Issue 78

R. Shivashankar et alii, Fracture and Structural Integrity, 78 (2026) 454-473; DOI: 10.3221/IGF-ESIS.78.26

where (b) represents the specimen width (mm), (d) represents the specimen thickness (mm), and (F) represents the maximum load at failure (N). The produced hybrid nanocomposites' fiber–matrix interfacial bonding and delamination resistance were assessed using the acquired ILSS values. Flexural test To test flexural properties of fabricated composites, a three-point bending test was performed according to ASTM D790 [23]. Tests were performed on a universal testing machine (Zwick/Roell 2020), having a load cell of 2.5 kN and a crosshead rate of 2 mm/min, with a support span of 70 mm. Specimens were loaded continuously until failure, and the recorded peak load was used to calculate flexural strength via Eqn. (2).

2 3 F L 2 b d



σ

(2)

f

In Eqn. (2), f σ is the flexural strength (MPa), (F) is the maximum applied load (N), (L) is the support span (mm), (b) is the specimen width (mm), and (d) is the specimen thickness (mm). Flexural modulus ( f E ) was determined using Eqn. (3):

3

mL

(3)



E

f

3

bd

4

where, m is the slope of the load-deflection curve (N/mm). Fracture toughness test

To minimize the amount of damage caused by the machining process, single-edge-notched (SEN) samples were cut from each composite laminate using water-jet machining. The crack length was taken with a stereomicroscope EMZ-5 (Meiji Techno Co. Ltd., Japan) at a magnification of 1.5× with a precision of ±0.1 mm, while the specimen dimensions were taken with a digital electronic caliper Powerfix (OWIM GmbH & Co. KG, Germany) with a precision of ±0.01 mm, and the average of the three measurements taken at each specimen was used in the calculations. Three-point bending was used to evaluate the Mode-I fracture toughness (K IC ) of the composites according to ASTM D5045 [24] on a universal testing machine (Zwick/Roell 2020) with a 2.5 kN load cell. The SEN specimen was loaded at a constant cross head speed of 0.5mm/min until the crack propagates and final fracture takes place. The critical fracture load obtained from the load–displacement curve was used to calculate the fracture toughness according to Eqn. (4).

FL

 

  

 

(4)

 

K

f x

IC

1.5

bw

where (F) is the fracture load, (L) is the support span, (b) is the specimen thickness, and (w) is the specimen width. (a) is the notch length. The geometry calibration factor f(x) was determined using Eqn. (5):

1 2







 

2

 3 2 6 1.99 1 (2.15 3.98 2.7 1 2 1 x x x x x x x         

 



(5)

f x

a

, 



0 1 x   (a) is the notch length and (x) represents the normalized crack length ratio.



x

W

Scanning electron microscopy Field-emission scanning electron microscopy (FESEM) was used to describe the fracture surface morphology of the composite specimens. A LEO Supra 35 FESEM system (Zeiss, Wetzlar, Germany) with a secondary electron detector and an accelerating voltage of 5 kV was used for the analysis. To improve surface conductivity and reduce charge effects during electron beam exposure, the fragmented specimens were vacuum sputter-coated with a thin layer of gold for two minutes prior to analysis. The fracture characteristics of the composites, such as fiber–matrix interfacial adhesion, Si ₃ N ₄ nanoparticle

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