Evaluating Methodologies in India – Advances and Gaps

NDT in India has witnessed significant progression over the past two decades. The earliest facilities for NDT were estab­lished in government research laboratories. Towards the end of the twentieth century, a number of organisations like National Aerospace Laboratory (NAL), Electronics Corporation of India Limited (ECIL), Hindustan Aeronautics Laboratory (HAL), Aeronautical Development Agency (ADA) and others commenced their tryst with non-destructive tests for composites. All these estab­lishments delved into producing varying kinds of composites which could not be evaluated, certified and supplied for use unless subject­ed to rigorous NDT tests.

In India, NDT is relied upon by a multitude of companies for prod­uct quality assessment. This is seen in industries ranging from wind energy to aerospace and defence. Ultrasonic Testing (UT) is used by more than 80% of businesses for the NDT of composites. Pulse echo and through transmission ultrasonics can reveal delamination, mac­ro porosities, trans-laminar cracks, and detect foreign components. The B scan method also aids identification of anomaly depth. These approaches provide a comprehensive range of defect detection for composites. The helicopter division of HAL also uses computed to­mography (CT). CT scan helps evaluate post process verification of honeycomb structure. The method is also very helpful for sub-sur­face flaw detection in composites. Various defects that can be detect­ed are blown core, porosity and even entrapped fluid.

At the research level, an in-depth understanding of defects is re­quired. This includes the type, extent, root cause, stage of defect and others. Thus, many Indian research laboratories, including HAL and the Defence Research and Development Organisation (DRDO), employ radiography, thermography, X-ray diffraction (XRD) as other NDT methods. Some of the research institutes have also used Raman spectroscopy for determination of residual stresses in composites. The interlaminar residual stresses due to difference in Coefficient of thermal expansion (CTE) of reinforcement and ma­trix can also be studied. The technique is very successful in crystalline structures like carbon and thermoplastic matrix. However, Raman peaks for amorphous structures like glass, thermosetting resins are highly irregular and difficult to evaluate.

In addition to above, there are some performance specific NDTs required in composites like dielectric loss tangent, helium leak test, etc. These NDTs and equipment, however, are specific to the perfor­mance that is desired from these composite parts and are not generic for all composites produced. Hence, they are found with laboratories or select industries only.

A number of tier 1 Indian companies, including TATA Advanced Systems Limited, Laxmi Metal Works, Larsen and Toubro, etc., have ultrasonic test machines. Many of these machines are automated for speed and repeatability. CT is also available at leading research insti­tutes and development agencies, along with UT, which is used to test advanced composites.

Ultrasonic NDT – detection and evaluation challenges
Conventional ultrasonic NDT uses piezoelectric transducers. The high acoustic scattering owing to anisotropic material causes strong attenuation. The low back wall echo implicates many Indian firms to rely on through-transmission technique for UT. However, the through-transmission technique can only determine extent and location of defect, and not the depth of defect. A pulse-echo based detection is used for better definition of defect. Another important variable in detection is coupling agent. The water has been an ideal media as the acoustic impedances are in same dimension. This en­ables better sound penetration. However, it poses a challenge for larger sized components and hygroscopic composites. The squirter technique, for instance, would require constant pressure and precise adjustments, thus, complicating the detection.

The evaluation of NDT in composites is as challenging as the detec­tion itself. The ultrasonic detection in composites is mostly at low frequency. The probes range from 75 KHz to 2 MHz. The signal resolution greatly decreases with the probe frequency. The reduced resolution increases risk of noise inclusion in evaluation. Thus, UT machines for composites require far superior signal-to-noise ratio (SNR), keeping machine noise to the lowest level and compensating the material noise.

Many companies use B scan and C scan machines. However, 60 to 65% of Indian companies in the Micro, Small, and Medium Enterprise (MSME) sector continue to utilise A scan machines for composites. The evaluation of raw A scan data is tricky. Figure 1 il­lustrates how key defects affect ultrasonic waves in the pulse-echo technique. It is obvious that interpreting a signal often depends on the return from many points and not just one. Thus, evaluation of de­fect type in A scan would require immense experience and scientific understanding of the product and detection technique.

There is, however, a slight advantage to evaluating defect type and depth with a B scan. Machines with development laboratories, OEMs, and tier 1 composite manufacturers are typically C scan or a combination of B scan and C scan. Compared to A scan machines, these machines simplify the evaluation process.

Despite the advanced capabilities of machines, composite evaluation is still a specialised area. Every composite yields different results, with the same materials used to form them. Same input materials can produce variety of composites. Thus, there is no single stand­ard that can cover all the composites for evaluation. The standards like ASTM E 2580:2017 offer a governing framework for detection and assessment. The differences in each composite necessitate the production of specimens containing known defects and calibrating the process accordingly. This makes ultrasonic NDT an incredibly skill-demanding task.

Efforts are being made to use AI to evaluate ultrasonic NDT. However, these are currently limited to mass-produced products. The machines must iteratively detect and evaluate the same parts. India is still far from developing generic artificial intelligence that covers composites ultrasonics NDT in a broader way.

Ultrasonic NDT service providers network for composites
The technology fully matures, once it reaches medium and small-scale companies. However, such companies can only utilise such sophisti­cated technology, once the whole spectrum of resources for that tech­nology has been developed in that location. Therefore, in addition to sophisticated machines, it also requires standards, skilled manpower, and market demands.

It is estimated that 12 to 15 small-scale freelancers offer services in ul­trasonic NDT for composites. These service providers involve level II UT experts, who are equipped to detect and evaluate defects in composites with ultrasonics. Most of these small-sized companies work with A scan machines, as they are highly portable.

Ultrasonic NDT machine builders
In the last two decades, India witnessed incubation of many ultrasonic machine builders. At first, most machine builders imported probes but integrated to robotics domestically. This changed with increasing demand and especially variation in demand. There are couple of probe manufacturers in India now.

The first generation of machines majorly used water-coupled or water-immersed inspection. However, the hygroscopic composites needed a different probe. This welcomed soft tip probes in operations. The soft tip probes in automation further required…

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