PSI - Issue 28

S.V. Suknev et al. / Procedia Structural Integrity 28 (2020) 903–909 Author name / Structural Integrity Procedia 00 (2019) 000–000

907

5

Eq. (2) yields expression for the relationship between the critical value of the stress 2  and the loading biaxiality ratio 1 2 / k k   :   

2 16 1 

  

.

(4)



0 2  

c

J

12 J J



1

2

4. Experimental study The experimental research program includes two series of tests of specimens made of plaster of Paris with a circular hole under non-uniformly distributed compression. The load p is applied to the specimen through rigid inserts placed between the specimen and loading platens. The inserts are placed at the center of the upper and lower faces of the specimen (Fig. 1). In this case, as shown in the work by Suknev (2015), there is a fairly uniform biaxial stress state in the central part of the specimen (outside of the region affected by the hole): tension along the horizontal axis and compression along the vertical axis of the specimen.

Fig. 1. Loading of the specimen with a circular hole: (a) geometry of the specimen; (b) stress distribution at the center of the specimen.

In the first series of tests, the effect of the hole size on the initiation of tensile cracks at the hole is investigated under constant boundary conditions. In the second series of tests, the effect of boundary conditions on the initiation of tensile cracks at the hole of fixed diameter is studied. For the first series of tests, the specimens are made in the form of 200 x 200 x 36-mm plates. Before the test, circular holes with a diameter 2 a = 1, 2, 5, 10, 15, and 20 mm are drilled at the center of the specimens. Five specimens are made and tested with holes of each diameter. The specimens are loaded using inserts with a size of 120 mm. During testing, tensile cracks initiation in stress concentration zones at the hole is observed, which occurs suddenly and is accompanied by a characteristic click. In specimens, whose hole diameters range from 5 mm to 20 mm, cracks instantly propagate to a distance of about 50 mm along the compressive load line, and their growth stabilizes upon further loading. In specimens, whose hole diameters are 1 mm and 2 mm, cracks instantly propagate over almost the entire vertical section of the specimen. Crack initiation is accompanied by partial unloading of the specimen, thereby forming a tooth in the loading diagram. The critical load at the instance of crack initiation is determined by the tooth tip in the diagram. For the second series of tests, 100 x 100 x 36-mm specimens are manufactured. Before the tests, circular holes with a diameter 2 a = 2 mm are drilled at the center of the specimens. The specimens are loaded using inserts of sizes B = 20, 40, 60, and 80 mm. We tested 5 to 7 specimens with inserts of each size. As in the first series of tests, the initiation of tensile cracks in stress concentration zones at the hole is observed, which occurs suddenly and is accompanied by a characteristic click and partial unloading of the specimen.