Transitional Fractures

The distal tibial epiphysis ossifies between 6 and 24 months of age. The medial malleolus appears at 7-8 years and is complete at 10 years. It usually ossifies as a downward extension of the distal tibial ossific nucleus but may develop as a separate centre of ossification and thus be mistaken for a fracture line. The distal tibial physis closes first centrally, then medially and finally anterolaterally (Fig. 15.1), with the entire process lasting about 18 months. This sequence of closure of the distal tibial physis is important in the pattern of transitional fractures (triplane and juvenile Tillaux). Completion of distal tibial physeal closure is at around 14 years in girls and 16 years in boys. 

Magnetic resonance (MR) imaging for acute paediatric ankle injuries conveys no real advantage over radiography (Lohman et al. 2001). It is probably as useful as CT in the assessment of transitional fractures and is also helpful in the evaluation of the articular cartilage in traumatic osteochondral lesions of the talus. Where it is perhaps most useful is in the later stages, following physeal injury, to identify and characterise physeal arrest (Futami et al. 2000 Sailhan et al. 2004). 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Fig. 13. Transition between ductile fracture and brittle fracture when Al QFe Gd metallic glass is aimealed at 170°C.

Oxides such as CaO, MgO, and Y2O2 are added to Zr02 to stabili2e the tetragonal phase at temperatures below the tetragonal to monoclinic phase-transition temperature. Without stabili2et, the phase transition occurs spontaneously at temperatures below 850—1000°C, and no fracture toughness enhancement occurs (25). 

When pressure tests are conducted at metal temperatures near the ductile-to-brittle transition temperature of the material, the possibility of brittle fracture shall be considered. 

Figure 15.27 Fracture face (right) contrasted with corresponding, intact weldment. Note that the transition from a rough to a smooth fracture surface occurs abruptly at the junction of the weld metal with the base metal.

Figure 8.14. Strain-rate dependence of ideal brittle (fracture dominated) and ideal ductile (flow dominated) spall energies. A postulate of minimum fragmentation energy leads to a transition in spall behavior.

Dynamic Fracture and Fragmentation 289 an expression for the transition strain rate is obtained... 

It is somewhat disconcerting that the MYD analysis seems to present a sharp transition between the JKR and DMT regimes. Specifieally, in light of the vastly different response predicted by these two theories, one must ponder if there would be a sharp demarcation around /x = 1. This topic was recently explored by Maugis and Gauthier-Manuel [46-48]. Basing their analysis on the Dugdale fracture mechanics model [49], they concluded that the JKR-DMT transition is smooth and continuous. 

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