Identification of the effect of nanofiller morphology on interlaminar fracture toughness of hybrid composites

Nanoscale reinforcements have the potential to improve mechanical properties of fiber reinforced composite. Here, effect of nanofiller morphology and dispersion in augmenting mode I fracture toughness of unidirectional carbon fiber reinforced composite materials is studied. The nanofillers used is electrospun carbon nanofibers (CNFs). Unlike most nanofillers which are in particulate form, CNFs exist in both continuous nanofiber mat and particulate forms. This trait allowed us to compare the effect of particulate nanofillers (CNFs dispersed in B-staged epoxy) vs. dry mats on fracture toughness of composites while all other parameters are kept constant. To enhance CNFs-matrix interactions, a novel approach was utilized to functionalize CNFs surface with melamine, so that epoxy functional groups can form strong bonds to matrix. The improvement in mode I initiation fracture toughness with CNF mats was statistically significant, while in B-staged samples, statistical analysis revealed insignificant improvement. In addition, in both CNFs reinforced samples, crack propagation fracture toughness decreased with crack growth and approached that of the composites with no CNFs. The decline was steeper in samples with B-staged CNFs. This behavior was explained by evaluating fracture path via SEM imaging. It was concluded that while CNFs bridge crack tip initially and delay crack initiation, crack deflects towards a lower resistance path by tearing CNFs mat and propagating along resin-rich interface between CNFs and microfibers. These alternative and weaker fracture planes are more readily available in B-staged samples due to poor integration of the B-staged epoxy with the rest of the composite.

Crack Initiation and Propagation in Static Loaded Fracture Mechanics Tests in Steels Containing Atomic Hydrogen

Hydrogen is well known to have a detrimental influence on the ductility of low alloy steels, reducing the fracture toughness. Standard test methods to characterize fracture toughness of steels in terms of ductile tearing resistance curves have not been developed to account for any hydrogen-driven contribution to the crack extension, Δa. Simply plotting J or CTOD against Δa is not necessarily appropriate for defining the initiation fracture toughness for tests performed in a hydrogen-charging environment. This paper explores a method to further analyse experimental data collected during fracture toughness tests, which allows the contribution of plasticity (i.e. when blunting precedes ductile tearing) to be considered separately from the initiation of crack extension (which could be by stable tearing and/or by hydrogen-driven crack extension). The principle is based on the assumption that a crack growing by a hydrogen-driven mechanism in a quasi-static fracture mechanics test performed in environment may not be associated with significant ductility in the plastic zone (which would accompany crack growth by stable tearing). The analytical method presented in this paper compares the different points of deviation from linear behavior of the components of J, to isolate the effects of ductility within the plastic zone from pure crack extension. In this way, the point of crack initiation can be defined in order to determine the relevant initiation fracture toughness; whether by blunting and stable tearing, or by hydrogen-driven crack growth. This approach offers a screening method which is illustrated using examples of fracture mechanics specimens tested in environments of varying severity (air, seawater with cathodic protection, and sour service). This method can be used to identify the relevant definition of initiation fracture toughness while allowing for a combination of ductile tearing, hydrogen-driven crack extension, or both, to be present during the test.

Damage monitoring of glass/epoxy laminates with different interply fiber orientation using acoustic emission

This article investigates the effects of interface fiber orientation on the mode-I interlaminar fracture toughness, and the evolution of damage was monitored using acoustic emission technique. The results show that changing the interply fiber orientation improved the fracture toughness and delamination resistance significantly. The GIC initiation fracture toughness was found to increase by 87%, 71%, and 69% for 0°/45°, 0°/90°, and ±45° interfaces, respectively. The improvement in fracture toughness was attributed to the crack path diversion along the interply fiber orientation which offered more resistance to crack propagation. In addition, sentry function was also computed based on the correlation between the mechanical strain energy accumulated in the laminates, and the acoustic energy that propagates by fracture events made it possible to evaluate the delamination resistance. The results show that 0°/45° has high resistance to crack propagation followed by 0°/90°, ±45°, and 0°/0°. Finally, scanning electron microscopic analysis was used to determine the fracture surface in different interply fiber orientation of glass/epoxy laminates.

