EXPERIMENTAL STUDY OF THE EFFECTS OF LAYERS ORIENTATION ON MODE I INTERLAMINAR FRACTURE TOUGHNESS USING FULLY UNCOUPLED MULTIDIRECTIONAL SPECIMENS

Nowadays, composite materials are used for a variety of applications, including safety-critical structures, whose design requires a damage tolerance approach. For laminated composites, delamination is one of the most critical damage modes. In literature, delamination is studied using Linear Elastic Fracture Mechanics (LEFM) concepts, and interlaminar fracture toughness of a material is determined, in pure mode and mixed-mode conditions, by means of standard test procedures [1-3]. However, the validity of such standards is restricted to unidirectional (UD) specimens in which all layers have fibres oriented along the longitudinal direction of the specimen (0°). Real structures, instead, are built using multidirectional (MD) layups and delaminations may occur at any interface. Hence, characterisation of interlaminar fracture toughness of the interface between plies oriented at different angles is required. Unluckily, experimental characterisation of interlaminar fracture toughness in such interfaces is still an open problem due to the following issues: • During delamination, additional damage mechanisms may appear such as intra-ply matrix cracking within off-axis plies; this may cause the delamination to jump to another interlaminar plane. • MD laminates show a complex coupled mechanical behaviour. Due to couplings, thermal residual stresses may appear during curing and they affect fracture toughness evaluation; additionally, couplings induce displacement and strain fields that may introduce undesired parasite modes contributions2. • Most data reduction techniques are based on 2-dimensional theoretical formulations, whose validity for MD laminates is not granted To overcome such problems, a special class of stacking sequences, called Fully-Uncoupled Multi-Directional (FUMD), has been developed and presented in [4]. These sequences are conceived to have null coupling terms, in the framework of Classic Laminated Plate Theory (CLPT), for the entire sequence itself and for its upper and lower halves, which form the arms of a typical standard delamination specimen [1-3]. In this study, for the first time, FUMD stacking sequences were used to design Double Cantilever Beam (DCB) specimens for mode I interlaminar fracture toughness characterisation. Five FUMD specimen sets were fabricated with different delamination interfaces (indicated by a double slash): 1. FUMD 0°//0°; 2. FUMD 0°//15°; 3. FUMD 0°//30°; 4. FUMD 0°//45°; 5. FUMD -45°//45°; where orientation 0° is aligned with the longitudinal direction of the DCB specimen. In addition, standard UD specimens (thus having 0°//0° delamination interface) were fabricated as well. A glass/epoxy prepreg fabric with 10% fibres in the fill direction was used, as this is expected to reduce matrix cracking problems in off-axis plies and hence prevent delamination jump phenomena. Sequences FUMD 0//0 and UD have the same delamination interface but different global stiffness: results from their tests allow to assess if, for a fixed delamination interface, global stiffness of the specimen play a relevant role in fracture toughness evaluation, as some authors suggest. On the other hand, sequences FUMD 0//0, FUMD 0//45 and FUMD -45//45 have different delamination interfaces, but identical global stiffness: their tests allow to observe whether fracture toughness is affected by local effects, such as orientation of plies embedding the delamination plane and of adjacent plies at most. Results show that global stiffness has not relevant effects, while layers orientations plays a major role in changing interlaminar fracture toughness of the interface. REFERENCES [1] ASTM-D5528-13, Standard Test Method for Mode I Interlaminar FractureToughness of unidirectional Fiber-Reinforced Polymer Matrix Composites, ASTM Interntional, West Conshohocken, PA. [2] ASTM D7905/7905M-14, Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer, ASTM Interntional, West Conshohocken, PA. [3] ASTM 6671/D 6671M-06, Standard Test Method for Mixed Mode I-Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites, ASTM International, West Conshohocken, PA. [4] T. Garulli, A. Catapano, D. Fanteria, J. Jumel, E. Martin, Design and finite element assessment of a Fully-Uncoupled Multi-Directional (FUMD) specimen for delamination tests, Composites Part B, paper submitted