JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011 - 42

Tab. 2: Static properties of the single-lap joints

Fig. 4: Shear stress extensometer displacement curves for the lap joints

joining

Specimen

Bond line shear strength (τinit, MPa) 12,55

Ultimate shear strength (τdeb, MPa) 12.55

Maximum elongation (mm) 0.38

Reference 0.2 RHEA steel

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The results for the RHEA joints demonstrate a similar linear behaviour at the beginning, showing that the initial joint stiffness is not drastically diminished by the presence of the metallic spikes. An initial drop in stiffness is encountered between 7 and 9 MPa, which denotes the initiation of a debond crack. The bond line initiation strength described in Table 2 corresponds to the shear stress values for each specimen at the initiation of this debond crack. The joints containing the metal reinforcements undergo two other stiffness changes, the trend being that the joints c d a b become more compliant as the damage progresses. This signals a gradual failure mechanism of the joint, with the metallic Fig. 5: Strain distribution for reference specimens at a) 7,500 N; b) 11,000 N; c) 14,500 N; d) 16,000 N spikes bridging the debond crack as it c d a b propagates across the joint. Similar crack bridging mechanisms have been observed for Z-pinned reinforced joints Fig. 6: Strain distribution for specimens reinforced with 0.2-mm steel at a) 7,500 N; b) 13,000 N; c) (Ref. 17-18), where small load drops 16,000 N; d) 18,600 N followed by an increase in load capacity c d a b correspond to the initiation and arrest of debond cracks. No clear trends were found regarding Fig. 7: Strain distribution for specimens reinforced with 0.4-mm steel at a) 7,500 N; b) 15,700 N; c) 18,800 N; d) 20,000 N what governs the bond line initiation
8.81 (-30%) 14.00 (+11%) 0.51 (+35%) 8.21 (-35%) 15.73 (+25%) 0.58 (+53%) 13.72 (+9%) 0.50 (+32%) 0.4 RHEA steel 32,559 33,032 33,087 0.2 RHEA titanium 7.36 (-41%) 0.4 RHEA titanium 8.72 (-31%) 14.58 (+16%) 0.58 (+53%)

strength values for the RHEA joints. However, a tendency is encountered when correlating the ultimate shear strength and maximum elongation to the physical and mechanical properties of the metal reinforcements. The joints containing steel reinforcements were able to withstand greater shear stresses than those with titanium due to the higher values of Young’s and shear moduli in steel. In the same way, the joint containing 0.4-mm reinforcements also showed a better performance than the 0.2-mm ones, achieving higher shear stresses and greater maximum elongations. The highest shear stresses were always experienced by the 0.4-mm reinforced specimens. Although steel has a much higher maximum elongation (45%) than titanium (~20%), no distinction was found between the specimens containing different metal reinforcements. The only factor that seems to be important in how much strain the joints can undertake is the thickness of the reinforcements. The RHEA joints Secondary Tertiary Initial elastic with 0.4-mm metal sheets achieved elastic elastic stiffness stiffness stiffness values over 10% higher than those (N/mm) (N/mm) (N/mm) containing 0.2-mm reinforcements, 34,340 which correspond to 53% and over 30% in comparison to the reference 32,608 22,859 23,167 specimens, respectively. DICM investigations Fig. 5-9 illustrate the displacements 27,599 23,500 and strains of the overlaps during tensile testing for all the joint 21,265 20,611 specimens. In all DICM investigations, the highest strains were always recorded at the ends of the overlap region. The reference specimens were able to withstand a crack of about 10.2 mm before the ultimate failure of the joint. The crack was initiated from one end of the overlap and propagated from that point on until the specimen was completely delaminated. It was not possible to accurately assess the crack length for each reinforced specimen. However, it is believed that ultimate failure only occurs once the crack front has passed the reinforced area at least at one end of the overlap. The RHEA specimens showed a slightly different behaviour. On
26,160 21,477

the damage is initiated, its propagation is bound to be catastrophic.

jec composites magazine / No69 November - December 2011
JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011

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