Standard Compact Tension Specimens Research Articles

Creep crack growth modeling and near tip stress fields

The problem of near tip stress fields in a cracked body subjected to Mode I loading at elevated temperatures is studied. Specifically, the superalloy, IN 718, is examined in the standard compact tension specimen geometry. The simulation is at 650°C. The specimen is assumed to be under dead load conditions. For a stationary crack, the near tip stress fields are calculated and compared with the asymptotic solutions available in the literature. While the results assuming small strains agree very well with the asymptotic solutions, the large strain analysis does not. The results indicate that both the amplitude and the asymptotic exponent are dependent on the applied load level which is in disagreement with the asymptotic predictions. In addition, the zone effected by creep deformation is larger when large strains are considered. An algorithm is developed and tested for the modeling of stable crack growth. Both convergence and stability are investigated. Explicit time integration is used for crack growth studies as it is demonstrated to be computationally more efficient. The algorithm is employed to study the near tip stress fields for a growing crack. The near tip stress fields for a growing crack (with constant velocity) are generated using the developed algorithm. The results demonstrate that the asymptotic behavior of the stress field is load dependent. Comparison is made with the limited analyses available. Recommendations for future research are discussed.

Use of COD in KIC determination

It has been theoretically postulated that the crack opening displacement (COD) technique is applicable in characterising fracture of material in the generally yielded as well as linear elastic regimes. Hence, it is possible to determine the plane strain fracture toughness, K IC , of a material using the COD method if the onset of crack propagation and the location of the centre of rotation can be evaluated. In the present investigation, COD values at crack initiation, δ i , for Rochling Moulrex A steel were obtained with 3-point bend specimens at six different tempering temperatures using the multiple specimens load-unload extrapolation technique. It is found that by using the BS 5762:1979 analysis, K IC values were grossly over-estimated. The use of a constant rotational factor is observed to be inadequate. Under small scale yielding conditions, the centre of rotation is postulated from the present results to be located at approximately the end of the stretched zone. This supposition was applied to estimate the K IC values of standard compact tension specimens made from Rochling Moulrex A, Assab 25X and Comsteel En25 steels. Where the material was linear elastic, the estimation was good with discrepancy of less than 7%. Overestimation was seen at high tempering temperatures due to the increasing amount of plasticity in the material that shifted the location of the centre of rotation from the end of the stretched zone to a position closer to that suggested in the British Standard.

Fracture toughness of aluminium alloy 7075-T6 in the as-cast condition

The fracture toughness of high strength aluminium alloy 7075-T6 in the as-cast condition was studied using standard compact tension specimens with thicknesses ranging from 4 to 25 mm with a 3 mm incremental interval. ASTM Standard E 399-74 for plane strain fracture toughness determination was adhered to in the investigation. The results show that the ASTM-E 399 Standard minimum thickness requirement occurs before the fracture toughness K c reaches the limiting value K Ic but that for the material tested, although K c decreases as the specimen thickness increases within the valid thickness range K Q remains constant and valid over this thickness range and thus K Ic can be determined. The plane strain fracture toughness is 27.5 MPa m 1 2 . K Ic values obtained from specimens of different crack plane orientations were found to remain constant, indicating the lack of anisotropy in the material. This is expected, however, for material in the as-cast condition.

Mode I and II fracture of adhesive joints

The compliance method is used to determine strain energy release rates in mode I and II fracture of adhesive bonded joints. A strong dependence of the strain energy release rates upon the adhesive thickness is found. Standard compact tension and compact shear specimens are used. It is found that the former are well suited to the investigation of mode I fracture, but that the latter are less well suited to mode II investigations. Attention is focussed on bond failures caused by cracks propagating between an adhesive and an adherend. Experimental and theoretical arguments are combined to determine the critical strain energy release rates at which such failures occur. The critical values are shown to be strongly dependent upon the method of surface preparation as well as upon the physical properties of the materials used.

The viscoplastic crack growth behavior of a compact tension specimen using the Bodner-Partom flow law

Creep crack growth in IN-100 was studied using a hybrid experimental-numerical procedure (HEN). This procedure couples displacement data generated during creep crack growth tests to a finite element model of the test specimen. A standard compact tension specimen geometry was used in all cases. The computer program used herein is a 2-dimensional plane stress/plane strain code that uses constant strain triangular elements and has the ability to release fixed nodes to simulate crack growth. This procedure is facilitated by using a Gauss-Seidel iterative solution technique that allows appropriate terms in the stiffness matrix to be changed between timesteps to accommodate changing boundary conditions due to crack growth. A variable timestep algorithm maximizes the timestep size during the analysis while maintaining good accuracy. Constitutive equations proposed by Bodner and Partom were used to account for the nonlinear, viscoplastic material behavior exhibited by IN-100 at high temperatures. These equations were integrated through time by a linear Euler extrapolation technique. Crack growth estimates generated with the HEN procedure were within 20% of the actual crack growth in the test specimens after times ranging from 5–14 hr. Additionally, a critical stress and an accumulated stress history criterion for crack growth were both found to show promise. Both of these criteria merit further investigation.

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.

Crack problems for a rectangular plate and an infinite strip

In this paper, the general plane problem for an infinite strip containing multiple cracks perpendicular to its boundaries is considered. The problem is reduced to a system of singular integral equations. Two specific problems of practical interest are then studied in detail. The first is the investigation of the interaction effect of multiple edge cracks in a plate or beam under tension or bending. The second problem is that of a rectangular plate containing an arbitrarily oriented crack in the plane of symmetry. Particular emphasis is placed on studying the problem of a plate containing an edge crack and subjected to concentrated forces. The plate has the dimensions of a standard compact tension specimen and is intended to simulate the CTS.

