Structural Integrity

Investmech specialises in structural integrity and was involved in a quite a number of projects over the past years. At Investmech, we ensure structural integrity of machinery and structures applying experience, standards and mission profile driven requirements. In most cases analysed structure performance is confirmed by static and variable amplitude testing. Transient and quasi-static finite element analyses are done, and where required, transient dynamic finite element analysis.

We offer the following with regards to structural integrity:

• Limit state design
• Fatigue life expectancy
• Fracture mechanics
• Fracture control programmes
• Mission profile driven design and re-design
• Design for construction
• Certification
• Audit
• Welding procedure specifications
• Procedures & Specifications

Vibrating structures and structural integrity

Investmech’s Managing Director was part of a team that developed a unique method to reconstruct field measured responses in a test laboratory or on a dynamic simulation of the test specimen, using a state-of-the-art application of NARX models and forward-inverse-reverse-inverse system identification techniques. This technique is used extensively to define new boundaries in the field of Structural Integrity in the world. The technique allows Investmech to accurately describe how a client’s product will perform subject to a given or prescribed mission profile. A typical example of how the technique is used is where the input forces of the sea are reconstructed on a ship. This enables Investmech to prescribe new design specifications for the ship because structural inputs are known. The same technology is applied on motor vehicles and automotive components, trucks, structures, heavy equipment, buildings, etc.

Vibrating structures all over the world may suffer premature failures due to the dynamic loading caused by variable input loads. Further, with increasing pressure on production, many structures are loaded more than was anticipated in the design phase.

Investmech consultants investigate the vibration response of the structure and, based on an analysis, redesign the structure or use vibration control techniques to reduce the strain response of the structure to acceptable and safe levels. Strain response is the important objective of this process as the damage on the structure is related to the strain response and the applicable material’s fatigue characteristics.

The techniques that we use include the following:

1. Vibration measurement and analysis is done by performing operating and natural mode shape, natural frequency, and maximum allowable vibration level analysis on measured signals. Frequency Response Functions, for example, are used to design isolation between screens and supporting structures. FRF’s also characterizes the responses between two channels.
2. Strain gauges are applied, and strain responses measured to calculate damage at the measurement points and to infer damage at other critical points on the structure. The strain responses are also used in conjunction with acceleration responses to quantify the input forces on a test-rig or for a Finite Element Model that will yield the same time domain responses as was measured.
3. Structural analysis and redesign are done using structural packages that predict the natural modes and internal loads. Redesign is then done on the package to ensure structural integrity, and manufacturing drawings are delivered.
4. Structural modifications are then proposed and Investmech offers project management to upgrade the structure on site.
5. Continuous condition monitoring of structures ensures that structural deterioration is detected at an early stage and condition–based maintenance scheduled accordingly.

Transient strain and acceleration responses of a headgear

Investmech measured strain and acceleration responses of a headgear during various mission profile driven operating conditions (trips, emergency stops, normal operation, etc.). These responses were used to predict axial, shear and bending moments on members and connections. From these, compliance with strength and fatigue requirements were confirmed. Results showed that on the structure below, thermal strains dominated the daily response.

Structural dynamics

Investmech’s experience in structural dynamics cover the modelling and integrity assurance of super structures for ships, aeroplanes, trains, plants, incline conveyors, large material handling machines and the simulation of the transient dynamics of structures during demolition.

Raise bore drill string structural integrity

Measured strains were used with a non-linear time and state dependent transient multi-degree of freedom mathematical model solved by the Runge-Kutta integration algorithm to find maximum and operating torque conditions. Start-up and post-stalling responses dominate design requirements.

Investmech offers the structural integrity assessment and certification of structures subject to transient loads and consequential responses. Where the transient inputs are not known, responses can be measured on existing structures and inputs inferred using autoregressive calculations. Transient analysis of non-linear contact bodies and coupled systems are time consuming and may take a long time to complete. 

Structures under steady-state vibrating conditions

According to the lumped mass theory, the dynamic response of a super structure is the sum of the responses of the individual natural modes at their respective natural frequencies. Resonance is in most cases the cause of excessive structural responses, even a distance away from the source. Investmech ensures that vibrating sources in structures are positioned such to minimize excitation of structural natural modes (resonance). A harmonic modal analysis is sufficient for this purpose.