Material and geometry effects on crack shape in double torsion testing

Previous work has established a model for expressing the curved crack front shape observed in a Double Torsion test in a single parameter, the shape factorS. This paper demonstrates the ability of this model to account for crack shape effects on measured toughness against crack speed data, using a direct measurement ofS, in tests on polymethyl methyacrylate (PMMA) of various molecular weights. The analysis is then extended to enableS to be predicted without reference to the material crack front size parameter referred to in earlier work: only specimen geometry significantly influences the crack front shape in a wide variety of materials.     

Fracture Behaviour Investigation into a Polymer‐Bonded Explosive

Abstract: Polymer‐bonded explosive (PBX) is used widely in weapon systems. Failure of PBX caused by mechanical damage is one of the sources of accidental ignitions. A brittle crack of PBX produces local heating, creating a ‘hot spot’ finally. Investigation into the tensile fracture behaviour of PBX is one of the main works to determine the failure mechanism. Although many researchers have carried out the quasi‐static Brazilian test to understand the damage evolution of PBX, the fracture feature of PBX under dynamic impact is rarely reported. In this article, dynamic Brazilian tests were conducted. A single‐pulse loading apparatus was used to ensure that specimen was loaded only once during a dynamic Brazilian test. High‐speed camera, digital image correlation and micro‐observation techniques were adopted for strain measurement and microfracture observation. All the dynamic tensile crack exhibits transgranular fracture, which indicates more heat would released by the propagation of crack and more friction between fractured crystal surfaces. On the basis of the theories of interface debonding and transgranular fracture, larger crystals are more prone to crack, whereas smaller crystals simply debond with neighbouring binders. Discrete element method simulation results show that specimen with interface debonding microcracks was able to sustain additional load until transgranular fracture begins.     

Determination of the Parameters of the Two-Parametric Fracture Mechanics along the Crack Front Based on the Digital Image Correlation Data

A mathematical processing technique and an algorithm for the numerical correction of experimental displacement fields on the specimen surface in the vicinity of a crack tip with consideration of the displacement of a body as a rigid whole under its loading and the actual position of a crack tip were proposed. The initial experimental displacement fields in the vicinity of a fatigue crack tip of a compact specimen were determined by digital image correlation. This approach was successfully used for a compact specimen and the possibility of its expansion to obtain the distributions of the stress intensity factors and the nonsingular T stresses along the spatial crack front was shown.     

Crack tip opening angle measurement through a girth weld in an X100 steel pipeline section*

Crack tip opening angle (CTOA) is becoming one of the most accepted methods for characterizing fully plastic fracture. It provides a measure of the resistance to fracture for a material in cases where there is a large degree of stable‐tearing crack extension during the fracture process. Our current pipeline research uses the CTOA test as an alternative, or addition, to the CTOD (crack tip opening displacement) and the fracture energy characterization provided by the J‐integral approach. A test technique was developed for measurement of CTOA that uses a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate the CTOA. In this article, CTOA data on crack growth orientations perpendicular to pipeline girth welds are presented. The CTOA for X100 high strength bainitic gas pipeline steel is reported. Two different specimen gauge sections, 3 mm and 8 mm, were used and the effect of the specimen thickness on the CTOA is discussed. The results show a change in the CTOA as the crack grows into the heat affected zone (HAZ). A slight improvement in the fracture resistance is measured, and through the weld, a slight decrease in fracture resistance is observed.     

Crack front curvature: Influence and effects on the crack tip fields in bi-dimensional specimens

Crack front curvature is evidence in most experimental crack advance test. When classical linear elastic fracture mechanic theory deals with bi-dimensional crack configurations, it ignores the three-dimensional effects of crack propagation. Issues as the influence of the specimen thickness and the crack front curvature are not considered. Previous numerical studies have shed light on out-of-plane plastic zone development or stress state. Nevertheless, these numerical studies are based on the assumption that the crack front is ideally straight; despite it is well known that the crack front has some kind of curvature. In the present work, a CT aluminium specimen has been modelled three-dimensionally and several calculations have been made considering a huge combination of different single load levels, specimen thicknesses and crack front curvatures. Due to the abrupt transition from plane strain to plane stress, an ultrafine mesh along the thickness has been applied. The analysis of the evolution of the plastic zone and the stress state along the thickness provides information about the combined influence of these parameters on fracture mechanics.     

Testing and modelling of creep crack growth in compact tension specimens from a P91 weld at 650 °C

Creep crack growth tests were performed, at 650 °C, on compact tension (CT) specimens machined from the parent material and from the weld region of a P91 weldment. Parent material tests were performed on a number of different CT specimen designs in order to investigate the effects of side grooves on the shape of the crack front. Tests of CT specimens machined from the weld region were performed with the initial cracks located within the heat-affected zone (HAZ) along the interface with the parent material (i.e. the type IV position). All of the specimens were prepared with initial cracks created by wire spark erosion. Good correlations between creep crack growth rates and C^∗ were obtained for both the parent and type IV test results. The results indicate that the crack growth rates in the weld specimens are about four times higher than those of the parent material specimens, at the same C^∗. Microstructural investigations of the fracture surfaces using SEM and hardness measurements have shown that the exact location of the initial crack within the weldment has a large effect on the crack growth rate, at various loading levels. The results of Finite Element (FE) analyses of the parent material specimen tests, using a creep continuum damage material model, compared favourably with those obtained from the experiments.     

