Pure Mechanical Impact Trimming for Ultra High-strength Steels: A Strain Rate-managed Approach to Enhance Shear Edge Quality Without Thermal Effects

Mechanical trimming is preferred in industrial applications due to its energy efficiency and high productivity compared to laser trimming. However, ultra high-strength steel (UHSS) for lightweight structures exhibits poor shear quality and is prone to material damage during mechanical trimming. Heat-assisted mechanical trimming can be employed for UHSS to address this issue; however, it has drawbacks, including high energy consumption and thermal damage to the material. Recently, an ultra high-speed trimming (UHST) method was introduced for UHSS to mitigate thermal damage. This technology achieves high-quality shear edges by converting the high kinetic energy of the tool into localized thermal energy in a narrow adiabatic shear band (approximately 800 °C in the adiabatic band) at very high cutting speeds (exceeding 10 m/s). However, equipment capable of operating at speeds exceeding 10 m/s requires significant capital investment, consumes considerable energy, and retains the thermal effects within the adiabatic band. This study introduces a purely mechanical impact trimming technique that manages the strain-rate effect to improve shear edge quality while reducing material damage. The central concept is to perform trimming at a speed that is sufficiently high to reduce the fracture strain of the material but considerably lower than the speed at which adiabatic heating occurs. In this method, thermal damage is avoided, and a lower fracture strain helps minimize material damage during the cutting process. This approach, termed intermediate-speed impact trimming (IST) in this work, is a purely mechanical impact trimming method, which avoids thermal effects. Experimental and numerical investigations of the proposed method were conducted using UHSS sheets (CP1180-1.4t and MS1480-1.5t). In order to apply this approach to trimming, fracture limit experiments with varying strain rates were conducted to determine the trimming tool speed that reduces the fracture strain; subsequently, the IST tests were conducted with different clearances (5%, 10%, and 15%) and impact trimming speeds (1.1 and 2.0 m/s). The experimental results indicate that the shear edge produced by the IST exhibits a uniform shape and minimal variation in hardness, suggesting that this method reduces damage to the shear edge. Microstructural analysis revealed that the shear edge was formed by pure mechanical deformation. To analyze the trimming mechanism, a rate-dependent fracture model was developed to capture the material test results. This model was implemented in a Vectorized User Material (VUMAT) subroutine of the ABAQUS software to simulate the trimming process and successfully replicate the shear edge observed in the experimental results. Furthermore, the application of the proposed method to a door-impact beam was explored. Both the experimental and analytical results confirmed that the IST method reduces material damage and produces a high-quality shear edge devoid of thermal effects.