Enhancing the Fracture Toughness Properties by Introducing Anchored Nano-Architectures at the Metal–FRP Composite Interface

This paper presents a novel technique for improving aluminium–glass/epoxy composite interfacial bonding through the generation of metallic nano-architectures on the metal surface. Silver nanowires (AgNWs) deposited via solution casting at varying concentrations and annealed at different temperatures in an air atmosphere improved the aluminium-glass/epoxy composite fracture toughness as measured via mode I experiments. For AgNW concentrations of 1 and 3 g/m2 deposited via a single-stage process and annealed at 375 °C, the initiation fracture toughness of the aluminium-glass/epoxy composite improved by 86% and 157%, respectively, relative to the baseline composite without AgNWs. The corresponding steady-state fracture toughness of these nano-modified fibre metal laminates (FMLs) were at least seven times greater than the baseline composite. The FML variant in which AgNWs were deposited at a concentration of 3 g/m2 through a two-stage process followed by annealing at 375 °C and 300 °C, respectively after each deposition, achieved the highest steady-state fracture toughness of all nano-modified composites—a fracture toughness value that was 13 times greater than the baseline composite. Intrinsic and extrinsic toughening mechanisms dictated by the morphology of the silver nano-architectures were found to be responsible for the improved initiation and steady-state fracture toughness in nano-modified FMLs.

Study of Testing Method for Dynamic Initiation Toughness of Sandstone under Blasting Loading

In this paper, an internal central single-cracked disk (ICSCD) specimen was proposed for the study of dynamic fracture initiation toughness of sandstone under blasting loading. The ICSCD specimen had a diameter of 400 mm sandstone disc with a 60 mm long crack. Blasting tests were conducted by using the ICSCD specimens. The blasting strain-time curve was obtained from the radial strain gauges placed around the blast hole. The fracture initiation time was determined by circumferential strain gauges placed around the crack tip. The stress history on the blast hole of the sandstone specimen was then derived from measured strain curve through the Laplace transform. The numerical solutions were further obtained by the numerical inversion method. A numerical model was established using the finite element software ANSYS. The type I dynamic stress intensity factor curves of sandstone under blasting loading were derived by the mutual interaction integration method. The results showed that (1) the ICSCD specimen can be used to measure dynamic initiation fracture toughness of rocks; (2) the stress on the blast hole wall can be obtained by the Laplace numerical inversion method; (3) the dynamic initiation fracture toughness of the ICSCD sandstone specimen can be calculated by the experimental-numerical method with a maximum error of only 7%.

Fracture toughness of unidirectional flax fiber composites with rigid gliadin matrix

The aim of this study is to improve the interlaminar fracture toughness of flax/gliadin composites. Some key parameters are used such as processing conditions, matrix plasticization, and fiber treatment. These parameters affect the crack-growth resistance since they are, respectively, related to the cross-linking density by varying the cooling conditions, to the reduction of the brittleness by adding a low amount of plasticizer, and to the quality of the fiber–matrix interface. Results show that interface quality is more dominant than the effect of a low amount of plasticizer for the initiation fracture toughness value. While crack initiation is closely related to weak links such as the fiber–matrix interface, crack propagation appears to be mainly in the matrix.

Inter-Laboratory Results and Analyses of Mini-C(T) Specimen Testing of an Irradiated Linde 80 Weld Metal

An irradiated low-upper-shelf Linde 80 weld metal has been tested by four laboratories as part of an inter-laboratory assessment of use of the miniature compact tension [mini-C(T)] test specimen for Master Curve fracture toughness evaluation following ASTM E1921. The preliminary results from each of the laboratories have been compiled and evaluated together to assess the validity and use of the mini-C(T) specimen for an irradiated reactor pressure vessel material which can exhibit ductile crack growth at low temperatures relative to cleavage initiation fracture toughness. The preliminary results from this mini-C(T) testing can also be compared to extensive specimen test results from larger C(T) specimens of the same irradiated material. Comparisons of the results from each of the laboratories and some inter-laboratory differences in the fracture testing are assessed. The evaluations indicate reasonable agreement between the mini-C(T) and larger specimen results, but the selection of test temperature and the number of test specimens needed to obtain reliable results are more difficult when testing a low-upper-shelf toughness material.