{"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\/pt\/august-wohlers-experiment-statics-mostrando-voce-como-os-4-elementos-impacto-na-fadiga-rachadura\/","title":{"rendered":"Est\u00e1tica do experimento de August W\u00f6hler mostrando como os 4 elementos impactam na rachadura por fadiga"},"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 \u00e9 o pioneiro da pesquisa em fadiga e apresenta um m\u00e9todo emp\u00edrico. Entre 1852 e 1870, W\u00f6hler estudou a falha progressiva de eixos ferrovi\u00e1rios. Ele construiu o banco de ensaio mostrado na Figura 1. Este banco de ensaio permite que dois eixos ferrovi\u00e1rios sejam girados e dobrados ao mesmo tempo. W\u00f6hler tra\u00e7ou a rela\u00e7\u00e3o entre a tens\u00e3o nominal e o n\u00famero de ciclos que levam \u00e0 falha, que mais tarde \u00e9 conhecido como diagrama SN. Cada curva ainda \u00e9 chamada de linha aw \u00f6 hler. O m\u00e9todo Sn ainda \u00e9 o m\u00e9todo mais utilizado atualmente. Um exemplo t\u00edpico desta curva \u00e9 mostrado na Figura 1.<\/p>

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Figura 1 teste de fadiga de flex\u00e3o de rota\u00e7\u00e3o de W \u00f6 hler<\/figcaption><\/figure>

<\/a><\/p>

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>

Com o surgimento da moderna tecnologia de amplifica\u00e7\u00e3o, as pessoas podem estudar as rachaduras de fadiga com mais detalhes. Sabemos agora que o surgimento e propaga\u00e7\u00e3o de trincas de fadiga podem ser divididos em duas etapas. No est\u00e1gio inicial, a trinca se propaga em um \u00e2ngulo de cerca de 45 graus em rela\u00e7\u00e3o \u00e0 carga aplicada (ao longo da linha de tens\u00e3o de cisalhamento m\u00e1xima). Ap\u00f3s cruzar dois ou tr\u00eas contornos de gr\u00e3o, sua dire\u00e7\u00e3o muda e se estende ao longo da dire\u00e7\u00e3o de cerca de 90 graus em rela\u00e7\u00e3o \u00e0 carga aplicada. Esses dois est\u00e1gios s\u00e3o chamados de rachadura de est\u00e1gio I e rachadura de est\u00e1gio II, conforme mostrado na Figura 2.<\/a><\/a><\/a><\/a><\/p>

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Figura 2 Diagrama esquem\u00e1tico de crescimento de trincas no est\u00e1gio I e est\u00e1gio 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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Figura 3 Diagrama esquem\u00e1tico da faixa de deslizamento cont\u00ednua<\/figcaption><\/figure>

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Ap\u00f3s cruzar dois ou tr\u00eas contornos de gr\u00e3o, a dire\u00e7\u00e3o de propaga\u00e7\u00e3o da trinca agora entra no modo fase II. Neste est\u00e1gio, as propriedades f\u00edsicas de propaga\u00e7\u00e3o de trincas mudaram. A pr\u00f3pria trinca constitui um macro obst\u00e1culo ao fluxo de tens\u00f5es, causando alta concentra\u00e7\u00e3o de tens\u00f5es pl\u00e1sticas na ponta da trinca. Conforme mostrado na Figura 4. Deve-se notar que nem todas as trincas do est\u00e1gio I ir\u00e3o evoluir para o est\u00e1gio 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 Diagrama esquem\u00e1tico de propaga\u00e7\u00e3o de trincas no est\u00e1gio II<\/figcaption><\/figure>

<\/p>

Fatores que afetam a taxa de crescimento de trincas por fadiga<\/h2>

A influ\u00eancia dos seguintes par\u00e2metros na taxa de crescimento de trincas por fadiga \u00e9 estudada e explicada conceitualmente:<\/p>

