温度不均匀引起KDP晶体开裂的实验与模拟

Crack of KDP Crystals Caused by Temperature-nonuniformity

KDP（KH2PO4）晶体内部温度不均匀可能导致其开裂。采用降温实验和有限元计算研究了直径为3040 mm的KDP晶体内部温度分布、应力与开裂的关系。结果表明：晶体开裂发生在降温早期（10150 s）,晶体主裂纹平行于（100）面或（001）面,表面裂纹遍布于晶体的多个表面,高宽比大的晶体更倾向于产生（001）面裂纹。模拟发现,当晶体内温差超过4℃时,其内部最大热应力接近晶体断裂强度,晶体有开裂的可能。4个晶体样品开裂时刻的应力分别为4.76、5.36、7.69、5.74 MPa,与断裂强度6.67 MPa相当;应力最大值出现在晶体底面棱中点或柱面棱中点,与晶体实际开裂的起始点一致。

Cracks are likely to appear in bulk KDP crystals with temperature-nonuniformity. The cooling experiments on KDP crystals with the sizes of 30-40 mm were carried out to investigate the influence of temperature-nonuniformity on the crystal cracking. Temperature and stress distributions on crystals during the cooling process were simulated by the finite-element method. The results show that cracking occurs in the early period（10-150 s）. Deep main cracks and tiny cracks both appear on the experimental crystals. Main cracks usually originate from crystal periphery and propagate to the interior with a direction that parallels to（100） or（001） plane. Tiny cracks are just on the crystal surface. Cracks that parallel to（001） plane appear in the samples with a large height-width ratio. The simulation results reveal that crystals with the sizes of 30-40 mm likely crack when a difference between the maximum temperature and minimum temperature inside is greater than 4 ℃. The maximum stress at the moment of cracking for each sample is 4.76, 5.36, 7.69, 5.74 MPa, respectively, which is close to 6.67 MPa（i.e., the fracture strength of KDP crystal）. The location of the maximum stress is in the middle point of the bottom edge or prism edge, which is in accord with the crack origination.