Standard Practice for Part-to-Itself Examination Using Process Compensated Resonance Testing Via Swept Sine Input for Metallic and Non-Metallic Parts
5.1 PCRT Applications and Capabilities—PCRT PTI examination has been applied successfully to a wide range of parts in manufacturing and maintenance environments. Examples of manufacturing processes, repair processes, and in-service damage mechanisms evaluated with PTI are discussed in 1.1. PCRT has been shown to provide cost effective and accurate PTI-based NDT, process monitoring, and life monitoring in many industries including automotive, aerospace, and power generation. Examples of successful applications currently employed in commercial use include, but are not limited to:
(1) Heat treatment operations:
(a) Aerospace gas turbine engine components (blades, vanes, disks)
(b) Additively manufactured components
(c) Steel mechanical components
(d) Industrial gas turbine blades
(2) Induction hardening and carburization (both case-hardened and through-hardened parts):
(3) Hot Isostatic Pressing (HIP):
(a) Gas turbine engine components (blades, vanes, disks)
(4) Shot peening:
(a) Steel mechanical components
(5) In-service thermal history, aging, creep damage, fatigue:
(b) Industrial gas turbine blades
(c) Aircraft landing gear wheels
(6) Maintenance repair/rejuvenation processes:
(c) Aircraft landing gear wheels.
5.2 General Approach and Equipment Requirements for PCRT via Swept Sine Input:
5.2.1 PCRT systems comprise hardware and software capable of inducing vibrations, recording the component response to the induced vibrations, and analyzing the data collected. Inputting a swept sine wave into the part has proven to be an effective means of introducing mechanical vibration and can be achieved with a high-quality signal generator coupled with an appropriate active transducer in physical contact with the part. Collection of the part’s resonance response is achieved by recording the signal received by an appropriate passive vibration transducer. The software required to analyze the available data may include a variety of suitable statistical analysis and pattern recognition tools. Measurement accuracy and repeatability are extremely important to the application of PCRT.
5.2.2 Hardware Requirements—A swept sine wave signal generator and response measurement system operating over the desired frequency range of the test part are required with accuracy better than 0.002 %. The signal generator should be calibrated to applicable industry standards. Transducers must be operable over same frequency range. Three transducers are typically used; one Drive transducer and two Receive transducers. Transducers typically operate in a dry environment, providing direct contact coupling to the part under examination. However, noncontacting response methods can operate suitably when parts are wet or oil-coated. Other than fixturing and transducer contact, no other contact with the part is allowed as these mechanical forces dampen certain vibrations. For optimal examination, parts should be placed precisely on the transducers (generally, ±0.062 in. (1.6 mm) in each axis provides acceptable results). The examination nest and cabling shall isolate the Drive from Receive signals and ground returns, so as to not produce (mechanical or electrical) cross talk between channels. Excessive external vibration or audible noise, or both, will compromise the measurements.
5.3 Constraints and Limitations:
5.3.1 PCRT cannot separate parts based on visually detectable anomalies that do not affect the structural integrity of the part. It may be necessary to provide additional visual inspection of parts to identify these indications.
5.3.2 Excessive variation in part geometry or base material properties may limit the sensitivity of PCRT PTI examination.
5.3.3 A direct measurement of a single geometric dimension of a region undergoing a material state change, such as the case depth (in centimeters or inches) of an induction hardened region, is generally not possible with PCRT PTI. The frequency changes are dependent on the total volumetric effect of the process that causes the material state change. With accurately trained acceptability limits, however, PCRT PTI is very effective at screening populations of components for acceptable and unacceptable processing.
5.3.4 PCRT will only work with stiff objects that provide resonances whose peak quality factor (Q) values are greater than 500. Non-rigid materials or very thin-walled parts will not yield useful Q values.
5.3.5 While PCRT can be applied to painted and coated parts in many cases, the presence of some surface coatings such as vibration absorbing materials and heavy oil layers may limit or preclude the application of PCRT.
5.3.6 While PCRT PTI examination can be applied to parts over a wide range of temperatures, it cannot be applied to parts that are rapidly changing temperature. The part temperature should be stabilized before collecting resonance data.
5.3.7 Misclassified parts in the teaching set, along with the presence of unknown anomalies in the teaching set, can significantly reduce the accuracy and sensitivity of PCRT.
1.1 This practice covers a general procedure for using the Process Compensated Resonance Testing (PCRT) via swept sine input method to perform Part-to-Itself (PTI) examination on populations of newly manufactured and in-service parts. PCRT detects resonance pattern differences in metallic and non-metallic parts. Practice E2534 for Defect Detection with PCRT and Practice E3081 for Outlier Screening with PCRT cover the development and application of PCRT sorting modules that inspect a part at a single point in time. These methods use the resonance frequency spectra recorded from test parts and perform different statistical analyses to compare test parts to reference populations. These comparisons include, and must compensate for, the normal geometric, material, and processing variations present in any population of parts. In many applications, however, the user may need to evaluate the effects of a single processing step or in-service load in isolation from other sources of variation. For example, a manufacturer may want to perform process monitoring and control on a heat treatment or hardening process. A maintainer may want to evaluate the effect of service cycles in an engine. A PCRT PTI examination measures the resonance frequency spectrum of a part at two points in time, such as before and after a manufacturing process step, and calculates the change in resonance frequencies to evaluate the effect of the intervening process. Control limits can be set on the frequency change to field a PTI PASS/FAIL inspection capability. The limits may be based on training populations of parts with acceptable and unacceptable levels of change, model predictions of the effects of part changes, or criteria derived from process control practices. Manufacturing processes and in-service loads that can be evaluated with a PCRT PTI inspection include, but are not limited to heat treatment, hot isostatic pressing (HIP), shot peening, induction hardening, carburization, coating, thermal history changes, residual stress changes, creep, plastic deformation, corrosion, and fatigue. This practice is intended for use with instruments capable of exciting, measuring, recording, and analyzing multiple, whole body, mechanical vibration resonance frequencies in acoustic or ultrasonic frequency ranges, or both.
1.2 Units—The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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