fracture mechanics presentation
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FRACTURE MECHANICSPresented by
Muhammad Shahid BS (hons) 4th semester
Institute OF chemistry,University of the Punjab
Overview
Brittle & Ductile Fracture
Modes of Failure
Energy Release Rate & Crack Resistance
Crack Growth
Stress Intensity Factor
parameters
Applications of Fractures Mechanics
Fracture means the cracking or breaking of a hard object or material.
What is fracture?
Breakingof
Materials
High sulphur in material
Cold waters
Welding instead of riveting
Continuity of the structure
Applied stress
Microcracks
Brittle deformation: it occurs (in rocks) when a rock breaks or fractures due to stress. This happens in high pressure but low temperature environments
Elastic deformation: An object can bend or flex temporarily and will “ spring back" to its original shape, but at some point it will also fail.
Ductile deformation: Any strain is irreversible and the rock deforms readily (Think taffy puller). Often in a single fault all three forms of deformation take place in that order as depth increases.
Brittle & Ductile Fracture
• Ductile materials - extensive plastic deformation and energy absorption (“toughness”) before fracture
• Brittle materials - little plastic deformation and low energy absorption before fracture
What is Fracture Mechanics?
Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.
Fracture mechanics is based on the implicit assumption that there exists a crack in a work component.
It is the study of flaws and cracks in materials.
Mostly deals with crack growth.
Importance of Fracture mechanics
In modern materials science, fracture mechanics is an important tool in improving the mechanical performance of mechanical components. It applies the physics of stress and strain, in particular the theories of elasticity and plasticity, to the microscopic crystallographic defect found in real materials.
Fractography is widely used with fracture mechanics to understand the causes of failures and also verify the theoretical failure predictions with real life failures.
CRACK INITIATION
Physically, cracks initiate from; An imperfection
An already existing crack
A damaged (locally weakened) area
A failure analysis must include; Stress analysis
Failure criterion
Three Modes of Fracture
Mode I denotes a symmetric opening (opening or tension mode) Mode II denotes an antisymmetric separation (In-plane shear
mode) Mode III denotes an antisymmetric separation (out-of-plane shear
or tearing mode) Crack growth usually takes place in mode I or close to it. The crack “adjusts” itself such that the load is perpendicular to the
crack faces.
Parameters of Fracture Mechanics
J-Integral
Energy-Release Rate
Stress-Intensity Factor
T-Stress
Material Force
C*-Integral
Energy Release Rate and J-Integral
The strain energy release rate (or simply energy release rate) is the energy consumed during fracture per unit of newly created fracture surface area. This quantity is central to fracture mechanics because the energy that must be supplied to a crack tip for it to grow must be balanced by the amount of energy dissipated due to the formation of new surfaces and other dissipative processes such as plasticity.
Strain energy release rate
where is the potential energy available for crack growth, is the work associated with any external forces acting, and is the crack area (crack length for two-dimensional problems). The units of are J/m2. where is the potential energy available for crack growth,
Where U is the potential energy available for crack growth, and V is the work associated with any external forces acting and A is the crack area. The units of G are J/m2.
The energy release rate failure criterion states that a crack will grow when the available energy release rate G is greater than or equal to a critical value Gc.
G >=Gc
The quantity Gc is the fracture energy and is considered to be a material property which is independent of the applied loads and the geometry of the body
Stress Intensity Factor
The stress intensity factor K is used in fracture mechanics to predict the stress state ("stress intensity") near the tip of a crack caused by a remote load or applied stresses.
Stress Intensity Factor is a quantity determined analytically and varies as a function of the crack configuration and the external loads are applied
Critical stress intensity factor is independent of the crack geometry and loading and may be regarded as a material constant.
T-Stress
T- Stress is the stress acting parallel to the crack faces
Unlike J-integral, it can have both negative and positive values
Positive T-stress values promotes fracture, where negative T-stress values results in larger plastic zones
The J-integral represents a way to calculate the strain energy release rate or work (energy) per unit fracture surface area, in a material.
Crack growth becomes a concern when structural components are operated at high temperatures i.e Nuclear Industry
Applications of Fracture Mechanics
Fracture mechanics can be used in different major areas:
(i) designing
(ii) material selection and alloy development
(iii) determining the significance of defects.
(iv) monitoring , control, and failure analysis.
Cont.…
Many large complex structures such as bridges, ships, buildings, aircraft and pressure vessels can have crack like imperfections, sharp notches and discontinuities of various kinds. Using fracture mechanics an engineer can quantitatively establish allowable stress levels and inspection requirements to design against the occurrence of fractures in such structures.in addition fracture mechanics is also used to analyze the growth of small cracks to critical size by loading or by stress cracking. therefore fracture mechanics techniques have several advantages and offer the designer a method of quantitative design to prevent fracture in structures.
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