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.