Defects cooling cracks

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Polymer preparations should not be scaled up without a careful review and a gradual scaleup to check exotherms. This will determine the proper equipment and cooling needed before starting. All glassware should be free of cracks, and defects before using. In most cases, ordinary laboratory glassware may be used, but resin kettles are sometimes desirable on a larger scale operation. 

WhUe you are casting in tins manner, another person with another ladle may catch some of the shot, pbcing the ladle four or five inches underneath the plate in the water, by W hich means you will see if they arc defective, and rectify tliem. Your chief care b to keep the lead in a just degree of heat, that it shall be not so cold as to st< up the lu)les in your plate, nor so hot as to cause die sliot to crack to remedy the heat, you must refrain working till it b of a proper coolness and to remedy the coolness... 

It is interesting to note that not all defects in product crystals arise from the crystallization process itself. Thermal stresses induced in crystals are sometimes greater during the cooling and drying than during the crystallization period, and can be so large as to crack the crystalline material. These thermal stresses, which can produce defect concentrations on the order of 10 -10 /cm, ... 

But what about defects The TFS-films certainly will possess defects arising from the fact that they were transferred at elevated temperature, but then cooled down to the crystalline bulk phase. Since the thermal expansion coefficients of the substrate and the film should be different, the formation of some defects, even besides the occurrence of grain boundaries due to crystallization, is to be expected. In fact cracks have been observed between very smooth areas from which molecular resolution could be obtained (57). However, these films appear perfectly transparent to the naked eye and macroscopic defects (besides dust particles) are not even visible under an optical microscope. LB-films of the same material are also initially transparent but already show some defects visible to the naked eye. After rearrangement to the monolayer phase, the defects become even more pronounced. Nevertheless, comparison of the rearranged LB-films with LB-films of... 

A highfy cross-linked epo] resin has a coejSicient of linear thermal expansion a Sx 10 K", G,c — 120 J 3.2 GPa, and > =0.35. A thick layer of resin is cured and is firmly bonded to an aluminium part (a = 2.5 X 10" K 0 at 180 C. Calculate the minimum defect size needed to initiate cracking in the resin on cooling to 20°C. Take Y = 2/tt for semi-circular edge-cracks of radius a in a wide sheet. 

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