Cracks causes

Most ceramics (as we have seen) contain flaws holes and cracks left by processing, cracks caused by thermal stress, corrosion or abrasion. Even if there are no cracks to start with, differences in elastic moduli between phases will nucleate cracks on loading. And most of these flaws have a size which is roughly that of the powder particles from which the ceramic was made. If the flaw size can be reduced, or if samples containing abnormally large flaws can be detected and rejected, the mean strength of the ceramic component is increased. 

Corrosion Fatigue failure by cracking caused by reversing alternating stress in the presence of a corrosive environment. 

The other effect of having a stretched area is a reduction in resistance to stress cracking. Crazing is a possibility in such areas such as in polystyrenes, and environmental stress cracking caused by solvent substances will occur in the stretched areas. This is a particularly important consideration in vacuum formed products used for packaging food that frequently has some solvent action on the plastics. 

Cracking caused by the combined actions of (i) mechanical stress and (ii) chemical agents or radiation or both. 

Explode a small charge in contact with a block of glass in the image path. The internal multiple cracking caused by the charge makes the glass effectively opaque... 

Cracks caused by contact of an elastic ball of indenter with an elastic brittle surface are given by Hertz s model (Fig. 6.2.7a, b). 

FATIGUE (Metals). Failure of metal parts by progressive cracking caused by repeated application of stress. Most fatigue failures start at the surface where discontinuities in section such as square shoulders, screw threads, or even tool marks cause a high concentration of stress. 

Figure 6. Polarized transmitted light photomicrograph at x 70 of a typical fiber break for an epoxx-sized H-glass fiber. The large initial matrix crack causes coupon failure due to continued matrix crack propagation into the matrix perpendicular to the fiber axis.

Brittle erosion is the loss of material from a solid surface due to fatigue cracking and brittle cracking caused by the normal collisional force Fn. Materials with very limited capacity for elastic and plastic deformation, such as ceramics and glass, respond to particle impacts by fracturing. The yield stress for brittle failure Fb for normal impacts is about... 

Discoidal stress cracks Disc-shaped cracks caused by stress in a material such as amber, when it is heated and cooled. Sometimes called sun spangles or lily pads . 

Roots with jagged cracks. Causes Freezing injury uneven soil moisture cavity spot. Temperatures below 30°F cause cracked roots with a water-soaked appearance. Protect roots with mulch before temperatures fall. 

Potatoes cracked. Cause Uneven soil moisture. If soil is alternately dry then wet. roots may split their skins. Prevent problems by keeping soil evenly moist and storage humidity constant. Some cultivars, such as Jeu ell and Puerto Rico 198 , are somewhat resistant to cracking. 

Root flesh black but firm skin rough and cracked. Cause Downy mildew. Control this fungal disease by spraying plants with a baking-soda-and-soap spray 1 teaspoon baking soda, 1 teaspoon liquid dish soap, 1 quart water) or copper if you have had problems in the past. 

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