Issue 73

A. Masmoudi et alii, Fracture and Structural Integrity, 73 (2025) 41-58; DOI: 10.3221/IGF-ESIS.73.04

Ultimate flatwise

Compressive strain (mm/mm)

Displacement (mm)

Pmax (kN)

compressive stress (MPa)

Sp 01

52.49

20.41

20.997

0.68

Sp 02

52.43

18.31

20.993

0.70

Sp 03

52.47

20.30

20.990

0.68

Average

52.48

19.67

20.99

0.69

SD

9.00E-03

1.18

3.50E-03

0.011

Table 6: Mechanical parameters obtained through flatwise compressive test of sandwich structures.

Edgewise compression test Fig.17a shows the failure modes of sandwich structures under edgewise compression loading for S-60. It is observable that Sp 03 behaved differently from the other specimens, this variability is a known characteristic of compression tests for sandwich structures, as slight differences in specimen alignment, material properties, or initial defects can significantly impact the failure mode and load response. An abrupt failure caused by skin buckling, succeeded by skin-core debonding on both sides for Sp 03 with a shear sliding damage of the skin on the left side. The other three specimens experienced the same behavior until final failure; localized buckling in the mid-section of the specimens followed by skin cracking on both sides and progressed throughout the width of the specimen and a small skin-core debonding on one side. Fig.17b depicts the load-displacement curves obtained from the test for S-60. The curve for Sp 03 demonstrates a decent linearity to ultimate load then the load decreased gradually to total failure. Meanwhile, the curves of the other three specimens exhibited a similar pattern and performed an initial phase of linear increase of the applied load, followed by a non-linear phase until the ultimate load is reached, then the load decreased gradually to final failure. The average ultimate load for those three is about 37.86 KN, which is roughly twice compared of Sp 03. Fig.18a illustrates the collapse modes of S-200, where a significant difference in failures mechanisms is noticed. For S-200, different behavior was detected, the specimens collapsed in the global buckling mode or as it known Euler buckling. A densification of the foam was observed during the test for both S-60 and S-200. Fig.18b represents the load-displacement curves obtained from the test for S-200. The curves exhibit a small linear pattern succeeded by a non-linear phase until the ultimate load reached its maximum. Followed by softening phase of post-buckling. The divergence in failure modes of S-60 and S-200 samples under edgewise compression load signifies that the lengths of samples have a crucial impact. When the length is higher than the width, Euler buckling becomes the dominant failure mode, and the load capacity of the samples decreases. The ultimate load decreased from 39.76 kN to 5.1 kN with the of the length of specimens increased. DIC engineering displacement and strain fields for edgewise compression test for Sp 02 for both S-60 and S-200 are shown in Fig.19. Point A in Fig.17a and 18a indicates where the images were taken, before the correlation is lost. The collapse modes of the specimens during this test are further clarified by the displacement along the horizontal direction U. The displacement U of S-60 is small compared to S-200. The maximum displacement for S-60 is 3.5 mm and it is concentrated in the region, where it first buckled and damaged. While the maximum displacement for S-200 is 14 mm in the middle part of the sample, with the top and bottom ends have minor displacement. Summarized results are listed in Tab. 7, for S-60 the average values of the test were calculated without including Sp 03. According to ASTM C 364M the ultimate flatwise compression strength was calculated using the Eqn. (2)

  max EC s P w t 2. .

(2)

where  EC the ultimate edgewise compressive strength (MPa). Pmax is the ultimate force prior to failure (N), w width of specimen (mm) and s t thickness of a single face sheet (mm).

53