Onset Of Stable Crack Growth Research Articles

超音波法による安定破壊開始点の検出と単一試験片による弾塑性破壊じん性の評価

An ultrasonic method was applied to elastic-plastic fracture toughness testing, and the relationship between the plastic blunting at the fatigue precrack tip or the onset of stable crack growth and the echo difference output voltage characteristics during fracture toughness loading were clarified. The elastic-plastic fracture toughness test procedure based on Top-on and End-on ultrasonic methods was proposed by which the JIc value can be determined using a single specimen. The JIc values for various metallic materials from medium strength steels to aluminum and titanium alloys were evaluated according to the proposed JIc test procedure. The JIc measurements were also compared with those determined according to the SZ method using multiple specimens and those determined according to the AE method using a single specimen. It is confirmed that the proposed JIc test procedure based on Top-on and End-on ultrasonic methods is very useful.

Determination of J‐integral R‐curves for Hastelloy X and inconel 617 up to 1223 K using the potential drop technique

AbstractFor Hastelloy X (T = 308 K and T = 1223 K) and for Inconel 617 (T = 308 K, 473 K, 673 K, 873 K, 973 K, 998 K, 1023 K, 1048 K, 1073 K, 1173 K, and 1223 K) the J‐integral R‐curves have been determined. For Hastelloy X at 308 K the multispecimen technique has been compared with the single specimen method using the potential drop technique. At 1223 K Hastelloy X delivered a low J0‐integral value. Inconel 617 showed a smooth increase in J0 with increasing temperature, up to 998 K; above this temperature even for large loadline displacements only blunting behaviour has been observed. The same tendency has been found for the evaluation of the tearing modulus. With increasing temperature the tearing modulus increases. In the temperature range from 998 K to 1223 K the tearing modulus increases by about one order of magnitude. For both alloys the onset of stable crack growth in most cases is earlier than the load maximum is reached in the load versus load‐line displacement curves.

Influence of notch root radius on ductile rupture fracture toughness evaluation with Charpy-V type specimens

The blunt notch fracture toughness of four types of carbon-manganese steel (ASTM A516 grade 70) has been determined by J-integral tests on Charpy-V type samples with different values of notch root radius, ρ. J-ρ plots, determined using specimens with a notch depth to width ratio, a/w, equal to 0.5, have shown the existence of a limiting ρ value ( ρ eff ) below which applied J-intergral values at fracture initiation are constant. These ρ eff values have been seen to depend only on second-phase particle distribution and not on their volume fraction or on the steel ferritic grain size. The procedure for deriving J-integral values at the onset of stable crack growth from J resistance curves in the case of notches has also been discussed. Experiments with Charpy specimens with a/w = 0.2 do not allow the derivation of meaningful J- ρ plots. In all cases, a ductile fracture criterion based on the constancy of the notch tip strain at rupture initiation has been proved when ρ > ρ eff .

On the Characterization of Elasto-Plastic Material Behavior Using the Concepts of Crack Opening Stretch and J Integral

Values of the crack tip opening stretch (COS), the stress intensity factor, Keff, and the energy integral J were determined for 22 Ni Mo Cr 3 7 steel at room temperature. A material-dependent linear relationship between COS and J resulted. The critical values of COS and J determined for the onset of stable crack growth using the electrical potential method and the interrupted loading procedure were independent of specimen type and crack length for thicknesses smaller than 30 mm. A dependence on crack length, although accompanied by large scatter, was indicated for 75-mm-thick specimens with an instability type of fracture behavior.

Fracture of longitudinally cracked ductile tubes

Various bulging factor and plasticity correction factor formulations are discussed and a new plasticity correction factor leading to a simple failure law is proposed. Failure stresses predicted by the usual LEFM formula corrected for plasticity are shown to be identical with the Dowling and Townley two-criteria approach if the relevant parameters are chosen in a suitable manner. Burst tests on AISI 304 stainless steel tubes performed at the Joint Research Centre, Ispra are described. The strengthening effect of the sealing patch was taken into account by replacing the Folias bulging factor by a smaller empirical factor determined by Bernard and Henry from fatigue crack growth tests. A flow stress \\ ̄ gs and a toughness K c were derived which apply to the prediction of the onset of stable crack growth in 304 stainless steel tubes at room temperature. For other ductile materials and temperatures tentative formulae are proposed.

Plastic deformation at a stably growing crack tip

The deformations around the tip of a straight, through-the-thickness crack in sheets of cellulose nitrate, celluloid, in tension have been studied, as well before as after the onset of stable crack growth. A tensile testing apparatus and a device for engraving a fine grid directly on the specimen sheet have been designed. The development of the plastic enclave also after the start of stable crack extension has been studied in connexion with investigations of the deformations in gross fields of size up to 30 × 40 mm2 performed on a digital computer (line element length 1 or 0.5 mm, virgin crack length 80 mm). Unexpected deformations very close to the crack tip were found in the near field where line elements of length 50 µm were analyzed. A modified Dugdale model where the cohesive stresses vary linearly is examined and is applied on polyvinylehloride, PVC, specimen the thickness of which is strongly reduced near the crack tip. A method for measurement of thickness changes of the order of a few µm by tracing isohypses is described. Finally, to enable comparison with the experimental results, the elastic deformation pattern for a square mesh in the plane strain and plane stress case is obtained by a digital computer.