Fracture overload

Mechanical fracture can be due to monotonic increase of the load overload fracture or forced fracture) or due to cyclic loads fatigue fracture). Overload... 

The main causes of failure in gear couplings are wear or surface fatigue caused by lack of lubricant, incorrect lubrication, or excessive surface stresses. Component fracture caused by overload or fatigue is generally of secondary importance. 

Fracture of components Overload or fatigue, shock loading... 

There are a number of fracture modes, the most important of which are ductile overload, which is fairly well understood and can be predicted reasonably accurately, and brittle fracture, which is less predictable from an engineering viewpoint and can cause catastrophic failures due to the speed of the fracture. 

The term shatter fracture applies to a situation where the applied energy is well in excess of that necessary for fracture. Many regions of the particle are overloaded under these conditions, yielding a comparatively large number of particles with a wide distribution of sizes. This type of fracture occurs under conditions of rapid loading such as those obtained in a high-velocity impact. 

With the exception of PC, amorphous, non-oriented polymers did not produce measurable amounts of broken segments when subjected to tension. As has been shown in previous paragraphs, large axial stresses capable of chain scission in amorphous polymers can only be transmitted into the chain by friction of slipping chains requiring strong intermolecular interactions. In addition, macroscopic fracture occurs before a widespread chain overloading and scission occurs, which is opposite to the behavior of semicrystalline polymers. 

Fracture processes become very different in the glassy state as compared with the rubbery state. In the glassy state, the sensitivity of fracture to network topology is lost. The chemical structure of the network, crosslink density and the type of bond overloading do not play a key role and defects of glassy samples become very important. 

Figure 5. Regions of stable crack growth and overload observed in the Zr-stabilized 2014 aluminum alloy tested at 125 MPa fractured after 3.5 105 cycles.

The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment. As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload. 

The total cost of material fracture is about 4% of gross domestic product in the United States and Europe (88,89). Fracture modes included in the cost estimates were stress-induced failures (tension, compression, flexure, and shear), overload, deformation, and time-dependent modes, such as fatigue, creep, SCC, and embrittlement. The environmentally assisted corrosion problem is very much involved in the maintenance of the safety and reliability of potentially dangerous engineering systems, such as nuclear power plants, fossil fuel power plants, oil and gas pipelines, oil production platforms, aircraft and aerospace technologies, chemical plants, and so on. Losses because of environmentally assisted cracking (EAC) of materials amount to many billions of dollars annually and is on the increase globally (87). 

Overload Bone fracture site Implant screw hole Screw thread Small size implant Unstable reduction Early weight bearing... 

Modulus of Toughness Modulus of toughness is a mechanical property of a material that indicates the ability of the material to handle overloading before it fractures. 

Although the incidence of patellar tendon rupture is reportedly low (0.06-0.24 %) [53, 59, 64], ruptured tendons are technically difficult to repair [65-69]. To prevent this, the patellar tendon should be cut along the longitudinal fibers so as to minimize the risk of tendon laceration under direct vision the edges of the residual patellar tendon at the tibial insertion site should not be undermined [52, 53, 65, 70]. Also, use of a double parallel blade is recommended to avoid tendon laceration [52, 53]. It should be noted that, when the contralateral, normal knee is used as the donor site, early mobilization with limited weight bearing on the reconstructed side may overload the contralateral donor knee, leading to patellar fracture and tendon rupture [54]. Since dismption of the knee-extensor apparatus is a severe complication, both patellar fracture and patellar tendon rupture should be avoided to the extent possible. 

Fig. 7.52 Representative scanning electron micrographs of the nominally identical fracture-surface morphologies obtained in Mg-PSZ (MS grade) for a overload fracture under monotonic loads at AK 11.5 MPa and (b) fatigue fracture under cyclic loads at AK 6 MPa m. Note, in contrast to metals, the absence of striations or crack-arrest markings on the fatigue fracture surface. Arrow indicates general direction of crack growth [6]. With kind permission of John Wiley and Sons...

---