A non-hypersingular time-domain BIEM for 3-D transient elastodynamic crack analysis

A three-dimensional (3-D) time-domain boundary integral equation method (BIEM) is presented for transient elastodynamic crack analysis. A non-hypersingular traction BIE formulation is used with the crack opening displacements and their derivatives as unknown quantities. A collocation method in conjunction with a time-stepping scheme is developed to solve the non-hypersingular time-domain BIEs. To simplify the analysis and to describe the proper behaviour of the unknown quantities at the crack front, a constant spatial shape function is applied for elements away from the crack front, while a spatial ‘square-root’ crack-tip shape function is adopted for elements near the crack front. A linear temporal shape function is used in the time-stepping scheme. Numerical calculations, have been carried out for penny-shaped and square cracks. Results for the elastodynamic stress intensity factors are presented as functions of the temporal and the spatial variables. For the test examples considered, good agreement between the present results and those of other authors is obtained.     

Fatigue crack initiation detection by an infrared thermography method

In this paper, the study of the temperature variation during fatigue tests was carried out on different materials (steels and aluminium alloys). Tests were performed at ambient temperature using a piezoelectric fatigue system (20 kHz). The temperature field was measured on the surface of the specimen, by means of an infrared camera.
Just at the beginning of the test, it was observed that the temperature increased, followed by a stabilization which corresponds to the balance between dissipated energy associated with microplasticity and the energy lost by convection and radiation at the specimen surface and by conduction inside the specimen. At the crack initiation, the surface temperature suddenly increases (whatever the localization of the initiation), which allows the determination of the number of cycles at the crack initiation and the number of cycles devoted to the fatigue crack propagation. In the gigacycle fatigue domain, more than 92% of the total life is devoted to the initiation of the crack.
So, the study of the thermal dissipation during the test appears a promising method to improve the understanding of the damage and failure mechanism in fatigue and to determine the number of cycles at initiation.     

Crack tip opening angle optical measurement methods in five pipeline steels

Crack tip opening angle (CTOA) is becoming one of the more widely accepted properties for characterizing fully plastic fracture. In fact, it has been recognized as a measure of the resistance of a material to fracture in cases where there is a large degree of stable-tearing crack extension during the fracture process.Our current pipeline research uses the CTOA concept as an alternative or an addition to the fracture energy characterizations provided by the Charpy V-notch (CVN) and drop weight tear test (DWTT). A test technique for direct measurement of CTOA was developed by use of a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate CTOA. In this report, different optical measurement methods are compared, three using the crack edges adjacent to the crack tip (defined in the ISO draft standard and ASTM standard) and one using the specimen surface grid lines. Differences in CTOA resulting from the various measurement methods are evaluated. The CTOAs for five different grades of gas pipeline steel are reported, and the effect of microstructure on CTOA is discussed.     

CTOD‐R Curve Tests of API 5L X90 By SENT Specimen Using a Modified Normalization Method

A modified normalization method is presented for obtaining crack tip opening displacement (CTOD) resistance curves of single edge‐notched tension specimens. For the applied load mode, the normalization method should be based on force‐crack mouth opening displacement. A formula is proposed for C_LL(i), the core parameter used in the normalization calculation. The modified method is validated using the unloading compliance method. The study of specimen geometry shows that side‐groove depth and crack depth have significant impacts on the obtained CTOD resistance curves, and the shape of the crack front affects the determination accuracy of crack length.     

Flexible three-dimensional measurement technique based on a digital light processing projector

A new method of 3D measurement based on a digital light processing (DLP) projector is presented. The projection model of the DLP projector is analyzed, and the relationship between the fringe patterns of the DLP and the fringe strips projected into the 3D space is proposed. Then the 3D shape of the object can be obtained by this relationship. Meanwhile a calibration method for this model is presented. Using this calibration method, parameters of the model can be obtained by a calibration plate, and there is no requirement for the plate to move precisely. This new 3D shape measurement method does not require any restrictions as that in the classical methods. The camera and projector can be put in an arbitrary position, and it is unnecessary to arrange the system layout in parallel, vertical, or other stringent geometry conditions. The experiments show that this method is flexible and is easy to carry out. The system calibration can be finished quickly, and the system is applicable to many shape measurement tasks.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

Effect of crack curvature on stress intensity factors for ASTM standard compact tension specimens

The stress intensity factors (SIF) are calculated using the method of lines for the compact tension specimen in tensile and shear loading for curved crack fronts. For the purely elastic case, it was found that as the crack front curvature increases, the SIF value at the center of the specimen decreases while increasing at the surface. For higher values of crack front curvatures, the maximum value of the SIF occurs at an interior point located adjacent to the surface. A thickness average SIF was computed for parabolically applied shear loading. It is assumed that it reflects the average stress environment near the crack edge. These results were used to assess the requirements of ASTM standards E399-71 and E399-81 on the shape of crack fronts.     

