18:55","modified_gmt":"2026-08-25T03:18:55","slug":"fatigue-failure-in-sheet-metal","status":"publish","type":"post","link":"https:\/\/shengenfab.com\/fr\/fatigue-failure-in-sheet-metal\/","title":{"rendered":"Rupture par fatigue des t\u00f4les : causes profondes et pr\u00e9vention"},"content":{"rendered":"\t\t
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Sheet metal fatigue failure is the progressive cracking of a thin metal part caused by repeated cyclic loading or vibration, even when stresses remain well below the material’s yield strength. Cracks almost exclusively initiate at manufactured stress concentrations like cut edges, tight bends, or weld toes.<\/p>\n

In the factory, a newly stamped bracket might pass every dimensional and static load check flawlessly. Yet, after three months attached to a 30Hz industrial motor, it snaps in half. This is the reality of fatigue: failures rarely happen during inspection; they happen in the field, causing costly warranty claims and machine downtime.<\/p>\n

This guide explains why fabricated sheet metal parts fail under cyclic loads. More importantly, it provides actionable DFM (Design for Manufacturability) fixes for cut edges, bends, and welds to help engineers eliminate fatigue risks before releasing CAD files for mass production.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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What Repeated Loading Does to Thin Sheet Metal\uff1f<\/h2>\n

Fatigue is a process of accumulating damage. It is essential to distinguish fatigue from static overload to determine the correct fix for your part.<\/p>\n

Cyclic Stress and Service Loads<\/h3>\n

Static overload occurs when a single force exceeds the material’s yield strength, resulting in immediate, visible deformation. Fatigue is driven by repeated loading, often at stress levels well below the material’s yield point.<\/p>\n

Fatigue life depends heavily on the stress range (Delta sigma) and the number of load cycles:<\/p>\n

Delta sigma = sigma_max – sigma_min<\/p>\n

(Engineering Note: You don’t need to calculate this manually, but understanding that the range<\/strong> of stress matters more than the peak<\/strong> stress is critical for design.)<\/em><\/p>\n

In sheet metal, cyclic stress usually comes from continuous vibration, equipment start-stop cycles, pressure pulses, or thermal cycling.<\/p>\n

Fatigue vs. Static Overload<\/strong><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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\r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n \r\n
Feature<\/th>\r\n Fatigue<\/th>\r\n Static Overload<\/th>\r\n <\/tr>\r\n <\/thead>\r\n
Load Type<\/td>\r\n Long-term, repeated<\/td>\r\n Single, excessive event<\/td>\r\n <\/tr>\r\n
Global Deformation<\/td>\r\n Usually minimal<\/td>\r\n Often obvious yielding or bending<\/td>\r\n <\/tr>\r\n
Crack Origin<\/td>\r\n Local stress concentration<\/td>\r\n Cross-section lacking capacity<\/td>\r\n <\/tr>\r\n
Investigation Focus<\/td>\r\n Load spectrum and crack origin<\/td>\r\n Peak load and cross-sectional area<\/td>\r\n <\/tr>\r\n <\/tbody>\r\n <\/table>\r\n<\/div>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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(Engineering Note: Mean stress, stress ratio (R), and Miner’s rule are used to calculate cumulative damage when loads are random or variable, but identifying the crack origin remains the priority on the floor.)<\/em><\/p>\n

Panel Flexing and Resonance<\/h3>\n

Thin sheet metal behaves differently than heavy structural steel. Large, flat panels have low out-of-plane stiffness and flex repeatedly. The way a panel is mounted significantly changes this stiffness.<\/p>\n

When continuous excitation\u2014such as a motor or fan\u2014matches the panel’s natural frequency, resonance occurs. For example, an industrial motor running at 1800 RPM creates a continuous 30Hz vibration. If the bracket’s natural frequency is near 30Hz, the vibration amplifies, and even a 2mm thick stainless steel panel can tear in weeks.<\/p>\n

