JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011 - 41

In order to reduce fibre kinking and misalignment, a preliminary square array of spikes was chosen for the metallic sheet. This is also expected to reduce the amount of stress concentrations that would otherwise be present if the prepreg was consolidated with more fibre waviness. Optimizations of spike geometry and array are currently under investigation.

Materials and experimental techniques
Specimen fabrication Five series of single-lap shear joint specimens were manufactured to evaluate the RHEA joint performance: a reference series where the CFRP specimens were purely bonded, specimens with 0.2- and 0.4mm metallic reinforcements of steel and titanium. Five specimens of each series were tested. The CFRP adherend materials employed were uni-directional prepregs. A Hexply M21/194/T800S prepreg was used for the reference samples. The metal-reinforced RHEA specimens were manufactured from Hexply M21/134/T800S. Both materials were supplied by Hexcel. All specimens received an additional adhesive film (FM300M from Cytec) at the bond line. The reference samples had a [+45°/45°/0°/90°/0°]s stacking sequence, whereas the 3D reinforced samples had the quasi-isotropic sequence [+45°/-45°/0°/90°/45°/+45°/0°/90°]s. The metal reinforcements used were a standard stainless steel (304/ DIN1.4301 at 0.2 and 0.4 mm) and 2 cold-formable titanium alloys (Ti 3-2.5 at 0.2 mm and Ti 15-3-3-3 at 0.4 mm). The properties of the metal materials are shown in

Tab. 1: Mechanical properties of the metal materials 1.4310 /304 Ti 3-2.5 620 20 95-105 43-45 4.5

most likely to initiate. After the first curing cycle, the overlap region was pre-treated to ensure a good bonding quality. The cured laminate was then bonded to an uncured prepreg laminate, using an additional adhesive film in a special mould, therefore ensuring the correct geometry of the single-lap shear joint. The geometry of the tested joints is illustrated in Fig. 3. Tensile test set-up All tensile tests were executed on a Zwick Z250 machine with a load range up to 250 kN, which records reaction forces with a tolerance of 0.5%. The bond-line delamination strength (as described in Ref. 15), ultimate failure strength and elongation limit of the single-lap joints were verified under displacement-controlled static loading with a crosshead velocity of 13 mm/min. The crosshead displacements and the specimen displacements at the overlap region were recorded with an 80-mm extensometer. The Digital Image Correlation Method (DICM) This technique was utilized for measuring surface displacements and strains from digital images during the tensile tests of one specimen from each series, using a measuring system called ARAMIS. One of the specimen’s edges in the overlap region was sprayed with a silver paint to form a speckle pattern on the surface. This surface was then monitored by two cameras and an image analysis algorithm was used to compute the three-dimensional displacement vector of each point in the speckle pattern over two consecutive images (Ref. 16).

Table 1. The single-lap shear specimens were manufactured following the co-bonding procedure described previously. The RHEA reinforcements were strategically applied to the ends of the overlap joint configuration. These areas correspond to the regions where the highest shear and peel stresses occur in a single-lap joint (Ref. 14), hence the area where delaminations and debond cracks are

Fig. 3: Geometry of the single-lap joints Elongation % 45 Young´s modulus (GPa) Shear modulus (GPa) Density (g/cm3) 86 7.9

Tensile strength (MPa)

520-720

200-210

No69 November - December 2011 / jec composites magazine
Ti 15-3-3-3 787 21 81-84 39-42 4.8

Results and discussion

Static tensile tests The shear stress displacement curves for all the joint specimens are illustrated in Fig. 4. The displacement data shown are those recorded by the extensometer during testing. The shear stress is calculated by dividing the applied tensile load by the overlap area. Table 2 presents the properties and failure modes of the different specimens. It was observed that the reference sample curve rises almost linearly until a peak failure stress of 12.56 MPa is reached. Once the resin bond line at the end of the overlap has failed, the damage progresses rapidly until the entire overlap region is completely debonded. This illustrates unstable damage propagation for the purely bonded joint. Damage initiation is governed by the fracture properties of the epoxy matrix and the stress concentrations present at the ends of the overlap in a single-lap joint. Given the brittle nature of the epoxy matrix and an energy release rate that increases with crack length (a geometric factor of the joint), once

Research & development

JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011