{"id":446,"date":"2026-05-26T06:18:01","date_gmt":"2026-05-25T21:18:01","guid":{"rendered":"https:\/\/www.nichidai.jp\/english\/?post_type=column&#038;p=446"},"modified":"2026-09-11T08:29:52","modified_gmt":"2026-09-10T23:29:52","slug":"what-is-fatigue-failuremechanism-life-calculation-methods-and-prevention-strategies-explained","status":"publish","type":"column","link":"https:\/\/www.nichidai.jp\/english\/column\/446\/","title":{"rendered":"What Is Fatigue Failure?Mechanism, Life Calculation Methods, and Prevention Strategies Explained"},"content":{"rendered":"\n<p>Fatigue failure is a phenomenon in which materials fracture due to repeated cyclic loading\u2014even when the applied stress is lower than the material\u2019s tensile strength or yield strength. Over time, microscopic cracks initiate and propagate until sudden fracture occurs.<br>Because fatigue failure often occurs without visible deformation before failure, it is difficult to detect through visual inspection. Therefore, properly evaluating fatigue risk during the design stage is essential for ensuring product reliability.<br>This article provides a comprehensive explanation of.<\/p>\n\n\n\n<p>\u2022 The microscopic mechanism of fatigue failure<br>\u2022 How to interpret S-N curves for life prediction<br>\u2022 Practical prevention strategies for design and manufacturing<\/p>\n\n\n\n<p><strong>\u3010What You\u2019ll Learn\u3011<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>The three stages of fatigue failure: initiation, propagation, and final fracture<\/li>\n\n\n\n<li>Differences in fatigue limits between steel and aluminum<\/li>\n\n\n\n<li>Key factors affecting fatigue life such as stress concentration and mean stress<\/li>\n\n\n\n<li>Practical countermeasures including shot peening and geometric optimization<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">What Is Fatigue Failure?<br>Definition and Comparison with Other Failure Modes<\/h2>\n\n\n\n<p>Understanding fatigue failure begins by recognizing that it differs fundamentally from failures caused by a single large load.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Why Failure Occurs Below Tensile Strength<\/strong><\/h3>\n\n\n\n<p>In mechanical design, strength is typically evaluated based on:<br>\u2022Yield strength<br>\u2022Tensile strength<br>However, fatigue failure can occur even within the elastic range, below these limits, due to repeated loading cycles.<br>Additionally:<br>\u2022There is little to no visible plastic deformation before fracture<br>\u2022Damage progression is difficult to detect externally<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Comparison of Failure Modes<\/strong><\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong><strong>Item<\/strong><\/strong><\/td><td><strong><strong>Fatigue Failure<\/strong><\/strong><\/td><td><strong><strong>Ductile Failure<\/strong><\/strong><\/td><td><strong><strong>Brittle Failure<\/strong><\/strong><\/td><\/tr><tr><td><strong>Cause<\/strong><\/td><td>Repeated loading<\/td><td>Excessive load<\/td><td>Impact or low temperature<\/td><\/tr><tr><td><strong>Deformation before fracture<\/strong><\/td><td>Minimal<\/td><td>Significant plastic deformation<\/td><td>Minimal<\/td><\/tr><tr><td><strong>Fracture surface<\/strong><\/td><td>Striations, beach marks<\/td><td>Dimples<\/td><td>Cleavage surface<\/td><\/tr><tr><td><strong>Stress level<\/strong><\/td><td>Below yield strength<\/td><td>Near tensile strength<\/td><td>Near tensile strength<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Mechanism of Fatigue Failure<br>Three Stages of Crack Development<\/h2>\n\n\n\n<p><strong>Fatigue failure occurs progressively through three stages<\/strong><\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Stage 1: Crack Initiation<\/strong><\/h3>\n\n\n\n<p>\u2022Begins at stress concentration points:<br>Thread roots<br>Holes<br>Surface defects<br>Inclusions<br>\u2022Repeated shear stress causes microscopic slip at the atomic level<br>\u2022Surface irregularities form (extrusions and intrusions)<br>\u2022These evolve into microcracks<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Stage 2: Crack Propagation<\/strong><\/h3>\n\n\n\n<p>\u2022Cracks grow gradually under cyclic loading<br>\u2022Characteristic fracture patterns appear<\/p>\n\n\n\n<p>Key Features:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Striations<\/strong>\uff1aFine lines formed per stress cycle (visible under a microscope)<\/li>\n\n\n\n<li><strong>Beach Marks<\/strong>\uff1aMacroscopic concentric patterns indicating crack growth<\/li>\n<\/ul>\n\n\n\n<p>These are critical clues in failure analysis.