Cyclic cracking resistance of brittle materials under compressive loading

Fatigue behavior of brittle materials under compression is considered. The findings should be taken into account in the failure probability assessment of components made of materials with limited plasticity, which are used in various stress states. __________ Translated from Problemy Prochnosti, No. 1, pp. 83–87, January–February, 2009.     

Bearing strength of carbon epoxy laminates under static and dynamic loading

An experimental study has been carried out to investigate both the static and dynamic bearing strengths of a pinned-joint carbon epoxy composite plate with [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking configurations. The static and dynamic experiments have been carried out according to the ASTM D953 standards and ASTM STP 749, respectively [ASTM D 953-D, Standard Test method for Bearing Strength of Plastics, ASTM Designation. 342; Joining of Composite Materials, ASTM STP 749, American Society for Testing and Materials (1981) 131]. The ratio of the edge distance to the pin diameter (E/D), and that of the width to the pin diameter (W/D) of the specimens were varied to obtain the static bearing strength and the S–N fatigue curve. The experiments show that the static bearing strengths reach their upper limit when E/D and W/D ratios are equal to or greater than 4 for both [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking sequences. The fatigue strength, on the other hand, reduces by up to 65% as E/D and W/D ratios increase for both stacking configurations.     

Method of measurement of the dynamic strength of concrete under explosive loading

Within the framework of the search for the method of determination of the strength properties of concrete under the action of an explosion or high-velocity impact, suitable for large scale concrete samples, the evolution of the compression pulse in plates or rods made of concrete with compressive strength of 30 MPa was investigated. It was found that wave configuration consisting of the ramped elastic precursor with insignificant stress jump at the front followed by a dispersed plastic shock wave is formed in the plates under uniaxial shock compression. In this experimental configuration, the compressive strength of the material is not identified. Experiments with concrete rods of various diameters have demonstrated the scalability of the wave process. It was established that the compressive fracture of the rods occurs at a distance of around twice their diameters and is accompanied by the fast decay of the load pulse after that weakly decaying elastic wave was propagated along the rods. The measurements of parameters of the compression pulse at the end of the fracture zone allowed us to determine the value of the dynamic compression strength of concrete equal to \(105\pm 20~\hbox {MPa}\), which turned out to be 3.5 times higher than the static strength.     

Aggregate breakage under dynamic loading

Numerical simulations with the Discrete Element Method are used to study agglomerate breakage under two different kinds of dynamic loading: normal impact and shear loading. Simple mechanical models based on energy balance are developed herein for each one and show good agreement with the results of the simulations. For impact, damage is found to depend on a dimensionless number N _i , which describes the ratio of the incoming kinetic energy to the internal bonding energy. For shear loading, damage is shown to depend on another dimensionless number N _f which describes the ratio of the frictional work to the internal bonding energy. The friction force is first modelled as a solid-like friction force, then the model is improved by using a granular frictional force. The two types of loading as damaging processes are then compared. These results appear to be consistent with the available experimental data on impact and abrasion wear tests.     

Effects of loading rate and temperature on cracking resistance in steel

A study has been made on the effects of loading rate and temperature on cracking resistance in steels. A method has been devised for estimating the carrying capacity of a steel structure under actual working conditions. Test data and published evidence give general temperature-dependence curves for the cracking resistance in static and dynamic loading, and a criterion for the carrying capacity is devised.Translated from Problemy Prochnosti, No. 7, pp. 30–33, July, 1992.     

Dependence of the dynamic strength on loading rate

Document presented at the 1st All-Union Conference on Fracture Mechanics of Materials, Lvov, October 20–22, 1987.     

Low-temperature cyclic cracking resistance of higher strength austenitic steels

Translated from Fiziko-Khimicheskaya Meckhanika Materialov, No. 6, pp. 27–32, November–December, 1989.     

