Fracture mechanics: an introduction
When a rigid line inclusion is considered, a similar asymptotic expression for the stress fields is obtained. Irwin was the first to observe that if the size of the plastic zone around a crack is small compared to the size of the crack, the energy required to grow the crack will not be critically dependent on the state of stress the plastic zone at the crack tip. The energy release rate for crack growth or strain energy release rate may then be calculated as the change in elastic strain energy per unit area of crack growth, i.
Either the load P or the displacement u are constant while evaluating the above expressions. Irwin showed that for a mode I crack opening mode the strain energy release rate and the stress intensity factor are related by:. Irwin also showed that the strain energy release rate of a planar crack in a linear elastic body can be expressed in terms of the mode I, mode II sliding mode , and mode III tearing mode stress intensity factors for the most general loading conditions. Next, Irwin adopted the additional assumption that the size and shape of the energy dissipation zone remains approximately constant during brittle fracture.
This assumption suggests that the energy needed to create a unit fracture surface is a constant that depends only on the material. This new material property was given the name fracture toughness and designated G Ic. Today, it is the critical stress intensity factor K Ic , found in the plane strain condition, which is accepted as the defining property in linear elastic fracture mechanics.
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In theory the stress at the crack tip where the radius is nearly zero, would tend to infinity. This would be considered a stress singularity, which is not possible in real-world applications. For this reason, in numerical studies in the field of fracture mechanics, it is often appropriate to represent cracks as round tipped notches , with a geometry dependant region of stress concentration replacing the crack-tip singularity. An equation giving the stresses near a crack tip is given below: . Nevertheless, there must be some sort of mechanism or property of the material that prevents such a crack from propagating spontaneously.
The assumption is, the plastic deformation at the crack tip effectively blunts the crack tip. This deformation depends primarily on the applied stress in the applicable direction in most cases, this is the y-direction of a regular Cartesian coordinate system , the crack length, and the geometry of the specimen. From this relationship, and assuming that the crack is loaded to the critical stress intensity factor, Irwin developed the following expression for the idealized radius of the zone of plastic deformation at the crack tip:.
Models of ideal materials have shown that this zone of plasticity is centered at the crack tip. The ratio of these two parameters is important to the radius of the plastic zone. This implies that the material can plastically deform, and, therefore, is tough. The same process as described above for a single event loading also applies and to cyclic loading.
If a crack is present in a specimen that undergoes cyclic loading, the specimen will plastically deform at the crack tip and delay the crack growth. In the event of an overload or excursion, this model changes slightly to accommodate the sudden increase in stress from that which the material previously experienced. At a sufficiently high load overload , the crack grows out of the plastic zone that contained it and leaves behind the pocket of the original plastic deformation. Now, assuming that the overload stress is not sufficiently high as to completely fracture the specimen, the crack will undergo further plastic deformation around the new crack tip, enlarging the zone of residual plastic stresses.
This process further toughens and prolongs the life of the material because the new plastic zone is larger than what it would be under the usual stress conditions. This allows the material to undergo more cycles of loading. This idea can be illustrated further by the graph of Aluminum with a center crack undergoing overloading events. But a problem arose for the NRL researchers because naval materials, e. One basic assumption in Irwin's linear elastic fracture mechanics is small scale yielding, the condition that the size of the plastic zone is small compared to the crack length.
However, this assumption is quite restrictive for certain types of failure in structural steels though such steels can be prone to brittle fracture, which has led to a number of catastrophic failures. Linear-elastic fracture mechanics is of limited practical use for structural steels and Fracture toughness testing can be expensive. In general, the initiation and continuation of crack growth is dependent on several factors, such as bulk material properties, body geometry, crack geometry, loading distribution, loading rate, load magnitude, environmental conditions, time effects such as viscoelasticity or viscoplasticity , and microstructure.
In addition, as cracks grow in a body of material, the material's resistance to fracture increases or remains constant.
BASIC fracture mechanics: including an introduction to fatigue - R. N. L. Smith - Google книги
In the prior section, only straight-ahead crack growth from the application of load resulting in a single mode of fracture was considered. In mixed-mode loading, cracks will generally not advance straight ahead. Maximum hoop stress theory predicts the angle of crack extension in experimental results quite accurately and provides a lower bound to the envelope of failure.
Other factors can also influence the direction of crack growth, such as far-field material deformation e. In anisotropic materials, the fracture toughness changes as orientation within the material changes. The above can be considered as a statement of the maximum energy release rate criterion for anisotropic materials.
