PSI - Issue 6

V.A. Morozov et al. / Procedia Structural Integrity 6 (2017) 154–160 Morozov et al. / Structural Integrity Procedia 00 (2017) 000–000

158

5

From the radial pressure oscillograms, it is possible to determine the strain rate of the samples: d ε dt = 1 E dp dt .

(1)

Here E is the Young’s modulus of the sample material and p is the radial pressure. The results of processing radial pressure pulses in fluoroplast and PMMA samples showed that on loading a low voltage scheme the deformation rate was 10 4 s − 1 . And it was at the order of 10 5 s − 1 with a high-voltage loading scheme when the pulse front is much shorter. It should be noted that the radial pressure decreases as the diameter of the sample increases. Longitudinal cracks appear on samples made of fluoroplast with small diameter (8 mm ) leading to their fracture. This is clearly illustrated by the photo shown on Fig. 8a. A system of radial cracks is clearly visible (Fig. 8b) on samples of PMMA with a large diameter (36 − 50 mm ). But the sample has not yet been divided into parts. Samples of small diameter are destroyed by two or more fragments in this case.

Fig. 8. Photo of fracture of a fluoroplast (a) and PMMA (b) samples.

4. Fracture analysis

A microscopic study of the fracture surfaces for PMMA and fluoroplast was carried out, on the basis of which a comparative analysis of the mechanisms of fracture was made as a function of the rate of deformation. The processes that occur when solid bodies are destroyed are reflected in the character of the fracture surface. The area of polymer formed by the slow development of a crack from the source of failure during the tension of the sample is smooth and has a high reflectivity. This region is called a mirror zone or regions with cracks of “silver” - thin interlayers of highly deformed, partially stratified material capable of mirroring light (Atroshenko et al. (2002)). Outside the mirror region, or the area of slow crack growth, there is a transition zone, which is usually just a region with an increased roughness on the surface of the fracture. In samples with a notch this area is more extensive than in samples without a notch, and is dotted with uneven lines oriented in the direction of crack propagation. The next area contains a series of geometric figures, similar mainly to parabolas and hyperbolas. They arise when a secondary crack starts to grow from a defect lying in front of the front of the primary growing crack. The secondary crack extends uniformly in the form of a system of concentric circles diverging radially from numerous local defects that are activated by a propagating stress wave and, when they are intersected with the primary crack, form parabolas if the growth rates of the cracks are equal. The defect from which the secondary crack arises is the focus of the parabola. A small di ff erence in the levels of the two propagating disruptions makes these figures visible and causes a significant absorption of energy. The features of fracture surfaces reflect the main deformation processes, on which the energy of destruction is consumed. Outside this region, the stress level and crack propagation rate increase, and high-speed (explosion-like) failure occurs. Under conditions of high dynamic overstrain, many defects act simultaneously, and the material undergoes brittle fracture regardless of its behavior under static loading.