Jeremy Blake Renshaw

Measurement of crack opening stresses and crack closure stress profiles from heat generation in vibrating cracks

A method is described to measure crack opening stresses and closure stress profiles of a surface-breaking crack. Vibration is used to generate frictional heat by rubbing crack face asperities. Heat is generated at regions of contacting crack asperities under low, but nonzero, closure stress. Increasing force is applied to incrementally open the crack and measure the locations of crack heating as a function of applied load. Surface crack closure stresses are approximated from the heating locations as the load is varied and the crack opening stress is measured from the load required to fully open the crack and terminate heat generation.

Toward a Viable Strategy for Estimating Vibrothermographic Probability of Detection

Vibrothermography is a technique for finding cracks and delaminations through infrared imaging of vibration‐induced heating. While vibrothermography has shown remarkable promise, it has been plagued by persistent questions about its reproducibility and reliability. Fundamentally, the crack heating is caused by the vibration, and therefore to understand the heating process we must first understand the vibration process. We lay out the problem and begin the first steps toward relating detectability to the local motion around a crack as well as the crack size. A particular mode, the third‐order free‐free flexural resonance, turns out to be particularly insensitive to the presence of clamping and transducer contact. When this mode is excited in a simple bar geometry the motions of the part follow theoretical calculations quite closely, and a single point laser vibrometer measurement is sufficient to evaluate the motion everywhere. Simple calculations estimate stress and strain anywhere in the bar, and these can then be related to observed crack heating.

Vibrothermographic Crack Heating: A Function of Vibration and Crack Size

Vibrothermography is an inspection technique that detects cracks by observing vibration induced crack heating. Frictional crack heating in a vibrating specimen is directly linked to the resonant vibrational stress on the crack. In simple geometries we can measure the vibrational mode structure and intuit the dynamic vibrational stress field on the crack. This is used to establish a relationship between crack heating and vibration. Such a relationship will be critical for vibrothermography to be accepted as a viable inspection technology. We correlate stress to heating by exciting specimens in a well understood and repeatable resonant vibration mode. Our sample set consists of 65 Titanium and 63 Inconel specimens with low cycle fatigue cracks. Through knowledge of the mode shape, a single point surface velocity measurement is sufficient to calculate the deformed shape of the entire specimen. The loads and stresses within the specimen are calculated from the deformed shape and used to identify the relationship between crack heating and vibration. The observed relationship between normal stress, crack size, and crack heating is presented. This relationship may eventually prove viable for quantifying crack detectability in vibrothermography.Vibrothermography is an inspection technique that detects cracks by observing vibration induced crack heating. Frictional crack heating in a vibrating specimen is directly linked to the resonant vibrational stress on the crack. In simple geometries we can measure the vibrational mode structure and intuit the dynamic vibrational stress field on the crack. This is used to establish a relationship between crack heating and vibration. Such a relationship will be critical for vibrothermography to be accepted as a viable inspection technology. We correlate stress to heating by exciting specimens in a well understood and repeatable resonant vibration mode. Our sample set consists of 65 Titanium and 63 Inconel specimens with low cycle fatigue cracks. Through knowledge of the mode shape, a single point surface velocity measurement is sufficient to calculate the deformed shape of the entire specimen. The loads and stresses within the specimen are calculated from the deformed shape and used to identify the relations...

The Effect of Crack Closure on Heat Generation on Vibrothermography

Vibrothermography is a nondestructive evaluation (NDE) technique that has shown great promise in detecting tight cracks that can often be missed using other NDE methods. Vibration applied to a structure containing cracks forces crack faces to rub together and generates frictional heat imaged with an infrared camera. The closure state of a crack controls the locations and magnitude of heat generation in a vibrated crack. Noncontacting regions of cracks and regions under large closure stresses generally do not rub together to generate heat. Heat is generated at contacting regions of crack faces under low closure stresses. Regions along a crack that generate heat can be modulated based on externally applied stresses. The closure state of a crack, the level of applied vibration, and externally applied stresses influence the regions of a crack that will generate heat and those that will not. Due to the nature of the heat generation process, some cracks that are not detectable using other NDE methods are readil...

FREQUENCY DEPENDENCE OF VIBROTHERMOGRAPHY

It has long been postulated that vibrothermographic heating—the heating of cracks due to sound or vibration‐induced rubbing—may be frequency dependent. It has been difficult to factor out the innate frequency dependence of the heat‐generation process from the geometry‐dependent mode structure. We present experiments showing the heating of cracks in slender Inconel/Titanium specimens at transverse resonance. Different resonant modes vibrate at different frequencies but load the crack in the same way (Mode I). The results show a clear increase of heating with vibration frequency.

FULL‐FIELD VIBRATION MEASUREMENT FOR VIBROTHERMOGRAPHY

Vibrothermography is a nondestructive technique for finding defects through vibration‐induced heating imaged with an infrared camera. To model the crack heating process in Vibrothermography, it is essential first to understand the vibration that causes heat generation. We describe a method for calculating internal motions from surface vibrometry measurements. A reciprocity integral and Gausss law allow representation of internal motion by a surface integral of boundary motion times the Greens Function. We present experimental results showing internal motions calculated from measured surface motions of a vibrating sample. This will ultimately allow estimation of the detectability of a hypothetical crack at an arbitrary location in a specimen.

Measurement of dynamic full-field internal stresses through surface laser Doppler vibrometry

We present a method for evaluating internal dynamic stresses in a solid vibrating body from measurements of surface motion. The method relies on the same mathematics as boundary element method: A boundary reciprocity integral represents interior motion as a surface integral of boundary motion times the Green’s function. The surface motions are measured with a laser vibrometer rather than simulated, giving a direct measurement of internal motions and internal dynamic stresses. Experimental results on a flexing beam demonstrate that stresses measured in this fashion match those calculated from elementary theory.

SYNTHETIC DEFECTS FOR VIBROTHERMOGRAPHY

Synthetic defects are an important tool used for characterizing the performance of nondestructive evaluation techniques. Viscous material‐filled synthetic defects were developed for use in vibrothermography (also known as sonic IR) as a tool to improve inspection accuracy and reliability. This paper describes how the heat‐generation response of these VMF synthetic defects is similar to the response of real defects. It also shows how VMF defects can be applied to improve inspection accuracy for complex industrial parts and presents a study of their application in an aircraft engine stator vane.