{"id":21250,"date":"2022-07-23T16:07:37","date_gmt":"2022-07-23T08:07:37","guid":{"rendered":"https:\/\/www.meetyoucarbide.com\/?p=21250"},"modified":"2022-07-27T11:24:16","modified_gmt":"2022-07-27T03:24:16","slug":"august-wohlers-experiment-statics-showing-you-how-the-4-elements-impact-on-fatigue-crack","status":"publish","type":"post","link":"https:\/\/www.meetyoucarbide.com\/fr\/august-wohlers-experiment-statics-show-how-the-4-elements-impact-of-fatigue-crack\/","title":{"rendered":"Statique de l'exp\u00e9rience d'August W\u00f6hler vous montrant comment les 4 \u00e9l\u00e9ments ont un impact sur la fissure de fatigue"},"content":{"rendered":"

Fatigue cracks are generally the result of periodic plastic deformation in local areas. Fatigue is defined as “failure under repeated load or other types of load conditions, and this load level is not sufficient to cause failure when applied only once.” This plastic deformation occurs not because of the theoretical stress on the ideal component, but because the component surface can not be actually detected.<\/a><\/a><\/a><\/a><\/a><\/a><\/p>

August W\u00f6hler est le pionnier de la recherche sur la fatigue et propose une m\u00e9thode empirique. Entre 1852 et 1870, w\u00f6hler \u00e9tudie la rupture progressive des essieux ferroviaires. Il a construit le banc d'essai illustr\u00e9 \u00e0 la figure 1. Ce banc d'essai permet \u00e0 deux essieux ferroviaires d'\u00eatre tourn\u00e9s et pli\u00e9s en m\u00eame temps. W\u00f6hler a trac\u00e9 la relation entre la contrainte nominale et le nombre de cycles conduisant \u00e0 la rupture, qui sera plus tard connue sous le nom de diagramme SN. Chaque courbe est encore appel\u00e9e ligne aw \u00f6 hler. La m\u00e9thode Sn est encore aujourd'hui la m\u00e9thode la plus utilis\u00e9e. Un exemple typique de cette courbe est illustr\u00e9 \u00e0 la figure 1.<\/p>

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Figure 1 essai de fatigue en flexion par rotation de W \u00f6 hler<\/figcaption><\/figure>

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Several effects can be observed through the w \u00f6 hler line. First, we note that the SN curve below the transition point (about 1000 cycles) is invalid because the nominal stress here is elastoplastic. We will show later that fatigue is caused by the release of plastic shear strain energy. Therefore, there is no linear relationship between stress and strain before fracture, and it cannot be used. Between the transition point and the fatigue limit (about 107 cycles), the Sn based analysis is valid. Above the fatigue limit, the slope of the curve decreases sharply, so this region is often referred to as the “infinite life” region. But this is not the case. For example, aluminum alloy will not have infinite life, and even steel will not have infinite life under variable amplitude load.<\/a><\/p>

Avec l'\u00e9mergence de la technologie d'amplification moderne, les gens peuvent \u00e9tudier les fissures de fatigue plus en d\u00e9tail. Nous savons maintenant que l'\u00e9mergence et la propagation des fissures de fatigue peuvent \u00eatre divis\u00e9es en deux \u00e9tapes. Au stade initial, la fissure se propage \u00e0 un angle d'environ 45 degr\u00e9s par rapport \u00e0 la charge appliqu\u00e9e (le long de la ligne de contrainte de cisaillement maximale). Apr\u00e8s avoir travers\u00e9 deux ou trois joints de grains, sa direction change et s'\u00e9tend le long de la direction d'environ 90 degr\u00e9s par rapport \u00e0 la charge appliqu\u00e9e. Ces deux \u00e9tapes sont appel\u00e9es fissure de stade I et fissure de stade II, comme le montre la figure 2.<\/a><\/a><\/a><\/a><\/p>

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Figure 2 Diagramme sch\u00e9matique de la croissance des fissures au stade I et au stade II<\/figcaption><\/figure>

If we observe a stage I crack at high magnification, we can see that the alternating stress will lead to the formation of a continuous slip band along the maximum shear plane. These slip bands slide back and forth, much like a deck of cards, resulting in uneven surfaces. The concave surface finally forms a “budding” crack, as shown in Figure 3. In phase I, the crack will expand in this mode until it meets the grain boundary and will stop temporarily. When enough energy is applied to the adjacent crystals, then the process will continue.<\/p>

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Figure 3 Sch\u00e9ma de principe d'une bande de glissement continue<\/figcaption><\/figure>

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Apr\u00e8s avoir travers\u00e9 deux ou trois joints de grains, la direction de propagation de la fissure passe maintenant en mode phase II. A ce stade, les propri\u00e9t\u00e9s physiques de propagation des fissures ont chang\u00e9. La fissure elle-m\u00eame constitue un macro-obstacle au flux de contraintes, provoquant une forte concentration de contraintes plastiques en pointe de fissure. Comme le montre la figure 4. Il convient de noter que toutes les fissures de stade I ne se d\u00e9velopperont pas au stade II.<\/a><\/p>

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Fig4<\/figcaption><\/figure>

