Fracture surface BAC-30wt%SiC B4C inner layer outer layer BAC la XL30 EFEG Cleavage steps Sic grain 4c rain Figure 4 Fracture surface of a three layered tile. (A)An interface between B4 C-30 wt%SiC outer layer and pure B4C inner layer;(B)A fracture urface of B C layer;(C)A fracture surface of B C-30 wt% SiC; (D)Cleavage steps on the B C fracture surface individual layers of the same composition completely mance of the laminates. As a result of the hardness and disappeared and only the interface between B4 C-30 Youngs modulus decrease, the mterial with a residual wt%SiC and B4C layers could be distinguished. porosity more then 2%o cannot be considered as a can- A fracture surface of a three-layer tile hot pressed at didate for ballistic protection. 2200C for I h is shown in Fig 4. The layered compos ite demonstrates typical brittle fracture. The interface 6. What residual stresses can do between the B4 C-30 wt%SiC outer layer and the pure with a laminate B4 Cinner layer is shown in Fig 4a. The fracture surface During the assembly of one 100 x 100 x 12 mm mt of the B4 C layer is presented in Fig. 4b. Fig 4c shows tilayered tile, inner thin B4C-30 wt %SiC layers were the fracture surface of the B C-30 wt%SiC layer. The mistakenly replaced with pure B4C thin layers. As a cleavage steps on the B4 C fracture surface are presented result, instead of a multilayered tile, 3 layered laminate in Fig. 4d. Such cleavage mode plays an important role was produced. The parameters of this 3-layered tile, in both in fracture and in the fragmentation event during cluding a thickness of layers and calculated stresses are ballistic impact [37] presented in Table V. The outer B4 C-30 wt%SiC layers As one can see from Fig. 4, the B4C grain size in had their thickness of 1650 um, and the thick B4C layer BC-30 wt %SiC layers was in the range of 4-6 um, had a thickness of 9000 um. For such design the level of Sic grain size was in the range of 2-5 um. BC grain residual tensile stress has been raised to 210 MPa after size in pure B4 C layers could not be calculated because cooling from THP= 2200C. Such a high residual ten of a pure transgranular fracture mode with no grains sile stress leads to a complete fracture of the tile during or grain boundaries revealed after fracture. Significant decompression of the graphite die to separate a tile af grain growth of boron carbide is expected during hot ter hot pressing(Fig. 5). The failure apparently started pressing at 2200C. However, in B4 C-30 wt%SiC lay- from the tile edges with cracks propagated further into ers, the existence of the Sic phase prevented the ex- the tile body aggerated grain growth and the grain size distribution This example shows the importance of determination was homogeneous. Tiles hot pressed at 2200C for I h of a critical value of the tensile stress in a layer. Certain were fully dense. Tiles hot pressed at 2150C for 30 difficulties exist to find this critical value. One of the or 45 min contained some amount of porosity(2-5%) problems is that the mechanical properties of an indi- that was concentrated along the interfaces and mostly vidual layer can significantly deviate from the ones of in pure B4 C layers. Such porosity could be detrimental a corresponding bulk material. We can easily calculate for material hardness, affecting Youngs modulus and the critical tensile stress if intrinsic fracture toughness tensity, thus significantly lowering the ballistic perfor- of layer and size of critical flaw inside the layer are 5488Figure 4 Fracture surface of a three layered tile. (A) An interface between B4C-30 wt%SiC outer layer and pure B4C inner layer; (B) A fracture surface of B4C layer; (C) A fracture surface of B4C-30 wt%SiC; (D) Cleavage steps on the B4C fracture surface. individual layers of the same composition completely disappeared and only the interface between B4C-30 wt%SiC and B4C layers could be distinguished. A fracture surface of a three-layer tile hot pressed at 2200◦C for 1 h is shown in Fig. 4. The layered compos￾ite demonstrates typical brittle fracture. The interface between the B4C-30 wt%SiC outer layer and the pure B4Cinner layer is shown in Fig. 4a. The fracture surface of the B4C layer is presented in Fig. 4b. Fig. 4c shows the fracture surface of the B4C-30 wt%SiC layer. The cleavage steps on the B4Cfracture surface are presented in Fig. 4d. Such cleavage mode plays an important role both in fracture and in the fragmentation event during ballistic impact [37]. As one can see from Fig. 4, the B4C grain size in B4C-30 wt%SiC layers was in the range of 4–6 µm, SiC grain size was in the range of 2–5 µm. B4C grain size in pure B4C layers could not be calculated because of a pure transgranular fracture mode with no grains or grain boundaries revealed after fracture. Significant grain growth of boron carbide is expected during hot pressing at 2200◦C. However, in B4C-30 wt%SiC lay￾ers, the existence of the SiC phase prevented the ex￾aggerated grain growth and the grain size distribution was homogeneous. Tiles hot pressed at 2200◦C for 1 h were fully dense. Tiles hot pressed at 2150◦C for 30 or 45 min contained some amount of porosity (2–5%) that was concentrated along the interfaces and mostly in pure B4C layers. Such porosity could be detrimental for material hardness, affecting Young’s modulus and density, thus significantly lowering the ballistic perfor￾mance of the laminates. As a result of the hardness and Young’s modulus decrease, the mterial with a residual porosity more then 2% cannot be considered as a can￾didate for ballistic protection. 6. What residual stresses can do with a laminate During the assembly of one 100 × 100 × 12 mm mul￾tilayered tile, inner thin B4C-30 wt%SiC layers were mistakenly replaced with pure B4C thin layers. As a result, instead of a multilayered tile, 3 layered laminate was produced. The parameters of this 3-layered tile, in￾cluding a thickness of layers and calculated stresses are presented in Table V. The outer B4C-30 wt%SiC layers had their thickness of 1650 µm, and the thick B4C layer had a thickness of 9000µm. For such design the level of residual tensile stress has been raised to 210 MPa after cooling from THP = 2200◦C. Such a high residual ten￾sile stress leads to a complete fracture of the tile during decompression of the graphite die to separate a tile af￾ter hot pressing (Fig. 5). The failure apparently started from the tile edges with cracks propagated further into the tile body. This example shows the importance of determination of a critical value of the tensile stress in a layer. Certain difficulties exist to find this critical value. One of the problems is that the mechanical properties of an indi￾vidual layer can significantly deviate from the ones of a corresponding bulk material. We can easily calculate the critical tensile stress if intrinsic fracture toughness of layer and size of critical flaw inside the layer are 5488