Initial Crack Formation Research Articles

In-situ Observation of Compressive Fracture Processes in Flat Woven Carbon Fabric Reinforced Carbon Composite Materials

The compressive fracture processes in flat woven carbon fabrics reinforced carbon composite materials (C/C composites) were studied “in-situ” by means of optical microscopy. The compressive fracture strength obtained was 110MPa and 90MPa for the C/C composites with the final heat treatment at 1873K and 3273K, respectively. For the former composite, the specimen size dependence of fracture strength was seen with the specimen length smaller than 25mm, where the fracture strength was increased with decreasing the specimen length. The initial crack formation between fiber bundles crossed each other was observed at a quarter of the fracture stress. At about 95 % of the fracture stress, inter-laminar de-bonding was observed. The second cracking stage was very fast, followed by catastrophic fracture. For the latter composite, the catastrophic fracture was followed immediately after the first crack initiation. These fracture characteristics were interpreted mainly in terms of the difference in graphitization at interfaces caused by the final heat treatment.

Mechanical behaviour and failure of AISI 4130–lncoloy 825 bimetallic composite under uniaxial tension

The present paper is an analysis of the results from a finite element method (FEM) study of the mechanical behaviour and state of stresses developed during the tensile testing of AISI 4130 steel–Incoloy 825 bimetallic composite specimens. The geometries of the specimens simulated are identical to those tested and machined from a commercial bimetallic tube heat treated under several conditions. The resulting microstructures have also been analysed. The FEM analysis shows that before necking, the rule of mixtures correctly predicts the flow behaviour of the bimetallic composite. After necking, stresses normal to the interface are developed. Shear stresses at 45° seem to be responsible for the formation of initial intergranular cracks along a zone of high density carbides precipitated inside the Incoloy 825. The normal stresses finally cause fracture of the composite along this region of carbides.MST/3089

Cracking Analysis Based on Slip and Bond Stresses

A theory for crack formation in reinforced concrete elements based on an analysis of slip, bond stresses, and steel stresses is presented. The problem is treated as an initial value problem both for initial crack formation when the cracks are independent, and for stabilized crack formation when the zones of bond stresses reach each other. Simple rules for analysis are presented for initial crack formation derived from the analytical solution of the mathematical model using homogeneous initial values. As an application, the spalling crack width control of a prestresesd pretensioned bridge girder is estimated. Numerical analysis is presented for stabilized crack formation. Tension stiffening is analyzed as a result of the interface behavior.

Sliding damage of silicon nitride in plane contact

Experiments are described in which sintered silicon nitride was slid against itself in plane contact in oil and in various gaseous atmospheres. Results of observation of wearing surfaces by microscopy, profilometry etc. which show the history of surface damage are presented. First, subsurface defects seem to act as preferential sites of origin of initial crack formation. The cracked material is then removed and, while being dragged, causes the damage to propagate in the direction of sliding. Finally, a nearly two-dimensional structure, similar to that observed on wearing surfaces of metals is established in the microtopography. Removal of material from the ridges creates valleys and successive removal occurs at the new ridges. Repetition of this process leads to steady wear. Effects of environments on wear morphology are also discussed.

A comparative study of the strength of aluminous porcelain and all-ceramic crowns

This study compared the breaking strengths of all-ceramic crowns constructed with aluminous porcelain, the Cerestore system, and the Dicor system on an anterior tooth preparation. Fifteen crowns from each group were constructed to comparable dimensions on dies from the same master die. Ten crowns from the same group were cemented on the master die and loaded until catastrophic failure. Five crowns from each group were embedded and sectioned to examine internal adaptation. No significant difference was found between the load necessary to produce failure in the aluminous porcelain and Dicor crowns and the load needed to initiate crack formation in the Cerestore crowns. However, seven of the 10 Cerestore crowns showed a two-phase failure pattern; the difference between crack initiation and catastrophic failure was significant. Within the limitation of this study, neither the Cerestore (high alumina core) nor the Dicor (castable glass-ceramic) produced an all-ceramic crown superior to the current aluminous porcelain crown.

Crack Initiation in Perforated Propellants under high strain rate impact conditions

AbstractThe fracture behaviour of gun propellant grains during impact has been measured on a modified Hopkinson Bar which uses a projectile as the input bar. Evidence for the detection of initial minor crack formation in seven‐perforated cylindrical specimens of single base, double base and triple base propellant is presented. Under compressive load (pressurization rates of 100 GPa/s‐200 GPa/s), minor crack formation does not immediately result in large scale crack propagation or fragmentation of the specimens. The single base and triple base propellant are equally more resistant to crack initiation than the double base propellant when tested in either the end‐on or side‐on modes. Implications for munition survivability are discussed in terms of mechanical frictional effects caused during compressive loading of cracked propellant or compressive heating in propellant cracks during the ballistic cycle.

Interfacial de-bonding caused by a screw dislocation pile-up at a circular cylindrical rigid inclusion

An array of continuously-distributed screw dislocations piled up against a circular cylindrical rigid inclusion is analyzed by the complex-variable method. Both uniformly applied shearing load at infinity and internal friction stress opposing the movement of dislocations are taken into account. The pile-up tip is away from the matrix-inclusion interface, its distance from the interface being determined by the condition that the stresses should be finite everywhere in the solid. Stress distributions on the interface are determined, and de-bonding of the interface, namely the formation of initial voids or cracks, is discussed. Stress and displacement near the tip of these initial voids are then analyzed. This analysis is combined with the virtual work argument of A.A. Griffith (1920) to yield a criterion for the initial voids to grow along the interface. The critical void-growth load is expressed by the sum of two terms, one proportional to the friction stress and the other inversely proportional to the square-root of the inclusion radius.