WeeLiam is a Charted Engineer (CEng) and has a Ph.D. in Mechanical Engineering, funded by TWI and Brunel University. His research focuses primarily on Fracture Mechanics and mechanical testing. Some research findings contributed to the improvement of fracture toughness standards ISO 12135 and ISO 15653. He's currently a member of the BSI's toughness testing group and also IOP's Applied Mechanics Group.
Currently, WeeLiam's been working on:
- the assessment of fracture toughness of metal matrix composites.
- fracture, fatigue, corrosion and integrity assessment of cast iron structures.
Areas of specialism
Study of Crack Tip Opening Displacement (CTOD) in Single Edge Notched Bend (SENB) specimens.
- Fracture mechanics
- Mechanical testing
- Fracture toughness testing
- Finite Element modelling of cracked components
Indicators of esteem
Engineering module lead for the 100 Scientists of Malaysia
Variation of Crack Tip Opening Displacement (CTOD) test values can have a significant effect on the Engineering Critical Assessment of a structure. This paper examines the development of CTOD with increasing load in an austenitic stainless steel. The silicone replication method giving variation of CTOD across the specimen thickness, and Digital Image Correlation (DIC) are compared to each other, and in turn to clip gauge measurements from tests. Results from Finite Element models are also presented. Estimations of CTOD from BS 7448‐1, ISO 12135 and ASTM E1820, and a proposed modification from JWES are compared to the experimental data from the crack cast in silicone compound – assumed to be the actual CTOD. The DIC measurement showed consistency with crack replicas, and a formula is given to estimate CTOD using DIC. For high strain hardening austenitic stainless steel, both the JWES and ASTM E1820 estimations provide adequate accuracy for CTOD.
Methods for determining crack tip opening displacement (CTOD) given in national and international standards are compared for steels with a range of strain hardening characteristics. Crack tip opening displacement measurements were made from single‐edge notched bend notches using a silicone rubber casting method. The finite element model produced good agreements with predictions of these CTOD measurements. The versatility of the finite element model enabled CTOD from the original crack tip and the 45° intercept method to be compared. The 45° CTOD generally underestimates the origi nal crack tip CTOD, and is less useful for conditions with stable crack extension. Apart from the high strain hardening material, CTOD calculated using BS 7448‐1, WES 1108 (JWES), and ASTM E1820 was slightly lower than the values determined from silicone measurements and modelling, which is conservative. ASTM E1820 gave the largest underestimation of CTOD, whilst BS 7448‐1 may be unsuitable for higher strain hardening steels, where the standard predicts higher CTOD than measured from the replica. JWES gives the most consistent estimation of CTOD for steels with a wide range of strain hardening values.
In ductile materials the fracture toughness is usually characterised by a tearing resistance curve, or R-curve, plotting the fracture toughness in terms of J or CTOD against crack extension. Recent research has evaluated the methods to determine CTOD in engineering alloys with a wide range of yield to tensile (Y/T) ratios for single point CTOD. This work develops the investigation into R-curves, and reviews the assumptions about SENB specimens deforming under rigid rotation, the evaluation of CTOD from J for R-curves, and the nature of tearing initiation in low Y/T ratio stainless steel, from comparisons against a series of silicone replicas cast from the SENB specimen notch during fracture toughness tests. For CTOD R-curves, the methods based on CTOD from J in ISO 12135 and ASTM E1820 gave lower and less accurate R-curves than the rigid rotation methods in BS 7448-4 and WES 1108. However, the accuracy of the BS 7448-4 formula varied for the different strain hardening materials, and overestimated the R-curves in the low tensile ratio stainless steel. Investigations into the effect of the assumption about rigid rotation in different strain hardening materials led to a rotational factor function of tensile ratio, rp sh, to be developed from numerical modelling. When this function was substituted into standard equations in place of the fixed value of rp an improvement in the accuracy of BS 7448-4 R-curves compared to replica measurements was seen for the range of strain hardening investigated, but it did not significantly improve the accuracy of the WES 1108 formula, which accounts for strain hardening in other parameters. A combination of the elastic CTOD part of the WES 1108 formula, with the plastic CTOD incorporating the modified rotational factor, was concluded to offer the optimum method to determine CTOD R-curves for a range of strain hardening materials.