An in situ technique for the assessment of adhesive properties of a joint under load

Slow crack propagation in adhesive bonded joints has been characterised using an asymmetric wedge test. Crack position was evaluated from strain gauge measurements, both in the debonded part of the joint and in the bonded zone. Test temperature was changed during loading, giving insight into bond evolution. The technique allows accurate, and virtually continuous, determination of crack position to be made, and therefore the evaluation of crack speed versus fracture energy curves, as well as elastic properties of the adhesive layer. This technique also enables the monitoring of crack propagation in controlled environmental conditions to be performed, without interruption of exposure for measurements. By using a Winkler elastic foundation model to analyse results, the method seems to be the first to describe a process zone, or region where the adhesive is significantly strained under load, and a finite length specimen effect, manifested by crack front acceleration during the final stage of the test. The method was found to offer great potential to study in situ fracture and bulk adhesive properties.     

Quantitative measurement of filament ruptures of a multi-filament AR-glass yarn embedded in concrete

This paper presents a new digital image analysis method for quantitative and online measurement of filament ruptures of a multi-filament AR-glass yarn embedded in concrete during pullout loading. The proposed method was developed based on an existing test method for determination of filament ruptures occurring during the loading called failure investigation using light transmitting (FILT) property test, which uses light transmitting property of AR-glass fibers. Artificial light is exposed on the glass filaments from one side of the specimen. On the opposite side, a charge-coupled device (CCD) camera with an optical microscope records the lighted filament cross-sections in the yarn. To detect filament ruptures during pullout loading, the light intensity time history of every individual filament of the yarn was investigated by a digital image analysis method. The number of broken filaments was also investigated by acoustic emission (AE) analysis simultaneously and the results were compared. Test results showed that the light transmitting property of AR-glass can be used to identify filament ruptures and it is possible to determine the failure of the filaments during pullout in the cross-section quantitatively by the improved FILT test.     

A test specimen with constant stress intensity factor for prescribed displacement

A test specimen has been developed that gives a constant value of the stress intensity factor in mode I under fixed displacement loading conditions. The test specimen is a double contoured cantilever beam specimen whose shape is derived from engineering beam theory. A final shape for a range of crack extensions where stress intensity is constant was established using a finite element analysis. It is believed that this specimen could be useful for many applications where a constant stress intensity factor is needed over a range of crack extension for constant displacement loading e.g. viscoelastic crack growth in an adhesive layer or stress corrosion cracking.     

Eine neue Methode zur Festigkeits- und Zähigkeitsbestimmung von dünnen Schichten

A New Method of Determining Strength and Fracture Toughness of Thin Films A method is presented for measuring strength and fracture toughness of thin films. The basic idea of this method is to use a compact steel specimen as a substrate, which is deeply precracked by means of fatigue loading prior to the coating process. Under bend loading of this composite crack opening occurs accompanied by straining the free-standing thin film which bridges the crack. Film rupture firstly occurs at the front face of the specimen followed by crack growth on both sides. The film properties can be derived by eliminating the substrate effect through determining the difference between the two load-displacement curves corresponding to the intact and the damaged film, respectively. The strength of the film can be calculated from the load difference at film rupture at the front face of the composite. The fracture toughness is evaluated from load and compliance change during successive crack growth on both sides of the specimen. First results on PACVD TiN-films are presented and compared with available bulk data, thus demonstrating the applicability of the proposed method.     

Image-based bidirectional reflectance distribution function measurement

We present a new image-based process for measuring a surface's bidirectional reflectance rapidly, completely, and accurately. Requiring only two cameras, a light source, and a test sample of known shape, our method generates densely spaced samples covering a large domain of illumination and reflection directions. We verified our measurements both by tests of internal consistency and by comparison against measurements made with a gonioreflectometer. The resulting data show accuracy rivaling that of custom-built dedicated instruments.     

Multiple‐view Shape and Deformation Measurement by Combining Fringe Projection and Digital Image Correlation

Abstract: We present a new method that combines the fringe projection and the digital image correlation (DIC) techniques on a single hardware platform to simultaneously measure both shape and deformation fields of three‐dimensional (3‐D) surfaces with complex geometries. The method in its basic form requires only a single camera and single projector, but this can be easily extended to a multi‐camera multi‐projector system to obtain complete 360° measurements. Multiple views of the surface profile and displacement field are automatically co‐registered in a unified global coordinate system, thereby avoiding the significant errors that can arise through the use of statistical point cloud stitching techniques. Experimental results from a two‐camera two‐projector sensor are presented and compared with results from both a standard stereo‐DIC approach and a finite element model.