Investigation of installation characteristics of ASTM-F1852 twist-off type tension control fasteners under various temperature levels

Twist-off tension control method is one of the available and recognized methods for bold pretensioning in the steel construction industry. Twist-off tension control bolts are commonly used in steel construction because they are fast, easy to utilize and convenient to inspect. Twist-off tension control is characterized by a spline at the end which can shear off at a circular notch (torque control groove) when the required preload is achieved. Preload in twist-off assemblies are based on torque control that can be affected by friction between components of the bolt assembly. This friction can be affected by any environmental condition or material property such as bolt material strength, geometric properties of the assembly, thread and lubrication conditions of the nut and bolt and installation temperature. Earlier studies have shown that twist-off bolt assemblies used in the field and left exposed to weather for a period of time prior to applying the pretensioning force, demonstrate degradation in the achieved tensile force. This phenomenon formed a series of research concerning effects of moisture, delay installation and temperature levels on the achieved tensile force in the bolt. The effect of temperature levels in degradation of achieved tensile force forms the principle parameter of concern in the current research. Results showed that the twist-off bolt assemblies failed to achieve the minimum specified pretension for installation temperatures less than 0°C. Empirical equations were developed to provide engineers and fabricators of an estimate of the reduction in the achieved pretensioning force in twist-off bolt assemblies with the change in temperature levels at time of installation.

Investigation of installation characteristics of ASTM-F1852 twist-off type tension control fasteners under various temperature levels

Twist-off tension control method is one of the available and recognized methods for bold pretensioning in the steel construction industry. Twist-off tension control bolts are commonly used in steel construction because they are fast, easy to utilize and convenient to inspect. Twist-off tension control is characterized by a spline at the end which can shear off at a circular notch (torque control groove) when the required preload is achieved. Preload in twist-off assemblies are based on torque control that can be affected by friction between components of the bolt assembly. This friction can be affected by any environmental condition or material property such as bolt material strength, geometric properties of the assembly, thread and lubrication conditions of the nut and bolt and installation temperature. Earlier studies have shown that twist-off bolt assemblies used in the field and left exposed to weather for a period of time prior to applying the pretensioning force, demonstrate degradation in the achieved tensile force. This phenomenon formed a series of research concerning effects of moisture, delay installation and temperature levels on the achieved tensile force in the bolt. The effect of temperature levels in degradation of achieved tensile force forms the principle parameter of concern in the current research. Results showed that the twist-off bolt assemblies failed to achieve the minimum specified pretension for installation temperatures less than 0°C. Empirical equations were developed to provide engineers and fabricators of an estimate of the reduction in the achieved pretensioning force in twist-off bolt assemblies with the change in temperature levels at time of installation.

Revisiting Failure of Brittle Materials

Failures of isotropic brittle materials were revisited to find out whether there are unified failure criteria which can be applied to a load-carrying structural component made of a brittle material regardless of whether the component has a crack, a circular notch, or neither of these. To this end, a set of failure criteria were considered to determine failure of a local point which may be located at the crack tip, a notch tip, or any other location. The proposed criteria consist of two conditions, of which both must be satisfied for failure to occur. The first condition is that the local stress must not be lower than the failure strength of the material. The second condition is the stress-gradient condition. Even if the stress at a local point far exceeds the failure strength of the material, failure will not initiate until the stress gradient condition is satisfied. Four different examples cases were presented to explain the proposed failure concept.

A Stress concentration factor for interacting surface notch and subsurface hole

We propose a conservative method for the calculation of the maximum stress concentration factor (SCF) for an interacting notch-hole pair and for a double semi-circular notch (i.e., a notch that has an additional small semi-circular notch ahead of its tip). The method is based on a linearly elastic Airy stress function solution for a circular hole. The notch-hole and double notch configurations are aligned vertically with respect to uniform uniaxial (horizontal) stress. This means, a uniform horizontal tension is applied to a notch-hole pair that lie on a vertical axis. For the notch-hole pair, the maximum interacting SCFs are calculated for edge to edge gaps equal to hole sizes of 2.5a, 5a, 10a and 15a, where a is the hole radius. The analytical results are validated by 2-D finite element calculations. The presented simple approach provides good results with errors well below 10% in most cases compared to the detailed finite element analyses. Fatigue notch factors that can be thought of as the effective SCFs in fatigue analyses are determined. By using the simple approach, computationally costly finite element analysis can be avoided.