Maraging steel (Grade 250) pressurized chambers are used in booster stages for the launch vehicles and missiles. These are designed & realized with ultra high strength steels like Maraging steels in order to gain in range and pay load capabilities with optimal Factor-of-Safety(FOS). Effective manufacturing methodology and fracture control is a prime requirement however manufacturing processes give rise to defects that are inevitable. Non-Destructive Engineering techniques viz. ultrasonic testing, Radiography testing, Eddy current testing etc. address the issue of detecting passive defects only. Defects that are active i.e. critical and that could cause catastrophe shall be detected possibly during Proof pressure testing Strain gauge method adopted is restricted for strain measurements and is in-effective in detecting flaws as it is a highly localized. Acoustic Emission Technique (AET), as a whole field method, has ability to detect unstable flaws effectively. Acoustic emissions are generated due to defects, microstructural variation, presence of inclusions and second phase particles in metallic materials. AET identifies defects and discontinuities in terms of Acoustic Emission parameters. Sources of Acoustic Emission (AE) can be distinguished by their AE signature in terms of amplitude, Count and Energy. The severity can be quantified in terms of AE Parameters. In this paper an attempt is made towards predicting tensile failure load of Maraging Steel weldment with varying extents of notch thereby representing equivalent tight cracks as per Linear Elastic Fracture Mechanics (LEFM) design approach. Customized specimens were fabricated and notches were made using Electric Discharge Machining process. Tensile load has been applied to the test specimens with AE data acquisition. The AE distribution obtained from each specimen has been correlated to an equivalent Weibull distribution and represented in terms of weibull parameters. The significant Weibull parameters viz. Skewness (b value) and centroid of distribution curve (θ) are estimated. The distribution at a load of 85% of failure load is used for the prediction process. An empirical relation connecting Weibull parameters b, θ, b * θ, is proposed. It is observed that the product b * θ is linearly correlated to tensile failure load. On comparing results, predicted tensile failure load is closely matching to the recorded tensile failure load of the specimen. The average prediction capability of the proposed model is within 5–6%.

Prediction of Tensile Failure Load for Maraging Steel Weldment by Acoustic Emission Technique

Safety and reliability are primary concerns in launch vehicle performance due to the involved costs and risk. Pressure vessels are one of the significant subsystems of launch vehicles. In order to have minimal weight, high strength material viz. maraging steel M250 grade is used in realizing the pressure vessel casing hardware. Despite the best efforts in design methodology, quality evaluation in production and effective structural integrity assessment is still a farfetched goal. The evolution of such a system requires, first, identification of an appropriate technique and next its adoption to meet the challenges posed by advanced materials like maraging steels. In fact, a quick survey of the available Non-Destructive Evaluation (NDE) techniques suggests Acoustic Emission (AE) as an effective structural integrity assessment tool capable of identifying any impending failure or degradation at an earlier stage. Experience shows that the longitudinal welds in the pressure vessels are quite vulnerable to failure due to the fact that they experience the maximum stress (i.e. hoop stress). Loading welded tensile samples are quite synonymous to the hoop stress experienced by longitudinal welds. An attempt is made to compare the Acoustic Emission data acquired during tensile deformation of maraging steel welded specimens. A total of 16 welded specimen’s with known defects were studied for their tensile behaviour is in connection with Acoustic Emission data. The lowest failure load was 70.5 kN and the highest being 84.8 kN. AE activity graphs viz. cumulative AE activity, hit rate, energy rate, count rate, AE amplitude history, AE count history, AE energy history, amplitude-count correlation and hit amplitude distribution have been investigated and salient features with respect to the data have been critically studied and relevant correlations are arrived at.

https://acoustics.ippt.pan.pl/index.php/aa/article/download/2680/pdf_489

Comparison of Acoustic Emission Data Acquired During Tensile Deformation of Maraging Steel M250 Welded Specimens

Acoustic emission test method for investigation of M250 maraging steel pressure vessels for aerospace applications

Acoustic emission studies have been carried out on a batch of welded and center cracked specimens made of 18% Ni M250 maraging steel where the crack depth to specimen thickness ratio varied from approximately 10/80 to 25/80. Broad band AE transducers providing maximum sensitivity in frequency range 135 to 310 KHz were used for the AE monitoring. The paper brings out the AE performance of the specimens with inserted surface cracks of different sizes when the latter become critically severe leading to failure. The studies indicate the prediction possibility for the hardware constructed out of this material reasonably well before their final rupture. The AE signatures are also presented in a form that would facilitate generation of an acceptance criteria for the evaluation of hardware in real time.

Acoustic emission response of 18% Ni maraging steel weldment with inserted cracks of varying depth to thickness ratio

Acoustic emission studies have been carried out on a batch of center cracked M250 maraging steel welded specimens, having crack depth to specimen thickness ratio of 0.11 to 0.17. Broad-band AE sensors with 100-360 kHz bandpass filter have been employed for the experiments. The studies promise the prediction possibility for the hardware constructed out of this material well before their final future and hence can facilitate generation of suitable acceptance criteria for the evaluation of hardware in real time

T. Chelladurai

Papers

Acoustic emission response of 18% Ni maraging steel weldment with inserted cracks of varying depth to thickness ratio

Acoustic emission response of centre cracked M250 maraging steel welded specimens