This is often called the criterion of local symmetry. Consider a semi-infinite crack in an asymmetric state of loading. This is considered as directionally unstable kinked crack growth. This is considered as neutrally stable kinked crack growth. This is considered as directionally stable kinked crack growth. Most engineering materials show some nonlinear elastic and inelastic behavior under operating conditions that involve large loads. Therefore, a more general theory of crack growth is needed for elastic-plastic materials that can account for:. Historically, the first parameter for the determination of fracture toughness in the elasto-plastic region was the crack tip opening displacement CTOD or "opening at the apex of the crack" indicated.
This parameter was determined by Wells during the studies of structural steels, which due to the high toughness could not be characterized with the linear elastic fracture mechanics model. He noted that, before the fracture happened, the walls of the crack were leaving and that the crack tip, after fracture, ranged from acute to rounded off due to plastic deformation. In addition, the rounding of the crack tip was more pronounced in steels with superior toughness.
There are a number of alternative definitions of CTOD. The two most common definitions, CTOD is the displacement at the original crack tip and the 90 degree intercept. The latter definition was suggested by Rice and is commonly used to infer CTOD in finite element models of such. Note that these two definitions are equivalent if the crack tip blunts in a semicircle.
Most laboratory measurements of CTOD have been made on edge-cracked specimens loaded in three-point bending. Early experiments used a flat paddle-shaped gage that was inserted into the crack; as the crack opened, the paddle gage rotated, and an electronic signal was sent to an x-y plotter. This method was inaccurate, however, because it was difficult to reach the crack tip with the paddle gage.
Fracture Mechanics: An Introduction (Solid Mechanics and Its Applications)
Today, the displacement V at the crack mouth is measured, and the CTOD is inferred by assuming the specimen halves are rigid and rotate about a hinge point the crack tip. An early attempt in the direction of elastic-plastic fracture mechanics was Irwin's crack extension resistance curve , Crack growth resistance curve or R-curve. This curve acknowledges the fact that the resistance to fracture increases with growing crack size in elastic-plastic materials. The R-curve is a plot of the total energy dissipation rate as a function of the crack size and can be used to examine the processes of slow stable crack growth and unstable fracture.
However, the R-curve was not widely used in applications until the early s. The main reasons appear to be that the R-curve depends on the geometry of the specimen and the crack driving force may be difficult to calculate. In the mids James R.
Introduction to Fracture Mechanics (2001)
Rice then at Brown University and G. Cherepanov independently developed a new toughness measure to describe the case where there is sufficient crack-tip deformation that the part no longer obeys the linear-elastic approximation. Rice's analysis, which assumes non-linear elastic or monotonic deformation theory plastic deformation ahead of the crack tip, is designated the J-integral. It also demands that the assumed non-linear elastic behavior of the material is a reasonable approximation in shape and magnitude to the real material's load response.
The elastic-plastic failure parameter is designated J Ic and is conventionally converted to K Ic using Equation 3. Also note that the J integral approach reduces to the Griffith theory for linear-elastic behavior. When a significant region around a crack tip has undergone plastic deformation, other approaches can be used to determine the possibility of further crack extension and the direction of crack growth and branching.
A simple technique that is easily incorporated into numerical calculations is the cohesive zone model method which is based on concepts proposed independently by Barenblatt  and Dugdale  in the early s. The relationship between the Dugdale-Barenblatt models and Griffith's theory was first discussed by Willis in A failure locus is defined for the material using basic mechanical properties. A factor of safety can be calculated by determining ratios of the applied stress to the yield strength and applied stress intensity to the fracture toughness, and then comparing these ratios to the failure locus.
Under small-scale yielding conditions, a single parameter e. Single-parameter fracture mechanics breaks down in the presence of excessive plasticity, and when the fracture toughness depends on the size and geometry of the test specimen. The theories used for large scale yielding is not very standardized. The following theories and approaches are commonly used among researchers in this field.
By using FEM, one can establish a parameter Q to modify the stress field for a better solution when the plastic zone is growing. A negative value greatly changes the geometry of the plastic zone. The J-Q-M theory includes another parameter, the mismatch parameter, which is used for welds to make up for the change in toughness of the weld metal WM , base metal BM and heat affected zone HAZ. This value is interpreted to the formula in a similar way as the Q-parameter, and the two are usually assumed to be independent of each other.
As an alternative to J-Q theory, a parameter T can be used. This only changes the normal stress in the x-direction and the z-direction in the case of plane strain. T does not require the use of FEM but is derived from constraint. It can be argued that T is limited to LEFM, but, as the plastic zone change due to T never reaches the actual crack surface except on the tip , its validity holds true not only under small-scale yielding.
Occasionally post-mortem fracture-mechanics analyses are carried out. In the absence of an extreme overload, the causes are either insufficient toughness K Ic or an excessively large crack that was not detected during routine inspection. Member Benefits. Committee Involvement. Volunteer Profiles. Volunteer Interest Form. Why and How to Volunteer. Volunteer Opportunities. Volunteer Awards. Volunteer Resources. Find Local Chapters.
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