Showing papers on "Fracture toughness published in 1979"

Residual stress effects in sharp contact cracking

Abstract: A study is made of residual stress effects in the mechanics of median fracture in sharp indenter contact. Starting with a simplistic treatment of the elastic-plastic indentation field, the problem is conveniently resolved into two separable parts, involving reversible (elastic) and irreversible (residual) components. The assumption of geometrical similarity in the residual field about the deformation zone, later backed up by stress birefringence measurements, leads to a stress intensity factor for median crack propagation containing the elastic and residual parts as the sum of two terms. The resulting formulation for equilibrium fracture shows some differences in the crack response during the loading and unloading half-cycles. By imposing certain stress states on the specimen surface during indentation the residual component of the field may actually cause the median crack to continue in downward extension as the indenter is withdrawn, a response which is especially amenable to experimental investigation. Direct observations of median crack evolution in soda-lime glass confirm this and other essential predictions of the fracture mechanics theory. The contribution of the residual component to the crack growth is found to be by no means secondary in importance to that of the elastic component.

Effect of microstructure on cleavage fracture toughness of quenched and tempered steels

Abstract: The cleavage fracture of quenched and tempered steels at a sharp crack is seen to involve a statistical competition between different sized crack nuclei in the rapidly changing stress gradient ahead of the crack tip. A procedure is proposed, based on this statistical fracture model, whereby it is possible to estimate the cleavage fracture toughness of a steel containing spheroidal carbide particles from a knowledge of the carbide particle radius distribution. Predictions so made are seen to be in good agreement with experimentally determined fracture toughness values over a range of different temperatures and microstructures.

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Abstract: Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (−190 to 200°C), although validJIc measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1.

Studies on Crack Initiation and Stable Crack Growth

Abstract: Experimental results are presented which suggest that parameters based on the J-integral and the crack opening tip displacement δ are viable characterizations of crack initiation and stable crack growth. Observations based on some theoretical studies and finite-element investigations of the extending crack revealed that J and δ when appropriately employed do indeed characterize the near-field deformation. In particular, the analytical and experimental studies show that crack initiation is characterizable by the critical value of J or 6, and stable crack growth is characterizable in terms of the J or δ resistance curves. The crack opening angle, d6/da, appears to be relatively constant over a significant range of crack growth. Thus, appropriate measures of the material toughness associated with initiation are J I c and δ I c , and measures of material toughness associated with stable crack growth are given by the dimensionless parameters T J [= (E/σ o 2 )(dJ/da)] and T δ [= (E/σ o )(dδ/da)]. The two-parameter characterization of fracture behavior by J I c and T J or δ I c and T δ is analogous to the characterization of deformation behavior by the yield stress and strain hardening exponent.

A fracture mechanics study of subcritical tensile cracking of quartz in wet environments

Abstract: Load relaxation and cross-head displacement rate-change experiments have been used to establish log10 stress intensity factor (K) versus log10 crack velocity (v) diagrams for double torsion specimens, of synthetic quartz cracked on thea plane in liquid water and moist air. For crack propagation normal toz and normal tor at 20°C,K Ic (the critical stress intensity factor) was found to be 0.852±0.045 MN·m−3/2 and 1.002±0.048 MN·m−3/2, respectively. Subcritical crack growth at velocities from 10−3 m·s−1 to 10−9 m·s−1 at temperatures from 20°C to 80°C is believed to be facilitated by chemical reaction between the siloxane bonds of the quartz and the water or water vapour of the environment (stress corrosion). The slopes, of isotherms in theK-v diagrams are dependent upon crystallographic orientation. The isotherms have a slope of 12±0.6 for cracking normal tor and 19.9±1.7 for cracking normal toz. The activation enthalpy for crack propagation in the former orientation in liquid water at temperatures from 20°C to 80°C is 52.5±3.8 kJ·mole−1. A discussion is presented of the characteristics of theK-v diagrams for quartz.

The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel

Abstract: The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite;i.e., localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic structures.

Fracture of a brittle particulate composite

Abstract: Previous models for crack-particle interactions in brittle composites are modified to account for penetrable obstacles, obstacle shape and secondary crack interactions. The modified model is applied to a glass-unbonded nickel sphere composite system, the experimental aspects of which were summarized in Part 1. Increases in fracture energy are explained in terms of local crack blunting. It is shown that these results fall, as expected, between those for an entirely sharp crack front and an entirely blunt one.

Tempered martensite embrittlement in SAE 4340 steel

Abstract: The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K 1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).

Mechanical Properties of Hot‐Pressed Silicon Nitride with Different Grain Structures

Abstract: During hot-pressing of α-Si3N4 powders, the equiaxed α micro-structure gradually transforms into a β structure characterized by needle-shaped prismatic grains which are closely entangled and linked together. With increasing amounts of the β fraction, the bend strength, fracture toughness, and work of fracture increase significantly, then decrease as grain growth occurs. The Klc, improves by a factor of >2 and the change in γF by a factor of >4. The crack resistance to achieve the same crack velocity in materials of different β contents shows a similar trend. The dependence of the mechanical properties on the microstructure is explained by linking and pullout of the β crystals and by grain coarsening.

Theory for determining K Ic from small, non-LEFM specimens, supported by experiments on aluminum

Abstract: A new method is presented for measuring the plane-strain fracture toughness of ductile materials. The method is much simpler than JIc measurements, yet retains the advantage that the specimen can be much smaller than is required for a valid KIc test. The small specimen advantage results from selecting the short rod specimen configuration for which the material along the crack tip is well constrained to the plane-strain state, and from the measurement and elastic-plastic analysis of the load-displacement curve (including at least two unloading slopes) to determine the energy per unit crack area which is required to slowly advance a steady-state crack.

