基于3D-ILC含水平内裂纹脆性固体三点弯断裂特性研究

Three-point bending fracture characteristics of brittle solid with horizontal internal cracks based on 3D-ILC

断裂力学是固体力学的重要分支,三点弯试验是力学领域的一个经典试验,含裂纹的三点弯研究多集中于穿透或表面裂纹,含纯封闭内裂纹的三点弯断裂研究较少。基于3D-ILC技术,在对试样表面无任何影响的情况下,生成任意参数的纯封闭内裂纹,开展含不同深度共勉内裂纹试样及完整试样三点弯断裂试验,分析应力双折射规律、破坏荷载、断口特征、断口特征区尺寸及破坏形态。通过对含不同深度内裂纹的试样进行数值模拟,得到裂纹尖端KⅠ、KⅡ、KⅢ分布规律。结果表明:(1)内裂纹改变了完整试样的应力双折射规律,云纹在裂纹上端呈半圆状,色差对比显著。(2)内裂纹的存在显著降低了试样三点弯破坏荷载,含深度10、20、30 mm单内裂纹试样及含深度20 mm双、三内裂纹试样与完整试样破坏荷载相比,下降百分比分别为11.8%、19.5%、78.4%、30.9%、35.5%。(3)内裂纹深度10、20mm试样与完整试样破坏形态均为动态裂纹分叉破坏,呈现出镜面雾化羽毛平纹特征;含深度30mm单内裂纹试样发生Ⅰ-Ⅱ-Ⅲ型复合扩展,断口为光滑的翼状扩展。(4)得到了破坏断口特征区尺寸与内裂纹深度关系的拟合公式。(5)KⅠ、KⅡ、KⅢ与内裂纹深度成反比关系;裂纹越远离试样中心,KⅠ越小,KⅡ、KⅢ越大;左右侧裂纹对中间裂纹的K值不影响。该研究表明3D-ILC可做为研究断裂力学中的内裂纹及复合裂纹问题的重要工具。

Fracture mechanics is an important branch of solid mechanics. The three-point bending test is a classic test in the field of mechanics. Most of previous studies on three-point bending tests with cracks focus on either through-wall or surface cracks, while limited studies have been performed on internal cracks. Based on the 3D-internal laser-engraved crack(3D-ILC) method, without any damage on the surface, the internal crack was created. Experiments were performed on specimens with horizontal internal cracks at different depths and intact specimens. The stress-induced birefringence, failure load, fracture characteristics, fractography size and fracture morphology were analyzed. The distribution of KⅠ, KⅡ, KⅢ around crack front was obtained by numerical simulation. The results show that: i) The internal crack changes the stress birefringence law of the intact sample, the moire is semi-circular at the upper end of the crack, and the color difference is significant. ii) The existence of internal cracks significantly reduce the failure load of the specimens. Compared with that of intact specimen, the failure loads of the single crack specimens with depths of 10, 20, and 30 mm and the double and triple crack specimens with depth of 20 mm, decrease by 11.8%, 19.5%, 78.4%, 30.9% and 35.5% respectively. iii) The fracture morphology of the specimens containing cracks with depths of 10 and 20 mm are similar to that of the intact specimen, all of which is belonged to dynamic crack-branching fracture, showing the characteristics of mirror atomization feather plain grain. The mixed mode Ⅰ-Ⅱ-Ⅲ fracture occurs in the specimen with single internal crack with a depth of 30 mm, the characteristic of fracture is a smooth wing-like extension. iv) A fitting formula of the relationship between the fractography size and the crack depth is obtained. v) The KⅠ, KⅡ, KⅢ are inversely proportional to the depth of internal crack. The farther away the crack is from the center of the specimen, the smaller the value of KⅠ and the larger the value of KⅡ and KⅢ. The cracks on the left or right side have no effect on the value of K of the middle crack. This study shows that 3D-ILC is an important tool for studying internal cracks and mixed mode in fracture mechanics.