The results show that the partial unmelted region, casting defect structure and secondary crack form in the welding seam, which result in the low stress breakage of the samples.
结果表明：在焊缝区域内存在着局部未熔区以及铸造疏松组织和二次裂纹等缺陷，这是导致试样低应力断裂的根本原因。
Rock in coal seam, suggest casting away sub-level caving.
对于煤层内夹石厚度比较大的情况，建议不采用放顶煤开采。
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