Ultimate fracture toughness through thickness engineering

Introduction

The design of critical structural metallic components in the aeronautic, biomedical, nuclear, or pipeline fields is often dictated by a fail-safe criterion established in the context of fracture mechanics. The design must prevent any pre-existing crack from propagating under nominal but also sometimes accidental loads.

Fracture Toughness

In this context, the primary material property is the fracture toughness, which quantifies the resistance to crack initiation. In ductile metals, the fracture toughness depends on plate thickness, with a peak value attained at an intermediate thickness in the range of a fraction of a millimetre up to ten millimetres or more.

Literature Gap

Although this thickness dependence has been known since the 60s, the literature is very silent regarding:

• The peak fracture toughness value
• What sets its magnitude
• The corresponding optimum thickness

Hence, there is no definitive rationale about how to use or account for such an optimum fracture toughness in structural design.

Vision of HAPI

The vision of HAPI is that the fracture resistance of critical metallic structural components can be significantly enhanced by:

1. Selecting and/or controlling the plate thickness for thin-walled applications
2. Controlling the constituent plate thickness for thick laminates

In particular, metal laminates with optimum thickness of the constituents, selected for their high strain hardening capacity, will lead to unattained levels of cracking resistance.

Research Requirements

This will require:

• Generating a range of new experimental fracture data
• Performing complex 3D finite element simulations relying on a rich micromechanical model with new enhancements
• Extending the materials selection approach
• Exploring the processing/assembling of novel ultra-tough metal laminates
• Developing a radically new concept of laminate pressure vessel

Expected Gains

Major gains are expected in the weight of structures, potentially up to a factor of five if fracture toughness is the dominating design factor, and this, without changing the chemistry or inventing new microstructures.