Designing a cyclic pressure test programme requires more than just "pressurise and depressurise repeatedly." Ramp rate, hold time, and cycle count interact with the specific fatigue mechanism being investigated. Here's the implementation approach.
Pressure Profile State Machine
The core of the system is a state machine with eight states: IDLE, FILLING, RAMPING_UP, HOLDING, DEPRESSURIZING, CYCLE_COMPLETE, TEST_COMPLETE, and FAULT. A CyclicPressureTest controller is configured with the target pressure, ramp time, hold time, total cycle count, and depressurisation time, and it tracks the current cycle number and a running log of cycle data.
Each cycle moves through three phases. In the ramp-up phase, the setpoint is calculated as a linear function of elapsed time relative to the programmed ramp duration — a simple linear ramp, though an s-curve profile could be substituted for gentler loading on sensitive components. The valve control output is updated continuously while the pressure sensor is sampled at 10Hz, with every reading logged alongside its timestamp.
In the hold phase, the valve is held at the target pressure while pressure is sampled continuously. This is also where fault detection happens: if the pressure drops more than 5% within a short window compared to a few samples earlier, the state machine immediately flags a FAULT and returns the failure reason and the cycle data collected so far — this is the mechanism that catches a sudden pressure drop indicating a seal failure or leak without needing a human watching the live graph.
In the depressurise phase, the valve is opened to vent the chamber and the system waits out the programmed depressurisation time before incrementing the cycle counter. Once the total cycle count is reached, the state machine reports TEST_COMPLETE; otherwise it reports CYCLE_OK and the next cycle begins.
Fatigue Data Analysis After Test Completion
Once the test finishes, the cycle log is analysed for consistency rather than just pass/fail. The analysis walks through every recorded cycle, extracting the peak pressure reached and the time taken to reach 95% of the target pressure (a practical proxy for ramp performance). From these two series, it computes the mean and standard deviation of peak pressure across the whole test, then compares the average of the first 10 cycles against the average of the last 10 cycles to calculate a drift percentage — a meaningful drift here can indicate a developing leak or an actuator that's losing performance over the course of thousands of cycles, well before it would show up as an outright failure.
The final fatigue report packages the component ID, the test standard used, the target pressure, the number of cycles completed, and the consistency analysis into a single record, with an automatic PASS/REVIEW REQUIRED verdict based on whether the peak-pressure drift stayed within 5%. This becomes the documentation trail for fatigue qualification sign-off.
The Neometrix PLC Controlled Autoclave Pressure Tester implements this state-machine architecture for automated, unattended cyclic pressure testing with full data logging for fatigue qualification programmes.
→ https://neometrixgroup.com/products/PLC-controlled-autoclave-pressure-tester
