At Proto we use the x-ray diffraction method to measure residual stress. X-ray diffraction is presently the only portable nondestructive method that can quantitatively measure residual stress in crystalline and semi-crystalline materials. Our high speed x-ray detector technology enables measurements to be performed easily on metals and ceramics; including traditionally difficult materials such as shot peened titanium. XRD uses the coherent domains of the material (the grain structure) like a strain gage which reacts to the stress state existing in the material. Residual stress and / or applied stress expands or contracts the atomic lattice spacing (d).
Actually, we measure strain and convert to stress. The d-spacings are calculated using Bragg's Law: λ = 2 d sin q. If a monochromatic (l) x-ray beam impinges upon a sample with an ordered lattice spacing (d), constructive interference will occur at an angle q. Changes in strain and thus the d-spacing translate into changes in the diffraction angle q measured by the x-ray detectors. The diffraction pattern is in the shape of a cone for polycrystalline materials. The shape of the diffraction peaks can also be related to the dislocation density and coherent domain size.
Unlike other single detector systems. Proto uses two (2) detectors for stress measurements thus capturing both sides of the diffraction cone. This means twice as much data is collected in the same amount of time simply by virtue of the design.
Proto offers a four (4) detector system that can be used for both the four peak % retained austenite method and in multiphase stress measurements.
Proto also offers 3 and 5 detector configurations for use in Simultaneous Stress and % Retained Austenite determination. Proto adheres to SAE SP-453 Retained Austenite and Its Measurement by X-ray Diffraction and ASTM E975-84 Standard Practice for X-ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation..
Proto uses fiber optic based solid state detectors. The fiber optics allow the detector electronics to be remote from the sensing head making them suitable for measurements in harsh environments. Proto detectors are maintenance free and do not degrade with exposure to x-rays, thus less down time, better productivity and no hidden maintenance costs. Direct expose solid state detectors and position sensitive proportional counters degrade with exposure to x-rays and eventually require replacement which can be extremely costly. Because of x-ray damage, these detectors and counters must constantly be re-calibrated. In addition, some position sensitive proportional counters require periodic (bi-annual) maintenance to refill the sealed gas filled detector housing.
Proto detectors are the fastest detectors on the market today. A stress measurement can be performed in less than 0.3 seconds, an order of magnitude faster than any other detector technology commercially available. Position sensitive proportional counters can only detect one x-ray event at a time. In addition, there is dead time associated with their signal processing which slows data collection. Proto detectors have no dead time associated with them. They are multi-channel solid state detectors that collect many x-ray events simultaneously resulting in unmatched data collection speed. This is particularly important for laboratories with high throughput demands and for industrial on-line and audit station applications.
Position sensitive proportional counters can drift if there is any fluctuation in the DC bias voltage thus causing errors in peak position determination. Ambient temperature fluctuations, gas pressure and oxides on connections, to name a few, can contribute to detector instability and drift. Proto detectors are solid state, thus there is no positional drift associated with them. This means they are much more stable in harsh environments and at elevated or cold temperatures.
Proto's wide 2q detector range, 18.7 degrees 2q for the 40 mm goniometer geometry offers increased accuracy on materials with broad diffraction peaks found in hardened tool and bearing steels.
Most systems, particularly one detector systems, offer only double exposure and multiple exposure sin ²y techniques. Proto systems offer the double exposure and multiple exposure sin ²y techniques as well as the single exposure technique and the multiple exposure sin ²c techniques. This translates into more flexibility for characterizing samples with complicated geometries.
With Proto equipment, unlike other diffraction systems, diffraction peaks can be fit using a number of mathematical functions including, Parabola, Gaussian, Cauchy, Pearson VII, centroid, and mid-chords. Proto also offers both the difference, and cross-correlation methods for peak position determination. This translates into both improved accuracy and flexibility.
Proto systems operate on a true center of rotation and are delivered pre-calibrated to meet exceed ASTM E915-90 "Standard Test Method for Verifying the Alignment of X-ray Diffraction Instrumentation for Residual Stress" and adhere to SAE J784a "Residual Stress Measurement by X-ray Diffraction" alignment specifications. All Proto systems operate using parafocusing optics thus eliminating the need for Sollier slits and allowing very fine positional accuracy in stress measurements inside 90 mm and 120 mm i.d. confinements (e.g. the i.d. of pipes and holes, or between parallel surfaces). The competition cannot offer access to such small holes.
Quick change apertures allow for easy adjustment of the irradiated area and sample setup (apertures can be changed in about 2 seconds) with beam dimensions (irradiated area) available from 0.3 mm to 5.0 mm.
Sample positioning and focusing can be performed easily using the standoff pointer provided with all systems and through the collimator laser pointer which allows the user to quickly locate measurement locations. This is particularly helpful when using the Automated Stress Mapping option.
The 4-Point bending fixture and Proto strain bridge are used for quick and easy determination of the effective x-ray elastic constant for new materials as per ASTM 1426-91, "Standard Test Method for Determining the Effective Elastic Parameter for X-ray Diffraction Measurements of Residual Stress".
The Proto Portable Electro Polisher is custom manufactured specifically for x-ray diffraction work, making material removal quick and efficient.
Truly portable systems are available weighing less than 18 kg (40 lbs).
Custom systems are available for customers with special requirements.
Comprehensive turnkey systems are offered by Proto to their customers to simplify and expedite their stress measurement needs.
Continuous Research and Development and a commitment to give you the best systems in the world.