Specimen design, manufacturing and testing procedures for flat carbon fiber reinforced plastic laminates under biaxial loading

Abstract

The optimization of a specimen design allowing the investigation of the biaxial strength of composite laminates over the full range of failure strain was the primary objective of this work. Multiaxial strength criteria are often found unreliable mainly as a result of the inherent complexity of biaxial tests and, in many cases, as a result of inefficient specimen designs. As a result of a development program combining numerical simulations and experimental measurements, a flat cruciform‐shaped specimen has been developed for carbon fiber reinforced plastic (CFRP) laminates. The design fulfills the basic criteria for such a specimen, namely allowing for a uniform biaxial stress/strain state to exist in the gauge area and for testing the virgin material up to failure in both the tension‐tension and tension‐compression quadrants of the strain/stress space. The fabrication of the specimen is described and a three‐step testing procedure for generating biaxial strength data is proposed. Typical results obtained from specimens of the proposed configuration tested in accordance with this procedure are presented. Results compare well with those obtained from tubular specimens, thus confirming the effectiveness of the proposed design. Experimental data obtained for the AS4/3501‐6 carbon/epoxy composite system are finally compared against strength predictions of recognized failure theories.

Comparison of Different Techniques for Strain Monitoring of a Biaxially Loaded Cruciform Specimen

“…Systems belonging to either category are used successfully on biaxially loaded planar specimens in the past and present. In [14, 15], strain gages in the centre of the specimen and on several well‐chosen locations are used to retrieve strain data and validate a FE model. A biaxial extensometer was first developed by Makinde et al.…”

“…A biaxial extensometer was first developed by Makinde et al. [16] and similar ones are used in more recent works [15, 17]. Also, full‐field techniques such as electronic speckle pattern interferometry (ESPI) [18] and digital image correlation (DIC) [19] are applied successfully to analyse the strains at the surface of cruciform specimens.…”

Recent Development of Planar Cruciform Experiment on Biaxial Tensile Deformation and Failure of Unidirectional Glass/Epoxy Composite

“…Proper design of multiaxial composite specimens is a complex task [13][14][15][16][17][18][19][20][21][22][23][24][25]. Basic requirements for the design are [1]:…”

“…Cruciform specimens are, in theory, well suited [5,6] for biaxial composite tests under combined in-plane transverse and longitudinal stresses, when the fiber orientation is aligned with specimen loading arms. Proper specimen design is complicated by load sharing and differential deformations between the gage section and adjacent loading arms due to Poisson's effect [13][14][15][16][17][18][19][20]. Cruciform specimens have been used successfully to study cross-ply and other multidirectional laminates [13][14][15].…”

“…Proper specimen design is complicated by load sharing and differential deformations between the gage section and adjacent loading arms due to Poisson's effect [13][14][15][16][17][18][19][20]. Cruciform specimens have been used successfully to study cross-ply and other multidirectional laminates [13][14][15]. Thickness-tapered cruciform specimens were successfully used for determination of the complete failure envelope for biaxial normal stresses of a cross-ply carbon/epoxy [14].…”