1 Tens\u00e3o de cisalhamento<\/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 tens\u00e3o e deforma\u00e7\u00e3o elastopl\u00e1stica ao longo da superf\u00edcie de deslizamento e na raiz da trinca<\/figcaption><\/figure>

<\/a><\/p>

2 estresse m\u00e9dio<\/h3>

A tens\u00e3o m\u00e9dia (tens\u00e3o residual) tamb\u00e9m \u00e9 um fator que afeta a taxa de falha por fadiga. Conceitualmente, se a tens\u00e3o de expans\u00e3o for aplicada \u00e0 trinca fase II, a trinca ser\u00e1 for\u00e7ada a abrir, ent\u00e3o qualquer ciclo de tens\u00e3o ter\u00e1 um efeito mais significativo. Pelo contr\u00e1rio, se a tens\u00e3o de compress\u00e3o m\u00e9dia for aplicada, a trinca ser\u00e1 for\u00e7ada a fechar, e qualquer ciclo de tens\u00e3o precisa superar a tens\u00e3o de pr\u00e9-compress\u00e3o antes que a trinca possa continuar a se expandir. Conceitos semelhantes tamb\u00e9m se aplicam \u00e0s trincas do est\u00e1gio I.<\/p>

3 acabamentos de superf\u00edcie<\/h3>

Como as trincas de fadiga geralmente aparecem primeiro na superf\u00edcie dos componentes onde h\u00e1 defeitos, a qualidade da superf\u00edcie afetar\u00e1 seriamente a probabilidade de ocorr\u00eancia de trincas. Embora a maioria das amostras de teste de material tenha acabamento espelhado, elas tamb\u00e9m alcan\u00e7ar\u00e3o a melhor vida \u00e0 fadiga. De fato, a maioria dos componentes n\u00e3o pode ser comparada com as amostras, ent\u00e3o precisamos modificar as propriedades de fadiga. O acabamento superficial tem um efeito maior na fadiga de componentes submetidos a ciclos de tens\u00e3o de baixa amplitude.<\/p>

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Figura 7 Diagrama esquem\u00e1tico da influ\u00eancia da sequ\u00eancia do ciclo A influ\u00eancia do acabamento superficial pode ser expressa por modelagem, ou seja, multiplicando a curva SN pelo par\u00e2metro de corre\u00e7\u00e3o da superf\u00edcie no limite de fadiga.<\/figcaption><\/figure>

4 tratamento de superf\u00edcie<\/h3>

O tratamento de superf\u00edcie pode ser usado para aumentar a resist\u00eancia \u00e0 fadiga dos componentes. O objetivo do tratamento de superf\u00edcie \u00e9 formar tens\u00f5es de compress\u00e3o residuais na superf\u00edcie. Sob o per\u00edodo de baixa amplitude, a tens\u00e3o na superf\u00edcie \u00e9 obviamente baixa e at\u00e9 mant\u00e9m o estado de compress\u00e3o. Portanto, a vida de fadiga pode ser significativamente prolongada. No entanto, como apontamos, esta situa\u00e7\u00e3o s\u00f3 \u00e9 v\u00e1lida para componentes sujeitos a ciclos de tens\u00e3o de baixa amplitude. Se for aplicado um per\u00edodo de alta amplitude, a pr\u00e9-compress\u00e3o ser\u00e1 superada pelo per\u00edodo de alta amplitude e suas vantagens ser\u00e3o perdidas. Tal como acontece com a qualidade da superf\u00edcie, o impacto do tratamento de superf\u00edcie pode ser demonstrado por modelagem.<\/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\/pt\/wp-json\/wp\/v2\/posts\/21250","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/comments?post=21250"}],"version-history":[{"count":0,"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/posts\/21250\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/media\/21253"}],"wp:attachment":[{"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/media?parent=21250"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/categories?post=21250"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/pt\/wp-json\/wp\/v2\/tags?post=21250"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}