High-rate, fracture testing methods

Date published

2024-11-13

Free to read from

2025-01-17

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Journal Title

Journal ISSN

Volume Title

Publisher

Cranfield University Defence and Security

Department

Type

Conference paper

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Citation

Sargeant B, Davies CM, Hooper P, Cox M. (2024) High-rate, fracture testing methods. DSDS24, Cranfield Defence and Security Doctoral Symposia 2024, 13-14 November 2024, STEAM Museum, Swindon, UK

Abstract

It is generally easier to break alloys at high speeds; in other words, fracture toughness reduces as strain rate increases. Understanding of these dynamic properties, by experimentation, reduces excessive conservatism in structural design, allowing for safe life extension of existing components and more efficient new constructions. This work investigates procedures to determine fracture toughness at elevated loading rates. Specific testing, used examples of different properties from Zinc, Aluminium and SA508-III pressure vessel steel, highlighting challenges in standard methods. Two methods of ‘rapid’ toughness testing have been contrasted – pendulum (Charpy) and servohydraulic (Very High Speed [VHS] Instron) test rigs. Charpy pendulums have a set energy input, hence a non-constant speed. Servohydraulic machinery adds energy to the system, maintaining speed. Both methods utilised sub-sized (10mm square bending specimens) specimens are recommended for high speed, minimising inertial effects and align with load capacities of high-rate instrumentation. Elevated speeds also make crack growth more difficult to monitor, compared to well understood static methods. Normalisation crack length monitoring (as recommended in standards) and a linear growth assumption from peak load have been utilised in this testing. Both methods produce similar crack length histories. Normalisation method anticipated the same crack growth initiation as peak load and showed a non-linear crack growth that aligns well with fracture surface features. However, the normalisation method is open to personal interpretation to fit data to a standard model. Results showed both testing methods to be dynamically invalid for brittle materials (Zinc and SA508-III steel at low temperature). Dynamic invalidity occurred as fracture occurred before (less than 93µs) kinetic energy effects are minimised - a requirement from standards for valid high-rate testing. Ductile materials (Aluminium and SA508-III steel at high temperature) were dynamically valid. Peak loads in Charpy data were clipped by relatively low sample rate. Peak loads in VHS testing were emphasised by high levels of ringing caused by impact shockwave resonating through the test rig. This let Charpy and VHS data to present different responses. Aluminium testing (low stiffness) loaded slow enough and damped ringing enough to produce comparable toughness data. Steel (higher stiffness) experienced these effects so much that data was incomparable. Improved system by higher sample rate Charpy instrumentation and higher mass and rigidity in the VHS test rig would reduce ringing (signal noise) and allow for better identification of accurate material response. The discrepancies between testing machines overwhelmed the effect of fixed energy input or speed controlled procedures.

Description

Software Description

Software Language

Github

Keywords

Rapid, High-rate, Fracture, Toughness, RPV, Steel, Charpy, VHS Instron

DOI

Rights

Attribution 4.0 International

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Resources

Funder/s

Engineering and Physical Sciences Research Council (EPSRC)
EPSRC ref. EP/S023844/1
AWE, DNV Energy Systems, UK, Nuclear Energy Futures CDT

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