Fracture toughness measurement by cylindrical specimen with ring-shaped crack

A method for measuring the plane strain fracture toughness of metals by means of cylindrical specimen in tension with axi-symmetrical ring-shaped crack is discussed. Owing to the fact that the crack tip of such a specimen is closer to ideal plane strain state, the K 1 c value measured is effective and reliable. This investigation has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen dimensions. In both safety evaluation and life estimation of engineering components by linear elastic fracture mechanics, it is necessary to measure the fracture resisting parameter—fracture toughness under plane strain. According to the ASTM-E399-74 standard[1], when measuring the fracture toughness K 1 c values of medium and low strength steels with a standard compact tension specimen or three-point bending specimen, it is necessary to use specimens of large dimensions, great tonnage fatigue testing machine and universal testing mechine. Naturally, this presents great difficulties to the investigation and application of fracture mechanics and it is precisely for the purpose of overcoming these difficulties that we have studied the method of measuring the plane strain fracture toughness by a cylindrical specimen in tension with axi-symmetrical ring-shaped crack. This method has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen size. Owing to the fact that the field around the crack tip of such a specimen is closer to ideal plane strain state, the results obtained are values smaller than those by using compact tension and three-point bending specimens and are more reliable fracture resisting constants for materials in linear elastic fracture mechanics analysis. Moreover, this method is more practical and economical because no expensive large fatigue testing machine is needed and the specimen size is small.

Effects of ‘Streaks’ on the Fracture Toughness of Chromium–Molybdenum Rail Steel

AbstractMicrostructural inhomogenities or ‘streaks’ of lower bainite were found in the webs of experimental ingot and continuously cast Cr–Mo steel rails. The effect of these inhomogenities on the fracture toughness behaviour of rail web steels was investigated by means of miniature compact tension specimens (W = 13.5 mm) and compared to results obtained from similar specimens taken from 1% Cr and carbon rail steels.It was concluded that the bainite ‘streaks’ did not lead to a loss of fracture toughness and that KQ values obtained from miniature specimens were comparable to those determined from standard compact tension specimens (W = 25.4 mm) taken from rail heads of corresponding rail steels. Resume Des non-homogeneites de microstructure ou “traits” de bainite, inferieure ont ete trouve dans les âmes des rails fabriques en aciers differents, Cr–Mo experimentaux, de coulee continue et de coulee en lingot. L'effet de ces non-homogeneites sur le comportement en tenacite a la rupture des aciers a âme de rai...

Discussion of a New Experimental Method in Measuring Fracture Toughness Initiation at High Loading Rates by Stress Waves

An experimental method is described for measuring the fracture initiation properties of metals and alloys over a wide range of loading rates, which can cover over six orders of magnitude in K˙I (1 MPam s−1 ≤ K˙I ≤ 106 MPam s−1). With some modification of the standard compact tension specimen, a large series of screening tests can be performed in the high loading region at a relatively low cost. At the lower loading rates a standard closed loop testing machine can be used. To evaluate fracture initiation at a very high loading rate, a special arrangement of the split Hopkinson pressure bar has been proposed. Specimens of the same geometry as those used in quasi-static tests are placed between the Hopkinson bars. Since the wedge is attached to the incident bar, and the specimen is backed by the transmitter bar (Fig. 2), the course of specimen loading and fracturing can be exactly monitored by recording the incident, reflected and transmitted longitudinal waves. Using this technique, fracture initiation of the prefatigued specimen has been achieved within ∼ 20 μs after the beginning of specimen loading. The effects of inertia acting on the specimen and an error introduced by friction are both considered. Experiments performed on some aluminum alloys as well as on medium carbon steel revealed a complicated pattern of the fracture toughness behavior. Generally, for the strain rate sensitive materials a substantial decrease in fracture toughness was observed under high loading rates.

The dynamic mechanical properties and fracture behaviour of phthalocyanine polymers

The dynamic mechanical properties of a series of phthalocyanine polymers with varying aliphatic chain length linking stable phthalocyanine nuclei have been studied. Dynamic shear modulus and polymer loss factor were determined using a torsion pendulum. Polymer fracture energies were also determined using both the standard compact tension specimens and double-tapered cantilever beam specimens. The results showed that both the dynamic shear modulus and the glass transition temperature increased with decreasing molecular flexibility and increasing extent of cure. The fracture energies increased with increasing molecular flexibility. It was also shown that extended thermal treatment could greatly embrittle the thermosetting polymers and hence reduce the fracture toughness, even though such treatments usually promoted crosslinking leading to higher moduli and glass transition temperatures for these polymers.

Fracture toughness concept applied to methyl methacrylate.

Polymethyl methacrylate bone cement is tested at the Institute of Fracture and Solid Mechanics Laboratory, Lehigh University. Standard tensile coupon and compact tension specimens are molded under varying conditons of temperature and pressure. Test results are obtained for the strength and toughness of bone cement involved in total hip replacement operations.

Three-dimensional speckle interferometric investigation of the stress-intensity factor along a crack front

The variation in Mode I stress-intensity factor throughout the thickness of an ASTM standard compact tension specimen was determined using scattered-light speckle interferometry. Two very thin sheets of coincident coherent light traveling in opposite directions were passed through a Plexiglas specimen normal to the crack faces. A double-exposed photograph of the scattered-light speckle pattern was taken while the specimen was subjected to a small load increment. From this double-exposed photograph, the change in the crack-opening displacement could be determined. From the information about the crack-opening displacement in the region of the crack tip, the stress-intensity factor was calculated for various interior planes and on the surface of the specimen. For the compact tension specimen tested, the stress-intensity factor did not vary throughout the specimen's thickness. The method of scattered-light speckle interferometry proved to be very powerful in solving this complex three-dimensional problem.