A.S. Tetelman

Embrittlement of a ferrous alloy in a partially dissociated hydrogen environment

Gaseous hydrogen embrittlement of quenched and tempered 4130 steel was studied as a function of temperature from −42° to 164°C in a partially dissociated hydrogen environment at low molecular hydrogen pressures (≈8 × 10−3 torr). Atomic hydrogen was created by dissociation of molecular hydrogen on a hot tungsten filament located near a crack opening. The presence of atomic hydrogen was found to increase the rate of hydrogen-induced, slow crack growth by several orders of magnitude and to significantly alter the temperature dependence of embrittlement from what is observed in the presence of molecular hydrogen alone. Based on a previous study, these observations are interpreted in terms of a difference between the hydrogen-transport reaction step controlling hydrogen-induced, slow crack growth in the molecular hydrogen and the atomic-molecular hydrogen environments. Finally, a comparison is made between the kinetics of hydrogen-induced, slow crack growth observed in the presence of atomic-molecular hydrogen and the kinetics of known, possible hydrogen-transport reactions in an effort to identify the reaction step controlling hydrogen embrittlement in the presence of atomic hydrogen.

Relation between KIc and microscopic strength for low alloy steels

Abstract A simple model has been developed to determine KIC in terms of the microscopic cleavage strength σ f ∗ and the tensile yield strength σY for low temperature cleavage fracture in A302B and A533 reactor grade quenched and tempered steels. The model applies at sufliciently low temperatures or in irradiated steels where σ f ∗ ≦ 3.4 σ Y . It was determined that σ f ∗ is independent of temperature below −150°F and then increases with increasing temperature. At this time, it appears that σ∗ is independent of irradiation. At temperatures above that at which σ f ∗ ≦ 3.4 σ Y , unstable fracture initiates when a critical plastic strain ϵf is achieved near to the crack tip. The critical local plastic strain for unstable fracture also increases with increasing temperature.

Non-Destructive characterization of hydrogen-embrittlement cracking by acoustic emission techniques

Abstract Acoustic emission techniques are used to determine the rate of cracking of high-strength steel due to hydrogen embrittlement, making it possible to predict failure in certain structural components undergoing hydrogen attack. The acoustic emission rate from a hydrogen-charged, linear-compliance, fracture-toughness specimen of high-strength 4340 steel was recorded as a function of the stress-intensity factor at the crack tip in the specimen. The structural component (a hydrogen-charged boh of the same material) was then torqued down in a simple fixture and the acoustic emission was recorded as a function of time. The acoustic emission rate obtained from a near-critical value of the stress-intensity factor of the fracture-toughness specimen was used to predict within ±15 per cent the time for the onset of unstable failure of the bolt The most significant result of this investigation is that regardless of the initial loading, nucteatkm time for cracking to begin, or any other factors connected with geometrical differences in specimens, the acoustic emission rate can be used to determine the time for and the stress intensity factor at the onset of rapid fracture.

Comparison of Various Methods of Measuring K1c on Small Precracked Bend Specimens That Fracture after General Yield.

An experimental investigation into the measurement of fracture toughness on small precracked bend specimens after general yield is described. Six parameters are compared on the basis of their accuracy and utility in predicting KIc. The parameters are: (1) the on-load crack tip opening displacement, COD, (2) the lateral (notch root, crack tip) contraction, NRC, (3) the angle of bend, (4) the stretch zone, (5) the J integral, and (6) the equivalent energy. The results show that, from the standpoint of accuracy and of ease of measurement, the plane strain COD at fracture initiation, (COD)Ic, is the most useful parameter for measuring KIc on small specimens that fracture after general yield. (Author)

The Use of Pre-Cracked Charpy Specimens to Determine Dynamic Fracture Toughness.

