**7. Conclusions**

This paper presents a comprehensive study of the fracture process in an Al–Ti laminate prepared by explosion welding. The testing methods used are modern and the results obtained are novelty. Mechanical load curves of SENB samples, *P–COD*, were examined in order to accurately interpret the fracture process; the fracture surfaces of the samples were thoroughly tested using SEM, and AE signals were recorded and subjected to various methods of analysis of wave packets of AE signals.

Based on the loading tests of the SENB specimens at *T*1 = 20 ◦C and *T*2 = −50 ◦C temperatures and the load curves, it was found that the *P–COD* curves had an atypical form, particularly at lowered temperatures.

Tests of the fracture surfaces of the specimens using optical and SEM microscopes found a delamination crack, which is usually initiated by the brittle phase fracture in the transition zone between the inner layer of the AA1050 alloy and the layer of the Ti6Al4V alloy. Then, the delamination crack develops primarily through the shear fracture mechanism. The propagation of the main crack in the base layers of the AA2519 and Ti6Al4V alloys occurs through the ductile mechanism of the growth of voids for two test temperatures.

The recording of AE signals and using different methods for the analysis of these signals, including non-hierarchical clustering methods (*k*-means) and analyses using Waveform Time Domain, Fast Fourier Transform (FFT Real), Waveform Continuous Wavelet using the Morlet wavelet and Waveform Time Domain (Autocorrelation), enabled the identification of four classes of signals and characterisation of the primary mechanisms of the fracture processes in the tested layered Al–Ti composite:


The recorded AE signals were used to determine the primary differences in the fracture process of the Al–Ti laminate. At temperature *T*1<sup>=</sup> 20 ◦C, the growth of the delamination crack and the main crack occur almost simultaneously. However, at temperature *T*2 = −50 ◦C, the delamination crack precedes the main crack, and the development of cracks in the base materials occurs without interaction between them. This explains the non-characteristic and illogical behaviours observe in the specimen load curves (*P–COD*) at different test temperatures.

These comprehensive tests indicated that the methods of analysing AE signals can be effectively used to identify the development of cracks in structural members. They can identify characteristic mechanisms of the formation and development of defects, also at very early stages of damage evolution, which cannot be achieved with other NDT methods. This fact can be used to create an automatic diagnostic system capable of determining the types and mechanisms of the development of potential damage at every stage of material use and assessing the reliability of the structure.

**Author Contributions:** Conceptualization, G.S., I.D. and A.K.; Methodology, G. ´ S., D.I. and A.K.; Software, S.L. and ´ A.K.; Validation, S.L. and T.P.; Formal Analysis, G.S., I.D. and A.K.; Investigation, T.P. and A.K.; Resources, G. ´ S. and ´ I.D.; Data Curation, A.K. and T.P.; Writing—Original Draft Preparation, G.S., I.D. and A.K.; Writing—Review and ´ Editing, G.S., I.D. and A.K.; Visualization, A.K. and S.L.; Supervision, G. ´ S. and I.D.; Project Administration, G. ´ S.; ´ Funding Acquisition, G.S. and I.D. All authors have read and agreed to the published version of the manuscript. ´

**Funding:** This research was funded by the National Science Centre, Poland (No. 2017/25/N/ST8/00179) and the Ministry of Science and Higher Education of Poland (No. 01.0.08.00/2.01.01.00.0000 MKPK 20.001 and 02.0.06.00/2.01.01.00.0000 SUBB. BKWB. 20.001.

**Conflicts of Interest:** The authors declare no conflict of interest and funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