A Procedure for the Determination of Ductile Fracture Toughness Values Using J Integral Techniques

Abstract: A procedure to determine the ductile fracture toughness value JIc, the value of the J integral at incipient crack growth, is described. The procedure was developed by the working committee of ASTM Task Group E24.01.09 on Elastic-Plastic Fracture under Committee E-24 on Fracture Testing. The principal goal of this report is to present a procedure for JIc determination that may be used as a basis for an ASTM standard on elastic-plastic testing. Although this procedure is described in terms of the multiple-specimen technique, any single-specimen method can be used as long as a good correspondence exists between the crack extension as measured by the heat tinting technique and that measured by single-specimen methods.

Fracture Toughness of Fresh-Water Ice

Abstract: The plane-strain fracture toughness of fresh-water ice was measured at various loading rates and temperatures. The fracture toughness of ice decreases as loading rate increases and as the test temperature approaches the melting point. The presence of liquid water seems to reduce the fracture toughness. The fracture toughness for crack arrest is slightly lower than the static fracture toughness.

A Simple and Sensitive Method of Monitoring Crack and Load in Compact Fracture Mechanics Specimens Using Strain Gages

Abstract: The calibrated strain on the back face (the face opposite that from which the slot is machined) of compact tension (CT) and T-type wedge-opening-loading (WOL) specimens provides a method for measuring crack length when the load is known or for measuring load when the crack length is known. The method is simple, reliable, sensitive, and inexpensive. A good correlation was achieved between strain measurements on a CT specimen and values computed from a two-dimensional finite element analysis. The method has good potential for developing into a more sensitive crack length measurement technique than has previously been achieved. Calibration tables and graphs are reproduced that describe the relationship between crack length, back-face strain, and load for CT and T-type WOL specimens of any size and thickness and for any linear elastic material. The method has several advantages over the closely related crack opening displacement (COD) technique for some test situations and these are described. In particular, the back-face strain increases linearly with crack length for constant stress intensity conditions except for very deep cracks in CT specimens. The overall characteristics render the technique ideal for incorporation into computerized/automated crack growth testing. For constant back-face strain, the stress intensity was shown to decrease with increase in crack length for both CT and T-type WOL specimens. This decrease is more pronounced than for corresponding constant COD testing and this provides a good technique for obtaining threshold fatigue or stress corrosion conditions.

Elastic-Plastic Fracture Toughness Based on the COD and J -Contour Integral Concepts

Abstract: The paper reviews the definition, fracture characterizing roles, and measurement of critical COD and J-values. It is proposed that COD should be defined as the opening displacement at the original crack tip position. Thisdefinition avoids much of the ambiguity of previous definitions based on the crack tip profile and the elastic-plastic interface. Attention is drawn to a fundamental problem which limits the general application of the J-contour integral concept to elastic-plastic descriptions of the crack tip environment when cracks occur in overmatching yield strength weld regions. A comparison of recent three-point single-edge notch bend (SENB) testing techniques, based on the standard instrumentation used in K I c tests, shows there is a close mathematical link between the estimated values of COD and J. Experimental data, obtained over a wide range of temperatures, are used to demonstrate how the critical values of COD and J for unstable fracture are affected by variations in specimen geometry. Also, it is shown that measurements of J I c may lead to overestimates of K I c in materials having yield strengths less than approximately 700 N/mm 2 .

Diametral Compressive Testing Method

Abstract: A new approach of diametral compressive testing with circular anvils is proposed. The circular anvils are used to select a suitable contact width or to avoid the collapse of the specimen in the contact edge. Furthermore, the statistical corrections on the diametral compressive strength for the effects of the size and stress distribution are explained by the application of Weibull’s statistical theory. The experimental results of the diametral compressive testing are compared with the uniaxial tensile strengths for some kinds of graphite and marble, and the discrepancies between the two strengths are discussed. According to our macroscopic brittle fracture criterion under biaxial stress state, which was proposed recently, the tensile strength can be deduced from the diametral compressive strength σHC* and the uniaxial compressive strength σC as follows, σt* = KICKIIC 12 σCσHC*(1+σx/σH)+σC {σHC*(1−σx/σH)−σC}+σC2 where σt* is the deduced tensile strength, σx /σH is the ratio of maximum and minimum principal stresses at the center of the disk, and KIC and KIIC are the values of Mode I and Mode II fracture toughness. The deduced values σt* are ascertained experimentally to agree very well with the uniaxial tensile strength in wide range of brittle materials, such as graphite and marble.

Crack propagation in and fractography of epoxy resins

Abstract: The relationship between the stability of crack propagation in, and the fracture surface appearance of, DGEBA epoxy resins cured with TETA has been investigated using a linear elastic fracture mechanics approach. In particular, the effect of varying the amount of curing agent and curing conditions and altering external variables such as testing rate, temperature and environment have been studied. Under certain conditions, propagation is found to be stable and fracture surfaces have a smooth appearance. Under other conditions the cracks propagate in an unstable ″stick-slip″ manner. In this case, arrest lines can be seen on the fracture surfaces and their intensity and roughness increases with the magnitude of the crack jumps. The roughness of the fracture surfaces has been measured using a Talysurf and this has been shown to be due principally to deviation of the cracks from the original fracture plane rather than any gross plastic deformation.

Abrasion resistance and fracture toughness of white cast irons

Abstract: The abrasive wear resistance and fracture toughness of the principal types of alloy white cast iron have been determined in laboratory tests. It was found that low-stress abrasion resistance is con...