In order to understand the propagation mechanism of stage II, we need to consider the situation of crack tip cross-section during the stress cycle. As shown in Figure 5. The fatigue cycle begins when the nominal stress is at point “a”. As the stress intensity increases and passes through point “B”, we notice that the crack tip opens, resulting in local plastic shear deformation, and the crack extends to point “C” in the original metal. When the tensile stress decreases through the “d” point, we observe that the crack tip closes, but the permanent plastic deformation leaves a unique serration, the so-called “cut line”. When the whole cycle ends at the “e” point, we observe that the crack has now increased the “Da” length and formed additional section lines. It is now understood that the range of crack growth is proportional to the range of applied elastic-plastic crack tip strain. A larger cycle range can form a larger Da.<\/a><\/p>

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Fig. 5 Repr\u00e9sentation sch\u00e9matique de la propagation des fissures au stade II<\/figcaption><\/figure>

<\/p>

Facteurs affectant le taux de croissance des fissures de fatigue<\/h2>

L'influence des param\u00e8tres suivants sur le taux de croissance des fissures de fatigue est \u00e9tudi\u00e9e et expliqu\u00e9e conceptuellement\u00a0:<\/p>

1Contrainte de cisaillement<\/h3>

From the diagram, we can see that a certain “amount” of shear stress is released during the periodic change of the strength of the nominal stress. And the larger the range of stress changes, the greater the energy released. Through the SN curve shown in Figure 1, we can see that the fatigue life decreases exponentially with the increase of the stress cycle range.<\/a><\/p>

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Fig. 6 contrainte et d\u00e9formation \u00e9lastoplastique le long de la surface de glissement et \u00e0 la racine de la fissure<\/figcaption><\/figure>

<\/a><\/p>

2 stress moyen<\/h3>

La contrainte moyenne (contrainte r\u00e9siduelle) est \u00e9galement un facteur affectant le taux de rupture par fatigue. Conceptuellement, si la contrainte d'expansion est appliqu\u00e9e \u00e0 la fissure de phase II, la fissure sera forc\u00e9e de s'ouvrir, de sorte que tout cycle de contrainte aura un effet plus significatif. Au contraire, si la contrainte de compression moyenne est appliqu\u00e9e, la fissure sera forc\u00e9e de se fermer et tout cycle de contrainte doit surmonter la contrainte de pr\u00e9compression avant que la fissure puisse continuer \u00e0 se dilater. Des concepts similaires s'appliquent \u00e9galement aux fissures de stade I.<\/p>

3 finition de surface<\/h3>

\u00c9tant donn\u00e9 que les fissures de fatigue apparaissent g\u00e9n\u00e9ralement d'abord sur la surface des composants o\u00f9 il y a des d\u00e9fauts, la qualit\u00e9 de la surface affectera s\u00e9rieusement la probabilit\u00e9 d'apparition de fissures. Bien que la plupart des \u00e9chantillons de test de mat\u00e9riaux aient une finition miroir, ils atteindront \u00e9galement la meilleure dur\u00e9e de vie \u00e0 la fatigue. En fait, la plupart des composants ne peuvent pas \u00eatre compar\u00e9s aux \u00e9chantillons, nous devons donc modifier les propri\u00e9t\u00e9s de fatigue. L'\u00e9tat de surface a un effet plus important sur la fatigue des composants soumis \u00e0 des cycles de contraintes de faible amplitude.<\/p>

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Figure 7 Sch\u00e9ma de principe de l'influence de la s\u00e9quence de cycles L'influence de l'\u00e9tat de surface peut \u00eatre exprim\u00e9e par mod\u00e9lisation, c'est-\u00e0-dire en multipliant la courbe SN par le param\u00e8tre de correction de surface \u00e0 la limite de fatigue.<\/figcaption><\/figure>

4 traitement de surface<\/h3>

Le traitement de surface peut \u00eatre utilis\u00e9 pour am\u00e9liorer la r\u00e9sistance \u00e0 la fatigue des composants. Le but du traitement de surface est de former une contrainte de compression r\u00e9siduelle sur la surface. Sous la p\u00e9riode de faible amplitude, la contrainte sur la surface est \u00e9videmment faible, et maintient m\u00eame l'\u00e9tat de compression. Par cons\u00e9quent, la dur\u00e9e de vie en fatigue peut \u00eatre consid\u00e9rablement prolong\u00e9e. Cependant, comme nous l'avons soulign\u00e9, cette situation n'est valable que pour les composants soumis \u00e0 des cycles de contraintes de faible amplitude. Si une p\u00e9riode de forte amplitude est appliqu\u00e9e, la pr\u00e9compression sera surmont\u00e9e par la p\u00e9riode de forte amplitude, et ses avantages seront perdus. Comme pour la qualit\u00e9 de surface, l'impact du traitement de surface peut \u00eatre montr\u00e9 par mod\u00e9lisation.<\/p><\/div>","protected":false},"excerpt":{"rendered":"

Fatigue cracks are generally the result of periodic plastic deformation in local areas. Fatigue is defined as “failure under repeated load or other types of load conditions, and this load level is not sufficient to cause failure when applied only once.” This plastic deformation occurs not because of the theoretical stress on the ideal component, but…<\/p>","protected":false},"author":2,"featured_media":21253,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[79],"tags":[],"class_list":["post-21250","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-materials-weekly"],"jetpack_featured_media_url":"https:\/\/www.meetyoucarbide.com\/wp-content\/uploads\/2022\/07\/\u56fe\u72472.png","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/posts\/21250","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/comments?post=21250"}],"version-history":[{"count":0,"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/posts\/21250\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/media\/21253"}],"wp:attachment":[{"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/media?parent=21250"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/categories?post=21250"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/fr\/wp-json\/wp\/v2\/tags?post=21250"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}