Bearing strength of autoclave and oven cured Kevlar/epoxy laminates under static and dynamic loading

The pin bearing behaviour of woven Kevlar fibre-reinforced epoxy laminates, prepared by autoclave and oven curing methods, was examined under static and dynamic loading. Static bearing strength was determined as a function of bolt constraint and specimen geometry with particular attention to failure modes. The performance was very sensitive to lateral bolt tightening: a bolt with only a ‘finger-tight’ nut produced ∼100% improvement in bearing strength, compared with the performance of a pin-loaded hole. Some specimens were hygrothermally conditioned and indicated a 10% deterioration in the bearing strength of the bolt-loaded hole. However, this was well below the magnitude of variation in strength which resulted under different methods of production. The fatigue endurance limit (N ∼ 10⁶) at 1 Hz was only achievable at stress levels equivalent to 25% of the maximum static bearing strength.     

Effect of loading history on stress corrosion cracking in high strength steel

The effect of preloading on crack nucleation time was examined with compact tension specimens having various notch radius in 0.1N-H²SO⁴ aqueous solution for 200°C tempered AISI 4340 steel. Crack nucleation time t_n increases by preloading for a given apparent stress intensity factor K_p2. The curve K_?2 vs. t_n deviates upward from the curve for the non preloading case. A linear relationship between the crack nucleation time and parameter is seen in semi-log diagram, where is taken as the value at t_n=α due to preloading. The apparent threshold stress intensity factor increases with K_?2 which is the apparent stress intensity factor of preloading. A detached crack is nucleated at some distance from the notch root and extends in a form of circle. This distance increases with increasing K_?2. The effect of load reduction during crack growth was examined. When the K-value was reduced from K₁ to K₂, an incubation time was observed before the crack started growing under the K₂-value. The incubation time t_m tends to increase with increasing ΔK = K₁-K₂. The threshold stress intensity factor was also found to increase for high load reduction.In order to explain these experimental results, a new dislocation model is proposed on the basis of stress induced diffusion of hydrogen in high stress region ahead of the notch root or a crack. This model suggests that the change in the crack nucleation time and the increase of the incubation time due to preloading or load reduction are caused by reducing the hydrostatic pressure and by spreading the hydrogen saturated region which requires more time for the hydrogen accumulation due to preloading or load reduction. The theory predicts the experimentally observed relations between and t_n and between log t_in and ΔK.     

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality.The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.     

Simulation of crystal plasticity under dynamic loading

Phenomenological constitutive equations of relaxation type have been constructed and applied to simulate plastic deformation of heterogeneous media. Both the dislocation kinetics and the viscous model with function of relaxation times were used to calculate the plastic strain rate. The deformation at the meso scale level of polycrystals subjected to dynamic loading (including shock waves) has been numerically investigated. The results obtained are reported and discussed.     

Polymeric foam behavior under dynamic compressive loading

Polymeric foams are commonly used in many impact-absorbing applications and thermal-acoustic insulated devices. To improve their mechanical performances, these structures have to be modeled. Constitutive equations (for their macroscopic behavior) have to be identified and then determined by appropriate tests.Tests were carried out on polypropylene foams under high strain rate compression. In this work, the material behaviour has been determined as a function of two parameters, density and strain rate. Foams (at several densities) were tested on a uniaxial compression for initial strain rates equal to 0.34 s⁻¹ and on a new device installed on a flywheel for higher strain rates. This apparatus was designed in order to do stopped dynamic compression tests on foam. With this testing equipment, the dynamic compressive behaviour of the polymeric foam has been identified in the strain rate range [6.7.10⁻⁴s⁻¹, 100s⁻¹].Furthermore, the sample compression was filmed with a high speed camera monitored by the fly wheel software. To complete this work, picture-analysis techniques were used to obtain displacement and strain fields of the sample during its compression. Comparisons between these results and stress-strain responses of polypropylene foam allow a better understanding of its behaviour. The multiscale damage mechanism, by buckling of the foam structure, was emphasised from the image analysis.     

Fatigue strength of steels under multicycle impact loading

The functional dependence of the fatigue limit under impact loading on the fatigue limit in harmonic loading was derived for structural steels, together with the dependence of the coefficient of the effect of impact on impact toughness. The impact toughness values at which multicycle impact loading is detrimental are determined.Translated from Problemy Prochnosti, No. 3, pp. 87–93, March, 1994.