An experimental approach that assesses in-situ micro-scale damage mechanisms and fracture toughness in thermoplastic laminates under out-of-plane loading

Abstract

Studying the response of laminated composites under out-of-plane loading routinely involves mechanical tests, such as quasi-static indentation or impact. The phenomenology during these tests is so complex that it is difficult to identify different material properties related to each failure mechanism (damage mode). We aim at providing an experimental approach, which is practical and fast, for assessing the in-situ micro-scale damage mechanism and extracting the fracture toughness in thermoplastic laminates under out-of-plane loading. To this end, we developed a dedicated, micro-scale, three-point bending (micro-3PB) test fitted inside a scanning electron microscope (SEM). In a single experiment, we were able: (i) to assess the initiation of a transverse crack, the transverse crack-to-delamination transition, delamination growth, development of shear-induced microcracks during delamination, and fibrillation, and (ii) to evaluate the effective fracture toughness during transverse cracking and delamination under a representative out-of-plane loading. We used this approach to rank two types of glass fiber-reinforced polypropylene cross-ply laminates, i.e., based on either homopolymer PP (ductile matrix) and copolymer PP (less-ductile matrix), according to their relative fracture parameters. We also performed short edge notch bending (SENB), double cantilever beam (DCB) and end-notch flexure (ENF) to obtain the standard fracture toughness values. We found that the relative fracture toughness values obtained by SENB, DCB and ENF are comparable with that of micro-3PB results. Furthermore, ENF results showed that the delamination process during micro-3PB is dominated by Mode-II fracture.

Revealing the effects of matrix behavior on low-velocity impact response of continuous fiber-reinforced thermoplastic laminates

“…Table 3 provides a summary of the tensile and fracture properties of GF-PP and GF-IPP, which was already reported in Refs. [39,31,42]. GF-PP is stronger and stiffer than GF-IPP in longitudinal, transverse and in-plane shear directions.…”

“…Tensile and fracture properties of GF-PP and GF-IPP[31,42]. Mode I intralaminar fracture toughness, kJ/m 2 0.37 ± 0.07 0.33 ± 0.04 Mode I interlaminar fracture toughness, kJ/m 2 0.74 ± 0.05 0.21 ± 0.02 Mode II interlaminar fracture toughness, kJ/m 2 3.92 ± 0.41 0.92 ± 0.02 elastic energy and its vulnerability towards fiber fracture and perforation.…”

Parallel Computing with the Thick Level Set Method

“…As a result, overall speedups are similar for the two cases. [31,4]. This process begins with an array of inclined cracks perpendicular to the maximum principal stress, which eventually merge to form a single crack on a higher level of observation.…”

“…The high computational demand comes from the fact that the TLS requires element sizes smaller than the width of the damaged band in order to achieve a desirable accuracy, especially for the computation of nonlocal quantities [3,11], giving rise to a system of equations with many degrees of freedom (DOFs). This issue can be amplified if the size of the width of the damage band is constrained to be small relative to the geometry of the problem being investigated like, for instance, interlaminar cusp formation in a polymer matrix of composite materials subjected to mixed mode loading condition [31], which takes place in a very narrow area and involves multiple cracks that eventually merge. Additionally, in order to guarantee numerical stability of the level set update, the damage front advance is constrained such that it does not move more than one element length per time step [3,11,30,18].…”

Numerical Simulation of Transverse Crack on Composite Structure Using Cohesive Element

“…The strength in the transverse 2 of 15 direction is determined mostly by the strength of the matrix and fiber-matrix bonding. For a multidirectional composite laminate, failure usually starts at the ply enduring the transverse load [5]. Failure due to transverse loading usually starts either with matrix transverse crack or fiber-matrix debonding.…”