Precracking Technique Research Articles

Effect of compression precracking on the near threshold fatigue crack propagation in an AISI 316L stainless steel

This work addresses an alternative test procedure based on fatigue precracking under compression to mitigate the potential effects of crack closure on near-threshold fatigue data and, thus, to obtain more consistent and reliable ΔKth-values. The method relies on performing a compression precracking, in which both maximum and minimum loads are compressive, thereby resulting in a tensile residual stress field at the crack tip. Fatigue crack growth (FCG) tests conducted on an AISI 316L stainless steel provide the experimental data upon which the near-threshold fatigue crack growth rate data for different test techniques can be determined. The experimental program includes FCG tests conducted on conventional C(T) specimens with crack size to specimen width ratio a/W=0.2, single edge notched SE(B) specimens under three-point bending with a/W=0.2 and two different span sizes, S, characterized by S/W=4 and 8. Overall, the present investigation shows that combining the compression precracking technique with the utilization of non-standard bend specimens having increased span improves evaluation of the near-threshold fatigue crack growth rate data for the tested AISI 316L steel, thereby providing more accurate ΔKth-values.

Influence of Precracking Techniques on Fracture Toughness of Carbon-Carbon Composites

Carbon-Carbon composites are one such material which give designers significant importance for advanced applications over conventional materials. The remarkable characteristics of carbon-carbon composites had made these products initially extremely useful in the field of aerospace and defense applications. Now, they are presently used in many applications such as biomedical implants, glass, and high temperature glass, etc. In material science, fracture toughness is a trait that depicts the ability of a material to withstand fractures and is one of the most important features in many design applications of any material. A precracked specimen is a sample that is used to accurately assess the distribution of cracks and it is a favored method. This paper describes a comparison of four precracking techniques for carbon-carbon composites using SENB specimen. The potential implications of these techniques on fracture toughness values have been evaluated. The outcome of this work indicates that precracking with a jewel saw is recommended over the other techniques.

Pre-Cracking techniques of polymeric materials: an overview

In fracture toughness tests, a number of notched specimens with identical artificial pre-cracks are essential to obtain accurate fracture parameters. The test results are critically depending on the initiation stage quality. Any slight variance in pre-cracks front shape, length and orientation could significantly affects the test results; therefore, producing identical pre-cracks is a critical issue to obtain accurate results. The pre-cracking technique should be selected carefully to fulfil controllably and repeatability requirements of the standard pre-cracks for a certain material while preserving the induced residual stresses at the crack tip at a minimal value. The notching and pre-cracking standards for metallic material have been well specified in ASTM E399. However, the case is more cumbersome for polymeric materials due their viscoelastic nature. ASTM D5045, ISO 13586:2000 and ASTM D6068 specified different procedures to prepare a sharp pre-crack for polymeric materials. Many pre-cracking techniques have been proposed in literature. The present work introduces an overview of the pre-cracking techniques for polymeric materials.

Application of Pre-Cracking in Semi-Rigid Base to Mitigate Reflective Cracking

Because of the high strength and sound stability, the cement stabilized base has been widely used in high grade pavement in China. But shrinkage cracking are easy to occur in the cement stabilized base. Under the repeating action of traffic loads and temperature loads, this kind of cracks are easy to propagate towards the asphalt surface, being called “reflective crack”. Shrinkage cracks are not normally avertable in the cement stabilized base, in contrast with a few wide cracks of broad spacing, numerous thin cracks of narrow spacing contribute little to the severe reflection cracking. On the basis of this thinking, the pre-cracking technique as a promising approach is brought forward. Several vibratory roller passes to the cement-treated base at a short curing stage, typically 1 to 3 days after placement, to create a fine network of cracks, which avoids the wide and/or long cracks and creates the ideal crack model. Comparing to wide cracking, the degree of the stress concentration resulting from thin cracking is by far mitigated. The Finite Element Method is used to predict the ideal crack model, and then the mechanical responses of the semi-rigid pavement of single wide cracking and net hairline cracking under vehicle loading are numerically simulated, simultaneously compared and analyzed. The simulation analyses indicate that the pre-cracking technique is a very useful tool to mitigate the reflective cracks.

