Essay: Fracture in materials science and engineering

Fatigue is the weakening or exhausted of material caused by frequently applied loads. Fatigue is a major concern to ensure the longevity of product to avoid under stress , as well as trying no to overburden the metal during production. It is the structural damage that happened when a material is undergo the cyclic loading.
Fatigue usually occurs when a material is undergo to repeated loading and unloading. The surface of metal undergo deformation which bring that fatigue begins to occurred.
Microscopic cracks will begin to form at the stress concentrators such as the grain interfaces, surface, and persistent slip bands if the load or above threshold. Fatigue life is greatly affected by the shape of the structure which are sharp corners or sqaure holes will bring to upraised stresses where fatigue cracks can instigate. Fillets or smooth transistions as well as round holes can be said that will increase the fatigue strength of the structure. The metal actually will easily tear apart as these cracks develop. A crack may in time will bring catastrophic failur by a fracture mechanism if the crack remain undetected. When a metal experiences or undergo stresses the over or exceeds its yield strength eventually fracture would occurred.
One of the most important and major concepts in the entire field of Materials Science and Engineering is fracture. In its simplest form, fracture can be described as a one body being separated into pieces by forced stress. For engineering materials there are only two possible characteristic of fractures which are brittle fractures and ductile fractures. In general, the main difference between brittle fracture and ductile fracture can be hold responsible to the amount of plasic deformation that the will material undergoes before fracture occurs.
Brittle fracture is without apparent of plastic deformation and usually takes place before fracture. Fracture can occur by cleavage in brittle crystalline as the results of tensile stress acting on the normal to crystallographic planes with low bonding. The lacking in crystalline structure results in a conchoidal fracture , with cracks normal to the applied tension. Brittle fracture is the sudden , extensive cleavage fracture with negligible distortian under certain tensile stress which it was unused and applied. It usually defects in ferritic steels which usually in thickness over 12mm and transition temperature. Brittle fracture usally occurs with brittle materials , for example material with high strength steels , cast iron , glass , ceramic and etc. Brittle fracture surface has few characteristic. Materials that will fail in a brittle manner will no fail in a ductile manner. Brittle fractures can be characteristic as having little or no plastic deformation prior to failure. Possible with catastrophic results , some metals usually ductile will fail in a brittle manner under some circumstances. Like ductile fractures , brittle fractures have a distinctive fracture surface. The surface of a brittle failur is usually smooth.
The crack propagates through material by a process called cleavage. The crack can run close to perpendicular to the applied stress in a brittle fracture. A flat surface at the break leave by a perpendicular fracture. Besides having a nearly flat fracture surface , brittle surface usually having a pattern on their fracture surfaces. At the beginning origin of the crack , brittle materials have lines and ridges as well as spreading out across the crack surface. There are two type of brittle fracture which are transgranular fracture. Transganular fracture travels through the grain of material. Due to the different lattic arrangment or placement of atoms in each grain , the fracture changes direction from grain to grain. In other words or meaning , it may have to find a new path , road or plane atoms to travel on because it is easier to change direction for the crack than to rip through when the crack reaches a new grain. The cracks alone will try to choose the path with most least resistance. You can tell or observe when a crack has change the direction through material , because there is a slightly bumpy crack on the surface of material. After that , intergranular fracture is the second type of brittle fracture. Intergranular fracture is the crack that travels along the grain boundaries , and not go through the actual grains. As intergranular fractures normally occurs when the phase in grain boundaries is weak and brittle. For example cementite in Iron’s grain boundaries. In conclusion transgranular fractures cuts through the puzzle pieces and intergranular fractures travels along the puzzple pieces pre-cut edges.
Ductile fracture is the most common type of fracture in metal and it is different with what brittle fracture undergo. The metal yields before it breaks in a ductile fracture which is unlike what occurs in a brittle fracture. Tensile strength is meant by when the peak stress of a metal can withstand before it breaks. Duticle fracture is occurred because of the stress exerted on the metal actually work hardening the metal as it yields , crack from fatigue develop. Those cracks will propagate rapidly through the metal until complete failure occurs. Extensive plastic deformation takes place before fracture in ductile fracture. The material been called ???pulls apart??? rather than cracking generally leaving a rough surface for a ductile fracture. There is an absorption of a large amout energy before fracture and making a slow propagation. Materials with high purity can sustain very high and large deformation or more strain before fracture under loading conditon when strain at which the fracture happens is controlled and adjusted by the purity of materials. Before the cracks actually propagate , some of the energy from stress concentrations or focus at the crack tips is dissipated or evaporated by the plastic deformation.
There are ductile fracture mechanism. Ductile materials can be attributed or assigned to cup and cone fracture because of the failure of many ductile materials. This form of ductile fracture happened in stages that begin after necking begins. Necking is meant by Extensive plastic deformation takes place before fracture in ductile fracture. At the first stage , small microvoids form in the interior of the material. Next , the deformation continues and the microvoids will enlarge to form a crack. As the crack continues to grow and it spreads towards the edges of specimen. Finally , 45 degree angle with the tensile stress axis make as the crack propagation is rapid along a surface. A very irregular appearance will form on the new fracture surface. The final shearing of specimen produces a cup type shape on one fracture surface and a cone shape on the adjacent connecting fracture surface.
