ABSTRACT:The apparent tensile strength of technical flax fibers was determined in single-fiber tests at various clamping lengths (20, 40, and 80 mm) and the outcome was compared with literature data. It was demonstrated that the strength of flax at each clamping length obeyed the twoparameter Weibull model. The failure mode and sequence were studied in situ (i.e., during loading) by SEM and acoustic emission (AE). The failure sequence (axial splitting of the technical fiber along its elementary constituents, radial cracking of the elementary fibers, multiple fracture of the elementary fibers) concluded reflected the hierarchical build-up of the flax bast fibers. To the above failure events AE amplitude ranges were assigned.

Thermoplastic starch (MaterBi®) based composites containing flax fibers in unidirectional and crossed‐ply arrangements were produced by hot pressing using the film stacking method. The flax content was varied in three steps, viz. 20, 40 and 60 wt.‐%. Static tensile mechanical properties (stiffness and strength) of the composites were determined on dumbbell specimens. During their loading the acoustic emission (AE) was recorded. Burst type AE signal characteristics (amplitude, width) were traced to the failure mechanisms and supported by fractographic inspection. The mechanical response and failure mode of the composites strongly depended on the flax content and the flax fiber lay‐up. It was established that the tensile strength increases until 40 wt.‐% flax fiber content but stays almost constant above this value. In the case of 40 wt.‐% unidirectional fiber reinforcement, the tensile strength of the composite was 3 times greater than that of the pure starch matrix. The flax fiber reinforcement increased the tensile modulus of the pure starch by several orders of amplitude.SEM picture of the fracture surface of a composite with UD flax reinforcement.magnified imageSEM picture of the fracture surface of a composite with UD flax reinforcement.

Abstract. In this work carbon fiber/epoxy composites and MWCNT (multiwalled carbon nanotube)/carbon fiber/epoxy hybrid composites with 0.1, 0.3, 0.5 and 1 weight% nanotube filling of the matrix have been prepared and compared in terms of interlaminar properties. For the dispersion of the carbon nanotubes in the epoxy resin three roll milling has been utilized. The mechanical characterization has been carried out using standard DCB (double cantilever beam) tests assisted by acoustic emission. The test results have been evaluated by the conventional method provided by the standard and a novel method implementing acoustic emission signal localization for crack propagation tracing. According to the tests carbon nanotube filling of the matrix of the composites has a beneficial effect on their interlaminar properties: the interlaminar fracture toughness values of the composites have increased by a maximum of 13 and 33% at a 0.3 weight% carbon nanotube content of the matrix according to the conventional and AE based evaluation method respectively.

This review aims at showing how the location of acoustic emission (AE) in loaded polymer composites can be used to get a deeper insight into the onset and growth of damage, and associated failure events and sequences. Different location methods (experimental and theoretical) are briefly introduced along with AE characteristics in time and frequency domains. Linear (1D), planar (2D), and spatial (3D) locations of AE are examined through selected examples. The cited works demonstrate the versatile use of AE. Apart from damage and failure assessment, AE can be used to reconstruct crack growth, and thereby help determine the accurate fracture mechanical parameters. Unlike the detection of the development of damage, the identification of failure mechanisms by analyzing selected AE signal parameters, including their clustering, requires further research. Unraveling the failure mode is, however, a key topic with respect to structural integrity, residual strength, and lifespan expectation of composite parts. A recent major challenge is to establish a reliable, real-time structural health monitoring system making use of located AE events monitored by built-in sensors.

Shape memory characteristics of a woven glass fibre fabric reinforced epoxy composite (reinforcement content: 38 vol.%) were assessed in three-point bending mode in a dynamic mechanical analysis device and compared to those of the parent epoxy resin. From unconstrained tests, the shape fixity and recovery ratios and the recovery rate, whereas from constrained tests the recovery stress were determined. The shape fixity and recovery rate decreased due to the glass fibre reinforcement which had, however, no effect on the shape recovery. Major benefit of the woven glass fibre fabric was that the recovery stress could be enhanced by two orders of magnitude in comparison to the neat epoxy. Glass fibre reinforcement was accompanied with a substantial decrease in the failure-free flexural deformability of the composite specimen.