Semi-rigid pavement performance and construction techniques for semiarid areas

This article explains a field study about the service properties of semi-rigid pavements attained in a climatic area that can be considered semiarid/arid, located in the west of Spain. A semi-rigid pavement includes a soil-cement (SC) base (or subbase) and a stabilised subgrade. Taking as a reference the works in two motorways of recent construction such as the A-66 and A-58, the criteria are presented that favoured the choice of the semi-rigid solution for its pavement and the specific techniques used for its building as well as its performance after having been opened to the traffic for 7–8 years. It has to be noted, however, that the treated layers, especially the SC base, need to be protected by a specific thickness of bituminous layers in order to achieve a homogeneous distribution of the traffic loads and provide protection against water, as well as to absorb part of the thermal cracks that will inevitably appear. Nevertheless, in the considered weather conditions, the use of pre-cracking techniques is necessary in order to maximise cement content and obtain higher bearing capacity. The SC course, resting on a quality subgrade, is responsible to a great extent for the pavement final stiffness, being able to reduce the deformability of the subgrade itself to over 40%. The results reached with the International Roughness Index regularity rate on the treated subbases are presented as well, displaying its evolution with regard to the pavement construction phases and remarking on the recommended strategies for its improvement.

Effect of the experimental technique onto R dependence of ΔKth

An experimental study on two structural steels (normalized C45 and 25CrMo4 grades) was conducted to determine the influence of the adopted technique onto the threshold SIF value. For both the tested materials, the standard approach used to pre-crack the specimens showed to have an influence on determining the thresholds, generating SIF higher than the ones estimated adopting compression pre-cracking techniques. This observation can have a huge effect on both life prediction and inspection intervals for a real structure. An example of life prediction was carried out applying the Nasgro equations, with the experimentally estimated parameters, to the propagation of a crack located at the T-transition of a generic railway axle, assumed to be built using the two tested steel grades.

Effect of Initial Debond Crack Location on the Face/core Debond Fracture Toughness

This paper studies the effect of initial crack location on the face/core debond fracture toughness under different mixed mode loading conditions. The mixed mode loading at the crack tip is defined in terms of the mode-mixity. In order to achieve the desired initial debond crack location, a pre-cracking technique is developed, where the mode-mixity, number of cycles, crack increment and load level are accurately controlled. Results show that the debond fracture resistance of foam-cored sandwich specimens depends on parameters such as loading condition (mode-mixity), core and face properties, as well as initial debond crack location. Lower fracture toughness values were measured for specimens with the initial crack location in the face laminate.

Development of a Fastener Structural Element Test for Certifying Navy Fastener Materials

Abstract New materials intended for use in critical U.S. Navy applications are evaluated by a Fracture Toughness Review Process (FTRP) for acceptability. In addition to developing data on the mechanical properties, fatigue life, and fracture toughness, the FTRP requires that a representative structural element test be performed to verify that the material can withstand plastic deformation at high loading rates in the presence of a detectable flaw. A Fastener Structural Element Test (FSET) has been developed to certify fastener materials such that the results of fracture mechanics testing on candidate alloys can be verified. The testing required the development of a suitable specimen that best represented a fastener, flaw seeding, and pre-cracking techniques, load and displacement measurement techniques suitable for high loading rates, and material performance acceptance criteria. The complete results of the testing on titanium alloy Ti-5AI-1Sn-1Zr-1V-0.8Mo (a.k.a. Ti-5111) and a Co-Ni-Cr-Mo-Al-Fe alloy will be presented along with the test methodology, data acquisition, and analysis details. The challenges encountered and areas that require further refinement will also be discussed.

Pre-cracking technique for fracture mechanics experiments along interface between thin film and substrate

Utilizing the difference in interface strength due to fabrication process, a technique for producing a sharp pre-crack between a thin film and a substrate is proposed. A cracked specimen for examining fracture toughness of interface between a sputtered copper (Cu) thin film and silicon (Si) is made by the method. A vacuum-evaporated Cu thin film, which has poor adhesion to Si, is inserted between the sputtered Cu thin film and the Si substrate as a release layer. The release layer debonds from the Si substrate at very low load, and the sharp pre-crack is successfully introduced along the interface. Using the pre-cracked specimen, the interface fracture toughness test is conducted and the critical J-integral, J C, is evaluated as about 1 J/m 2 for the sputtered Cu/Si interface.

Hot filament assisted metalorganic vapor-phase deposition of GaN

Hot tungsten filament is investigated as an ammonia precracking technique for the low temperature of MOVPE GaN. Samples were grown using different filament temperatures ( T f) and under different (N 2, H 2) carrier gas compositions. Results were discussed to take one's stand on a thermodynamic analysis of NH 3 decomposition as a function of temperature. It appears that the growth kinetics is controlled by the ammonia adsorption for low filament temperatures (<1200 K) and low nitrogen amounts. For medium T f (1200–1800 K), the use of hot tungsten filament improves the efficiency of NH 3 decomposition and its adsorption on the growth surface. The growth mechanism is controlled by the NH 2 adsorption and the ratio of growth rates obtained with and without filament is about 2. High efficiency of ammonia decomposition for T f>1800 K results in low growth rate. This was related to NH 2 decomposition into atomic nitrogen (N) and hydrogen (H). The recombination of atomic nitrogen before reaching the substrate seems to be significant enough so as to limit the deposition rate. Secondary ion mass spectrometry analysis revealed a decrease in the hydrogen contamination as T f increases.

