A system to monitor a thermal environment of a composite structure to facilitate optimization of a design of the composite structure, the system comprising: - a thermochromatic witness assembly to monitor a thermal environment of a composite structure, the thermochromatic witness assembly comprising: a polymeric material; and, one or more thermochromatic probes mixed into the polymeric material to form a thermochromatic probe mixture, the thermochromatic probe mixture applied to a transparent polymeric film, or formed into the transparent polymeric film with a pressure sensitive adhesive applied thereto, thus forming the thermochromatic witness assembly in a form of a thermochromatic applique (302, 302a, 302b), the thermochromatic applique (302, 302a, 302b) being configured to be applied directly and continuously to a surface of the composite structure to obtain a covered surface, and being configured to monitor the thermal environment of the composite structure by detecting one or more temperatures and one or more times the surface of the composite structure is exposed to the thermal environment during one or more tests performed in the thermal environment, to obtain an exposed surface having one or more maximum temperature locations; - a light source configured to fluoresce the plurality of thermochromatic probes of the covered surface and the exposed surface; - an imaging device configured to image and record one or more images of the covered surface and the exposed surface, after application of the light source; - a baseline map comprising one or more baseline colors and one or more baseline intensities, obtained by applying the light source to the covered surface and imaging the covered surface; - one or more thermal maps each comprising one or more exposed colors and one or more exposed intensities, and each obtained by applying the light source to the exposed surface and imaging the exposed surface; and - a time-temperature history of the composite structure, obtained by comparing color changes between the one or more exposed colors and the one or more baseline colors, and by comparing intensity changes between the one or more exposed intensities and the one or more baseline intensities. - a thermochromatic witness assembly to monitor a thermal environment of a composite structure, the thermochromatic witness assembly comprising: a polymeric material; and, one or more thermochromatic probes mixed into the polymeric material to form a thermochromatic probe mixture, the thermochromatic probe mixture applied to a transparent polymeric film, or formed into the transparent polymeric film with a pressure sensitive adhesive applied thereto, thus forming the thermochromatic witness assembly in a form of a thermochromatic applique (302, 302a, 302b), the thermochromatic applique (302, 302a, 302b) being configured to be applied directly and continuously to a surface of the composite structure to obtain a covered surface, and being configured to monitor the thermal environment of the composite structure by detecting one or more temperatures and one or more times the surface of the composite structure is exposed to the thermal environment during one or more tests performed in the thermal environment, to obtain an exposed surface having one or more maximum temperature locations; a polymeric material; and, one or more thermochromatic probes mixed into the polymeric material to form a thermochromatic probe mixture, the thermochromatic probe mixture applied to a transparent polymeric film, or formed into the transparent polymeric film with a pressure sensitive adhesive applied thereto, thus forming the thermochromatic witness assembly in a form of a thermochromatic applique (302, 302a, 302b), the thermochromatic applique (302, 302a, 302b) being configured to be applied directly and continuously to a surface of the composite structure to obtain a covered surface, and being configured to monitor the thermal environment of the composite structure by detecting one or more temperatures and one or more times the surface of the composite structure is exposed to the thermal environment during one or more tests performed in the thermal environment, to obtain an exposed surface having one or more maximum temperature locations; - a light source configured to fluoresce the plurality of thermochromatic probes of the covered surface and the exposed surface; - an imaging device configured to image and record one or more images of the covered surface and the exposed surface, after application of the light source; - a baseline map comprising one or more baseline colors and one or more baseline intensities, obtained by applying the light source to the covered surface and imaging the covered surface; - one or more thermal maps each comprising one or more exposed colors and one or more exposed intensities, and each obtained by applying the light source to the exposed surface and imaging the exposed surface; and - a time-temperature history of the composite structure, obtained by comparing color changes between the one or more exposed colors and the one or more baseline colors, and by comparing intensity changes between the one or more exposed intensities and the one or more baseline intensities.