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Stage 3: Final Fracture<\/strong><\/h3>\n\n\n\n<p>\u2022Effective cross-sectional area decreases<br>\u2022Remaining material can no longer support the load<br>\u2022Sudden fracture occurs<\/p>\n\n\n\n<p>Final fracture resembles static failure:<br>\u2022Ductile materials show dimples<br>\u2022Rapid crack propagation dominates<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Predicting Fatigue Life<br>Understanding the S-N Curve<\/h2>\n\n\n\n<p>Fatigue life is determined by the relationship between:<br>\u2022Stress amplitude (S)<br>\u2022Number of cycles to failure (N)<br>This relationship is represented by the S-N curve.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>How to Read the S-N Curve<\/strong><\/h3>\n\n\n\n<p>\u2022Vertical axis: Stress amplitude<br>\u2022Horizontal axis: Number of cycles (log scale)<\/p>\n\n\n\n<p>Trend:<br>\u2022 Higher stress \u2192 shorter life<br>\u2022 Lower stress \u2192 longer life<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Fatigue Limit vs Finite Life Design<\/strong><\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong><strong><strong>Material Type<\/strong><\/strong><\/strong><\/td><td><strong><strong>Fatigue Limit<\/strong><\/strong><\/td><td><strong><strong>Design Approach<\/strong><\/strong><\/td><\/tr><tr><td><strong>Steel \/ Titanium<\/strong><\/td><td><strong>Exists<\/strong><\/td><td>Infinite life below limit<\/td><\/tr><tr><td><strong>Aluminum \/ Copper<\/strong><\/td><td>Does not exist<\/td><td>Design based on finite cycles<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<figure class=\"wp-block-image is-resized\"><img decoding=\"async\" src=\"https:\/\/www.nichidai.jp\/column\/wp-content\/uploads\/2026\/05\/image1-840x458.png\" alt=\"\" class=\"wp-image-305\" style=\"width:900px;height:auto\"\/><\/figure>\n\n\n\n<p>Key Point<br>\u2022 Steel materials: Exhibit a fatigue limit where the curve becomes horizontal<br>\u2022 Non-ferrous metals: Continue degrading with increasing cycles<br>Engineers must choose:<br>\u2022 Infinite-life design (steel)<br>\u2022 Finite-life design (aluminum)<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>5 Factors Affecting Fatigue Life<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>1. Stress Concentrati<\/strong><\/h3>\n\n\n\n<p>\u2022 Occurs at geometric discontinuities<br>\u2022 Evaluated using stress concentration factor (Kt)<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>2. <strong>Mean Stress<\/strong><\/strong><\/h3>\n\n\n\n<p>Fatigue behavior depends on both:<br>\u2022 Stress amplitude (\u03c3a)<br>\u2022 Mean stress (\u03c3m)<br>Effects:<br>\u2022 Tensile mean stress \u2192 reduces fatigue life<br>\u2022 Compressive mean stress \u2192 improves fatigue life<br>Used in techniques like shot peening.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>3. Surface Condition<\/strong><\/h3>\n\n\n\n<p>\u2022 Rough surfaces reduce fatigue strength<br>\u2022 Smooth or polished surfaces improve performance<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>4. Size Effect<\/strong><\/h3>\n\n\n\n<p>\u2022 Larger components have lower fatigue strength<br>\u2022 More likely to contain internal defects<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>5.Environmental Factors<\/strong><\/h3>\n\n\n\n<p>\u2022 Corrosion accelerates crack growth<br>\u2022 High temperature and vibration also impact life<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>How to Prevent Fatigue Failure<\/strong><\/h2>\n\n\n\n<p>Effective prevention requires both:<br>\u2022 Design optimization<br>\u2022 Manufacturing improvements<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Design Strategies<\/strong><\/h3>\n\n\n\n<p>Design Strategies<br>\u2022 Add fillets (R) to reduce stress concentration<br>\u2022 Avoid abrupt geometry changes<br>\u2022 Optimize load distribution<br>\u2022 Use damage-tolerant design when necessary<\/p>\n\n\n\n<p><strong>\u3010Manufacturing and Surface Treatments\u3011<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong><strong>Method<\/strong><\/strong><\/td><td><strong><strong>Description<\/strong><\/strong><\/td><td><strong><strong>Effect<\/strong><\/strong><\/td><\/tr><tr><td><strong>Shot Peening<\/strong><\/td><td>Bombarding surface with steel shots<\/td><td>Introduces compressive stress<\/td><\/tr><tr><td><strong>Surface Hardening<\/strong><\/td><td>Carburizing, nitriding\u3002<\/td><td>Improves