Propagación subcrítica de fisuras asistidas por fatiga en aceros estructurales ASTM A36
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Date
2024
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Universidad Nacional de Trujillo
Abstract
La falla catastrófica de estructuras es causada por la propagación de fisuras más allá de un tamaño crítico. Estas fisuras están presentes de alguna forma, en todo componente estructural, y son el resultado de defectos de fabricación o de daños localizados, generados durante su servicio, y estos daños pueden crecer lentamente debido a procesos de fatiga, provocando una disminución de su resistencia estructural y generando hacia futuro una falla catastrófica. Uno de los objetivos en el análisis y evaluación de fallas, es la determinación de la tolerancia al daño, (propagación subcrítica), siendo esta una propiedad estructural que determina la capacidad para sobrellevar de manera segura una fisura durante la vida útil de una estructura. Adicionalmente, la determinación de la velocidad de crecimiento y el tamaño crítico de la fisura, son parámetros que permiten determinar la estabilidad de la fisura y vida residual de un componente estructural.
En el presente trabajo, se estudió la propagación subcrítica de fisuras asistidas por fatiga. El material de estudio es el acero estructural ASTM A36, el cual se sometió a ensayo de fatiga de flexión en tres puntos al aire utilizando una máquina de fatiga. Se ensayaron tres probetas cuyas dimensiones, según la norma ASTM 399, son: 1.26 cm de espesor, 5.05 cm de ancho, 20 cm de largo; con una ranura en el centro, hecha con una fresa pequeña, cuyas dimensiones de la ranura son de 0.3 cm de profundidad por 1.26 cm de largo.
Se obtuvieron datos, de profundidad de fisura “𝑎” y número de ciclos “𝑁”, construyéndose la gráfica 𝑎 𝑣𝑠 𝑁, a partir de la cual se construyó la gráfica 𝑑𝑎𝑑𝑁 𝑣𝑠 𝑎, que representa la velocidad de avance de fisura en función de su tamaño. Utilizando la ecuación de Newman-Raju, se determinó el factor de intensidad de tensiones 𝐾, luego el Δ𝐾, lo que permitió graficar 𝑑𝑎𝑑𝑁 𝑣𝑠 Δ𝐾; de esta curva se tomó la zona de propagación subcrítica de fisuras, en la cual se aplica la Ley de Paris 𝑑𝑎𝑑𝑁=𝐶(Δ𝐾)𝑚; del análisis de la gráfica se determinó experimentalmente los valores de 𝐶 𝑦 𝑚 de la Ley de Paris para el acero ASTM A36, realizado al aire, obteniéndose
𝑚=0.697±0.257 𝑦 𝐶=(4.010±0.358)×10−6 , donde 𝐶 da información del medio en que se realizó los ensayos de fatiga, y 𝑚 nos da información del comportamiento del material. Lo importante de estos valores es que servirán para, monitoreo de la propagación subcrítica de fisuras cuando el material ensayado esté en servicio.
Catastrophic failure of structures is caused by crack propagation beyond a safe size. These cracks, present in some form in every structural component, are the result of manufacturing defects or localized damage generated during service and can grow slowly due to fatigue processes, causing a decrease in its structural resistance and generating failure in the future catastrophic. One of the objectives in the analysis and evaluation of failures is the determination of damage tolerance (subcritical propagation), this being a structural property that determines the ability to safely harden a crack during the useful life of a structure. Additionally, the determination of the growth rate and the critical size are parameters that allow determining the stability of the crack and residual life of a structural component. In the present work, the subcritical propagation of fatigue-assisted cracks was studied. The study material is ASTM A36 structural steel, which was subjected to a three-point bending fatigue test in air using a “soft” fatigue machine. Three specimens were tested whose dimensions, according to ASTM 399, are: 1.26 cm thick, 5.05 cm wide, 20 cm long; with a slot in the center, made with a small bur, whose dimensions are 0.3 cm deep by 1.26 cm long. Data on crack depth “𝑎” and number of cycles “𝑁” were obtained, constructing the graph 𝑎 𝑣𝑠 𝑁 from which the graph 𝑑𝑎𝑑𝑁 𝑣𝑠 𝑎 was constructed, which represents the speed of crack advancement in function of the crack size, Using the Newman-Raju equation, the stress intensity factor K was determined, then the ΔK, which allowed 𝑑𝑎𝑑𝑁 𝑣𝑠 Δ𝐾 to be graphed; From this curve, the subcritical crack propagation zone was taken, in which the Paris Law 𝑑𝑎𝑑𝑁=𝐶(Δ𝐾)𝑚 is applied; From the analysis of the graph, the values of 𝐶 and 𝑚 of the Paris Law for ASTM A36 steel were experimentally determined, carried out in air, obtaining 𝑚=0.697±0.257 and 𝐶=(4.010±0.358)×10−6 , where C gives information about the medium in which the fatigue tests were carried out, and m gives us information about the behavior of the material. The important thing about these values is that they will serve to monitor the subcritical propagation of cracks when the tested material is in service.
Catastrophic failure of structures is caused by crack propagation beyond a safe size. These cracks, present in some form in every structural component, are the result of manufacturing defects or localized damage generated during service and can grow slowly due to fatigue processes, causing a decrease in its structural resistance and generating failure in the future catastrophic. One of the objectives in the analysis and evaluation of failures is the determination of damage tolerance (subcritical propagation), this being a structural property that determines the ability to safely harden a crack during the useful life of a structure. Additionally, the determination of the growth rate and the critical size are parameters that allow determining the stability of the crack and residual life of a structural component. In the present work, the subcritical propagation of fatigue-assisted cracks was studied. The study material is ASTM A36 structural steel, which was subjected to a three-point bending fatigue test in air using a “soft” fatigue machine. Three specimens were tested whose dimensions, according to ASTM 399, are: 1.26 cm thick, 5.05 cm wide, 20 cm long; with a slot in the center, made with a small bur, whose dimensions are 0.3 cm deep by 1.26 cm long. Data on crack depth “𝑎” and number of cycles “𝑁” were obtained, constructing the graph 𝑎 𝑣𝑠 𝑁 from which the graph 𝑑𝑎𝑑𝑁 𝑣𝑠 𝑎 was constructed, which represents the speed of crack advancement in function of the crack size, Using the Newman-Raju equation, the stress intensity factor K was determined, then the ΔK, which allowed 𝑑𝑎𝑑𝑁 𝑣𝑠 Δ𝐾 to be graphed; From this curve, the subcritical crack propagation zone was taken, in which the Paris Law 𝑑𝑎𝑑𝑁=𝐶(Δ𝐾)𝑚 is applied; From the analysis of the graph, the values of 𝐶 and 𝑚 of the Paris Law for ASTM A36 steel were experimentally determined, carried out in air, obtaining 𝑚=0.697±0.257 and 𝐶=(4.010±0.358)×10−6 , where C gives information about the medium in which the fatigue tests were carried out, and m gives us information about the behavior of the material. The important thing about these values is that they will serve to monitor the subcritical propagation of cracks when the tested material is in service.
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