As the core component of mechanical connections, the stress state of bolts directly affects the safety and reliability of equipment. The bolt stress tester uses techniques such as ultrasonic waves, strain gauges, or torque methods to detect the pre tension, axial stress, and fatigue damage of bolts. However, the accuracy of the test results is affected by various factors. The following analysis will be conducted from the aspects of instrument performance, environmental conditions, operating specifications, material characteristics, and installation conditions.

1、 Instrument performance and calibration

1. Sensor accuracy and stability

The accuracy of the core components of the detector, such as ultrasonic probes, strain gauges, or torque sensors, directly affects the measurement results. For example, the ultrasonic method relies on the propagation time of longitudinal waves in bolts to calculate stress. If the frequency response of the sensor is insufficient or the signal acquisition is delayed, it will lead to time measurement errors, which in turn will affect the stress calculation results. In addition, sensors may age or drift after long-term use and require regular calibration.

2. Calibration methods and standard components

Calibration is the key to ensuring the accuracy of instruments. If there is a significant difference in material and size between the reference sample used for calibration (such as a standard bolt with known stress) and the actual tested bolt, it will result in the failure of the calibration model. For example, in the ultrasonic method, the sound velocity is related to material properties, and the difference in sound velocity between different steels may reach 5%. If the material of the calibration piece does not match that of the measured piece, the error will significantly increase.

3. Data collection and processing system

Modern detectors often use digital processing, and the rationality of algorithms (such as identifying ultrasonic reflection waves and filtering noise) directly affects the results. For example, low-frequency noise may mask the true echo signal, leading to misjudgment; However, excessive filtering may result in the loss of effective signal details. In addition, the applicability of software compensation models (such as temperature correction and material nonlinearity correction) also needs to be verified.

2、 The impact of environmental conditions

1. Temperature effect

Temperature has a significant impact on the elastic modulus, sound velocity, and electronic components of bolt materials. For example, the elastic modulus of steel decreases with increasing temperature (approximately 3% -5% for every 100 ℃ increase), and if temperature compensation is not performed during testing, the calculated stress values may deviate from reality. In the ultrasonic method, the speed of sound is more affected by temperature, with a change of about 0.5% for every 1 ℃ increase at room temperature.

2. Humidity and corrosive environment

High humidity environments may cause rusting or corrosion on bolt surfaces, altering their cross-sectional dimensions and material properties. For example, corrosion can reduce the effective cross-sectional area of bolts, leading to an increase in actual stress, and the detector may not be able to accurately measure due to an increase in surface roughness. In addition, humid environments may affect the insulation performance of electronic components and cause measurement noise.

3. Vibration and electromagnetic interference

In construction machinery or dynamic load scenarios, external vibrations may cause unstable contact between sensors and bolts (such as poor ultrasonic coupling), resulting in false waveforms. Electromagnetic interference (such as nearby motors and frequency converters) may affect the voltage signal of strain gauges or wireless transmission data, and needs to be resolved through shielded wires or filtering circuits.

3、 Operating standards and human factors

1. Installation and coupling quality

The ultrasonic method requires the application of coupling agent (such as Vaseline) on the bolt end face. If the coupling is poor (such as the presence of bubbles or oil stains), the signal attenuation can reach more than 30%, resulting in insufficient echo amplitude. In the strain gauge method, deviations in the placement of the patch or weak adhesion may introduce bending stress errors. For example, when the strain gauge is not aligned with the bolt axis, the measured strain contains bending components and needs to be corrected through bridge compensation.

2. Loading method and boundary conditions

If the loading method during testing does not match the actual working conditions (such as the torque method not considering the thread friction coefficient), the results will deviate from the true stress. For example, in humid environments, the friction coefficient of threads decreases, and the preload force generated by the same torque may decrease by 10% -15%. In addition, insufficient support stiffness (such as fixture deformation when detecting slender bolts) can lead to uneven stress distribution.

3. Operator skills

Human operation has a significant impact on the results. For example, uneven pressure on the ultrasonic probe may cause changes in the coupling state; In the strain gauge method, incorrect wiring or poor welding may cause virtual soldering; In the torque method, a too fast loading rate may result in higher readings due to bolt hysteresis deformation. Human error needs to be reduced through training.

4、 Material characteristics and bolt status

1. Material uniformity and heat treatment status

The uniformity of bolt material directly affects stress distribution. For example, if there is uneven quenching hardness in medium carbon steel bolts, differences in ultrasonic propagation paths may lead to stress calculation errors. Residual stresses (such as incomplete release after cold processing or heat treatment) can also interfere with the detection results and need to be eliminated through annealing or aging treatment.

2. Thread parameters and surface condition

The quality of thread processing, such as pitch error and flank wear, affects the torque stress relationship. For example, if the actual contact angle of worn threads decreases, the preload force may decrease by 20% under the same torque. Surface coatings (such as galvanizing or Dacromet coating) can change the bolt diameter and friction coefficient, and need to be corrected in the detection model.

3. Fatigue damage and creep

The bolts in service may develop microcracks due to fatigue, leading to stress concentration. The ultrasonic method can identify cracks through reflected wave characteristics, but early small cracks (size<1mm) may not be detectable due to wavelength limitations. Under high temperature conditions, bolt creep can lead to stress relaxation, and the detection results need to be corrected based on the time factor.

5、 Installation conditions and boundary effects

1. Stiffness of the connected component

The stiffness of the connected component affects the force distribution of the bolt. For example, if the connected component is made of soft material (such as aluminum alloy), deformation may occur during loading, resulting in the bolt actually bearing a load lower than the theoretical value. The detector needs to be corrected based on the system stiffness, otherwise the error can reach 10% -30%.

2. Pre tightening force loss and looseness

Bolts may experience a decrease in preload force due to relaxation or vibration during long-term use. The detector needs to distinguish between the remaining preload force and the initial preload force, otherwise it may misjudge the remaining life. For example, the ultrasonic method measures the current stress, while the torque method may not reflect the true state due to thread wear.

3. Interaction of multi bolt systems

In multi bolt connections such as flanges and bearing seats, the stress of each bolt may interact with each other due to differences in installation sequence and preload force. For example, overloading adjacent bolts may lead to stress concentration, and the detector may ignore the overall system effect when measuring a single bolt separately.