crack resistance<\/td><\/tr><tr><td><strong>Surface Finishing<\/strong><\/td><td>Polishing<\/td><td>Reduces stress concentration<\/td><\/tr><tr><td><strong>CAE Analysis<\/strong><\/td><td>Simulation (FEM)<\/td><td>Identifies weak points<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>Important Insight<br>Shot peening is particularly effective because:<br>\u2022 It counteracts tensile stress<br>\u2022 Suppresses crack initiation and growth<\/p>\n\n\n\n<p>Conclusion<br>Fatigue failure is a complex phenomenon involving:<br>\u2022 Material properties<br>\u2022 Geometry<br>\u2022 Manufacturing processes<br>To ensure product reliability:<br>\u2022 Do not rely only on tensile strength<br>\u2022 Consider fatigue behavior comprehensively<\/p>\n\n\n\n<p>Key Takeaways<br>\u2022 Fatigue failure occurs under repeated stress below strength limits<br>\u2022 Crack initiation and propagation are gradual but lead to sudden failure<br>\u2022 S-N curves are essential for life prediction<br>\u2022 Multiple factors must be considered in design<br>\u2022 Preventive strategies must combine design and processing<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Nichidai\u2019s Engineering Support<\/strong><\/h2>\n\n\n\n<p>Nichidai offers:<br>\u2022 Advanced precision forging technologies<br>\u2022 High-quality tooling solutions<br>\u2022 Design support to reduce fatigue failure risks<br>From design to manufacturing, integrated solutions are provided to improve reliability.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>FAQ<\/strong><\/h2>\n\n\n\n<p>\u75b2\u52b4\u7834\u58ca\u306e\u8a2d\u8a08\u3084\u4e0d\u5177\u5408\u8abf\u67fb\u306b\u304a\u3044\u3066\u3001\u73fe\u5834\u306e\u6280\u8853\u8005\u304b\u3089\u3088\u304f\u5bc4\u305b\u3089\u308c\u308b\u8cea\u554f\u3092\u307e\u3068\u3081\u307e\u3057\u305f\u3002\u5b9f\u52d9\u3067\u306e\u5224\u65ad\u3084\u5bfe\u7b56\u306e\u691c\u8a0e\u306b\u5f79\u7acb\u3066\u3066\u304f\u3060\u3055\u3044\u3002<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-darkazure-color\">How can fatigue failure be detected early?<\/mark><\/strong><\/h3>\n\n\n\n<p>Use non-destructive testing (NDT):<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Magnetic particle testing (MT)<\/strong><\/li>\n\n\n\n<li><strong>Dye penetrant testing (PT)<\/strong><\/li>\n\n\n\n<li><strong>Ultrasonic testing (UT)<\/strong><\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\"><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-darkazure-color\">Difference between high-cycle and low-cycle fatigue?<\/mark><\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong><strong>Item<\/strong><\/strong><\/td><td><strong><strong>HCF<\/strong><\/strong><\/td><td><strong>LCF<\/strong><\/td><\/tr><tr><td><strong>Cycles<\/strong><\/td><td> 10\u2074\uff5e10\u2075 <\/td><td>Up to thousands<\/td><\/tr><tr><td><strong>Stress<\/strong><\/td><td>Elastic range<\/td><td>Plastic range<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h3 class=\"wp-block-heading\"><strong><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-darkazure-color\">Does higher hardness improve fatigue strength?<\/mark><\/strong><\/h3>\n\n\n\n<p>Generally yes, but:<br>\u2022 Too much hardness increases sensitivity to defects<br>\u2022 Balance between strength and toughness is important.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-darkazure-color\">Why are bolts prone to fatigue failure?<\/mark><\/strong><\/h3>\n\n\n\n<p>\u2022 High stress concentration at thread roots<br>\u2022 Improper tightening causes additional stress<\/p>\n","protected":false},"author":1,"template":"","column_cat":[9],"class_list":["post-446","column","type-column","status-publish","hentry","column_cat-technology-development"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/column\/446","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/column"}],"about":[{"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/types\/column"}],"author":[{"embeddable":true,"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":4,"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/column\/446\/revisions"}],"predecessor-version":[{"id":468,"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/column\/446\/revisions\/468"}],"wp:attachment":[{"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/media?parent=446"}],"wp:term":[{"taxonomy":"column_cat","embeddable":true,"href":"https:\/\/www.nichidai.jp\/english\/wp-json\/wp\/v2\/column_cat?post=446"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}