Relating Structural Loading Rate to Testing Rate for Fracture Mechanics Specimens

Abstract

It is very well-known that fracture toughness depends on loading rate. Higher strain rates can shift the ductile to brittle transition curve to higher temperatures, resulting in a more brittle structure at the same temperature. However, there is little effort to relate the testing rate to the loading rate within the offshore and maritime industry. For example, BS 7448-1 requires that the stress intensity factor loading rate be 0.5 MPa√m/s to 3.0 MPa√m/s. The loading rates of BS 7448-1 are very far away from the vibrational modes of the specimen, so these limitations are not necessary in order to assure a quasi-static test. In comparison, SSC 275 indicates that normal ship loading rates can be of the order of 220–440MPa√m/s. The results of SSC 275 are consistent with results obtained from a Dutch offshore equipment supplier, who indicates a time to maximum loading of 0.25–1.3 seconds. In general, a conservative loading scenario for the maritime and offshore industry is on the order of 200 times faster than the loading rate that is recommended by BS 7448-1. Testing at the standard rate has the consequence of artificially lowering the ductile to brittle transition temperature by 8–35°C in comparison to a real loading scenario, thus possibly giving a false impression of safety. This means that a CTOD measured as 0.2 mm for static testing conditions could be 0.08–0.15 mm for actual loading. The analysis is shown to be consistent with CTOD test data on a Quenched and Tempered (QT) and a Thermo-Mechanically Controlled Processed (TMCP) S690 grade steel.

Tensile behaviour of S690QL and S960QL under high strain rate

“…For the tensile tests carried out, the corresponding time to maximum force and fracture at 1 s -1 strain rate (the critical strain rate for offshore cranes) falls around 0.08 s and 0.12 s, respectively. This is slightly lower but similar to those given by Walters et al [11] for offshore structures. Based on this understanding more emphasis will be made for strain rates from QS to 4 s -1 , however discussion will still include the strain rate at 100 s -1 .…”

“…Yield stress is defined as the 0.2% proof strength for all samples, to enable the yield-to-tensile ratio to be determined. Information regarding offshore structures in-service scenarios under normal and high strain rate conditions revealed that time at maximum force could be around 1.3 s and 0.25 s, respectively [11]. For the tensile tests carried out, the corresponding time to maximum force and fracture at 1 s -1 strain rate (the critical strain rate for offshore cranes) falls around 0.08 s and 0.12 s, respectively.…”

“…In addition, very little information is available on the performance of HSSS (specifically, with Y/T in excess of 0.90) subjected to high loading rate scenarios. It is noteworthy that the effect of high loading rates is generally known to affect the strength and fracture performance of steels [8][9][10][11][12][13][14][15][16][17]. Invariably, the effect of loading rates on the tensile properties or strength of a steel is predicted to be specifically dependent on a particular steel grade [8,9] with the sensitivity depending on the strength level itself.…”

“…Typical examples of engineering loading rates expressed in terms of strain rate are given in Table 1. These values are taken from the information given by [11][12][13], and should be used as estimates only since the exact values will depend largely on loading configuration, local geometry, and flaw dimensions [12]. Whilst the fracture mechanical loading rate is mostly expressed in terms of stress intensity factor loading rate for linear elastic conditions, the loading rates in structural engineering are usually considered in terms of strain rates [8].…”

Inexpensive Fracture Toughness Testing of Welded Steel

“…• A shorter fatigue pre-crack, which reduces test specimen preparation time [2] • A higher load application rate, which is more representative of loading on real structures [3] • A fixed specimen size [4] • Simpler sensors [5] • No need for a temperature chamber for low-temperature testing [1] A good correlation was shown between this method and conventional CTOD tests on base materials for CTOD values less than 0.2 mm.…”