Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography

Summary

This method aims at locating vertical subsurface defects. Here, we couple a laser with a spatial light modulator and trigger its video input to heat a sample surface deterministically with two anti-phased modulated lines while acquiring highly resolved thermal images. The defect position is retrieved from evaluating thermal wave interference minima.

Abstract

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering thermal wave fields that are disturbed by the defect. This effect is measured and used to locate the defect. We form the destructively interfering wave fields by using a modified projector. The original light engine of the projector is replaced with a fiber-coupled high-power diode laser. Its beam is shaped and aligned to the projector's spatial light modulator and optimized for optimal optical throughput and homogeneous projection by first characterizing the beam profile, and, second, correcting it mechanically and numerically. A high-performance infrared (IR) camera is set up according to the tight geometrical situation (including corrections of the geometrical image distortions) and the requirement to detect weak temperature oscillations at the sample surface. Data acquisition can be performed once a synchronization between the individual thermal wave field sources, the scanning stage, and the IR camera is established by using a dedicated experimental setup which needs to be tuned to the specific material being investigated. During data post-processing, the relevant information on the presence of a defect below the surface of the sample is extracted. It is retrieved from the oscillating part of the acquired thermal radiation coming from the so-called depletion line of the sample surface. The exact location of the defect is deduced from the analysis of the spatial-temporal shape of these oscillations in a final step. The method is reference-free and very sensitive to changes within the thermal wave field. So far, the method has been tested with steel samples but is applicable to different materials as well, in particular to temperature sensitive materials.

Introduction

The laser projected photothermal thermography (LPPT) method is used to locate subsurface defects that are embedded in the volume of the test specimen and oriented predominantly perpendicular to its surface.

The method uses the destructive interference of two anti-phased thermal wave fields of the same elongation and frequency as shown in Figure 1b. In isotropic defect-free materials, the thermal waves neutralize destructively (i.e. zero temperature oscillation) at the symmetry plane by coherent superposition. In case of a material with a subsurface defect, the method takes advantage of the interaction of the latera....

Protocol

NOTE: Caution: Please pay attention to laser safety because the setup uses a class 4 laser. Please wear the correct protective glasses and clothes. Also, handle the pilot laser with care.

1. Couple the Diode Laser to the Projector Development Kit (PDK)

1. Prepare the breadboard.
   1. Preassemble all devices to the breadboard as shown in Figure 3. Place the breadboard with all preassembled devices in a laser laboratory.
2. Position the laser fiber mount on the breadboard.
   1. Attach the fiber to the laser fiber mount (cf. Figure 3).

Results

Following the protocol, side 1 of the steel sample with a subsurface defect at a depth of 0.25 mm was chosen to generate representative results. The defect was initially positioned approximately at the center of the illuminated area. The sample was then moved from -5 mm to 5 mm via the linear stage at a speed of 0.05 mm/s. Using these parameters, Figure 11a shows the scan data after extracting them from the depletion line. At this stage, the success of the experiment can .......

Discussion

The presented protocol describes how to locate artificial subsurface defects oriented perpendicular to the surface. The main idea of the method is to create interfering thermal wave fields which interact with the subsurface defect. The most important steps are (i) to combine an SLM with a diode laser in order to create two alternating high-power illumination patterns at the sample surface; these patterns are photothermally converted into coherent thermal wave fields, (ii) to let them destructively interfere whilst intera.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
500 W diode laser system, 940 nm	Laserline	LDM 500 - 20	Pilot laser class 2 @ 650 nm, diode laser is a class 4 laser system --> special laboratory needed
Laser control box	Laserline	Laser control box LDM	Add on to the laser system, used to switch electronically, laser threshold, shutter, laser on 0 V..5 V TTL
Control box scanner	Laserline		Add on to the laser system, used to adjust the optical output power via analog signal from 0 V..10 V
Fiber Laser Mount 2", f = 80 mm	Laserline		Add on to the laser system
Multifunction Data Aquisition (DAQ) Device + BNC Terminal	National Instruments	NI-USB 6251	The DAQ card is used to trigger the IR camera,  the DLP Light Commander 5500, control Laser and diode PDA 36A
Standard - PC			Control PC - graphic card for two screens, at least 4 x USB, Windows based
BNC cabel			Standard cable
HDMI cable			Standard cable
Micro USB to USB cable			Standard cable
LabVIEW 2013 SP1 Development System	National Instruments		Development environment for device control
LPPT control software	BAM		part of the LPPT software package by LabVIEW 2013 SP1
LPPT intensity  software	BAM		part of the LPPT software package by LabVIEW 2013 SP1
LPPT laser control software	BAM		part of the LPPT software package by LabVIEW 2013 SP1
Matlab 2016b	MathWorks		Postprocessing of the measurement data
LPPT postprocessing software	BAM		Postprocessing of the measurement data
IR camera control PC	InfraTec		Control PC is supplied by camera distributor
IR camera control software	InfraTec	Irbis 3 Professional
InfraTec SDK	InfraTec		Dynamic Link Library as interface between the native data aquisition format of Infratec and Matlab
IR camera	InfraTec	Image IR 8300	640 x 512, cooled InSb detector, wavelength 2 µm..5.7 µm, noise = 20 mK + accessories (LAN cable, Digital in/out cable, space ring, power supply, case)
Tripod	Manfrotto	161MK2B
IR camera mount	Manfrotto	405
Projector development kit (PDK) for digital light processing (DLP) technology (DLP Light Commander 5500)	Logic PD	DLP-LC-DLP5500-10R	DLP5500 Digital Micromirror Device from Texas Instruments included , light engine and case need to be disassembed
PDK control software	Logic PD		Included when delivered, DLP Light Commander control software
Mechanical platform for the PDK	BAM		Self made (140 x 230 x 420) mm3
Power meter control unit	Ophir	Vega	USB Interface
30 W power meter head	Ophir	30(150)A-LP1-18	Power meter head to determine Transmission of the projector system
500 W power meter head	Ophir	FL500A	Power meter for process supervision
Motion controller	Newport	ESP301	with USB Interface
Translation stage	Newport	M-ILS200CC	Connected to ESP301
Photodiode with amplifier	Thorlabs	PDA 36A-EC	1" mount
Reflective filter ND1	Thorlabs	ND10A	to be mounted to the PDA 36A
Pinhole 1"	Thorlabs	P1000S	to be mounted to the PDA 36A
Optical aluminium breadboard	Thorlabs	MB60120/M	(1,200 mm x 900 mm) base
Plano Convex Lens f = 200 mm	Thorlabs	LA1979-B	Coated for IR, first telescope lens
Plano Convex Lens f = 75 mm	Thorlabs	LA1145-B	Coated for IR, second telescope lens
xy-translation stage	Newport	M401	Used for adjusting the telecope
Beamsampler	Thorlabs	BSF20-B	Splits the optical output, used to reduce the optical input for the projector system
Mirror	Thorlabs	BB2-E03	Mirror for coupling the beam to the DLP Light Commander
Heavy duty lab jack	Thorlabs	L490	Used for the fiber mount and on top of the linear stage to position the sample (2x)
PDK-objective	Nikon	Nikon AF Nikkor 50 mm 1:1:8:D	Objective for DLP Light Commander, 50 mm
Plano Convex Lens f = 100 mm	Thorlabs	LA1050 -B	Lens is attached to the Nikon Objective
Bi-Convex Lens f = 60 mm	Thorlabs	LB1723 -B	Lens to be attached to the Nikon objective in order to determine the optical transmission with the 30 W measurement head
Square protected gold mirror	Thorlabs	PFSQ20-03-M01
High power IR sensor card	Newport	F-IRC-HP-M	Sensor card to check the optical pathway
2" crosshairs	BAM		Self-made
1" crosshairs	BAM		Self-made
Bullseye level	Thorlabs	LCL01
Translation Stage	Newport	M-UMR8.25	Used for measuring the beam profile
Micrometer screw	Newport	DM17-25	Used with translation stage M-UMR8.25
Mounted Zero Aperture Iris	Thorlabs	ID75Z/M	used to check the optical pathway
Bases and Post Holders Essentials Kit, Metric and Universal Components	Thorlabs	ESK01/M	Basis
Posts & Accessories Essentials Kit, Metric and Universal Components	Thorlabs	ESK03/M
M6 Cap Screw and Hardware Kit	Thorlabs	HW-KIT2/M
Construction Rails	Thorlabs	XE25L700/M
1" Construction Cube	Thorlabs	RM1G	Used to mount construction rails
Electrical discharge machining	Sodick	AG60L	www.sodick.de
St37 block of steel (100 x 100 x 40) mm3	BAM		self-made, hidden defect with remaining wall thicknesses of 0.25 mm, 0.5 mm, 0.70 mm, 1.25 mm (shown in Figure 5)
St37 block of steel (100 x 100 x 40) mm	BAM		self-made, hidden defect with remaining wall thicknesses of 1 mm, 1.5 mm, 1.75 mm, 2 mm (shown in Figure 5)
Graphite spray	CRC Industries Europe NV	GRAPHIT 33	Ref. 20760, 200 mL aerosol (Kontakt-Chemie)
Protective tape	Tesa	tesakrepp 4348	used to protect the hidden defects while coating