Slow stable crack growth in structural steel

There is continuing interest in defining a fracture parameter which is capable of characterising the ductile crack growth resistance o~" a material under conditions of large scale plasticity. The aim of this study is to assess the capability of a numberof, elastic-plastic fracture parameters for the case of a C-Mn steel. The fracture parameters considered are: the crack opening displacement at the original crack tip (CTOD); the crack opening angle at the growing crack tip (CTOA); and the crack opening displacement at the growing crack tip (6°). The parameters were assessed by determining experimentally whether they were affected by changes in test specimen size and geometry in cases where there was no change in the crack tip constraint. Changes in the latter were assessed from changes in the CTOD at crack initiation (CTOD i). Only 6~ was capable of characterising the crack growth resistance, as unlike both CTOD~ and 6a, dCTOD/da and CTOA were dependent on specimen geometry.

supporting

confidence: 80%

Section: ~supporting

confidence: 57%

An assessment of various crack tip displacement based elastic-plastic fracture parameters to characterise ductile crack growth resistance

Gibson¹,

Druce²

1987

“…Several methods of determination of the J-integral values were used. For the CT specimen, the J-integral value for a given crack extension was calculated using the incremental approach established by Garwood, Robinson and Turner [13] and derived from the deeply notched bend-type specimens method developed by Rice [14]. For the (n + 1)th increment of crack extension, the expression of the J-integral value is given by…”

Section: J-rcurves Analysmentioning

confidence: 99%

Ductile fracture characterization of polycarbonate by the R-curve method

Schmit¹,

Bouvart²,

François

1990

The R-curve method was used to characterize the stable crack growth and the unstable ductile fracture of 3 mm thick polycarbonate sheets. In order to delimit the field of application of this method, several parameters describing the crack growth resistance were employed. The effect of specimen size and geometry was also studied. Besides limitations linked to test specimens size, the different parameters used are shown to aptly describe the tearing process of this material. When the plane stress region is dominant, the J-integral appears to be as a parameter well adapted to characterize ductile fracture. NOMENCLATURE a 0 initial crack length B specimen thickness W specimen width ap, Aap physical crack length and extension ae, Aa e effective crack length and extension Rp plastic-zone length P load 6 load-line displacement U area under P-3 curve K elastic stress intensity factor K~ effective stress intensity factor J J integral /'2o, J0 K and J values at crack initiation Kpop_in K value at pop-in Jpop-in J value at pop-in K~, J, critical values of K and J (maximum load) ~ strain in the thickness direction T tearing modulus

“…However, the obtaine~ Jc data are corrected according to the amount of stable crack growth occurring, as is described by Garwood et al [7] and by Garwood and Turner [8]. The amount of stable crack growth may be obtained directly by measuring the length of the tearing region on the R224 fractured surface if the tearing region and rapid growth parts have distinct fracture surfaces.…”

mentioning

confidence: 99%

A single specimen JIc test method

Lereim

Lohne

1980

The proposed test method is a continuation of a simple Jic-test method developed by Lereim and Embury [i]. The method is based on the application of a silicone impression technique for determination of the point of initiation of stable crack growth and the corresponding JIc value. See Fig. i. The replication technique was initially applied by Robinson [2,3] for measuring the CODi values. The technique has also been used by Garwood [4,5] for measuring the opening displacement at the original crack tip and the growing crack front with corresponding amounts of slow stable crack growth. Consequently, JIc, CODi, CODc, and COD a may be obtained from a single specimen. However, the infiltration method is dependent on having a reasonably short curing time for the silicone replica. Thus the application of the infiltration method is limited to testing at temperatures above 0 deg C as the curing time for the silicone rubber in general increases drastically with decreasing temperature. Therefore alternative test methods are required if the test temperature is below 0 deg C, and a single specimen test method is preferred.The test method is based on the use of a single specimen, silicon infiltration rubber, and measurements of load, load point displacement, and clip gauge displacement at the mouth of the notch tip. However, one of the specimens in a series has to be tested at room temperature in order to establish the actual relation between the crack tip opening displacement and load point displacement, which depends on the specimen's compliance. This is shown in Fig. 2, where data from testing at different temperatures are included for the size and geometry of the applied test samples. The specimens are tested at the actual temperature and loaded to instability which may be followed by rapid crack growth and possible arrest in a displacement controlled testing machine. Subsequently, unloading is performed and the sample is removed from the real test conditions without breaking the specimen into two halves. A typical record of the load deflection curve is shown in Fig. 3a. When the specimen is heated to room temperature, the silicone rubber is poured into the notch and crack tip. The casting is removed after hardening, sectioned, and COD at initiation of stable crack growth is measured from the profile, as shown in Fig. 3b. Further, the obtained COD. is applied in Fig. 2 for the determination of the corresponding loa~ point displacement at initiation, u i. Finally, u. is used in Fig. 3a for exact determination of the area below the ~oad -load point displacement trace up to initiation of stable crack growth. Then the work done prior to initiation is known, and JI is readily calculated through the well known expression JIc = 2U/Bb ~6].