Technical Support

1. Delamination Defects

Abrupt Interface Acoustic Impedance Change

When delamination occurs inside composite materials (such as carbon fiber reinforced resin matrix), ultrasonic waves reflect at the interface between the resin matrix (high impedance) and air gap (low impedance), resulting in 180° phase reversal of the reflected wave.

In C-SAM, delamination appears as phase-reversed regions marked by specific colors such as red in color-coded images.

2. Voids and Bubbles

Gas-filled Defects

Voids and pores in metal castings or packaging materials are gas-containing defects. The acoustic impedance ratio between solid and air is extremely high (e.g., steel to air ≈ 30,000:1), inevitably causing 180° phase reversal of reflected waves.

Such defects show high-contrast boundary contours in ultrasonic images due to phase reversal signals.

3. Internal Cracks (Closed Defects)

Characteristics of Crack Interfaces

When ultrasonic waves encounter closed cracks filled with air or impurities, the abrupt acoustic impedance change at crack interfaces triggers phase reversal of reflected waves.

For transverse cracks tested with straight probes, the backwall echo disappears accompanied by phase reversal; longitudinal cracks present phase switching features when inspected with angle probes.

4. Shrinkage Cavity Residues

Internal Discontinuous Defects

Shrinkage cavities in castings form internal discontinuous voids. Ultrasonic reflection at metal-void interfaces is accompanied by phase reversal, with high-amplitude and highly continuous defect echoes.

5. Special Composite Defects

Inclusion Interfaces

Non-metallic inclusions (oxides, sulfides) have small impedance difference with the matrix and generally do not cause phase reversal.

If delamination or air gaps exist inside inclusions, local phase reversal can be used as a characteristic for identifying secondary defects.

Conclusion

Phase reversal mainly occurs in gas-containing defects or interfaces with abrupt acoustic impedance changes. Comprehensive evaluation shall be conducted by combining reflected wave amplitude, phase spectrum analysis and color-coded imaging.