Abstract : The use of an instrumented impact hammer on pre-cracked Charpy V specimens has led to an inexpensive method for determining the dynamic fracture toughness, K sub Id. Measurements were made on A-533 steel over a range of temperature (-125 to + 75F) at a loading rate K clot approximately equal to 1,000,000 ksi (in. to the 1/2 power)/sec. The data were found to be in excellent agreement with those obtained by other workers on much larger specimens. Analysis of the data leads to a new method for estimating the NDT temperature, which may be of practical value in nuclear reactor surveillance programs. (Author)

The role of fibrous reinforcements well bonded or partially debonded on the transverse strength of composite materials

Abstract The transverse strength of fibrous composite is studied analytically, numerically and experimentally, from fracture mechanics viewpoint. A detailed stress interaction of a single fiber, embedded in an elastic matrix, with a micro-crack situated along or near the interface, performs the basis for the suggested strength analysis. Extension to multi-fiber interaction (by Finite Element) with crack in different location and length is examined. The results quantify the effect of some fundamental parameters (geometrical and material) on the transverse strength. Finally, the strengthening capability of composite having a “soft” adhesive interlayer (surrounding each fiber) is considered and implications are discussed.

Crack arrest in transversely loaded elastic plates

Abstract This report describes an experimental study on arresting running cracks under oppositely located circular zones of externally applied transverse compression. Crack arrest experiments were conducted on precracked, single-edge-notched plate specimens of high-strength steel, subjected to both in-plane tension stress and transverse compression loads. Side loads of various magnitudes were applied and the specimens then pulled in tension. Crack arrest occurred if the side load was sufficiently large. The loads required for crack arrest were compared with theoretical predictions for this type of specimen, made by combining linear elastic fracture mechanics with the stress intensity factor for a cracked transversely loaded elastic plate. This factor had been previously determined by static analysis. Excellent agreement was observed between the experimental and theoretical results, thereby suggesting the validity of conclusions drawn from the theoretical analysis. The primary conclusions obtained from the theory were that transverse loading zone diameters equal to the plate thickness are nearly optimum for arresting cracks, and that the load required for arrest increases with fracture toughness. The technique is therefore most applicable to high-strength, low-toughness materials.

Characterization of the elevated temperature static load crack extension behavior of type 304 stainless steel

Abstract Creep crack extension rates in Type 304 stainless steel, obtained as a function of temperature over the range 650–800°C and as a function of specimen geometry at 750°C, are empirically correlated with both the net section stress and the apparent stress intensity factor. The results indicate that the stress intensity correlation is strongly dependent on specimen geometry, whereas the net section stress correlation appears to be generally valid. A direct correspondence between crack extension and local (crack tip) displacement is noted when creep crack extension rates at 750°C are compared with COD obtained from actual castings of the crack tip. By introducing the concept of a miniature creep specimen at the crack tip, a physical model for creep crack growth is developed, based on local stress relaxation and strain accumulation, that is consistent with both experimental observation and existing theories of steady state creep.

The application of the visioplasticity technique for derivation of the criterion for ductile rupture initiation in fully plastic notched bars

Abstract Recent studies have shown that the central region of a Charpy specimen in three point bend test remains in plane strain conditions well beyond general yield. This concept has been utilized to determine the stress and strain fields near the root of a notch by the visioplasticity technique, for fully plastic notched bars deformed in plane-strain bending. The solution allows for the actual work-hardening of the material and can be used to measure large strains (up to 70 per cent) at the notch root. A brief description of the analysis is presented. The angle of bend at which fracture initiation takes place has been measured experimentally and then employed to determine the local strains and stresses at fracture initiation. It was found that fracture of notched bars by ductile rupture occurs at a critical strain at the notch root, rather than at a critical maximum normal stress below the notch. The fracture strain is independent of notch geometry and correlates well with values of fracture toughness K Ic , critical crack opening displacement COD and the Charpy V-notch shelf energy determined by independent tests.

The effect of microstructure on the hot salt stress corrosion susceptibility of titanium alloys

It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application. Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement.