Delamination effect on the impact toughness of an ultrahigh carbon–mild steel laminate composite

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Abstract

A multilayer composite material composed of 70 alternate layers of ultrahigh carbon steel (UHCS) and mild steel (MS) has been processed by hot roll bonding in two steps. First, seven layers were rolled to obtain a block. Second, 10 blocks, each one containing seven layers, were rolled to obtain the composite. Therefore, two types of interfaces were present in the laminate. Damage tolerance properties of the laminated composite material have been studied by Charpy impact, three-point bending and shear tests. The impact toughness of the composite is 70 times higher than for the UHCS and 1.5 higher than for the MS. This improvement is due to delamination at the interfaces between the blocks. This mechanism obstructs the propagation of the dominant crack causing its deflection along the interface and forcing renucleation of cracks in the adjacent layer. Three-point bend and shear tests were performed to evaluate the fracture toughness of the laminate and the bond strength of the single layers in the composite. This study reveals the role of both types of interfaces on crack deflection for increasing the damage tolerance of the laminate composite material.

Introduction

Multilayer systems have been designed to enhance the damage tolerance of their constituent materials as a result of the presence of interfaces between layers in the laminate. The improvement of the impact behavior results from the ability of the laminate to absorb additional energy during damage development. With this purpose, a number of studies have previously been developed [1], [2], [3], [4].

Ultrahigh carbon steels (UHCS) containing high carbon concentration are known for its high strength and improved wear resistance but low damage tolerance such as fracture toughness and impact resistance [5], [6]. Recent studies have shown an improvement of the impact properties by combining these steels with other more ductile steels [7], [8], [9], [10].

Damage tolerance properties are essentially related to resistance to nucleation and crack propagation. Intrinsic factors of materials, influenced by its microstructure, control the nucleation of cracks. On the other hand, the crack propagation depends strongly on the extrinsic mechanisms of fracture such as crack blunting, crack deflection and crack bridging which are influenced by delaminations at the interfaces [4], [11].

The objective of this investigation is to study the delamination effect on the impact toughness of multilayer laminated composite materials. For this purpose, a laminate constituted by 70 alternate layers of ultrahigh carbon and mild steel was processed in two steps to obtain two types of interfaces in the laminate. The influence of these two types of interfaces on the mechanical behavior of the laminate composite has been studied by means of three-point bending and shears tests.

Section snippets

Materials and experimental procedure

A laminate composite material was prepared containing alternate layers of ultrahigh carbon steel (UHCS-1.1%C) and mild steel (MS-0.035%C). The chemical compositions of both steels are listed in Table 1.

The multilayer laminate was processed by hot roll bonding at temperatures higher than A1. This laminate, having a 68% volume fraction of UHCS, was processed in two steps as follows: in the first step, four layers of UHCS were piled up with three layers of MS alternately to form a block of seven

Microstructural characterization

Fig. 1 shows the microstructures of the constituent materials in the initial state. The MS-0.035%C (Fig. 1a) possesses a ferritic grain size of about 50 μm and small amounts of cementite (Fe3C)–iron carbide. In contrast, the UHCS-1.1%C material (Fig. 1b) exhibits a microstructure consisting of pearlite, where the pearlitic colonies are about 15 μm in size. Proeutectoid carbides are present at grain boundaries as well.

After processing the laminate by hot roll bonding, fine-grained and homogeneous

Conclusions

A laminate composite material composed by 70 alternate layers (10 blocks of seven layers) of ultrahigh carbon steel (UHCS-1.1%C) and mild steel (MS-0.035%C) has been produced in two steps by roll bonding. Due to this processing, two types of interfaces were formed: interfaces between layers subjected to both rolling steps (ε = 3.1) and interfaces between blocks subjected only to the second rolling step (ε = 1.1).

The multilayer laminate composite exceeded the impact toughness of the monolithic

Acknowledgements

The authors gratefully acknowledge the support of Comisión Interministerial de Ciencia y Tecnología (CICYT) under Grants MAT2000/2017 and MAT2003/1172. Also to the Comunidad de Madrid for the grants 07N/0006/2001 and 07N/0065/2002. Thanks are given to L. del Real Alarcón for the welding work, to F.F. González Rodríguez for assistance during hot rolling and to J. Chao Hermida for assistance with the Charpy impact tests.

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