Cracking under compression

Ashby M T, Hallam S D. The Failure of Brittle Solids Containing Small Cracks under Compressive Stress States. Acta Metall, 1986, 34(3) 497-510. Kemeny I, Cook N G W. 1986. Effective Moduli, Non-linear Deformation and Strength of a Cracked Elastic Solid. Int. J. Rock. Mech. Sci. 

M.F. Ashby, S.D. Hallam, The failure of brittle solids containing small cracks under compressive... 

O When cakes tend to crack under vacuum measures such as a fiapper, compression blanket or pressure roll may assist in sealing the cracks thus avoiding loss of vacuum. When such measures are used it is necessary to make sure that the belt supporting system can take these extra vertical loads. 

Several criticisms of these parameters have recently been pointed out. First, they have no specific association with a material plane (i.e., they are scalar parameters), despite the fact that cracks are known to nucleate on specific material planes. With traditional parameters it is difficult to account for the effects of crack closure under compressive loading. Traditional parameters have not been successful at unifying experimental results for simple tension and equibiaxial tension fatigue tests. Finally, a nonproportional loading history can always be constmcted for a given scalar equivalence parameter that holds constant the value of the scalar parameter, but which results in cyclic loading of material planes. For such histories, scalar parameters incorrectly predict infinite fatigue life. 

Crazes usually form under tensile stress when a critical strain is surpassed they do not occur under compressive stress applying hydrostatic pressure during tensile deformation can even inhibit their development. Crazes always nucleate preferentially at points of triaxial stress concentration. It is the dilatational strain which initiates crazes and cracks. 

Figure 4. Fracture surface of PS impacted under compressive shear stress. Crack propagated from top to bottom. Left magnification X50. Right a different point on same fracture surface,...

Cycles to craze breakdown and to fatigue fracture increase significantly upon changing from complete stress reversal to cycling in a tensile mode at the same maximum stress. Buckling of the craze fibrils under compressive stress is conducive to early crack formation within the craze. 

In a final chapter a closely related phenomenon, the formation of shear bands in semi-crystalline polymers under compressive load will be described. It is attempted to discuss under which conditions shear bands are formed in semi-crystalline materials and how they interact with each other or with certain microstructural features, finally leading to crack initiation and shear fracture of the bulk polymer. 

If the steel tanks expand more than the CRM lining, then the lining will crack under any internal loading or pressure, since the lining is weak in tension. Conversely, if the lining expands excessively and the allowable compression stress in the lining is exceeded, it will also fail. 

A machine screw (normally high-tensile steel) through the centre of the transducer clamps the ceramic discs together. The screw should be kept under compressive stress — even when the transducer is stretched to its maximum — in order to prevent the discs from cracking. 

In comparison the refractory intermetallic Ti5 Si3 compound is brittle at deformation temperatures below 950 °C and no macroscopic ductility has been observed under compressive load. At the test temperature of 1000 °C a remarkable high flow stress of about 1050 MPa was achieved. Crack initiation revealed after plastic deformation of Spi = 1.5%. 

One crucial aspect is the large difference in the thermal expansion coefficients of stainless steel and SiOx (more than 10-fold, Table 5.4.1). As a result, temperature changes create significant stress at the interface between the steel and the SiOx. During operation, mechanical stress is introduced in addition to the thermal stress. To prevent the formation of cracks, the internal stress of the SiOx layer has to be chosen so that the SiOx remains under compressive stress under all operating conditions. Typically, the internal stress needs to be compressive and on the order of 100 MPa. Stability against cracks is favored by a high density of the deposited layer [9],... 

The question of the original location of the cracks must be adressed. Cracks during drying occur from microflaws expected to be present on the surface of the gel. Regarded as a whole, the network is under compression stress. However, Scherer [22] demonstrated that at the extremity of the flaw tips the solid part remains in tension. 

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