Delaminations in composite structures: V. V. Bolotin Figure 1 Three types of delaminations: (a) internal,(b) near-surface and (c) multiple cracking khr (b Figure 2 Pioneering studies in mechanics of delaminations: (a) Obreimoff's study in splitting of mica, (b) Kachanov's problem of the compressed laminate tube 12 T-Tr bution of the residual specific fracture work along the 1-3 correspond to three increasing magnitudes of the Figure 3 Epox ies variation during the thermal treatment: 0 is shrinkage ratio, and E is the 10s Youngs modulus 3 DELAMINATIONS ORIGINATING IN THE MANUFACTURING PROCESS High strength of most laminated and fibrous composites in the direction of reinforcement is accompanied by low resistance against interlaminar shear and transverse ension. Therefore, the interlaminar cracks can originate both on the fabrication stage and on the stages of transportation, storage and service. Instabilities of the manufacturing process, imperfections of various natures, and thermal and chemical shrinkage of components may be the source of initial delaminations Figure 6 Distribution of summed cracked arca along the depth of a Delaminations in large-scale composite structures were specimen; coordinate is measured from the impacted surface met in the design of the deep underwater vehicle for ocean research. The vehicle was designed as a stiffened spheroidal that time which component of the shrinkage of the epoxy shell of glass/epoxy laminate. A set of multiple cracks was resin is more responsible for the occurrence of tensile stresses found in pilot specimens of these shells, and the stated thermal, chemical, or both. It depends, obviously, on how the objective was to avoid these defects. It was evident that these variation of shrinkage and compliance correlate in time 0, II cracks were produced when the transverse tensile stresses To study the shrinkage and compliance in situ, an occurred on the manufacturing stage, But it was not clear at amount of the liquid epoxy resin in a thin elastic shell was 130Delaminations in composite structures. V. V. Bolotin (a) (b) (c) Figure 1 Three types of delaminations: (a) internal, (b) near-surface and (c) multiple cracking I Figure 2 / (a) (b) Pioneering studies in mechanics of delaminations: (a) ObreimofFs study in splitting of mica, (b) Kachanov's problem of the compressed laminate tube 0 Figure 3 Epoxy resin properties variation during the thermal treatment: 0 is shrinkage ratio, and E is the 10 s Youn'g's modulus 3 DELAMINATIONS ORIGINATING IN THE MANUFACTURING PROCESS High strength of most laminated and fibrous composites in the direction of reinforcement is accompanied by low resistance against interlaminar shear and transverse tension. Therefore, the interlaminar cracks can originate both on the fabrication stage and on the stages of transportation, storage and service. Instabilities of the manufacturing process, imperfections of various natures, and thermal and chemical shrinkage of components may be the source of initial delaminations 9. Delaminations in large-scale composite structures were met in the design of the deep underwater vehicle for ocean research. The vehicle was designed as a stiffened spheroidal shell of glass/epoxy laminate. A set of multiple cracks was found in pilot specimens of these shells, and the stated objective was to avoid these defects. It was evident that these cracks were produced when the transverse tensile stresses occurred on the manufacturing stage. But it was not clear at 7,k//m 2 (a) (b) (c) Figure 4 Impact damage tests of specimens: (a) spheroidal-head impactor; (b) flat-head impactor; (c) three-point impact testing 2.0 1.6 1.2 0.8 20 .................. _o ......... _o__0__ ~|~"~O ~ l 0 • • 0 • 0 [] -{3 [] A O //° I I I I -40 -20 0 20 ~mm Figure 5 Distribution of the residual specific fracture work along the specimen; lines 1-3 correspond to three increasing magnitudes of the impactor energy 5 I I IO z~mm Figure 6 Distribution of summed cracked area along the depth of a specimen; coordinate z is measured from the impacted surface that time which component of the shrinkage of the epoxy resin is more responsible for the occurrence of tensile stresses: thermal, chemical, or both. It depends, obviously, on how the variation of shrinkage and compliance correlate in timel°'ll. To study the shrinkage and compliance in situ, an amount of the liquid epoxy resin in a thin elastic shell was 130