Prior Loading History Research Articles

Estimating the potential strength of concrete with prior load history

Recommended procedures in evaluating the results of concrete core testing to establish the estimated in situ cube strength or estimated potential strength of concrete are given in standards and guidance documents. Many factors have been considered to determine their combined influence in the interpretation of the conversion process, from a core strength to an equivalent moulded cube strength. Nonetheless, it is recognized that further research is required to determine the effects of other influencing factors. This experimental investigation examines the effects of different degrees of simulated uniaxial compressive load history within prismatic concrete specimen on the strength of cores drilled in the direction of the load history from different locations of the specimen. The results show that the strength of a core drilled from such parent concrete may be as much as 31·7% lower than that of a similar size core drilled from similar parent concrete which has no prior load history.

Subcritical Crack Growth in Y‐TZP and Al2O3‐Toughened Y‐TZP

Crack velocity curves for Y‐TZP and Al2O3‐toughened Y‐TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m1/2 for Y‐TZP and 3.6 MPa·m1/2 for Al2O3‐toughened Y‐TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m1/2. It is recognized that the dependence of the shielding term on the crack‐tip stress field renders transformation‐toughened materials particularly susceptible to stress‐corrosion cracking. This interrelation leads to the definition of a steady‐state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack‐tip stress field, not depending on prior loading history.

Stability behaviour of semi-rigid sway frames

Due to the inelastic behaviour of beam-to-column connections, the stability behaviour of semi-rigid steel frames is affected not only by the loading and unloading characteristics of connections at the critical points, but also by their prior load history up to these points. Thus, the analysis of inelastic critical and post-critical behaviour of semi-rigid frames becomes very complicated, when compared with that of the usual pinned or rigid frames. This paper utilizes a method developed by the authors to examine precisely how the stability behaviour of semi-rigid frames is influenced by factors such as modelling of connections, load conditions, geometrical imperfections, and the load history caused by cyclic wind load. Based on these results, the validity of the effective length factor as applied to the design of semi-rigid frames is discussed.

Dynamic Load Transfer in Virgin and Damaged Granular Rock Media

An experimental investigation is conducted to study dynamic load transfer in granular rock media. The granular rock media is modeled as a one-dimensional chain of disks fabricated from four different types of white Vermont marble. The study focuses on the effect of microstructure on transient pulse propagation. The transient pulse is generated by exploding a small amount of lead azide charge on top of the rock disk assembly. During wave propagation dynamic contact strains are recorded using electrical resistance strain gages. The location of the strain gages is chosen such that the averaging effects are minimized. This information is used to calculate wave velocity and attenuation as a function of the cumulative damage in the disk assembly. The results indicate a considerable influence of the microstructure and prior loading history on the wave propagation process. A correlation exists between the stress wave velocity and the microstructure of the rock material. Also the wave velocity increases initially and then decreases with repeated explosive loading.

Creep experiments for parameter identification of the stochastic model of steck with the powder metallurgical alloy AlMn10

Results from uniaxial creep experiments are used for identification of the parameters of the stochastic model of Steck for inelastic steady state strain rate andfor the transients after changes of stress or temperature formulated in the model. The identification procedure is shown for a powder metallurgical aluminum-alloy that can be taken as a model-material for a dispersion hardening steel. It is demonstrated that under changes of stress and temperature, respectively, the model predicts changes of the inelastic strain rate which are independent of the prior loading history.

Microscopic-damage-based criterion for fatigue failure prediction

A computer-aided X-ray rocking curve analyser was developed to monitor the accrued microstructural damage occurring during low cycle fatigue of an aluminium alloy. These results indicated a linear relationship between the X-ray rocking curve half-width and the fraction of life. The resultant damage-assessment curve provided a nondestructive means of estimating the time to failure in a sample with unknown prior loading history. This failure criterion was found to be independent of the strain amplitude and was equally applicable to either high cycle or low cycle fatigue.

Spectrum Loading and Crack Growth

An analytical device for improving the accuracy of crack growth predictions in metal subjected to variable amplitude cyclic loading is presented. A modification to the linear cumulative growth idea is proposed which incorporates a consideration of prior load history by taking into account the yield zone ahead of the crack tip. Correlation between analysis and experimental results for six different cases shows that the scheme, even though only a first order improvement on the Miner idea, is sound and can be used with confidence for design and analysis.

Behavior of Reinforced Concrete Under Repeated Load

An experimental investigation of strains, stresses, crack widths and end slips in axially reinforced concrete prisms subjected to repeated tension loads is presented. Cylindrical specimens were reinforced with deformed steel bars. To obtain the strain distribution along the length of the bars, these were sectioned longitudinally, grooved for installation of 30 strain gages along the length, and after wiring the gages, the sections were rewelded to form instrumented bars with effectively undisturbed exterior. The specimens were subjected to repeated loads at different stress levels. Residual effects after unloading and changes in strain distribution which results from prior loading history are analyzed. Results show that effectiveness of stress transfer between concrete and steel depends primarily on the magnitude of the previous maximum load. The greater the magnitude of this load, the lesser is the effectiveness of stress transfer at subsequent lower stress levels. Analytical evaluations of the effects of shrinkage and tension loading are also presented. The results of these evaluations give an insight into the phenomena of cracking and stress transfer between steel and concrete.