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The main aim of this research work is to study the influence of adhesive properties on the formability of adhesive-bonded steel sheets. The adhesive properties were varied by having two different adhesives, epoxy based and acrylic based, and by changing the hardener to resin ratios. The deep drawing quality cold rolled steel and stainless steel (SS 316L) sheets were used as base materials. The epoxy and acrylic adhesives show improved elongation with increase in hardener to resin ratio. This is because of changeover of resin-rich formulation to hardener-rich formulation, making the sample more ductile. The adhesive-bonded blanks show improved elongation as compared to double sheets, which is due to the presence of adhesive delaying the onset of necking. With increase in hardener to resin ratio of both the adhesives, the elongation of individual sheets has improved. This is due to the improvement in elongation of adhesives with increase in hardener to resin ratio. The strain hardening exponent (n) of adhesive-bonded blanks has improved with increase in hardener to resin ratio in all the regions of deformation. The limit strain of deep drawing quality and SS 316L sheets constituting adhesive-bonded blanks shows improvement with increase in hardener to resin ratio. The adhesive-bonded blanks with interface bonding exhibit better limit strain as compared to the case without interface bonding.

In this investigation, the influence of artificial finite adhesive defects with different shapes and sizes, characterized by (1) aspect ratios, a/b = 0.5, 1 and 1.5 (where a is transverse axis length of defect; b is longitudinal axis length of defect), (2) number of defects and (3) different locations, in the epoxy adhesive layer on the formability of adhesive-bonded blanks, is analysed. The deep drawing quality cold-rolled steel and stainless steel (SS 316L) sheets were used as base materials. The increase in aspect ratio of adhesive defect reduces the ductility of adhesive layer and thereby decreases the formability of adhesive-bonded blanks. This is due to the early failure of adhesive layer at the location of defect where strain is locally concentrated. The strain hardening exponent (n) of adhesive-bonded blanks has decreased with increase in aspect ratio of adhesive defect in all the regions of deformation. The limit strain of deep drawing quality and SS 316L sheets constituting adhesive-bonded blanks shows decrement with increase in aspect ratio of adhesive defect. It is postulated that the aspect ratio of the finite adhesive defect influences significantly the formability of adhesive-bonded blanks, but not shape of the finite adhesive defects. There is no considerable effect of number of adhesive defects and different locations of adhesive defects on the formability of adhesive-bonded blanks.

The mechanical properties of adhesive were evaluated by two different approaches to predict the formability of adhesive-bonded steel sheets by finite element (FE) simulations. The epoxy adhesive properties were varied by changing the hardener/resin (H/R) ratios. The true stress-strain behaviour of adhesive was evaluated from (i) direct tensile testing of adhesive samples (Approach-1) and (ii) rule of mixtures' concept applicable to adhesive-bonded sheets (Approach-2). The evaluated mechanical properties of adhesive through both the approaches were incorporated during simulation of tensile test and in-plane plane-strain formability test. The forming limit strains of adhesive-bonded sheets were evaluated during predictions by necking criteria, namely thickness gradient necking criterion (TGNC) and effective strain rate ratio based criterion (ESRC), and validated with the experimental limit strains. It is found that the limit strains predicted by both the approaches are moderately accurate with respect to experimental data. There is not much difference in limit strains of adhesive-bonded sheets obtained from Approach-1 and Approach-2. The TGNC shows better prediction as compared to ESRC. Further, the present study investigates the springback of adhesive-bonded sheets during V-bending with the influence of H/R ratio of adhesive, shims, and wire reinforcement. The results show that there is a considerable influence of H/R ratio on springback and about 8-20°reduction depending on the presence or absence of shim. But the wire reinforcement shows insignificant effect on V-bending of adhesive-bonded sheets.