Fatigue and Static Crack Propagation in Yttria‐Stabilized Tetragonal Zirconia Polycrystals: Crack Growth Micromechanisms and Precracking Effects

The influence of precracking techniques in the crack growth behavior of yttria‐stabilized tetragonal zirconia polycrystals (Y‐TZP) is investigated. Load‐bridge and cyclic‐compression precracking enhance subsequent tensile crack growth rates, in comparison to results that are found with precracks that are extended under four‐point bending prior to testing. The actual influence of these precracking techniques in the near‐threshold crack growth regime is remarkably different. Although load‐bridge precracking produces a pattern of crack growth fluctuations for stress intensity factors, K, lower than the effective crack‐growth threshold of the material, compression‐fatigue precracks start to propagate under far‐field tensile loads at very fast growth rates and for K values that are slightly higher than the effective threshold. Crack‐tip shielding by tetragonal‐to‐monoclinic transformation develops gradually, influencing the crack growth behavior in Y‐TZP. Proposed fatigue crack growth micromechanisms involve damage accumulation at the crack‐tip region. For Kmax &gt; 3 MPa·m1/2, fatigue crack growth rates are strongly affected by environmental interactions at the crack tip, and postulated fatigue micromechanisms include the cyclic degradation of crack‐tip shielding.

Application of fatigue precracking methods and their influence on fracture toughness values

The purpose of this work is to compare three fatigue precracking techniques for SENB specimens containing residual stresses. These are local compression, the use of a high R‐ratio in the fatigue cycle and reverse bending. The possible effects of these techniques on the values of fracture toughness are assessed.The main conclusion is that local compression reduces the measured fracture toughness up to 50 % and various materials have different sensitivities to 1 % plastic strain. Local compression produces also greater scatter of values. Reverse bending does not affect the fracture toughness but requires further research and development. A disadvantage caused by the use of a high R‐ratio is the essential increase in the total precracking time. Uniform fatigue crack front profiles can be obtained by all three methods.

An experimental technique to study the behaviour of small corner cracks

Innovative experimental procedures were developed to study the growth and closure behaviour of small corner cracks. These procedures include: (a) specimen preparation - employing a pre-crack specimen where a crack is grown in cyclic compression, to produce a final specimen with two shallow corner cracks; (b) compliance measurement — placing an active gauge on the mouth of the crack and subtracting the response of a dedicated far-field gauge to obtain a signal dependent on the crack; (c) opening load determination — fitting a curvilinear relation to the differential compliance curve to determine the opening load. The pre-cracking technique and final specimen fabrication minimize residual damage and stresses ahead of the crack tip, on its subsequent fatigue development. The size of the crack is monitored using a thin-film replica technique. The shape of the initial crack front is marked by heat coloration of the fatigue tension (FT) specimen prior to fatigue loading. The closure and growth behaviour of the small corner cracks in the FT specimens is observed in terms of shape and size.

Fracture surface interference effects in mode III

Abstract In this experimental investigation, the effects of fracture surface interference upon the torsional properties of steel alloys were quantitatively characterized. A cyclic compression pre-cracking technique was employed to produce microscopically fine crack fronts in circumferentially notched cylindrical test specimens. The abrasion and deformation that the resulting fracture surfaces underwent during the application of torsional loading was studied in detail. The interference of the fracture surfaces was observed to enhance the measured torsional stiffness of a mode III specimen by as much as 70%. The most intense abrasion effects occur close to the crack tip where the separation between adjacent fracture surfaces is relatively small. Compliance techniques, which are considered standard practice in the construction of crack resistance curves in mode I loading applications, were applied to determine their accuracy in mode III loading applications. These techniques, in comparison with direct post-fracture crack length measurements, were found to produce unreliable results in mode III due to the effects of fracture surface interference.

Residual Stresses and Fatigue Precracking Techniques for Weldment Fracture Toughness Specimens

Abstract This article examines the ability of the local compression, stepwise high R-ratio, and reverse bend weldment fatigue precracking techniques to remove the residual stresses in the uncracked ligaments of as-welded fracture mechanics test specimens. The stepwise high R-ratio and reverse bend techniques appeared to have no significant influence on the residual stresses in the ligament. This was demonstrated by the occurrence of nonuniform fatigue cracks when these techniques were followed by the standard fatigue precracking procedures. Specimens given these treatments, and other specimens fatigue precracked using the standard procedures, showed evidence of crack closure and initial compliances that were lower than normal for these specimen geometries. In contrast, local compression reduced the residual stresses in the uncracked ligaments to low, uniform levels. This was indicated by acceptable fatigue precrack front-straightness and normal specimen compliance. Therefore, of the three precracking techniques investigated, only the local compression technique is recommended.

