Predicting the influence of strain on crack length measurements performed using the potential drop method

Abstract

The potential drop (PD) crack growth measurement technique is sensitive to strain accumulation which is often erroneously interpreted as crack extension. When testing ductile materials these errors can be significant, but in many cases the optimum method of minimising or suppressing them remains unknown because it is extremely difficult to measure them experimentally in isolation from other sources of error, such non-ideal crack morphology. In this work a novel method of assessing the influence of strain on PD, using a sequentially coupled structural-electrical finite element (FE) model, has been developed. By comparing the FE predictions with experimental data it has been demonstrated that the proposed FE technique is extremely effective at predicting trends in PD due to strain. It has been used to identify optimum PD configurations for compact tension, C(T), and single edge notched tension, SEN(T), fracture mechanics specimens and it has been demonstrated that the PD configuration often recommended for C(T) specimens can be subject to large errors due to strain accumulation. In addition, the FE technique has been employed to assess the significance of strain after the initiation of stable tearing for a monotonically loaded C(T) specimen. The proposed FE technique provides a powerful tool for optimising the measurement of crack initiation and growth in applications where large strains are present, e.g. J-R curve and creep crack growth testing

Potential difference methods for measuring crack growth: A review

“…Several advantages of PD methods enable the wide application of the technique. The relatively cheap instrumentation and simple operation contribute to the preference [40,42,44,51,[117][118][119][120][121]. Moreover, PD methods are believed to provide reliable and accurate predictions [11,42,122,123] yet continuous detection of cracks thereby allows stable automation [42,44,51,117,118,120,[122][123][124].…”

“…The DCPD method has been widely accepted due to its easy application and hence relative cheap cost [41,131,134,137]. Because of the nature of DC, DCPD is independent of magnetic permeability of conductive specimens hence the capacitance problem (the capacitance of conductive specimen which is usually ignored actually exerts an effect on measurement of PD) concerned in ACPD is eliminated [13,52,121,131,137]. The wide use of DCPD offers another advantage, in that previous results and guidance are available for certain specimen geometries [41].…”

A Unified Potential Drop Calibration Function for Common Crack Growth Specimens

“…For each specimen type a 2D FE model was developed using COMSOL [16]. A typical model is shown in Fig.…”

Improvements in the measurement of creep crack initiation and growth using potential drop

“…Recently it has also been extended to include crack growth and has been used to confirm that the onset of stable tearing during fracture toughness testing coincides with an increase in gradient on a plot of PD vs. LLD (Tarnowski et al, 2017).…”