# Experimental Investigation of Delayed Fracture Initiation in Advanced High-Strength Steel Under Accelerated Bending

## TL;DR

This paper investigates how bending speed affects crack formation in high-strength steel, revealing that faster bending delays cracks.

## Contribution

The study introduces direct bending experiments to explore crack initiation in AHSS under accelerated conditions.

## Key findings

- Accelerated bending delayed crack initiation in all tested AHSS materials.
- Small-radius bending reduced strain localization due to strain rate hardening.
- Larger-radius bending increased fracture strain.

## Abstract

Predicting bending fractures in advanced high-strength steel (AHSS) is challenging due to complex microstructural behaviors and strain rate dependencies, particularly in industrial forming processes. Current models and standards primarily focus on quasi-static tension or slow bending speeds, leaving a gap in understanding the accelerated failure of AHSS without necking. In this study, direct bending experiments were conducted on dual-phase, complex-phase, and martensitic AHSS grades under varying bending speeds and radii. Since the bending crack is irrelevant to the load drop, surface crack evolution was measured using three-dimensional surface profile analysis. The results showed that accelerated bending significantly delayed crack initiation across all tested materials, with small-radius bending showing reduced strain localization due to strain rate hardening. Larger-radius bending benefited primarily from increased fracture strain.