Study of the fracture toughness and fracture morphology of polybenzimidazole

Polybenzimidazole (PBI) is a relatively new polymeric material exhibiting unusual properties that are attributable to its aromatic-heterocyclic monomer structure. Owing to its high strength, stiffness and excellent stability in hostile chemical and thermal environments, PBI is being used increasingly in critical applications. As a result, understanding the failure mechanisms of the material is vital. This paper presents the results of a study of the fracture toughness and fracture morphology of polybenzimidazole. The standard compact tension specimen was used as the basic experimental specimen in this study. The fracture tests were performed in an Instron tensile testing machine. The effects of varying the loading rate, and the ratio of the initial crack length,a, to the ligament length,W, were investigated. The fracture surface morphology was examined using optical and scanning electron microscopy. The results of this study indicate that the precracking technique significantly affects the measured fracture toughness. Also, an increase in the loading rate causes a significant decrease in fracture toughness. Examination of the fracture morphology reveals that PBI fracture surfaces exhibit many of the characteristics expected of a tough engineering plastic.

Valid KIC Determination via In‐Test Subcritical Precracking of Chevron‐Notched Bend Bars

The initiation of the crack in a chevronnotched bend bar was studied. Ideal testing conditions depend strongly on specimen preparation and material; valid KIC values do not always result. An in‐test precracking technique was developed for ceramics at room and elevated temperatures. A yttria‐partially‐stabilized zirconia ceramic was tested, and the results showed that ideal testing conditions were obtained, which guaranteed valid KIC determination.

The Effect of Precracking Techniques on Fracture Toughness of Polymers

The technique of introducing an initial crack in the specimen for fracture toughness tests has a significant effect on the fracture toughness. This study is mainly concerned with the technique of pressing a razor blade to make a pre-crack in PMMA, GPPS and PC which were commercial grades. A pre-crack of 5mm depth was introduced using a home-made notching machine at various temperatures. Then, three point bending tests were carried out to obtain the fracture toughness. For PMMA and GPPS, the crack length was difficult to be control at room temperature and 50°C. When the temperature was raised near Tg a pre-crack of 5mm length without crazes could be introduced which reduced fracture toughness. In such a case, the shape of the pre-crack was like a natural crack, but residual stress remained around the crack. A pre-crack of 5mm length could be introduced in PC even at room temperature. However, plastic deformation and residual stress were also generated around the crack. The fracture toughness was increased by the presence of this plastic deformation around the crack.

Low-temperature heteroepitaxial growth of InSb on CdTe by metalorganic chemical vapor deposition

We report the low-temperature metalorganic chemical vapor deposition of InSb on (001)CdTe. This low-temperature process was carried out by a precracking technique. Epitaxial growth with a substrate temperature as low as 185 °C can be obtained using a simple two-stage heater. The deposited films were examined by double-crystal x-ray diffraction, scanning electron microscope, and energy dispersive analysis of x ray. The films grown at 240 °C are stoichiometric, single crystal, and of specular surface morphology.

HgTe–CdTe superlattices and Hg1−xCdxTe grown by low-temperature metalorganic chemical vapor deposition

We report low-temperature metalorganic chemical deposition of Hg1−xCdxTe and HgTe–CdTe superlattices using a thermal precracking technique. The success of the precracking technique depends on decomposing the incoming metalorganic reactants at a high temperature while allowing the epitaxial growth to occur at a lower temperature. The precracking technique enables one to grow Hg1−xCdxTe and HgTe–CdTe superlattices at temperatures of 200 to 250 °C. The Hg1−xCdxTe epilayers show infrared transmission with sharp cutoffs. The as-grown Hg0.7Cd0.3Te is n type and has a room-temperature mobility of 12 200 cm2/V s with carrier concentration of 2.7×1017 cm−3 and 77 K mobility of 27 000 cm2/V s with carrier concentration of 1.0×1017 cm−3. For the HgTe–CdTe superlattices, HgTe layer thicknesses as small as 60 Å were obtained. All the samples examined using transmission electron microscopy had sharp HgTe–CdTe interfaces. The HgTe–CdTe superlattice structures also exhibited sharp infrared transmission cutoffs. In addition, compared to the band gap of Hg1−xCdxTe alloy with the same composition, a narrowing of the superlattice band gap was observed as predicted by theoretical studies.