Another issue is “oil-canning,” where a panel snaps back and forth between two states. While oil-canning is not fatigue itself, it forces the material to undergo severe cyclic deformation, which rapidly leads to cracking.<\/p>\n

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In our motor bracket example, the 30Hz excitation frequency from the motor causes the flat section of the bracket to repeatedly flex, accelerating damage near the bend.<\/em><\/p>\n<\/blockquote>\n

Crack Initiation and Growth<\/h3>\n

Fatigue failure progresses through three stages:<\/p>\n

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  1. Crack Initiation:<\/strong> Micro-cracks form at areas of high local stress, usually at a geometric feature or edge defect.<\/li>\n
  2. Stable Crack Growth:<\/strong> The crack advances incrementally with each load cycle. On many ductile metals, this leaves “beach marks” radiating outward from the origin.<\/li>\n
  3. Final Fracture:<\/strong> Once the crack reduces the load-bearing cross-section to a critical point, the remaining material tears under static overload.<\/li>\n<\/ol>\n

    When reading a fracture surface, the origin is usually smooth, followed by the flat growth region with beach marks, and ending in a rough fracture zone. Be aware that beach marks are not always clearly visible, and a visual check cannot replace a formal failure analysis.<\/p>\n

    Where Fatigue Cracks Start in Fabricated Parts<\/h2>\n

    Fatigue cracks rarely start in the middle of a flat panel. They initiate at features created during fabrication, where the nominal stress is multiplied.<\/p>\n

    Cut and Punched Edges<\/h3>\n

    The quality of the cut edge directly impacts fatigue life. Every cutting method leaves a specific signature that acts as a crack starter. You must evaluate the final edge condition relative to the direction of the principal load.<\/p>\n

    Common Edge Crack Starters<\/strong><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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    Process<\/th>\r\n Crack Starter<\/th>\r\n Inspection Point<\/th>\r\n <\/tr>\r\n <\/thead>\r\n
    Laser Cutting<\/td>\r\n Striations, dross, sharp internal corners<\/td>\r\n Edge roughness and HAZ cleaning<\/td>\r\n <\/tr>\r\n
    Punching<\/td>\r\n Rollover, burrs, fracture zone<\/td>\r\n Die clearance, tool wear, and burr direction<\/td>\r\n <\/tr>\r\n
    Shearing<\/td>\r\n Micro-tears, severe deformation<\/td>\r\n Blade condition and secondary finishing<\/td>\r\n <\/tr>\r\n <\/tbody>\r\n <\/table>\r\n<\/div>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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    Here is a manufacturing reality often missed on drawings: If a punched edge contains micro-cracks on the burr side, and that side is placed under tension<\/strong> during assembly, the crack will propagate rapidly. If the burr side is placed under compression<\/strong>, the fatigue life increases significantly.<\/p>\n

    Bends and Formed Features<\/h3>\n

    Forming<\/a> a bend stretches the outer fibers of the material and compresses the inner fibers, creating localized thinning and severe plastic strain.<\/p>\n

    This cold forming process leaves residual stress in the material. While residual stress alone does not cause fatigue, it alters how the part reacts to external loads. If a bend retains high residual tensile stress on its outer surface, any external vibration adds to it, pushing the material closer to failure.<\/p>\n

    Tight bend radii, bending parallel to the material’s rolling direction, or attempting to flatten and re-bend a part all introduce internal damage that drastically reduces fatigue life.<\/p>\n

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    In our bracket example, the mounting hole was placed too close to the bend line. The hardware installation disrupted the stress flow around the bend, creating an extreme stress concentration exactly where the material was already thinned and work-hardened.<\/em><\/p>\n<\/blockquote>\n

    Holes and Stiffness Transitions<\/h3>\n

    Fastener holes, slots, and cutouts are classic stress concentrators. Cracks frequently initiate at the transition points between rigid and flexible areas, rather than at the point of highest nominal load.<\/p>\n

    High-risk transition areas include:<\/p>\n