According to some standards, fracture toughness tests loaded under bending are assumed to deform around a fixed plastic hinge point within the ligament ahead of the notch tip. The rotation factor,rp, defines the proportion of the ligament ahead of the crack tip to where this hinge point is located. In this paper, the concept of an SENB specimen bending about a fixed rotational point under loading was investigated. Experimental SENB tests were carried out on three different strain hardening steels, and the geometrical point of rotation was determined experimentally throughout the tests using a double clip gauge and the similar triangles principle. The experimental results were then used to develop and validate a series of different strain hardening property numerical models. By extracting the rotational factor from the different strain hardening property models, a relationship between strain hardening and a strain hardening corrected rotational factor, rp sh was established. This corrected rotational factor function was used to propose an improved equation for the calculation of CTOD and CTOD R-curves, which gave good estimations of CTOD when compared to values measured experimentally from sections through silicone replicas of the specimen crack-tip. The improved R-curve equation will be proposed for future amendments to the ISO 12135 standard.
Whilst superseded by other materials, there are many instances of engineering structures in use today that are formed of grey cast iron. These aging assets need to be assessed for their abilities to continue to function, and to predict performance in the event of engineering works being carried out on the asset, or nearby. Grey cast iron does not exhibit an obvious elastic regime when loaded in tension and is often classified in terms of minimum guaranteed tensile strength. Grey cast irons typically fail in brittle cleavage with a strain limit of
Crack tip opening displacement (CTOD) from national and international standards was shown to give different values. This paper investigates the feasibility of CTOD determined based on the concept of rigid rotational factor in single‐edge notched bend (SENB) specimens. Based on validated modelling methods, finite element (FE) models were simulated for crack ratios 0.3 ≤ a0/W ≤ 0.7 and yield‐to‐tensile ratio 0.44 ≤ σys/σuts ≤ 0.98. This covers cases of shallow to deeply cracked specimens and a wide range of strain hardening properties. CTOD obtained from the FE models was used as the basis of a newly implemented strain hardening corrected rotational factor, which considers the effects of crack tip blunting due to strain hardening, rp sh. An improved equation considering strain hardening was implemented based on the rp sh. The equation gives accurate estimation of CTOD from the FE models compared with the equation from BS 7448‐1, ASTM E1820, and WES 1180.
Khor W, Moore PL, Pisarski HG, Brown CJ (2018) A modified approach to the rigid rotation plastic hinge point in SENB specimens in different strain hardening alloys and crack depth. In: BSI, ed. ISE/101/04 Toughness testing annual meeting.
Khor W, Moore PL, Pisarski HG, Brown CJ (2018) Determination of the rigid rotation plastic hinge point in SENB specimens in different strain hardening steels. In: Physics Io, ed. Modern Practice in Stress and Vibration Analysis. Institute of Physics, Clare College, Cambridge, UK.
Khor W (2018) Crack tip opening displacement (CTOD) in single edge notched bend (SEN(B)). Dept of Mechanical Aerospace and Civil Engineering Brunel University London, Brunel University London, http://bura.brunel.ac.uk/handle/2438/16198
Khor, W. (2016) Measurement and prediction of CTOD in austenitic stainless steel. In National Structural Integrity Research Centre Conference 2016. Cambridge.
Khor, W. (2015) Validation of Standardized CTOD Equations. In National Structural Integrity Research Centre Conference 2015. Cambridge.
Khor, W. & Pisarski, H. (2015) Comparison of CTOD estimation procedures with physical measurements. In International Institute of Welding Annual Assembly.