Modification and evaluation of a FRF-based model updating method for identification of viscoelastic constitutive models for a nonlinear polyurethane adhesive in a bonded joint

In this study, a Frequency Response Function (FRF) -based model updating method, was modified for the purpose of the identification of viscoelastic constitutive models. A steel beam, bonded to a heavy rigid steel block by a layer of Sikaflex-252 polyurethane adhesive, was employed as the test setup. Using the concept of Optimum Equivalent Linear FRF (OELF), accelerance FRFs were measured at different random excitation levels which demonstrated the nonlinear behavior of the adhesive. Using a finite element model, the sensitivity analysis showed that the selected FRFs are more sensitive to the storage and loss moduli of the adhesive near the resonances. Therefore, firstly, both of the storage and loss moduli were identified near each resonance separately and the results have been compared with the results based on Inverse Eigen-sensitivity Method (IEM). In continuation, five viscoelastic constitutive models were utilized and identified to characterize the dynamic mechanical properties of the adhesive at different excitation levels. Applying the identified models, the correlation between the FRFs of the FE models and the experimental ones were tested. The results show that amongst the identified models, The Standard Linear Solid (SLS) model in parallel with a viscous or constant structural damper (stiffness proportional) results in better correlation with experiments. Increasing the excitation level, the storage modulus of the adhesive decreases, whereas the loss modulus increases, especially at high frequencies

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

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 supressing 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

Creep-fatigue crack growth behaviour of P91 steels

The importance of predicting failure due to combined creep-fatigue crack growth in high temperature power-plant components has become of great importance importance due to the need for plant to ‘load follow’ in response to fluctuations in demands and the availability of renewables. P91 steel has been widely utilized in conventional plant components. Creep fatigue crack growth (CFCG) tests have been performed on compact specimens at temperatures ranging between 600° C to 625° C. The experimental results have been compared to static creep, high cycle fatigue and CFCG test data available in literature on P91 steel. The CFCG data has been characterised using stress intensity factor range parameter, ΔK and C* parameter. The crack growth per cycle and ∆K relationship shows that at high frequency, the CFCG behaviour tends to that of high cycle fatigue crack growth and at low frequency, the contribution of creep becomes increasingly more significant. The correlation between crack growth rate and C* parameter, shows that most CFCG data fall within the creep crack growth (CCG) P91 data band which may indicate that the crack growth behaviour is dominated by creep processes. Fractography has also shown an intergranular, ductile fracture surface indicating creep dominance for the conditions considered. A linear cummulative rule has ben used to predict the CFCG experimental result

Experimental and numerical investigation of the weld geometry effects on Type IV cracking behaviour in P91 steel

The focus of the present study is on creep crack growth behaviour in Type IV region of P91 steel weldments at 650 °C. In the experimental studies on small- and large-scale single-edge notched specimens in tension, SENT, the effects of weld dimensions and specimen size on the creep crack growth behaviour of the material are investigated. The experimental results demonstrate that the crack starts to propagate at an angle normal to the loading direction, subsequently deviates towards the Type IV region and the specimen eventually ruptures when the crack growth angle becomes parallel to the loading direction. The creep rupture data for SENT specimens compared well with those of the round bar specimens for P91 welded joints. In addition, the data for crack growth rates from the deviating crack path were correlated with the C* fracture mechanics parameter and showed good agreement with standard compact tension test data. To predict the creep crack growth behaviour in the Type IV region, finite element simulations were performed in conjunction with a multiaxial ductility damage criterion at the weld/base metal interface. Given that a lower failure strain along the Type IV region is prominent, it is shown that the cracking, in line with the experiments, followed the HAZ region and led to the final creep rupture in the net sectio

A unified potential drop calibration function for common crack growth specimens

Calibration functions, used to determine crack extension from potential drop measurements, are not readily available for many common crack growth specimen types. This restricts testing to a limited number of specimen types, typically resulting in overly conservative material properties being used in residual life assessments. This paper presents a unified calibration function which can be applied to all common crack growth specimen types, mitigating this problem and avoiding the significant costs associated with the current conservative approach. Using finite element analysis, it has been demonstrated that Johnson’s calibration function can be applied to the seven most common crack growth specimen types: C(T), SEN(T), SEN(B), M(T), DEN(T), CS(T) and DC(T). A parametric study has been used to determine the optimum configuration of electrical current inputs and PD probes. Using the suggested configurations, the error in the measurement of crack extension is <6% for all specimen types, which is relatively small compared to other sources of error commonly associated with the potential drop technique