There are ductile fracture to brittle fracture transitions. The major key of this and first factor of this transitions is temperature. At high temperature , the yield strength is low and the fracture is more ductile in nature. However , at low temperature the yield strength is high and the fracture is more brittle in nature. This relationship with temperature has to do with atom vibrations. The atoms in material vibrate with greatear frequency and amplitude as temperature increases which allowing atoms slip to a new places in material. As the opposite for the atoms when the atoms undergo low temperature. The material exhibits characteristic of both types of fracture when temperature reach a moderate or average level. Dislocation of density is another factor that determines the amount of brittle fracture or ductile fracture occurs in a material. The more brittle the fracture the higher the dislocation density. The plastic deformations comes from the movement of dislocations. As dislocations increase or getting high in material due to stresses above the yield point , it become difficults for the dislocation to move as they are pile or close packed to each other. A material that has a high dislocation density can only deform. The last factor is grain size , when grain size get smaller the fracture become more brittle which bring dislocatons have less space to move before they reach a grain boundary. Plastic deformation will decreases as the dislocations can not move very far before fracture. Thus creating the material’s fracture is more to the brittle side.
Creep which usually be called cold flow is the tendancy of a solid material to move slowly or deform permanently under the effect or influence of mechanical stresses. Due to long-term of exposure to high level stresses it occur creep. Because the stresses which in high level are below the yield strength of material. Creep is more and have larger chance of severe in materials that are exposed or subjected to heat for long periods of time and their melting point increases generally. The relationship of temperature can be related to the creep. As creep always increases with temperature. The function of a material properties , exposure time , exposure temperature and the applied structural load are determined by the rate of deformation. The deformation grow large that a compoonent can no longer to perform its function because of magnitude applied stress and its duration. For example , a creep on turbine blade will cause the blade to contact the sing result in failure. Creep is a deformation mechanism that may or may not be part of a failure mode. For example , moderate creep in conrete can be ignored because it relieves tensile stresses that lead to cracking.
Creep deformation is not like brittle fracture. They does not occur suddenly because of the application of stress. The strain that accumulates for a long time as a result causing long-term stress. Therefore , creep is a ???time-dependent??? deformation. Creep occurs as when a metal is exposed or put into to a constant tensile load at an elevated temperature which undergo a time-dependent increase in length. Since each and every materials have its own different melting point , thus each will creep when the homologous temperature > 0.5. Creep can be tested by using a Creep Test method. The creep test is carried out by applying a constant load to a tensile strength. And during the Creep Test method , a constant temperature is maintained. There are three stages for creep. In the primary creep which can be said initial stage , the strain rate is rather high , but quite slow with increasing time. The strain is high and become slow as time increasing because is due to work hardening. Primary creep aslo can be say is a period of transient creep. Material deformation has casued the creep resistance of the material increase. In secondary stage , the strain eventually reached it minimum and become more to constant because it is due to the balance between work hardening and thermal softening. The average value of the creep rate during this paticular period is called the minumum creep rate. Secondary stage aslo know as steady-state creep. Lastly , is the tertiary creep , the strain rate increases greatly with stress because of necking.
There will be some structural changes during ceep. There are three deformation processes at elevated temperature. The first one will be the deformation by slip. For it structure , there will be more slip systems operate at high temperature and slip bands are rough and widely spaced. For the second will be subgrain formation. In this formation , the structure will produces imhomoginiety especially around ground boundaries because creep deformation produces them which allowing dislocations to arrange themsleve into low-angle grain boundary. Lastly , is the grain boundary sliding. It produced by shear process and caused by increasing in temperature or decreasing of strain rate which results in grain boundary folding or migration.
There are five mechanism of creep. The first one will be dislocaton of creep. During high stresses , creep is fully controlled by the movement of dislocations. Therefore , there are strong dependence on the applied stresses and no grain size dependence for dislocation of creep. The second mechanism will be Nabarro-Herring creep which is some form of diffussion creep. During Nabarro-Herning Creep , atoms easily diffuse through the lattice endager grains to elongate along the stress axis. With the creep rate decreasing as grain size is increased , Nabarro-Herring creep hold a weak stress dependence and moderate grain size dependence. However , Nabarro-Herring creep is strongly temperature dependent. The third mechanism is coble creep which is a second form of diffusion controlled creep. Atoms diffuse along grain boundaries to elongate the grains along the stress axis which caused the Coble creep have a stronger grain size dependence than Nabarro-Herning creep. The fourth mechanism is Climb. For the fourth mechanism the strain actually accomplished by climb. And lastly , the fifth mechanism is Thermally activated glide which the it crosses via cross-slip. The creep aslo can be used for some applications. For example , the creep rate of hot pressure loaded components for a nuclear reactor can be design for constraint. The creep rate is enhance by flux of energetic particles. The design of tungsten light bulb filaments is an example of application of creep deformation. Metal paper clips are stronger than plastic because plastic can creep at room temperature.